U.S. patent application number 09/942664 was filed with the patent office on 2002-02-28 for wdm optical communication system with channels supporting multiple data formats.
Invention is credited to Jiang, Leon Li-Feng, Montalvo, Raul B., Shanton, John Lynn III, Yu, Wenli.
Application Number | 20020024698 09/942664 |
Document ID | / |
Family ID | 24765844 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020024698 |
Kind Code |
A1 |
Jiang, Leon Li-Feng ; et
al. |
February 28, 2002 |
WDM optical communication system with channels supporting multiple
data formats
Abstract
The present invention provides a flexible WDM optical
communication system in which each optical channel of the WDM
optical communication signal can simultaneously accept multiple
data formats. In one embodiment, the WDM optical system includes an
optical waveguide having an optical add-drop multiplexer to
selectively add and/or drop one or more optical channels to/from
the WDM signal carried on the waveguide. A first source of data
imparts information onto a first optical channel in a packet format
while a second source of data imparts information onto the first
optical channel in a time division multiplexed format. Other data
sources having other data formats may also be included. An optical
network interface electrically communicates with the data sources,
placing the data from these sources onto the first optical channel
which is generated from an optical source such as a laser. An
optical path carries the optical channel from the optical source to
the optical add-drop multiplexer. From there, it is multiplexed
onto the optical waveguide, merging with the other optical channels
of the WDM optical signal.
Inventors: |
Jiang, Leon Li-Feng;
(Princeton, NJ) ; Montalvo, Raul B.; (North
Potomac, MD) ; Shanton, John Lynn III; (Middletown,
MD) ; Yu, Wenli; (Gaithersburg, MD) |
Correspondence
Address: |
MARGARET BURKE
SENECA NETWORKS
SUITE 200
30 WEST GUDE DRIVE
ROCKVILLE
MD
20850
US
|
Family ID: |
24765844 |
Appl. No.: |
09/942664 |
Filed: |
August 31, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09942664 |
Aug 31, 2001 |
|
|
|
09688804 |
Oct 17, 2000 |
|
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|
Current U.S.
Class: |
398/83 ; 398/60;
398/75 |
Current CPC
Class: |
H04J 14/0216 20130101;
H04J 14/021 20130101; H04J 14/0209 20130101; H04J 14/0279 20130101;
H04J 14/0213 20130101; H04J 14/0294 20130101; H04J 14/0283
20130101; H04J 14/0219 20130101; H04J 14/02 20130101; H04J 14/0284
20130101 |
Class at
Publication: |
359/127 ;
359/124 |
International
Class: |
H04J 014/02 |
Claims
What is claimed is:
1. A wavelength division multiplexed optical communication system
configured to simultaneously accept multiple data formats from
voice and data sources on individual optical channels comprising:
an optical waveguide configured to carry a wavelength division
multiplexed optical communication signal comprising a plurality of
optical channels, each optical channel having a discrete
wavelength; an optical add-drop multiplexer optically communicating
with the optical waveguide configured to selectively add one or
more optical channels to the wavelength division multiplexed
optical communication signal; a first source of data in a first
data format selected from ATM, IP, MPLS, Gigabit Ethernet, and
Ethernet for imparting information to a first optical channel; a
second source of data comprising voice traffic for imparting
information to the first optical channel; an optical channel source
for producing an optical channel at a first optical channel
wavelength; an optical network interface electrically communicating
with the first and second sources and electrically communicating
with the optical channel source for placing data from the first and
second data sources onto the first optical channel such that voice
and data traffic are multiplexed onto the optical channel; an
optical path optically communicating with the optical channel
source and the optical add-drop multiplexer for transporting the
first optical channel to the optical add-drop multiplexer.
2. A wavelength division multiplexed optical communication system
as recited in claim 1 further comprising a cell format module
positioned between the first source of data for imparting
information onto the first optical channel in a cell format and
between the optical network interface for formatting the
information from the first data source to be output to the optical
network interface.
3. A wavelength division multiplexed optical communication system
as recited in claim 1 further comprising a TDM format module
positioned between the second source of data for imparting
information onto the first optical channel in a time division
multiplexed format and between the optical network interface for
formatting the information from the second data source to be output
to the optical network interface.
4. A wavelength division multiplexed optical communication system
as recited in claim 2 wherein the first data source has an
asynchronous transfer mode (ATM) format.
5. A wavelength division multiplexed optical communication system
as recited in claim 2 wherein the first data source has an Internet
protocol (IP) format.
6. A wavelength division multiplexed optical communication system
as recited in claim 2 wherein the first data source has a
multiprotocol label switching (MPLS) format.
7. A wavelength division multiplexed optical communication system
as recited in claim 2 further comprising additional data sources
electrically communicating with the cell format module.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation in part of U.S. patent
application Ser. Nos. 09/688,804, 09/731,760, and 09/731,761, the
disclosures of which are incorporated by reference herein. This
application also claims priority to provisional patent application
No. 60/229,126, the disclosure of which is incorporated by
reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to wavelength division
multiplexed optical communication systems in general and, more
particularly, to wavelength division multiplexed optical
communication systems having individual optical channels which are
capable of simultaneously supporting multiple electronic data
formats such as TDM, ATM, and IP.
[0004] 2. Description of the Related Art
[0005] As the need for communication signal bandwidth increases,
wavelength division multiplexing (WDM) has progressively gained
popularity for multiplying the transmission capacity of a single
optical fiber. A review of optical networks, including WDM
networks, can be found in Ramaswami et al., Optical Networks: A
Practical Perspective (Morgan Kaufinan, .COPYRGT. 1998), the
disclosure of which is incorporated herein by reference. Typically,
wavelength division multiplexed optical communication systems have
been designed and deployed in the long-haul, interexchange carrier
realm. In these long-haul optical systems, a wavelength division
multiplexed optical communication signal comprising plural optical
channels at different wavelengths travels in a single direction on
a single fiber (unidirectional transmission). Because the
communication traffic in such systems commonly travels many
hundreds of kilometers, the need for add-drop multiplexing of
individual channels is infrequent, occurring at widely-spaced
add-drop nodes.
[0006] Although the optical infrastructure of long-haul WDM optical
systems can accommodate future traffic needs created by increased
demand from traditional and multimedia Internet services, this
traffic must first be collected and distributed by local networks.
Currently, such local networks are structured to carry a single
wavelength, time-division multiplexed (TDM) optical signal along a
fiber network organized into various ring structures. To route the
various components of the TDM signal, numerous electronic add-drop
multiplexers are positioned along the fiber network. At each
add-drop location, the entire optical signal is converted into an
electrical signal; the portions of the electrical signal which are
destined for that add-drop point are routed accordingly. The
remaining portions of the electrical signal are converted back to a
new TDM optical signal and are output through the electronic
add-drop multiplexer. Thus, before a user can access the
bandwidth-rich WDM long-haul transport networks, he must first pass
through the bottleneck of the local networks.
[0007] To increase capacity on these local networks, e.g., by using
higher-rate optical transmitters, all of the equipment positioned
on an optical ring must be upgraded. Further, providing additional
add-drop nodes along a ring requires a re-examination of the
optical power budget for the entire ring structure. Although WDM
may be "overlaid" on such a local network to increase capacity, an
all-optical solution is insufficient to meet the needs of future
service demands. In particular, conventional WDM networks cannot
handle the rigorous add-drop requirements of local networks to
provide adequate routing of traffic. Further, current WDM solutions
do not address the problems posed by the need to carry traffic
having various data formats such as TDM, ATM, IP, MPLS, etc.
simultaneously on the same optical network. Depending upon network
usage patterns, a particular system may be confronted with heavy
loads of data in an IP format during certain hours while the
majority of traffic during other hours may be voice traffic. Thus,
the IP capacity is underutilized during peak voice traffic periods
and the voice capacity is underutilized during peak IP traffic
periods.
[0008] Several attempts have been made to remedy the problems of
conventional optical networks. In U.S. Pat. No. 5,751,454, a
wavelength bypassed ring network is proposed in which the
wavelength channels are arranged so that some bypass each node and
terminate further along the ring. Signals on bypass routes are not
processed by intermediate nodes. While this system allows for fixed
WDM add-drop on ring networks, it does not address the need for
various data formats to be able to access the optical network.
[0009] U.S. Pat. No. 6,069,892 describes a wavelength division
multiplexed optical communication system configured to carry
fixed-length cells such that the system is optimized as an ATM cell
transmission system. Because this system is optimized for ATM
traffic, each optical channel of the WDM signal carries cell-based
data, i.e., data having a single format. While such a technique
enhances the use of wavelength division multiplexing with
cell-formatted protocols, the formats for other protocols are not
carried by the system.
[0010] In U.S. Pat. No. 6,084,694, a WDM communications network
having a plurality of nodes is described. The wavelengths carried
by the network are organized into wavebands of four channels; each
node includes a filter for statically dropping a waveband and
passively forwarding the remaining bands. To create what is termed
a "protocol independent" network, each optical wavelength may be
connected to a different data source. Thus, as shown in FIG. 9 of
the patent, a SONET OC-3 signal may be sent from node Z to node B
without conversion to an electrical signal by intermediate nodes.
While the '694 patent depicts potential solutions to some optical
network problems, it does not describe a system with sufficient
flexibility to route any type of data format onto any channel
wavelength and deliver it to any node within the optical network.
This type of solution also fails to address the capacity issues
surrounding the different peak demand periods for voice and data
traffic; since voice and data traffic are carried on separate
wavelengths, the voice traffic cannot use surplus capacity on the
data wavelength during non-peak data periods nor can the data
traffic use surplus capacity on the voice wavelength during peak
periods.
[0011] Thus, there is a need in the art for a wavelength division
multiplexed optical network which is capable of transporting
multiple data formats simultaneously on an individual optical
channel. In particular, there is a need in the art for wavelength
division multiplexed optical networks in which both voice and data
can be carried on each optical channel to maximize the capacity of
the network and to alleviate overcapacity by either voice or data
traffic during peak periods. Such an optical network would impart
the flexibility required to provide access to any type of data
format to any customer at any point along an optical network.
SUMMARY OF THE INVENTION
[0012] The present invention provides a flexible wavelength
division multiplexed optical communication system capable of
supporting any data format from any customer along an optical
network. Each optical channel of the wavelength division
multiplexed optical communication signal can simultaneously accept
multiple data formats; in this manner, all types of data formats
can be placed on all of the optical channels in the VDM system.
[0013] In one embodiment, the WDM optical system includes an
optical waveguide configured to carry a wavelength division
multiplexed optical communication signal composed of plural optical
channels, each of which has a discrete wavelength. An optical
add-drop multiplexer optically communicates with the optical
waveguide to selectively add and/or drop one or more optical
channels to/from the WDM signal carried on the waveguide.
[0014] A first source of data imparts information onto a first
optical channel in a packet format while a second source of data
imparts information onto the first optical channel in a time
division multiplexed format. Other data sources having other data
formats may also be included. An optical network interface
electrically communicates with the data sources, placing the data
from these sources onto the first optical channel which is
generated from an optical source such as a laser. An optical path
carries the optical channel from the optical source to the optical
add-drop multiplexer. From there, it is multiplexed onto the
optical waveguide, merging with the other optical channels of the
WDM optical signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a wavelength division multiplexed optical
communication system configured to simultaneously accept multiple
data formats on an individual optical channel according to a first
embodiment of the present invention.
[0016] FIG. 2 is a bidirectional wavelength division multiplexed
optical communication system configured to simultaneously accept
multiple data formats on an individual optical channel according to
a further embodiment of the present invention.
[0017] FIG. 3 is a bidirectional add-drop multiplexer which may be
used in the bidirectional optical system of FIG. 2.
[0018] FIG. 4 shows details of an optical network interface used in
the optical communication system of FIG. 2.
DETAILED DESCRIPTION
[0019] Turning now to the drawings in detail in which like numerals
indicate the same or similar elements, FIG. 1 depicts a wavelength
division multiplexed optical communication system 10 according to a
first embodiment of the present invention. Optical system 10
includes optical waveguide 12 which is configured to carry a
wavelength division multiplexed optical signal composed of plural
optical channels, each channel having a discrete wavelength. As
used herein, the expression "wavelength division multiplexed" or
"WDM" refers to any optical system or signal composed of plural
optical channels having different wavelengths, regardless of the
number of channels in the system or signal. As such, the term
"wavelength division multiplexing" or "WDM" encompasses all
categories of WDM such as DWDM (dense wavelength division
multiplexing) and CWDM (coarse wavelength division multiplexing).
For clarity of presentation, only one system--the "work" system--is
shown here; it is understood that a substantially similar system is
provided for the "protect" waveguide.
[0020] Waveguide 12 may form part of a ring network, mesh network,
point-to-point network, subtended ring network, or any other
network topology. Typically, in local networks, ring structures are
employed, using "work" and "protect" rings. Along the ring, several
electrical add-drop nodes are provided (as discussed in the
"Background" section above). In such a topology, optical add-drop
multiplexers 20 would replace the conventional electrical
nodes.
[0021] Optical add-drop multiplexer 20 is interposed along
waveguide 12 to optically communicate with the waveguide for
receiving a wavelength division multiplexed optical signal. As used
herein, the expression "optically communicates" designates an
optical path between two elements. The optical path may be a direct
path or it may route through intermediate optical devices (e.g.,
optical isolators, additional optical circulators, filters,
amplifiers, etc.). Optical add-drop multiplexer 20 may be selected
from a number of devices depending upon the overall configuration
of optical system 10. Considerations include the number of optical
channels in the system, whether the channels propagate
unidirectionally or bidirectionally along waveguide 12, whether it
is desired to drop a fixed number of channels of fixed wavelengths
at the same drop point (or, conversely, a variable number of
channels of different wavelengths), etc. In the simplest case,
optical add-drop multiplexer 20 is configured to drop or add a
single optical channel of a fixed wavelength, as depicted in the
exemplary embodiment of FIG. 1. Such an add-drop multiplexer can
take the basic configuration of a three-port optical circulator and
an optical coupler with an in-fiber Bragg grating disposed in a
fiber connecting the devices. A unidirectional WDM signal enters
the first circulator; a channel to be dropped is reflected by the
grating to a drop port while the remaining channels of the WDM
signal pass through to the coupler. A channel to be added enters
the coupler and is output to the transmission waveguide where it
joins the remaining channels of the WDM optical signal. Such a
configuration is depicted in Optical Networks: A Practical
Perspective, incorporated by reference above. While this is an
example of a single channel add-drop multiplexer which may be used
with the present invention, it is understood that any device
capable of selecting one or more optical channels from a WDM
optical signal and/or adding an optical channel to a WDM optical
signal is contemplated for use in the optical systems of the
present invention.
[0022] A channel to be added to the optical communication system is
produced by optical source 30. Optical source 30 can be selected
from any device which produces an optical signal at the desired
channel wavelength. Such optical sources include, but are not
limited to, DFB lasers, Bragg grating lasers, etc. In the
embodiment depicted in FIG. 1, optical source 30 produces a channel
having a wavelength designated .lambda..sub.1, preferably selected
to be a wavelength within the gain band of an optical fiber
amplifier such as an erbium-doped fiber amplifier (EDFA).
[0023] Data to be placed on the optical channel are associated with
a variety of protocols. The term "data," as used herein, broadly
represents any type of information to be transmitted over an
optical communication system including, but not limited to, voice,
images, video, music, text, etc. As defined in Telecommunication
Transmission Systems, (Robert Winch, second edition, McGraw-Hill,
NY.COPYRGT. 1998), the disclosure of which is incorporated by
reference herein, a protocol is "a set of rules that control a
sequence of events which take place between equipment or layers on
the same level." ATM (Asynchronous Transfer Mode), IP (Internet
Protocol), MPLS (MultiProtocol Label Switching), TDM (Time Division
Multiplexing) are all examples of protocols used to carry data over
optical networks. Within these protocols are various data formats
which define how the individual bits of information are grouped in
a signal (e.g., header bits, payload bits, identifier bits, routing
information bits, etc.). Thus, for each protocol (e.g., ATM, IP,
MPLS, TDM, etc.) there is an associated data format for that
protocol. In the context of the present invention, the use of the
terms ATM, IP, MPLS, TDM, etc. refer to the data format associated
with that protocol unless otherwise indicated.
[0024] The information to be placed on optical channel
.lambda..sub.1 includes data configured in a variety of the data
formats set forth above; the optical system is constructed so that
plural data formats can be simultaneously and independently placed
on a single optical channel without conversion to another data
format prior to placement on that channel. To facilitate the
placement of each of these data formats on the optical channel,
optical network interface 40 is provided. Optical network interface
40 electrically communicates with plural data sources each of which
is configured using a different data format--ATM formatted data
source 50, IP formatted data source 60, MPLS formatted data source
70, and TDM formatted data source 70. The optical network interface
intelligently groups the information from data sources 50, 60, 70,
80 etc. for placement on the optical channel, .lambda..sub.1. Note
that .lambda..sub.1 is used as an exemplary channel; through the
use of the optical network interface, the information may be placed
on any optical channel to be added to the system. When the optical
channel is selected in accordance with SONET standards, the data
groups created by the optical network interface place each data
group into a SONET--compatible slot on the optical channel.
Alternatively, other types of optical channels may be selected such
as those which use the digital wrapper standard. Optical network
interface 40 may comprise a single device or, optionally, plural
devices which perform the functions described above.
[0025] The formatted data groups are electrically transmitted to
the optical source 30 where an appropriate modulator places the
information onto the optical channel through either direct
modulation techniques (e.g., varying a current source to a laser)
or external modulation techniques (e.g., through Mach-Zehnder
modulators, electroabsorption modulators, etc.). Alternatively, the
optical source may form part of the optical network interface 40.
The modulated optical channel is routed via optical path 32 to
optical add-drop multiplexer 20 where it joins the WDM optical
signal propagating on the transmission waveguide.
[0026] Turning to FIG. 2, a WDM optical communication system 110 is
depicted according to a further embodiment of the present
invention. WDM system 110 is a bidirectional, 64-channel optical
system having two counter-propagating 32-channel WDM optical
signals. As is common in commercially-deployed optical
communication systems, there is a "work" optical waveguide 112 may
and a "protect" optical waveguide 114. As is known in the art, the
protect waveguide 114 is used to transport traffic during failure
of waveguide 112 (e.g., through a break in the optical waveguide,
transmission equipment failure, etc.) in order to prevent a
disruption in service.
[0027] In accordance with traditional industry nomenclature, one of
the WDM signals propagating in a first direction is designated the
west-east WDM signal while the WDM signal propagating in the
opposite direction is designated the east-west WDM signal. The
individual optical channels in the west-east WDM optical signal are
denoted by the symbols .lambda..sub.1, .lambda..sub.2,
.lambda..sub.3 etc., while the individual optical channels in the
east-west WDM optical signal are denoted by the symbols
.lambda..sub.a, .lambda..sub.b, .lambda..sub.c, etc. for clarity of
presentation. Waveguide 112 is a bidirectional work waveguide while
waveguide 114 is a bidirectional protect waveguide. The identical
traffic is carried over each bidirectional waveguide to prevent
interruption of service caused by failure of one waveguide. It is
noted that although the embodiment of FIG. 2 is described in the
context of a bidirectional optical system, the system of FIG. 2 can
also be employed in a unidirectional optical communication
system.
[0028] Optical add-drop multiplexers 300 are positioned along each
bidirectional waveguide for adding and dropping optical channels.
In this embodiment, the optical add-drop multiplexers are selected
to be four-channel bidirectional add-drop multiplexers. Such
multiplexers are configured to add-drop two channels to/from each
of the counter-propagating WDM optical signals. An exemplary
bidirectional add-drop multiplexer 300 is depicted in FIG. 3 and is
further described in co-pending U.S. patent application Ser. No.
09/677,764 filed Oct. 3, 2000 (Attorney Docket No. SEN100),
assigned to the assignee of the present invention, the disclosure
of which is incorporated by reference herein. Bidirectional
add-drop multiplexer 300 will be discussed in further detail in
connection with FIG. 3, below. For illustrative purposes,
.lambda..sub.1 and .lambda..sub.2 are indicated as being
add-dropped from the west-east WDM optical signal, while
.lambda..sub.a and .lambda..sub.b are indicated as being
add-dropped from the east-west WDM optical signal.
[0029] Channels to be added to the optical communication system
through add-drop multiplexers 300 are produced by transponders 130.
Transponders 130 both receive the optical channels dropped by
add-drop multiplexers 300 and produce the optical channels to be
added by the add-drop multiplexers. Transponders 130 include a
short-reach optical interface and interact with the optical network
interface 140 through these short-reach optical signals which are
carried by optical paths 142, 143, 144, and 145.
[0030] As in the embodiment depicted in FIG. 1, optical network
interfaces 140 combine data from plural data sources having
different data formats and place that information on individual
optical channels such that each optical channel carries information
having different data formats. In the embodiment of FIG. 2,
additional elements, cell format module 150 and TDM format module
155 are provided so that data from the various individual data
sources can be intelligently routed and arranged on a particular
optical channel. Although a single cell format module 150 and TDM
format module 155 are depicted in FIG. 2, a pair of cell format and
TDM modules are associated with each of the four optical channels
being add-dropped by optical add-drop multiplexer 300; the
remaining modules have been omitted for clarity of
presentation.
[0031] As seen in FIG. 2, the TDM format module 155 takes data
which is already TDM formatted-schematically depicted as the
electrical communication of this module with TDM data sources such
as DS-3, OC-3, and OC-12 signals. As with the expression
"poptically communicates" the expression "electrically
communicates" denotes an electrical path between two elements. The
electrical path may be a direct path or it may route through
intermediate electrical devices. In this way, signals which already
contain TDM formats are not broken up into packets or cells,
incurring additional overhead bits identifying the respective
payloads. Alternatively, the TDM signal sources may be further
broken down into their component signals prior to the format
modules and traffic routed through the system based on the format
of the component signals.
[0032] Conversely, the cell format module 150 takes data which is
organized into packets or cells and transports it to optical
network interface 140. Cell format module electrically communicates
with cell or packet-formatted data sources such as ATM-formatted
data source 162, IP format data source 160, MPLS format data source
170, Gigabit Ethernet format data source 172, Ethernet format data
source 174, etc. The data from data sources 160, 162, 170, 172,
174, etc. are thus intelligently arranged for presentation to
optical network interface 140 which can then group the data
efficiently for output onto the optical channel. Cell format module
150 sends this information to optical network interface 140 via
electrical path 143 while TDM format module 155 sends this
information to the optical network interface via electrical path
145.
[0033] Note that although cell format module 150 and TDM format
module 155 are depicted as separate units, the modules may
optionally be combined into a single formatting module which
performs both the functions of grouping the cell-based data from
the various cell/packet based data sources and grouping the TDM
data from the various TDM based data sources. In this manner, a
single switching module can receive data from all types of data
sources and intelligently route that data to the optical network
interface.
[0034] In the optical network interface module 140, the information
from the cell module 150 and the TDM module 155 is arranged for
placement onto a short-reach optical signal for communication along
optical path 132 to transponder 130. Transponder 130 converts the
short-reach optical signal into a modulated optical signal having a
wavelength corresponding to .lambda..sub.1 of the channel plan. The
optical channel to be added, designated as .lambda..sub.1, is
transported along optical path 122 to optical add-drop multiplexer
300 for addition to the west-east wavelength division multiplexed
optical signal. Again, note that .lambda..sub.1 is used as an
exemplary channel; through the use of the optical network
interface, the information may be placed on any optical channel to
be added to the system. Further, incoming data may be separated
into its fundamental units and placed on plural optical channels.
For example, an incoming OC-12 signal may be decomposed into four
OC-3 signals; each of these signals may be independently placed on
any of the outgoing optical channels.
[0035] An exemplary optical network interface 140 is depicted in
FIG. 4. Information 143 from cell format module 150 enters optical
network interface 140 and is placed in cell scheduler 191 and
packet scheduler 192. Each of these respectively communicates with
cell buffer 193 and packet SAR (segregation and reassembly) 194 to
arrange the data from module 150 prior to placement in SONET mapper
195. SONET mapper 195 receives this information and combines it
with incoming data stream 145 from TDM format module 155 to create
a SONET-framed signal having multiple data formats encoded in the
signal. This signal is output to electrical-to-optical conversion
element 401 which encodes the information on an optical signal
(typically, a short reach optical laser having an output at 1310 nm
although any optical signal may be used to communicate with the
corresponding transponder or, when transponders are not employed,
the electrical-to optical conversion element may be a directly or
externally-modulated transmitter laser used to create the optical
channel to be carried to the add-drop multiplexer such as in the
embodiment of FIG. 1. Note that a set comprising a SONET mapper,
cell scheduler, packet scheduler, cell buffer, and packet SAR are
provided for each outgoing optical channel to created using the
optical network interface 140; those elements required to provide
input for electrical-to-optical conversion elements 403, 405, and
407 are not shown for the sake of clarity. Similarly, optical
signals received by the optical network interface (either from a
transponder or directly from an optical add-drop multiplexer) are
converted to electrical signals by optical-to-electrical conversion
elements 402, 404, 406, and 406. From the optical-to-electrical
conversion elements, each signal travels a path substantially
opposite to the one described above to reach the cell format or TDM
format modules for routing to the end user.
[0036] Although not shown in FIG. 2, a cell format module 150 and
TDM format module 155 are provided for each optical channel;
similarly, the data sources shown are those for each optical
channel. Further, in order to populate the protect ring, the
incoming data is duplicated and sent to protect cell and TDM
modules 150' and 155'; such duplication for work and protect paths
is known in the art (it is not shown in FIG. 2 for clarity of
presentation). In order to provide additional system survivability
in the event of a failure, an electrical cross-connect may
optionally be provided interconnecting the cell and TDM format
modules of the work system with the optical network interface of
the protect system and interconnecting the cell and TDM format
modules of the protect system with the optical network interface of
the work system. In this way, data can be efficiently routed to the
surviving optical path to prevent service interruption.
[0037] When an optical channel is dropped by optical add-drop
multiplexer 300, the optical signal follows the same path outlined
above in reverse. The optical channel is converted in transponder
130 from an optical signal at the channel plan wavelength to a
short-reach interface optical signal which is output along optical
path 132 to optical network interface 140. In optical network
interface 140, the optical signal is converted to an electrical
signal. Cell-based data is routed to cell format module 150 via
electrical path 142 while TDM-based data is routed to TDM format
module 155 through electrical path 144.
[0038] Based on the information in the signal, cell format module
150 routes the information to the corresponding ATM, IP, MPLS, etc.
module; similarly, TDM format module routes the information to the
corresponding DS-3, OC-3, OC-12, etc. module to be sent to the end
user. In some instances, it may be desirable to re-route traffic
from a dropped optical channel onto another optical channel to be
added to the WDM signal such that the traffic continues along the
transmission path and is not terminated at the add-drop point. This
information is identified by the respective TDM format or cell
format module and is routed back to the optical network interface
module for placement on an outgoing optical channel.
[0039] FIG. 3 depicts a bidirectional optical add-drop multiplexer
which may be employed in the optical system of FIG. 2. As seen in
FIG. 3, bidirectional add-drop multiplexer 300 includes ten
three-port optical circulators 210, 220, 230, 240, 250, 260, 270,
280, 290, and 310. The bidirectional add-drop multiplexer is
interposed along bidirectional waveguide 112. Bidirectional
waveguide 112 places the west-east wavelength division multiplexed
optical signal into the first port of first circulator 210 where it
is routed to the first port of the second optical circulator 270.
Optical channel selector 275 is positioned between the circulator
270 and circulator 280 for add/dropping the optical channel
designated .lambda..sub.2. The through channels and any added
channels exit through the second port of circulator 280 and enter
circulator 220. Again, the drop-add channel pair is selected by
channel selector 225 and the added channels and through channels
exit through port 2 of circulator 230. The through west-east
channels with the added channels exit the add-drop multiplexer
through port 1 of circulator 240 as the west-east channels enter
through port 1 of circulator 240. As with the west-east channels,
the circulators 240, 290, 310, 260 and channel selectors 295 and
255 add-drop the optical channels .lambda..sub.a and .lambda..sub.b
in the same manner as .lambda..sub.2 and .lambda..sub.1 were
add-dropped for the west-east channels. In this manner, two channel
pairs for each counter-propagating WDM optical signal are
add-dropped by add-drop multiplexer 300.
[0040] The channel selectors 225, 255, 275, and 295 may either be
fixed, i.e., configured to always add-drop a particular optical
channel having a given channel wavelength, or they may be
dynamically reconfigurable, i.e., capable of add-dropping an
optical channel at any optical wavelength to which the selector can
be set. Depending upon the choice of fixed or reconfigurable
channel selectors, the channel-selecting element may be chosen from
devices including, but not limited to, Bragg gratings, tunable
Bragg gratings, Fabry-Perot filters, acousto-optic tunable filters,
multilayer dielectric thin film filters, arrayed waveguide gratings
(AWGs) and/or combinations of these devices. Detailed descriptions
of such optical selection devices are found in chapter 3 of Optical
Networks: A Practical Perspective, incorporated by reference
above.
[0041] Advantageously, plural circulator/channel selector systems
may be added as needed to bidirectional add-drop multiplexer 300
when desiring to create a bidirectional add-drop multiplexer
capable of add-dropping more optical channels. By separating the
add/dropping of a single optical channel at a time with multiple
circulator/channel selector sub-systems, the traffic may be more
readily routed to diverse locations (e.g., different SONET rings,
interexchange vs. local destinations, unidirectional local networks
on customer premises, etc.). Because bidirectional optical
waveguides 112 and 114 must be interrupted at many locations in a
local network to interpose the bidirectional add-drop multiplexers,
it may be advantageous to include optical amplification in the
add-drop multiplexer to minimize the need to further insert optical
amplifiers at other locations along the bidirectional transmission
line.
[0042] Various channel plans can be accommodated by the systems of
the present invention. For example, the west-east WDM signal may be
selected to include only optical channels within the C-band
(nominally defined as wavelengths from approximately 1530-1565 nm);
conversely, the east-west WDM signal may be selected to include
only optical channels within the L band (nominally defined as
wavelengths from approximately 1565-1610 nm). For such a channel
plan, implemented in the optical system of FIG. 2, the optical
add-drop multiplexer would drop add-drop two C, L channel pairs
to/from bidirectional waveguide 112. Such a channel plan simplifies
optical amplifier selection since the amplifier chosen to amplify
each signal band would be optimized to have a flat gain profile
across that band.
[0043] Alternatively, the west-east channels may be selected from
wavelengths across the entire wavelength spectrum to provide
maximum interchannel spacing distance (and minimize potential cross
talk. In such an embodiment, the east-west channel wavelengths
would alternate with the west-east channel wavelengths in an
interleaved manner (e.g., west-east channel wavelengths of 1528,
1532, 1536, 1540, etc. and east-west channel wavelengths of 1530,
1534, 1538, 1542, etc.). In either case, the west-east and
east-west channels plans will likely be dictated by overall system
considerations, such as the network topology in which the system is
deployed. Further, because the west-east and east-west WDM optical
signals are routed along different paths within the add-drop
multiplexers, it is possible that one or more of the optical
channel wavelengths in each of the counter-propagating WDM signals
may be the same.
[0044] Advantageously, use of the present invention allows all of
the capacity on an optical network to be used to the fullest extent
regardless of whether the information to be carried by an optical
signal is voice or data. By not segregating voice and data to
different optical channels, the capacity of each optical channel
can be fully loaded with voice and data information, avoiding voice
or data bottlenecks at each of their respective peak capacity
times. In this manner layer 3 and 4 attributes are used to optimize
layer 1 and 2 services within a network.
[0045] While the above invention has been described with reference
to the particular exemplary embodiments, many modifications and
functionally equivalent elements may be substituted without
departing from the spirit and contributions of the present
invention. Accordingly, modifications and functionally equivalent
elements such as those suggested above, but not limited thereto,
are considered to be within the scope of the following claims.
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