U.S. patent application number 11/949636 was filed with the patent office on 2008-03-27 for method and apparatus for scheduling communication using a star switching fabric.
Invention is credited to Mohammed N. Islam.
Application Number | 20080075460 11/949636 |
Document ID | / |
Family ID | 36974581 |
Filed Date | 2008-03-27 |
United States Patent
Application |
20080075460 |
Kind Code |
A1 |
Islam; Mohammed N. |
March 27, 2008 |
Method and Apparatus for Scheduling Communication using a Star
Switching Fabric
Abstract
In one embodiment, a scheduler for use with a star switching
fabric includes a scheduling star switching fabric operable to
receive a plurality of packets each associated with one of a
plurality of wavelengths and a plurality of selecting elements
associated with the scheduling star switching fabric. Each of the
plurality of selecting elements is operable to contribute to
selectively passing packets from the scheduling star switching
fabric for receipt by a transmission star switching fabric. Packets
received at the transmission star switching fabric over a given
time period comprise a more uniform load distribution than packets
received at an input to the scheduler over the same period of
time.
Inventors: |
Islam; Mohammed N.; (Allen,
TX) |
Correspondence
Address: |
BAKER BOTTS L.L.P.
2001 ROSS AVENUE
SUITE 600
DALLAS
TX
75201-2980
US
|
Family ID: |
36974581 |
Appl. No.: |
11/949636 |
Filed: |
December 3, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11531605 |
Sep 13, 2006 |
7305186 |
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11949636 |
Dec 3, 2007 |
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10005998 |
Dec 3, 2001 |
7110671 |
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11531605 |
Sep 13, 2006 |
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Current U.S.
Class: |
398/45 ;
370/351 |
Current CPC
Class: |
H04Q 11/0066 20130101;
H04J 14/0212 20130101; H04Q 2011/005 20130101; H04Q 2011/0018
20130101; H04Q 2011/0015 20130101; H04Q 11/0005 20130101; H04Q
2011/0039 20130101; H04Q 2011/0011 20130101; H04J 14/0206 20130101;
H04Q 2011/0094 20130101; H04Q 2011/0016 20130101 |
Class at
Publication: |
398/045 ;
370/351 |
International
Class: |
H04J 14/00 20060101
H04J014/00 |
Claims
1. (canceled)
2. (canceled)
3. (canceled)
4. (canceled)
5. (canceled)
6. (canceled)
7. (canceled)
8. A network operable to direct optical signals, the network
comprising: a scheduler engine operable to generate one or more
control signals; one or more control lasers coupled to the
scheduler engine, each of the one or more control lasers operable
to generate an optical control signal based at least in part on a
portion of the one or more control signals, wherein the optical
control signal comprises at least a portion of information of the
one or more control signals; a transmission star fabric coupled to
the one or more control lasers, the transmission star fabric
operable to distribute the optical control signal to a first
plurality of line cards, each of the first plurality of line cards
comprising one or more receivers coupled to an electronic processor
and further coupled to one or more transmitters, the one or more
receivers adapted to receive the optical control signal, the one or
more optical transmitters adapted to communicate one or more
optical output signals to the transmission star fabric, wherein the
one or more optical output signals are communicated based at least
in part on the portion of information of the one or more control
signals, and wherein the optical control signal is at a different
optical wavelength than at least some of the one or more optical
output signals; wherein the transmission star fabric is operable to
combine at least some of the optical output signals into one or
more combined optical output signals and to direct at least a
portion of the one or more combined output signals to an input
buffer; wherein the one or more control signals generated by the
scheduler engine control firing of the one or more transmitters
using a scheduling algorithm that operates to avoid collisions
between the optical output signals in the transmission star fabric;
wherein the scheduler engine is further coupled to an optical
switching fabric that communicates at least some of the information
of the at least a portion of the one or more combined output
signals with a second plurality of line cards; and wherein traffic
transmitted through the transmission star fabric comprise at least
in part an Internet Protocol (IP) or a Transmission Control
Protocol (TCP) packet.
9. The network of claim 8, wherein the one or more optical output
signals comprise variable length packets, each of the variable
length packets comprising a header and a variable length
payload.
10. The network of claim 8, wherein the input buffer is further
coupled to a wavelength division multiplexer capable of combining
different wavelength signals into a common path, and wherein the
wavelength division multiplexer is further coupled to an optical
amplifier adapted to at least partially compensate for loss
associated with the transmission star fabric.
11. The network of claim 8, wherein the optical switching fabric is
further coupled to one or more tunable lasers.
12. The network of claim 8, wherein the optical switching fabric
comprises one or more optical devices selected from a group
consisting of micro-electromechanical switches (MEMS) and liquid
crystal devices.
13. The network of claim 8, wherein traffic through the optical
switching fabric comprises one or more express channels that bypass
optical-to-electrical conversion at a location associated with the
optical switching fabric.
14. The network of claim 8, wherein traffic transmitted through the
optical switching fabric comprise at least in part Multi-Protocol
Label Switching (MPLS) or Generalized Multi-Protocol Label
Switching (GMPLS) packets.
15. A method of directing optical signals, the method comprising:
generating, at a scheduler engine, one or more control signals;
converting at least a portion of the one or more control signals
into an optical control signal; distributing the optical control
signal using a transmission star fabric to a first plurality of
line cards, each of the first plurality of line cards comprising
one or more receivers coupled to an electronic processor and
further coupled to one or more optical transmitters; receiving, at
the one or more receivers, the optical control signal;
communicating, from the one or more optical transmitters, one or
more optical output signals to the transmission star fabric,
wherein the one or more optical output signals are communicated
based at least in part on a portion of information of the one or
more control signals, and wherein the optical control signal is at
a different optical wavelength than at least some of the one or
more optical output signals; combining, in the transmission star,
at least some of the optical output signal into one or more
combined optical output signals; and receiving, at the scheduler
engine, at least some information of the one or more combined
output signals; communicating at least some of the information from
the combined output signals from the scheduler engine to a second
plurality of line cards through an optical switching fabric;
producing a more uniform traffic distribution at one or more
outputs from the optical switching fabric compared with one or more
inputs to the optical switching fabric; wherein the one or more
control signals generated by the scheduler engine control firing of
the one or more transmitters using a scheduling algorithm that
operates to avoid collisions between the optical output signals in
the transmission star fabric.
16. The method of claim 15, wherein the one or more optical output
signals comprise variable length packets, each of the variable
length packets comprising a header and a variable length
payload.
17. The method of claim 15, wherein the transmission star fabric is
further coupled to an optical amplifier operable to at least
partially compensate for loss associated with the transmission star
fabric.
18. The method of claim 15, wherein traffic through the optical
switching fabric comprises one or more express channels that bypass
optical-to-electrical conversion at a location associated with the
optical switching fabric.
19. The method of claim 15, wherein the optical switching fabric
comprises one or more optical devices selected from a group
consisting of micro-electromechanical switches (MEMS) and liquid
crystal devices.
20. The method of claim 15, wherein the transmission star fabric is
further coupled to an Ethernet network.
21. The method of claim 15, wherein traffic transmitted through the
optical switching fabric comprise at least in part an Internet
Protocol (IP) packet, a Transmission Control Protocol (TCP) packet,
a Multi-Protocol Label Switching (MPLS) packet, or a Generalized
Multi-Protocol Label Switching (GMPLS) packet.
22. A network operable to direct optical signals, the network
comprising: a scheduler engine operable to generate one or more
control signals; one or more control lasers coupled to the
scheduler engine, each of the one or more control lasers operable
to generate an optical control signal based at least in part on a
portion of the one or more control signals, wherein the optical
control signal comprises at least a portion of information of the
one or more control signals; a transmission star fabric coupled to
the one or more control lasers, the transmission star fabric
operable to distribute the optical control signal to a first
plurality of line cards, each of the first plurality of line cards
comprising one or more receivers coupled to an electronic processor
and further coupled to one or more optical transmitters, the one or
more receivers adapted to receive the optical control signal, the
one or more optical transmitters adapted to communicate one or more
optical output signals to the transmission star fabric, the one or
more optical output signals communicated based at least in part on
the portion of information of the one or more control signals,
wherein the optical control signal is at different optical
wavelengths than at least some of the one or more optical output
signals; wherein the one or more optical output signals comprise
variable length packets, each of the variable length packets
comprising a header and a variable length payload; wherein the
transmission star fabric is operable to combine at least some of
the optical output signals into one or more combined optical output
signals and to direct at least a portion of the one or more
combined output signals to an input buffer; wherein the one or more
control signals generated by the scheduler engine control firing of
the one or more transmitters using a scheduling algorithm that
operates to avoid collisions between the optical output signals in
the transmission star fabric; wherein the scheduler engine is
further coupled to an optical switching fabric, wherein the
scheduler engine is operable to receive at least some of the
information of the one or more combined output signals and to
communicate at least some of the information of the one or more
combined output signals to a second plurality of line cards through
the optical switching fabric; and wherein the optical switching
fabric comprises one or more optical devices selected from a group
consisting of micro-electromechanical switches (MEMS) and liquid
crystal devices.
23. The network of claim 22, wherein traffic transmitted through
the optical switching fabric comprise at least in part
Multi-Protocol Label Switching (MPLS) or Generalized Multi-Protocol
Label Switching (GMPLS) packets.
24. The network of claim 23, wherein traffic through the optical
switching fabric comprises one or more express channels that bypass
optical-to-electrical conversion at a location associated with the
optical switching fabric.
25. The network of claim 24, wherein the optical switching fabric
is further coupled to one or more tunable lasers.
26. The network of claim 24, wherein the input buffer is further
coupled to a wavelength division multiplexer capable of combining
different wavelength signals into a common path, and wherein the
wavelength division multiplexer is further coupled to an optical
amplifier adapted to at least partially compensate for loss
associated with the transmission star fabric.
27. The network of claim 24, wherein the transmission star fabric
is further coupled to an Ethernet network.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
11/531,605 by Mohammed N. Islam, filed Sep. 13, 2006, entitled
"METHOD AND APPARATUS FOR SCHEDULING COMMUNICATION USING A STAR
SWITCHING FABRIC," currently pending, which is a continuation of
application Ser. No. 10/005,998 by Mohammed N. Islam, filed Dec. 3,
2001, entitled "METHOD AND APPARATUS FOR SCHEDULING COMMUNICATION
USING A STAR SWITCHING FABRIC," now U.S. Pat. No. 7,110,671B1.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to the field of communication
systems, and more particularly to an apparatus and method for
scheduling communication through a star switching fabric.
BACKGROUND
[0003] As optical systems continue to increase the volume and speed
of information communicated, the need for methods and apparatus
operable to facilitate high speed optical signal processing also
escalates. Router and switch cores performing optical switching
generally implement schedulers to assist in avoiding contention for
common system resources. In prior approaches, there has generally
been a tension between the complexity of the scheduler used and the
delay experienced in the switching fabric. More complex schedulers
generally require significant system resources and can be difficult
to implement. Trivial schedulers, while simple to implement, have
generally resulted in unsatisfactory switching delays.
OVERVIEW OF VARIOUS EXAMPLE EMBODIMENTS
[0004] The present invention recognizes a need for a method and
apparatus operable to efficiently and effectively facilitate
scheduling of communication through a star switching fabric. In one
embodiment, a scheduler for use with a star switching fabric
comprises a scheduling star switching fabric operable to receive a
plurality of packets each associated with one of a plurality of
wavelengths, and a plurality of selecting elements associated with
the scheduling star switching fabric. Each of the plurality of
selecting elements is operable to contribute to selectively passing
packets from the scheduling star switching fabric for receipt by a
transmission star switching fabric. Packets received at the
transmission star switching fabric over a given time period
comprise a more uniform load distribution than packets received at
an input to the scheduler over the same period of time.
[0005] In a method embodiment, a method of scheduling operation of
a star switching fabric comprises receiving at a scheduler a
plurality of packets each having a wavelength and communicating
from a scheduling star switching fabric of the scheduler a
plurality of substantially similar sets of the plurality of
packets. The method further comprises selectively passing packets
having selected wavelengths from the scheduling star switching
fabric for receipt by a transmission star switching fabric. Packets
received at the transmission star switching fabric over a given
time period comprise a more uniform load distribution than packets
received at an input to the scheduler over the same time
period.
[0006] Depending on the specific features implemented, particular
embodiments may exhibit some, none, or all of the following
technical advantages. One embodiment provides a way to schedule
communication of optical signals through a star switching fabric
using a simple scheduling algorithm while maintaining good
throughput. Other technical advantages are readily apparent to one
of skill in the art from the attached figures, description, and
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] For a more complete understanding of the present invention,
and for further features and advantages thereof, reference is now
made to the following description taken in conjunction with the
accompanying drawings, in which:
[0008] FIG. 1 is a block diagram illustrating an exemplary
communication system implementing aspects of the present
invention;
[0009] FIG. 2 is a block diagram of one example embodiment of an
optical implementing aspects of the present invention;
[0010] FIG. 3 is a block diagram of another example embodiment of
an optical implementing aspects of the present invention;
[0011] FIGS. 4a-4b are block diagrams illustrating example star
switch fabric architectures;
[0012] FIGS. 5a-5d are block diagrams illustrating example
scheduling mechanisms for use with star switching fabrics,
including those described herein;
[0013] FIGS. 6a-6d are block diagrams illustrating additional
example scheduling mechanisms for use with star switching fabrics,
including those described herein;
[0014] FIG. 7 is a block diagram illustrating an example embodiment
of a continuum optical source for use with a star switching fabric,
including those described herein;
[0015] FIGS. 8a-8b are block diagrams illustrating example
mechanisms useful in increasing the speed of optical routers
including those described herein;
[0016] FIGS. 9a-9c are block diagrams illustrating additional
example mechanisms useful in increasing the speed of optical
routers including those described herein;
[0017] FIG. 10 is a flow chart showing one example of a method of
routing optical signals using a star switching fabric;
[0018] FIG. 11 is a flow chart showing one example of a method of
scheduling communications through a star switching fabric;
[0019] FIG. 12 is a flow chart illustrating one example of a method
of enhancing the effective switching speed of an optical router by
reducing the duration of packets communicated through a star
switching fabric of the router;
[0020] FIG. 13 is a flow chart showing one example of a method of
enhancing the effective switching speed of an optical router by
aggregating packets bound for a common output communication
path;
[0021] FIG. 14 is a flow chart showing one example of a method of
enhancing the effective switching speed of an optical router using
a star switching fabric by providing express lanes that bypass line
cards performing electronic signal processing of some of the
optical signals received;
[0022] FIG. 15 is a flow chart showing one example of a method for
enhancing the effective switching speed of an optical router using
a star switching fabric by assigning a plurality of tunable filters
to each output link from the router; and
[0023] FIG. 16 is a flow chart showing one example of a method of
enhancing the effective switching speed of an optical router using
a star switching fabric by assigning a plurality of tunable
transmitters to an input link to the optical router.
DETAILED DESCRIPTION OF VARIOUS EXAMPLE EMBODIMENTS
[0024] FIG. 1 is a block diagram illustrating an exemplary
communication system 10 operable to facilitate communication of
optical signals. In this example, system 10 includes a router 12
coupled to a plurality of network elements 20a-20n. Router 12
facilitates directing optical communication signals between various
elements within and/or coupled to system 10. Throughout this
document, the term "coupled" denotes any direct or indirect
communication between two or more elements said to be "coupled" to
one another. Elements coupled to one another may, but need not, be
physically connected to one another. Additional elements may or may
not reside between two elements said to be "coupled" to one
another.
[0025] As used throughout this document, the term "router" refers
to any hardware, firmware, software, or combination thereof
operable to receive signals from various sources and to direct
signals received toward one or more destinations depending at least
in part on an identifier associated with the signal and its
destination.
[0026] In one particular embodiment, signals received by router 12
comprise packets. As used throughout this document, the term
"packet" refers to signals having fixed or variable size, each
comprising an identifier associated with a destination network
element. While some of the packets may comprise traffic terminating
at router 12, at least some of the packets contain identifiers
identifying destination elements external to router 12. The packets
could comprise, for example Internet Protocol (IP) packets or a
Transmission Control Protocol (TCP) packets including an address
identifying a destination network element. As another example, each
incoming optical signal could comprise a Multi-Protocol Label
Switching (MPLS) packet or Generalized Multi-Protocol Label
Switching (GMPLS) packet comprising a tag identifying a destination
network element.
[0027] In some cases, the "destination network element" may
comprise a node within or coupled to system 10, but external to
router 12, to which information in the optical signal is ultimately
destined. In other cases, the "destination network element" may
comprise a node external to router 12 in a communication path
between router 12 and an element to which the information is
ultimately destined. In that case, the "destination network
element" comprises an intermediate network element facilitating
further routing of the information to the ultimate destination
network element. In still other cases, router 12 may comprise the
destination element.
[0028] Network elements 20a-20n communicate optical signals over
system 10. Network elements 20 may comprise any hardware, software,
firmware, or combination thereof operable to transmit and/or
receive information via communication system 10. Router 12
communicates with network elements 20 via communication links
22a-22n. Communication links 22 may comprise, for example, optical
fibers. Communication links 22a-22n could, however, comprise any
land based or space based communication medium, or combination of
such media operable to communicate one or more optical signals.
[0029] Network elements 20 can couple directly to communication
links 22, or may couple to communication links 22 through one or
more networks 24. Each of networks 24 could comprise, for example,
a data network, a public switched telephone network (PSTN), an
integrated services digital network (ISDN), a local area network
(LAN), a wide area network (WAN), or other communication system or
combination of communication systems at one or more locations.
Networks 24 may comprise wireless networks, wireline networks, or
combinations of wireless and wireline networks. Network elements 20
and/or router 12 can reside with networks 24 or externally to those
networks.
[0030] In this particular example, router 12 comprises a plurality
of line cards 30a-30n. As used throughout this document, the term
"line card" can include any hardware, software, firmware, or
combination thereof operable to receive incoming optical signals
from communication links 22 and to convert at least a portion of at
least some of the incoming optical signals to electrical signals to
facilitate electronic decision making with respect to those
signals. In the illustrated embodiment, each line card 30 is
associated with an optical transmitter operable to generate, based
at least in part on the electrical signals received, an optical
router signal for transmission within router 12. The optical
transmitters may comprise, for example, laser diodes, light
emitting diodes, or other light emitting sources.
[0031] Line cards 30 may reside in one or more physically separate
locations. In this particular example, a first plurality of line
cards 30a-30m reside in a first rack 32, while a second plurality
of line cards 30m+1-30n reside in a second rack 34. As one specific
example, first rack 32 and second rack 34 may each hold sixteen
line cards 30. Additional or fewer numbers of line cards and
numbers of racks could be used without departing from the scope of
the invention.
[0032] In this example, racks 32 and 34 are physically separated
from one another. In one embodiment, racks 32 and 34 may be
separated by a distance where communication speed considerations
make it desirable to implement optical communication between racks
32 and 34. Line cards 30 in racks 32 and 34 advantageously use
optical communication links 42a-42n to facilitate high speed
communication. In this particular example, optical communication
links 42a-42n interconnect through an optical switching fabric
40.
[0033] Switching fabric 40 comprises hardware, software, firmware,
or combinations thereof operable to facilitate directing optical
router signals between line cards 30 and/or express channels (not
explicitly shown in this figure), which bypass line cards 30. In a
particular example, switching fabric 40 comprise a star switching
fabric. Throughout this document, the term "star switching fabric"
refers to a device and/or functionality operable to receive a
plurality of input optical signals from a plurality of sources and
to communicate a substantially similar set of at least some of the
input optical signals to each of a plurality of destinations. In
one particular embodiment, star switching fabric 40 resides within
one of racks 32 or 34.
[0034] In the example illustrated in FIG. 1, switching fabric 40
could comprise a star switching fabric operable to receive a
plurality of input optical router signals from plurality of line
cards 30 and to communicate substantially similar sets each
comprising at least some of the input optical router signals back
to at least some of the plurality of line cards 30 and/or express
channels bypassing line cards 30. Fused fiber couplers, waveguide
star couplers, arrayed waveguide gratings, power splitters,
wavelength division multiplexers, cascaded 2.times.2 couplers,
n.times.n couplers and cascades of these couplers are just a few
examples of devices that could form star switching fabric 40.
[0035] In a particular embodiment, switching fabric 40
advantageously interconnects line cards 30 residing within
different racks 32 and 34, and facilitates communicating optical
router signals between line cards 30 without requiring
electrical-to-optical or optical-to-electrical signal conversions
within switching fabric 40. This design can increase the speed of
the router, and could also reduce the physical size, power
dissipation, and cost of the router. In one particular embodiment,
switching fabric 40 could occupy less than one third of the space
of rack 32 or 34, leaving substantial room for additional line
cards and other processing elements.
[0036] In one example embodiment, router 12 includes a plurality of
tunable filters. A tunable filter can comprise any hardware,
software, and/or firmware operable to selectively substantially
communicate one or more wavelengths of light while substantially
rejecting other wavelengths of light. In this example, each tunable
filter is associated with one of line cards 30 or with an express
channel.
[0037] Each tunable filter is operable to receive a plurality of
optical signals and to select one or more signals for processing by
tuning to a wavelength associated with the selected signals. The
use of tunable filters in router 12 advantageously facilitates
efficient multicast and/or broadcast operation simply by tuning
multiple filters, each associated with a separate line-card or
express channel, to a common wavelength.
[0038] In operation, router 12 receives a plurality of input
optical signals from communication links 22. One or more optical
links can carry signals at wavelengths designated as express
channels within router 12. Express channels route directly through
switching fabric 40 from inputs of router 12 to outputs of router
12, bypassing line cards 30.
[0039] With respect to non-bypass traffic, line cards 30 receive at
least some of the input optical signals and convert all or a
portion of those signals to an electronic format to facilitate
electronic decision making processing. As one particular example,
one or more line cards 30 receive packets and convert at least a
destination identifier portion of the packet into an electronic
format. Line cards 30 then use the electronic destination
identifier information to assist in directing the packet to a
destination network element.
[0040] Optical transmitters associated with line cards 30 generate
input optical router signals based at least in part on processing
of the electronic signals. Router 12 communicates the input optical
router signals and any bypass traffic to switching fabric 40, where
a plurality of input optical router signals and any bypass traffic
are combined to form an output optical router signal. The output
optical router signal comprises information from some or all of the
plurality of input optical router signals and/or express channel
signals.
[0041] Switching fabric 40 facilitates communicating the output
optical router signal to at least some of a plurality of tunable
filters, each associated with an output link from router 12.
Tunable filters receive the output optical router signal and tune
to a selected wavelength associated with a portion of the output
optical router signal destined for a line card 30 associated with
that filter or an express channel output link associated with that
filter. The selected portion of the output optical router signal
can carry the packet bound for the destination network element.
[0042] Where a line card 30 is associated with the tunable filter,
the line card facilitates communication of the received packet from
the associated filter to the destination network element. This may
include, for example, passing the packet in optical form to an
output communication link, or converting the packet to an
electrical format for further processing within router 12. Router
12 may also perform wavelength conversion prior to passing the
signals toward the destination network element.
[0043] FIG. 2 is a block diagram of one particular embodiment of
router 112. In this example, router 112 includes a plurality of
wavelength division multiplexer/demultiplexers (WDM) 110a-110n.
Each WDM is associated with one or more optical links 122 carrying
wavelength division multiplexed optical signals. Wavelength
division multiplexers/demultiplexers 110 receive incoming WDM
signals from optical links 122 and separate the incoming signal
into a plurality of channels .lamda..sub.1-.lamda..sub.n for
processing within line cards 130. On the output side, wavelength
division multiplexers/demultiplexers 110 combine a plurality of
signals into one or more multiple wavelength output signals.
[0044] In this particular example, incoming signals received at
links 122 also include one or more express channels .lamda..sub.Ex,
which traverse router 112 over bypass links 155 without being
processed by line cards 130. Express channels .lamda..sub.Ex are
communicated directly to switching fabric 140 without any
optical-to-electrical conversion. Implementing express channels can
provide significant advantages in avoiding unnecessary processing
of particular groups of optical signals. Although this example
shows just one express link, any number of express links could be
provided. Traffic entering router 12 can be divided between
processed traffic and express traffic, for example, by designating
particular wavelengths in WDM signals 122 accordingly.
[0045] In the illustrated embodiment, router 112 comprises a
plurality of racks 132a-132n of line cards 130 each coupled to a
switching fabric 140. In this example racks 132a and 132n are
physically separated from one another and switching fabric 140
serves as an all-optical interconnect between line cards 130 in
racks 132a and 132n. In other embodiments, all line cards 130 could
reside locally to one another, for example, in a single rack.
[0046] In the illustrated example, each line card 130 includes a
processor 136. Alternatively, some of all of line cards 130 could
share central processing resources accessible to line cards 130. In
any case, processor or processors 136 operate to convert at least a
portion of an input optical signal 128 arriving from one of
communication links 22 to an electrical format. For example, input
optical signal 128 may comprise a packet having a destination
identifier, such as a TCP address, an IP address or, an MPLS or
GMPLS tag. Processor 136 operates to convert at least the
destination identifier portion of the packet to an electrical
format to facilitate electronic decision making functions with
respect to that packet.
[0047] In this example, each line card 130 comprises a memory 138.
Memory 138 may comprise any hardware, software, and/or firmware
operable to facilitate storage and/or retrieval of electronic
information. Although in this example memory 138 is shown as
residing entirely within line card 130, all or a portion of memory
138 could alternatively reside at another location remote from but
accessible to line card 130.
[0048] Each memory 138 stores a look-up table 144 operable to
facilitate electronic decision making to result in communicating
incoming optical signals 128 from router 112 toward destination
network elements residing externally to router 112. Look-up table
144 may comprise any data structure, compilation, or other
arrangement of information facilitating generation of instructions
based at least in part on information contained in a signal to be
routed. As one particular example, using an identifier of the
destination element from a received packet, processor 136 may index
look-up table 144 to obtain instructions on directing the packet
through router 112 and toward the destination element. Look-up
table 144 can, for example, facilitate TCP/IP routing based on an
address associated with the destination element. Alternatively,
look-up table 144 can facilitate label switching based on an MPLS
or GMPLS routing protocol.
[0049] In some cases, router 112 may comprise an edge router
facilitating communication of packet traffic received in one format
through a subnetwork operating with another format. For example,
router 112 could receive IP or TCP packets from an IP network and
convert those packets to an MPLS or GMPLS format for transmission
through a label switching portion of a network. In that case, the
packets traversing switch fabric 140 would comprise MPLS or GMPLS
packets.
[0050] Each line card 130a-130n further comprises an optical
transmitter 146a-146n operable to receive an electronic signal
129a-129n and to generate an input optical router signal 152a-152n,
respectively, based at least in part on the received electronic
signal 129a-129n. Each optical transmitter may comprise, for
example, a laser diode, although any optical transmitter could be
used without departing from the scope of the invention. Optical
transmitters 146 may comprise directly modulated or externally
modulated lasers. Alternatively, one or more of optical
transmitters 146 may comprise lasers having integrated modulators,
such as electro-absorbtion modulators.
[0051] In one particular embodiment, each optical transmitter 146
comprises a fixed wavelength laser. Throughout this document, the
term "fixed wavelength laser" denotes a laser operable to generate
optical signals at approximately one predetermined wavelength or
range of wavelengths, and which does not during operation perform
selective adjustment of the output wavelength. Lasers whose output
wavelength varies during operation due to, for example,
fluctuations in environmental conditions are not intended to be
excluded from the definition of a "fixed wavelength" laser.
Moreover, tunable lasers operated without intentionally selectively
varying the output wavelength of the laser during operation are
intended to be within the definition of a "fixed wavelength"
laser.
[0052] Although some embodiments of the invention implement tunable
lasers, using fixed wavelength lasers 146 provides an advantage of
reducing cost and complexity of router 112 compared to solutions
requiring tunable lasers. In addition, one aspect of the invention
recognizes that using fixed wavelength lasers, each transmitting at
a different wavelength, reduces or eliminates collisions in the
switching fabric.
[0053] In this example, each optical link 128 is associated with a
tunable filter. In the illustrated embodiment, each of line cards
130a-130n includes a tunable filter 148a-148n, respectively. Each
express channel 127 also includes a tunable filter 148ex1-148exn.
Tunable filters 148 may each comprise, for example, a tunable
optical filter operable to selectively communicate particular
optical router signals 152 from output optical router signal 154.
As one example, tunable filters 148 could each comprise a Fabry
Perot interferometric device. In a particular embodiment, the
filter could comprise a micro-electromechanical switch (MEMS)
device capable of tuning at speeds faster than once each one
hundred nanoseconds.
[0054] Although many other tunable filter designs could be
implemented without departing from the scope of this disclosure,
the following provides a brief description of one such device.
[0055] A Fabry Perot interferometric micro electromechanical
switching (MEMS) device typically implements a stationary mirror
structure and a moveable mirror structure, which form between them
an optical cavity having a depth that can be selectively altered by
applying a force to the moveable mirror structure. In one
particular novel design, the moveable mirror structure can be
supported by actuators surrounding the moveable mirror
structure.
[0056] The actuators can comprise, for example, a stationary
conductor and a moveable conductor, which form between them an
electrode gap. A voltage difference applied between the two
conductors creates an electrostatic force tending to move the
moveable conductor toward the stationary conductor.
[0057] The actuators can be placed in symmetric locations around
the moveable mirror and coupled to the moveable mirror. Locating
the actuators around the mirror facilitates independent selection
of the nominal optical cavity depth and the electrode gap depth.
Thus, this design facilitates optimizing both the optical
characteristics of the interferometer through selection of the
optical cavity depth, and separate optimization of the electrical
characteristics of the device through independent selection of the
electrode gap depth. Moreover, by forming the interferometer and
actuators in this manner, the dimensions of the moveable conductor
can be optimized to provide high speed and low drive voltage.
[0058] In some embodiments, the moveable mirror assembly of the
interferometer can be supported by a frame that substantially
surrounds and/or covers the moveable mirror. The frame and location
of the actuators help to avoid deformation of the moveable mirror
structure during actuation, resulting in better optical
characteristics for the device. Although details of one particular
tunable filter have been described here, other tunable filter
designs could be used. Other MEMs designs, lithium niobate tunable
filters, and liquid crystal tunable filters provide a few
examples.
[0059] Line cards 130 can also include a converter 149 operable to
convert the recognized portion 152 of output optical router signal
154 into an electrical signal 129 for further processing within
router 112.
[0060] Router 112 includes a control network 160 operable to
communicate control signals 162 to facilitate selection of a
communication path through router 112 and on to the destination
element. In one embodiment, control signals 162 direct tunable
filters 148 to tune to a specified wavelength or range of
wavelengths to facilitate selection of an appropriate optical
router signal 152 from multiple wavelength output optical router
signal 154. As a particular example, control network 160 could
comprise an Ethernet. Although other control network configurations
could be used without departing from the scope of the invention, an
Ethernet provides an advantage of efficient and economical
operation at speeds sufficient to control and reset filters 148
between receipt of sequential optical router signals.
[0061] In an alternative embodiment, control network 160 could
comprise a plurality of control lasers each operable to generate
and communicate to filters 148 an optical control signal 162 at,
for example a designated control frequency. In this embodiment
optical control signals are communicated via switching fabric 140.
Router 112 may, for example, communicate control signals to filters
148 prior to communicating optical router signals to filters 148.
In that way, filters 148 can be provisioned to accept selected
optical router signals 152 depending on the state of an optical
control signal 162.
[0062] Router 112 may include a scheduler 164 coupled to control
network 160. Scheduler 164 can operate to provide scheduling
functionality to avoid or reduce contention in transmission of
control signals 162 to filters 148. FIGS. 5a-5d discussed below
provide details of example scheduling mechanisms useful with any
star switching fabric, including the design discussed herein with
respect to FIGS. 2 and 3.
[0063] Router 112 interconnects line cards 130 using switching
fabric 140 including communication links 143 and 145. Communication
links 143 couple lasers 146 to switching fabric 140, while
communication links 145 couple filters 148 to switching fabric 140.
In this example, communication links 143 and 145 comprise single
mode fibers.
[0064] In operation, wavelength division multiplexer/demultiplexers
110 receive one or more multiple wavelength signals 122 and
separate input signals 128a-128n including express channels 127
from one another. Express channels 127a-127n are directed to
switching fabric 140 without performing optical-to-electrical
conversions on those signals.
[0065] Processor(s) 136 associated with line cards 130 receive
input optical signals 128a-128n and converts at least a portion of
each signal to an electronic format. In one embodiment,
processor(s) 136 can operate to convert to an electronic form the
entire contents including the header and payload portions of
incoming optical signal 128. Processor(s) 136 apply at least a
destination identifier portion of the electronic signal 129 to
look-up table 138 to determine communication instructions for the
signal. Optical transmitter 146 can then form an optical router
signal 152 by transforming electronic information into optical
router signal 152.
[0066] In another embodiment, processor(s) 136 may convert only a
header portion of input optical signal 128 to electronic form
leaving the payload portion in optical form. In that case,
processor(s) 136 may perform electronic processing on the header to
determine routing of the signal, and then pass the header or a
modified version thereof to optical transmitter 146. In that
embodiment, optical transmitter 146 produces an optical header,
which is then combined with the optical payload portion of the
signal to form an optical router signal for transmission through
switching fabric 140. In that embodiment, the portion of the input
optical signal that is not converted to an electronic format can be
passed through a delay element, such as a buffer or a delay line,
to facilitate delay while the identifier portion of the packet is
electronically processed.
[0067] Each optical transmitter 146 communicates to switching
fabric 140 an optical router signal 152 at a particular wavelength.
Where optical transmitters 146 comprise fixed wavelength lasers,
each optical transmitters 146 transmits its optical router signal
152 at a predetermined specified wavelength associated with that
particular transmitter 146, which is different from wavelengths
transmitted from other transmitters 146. Where optical transmitters
146 comprise tunable lasers, each laser communicates its optical
router signal 152 at a wavelength determined by a control signal
from, for example, processor 136.
[0068] In this particular embodiment, each processor 136 determines
a control signal 162 based at least in part on applying a
destination identifier to the look-up table 144 associated with
that line card 130. In some embodiments control signal 162 may
identify an output communication link 128 coupling to the
destination network element. In other cases, control signal 162 may
identify a filter 148 associated with the identified output link
128. Router 112 communicates control signals 162 via control
circuitry 160 to tunable lasers 146 and/or tunable filters 148 to
selectively enable communication paths through router 112.
[0069] Transmitters 146 each communicate an optical router signal
152 to switching fabric 140. In this particular example, switching
fabric 140 comprises a star coupler switching fabric. Star coupler
switching fabric 140 receives a plurality of optical router signals
152 and may also receive one or more express channels 127 each
having substantially different wavelengths. Switching fabric 140
combines information from at least some of the optical router
signals 152 and/or at least some of the express channels 127 into
an output optical router signal 154. Each output optical router
signal 154 comprises a substantially similar set of optical router
signals 152 and/or express channels 127. Star switching fabric
communicates optical router signal 154 to some or all of filters
148.
[0070] In a particular embodiment, transmitters 146 comprise fixed
wavelength lasers while filters 148 comprise tunable filters. This
embodiment provides an advantage of minimizing cost by implementing
low cost tunable filters as compared to relatively higher cost
tunable lasers. In addition, implementing tunable filters readily
facilitates multicast and/or broadcast operation simply by
provisioning the tunable filters to receive a plurality of the
optical router signals communicated from switching fabric 140.
[0071] In this example, router 112 communicates control signals 162
to scheduler 164 and/or to a tunable filter 148 associated with a
communication path leading to the destination network element.
Filters 148 receive control signals 162 and selectively tune to
receive particular wavelengths as directed by control signals 162.
In this manner, tunable filters 148 selectively receive only the
portion of output optical router signal 154 communicated from
switching fabric 140 that is intended for further transmission
toward the destination element.
[0072] In an alternative embodiment, transmitters 146 may comprise
tunable optical lasers. In that embodiment, lasers 146 may receive
control signals 162 and communicate optical router signals 152 to
switching fabric 140 at selected wavelengths predetermined to match
wavelengths of filters 148 associated with communication paths
leading to the destination network elements.
[0073] Filters 148 receive specified portions of output optical
router signal 154 corresponding to the packet desired for
transmission to the destination network element. In one embodiment,
each filter 148 comprises an optical filter operable to communicate
only optical router signals having a specified wavelength. In a
particular embodiment, the received optical router signal can be
communicated without further processing in router 112 to the
destination network element. In another embodiment, each line card
130 may also include a converter 149 operable to convert an optical
router signal received from an associated filter 148 to an
electronic format for further processing within router 112 before
conversion back to an optical format to be communicated toward the
destination network element.
[0074] FIG. 3 is a block diagram illustrating another embodiment of
a router 212. Router 212 is similar in structure and function to
router 112 shown in FIG. 2, except that in this case, tunable
filters 248 reside remotely from line cards 230 and in close
proximity to or integrally with switching fabric 240.
[0075] Router 212 includes a plurality of line cards 230 each
associated with an optical transmitter 246 and a tunable filter
248. Each line card 230 is coupled to a switching fabric 240 via
communication links 243 and 245. Switching fabric 240 operates to
receive a plurality of input optical router signals 252a-252n from
optical transmitters 246a-246n and one or more express channel
signals 227 and to generate an output optical router signal 254
comprising information from at least some of the input optical
router signals 252a-252n and/or express channel signals 227.
[0076] In one particular embodiment, optical transmitters 246
comprise fixed wavelength transmitters each operable to generate a
particular wavelength signal. In this embodiment, filters 248 each
comprise a tunable optical filter operable to receive multiple
signals each having different wavelengths and to tune to receive
only a selected wavelength signal in response to a control signal
262. In this example, tunable filters 248 selectively tune to a
particular wavelength or range of wavelengths based on control
signal 262 from control network 260. Control network 260 may
comprise, for example, an Ethernet or other suitable network or
combination of communication links operable to communicate an
electronic control signal 262. Alternatively, control network 260
could comprise control lasers operable to communicate optical
control signals 262 via switching fabric 240.
[0077] In this embodiment, optical transmitters 246 reside on their
associated line cards 230, while tunable filters reside remotely
from line cards 230. In this example, tunable filters 248 and
switching fabric 240 comprise a router core 245 for router 256. In
this embodiment, router core 245 includes switching fabric 240
combined with closely coupled tunable filters 248. Removing tunable
filters 248 from line cards 236 and integrating those filters into
router core 245 can provide significant advantages. For example,
removing tunable filters 248 from line cards 236 provides
additional space on each line card for other processing elements,
or facilitates reducing the physical size of each line card. This
allows for additional line cards to reside in any given rack.
Moreover, integrating filters 248 within router core 245 at or near
switching fabric 240 facilitates the use of arrays of filters,
rather than individually packaged filters for each channel.
Coupling switching fabric 240 to an array of tunable filters can
significantly reduce packaging costs and, thus, the overall cost of
the router.
[0078] Filters 248, in this example, are coupled to switching
fabric 240 using optical connections 255. Each optical connection
255 may comprise, for example, a short length of fiber or a planar
waveguide. In the illustrated embodiment, each of communication
links 243 coupling optical transmitters 246 to switching fabric 240
comprises a single mode fiber. Communication links 245 coupling
filters 248 to line cards 230 may comprise single mode or
multi-mode fibers. Communication networks using star couplers have
traditionally used single mode fibers to couple network elements
both to and from the star coupler. One aspect of the invention
recognizes that in certain embodiments, such as where filters 248
reside remotely from line cards 230, the use of multi-mode fibers
to couple one or more filters 248 to associated line cards 230 can
provide an advantage of reducing cost of router 212 without
significantly degrading performance of the device.
[0079] As discussed above, star switching fabric 40 can assume any
of a variety of physical embodiments. For example, a plurality of
fibers can be physically fused together to provide star switching
capabilities. In addition, wave guide star couplers and arrayed
wave guide gratings can be used to provide star switching
functionality. FIGS. 4A-4B depict two particular embodiments of
novel star switching architectures that can be implemented in any
system using star switching functionality, including the optical
routers described herein. In particular, FIG. 4A shows a
wavelength-based star switching fabric 40a. Wavelength-based star
switching fabric 40a includes a wavelength division multiplexer 41.
Wavelength division multiplexer 41 receives a plurality of
individual wavelength signals and combines those signals into a
wavelength division multiplexed signal. Wavelength division
multiplexer 41 may receive individual wavelength signals, for
example, from line cards at input ports to a router, or may receive
express lane traffic directly from input ports to the router.
[0080] Wavelength-based switching fabric 40a includes at least one
optical amplifier 43 operable to receive and amplify the wavelength
division multiplex signal generated by wavelength division
multiplexer 41. Optical amplifier 43 could comprise any of a
variety of amplifier types, such as a distributed Raman amplifier,
a discrete Raman amplifier, a rare earth-doped amplifier, a
semiconductor amplifier, or a combination of these or other types
of amplifiers. Amplifier 43 can be selected, for example, to offset
losses associated with distributing signals through star switching
fabric 40 and/or to provide unity gain for bypass traffic
traversing router 12.
[0081] Wavelength-based switching fabric 40a also includes a
cascade of splitters 45. Cascade of splitters 45 is operable to
receive the wavelength division multiplexed signal from amplifier
43 and to split that signal into a plurality of output signals. In
a particular embodiment, each splitter in cascade 47 operates to
approximately equally split each signal received into two output
signals, each comprising substantially the same wavelength set
output from wavelength division multiplexer 41. Multiple wavelength
signals are then communicated from the outputs of cascade 47 to
output links of the router or back to line cards for further
processing.
[0082] In operation, wavelength-based star switching fabric 40a
receives a plurality of signals each having a distinct center
wavelength. Some of these signals can be the result of signals
generated at line cards within a router, while others may be
express traffic designated to pass through the router without
electrical processing. Wavelength division multiplexer 41 combines
some or all of these wavelengths into a multiple wavelength signal.
The multiple wavelength output signal is amplified by amplifier 43
and communicated to a cascade 47 of splitters 45. Cascade 47
separates the incoming multiple wavelength signal into a plurality
of output signals each carrying a substantially similar set of
wavelengths as the input signal to the cascade.
[0083] FIG. 4B shows another embodiment of a star switching
architecture, in this case a power-based star switching fabric 40b.
Power-based star switching fabric 40b includes a power combiner 44
operable to receive a plurality of input signals. In this
particular example, some or all of the input signals have center
wavelengths distinct from other input signals. Power combiner 44
combines the input signals based on their power to create a
combined signal carrying all information received at the inputs of
power combiner 44. Power-based star switching fabric 40b also
includes at least one optical amplifier 46 operable to receive the
combined signal from power combiner 44, to amplify that signal, and
to communicate the amplified signal to a power splitter 48.
Amplifier 46 may be similar in structure and function to amplifier
43 described with respect to FIG. 4A. Power splitter 48 comprises a
device, or combination of devices operable to separate the power
combined signal into a plurality of output signals each containing
substantially the same set of wavelengths output by power combiner
44. Signals output by power combiner 48 may be communicated
directly to output links of a router, or may be communicated to
line cards for additional processing.
[0084] To resolve contention between signals competing for the same
system resources, it is helpful to implement a scheduling mechanism
for use with star switching fabrics. Although complex scheduling
mechanisms can be implemented without departing from the scope of
the invention, the following figures address relatively simple
scheduling mechanisms that can be implemented in conjunction with
any star switching fabric, including those described herein. These
scheduling mechanisms provide adequate contention resolution
capabilities while utilizing minimum processing resources.
[0085] FIG. 5a is a block diagram showing one example of a
scheduling mechanism 300 useful in conjunction with any star
switching fabric. This example depicts scheduling mechanism 300
operating within router 112 shown in FIG. 2. Scheduling mechanism
300 could, however, be useful with any router or switch using a
star switching fabric. In this particular example, scheduling
mechanism 300 includes a scheduling star switching fabric 340
configured to receive input signals 252. Signals received at inputs
to scheduling star switching fabric 340 comprise a non-uniform load
distribution, where some inputs receive more traffic than others.
In a particular example, each input to scheduling star switching
fabric 340 is associated with a particular wavelength and operates
to receive traffic corresponding to the associated wavelength. In
one particular example, each of the inputs to scheduling star
switching fabric 340 may receive input optical router signals from
an associated line card 230.
[0086] Scheduling star switching fabric 340 communicates signals
235 to a transmission star switching fabric 240. Transmission star
switching fabric 240 communicates output router signals 254 toward
line cards 230 and/or output links 228 from router 112. Scheduling
star switching fabric 340 facilitates creating a more uniform load
distribution of wavelength signals at the input to transmission
star switching fabric 240 compared to the load distribution
received at scheduling star switching fabric 340. Scheduling star
switching fabric 340 helps to more evenly distribute the traffic
load across the inputs to transmission switching fabric 340 to
allow scheduling of communication through switching fabric 240
using a relatively trivial scheduling algorithm.
[0087] Scheduling mechanism 300 includes one or more scheduling
engines 364. Scheduling engine 364 comprises any hardware,
software, firmware, or combination thereof operable to instruct
operation of tunable switching elements, such as tunable
transmitters or tunable filters, within router 112. In this
particular example, scheduling engine 364 communicates control
signals to a plurality of tunable filters 348 in scheduling star
switching fabric 340 and to a plurality of tunable filters 248 in
transmission star switching fabric 240. Although this example
illustrates a single scheduling engine communicating with filters
248 and filters 348, separate scheduling engines could be
implemented.
[0088] Scheduling engine 364 executes a scheduling algorithm to
determine the order in which filters 248 and 348 will be operated
and the center wavelength to which each filter will tune. In this
particular example, scheduling engine 364 executes a trivial
control algorithm, such as a round robin algorithm. A round robin
scheduling algorithm is simple to implement and requires minimal
system resources for execution. Round robin scheduling algorithms
exhibit good throughput for approximately uniform traffic patterns.
A single stage round robin scheduling scheme used in combination
with a star switching fabric can, however, experience a 1/N delay
when confronted with N channels of non-uniform traffic.
[0089] One embodiment overcomes this difficulty by using one or
more initial scheduling stages of scheduling star switching fabric
to establish more uniform traffic at the inputs to a transmission
star switching fabric 240. In particular, scheduling engine 364
instructs each of filters 348 to tune to alternating wavelengths so
that no one of the outputs from scheduling star switching fabric
340 overwhelms transmission star switching fabric 240 with any
particular wavelength signal. For example, on a first pass, each of
filters 348a-348n may communicate in round robin fashion optical
router signals 245 having wavelengths .lamda..sub.1-.lamda..sub.n,
respectively. On a second pass, each of filters 348a-348n-1 may
communicate in a round robin fashion optical router signals 245
having wavelengths .lamda..sub.2-.lamda..sub.n, respectively, while
filter 348n communicates signal 245 having wavelength .lamda..
Filters 348a-348n can continue to cycle through wavelengths
.lamda..sub.1-.lamda..sub.n so that the wavelength signals 245 are
more uniformly distributed to the input of transmission star
switching fabric 240. Although the illustrated embodiment depicts a
single stage of scheduling star switching fabric, multiple
scheduling star switching fabrics could be cascaded to further
normalize the load distribution entering transmission switching
fabric 240.
[0090] Establishing a more uniform traffic pattern at the input of
transmission star switching fabric 240 allows the use of a round
robin algorithm to control filters 248 associated with transmission
star switching fabric 240 without the 1/N delay penalty. Thus,
scheduling mechanism 300 provides a way to schedule non-uniform
traffic, such as packet traffic, using a trivial scheduling
algorithm for the transmission fabric, which occupies minimal
system resources while avoiding 1/N delay penalties traditionally
associated with simple routing algorithms and non-uniform
traffic.
[0091] Numerous modifications can be made to the example discussed
with respect to FIG. 5a. For instance, this example shows tunable
filters 248 and 348 as residing in close proximity to or integrally
to their respective switching fabrics 240 and 340. This provides an
advantage of saving space, for example, on line cards in router
112. Moreover, this technique provides an advantage of facilitating
the economical use of arrays of filters rather than individually
packaged filters for each output link. Filters 248 and 348 could,
however, reside remotely from switch fabrics 240 and 340.
[0092] In addition, although this example shows the use of tunable
filters 248 and 348, tunable optical transmitters could
alternatively be used in conjunction with fixed wavelength or
tunable wavelength filters 248 and/or 348. FIG. 5b is a block
diagram illustrating an example embodiment of a scheduling
mechanism 305 implementing tunable optical transmitters as
selecting elements for the scheduling star switching fabric.
[0093] Scheduling mechanism 305 includes a plurality of tunable
optical transmitters 346a-346n, which feed into scheduling star
switching fabric 340. Each tunable optical transmitter could
reside, for example, on a line card within router 112. Scheduling
mechanism 305 also includes a plurality of filters 348a-348n. In
this particular example, filters 348 comprise fixed wavelength
filters, each associated with a particular center wavelength.
Filters 348, in this example, reside within scheduling star
switching fabric 340. Filters 348, however, could reside remotely
from switching fabric 340.
[0094] In this embodiment, outputs of filters 348 are coupled to
inputs of a transmission star switching fabric 240. Transmission
star switching fabric 240 is associated, in this example, with a
plurality of tunable filters 248a-248n, each associated with an
output link 254a-254n from the router.
[0095] Scheduling mechanism 305 further includes one or more
scheduling engines 364. Scheduling engine 364 instructs selecting
elements 346 and 248 as to the order of tuning and the center
wavelength appropriate for tuning. Although a single scheduling
engine 364 is depicted, separate engines could be implemented for
elements 346 and 248.
[0096] In operation, tunable optical transmitters 346a-346n
generate optical signals 252a-252n having center wavelengths
determined by scheduling engine 364. Scheduling star switching
fabric receives signals 252a-252n and communicates substantially
similar sets of at least some of those signals to each of filters
348. In this example, each filter comprises a fixed wavelength
filter operable to pass signals having a particular center
wavelength.
[0097] Signals 235 passed by filters 348 are then communicated to
transmission star switching fabric 240. tunable filters 248 of
transmission star switching fabric 240 tune to receive selected
wavelengths according to instructions from scheduler 364. As a
result, selected wavelength signals are passed from transmission
star switching fabric 240 to output links 254.
[0098] FIG. 5c is a block diagram illustrating another example of a
scheduling mechanism 310 useful in conjunction with any star
switching fabric. Like the example shown in FIG. 5a, this example
depicts scheduling mechanism 310 operating within router 112 shown
in FIG. 2. Scheduling mechanism 310 could, however, be useful with
any router or switch using a star switching fabric. Scheduling
mechanism 310 is similar in structure and function to scheduling
mechanism 310 shown in FIG. 5a.
[0099] Scheduling mechanism 310 implements a buffering stage 230
between scheduling star switching fabric 340 and transmission star
switching fabric 240. Buffering stage 230 facilitates
synchronization and aids in scheduling communications between
scheduling star switching fabric 340 and transmission star
switching fabric 240. As a particular example, buffering stage 230
could comprise a plurality of line cards, each associated with an
input to transmission star switching fabric 240. Buffering stage
230 may also include memory used to avoid missequencing of packets
received by and communicated from scheduling star switching fabric
340.
[0100] In this example, scheduling switching fabric 340 receives
the multiple wavelength signal from input link 222 and communicates
separate wavelength signals 228a-228n (along with any express
traffic 228ex) from switching fabric 340. In the illustrated
embodiment, wavelength signals 228a-228n are communicated to line
cards 230 for buffering and/or electronic decision making with
respect to routing those signals through switching fabric 240.
Transmission star switching fabric 240 receives input router
signals 252a-252n and communicates those signals toward destination
elements associated with those signals.
[0101] Scheduling switching fabric 340 operates to separate the
multiple wavelength signal received at input 222 into a plurality
of wavelength signals each having a center wavelength. In this
particular example, Scheduling switching fabric 340 includes or is
closely coupled to a plurality of tunable filters 348a-348n, and
348ex. Tunable filters 348 selectively pass wavelength signals
228a-228n toward transmission star switching fabric 240. In this
particular embodiment, scheduling star switching fabric 340 passes
selected signals 228 to line cards 230 for processing.
[0102] Like the example in FIG. 5a, scheduling engine 364 operates
to provision tunable filters 348a-348n in a round-robin fashion so
that each filter 348 alternates the wavelength it passes toward
transmission star switching fabric 240. In this manner, scheduling
switching fabric 340 operates to make non-uniform traffic received
at input 222 more uniform at the inputs to transmission star
switching fabric 240. Because the incoming signals 252a-252n to
switching fabric 240 are more uniform in load distribution,
scheduling mechanism 310a can ensure reasonable throughput through
switching fabric 240 while utilizing a relatively simple scheduling
algorithm, such a round-robin scheduling algorithm.
[0103] The particular embodiment shown in FIG. 5c is just one
example of an implementation of scheduling mechanism 310 in an
optical router. Various modifications can be made without departing
from the scope of this aspect of the invention. For example, rather
than using tunable filters in both switching fabrics 240 and 340,
tunable lasers could be implemented in conjunction with fixed or
tunable filters to achieve similar operational effects. For
example, line cards 230 could include tunable lasers operable to
selectively communicate optical router signals 252a-252n at
selected wavelengths to fixed wavelength transmitters 248a-248n
associated with particular output links from router 112.
[0104] Moreover, although this example shows filters 248 and 348 as
residing integrally to or in close proximity with switching fabrics
240 and 340, respectively, filters 248 and/or 348 could
alternatively reside remotely from their associated switching
fabrics. In one particular example, filters 248 and/or 348 could
reside on line cards associated with those filters, or in another
location remote from their associated switching fabrics.
[0105] As another example of a potential modification to the
embodiment shown in FIG. 5c, processing capabilities and look-up
tables of line cards 230 could be eliminated, while electronic or
optical memory structures resident on the line cards could remain.
These memory structures could serve as buffers to optical signals
received from scheduling switching fabric 340 and awaiting
transmission to transmission switching fabric 240. These buffers
could further enhance the uniformity of wavelengths communicated to
star switching fabric 240.
[0106] FIG. 5d is a block diagram showing yet another example of a
scheduling mechanism 320 useful in conjunction with any star
switching fabric. Like the example shown in FIGS. 5a-5c, this
example depicts a scheduling mechanism 320 operating within router
112 shown in FIG. 2. Scheduling mechanism 320 could, however, be
useful with any router or switch using a star switching fabric.
[0107] Scheduling mechanism 320 is similar in structure and
function to scheduling mechanism 320 shown in FIG. 5c. Scheduling
mechanism 320, however, implements an input buffer stage 330
operable to receive wavelength signals from a wavelength division
multiplexer 325 and an output buffer stage 332 operable to operable
to receive wavelength signals 254 output from transmission star
switching fabric 240.
[0108] Input buffer stage 330 facilitates segmentation,
synchronization, buffering, and/or scheduling of communications to
scheduling star switching fabric 340. Input buffer stage 330 could
comprise any hardware, software, firmware, or combination thereof
operable to facilitate storage and retrieval of signals received.
In some embodiments, input buffer stage 330 could comprise an
optical memory comprising, for example, one or more delay loops. In
other embodiments, input buffer stage could comprise an electronic
memory. Input buffer stage 325 could reside, for example on one or
more line cards operable to convert at least a portion of incoming
optical signals to an electronic format and to generate optical
signals for retransmission to scheduling switching fabric 340. In
one particular embodiment, input buffer stage 325 could reside on
line cards 230.
[0109] Input buffer stage 325 can facilitate creating an even more
uniform load distribution of wavelength signals at the input to
star switching fabric 240. Moreover, input buffer stage 325 can
provide a mechanism to help alleviate missequencing of packets at
the outputs from star switching fabric 240. This technique can be
particularly effective when used in combination with a Full Frames
First algorithm to control the buffers in the system.
[0110] In operation, scheduling mechanism 320 receives at
wavelength division multiplexer 325 a multiple wavelength input
signal from input link 222. Wavelength division multiplexer 325
separates the multiple wavelength input signal into a plurality of
optical signals, each having a center wavelength. Input buffer
stage 325 stores incoming wavelength signals until those signals
are communicated toward scheduling switching fabric 340. Switching
fabric 340 communicates substantially similar sets of some or all
of the wavelength signals received to filters 348.
[0111] In this example, filters 348 comprise tunable filters
residing in close proximity to or integrally with switching fabric
340. Scheduling engine 364 instructs each of filters 348a-348n in a
round robin fashion to alternately communicate signals having
various selected wavelengths. This reduces the nonuniformity of
wavelengths of incoming signals.
[0112] Transmission star switching fabric 240 receives input router
signals 252 having a more uniform load distribution, and
communicates substantially similar sets of some or all of the
wavelength signals received to filters 248. Each of filters 248 is
provisioned in a round robin fashion to pass selected wavelength
signals toward output links associated with appropriate destination
elements.
[0113] As in the examples described in FIGS. 5a-5c, the example
shown in FIG. 5d could be modified in any number of ways. For
example, tunable optical transmitters could be used in place of
some or all of the tunable filters implemented. Moreover, filters
248 and 348 could reside remotely from their associated switching
fabrics.
[0114] Each of the embodiments of scheduling mechanisms depicted in
FIGS. 5a-5d provides a way to provide adequate throughput through
switching fabric 240 while utilizing a relatively simple scheduling
algorithm.
[0115] FIGS. 6a-6d provide additional nonlimiting examples of
implementations of scheduling mechanisms useful with star switching
fabrics. FIG. 6a is a block diagram showing an example of a
multiple buffer embodiment 315 utilizing tunable optical filters as
selecting elements within a scheduling star switching fabric. In
particular, embodiment 315 includes a plurality of line cards 230
which serve as an input buffer stage 230a, an intermediate buffer
stage 230b, and an output buffer stage 230c. Although this
embodiment depicts the use of different sets of cards 230a-230c to
serve as input, intermediate, and output buffer stages, the same
set of line cards could likewise be used for some or all of the
buffer stages, or one or more buffer stages could be
eliminated.
[0116] In this example, input buffer stage 230a operates to segment
incoming information into, for example, fixed length frames or
cells for transmission through transmission switching fabric 240.
Input buffer stage 230a can also perform a temporary storage
function while packets are scheduled for transmission through
scheduling star switching fabric 340.
[0117] In the illustrated embodiment, scheduling star switching
fabric 340 comprises or is coupled to a plurality of tunable
optical filters 348a-348n, each associated with an output from
scheduling star switching fabric 340. Under the control of a
scheduling engine 364 (located, for example, on one or more line
cards 230), tunable filters 348a-348n tune, in a round robin
fashion, to particular wavelengths to be transmitted toward the
inputs of transmission star switching fabric 240. Scheduling engine
364 instructs each filter 348 to alternate the wavelength of
information communicated so that the inputs to transmission star
switching fabric 240 experience a more uniform traffic load than
the inputs to scheduling star switching fabric 340.
[0118] In this example, optical transmitters associated with each
line card 230b generate input optical router signals 252 at
particular wavelengths associated with each line card 230b. Signals
252 are communicated to transmission star switching fabric 240,
where substantially similar sets of at least some of input optical
router signals 252 are communicated to each of a plurality of
tunable filters 248a-248m, each associated with an output link from
the device. Processors on or associated with line cards 230b
perform electronic decision making on signals 228 received to
determine an appropriate path for each signal from transmission
star switching fabric 240. Based on this determination, the
processors instruct tunable filters 248 to tune to particular
wavelengths so that signals destined for the output link associated
with that tunable filter 248 are passed by that filter.
[0119] Because scheduling star switching fabric 340 has created a
more uniform traffic distribution at the inputs of transmission
star switching fabric 240, the scheduling engine that schedules
communication through transmission star switching fabric 240 can
implement a trivial scheduling algorithm, such as a round robin
algorithm, to effectively administer system resources.
[0120] FIG. 6b is a block diagram of an example multiple buffer
embodiment 316 utilizing tunable optical transmitters 346 as
selecting elements within a scheduling star switching fabric
340.
[0121] Embodiment 316 includes a plurality of line cards 230 which
serve as an input buffer stage 230a, an intermediate buffer stage
230b, and an output buffer stage 230c. Buffer stages 230a-230c can
serve similar functions to like stages described above with respect
to FIG. 6a. Although this embodiment depicts the use of different
sets of cards 230a-230c to serve as input, intermediate, and output
buffer stages, the same set of line cards could likewise be used
for some or all of the buffer stages, or one or more buffer stages
could be eliminated.
[0122] In the illustrated embodiment, scheduling star switching
fabric 340 comprises or is coupled to a plurality of tunable
optical transmitters 346a-346n, each associated with an input to
scheduling star switching fabric 340. Under the control of a
scheduling engine 364 (located, for example, on one or more line
cards 230), tunable transmitters 346a-346n tune, in a round robin
fashion, to particular wavelengths to be transmitted toward the
inputs of scheduling star switching fabric 340. Scheduling engine
364 instructs each transmitter 346 to alternate the wavelength of
information communicated so that the inputs to transmission star
switching fabric 240 (received from outputs of scheduling star
switching fabric 340) experience a more uniform traffic load than
the inputs to scheduling star switching fabric 340.
[0123] Scheduling star switching fabric 340 receives the plurality
of incoming signals and communicates substantially similar sets of
at least some of the signals received to each of a plurality of
fixed wavelength filters 348a-348n. Each filter 348 is tuned to a
particular wavelength and communicates signals 228 having the
associated wavelength to an associated one of line cards 230.
[0124] Processors on or associated with line cards 230b perform
electronic decision making on signals 228 received to determine an
appropriate path for each signal from transmission star switching
fabric 240. In this example, each line card 230 includes or is
associated with a tunable optical transmitter 246a-246n,
respectively. Tunable optical transmitters 246 tune to selected
wavelengths under the direction of scheduling engine 364 executed
by the processors. Scheduling engine 364 instructs each tunable
transmitter 246 to tune, in a round robin fashion, to a particular
wavelength. Signals are communicated from tunable transmitters 246
to transmission star switching fabric 240, which communicates a
substantially similar set of at least some of the signals received
toward each of a plurality of fixed wavelength filters 248 within
or coupled to transmission star switching fabric 240. The
wavelength selected for each transmitter will determine the output
link over which the generated signal will pass, as each of the
fixed wavelength filters 248 passes a wavelength associated with a
particular output link associated with that filter.
[0125] Because scheduling star switching fabric 340 has created a
more uniform traffic distribution at the inputs of transmission
star switching fabric 240, the scheduling engine that schedules
communication through transmission star switching fabric 240 can
implement a trivial scheduling algorithm, such as a round robin
algorithm, to effectively administer system resources.
[0126] FIG. 6c is a block diagram showing yet another embodiment
317 of a multiple buffer stage switching fabric using tunable
optical filters as selecting elements.
[0127] This embodiment is similar in structure and function to the
embodiment depicted in FIG. 6a, but introduces input signals
directly to scheduling star switching fabric 340 without an input
buffer stage preceding scheduling star switching fabric 340.
[0128] FIG. 6d is a block diagram showing yet another embodiment
318 of a multiple buffer stage switching fabric using tunable
optical filters as selecting elements.
[0129] This embodiment is similar in structure and function to the
embodiment depicted in FIG. 6c, but implements a power combiner 333
in place of wavelength division multiplexers 210 shown in FIGS.
6a-6c. In addition, this embodiment can use one or more optical
amplifiers 337 prior to the input to scheduling star coupler 340.
Optical amplifiers 337 operate to compensate for at least a portion
of the loss otherwise caused by power combiner 333.
[0130] As discussed above, various embodiments of devices
implementing star switching fabrics implement optical transmitters
to generate signals destined for the star switching fabric. Some
embodiments described herein have discussed implementing optical
transmitters having fixed or tunable wavelength capabilities on
line cards within the devices. As the number of channels serviced
by the system increases, difficulties can arise with respect to
implementation of conventional optical transmitter technology.
[0131] For example, implementing a conventional laser diode on each
line card servicing a transmission channel can be prohibitively
expensive as the number of channels become large. Moreover,
conventional lasers and associated control circuitry can take up
significant space on each line card, leaving less space for other
processing elements, or requiring larger line cards. Requiring
larger line cards typically reduces the number of cards that can be
placed in any given rack.
[0132] In addition, as the number of channels increases, it becomes
increasingly difficult to administrate accurate assembly of line
cards using fixed wavelength transmitters. In that case, it becomes
necessary to ensure that each line card receives a transmitter
operating at a wavelength specified for that card. Increased
numbers of channels make it difficult to accurately associate
transmitter part numbers with particular line cards.
[0133] Furthermore, as the number of channels increases, the
channel spacing typically becomes more narrow. It becomes
increasingly difficult to stabilize the wavelength of each
individual transmitter to ensure proper channel spacing.
[0134] FIG. 7 is a block diagram of an optical transmitter system
380 particularly useful, for example, in conjunction with a star
switching fabric implementing large number of channels, for
instance 64 or more channels. Optical transmitter system 380
comprises a continuum source. In a particular embodiment, system
380 could comprise a supercontinuum source. Supercontinuum
generation describes extreme, nearly continuous spectral broadening
induced by high-intensity picosecond and sub-picosecond pulse
propagation through a nonlinear medium.
[0135] In this example, system 380 includes a modelocked source 382
operable to generate a series of optical pulses. As a particular
example, modelocked source 382 could comprise an erbium doped fiber
laser operable to generate pulses at a rate of, for example, forty
gigabits per second. Other modelocked sources operating at other
rates could likewise be used.
[0136] System 380 further includes a continuum generator 384
operable to receive a train of pulses from modelocked source 382
and to spectrally broaden the pulses to form an approximate
spectral continuum of optical signals. In this example, continuum
generator 384 includes an optical amplifier 383 coupled to one or
more lengths of optical fiber 385. Optical amplifier 383, in this
particular example, comprises an erbium doped amplifier. Other
amplifier types or combinations of amplifier types could likewise
be used. In this example, fiber 385 comprises a two stage
solution-effect compressor including approximately two meters of
standard fiber followed by approximately two meters of dispersion
shifted fiber. Other lengths of fiber and fiber types could be
used, depending on the spectral characteristics desired. Moreover,
although this example relies on the solution effect to broaden the
spectrum of the plurality of optical pulses, other pulse
compression techniques, such as adiabatic solution compression,
could alternatively be used.
[0137] System 380 also includes a signal splitter 386. Signal
splitter 386 receives the continuum from continuum generator 384
and separates the continuum into individual signals 389a-389n each
having a wavelength or a range of wavelengths. Signal splitter 386
could comprise, for example, a passive wavelength division
multiplexer, a power splitter followed by fixed wavelength filters,
or any other mechanism operable to separate a continuum or near
continuum of signals into a plurality of individual signals.
[0138] Mode locked source 382, continuum generator 384, and signal
splitter 386 can comprise common bay equipment--in other words,
equipment shared by plurality of line cards 390. Where it is
desired to generate a larger bandwidth of optical signals, multiple
sets of common bay equipment 381 can be implemented, each set
serving a separate set of line cards 390 and each generating a
separate range of wavelengths.
[0139] Signal splitter 386 communicates signals 389a-389n to one of
a plurality of modulators 392a-392n, respectively. Modulators 392
operate to encode information onto the optical signals received to
produce optical wavelength signals 393 for transmission to a star
switching fabric. In this particular example, each modulator 392
resides on a line card 390. When used with a continuum source, each
of the plurality of transmitters in system 380 can be viewed as one
of modulators 392 in combination with equipment, such as common bay
equipment 381, used to generate the unmodulated signal received by
each modulator 392.
[0140] In some embodiments, system 380 further comprises a pulse
rate multiplexer 387, such as a time division multiplexer. Pulse
rate multiplexer 387 operates to multiplex pulses received from
mode locked source 382 to increase the bit rate of the system.
Pulse rate multiplexer 387 could alternatively reside downstream
from modulators 392 and operate to time division multiplex signals
received from modulators 392.
[0141] In operation, modelocked source 382 generates a plurality of
optical pulses at a given rate. Continuum generator 384 receives
the train of pulses from modelocked source 382 and compresses those
pulses to form an approximate continuum of optical signals. Signal
splitter 386 receives and separates the continuum into a plurality
of optical signals 389a-389n each comprising a wavelength or range
of wavelengths. Each modulator 392 receives one of signals 392 from
signal separator 386 and encodes information onto the optical
signal received to generate signals 393 for transmission to a star
switching fabric.
[0142] Transmitter system 380 can support generation of fixed
wavelength signals or selectively tuned wavelength signals. To
facilitate generation of selectively tuned wavelength signals,
system 380 could include, for example, a signal selector 395
operable to selectively pass particular wavelength signals to
particular modulators 393, depending on the wavelength signal
desired to be transmitted from that modulator 393. Signal selector
395 could comprise any hardware, software, firmware, or combination
thereof operable to send particular wavelength signals to
particular modulators in response to, for example, a control signal
generated by a scheduling engine.
[0143] System 380 provides numerous benefits over systems
implementing separate optical transmitters for each channel. For
example, implementing one or more common modelocked sources to
generate numerous wavelength signals, saves considerable space on
each line card, and reduces cost by eliminating numerous individual
transmitters. Moreover, system 380 facilitates using common parts,
such as modulators, for a number of different line cards serving
different channels. This makes it easier to match parts to each
line card. Furthermore, stabilization issues can be alleviated
because system 380 allows stabilization of one or a few common
transmitter elements, rather than requiring stabilization of
separate transmitters each associated with one of the channels.
[0144] FIGS. 8-9 are block diagrams illustrating example mechanisms
useful enhancing the effective switching speed of devices using
star switching fabrics. For the purposes of illustration, these
mechanisms will be described with reference to router 112 shown in
FIG. 2. These mechanisms could, however, equally apply to many
other device designs implementing star switching fabrics.
[0145] FIG. 8a illustrates the use of a speed-up mechanism 125 at
line card 130. In this example, line card 130 receives incoming
optical signal 128, which includes packets having a first duration,
say fifty nanoseconds each. Each optical packet 128 is converted to
an electronic signal within line card 130 and then placed into an
optical format 152 for transmission to the router switching
fabric.
[0146] Speed-up mechanism 125 of line card 130 operates to decrease
the duration of each optical packet 128. For example, speed-up
mechanism 150 may increase the speed at which a modulator of line
card 130 encodes information onto optical signal 152. As a
particular example, information can be modulated onto optical
signal 152 at an increased rate resulting in the information
received in optical signal 128 being modulated in an optical signal
152 having one half the duration of signal 128. Other speed-up
ratios could be used without departing from the scope of the
invention.
[0147] FIG. 8b is a block diagram showing one example of an
aggregator 135 operable to aggregate a plurality of incoming
packets 131 into a single aggregated frame 137. Each aggregated
frame includes an identifier identifying a destination element
common to each packet 131 in the aggregated frame 137. Aggregator
135 can aggregate multiple packets 131, for example, by
encapsulating a plurality of packets within a single aggregation
frame having a common aggregation header.
[0148] Aggregator 135 can assemble aggregated frames 137 in a
variety of ways. For example, aggregator 135 can aggregate optical
packets received at line card 130 from input link 128, associating
an identifier with each frame 137. Line card 130 can then convert
at least the identifier portion of the frame 137 to an electronic
format to facilitate electronic processing of that information.
Line card 130 could then generate an optical aggregation header and
reform an aggregated frame for transmission to star switching
fabric 140. As another example, aggregator 135 could form
aggregated frames 137 after each packet 131 of that frame or
portions thereof are processed by processor 136. In that case,
processor 136 converts all or a portion of each packet received to
an electrical signal to facilitate electronic processing.
Transmitter 146 forms optical router packets, and aggregator 135
combines optical router packets into aggregated frames 137.
[0149] Allowing switching fabric 140 to switch a smaller number of
larger frames rather than numerous individual packets can provide
significant switching efficiencies.
[0150] FIGS. 9a-9c are block diagrams showing various embodiments
of filter and transmitter configurations operable to enhance the
effective switching speed of router 12 without modifying the
switching speed of any individual components, such as filters 148
or transmitters 146. In particular, FIG. 9a is a block diagram of a
multiple filter configuration. The speed of router 112 can be
limited in some cases by the switching speed of filters 148. That
is, each filter requires some finite time to tune between different
wavelengths desired to be processed. If router 112 is forced to
wait while filters 148 reset between wavelengths, the speed of
router 112 can be significantly hindered.
[0151] The example embodiment in FIG. 9a helps to alleviate this
problem without requiring increased switching speed of any one
filter 148, by assigning a plurality of filters 148a1-148ax to each
optical link 128. Filters 148a1-148ax operate in parallel so that
while one filter 148a1 is processing output optical router signal
154 from switch fabric 140, other filters 148a2-148ax can be
retuned to another wavelength to receive packets carried over other
channels. By switching between the multiple parallel filters
148a1-148ax, switching delay that might otherwise be caused when
waiting for filters 148 to retune can be significantly reduced.
[0152] In the illustrated embodiment, an optical splitter 141
receives output optical router signal 154 from switch fabric 140
and communicates a portion 154a1-154ax to each of filters
148a1-148ax, respectively. In this particular example, a switch 151
cycles between signals received from filters 148a1-148ax so that
only one of the signals from filters 148a is output to line card
136. Although this example shows use of a sequential control
algorithm that switches from one filter output to another, a
variety of control algorithms can be used to determine an active
filter 148a. For example, switch 151 could receive a control signal
instructing switch 151 as to which filter output to accept.
[0153] In the embodiment shown in FIG. 9a, optical signals output
from filters 148a are converted to electrical signals at receivers
149a, each associated with one of filters 148a. Switch 151 operates
to process electrical signals received from converters 149a and to
pass an electrical output to an associated line card 136. The
embodiment shown in FIG. 9b is similar in structure and function to
that shown in FIG. 9a, except that electrical switch 151 is
replaced with an optical switch 153. Optical switch operates to
receive optical signals from filters 148a and to select one of
those optical signals for communication to converter 149a.
Converter 149a converts the selected signal to an electrical signal
and passes the converted electrical signal to an associated line
card 136.
[0154] Although the example shown in FIG. 9b depicts the use of
multiple filters per line card 136, a similar concept could be
applied to filters associated with express channels 127. In that
case, converters 149 could be eliminated so that optical signals
output from optical switch 153 pass to wavelength division
multiplexer/demultiplexer 110 from optical link 127.
[0155] FIG. 9c is a block diagram showing yet another mechanism
operable to reduce switching delay of router 112 without modifying
the switching speed of individual switching components. This
example implements a plurality of tunable lasers 146a1-146ax
associated with each line card 136.
[0156] While one of optical transmitters 146a1-146ax generates an
optical router signal having one particular wavelength, other
transmitters 146a2-146ax can be retuned to another wavelength to
communicate packets bound for other destinations. By switching
between the multiple parallel transmitters 146a1-146ax, switching
delay that might otherwise be caused when waiting for transmitters
146 to retune can be reduced or avoided.
[0157] In the illustrated embodiment, a splitter 143 receives
electrical signal 129a from processor 136 and communicates a
portion 129a1-129ax to each of transmitters 146a1-146ax,
respectively. At least one of transmitters 146a1 generates an
optical router signal at a specified wavelength. Other transmitters
146a2-146ax can retune without emitting light during the time that
active transmitter 146a1 generates the optical signal. For example,
where optical transmitters 146 comprise multiple stage lasers
including tuning stages and lasing stages, the lasing stages of
those transmitters can remain inactive while tuning stages adjust
to process a new wavelength.
[0158] A switch 155 selects an appropriate optical router signal
from lasers 146a and communicates that signal to switching fabric
140. In one embodiment, switch 155 can sequentially cycle between
signals received from transmitters 146a1-146ax. A variety of
control algorithms can be used to determine an active transmitter
146a. For example, switch 155 could receive a control signal
instructing switch 155 as to which transmitter output to accept and
communicate to switch fabric 140.
[0159] Each of the efficiency enhancing mechanisms described with
respect to FIGS. 4 and 5 could be used independently or in
combination with one, some, or all others of those mechanisms to
further enhance operation of the router.
[0160] FIG. 10 is a flow chart illustrating one example of a method
400 of routing optical signals. For the purposes of illustration,
method 400 will be described with reference to router 112 shown in
FIG. 2. Method 400, however, could equally apply to alternative
router designs, such as router 212 shown in FIG. 3 including
enhancements shown in FIGS. 8 and/or 9. Method 400 begins at step
405 where line card 130a receives a first packet 128a comprising an
identifier of a destination element.
[0161] Processor 136a of line card 130a converts at least the
identifier portion of first packet 128a to an electronic format.
Processor 136a applies the identifier to look-up table 144a to
determine control signal 162a at step 410. In this example, control
signal 162a instructs a particular tunable filter, for example,
filter 148n to tune to a wavelength transmitted by transmitter 146a
of first line card 130a. Alternatively, processor 136a could
communicate control signal 162 to scheduling engine 164 to
facilitate scheduling and arbitration among control signals 162
before transmitting those signals to filters 148.
[0162] The identification of a destination tunable filter 148 could
comprise identification of a plurality of tunable filters operating
in parallel to service a single optical link and/or line card.
Embodiments discussed with respect to FIGS. 9a-9b provide examples
of this type of operation. In this manner, one of the filters 148
can process the optical signal while other filters in that group
retune to or from other wavelengths. This can help to enhance the
effective switching speed of router 112.
[0163] Transmitter 146a generates an optical router signal 152a and
communicates that signal to star coupler switch fabric 140 at step
415. In a particular embodiment, transmitter 146a comprises a fixed
wavelength transmitter operable to generate optical router signal
152a at a particular fixed wavelength. Generating optical router
signal 152 could comprise, for example, generating optical router
signal 152 using a laser/modulator combination residing on the same
line card. In another example, a modulator 393 resident on line
card 130a could receive from common bay equipment (see e.g., FIG.
7) an unmodulated optical signal having a particular wavelength.
Modulator 393 could modulate information onto the unmodulated
signal to generate optical router signal 152.
[0164] The process by which transmitter 146a generates optical
router signal 152a depends, in part, on the level of conversion
experienced by incoming packet 128a. Where processor 136a converts
the entire optical signal 128a into an electronic format,
transmitter 146a information for the entire optical signal
including header and payload information for optical router signal
152a.
[0165] Where, on the other hand, processor 136a converts only a
portion of optical signal 128a, transmitter 146a merely converts
that portion of the signal back to an optical signal, and
recombines that portion with the original optical portion of signal
128a to form optical router signal 152a. As a particular example,
processor 136a may convert only a header portion, or only the
identifier portion of a header portion of signal 128a to an
electronic format, while temporarily storing or delaying the
remainder of optical signal 128a until it can be combined with an
optical signal leaving transmitter 146a.
[0166] Generation of optical router signal 152a may include
aggregating individual packets 131 into larger frames 133 and/or
may include reducing the duration of each packet by implementing a
speed-up mechanism such as that described with respect to FIG.
8a.
[0167] Star coupler switching fabric 140 receives the first optical
router signal 152a and at least one other optical router signal
152b having a wavelength that is different than first optical
router signal 152a, and communicates both optical router signals
152a to a plurality of tunable filters 148 at step 420. In this
example, tunable filter 148n is associated with a line card coupled
to an optical path facilitating communication with the destination
network element. In this case, router 112 communicates control
signal 162 to tunable filter 148n at step 425.
[0168] Based at least in part on control signal 162a, filter 148n
associated with line card 130n tunes to the wavelength associated
with optical router signal 152a.
[0169] As a result, filter 148n accepts the first packet carried by
optical router signal 152a at step 430 and facilitates
communication of the first packet toward the destination element.
Tunable filter 248n comprises a tunable optical filter operable to
selectively accept one or more specified wavelengths while
rejecting others. Filter 148n may communicate the first packet
toward the destination element without further conversion, or may
pass optical router signal 152a to an optical-to-electrical
converter 149n to facilitate additional processing before
communicating the first packet toward the destination network
element.
[0170] FIG. 11 is a flow chart showing one example of a method 350
of scheduling communications through a star switching fabric.
Method 350 will be described with respect to scheduling mechanism
300 shown in FIG. 5a. Method 350 could apply, however, to any
scheduling mechanism described herein.
[0171] Method 350 begins at step 355, where scheduler 300 receives
a plurality of packets having a first load distribution. Scheduler
300 could receive, for example a plurality of packets in an optical
format, where each packet is associated with a wavelength.
Typically, packet-based traffic will exhibit a non-uniform load
distribution.
[0172] In this particular example, scheduling star switching fabric
340 of scheduler 300 receives packets 252a-252n, and communicates a
substantially similar set of at least some of packets 252 toward
each of a plurality of filters 348 at step 360. In this example,
filters 348 each comprises a tunable filter operable to selectively
tune to a wavelength to be passed. Alternatively, filters 348 could
comprise fixed-wavelength filters used in combination with tunable
wavelength optical transmitters, such as transmitters 346 shown in
FIG. 5b.
[0173] Scheduler 300 selectively passes packets associated with
selected wavelengths for receipt by transmission star switching
fabric 240 at step 365. In this example, under the direction of
scheduling engine 364, filters 348 selectively tune to alternating
wavelengths in a round robin fashion to ensure that no one
particular wavelength overwhelms transmission switching fabric 240.
The result of the selective alternate tuning of filters 348
culminates in a more uniform load at the input to transmission star
switching fabric 240.
[0174] As a result, scheduler 300 schedules communication of
packets from transmission switching fabric 240 at step 370 using a
trivial scheduling algorithm. Scheduler 300 may implement, for
example, a round robin algorithm for scheduling tuning of
selectable elements, such as filters 248, associated with
transmission star switching fabric 240. By establishing a more
uniform load at the input to transmission star switching fabric
240, scheduler 300 avoids the 1/N delay penalty that would
otherwise be associated with using a trivial scheduling algorithm
on non-uniform traffic.
[0175] FIGS. 12-16 are flow charts illustrating example methods of
enhancing the effective switching speed of a router utilizing a
star switching fabric without increasing switching speed of the
individual switching components of the router. For brevity of
description, the following methods will be described with reference
to router 112 depicted in FIG. 2. The methods described with
respect to FIGS. 12-16 could, however, apply to any router design
utilizing a star switching fabric, and are not intended to be
limited only to the example router embodiments explicitly described
herein.
[0176] FIG. 12 is a flow chart illustrating one example of a method
450 of enhancing the effective switching speed of router by
reducing the duration of packets communicated through a star
switching fabric of the router. Method 450 begins at step 455 where
router 112 receives at a first line card 130 an optical packet
comprising a payload and having a first duration. Referring to FIG.
8a, optical packet 131 may comprise a duration of, for example, 50
nanoseconds. Line card 130 generates at step 460, an optical router
packet 133 having a second duration shorter than the first
duration. Optical router packet 133 comprises the payload of
optical packet 131 received by line card 130, and comprises a
second duration shorter than the first duration associated with
packet 131. In this particular example, the second duration of
packet 133 comprises approximately one half the duration of input
packet 131.
[0177] Line card 130 communicates the optical router packet 133 to
star switching fabric 140 at step 465. Star switching fabric 140
communicates at step 470 a plurality of optical router packets to
each of a plurality of tunable filters 148. Each tunable filter 148
is associated with a separate output link from router 112. Router
112 communicates at step 475 a control signal 162 to a selected
tunable filter 148 to facilitate communicating at least the payload
of the optical router packet 133 toward the destination element
associated with that packet. The control signal 162 causes tunable
filter 148 to tune to a wavelength associated with optical packet
133, and to substantially communicate packet 133 toward a
destination element associated with that optical filter 148. Prior
to communicating optical packet 133 from router 112, router 112 may
expand the duration of packet 133 to recover its original
duration.
[0178] By reducing the duration of packets received at line cards
130, router 112 can increase switching speed and throughput
associated with the router without modifying the switching speeds
of any particular switching components in router 112.
[0179] FIG. 13 is a flow chart showing one example of a method 500
of enhancing the effective switching speed of an optical router by
aggregating packets bound for a common destination element. Method
500 begins at step 510 where router 112 receives a plurality of
optical packets each comprising a payload and each comprising an
identifier of the same destination element. Referring to FIG. 8b,
router 112 generates at step 520 an aggregated frame 137 comprising
an identifier 139 of the destination element shared by packets
131a-131n.
[0180] Router 112 communicates at step 530 aggregated frame 137 to
star switching fabric 140. In this example, star switching fabric
140 communicates at step 540 aggregated frame 137 to each of a
plurality of tunable filters 148. Each tunable filter is associated
with a separate output link from router 112. Alternatively,
aggregated frames 137 could be generated by tunable optical
transmitters and communicated to a plurality of fixed wavelength
filters through star switching fabric 140.
[0181] In the illustrated example, router 112 communicates a
control signal to at least a selected tunable filter 148 at step
550. The selected tunable filter 148 is associated with a
communication path to a destination element for each of the optical
packets 131a-131n within aggregated frame 137. The selected tunable
filter 148 receives a control signal and tunes to a wavelength
associated with aggregated frame 137, facilitating communication of
aggregated frame 137 toward the destination element. A line card
130 associated with the output link 128 leading to the destination
element may disassemble aggregated frame 137 to facilitate
communication of individual packets 131a-131n toward the
destination element.
[0182] FIG. 14 is a flow chart showing one example of a method 600
of enhancing the effective switching speed of an optical router
using a star switching fabric by providing express lanes that
bypass line cards that facilitate electronic signal processing of
some of the optical signals received. Method 600 begins at step 610
where router 112 receives an input optical packet at optical link
128. A line card 130 converts at least a portion of the optical
packet received to an electronic form at step 620. Line card 130
generates, based at least in part on the electronic signal, an
optical router signal having a first wavelength at step 630.
[0183] Router 112 also receives at an express lane 127 an express
optical packet having a second wavelength at step 640. Router 112
communicates at step 650 the optical router packet generated at
line card 130 and the express packet received at express lane 127
to star switching fabric 140. Star switching fabric 140
communicates the optical router packet and the express packet to
each of a plurality of tunable filters at step 660. Router 112
communicates a control signal to a selected tunable filter at step
670 to facilitate communicating the express optical packet toward a
destination element associated with that filter. The express
optical packet is communicated from an input to router 112, through
switching fabric 140, to an output of router 112 without ever
having been converted to an electronic form. Facilitating bypassing
line cards 130 depending, for example, on the wavelength of the
optical packets received, can provide significant efficiencies.
Packets that do not require electronic processing can transparently
pass through router 112, saving system resources and reducing delay
that would otherwise accompany having to convert all packets
received between optical and electrical formats.
[0184] Again, although this example discusses the use of tunable
filters and fixed wavelength transmitters, the concepts also apply
to embodiments utilizing tunable optical transmitters and fixed
wavelength filters.
[0185] FIG. 15 is a flow chart showing one example of a method 700
for enhancing the effective switching speed of an optical router
using a star switching fabric by assigning a plurality of tunable
filters to each output link from the router. Method 700 begins at
step 710 where router 112 receives at star switching fabric 114 a
plurality of optical signals each having a wavelength. Although
some of the optical signals may have the same wavelengths, at least
some of the signals received have different wavelengths from other
signals received. Star switching fabric 140 communicates at step
720 a plurality of substantially similar sets of the optical
signals. In some embodiments, each of the substantially similar
sets of optical signals may comprise a combination of all signals
received by the star switching fabric 140. In other embodiments,
star switching fabric 140 may communicate only some of the optical
signals received.
[0186] A group of tunable filters 148 associated with a common
output from router 112 receives one of the plurality of
substantially similar sets of optical signals at step 730.
Referring for example to FIG. 9a, a first tunable filter 148a1 of
the group of tunable filters associated with the output link is
tuned to a first wavelength to process one of the optical signals
received having primarily the first wavelength at step 740. While
the first filter 148a1 processes the optical signal primarily
comprising the first wavelength, a second tunable filter 148an of
the same group tunes to a second wavelength at step 750. In a
particular embodiment, the second tunable filter 148an can
substantially complete tuning to the second wavelength before the
first tunable filter 148a1 completes processing the optical signal
having primarily the first wavelength.
[0187] Router 112 communicates the optical signal having primarily
the first wavelength from first tunable filter 148a1 to an output
link associated with that filter at step 760. Subsequently, the
group of tunable filters, and in particular, second tunable filter
148an tuned to the second wavelength may receive another set of
optical signals and facilitate communication of an optical signal
comprising primarily the second wavelength toward the output link
associated with that group of filters.
[0188] Assigning a plurality of tunable filters to a single output
link allows router 112 to conceal delay that would otherwise be
associated with having to retune filters to process different
wavelength signals. Using a multiple filter configuration, router
112 can conceal delay by reconfiguring one filter associated with
the output link while another filter associated with that same
output link processes signals being received.
[0189] FIG. 16 is a flow chart showing one example of a method 800
of reducing delay by assigning a plurality of tunable transmitters
to an input link to the router. In this example, method 800 beings
at step 810 where a first tunable transmitter of a group of tunable
transmitters associated with a single input to the router generates
an optical router signal having primarily a first wavelength.
Referring to FIG. 8c for exemplary purposes, while first
transmitter 146a1 generates the optical router signal having
primarily the first wavelength, a second tunable transmitter 146an
tunes to a second wavelength at step 820. In a particular
embodiment, second tunable transmitter 146an substantially
completes tuning to the second wavelength before first tunable
transmitter 146a1 completes generation of the first optical router
signal. This process can be repeated at multiple groups of tunable
transmitters, each group associated with one input to router
112.
[0190] Router 112 communicates at step 830 a signal from each of
the groups of tunable transmitters to star switching fabric 140.
Star switching fabric 140 communicates substantially similar sets
of optical signals received to each of a plurality of filters. In
this particular example, each of the filters comprises a fixed
wavelength filter operable to substantially communicate a
predetermined wavelength or range of wavelengths and to reject
other wavelengths. Each filter can be associated with an output
from router 112. Router 112 can facilitate selectively directing
signals through switching fabric 140 by selectively tuning
transmitters 146 to wavelengths of filters associated with desired
output links from router 112. Like the method implementing multiple
tunable filters for each output link, using multiple tunable
transmitters for each input link conceals delay otherwise
associated with reconfiguring tunable lasers of router 112.
[0191] In various embodiments, one or more switching time enhancing
techniques, such as those described in FIGS. 12-16 can be combined
to further increase the switching time of the router.
[0192] Although various aspects of the present invention have been
described in several embodiments, a myriad of changes, variations,
alterations, transformations, and modifications may be suggested to
one skilled in the art, and it is intended that the present
invention encompass such changes, variations, alterations,
transformations, and modifications as fall within the spirit and
scope of the appended claims.
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