U.S. patent application number 12/328373 was filed with the patent office on 2009-04-30 for method and apparatus for reducing data burst overhead in an ethernet passive optical network.
This patent application is currently assigned to TEKNOVUS, INC.. Invention is credited to Glen Kramer.
Application Number | 20090110403 12/328373 |
Document ID | / |
Family ID | 34198053 |
Filed Date | 2009-04-30 |
United States Patent
Application |
20090110403 |
Kind Code |
A1 |
Kramer; Glen |
April 30, 2009 |
METHOD AND APPARATUS FOR REDUCING DATA BURST OVERHEAD IN AN
ETHERNET PASSIVE OPTICAL NETWORK
Abstract
One embodiment of the present invention provides a system that
reduces data burst overhead in an Ethernet passive optical network
which includes a central node and at least one remote node, wherein
downstream data from the central node is broadcast to the remote
nodes, and wherein upstream data from a remote node is transmitted
to the central node in a unicast manner. During operation, the
central node transmits grant messages to a number of remote nodes,
wherein a grant message for a specified remote node assigns a start
time and a duration of a transmission timeslot in which the
specified remote node may transmit an upstream data burst. In
response to the grant messages, the central node then receives a
number of upstream data bursts, wherein the time gap between two
consecutive upstream data bursts is less than the summation of a
default laser turn-on time, a default laser turn-off time, an AGC
period, and a CDR period.
Inventors: |
Kramer; Glen; (Petaluma,
CA) |
Correspondence
Address: |
PARK, VAUGHAN & FLEMING LLP
2820 FIFTH STREET
DAVIS
CA
95618-7759
US
|
Assignee: |
TEKNOVUS, INC.
Petaluma
CA
|
Family ID: |
34198053 |
Appl. No.: |
12/328373 |
Filed: |
December 4, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10820663 |
Apr 7, 2004 |
7477845 |
|
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12328373 |
|
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60495649 |
Aug 18, 2003 |
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Current U.S.
Class: |
398/98 |
Current CPC
Class: |
H04Q 11/0067 20130101;
H04Q 2011/0064 20130101; H04J 3/0682 20130101 |
Class at
Publication: |
398/98 |
International
Class: |
H04J 14/08 20060101
H04J014/08 |
Claims
1. A method for reducing data burst overhead in an Ethernet passive
optical network, the method comprising: transmitting grant messages
to a number of optical network units (ONUs), wherein a grant
message for a specified ONU assigns a start time and a duration of
a transmission timeslot in which the specified ONU may transmit a
upstream data burst; and receiving a number of upstream data
bursts, wherein the time gap between two consecutive upstream data
bursts is less than the summation of a default laser turn-on time,
a default laser turn-off time, an automatic gain control (AGC)
period, and a clock and data recovery (CDR) period.
2. The method of claim 1, wherein a preceding upstream data burst's
laser turn-off period overlaps with a subsequent data burst's laser
turn-on period.
3. The method of claim 1, wherein the non-overlapping portion of
the preceding data burst's laser turn-off period is equal to or
greater than twice the allowed maximum jitter of the round-trip
time between the central node and an ONU; and wherein the
non-overlapping portion of the subsequent data burst's laser
turn-on period is equal to or greater than twice the allowed
maximum jitter of the round-trip time between the central node and
an ONU.
4. The method of claim 1, wherein a grant message specifies a
transmission timeslot start time that is earlier than the ending
time of an immediately preceding transmission timeslot.
5. The method of claim 1, wherein receiving a number of upstream
data bursts involves receiving a number of consecutive data bursts
from an ONU, and wherein the ONU is allowed to transmit the number
of consecutive data bursts without turning off and turning on its
laser between two consecutive data bursts.
6. The method of claim 5, further comprising detecting the time gap
between two consecutive transmission timeslots assigned to the ONU;
and if the time gap is less than a pre-defined value, allowing the
ONU to transmit upstream data during the time gap without turning
off and turning on its laser.
7. The method of claim 1, wherein if one or more ONUs are virtual
remote nodes located in a common physical ONU, and if these virtual
ONUs transmit upstream data through a common laser belonging to the
common physical ONU, the method further comprises: allowing the
common laser to keep transmitting upstream data without being
turned off between consecutive transmission timeslots assigned to
one or more virtual ONUs located in the common physical remote
node.
8. The method of claim 7, wherein a grant message contains a
laser-turn-on flag and a laser-turn-off flag; wherein if a grant
message's laser-turn-on flag is true, the corresponding ONU turns
on its laser at the start time of its assigned transmission
timeslot and transmits an AGC bit sequence and a CDR bit sequence
before transmitting upstream data; wherein if a grant message's
laser-turn-on flag is false, the corresponding ONU immediately
starts transmitting upstream data at the start time of its assigned
transmission timeslot without transmitting an AGC bit sequence and
a CDR bit sequence; wherein if a grant message's laser-turn-off
flag is true, the corresponding ONU turns off its laser after
transmitting upstream data; and wherein if a grant message's
laser-turn-off flag is false, the corresponding ONU continues
transmitting data until the end of its assigned transmission
timeslot without turning off its laser.
9. The method of claim 7, wherein if one or more ONUs are virtual
ONUs located in a common physical ONU, and if these virtual ONUs
transmit upstream data through a common laser belonging to the
common physical ONU, the method further comprises allowing the
common laser to keep transmitting the upstream data bursts without
being turned off between consecutive transmission timeslots
assigned to one or more virtual ONUs located in the common physical
ONU.
10. The method of claim 1, further comprising receiving an actual
laser turn-on time and an actual laser turn-off time from an ONU;
wherein the actual laser turn-on and turn-off times specify the
amount of time required by the ONU to turn on and turn off its
laser, respectively.
11. The method of claim 10, wherein the actual laser turn-on and
turn-off times are transmitted with a registration message from the
ONU when the central node initially registers the ONU.
12. The method of claim 10, wherein a grant message assigns a start
time and a duration of a transmission timeslot based on the actual
laser turn-on and turn-off times of the ONU to which the grant
message is destined.
13. An apparatus for reducing data burst overhead in an Ethernet
passive optical network, comprising: at least one ONU; and an
optical line terminal (OLT) configured to, transmit grant messages
to a number of ONUs, wherein a grant message for a specified ONU
assigns a start time and a duration of a transmission timeslot in
which the specified ONU may transmit a upstream data burst; and
receive a number of upstream data bursts, wherein the time gap
between two consecutive upstream data bursts is less than the
summation of a default laser turn-on time, a default laser turn-off
time, an AGC period, and a CDR period.
14. The apparatus of claim 13, wherein a preceding upstream data
burst's laser turn-off period overlaps with a subsequent data
burst's laser turn-on period.
15. The apparatus of claim 13, wherein the non-overlapping portion
of the preceding data burst's laser turn-off period is equal to or
greater than twice the allowed maximum jitter of the round-trip
time between the central node and an ONU; and wherein the
non-overlapping portion of the subsequent data burst's laser
turn-on period is equal to or greater than twice the allowed
maximum jitter of the round-trip time between the central node and
an ONU.
16. The apparatus of claim 13, wherein a grant message specifies a
transmission timeslot start time that is earlier than the ending
time of an immediately preceding transmission timeslot.
17. The apparatus of claim 13, wherein an ONU is configured to
transmit a number of consecutive data bursts without turning off
and turning on its laser between two consecutive data bursts.
18. The apparatus of claim 17, wherein the ONU is further
configured to detect the time gap between two consecutive
transmission timeslots assigned to the ONU; and if the time gap is
less than a pre-defined value, allow the ONU to transmit upstream
data during the time gap without turning off and turning on its
laser.
19. The apparatus of claim 13, wherein if one or more ONUs are
virtual ONUs located in a common physical ONU, and if these virtual
ONUs transmit upstream data through a common laser belonging to the
common physical ONU, the common physical ONU is configured to:
allow the common laser to keep transmitting upstream data without
being turned off between consecutive transmission timeslots
assigned to one or more virtual ONUs located in the common physical
ONU.
20. The apparatus of claim 19, wherein a grant message contains a
laser-turn-on flag and a laser-turn-off flag; wherein if a grant
message's laser-turn-on flag is true, the corresponding ONU is
configured to turn on its laser at the start time of its assigned
transmission timeslot and transmits an AGC bit sequence and a CDR
bit sequence before transmitting upstream data; wherein if a grant
message's laser-turn-on flag is false, the corresponding ONU is
configured to start immediately transmitting upstream data at the
start time of its assigned transmission timeslot without
transmitting an AGC bit sequence and a CDR bit sequence; wherein if
a grant message's laser-turn-off flag is true, the corresponding
ONU is configured to turn off its laser after transmitting upstream
data; and wherein if a grant message's laser-turn-off flag is
false, the corresponding ONU is configured to continue transmitting
data until the end of its assigned transmission timeslot without
turning off its laser.
21. The apparatus of claim 19, wherein if one or more ONUs are
virtual ONUs located in a common physical ONU, and if these virtual
ONUs transmit upstream data through a common laser belonging to the
common physical ONU, the physical ONU is further configured to
allow the common laser to keep transmitting the upstream data
bursts without being turned off between consecutive transmission
timeslots assigned to one or more virtual ONUs located in the
common physical ONU.
22. The apparatus of claim 13, wherein the central node is further
configured to receive an actual laser turn-on time and an actual
laser turn-off time from an ONU; and wherein the actual laser
turn-on and turn-off times specify the amount of time required by
the ONU to turn on and turn off its laser, respectively.
23. The apparatus of claim 22, wherein the actual laser turn-on and
turn-off times are transmitted with a registration message from the
ONU when the central node initially registers the ONU.
24. The apparatus of claim 22, wherein a grant message assigns a
start time and a duration of a transmission timeslot based on the
actual laser turn-on and turn-off times of the ONU to which the
grant message is destined.
Description
RELATED APPLICATION
[0001] This application is a continuation of application Ser. No.
10/820,663, Attorney Docket Number TEK03-1007, entitled "METHOD AND
APPARATUS FOR REDUCING DATA BURST OVERHEAD IN AN ETHERNET PASSIVE
OPTICAL NETWORK," by inventor Glen Kramer, filed 7 Apr. 2004, which
claims the benefit of U.S. Provisional Application No. 60/495,649,
Attorney Docket Number TEK03-1007PSP, entitled "METHOD FOR TIMESLOT
ALLOCATION TO REDUCE GUARD BAND OVERHEAD IN ETHERNET PASSIVE
OPTICAL NETWORKS," by inventor Glen Kramer, filed 18 Aug. 2003.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The present invention relates to the design of Ethernet
passive optical networks. More specifically, the present invention
relates to a method and apparatus for reducing data burst overhead
in an Ethernet passive optical network.
[0004] 2. Related Art
[0005] In order to keep pace with increasing Internet traffic,
optical fibers and associated optical transmission equipment have
been widely deployed to substantially increase the capacity of
backbone networks. However, this increase in the capacity of
backbone networks has not been matched by a corresponding increase
in the capacity of access networks. Even with broadband solutions,
such as digital subscriber line (DSL) and cable modem (CM), the
limited bandwidth offered by current access networks creates a
severe bottleneck in delivering high bandwidth to end users.
[0006] Among the different technologies that are presently being
developed, Ethernet passive optical networks (EPONs) are one of the
best candidates for next-generation access networks. EPONs combine
ubiquitous Ethernet technology with inexpensive passive optics.
Hence, they offer the simplicity and scalability of Ethernet with
the cost-efficiency and high capacity of passive optics. In
particular, due to the high bandwidth of optical fibers, EPONs are
capable of accommodating broadband voice, data, and video traffic
simultaneously. Such integrated service is difficult to provide
with DSL or CM technology. Furthermore, EPONs are more suitable for
Internet Protocol (IP) traffic, because Ethernet frames can
directly encapsulate native IP packets with different sizes,
whereas ATM passive optical networks (APONs) use fixed-size ATM
cells and consequently require packet fragmentation and
reassembly.
[0007] Typically, EPONs are used in the "first mile" of the
network, which provides connectivity between the service provider's
central offices and business or residential subscribers. Logically,
the first mile is a point-to-multipoint network, with a central
office servicing a number of subscribers. A tree topology can be
used in an EPON, wherein one fiber couples the central office to a
passive optical splitter, which divides and distributes downstream
optical signals to subscribers and combines upstream optical
signals from subscribers (see FIG. 1).
[0008] Transmissions within an EPON are typically performed between
an optical line terminal (OLT) and optical networks units (ONUs)
(see FIG. 2). The OLT generally resides in the central office and
couples the optical access network to a metro backbone, which is
typically an external network belonging to an Internet Service
Provider (ISP) or a local exchange carrier. An ONU can be located
either at the curb or at an end-user location, and can provide
broadband voice, data, and video services. ONUs are typically
coupled to a one-by-N (1.times.N) passive optical coupler, where N
is the number of ONUs, and the passive optical coupler is typically
coupled to the OLT through a single optical link. (Note that one
may use a number of cascaded optical splitters/couplers.) This
configuration can save significantly in the number of fibers and
amount of hardware required by EPONs.
[0009] Communications within an EPON can be divided into downstream
traffic (from OLT to ONUs) and upstream traffic (from ONUs to OLT).
In the downstream direction, because of the broadcast nature of the
1.times.N passive optical coupler, downstream data frames are
broadcast by the OLT to all ONUs and are subsequently extracted by
their destination ONUs. In the upstream direction, the ONUs need to
share channel capacity and resources, because there is only one
link coupling the passive optical coupler with the OLT.
[0010] Correspondingly, an EPON typically employs some arbitration
mechanism to avoid data collision and to provide fair sharing of
the upstream fiber-channel capacity. This is achieved by allocating
a transmission timeslot to each ONU. An ONU typically buffers data
it receives from a subscriber until it reaches the start time of
its transmission timeslot. When its turn arrives, the ONU "bursts"
all stored frames to the OLT at full channel speed.
[0011] Due to unequal distances between an OLT and ONUs, optical
signal attenuation in an EPON is not the same for each ONU. The
power level received at the OLT could be different for each
transmission timeslot. This is called the near-far problem. If the
receiver in the OLT is adjusted to receive a high-power signal from
a closely located ONU, it may mistakenly read a "one" as a "zero"
when receiving a weaker signal from a distant ONU. Similarly, if
the receiver is adjusted to a weak signal, it may read a "zero" as
a "one" when receiving a stronger signal. To detect an incoming
signal properly, the OLT receiver is ideally given a short period
to adjust its zero-one threshold, which is called the automatic
gain control (AGC) period, at the beginning of each timeslot. In
addition, another period is usually reserved after the AGC period
for the receiver to synchronize its clock with the incoming bits. A
clock and data recovery (CDR) circuit is responsible for the
bit-synchronization.
[0012] Another issue is that it is not enough just to disallow an
ONU from sending data outside its assigned transmission timeslot.
Even in the absence of data transmission, an ONU's laser generates
spontaneous emission noise when powered on. Accumulated spontaneous
emission noise from several ONUs close to the OLT can easily
obscure the signal from a distant ONU (this is called the capture
effect). Thus, an ONU ideally shuts down its laser between its
transmission timeslots. Because a laser takes time to cool down
when turned off, and to warm up when turned on, its emitted power
may fluctuate at the beginning and the end of a transmission.
Therefore, a laser turn-on period and a laser turn-off period are
typically reserved for the laser to stabilize.
[0013] During the laser turn-on, turn-off, AGC, and CDR periods an
ONU cannot transmit payload data. This data burst overhead makes
the upstream bandwidth utilization less efficient. Hence, what is
needed is a method and apparatus for reducing data burst overhead
in an Ethernet passive optical network.
SUMMARY
[0014] One embodiment of the present invention provides a system
that reduces data burst overhead in an Ethernet passive optical
network which includes a central node and at least one remote node,
wherein downstream data from the central node is broadcast to the
remote nodes, and wherein upstream data from a remote node is
transmitted to the central node in a unicast manner. During
operation, the central node transmits grant messages to a number of
remote nodes, wherein a grant message for a specified remote node
assigns a start time and a duration of a transmission timeslot in
which the specified remote node may transmit an upstream data
burst. In response to the grant messages, the central node then
receives a number of upstream data bursts, wherein the time gap
between two consecutive upstream data bursts is less than the
summation of a default laser turn-on time, a default laser turn-off
time, an AGC period, and a CDR period.
[0015] In a variation of this embodiment, a preceding upstream data
burst's laser turn-off period overlaps with a subsequent data
burst's laser turn-on period.
[0016] In a further variation, the non-overlapping portion of the
preceding data burst's laser turn-off period is equal to or greater
than twice the allowed maximum jitter of the round-trip time
between the central node and a remote node. In addition, the
non-overlapping portion of the subsequent data burst's laser
turn-on period is equal to or greater than twice the allowed
maximum jitter of the round-trip time between the central node and
a remote node.
[0017] In a further variation, a grant message specifies a
transmission timeslot start time that is earlier than the ending
time of an immediately preceding transmission timeslot.
[0018] In a variation of this embodiment, a remote node is allowed
to transmit the number of consecutive data bursts without turning
off and turning on its laser between two consecutive data
bursts.
[0019] In a further variation, a remote node detects the time gap
between two consecutive transmission timeslots assigned to the
remote node. If the time gap is less than a pre-defined value, the
remote node transmits upstream data during the time gap without
turning off and turning on its laser.
[0020] In a variation of this embodiment, if one or more remote
nodes are virtual remote nodes located in a common physical remote
node, and if these virtual remote nodes transmit upstream data
through a common laser belonging to the common physical remote
node, the physical remote node allows the common laser to keep
transmitting upstream data without being turned off between
consecutive transmission timeslots assigned to one or more virtual
remote nodes located in the common physical remote node.
[0021] In a further variation, a grant message contains a
laser-turn-on flag and a laser-turn-off flag. If a grant message's
laser-turn-on flag is true, the corresponding remote node turns on
its laser at the start time of its assigned transmission timeslot
and transmits an AGC bit sequence and a CDR bit sequence before
transmitting upstream data. If a grant message's laser-turn-on flag
is false, the corresponding remote node immediately starts
transmitting upstream data at the start time of its assigned
transmission timeslot without transmitting an AGC bit sequence and
a CDR bit sequence. If a grant message's laser-turn-off flag is
true, the corresponding remote node turns off its laser after
transmitting upstream data. If a grant message's laser-turn-off
flag is false, the corresponding remote node continues transmitting
data until the end of its assigned transmission timeslot without
turning off its laser.
[0022] In a further variation, if one or more remote nodes are
virtual remote nodes located in a common physical remote node, and
if these virtual remote nodes transmit upstream data through a
common laser belonging to the common physical remote node, the
physical remote node allows the common laser to keep transmitting
the upstream data bursts without being turned off between
consecutive transmission timeslots assigned to one or more virtual
remote nodes located in the common physical remote node.
[0023] In a variation of this embodiment, the central node receives
an actual laser turn-on time and an actual laser turn-off time from
a remote node; wherein the actual laser turn-on and turn-off times
specify the amount of time required by the remote node to turn on
and turn off its laser, respectively.
[0024] In a further variation, the actual laser turn-on and
turn-off times are transmitted with a registration message from the
remote node when the central node initially registers the remote
node.
[0025] In a further variation, a grant message assigns the start
time and duration of a transmission timeslot based on the actual
laser turn-on and turn-off times of the remote node to which the
grant message is destined.
BRIEF DESCRIPTION OF THE FIGURES
[0026] FIG. 1 illustrates an Ethernet passive optical network
wherein a central office and a number of subscribers are coupled
through optical fibers and an Ethernet passive optical splitter
(prior art).
[0027] FIG. 2 illustrates an EPON in normal operation mode (prior
art).
[0028] FIG. 3 illustrates bridged Ethernet segments (prior
art).
[0029] FIG. 4A illustrates transmission of downstream traffic with
point-to-pint emulation in an EPON (prior art).
[0030] FIG. 4B illustrates transmission of upstream traffic with
point-to-pint emulation in an EPON (prior art).
[0031] FIG. 5 illustrates bridging between ONUs with point-to-point
emulation in an EPON (prior art).
[0032] FIG. 6 illustrates virtual ONUs (VONUs) with logical links
in an EPON (prior art).
[0033] FIG. 7 illustrates the structure of a transmission timeslot
in an EPON (prior art).
[0034] FIG. 8 illustrates an overlap of a laser turn-off period
with the next transmission timeslot's laser turn-on period in
accordance with an embodiment of the present invention.
[0035] FIG. 9 illustrates an overlap of two transmission timeslots
corresponding to two different VONUs in accordance with one
embodiment of the present invention.
[0036] FIG. 10 illustrates the merging of two consecutive
transmission timeslots assigned to one VONU in accordance with one
embodiment of the present invention.
[0037] FIG. 11 illustrates the merging of two consecutive
transmission timeslots assigned to two VONUs located in a common
physical ONU in accordance with one embodiment of the present
invention.
[0038] FIG. 12 presents a time-space diagram illustrating the
merging of transmission timeslots assigned to multiple VONUs
located in a common physical ONU in accordance with one embodiment
of the present invention.
[0039] FIG. 13A illustrates a transmission timeslot the size of
which is based on default laser turn-on and turn-off times.
[0040] FIG. 13B illustrates a transmission timeslot the size of
which is reduced based on actual laser turn-on and turn-off times
in accordance with an embodiment of the present invention.
DETAILED DESCRIPTION
[0041] The following description is presented to enable any person
skilled in the art to make and use the invention, and is provided
in the context of a particular application and its requirements.
Various modifications to the disclosed embodiments will be readily
apparent to those skilled in the art, and the general principles
defined herein may be applied to other embodiments and applications
without departing from the spirit and scope of the present
invention (e.g., general passive optical network (PON)
architectures). Thus, the present invention is not intended to be
limited to the embodiments shown, but is to be accorded the widest
scope consistent with the principles and features disclosed
herein.
[0042] The data structures and procedures described in this
detailed description are typically stored on a computer readable
storage medium, which may be any device or medium that can store
code and/or data for use by a computer system. This includes, but
is not limited to, application specific integrated circuits
(ASICs), field-programmable gate arrays (FPGAs), semiconductor
memories, magnetic and optical storage devices such as disk drives,
magnetic tape, CDs (compact discs) and DVDs (digital versatile
discs or digital video discs), and computer instruction signals
embodied in a transmission medium (with or without a carrier wave
upon which the signals are modulated).
Passive Optical Network Topology
[0043] FIG. 1 illustrates a passive optical network, wherein a
central office and a number of subscribers are coupled together
through optical fibers and a passive optical splitter (prior art).
As shown in FIG. 1, a number of subscribers are coupled to a
central office 101 through optical fibers and a passive optical
splitter 102. Passive optical splitter 102 can be placed in the
vicinity of end-user locations, so that the initial fiber
deployment cost is minimized. Central office 101 can be coupled to
an external network 103, such as a metropolitan area network
operated by an Internet service provider (ISP). Note that although
FIG. 1 illustrates a tree topology, a PON can also be based on
other topologies, such as a ring or a bus.
Normal Operation Mode in EPON
[0044] FIG. 2 illustrates an EPON in normal operation mode (prior
art). To allow ONUs to join an EPON at arbitrary times, an EPON
typically has two modes of operation: a normal operation mode and a
discovery (initialization) mode. Normal operation mode accommodates
regular upstream data transmissions, where an OLT assigns
transmission opportunities to all initialized ONUs.
[0045] As shown in FIG. 2, in the downstream direction, OLT 201
broadcasts downstream data to ONU 1 (211), ONU 2 (212), and ONU 3
(213). While all ONUs may receive the same copy of downstream data,
each ONU selectively forwards only the data destined to itself to
its corresponding users, which are user 1 (221), user 2 (222), and
user 3 (223), respectively.
[0046] In the upstream direction, OLT 201 first schedules and
assigns transmission timeslots to each ONU according to the ONU's
service-level agreement. When not in its transmission timeslot, an
ONU typically buffers the data received from its user. When its
scheduled transmission timeslot arrives, an ONU transmits the
buffered user data within the assigned transmission window.
[0047] Since every ONU takes turns in transmitting upstream data
according to the OLT's scheduling, the upstream link's capacity can
be efficiently utilized. However, for the scheduling to work
properly, the OLT needs to discover and initialize a newly joined
ONU. During discovery, the OLT may collect information critical to
transmission scheduling, such as the ONU's round-trip time (RTT),
its media access control (MAC) address, its service-level
agreement, etc. (Note that in some cases service-level agreement
may already be known to the OLT),
General Ethernet Requirement
[0048] FIG. 3 illustrates bridged Ethernet segments (prior art).
The IEEE 802 standards allow an Ethernet segment to operate in a
point-to-point mode. In a point-to-point Ethernet segment, a link
couples two hosts, or a host and an Ethernet bridge. Point-to-point
mode is a common form of operation in a switched Ethernet, such as
Gigabit Ethernet.
[0049] When multiple Ethernet hosts need to communicate with one
another, an Ethernet bridge typically couples and switches between
multiple point-to-point Ethernet segments to allow inter-segment
communications. As shown in FIG. 3, Ethernet bridge 310 has
multiple ports. Point-to-point segments 321 and 322 are coupled to
ports 311 and 312, respectively. Shared-medium segment 323 is
coupled to port 313. If the host on segment 322 sends a data frame
to the host on segment 321, the data frame will be switched by
Ethernet bridge 310 from port 312 to port 311 according to its
destination Ethernet (MAC) address.
[0050] Shared-medium segment 323 operates differently from
point-to-point segments. The IEEE 802 architecture generally
assumes that all devices connected to the same medium can
communicate to each other directly. Relying on this assumption,
bridges never forward a frame back to its ingress port. In the
example shown in FIG. 3, if the hosts on segment 323 needs to
communicate with each other, Ethernet bridge 310 does not forward
any of these frames, because it assumes that all the hosts coupled
to the same port can directly communicate with one another over the
shared medium.
Point-To-Point Emulation (PtPE) in EPON
[0051] In an EPON, because the upstream transmission from an ONU to
the OLT is point-to-point communication, the operation of EPON
ideally conforms to the point-to-point Ethernet operation as
defined by the IEEE 802 standard. However, the EPON architecture
does not automatically satisfy the requirement of bridged
point-to-point Ethernet: if the EPON upstream link is coupled to
one Ethernet bridge port, and all the upstream traffic is received
at that port, users connected to different ONUs on the same EPON
will be unable to communicate with one another. The Ethernet bridge
located within the OLT will not switch among the upstream data,
because they are received at the same port. Such a configuration
forces data traffic among ONUs within the same EPON to be processed
on layer 3 (network layer) and switched by equipment that resides
outside the EPON (e.g., an IP router to which the OLT is
connected). This is a very inefficient way of delivering intra-EPON
traffic.
[0052] To resolve this problem, and to ensure seamless integration
of an EPON with other Ethernet networks, devices attached to the
EPON medium ideally have an additional sub-layer that can emulate a
point-to-point medium. This sub-layer is referred to as
Point-to-Point Emulation (PtPE) sub-layer. This emulation sub-layer
resides below the MAC layer to preserve existing Ethernet MAC
operation defined in the IEEE P802.3 standards. Operation of this
emulation layer relies on tagging Ethernet frames with tags unique
for each ONU. These tags are called logic link IDs (LLIDs) and are
placed in the preamble before each frame.
[0053] FIG. 4A illustrates transmission of downstream traffic with
point-to-point emulation in an EPON (prior art). In PtPE mode, OLT
400 has multiple MAC ports (interfaces), each of which corresponds
to an ONU. When sending an Ethernet frame downstream from MAC port
431, PtPE sub-layer 440 in OLT 400 inserts LLID 461 which is
associated with MAC port 431. Although the frame is broadcast
through the passive optical coupler to every ONU, only the PtPE
sub-layer module located within an ONU with a matching LLID (ONU
451 with LLID 461 in this example) will accept the frame and pass
it to its MAC layer for further verification. MAC layers in other
ONUs (ONU 452 with LLID 462, and ONU 453 with LLID 463) will never
receive that frame. Accordingly, it appears as if the frame was
sent on a point-to-point link to only the destination ONU.
[0054] FIG. 4B illustrates transmission of upstream traffic with
point-to-pint emulation in an EPON (prior art). In the upstream
direction, ONU 451 inserts its assigned LLID 461 in the preamble of
each transmitted frame. Accordingly, PtPE sub-layer 440 of OLT 400
disseminates the frame to MAC port 431.
Bridging in EPON
[0055] FIG. 5 illustrates bridging between ONUs with point-to-point
emulation in an EPON (prior art). In general, all frames
transmitted (upstream and downstream) between OLT 400 and a certain
ONU always have the LLID assigned to that ONU. Note that an LLID is
only used to emulate a point-to-point link, not for switching or
relaying frames. In this example, ONU 451 intends to send a frame
to ONU 452. When the PtPE sub-layer 400 in OLT 400 receives this
frame, it determines to which Ethernet-bridge port this frame
should go, which is MAC port 431 and which is associated with LLID
461. PtPE sub-layer 400 also removes the frame's LLID 461.
Subsequently, Ethernet bridge 510 inspects the destination MAC
address of the frame and determines to which port the frame should
be switched, as regular Ethernet bridge would do. It then forwards
the frame to the port associated with ONU 452. PtPE sub-layer 400
in turn attaches to the downstream frame LLID 462, which is
associated with ONU 452. Based on LLID 462, PtPE sub-layer in ONU
452 accepts this frame and delivers the frame to ONU 452.
Virtual ONUs
[0056] FIG. 6 illustrates virtual ONUs (VONUs) with logical links
in an EPON (prior art). One implementation of EPON may allow more
than one LLID to be assigned to a physical ONU, wherein each LLID
corresponds to an entity (e.g., a network device or an application)
which needs a separate communication channel with the OLT. As shown
in FIG. 6, a physical ONU 650 accommodates two virtual ONUs (VONUs)
651 and 652. VONU 651 and 652 have LLIDs 661 and 662, respectively.
Correspondingly, ONU 650 has two MAC ports associated with VONU 651
and 652 respectively. In the same EPON, there may also exist
separate physical ONUs, such as ONUs 653, 654, and 655 (with LLIDs
663, 664, and 665, respectively). During actual operation, OLT 400
does not distinguish VONUs from separate physical ONUs, and grants
transmission slots to each VONU as if it was a separate physical
ONU. For the reason stated above, the terms "VONU" and "ONU" are
used interchangeably in the present invention.
Reducing Data Burst Overhead
[0057] FIG. 7 illustrates the structure of a transmission timeslot
in an EPON (prior art). An upstream data burst contained in a
transmission timeslot is comprised of several parts beside its data
payload. As shown in FIG. 7, the transmission timeslot may contain
a laser turn-on period 701, an AGC bit sequence 702, a CDR bit
sequence 703, a data/idle payload 704, and a laser turn-off period
705. Obviously, the useful portion of a transmission slot is the
data/idle payload portion 704 which actually carries user data.
[0058] The non-payload portions of a transmission usually do not
carry user data. In particular, the time gap comprising of laser
turn-on, turn-off, AGC, and CDR periods imposes a non-negligible
overhead to the transmission. It is desirable to reduce this data
burst overhead to achieve higher bandwidth utilization.
[0059] FIG. 8 illustrates an overlap of a laser turn-off period
with the next transmission timeslot's laser turn-on period in
accordance with an embodiment of the present invention. One way to
reduce data burst overhead, as shown in FIG. 8, is to schedule
consecutive transmission timeslots 810 and 820 such that the laser
turn-off period 815 of the preceding timeslot 810 overlaps the
laser turn-on period 821 of the subsequent timeslot 820. The net
result is reduced time gap between data bursts and hence a reduced
data burst overhead.
[0060] The overlap of laser turn-off and turn-on periods can be
complete or partial. In one embodiment of the present invention,
there is a portion of the laser turn-on or turn-off period that is
prohibited from overlapping. This portion is called "dead zone," as
shown in FIG. 8. A dead zone can provide some buffer time for time
jitter in the measured RTT between the OLT and a transmission ONU.
Note that time jitter in the measured RTT during transmission may
be caused by actual variations in the propagation delay and/or by
devices from several networking layers, such as the physical layer
(laser and receiver) and the MAC layer. A dead zone ensures that
such time jitter does not corrupt the data payload of a preceding
or subsequent transmission timeslot. In one embodiment of the
present invention, the dead zone is at least twice the allowed
maximum RTT jitter between the OLT and any ONU, such that the worst
jitter scenario (wherein the preceding transmission is delayed and
the subsequent transmission is early) can be accommodated.
[0061] FIG. 9 illustrates an overlap of two transmission timeslots
corresponding to two different VONUs in accordance with one
embodiment of the present invention. This example shows an EPON
comprising OLT 900, VONUs 921, 922, 923, 924, and 925. First, OLT
900 sends out two grant messages 931 and 932, which assign two
consecutive timeslots to VONUs 921 and 922, respectively. The start
time of the second timeslot (assigned to VONU 921) is earlier than
the end time of the first timeslot (assigned to VONU 922). As a
result, there is an overlap between the laser turn-off period of
data burst 942 from VONU 922 and the laser turn-on period of data
burst 941 from VONU 921.
[0062] FIG. 10 illustrates the merging of two consecutive
transmission timeslots assigned to one VONU in accordance with one
embodiment of the present invention. Some scheduling protocols in
an OLT may allow the OLT to grant consecutive transmission
timeslots to one VONU. In such case, the VONU does not need to turn
off its laser and then immediately turn it on. In addition, the
VONU does not need to generate AGC and CDR bits, because the
receiver in the OLT remains properly adjusted and synchronized.
This approach may eliminate the data burst overhead between two
consecutive timeslots assigned to the same VONU.
[0063] In the example shown in FIG. 10, OLT 900 issues two grant
messages, 1031 and 1032, to the same VONU 921. As a result, VONU
921 keeps transmitting data bursts 1041 and 1042 without any break
during the two timeslots.
[0064] In one embodiment of the present invention, a VONU may have
knowledge of a minimum timeslot size. Therefore, if the time gap
between two assigned timeslots to the same VONU is less that the
minimum timeslot size, the VONU may conclude that it is granted
consecutive timeslots and may transmit data continuously across the
timeslot boundaries.
[0065] FIG. 11 illustrates the merging of two consecutive
transmission timeslots assigned to two VONUs located in a common
physical ONU in accordance with one embodiment of the present
invention. As mentioned above, AGC and CDR bit sequences allow an
OLT's receiver to adjust to a proper power level and to lock into
the bit frequency of an incoming signal. However, sometimes
multiple VONUs may belong to the same physical ONU and share a
common laser. In this case both the power level and bit frequency
remain the same for two consecutive timeslots assigned to VONUs
located within the same physical ONU. Hence, it is possible to
eliminate the data burst overhead between consecutive timeslots
assigned to VONUs in one physical ONU.
[0066] In the example in FIG. 11, OLT 900 sends two consecutive
grant messages 1032 and 1031 to VONUs 1122 and 1121, which belong
to the same physical ONU 1101. Consequently, physical ONU 1101
transmits upstream data burst 1142 from VONU 1122 without turning
off its laser at the end of the first timeslot. ONU 1101 then
transmits upstream data burst 1141 from VONU 1121 without having to
turn on its laser and transmit the AGC and CDR bit sequences at the
beginning of the second timeslot.
[0067] In one embodiment of the present invention, to merge
consecutive transmission timeslots assigned to VONUs located within
the same physical ONU, a grant message may contain a START_ENABLED
flag and a STOP_ENABLED flag. If the START_ENABLED flag is true,
the corresponding VONU will perform a normal start sequence by
turning on the laser and transmitting the AGC and CDR bit
sequences. If the START_ENABLED flag is false, the VONU will start
transmitting payload data immediately upon the start time of the
assigned transmission timeslot.
[0068] Similarly, if the STOP_ENABLED flag is true, the VONU will
turn off its laser such that the laser is completely off by the end
time of the assigned transmission timeslot. If the STOP_ENABLED
flag is false, the VONU will keep transmitting payload data until
the end time of the assigned transmission timeslot without turning
off the laser.
[0069] In the embodiment described above, the OLT ideally has
knowledge of which VONUs belong to the same physical ONU. The OLT
can either obtain this knowledge through a management channel, or
through external configuration.
[0070] In another embodiment of the present invention, VONUs
located within one physical ONU may transmit their upstream data
through a common laser within the physical ONU. When the OLT
assigns consecutive timeslots to these VONUs, the physical ONU will
not turn off its laser between the slots. This approach does not
require the OLT to be aware of which VONUs are within the same
physical ONU, and does not require modification to the grant
message.
[0071] FIG. 12 presents a time-space diagram illustrating the
merging of transmission timeslots assigned to multiple VONUs
located in a common physical ONU in accordance with one embodiment
of the present invention. In this example, physical ONU 1 and
physical ONU 2 each have three VONUs. The VONUs in physical ONU 1
have LLIDs 1, 2, and 3, respectively. The VONUs in physical ONU 2
have LLIDs 4, 5, and 6 respectively. An OLT assigns consecutive
timeslots 1201, 1202, and 1203 to LLIDs 1, 2, and 3, respectively.
The OLT also assigns consecutive timeslots 1204, 1205, and 1206 to
LLIDs 4, 5, and 6, respectively.
[0072] The laser within ONU 1 will keep transmitting data bursts
according to the timeslots assigned to each LLID, without being
turned off between the timeslots. Similarly, the laser in ONU 2
will keep transmitting upstream data within timeslots 1204, 1205,
and 1206 without being turned off between the timeslots. In effect,
transmissions from multiple VONUs in one physical ONU will be
concatenated together and will look like one large timeslot. Thus,
a physical ONU may reduce the overhead in its transmission.
[0073] FIG. 13A illustrates a transmission timeslot the size of
which is based on default laser turn-on and turn-off times.
Typically, as shown in FIG. 13A, an OLT assumes conservative
default values for laser turn-on and turn-off times. In this
example, both laser turn-on time 1301 and laser turn-off time 1302
are 512 ns.
[0074] FIG. 13B illustrates a transmission timeslot the size of
which is reduced based on actual laser turn-on and turn-off times
in accordance with an embodiment of the present invention. In
certain implementations, laser drivers may turn on and off a laser
faster than the default times. Therefore, the data burst overhead
can be reduced if an ONU communicates to the OLT its actual laser
turn-on and turn-off times. The OLT may then place timeslots closer
to each other. As shown in FIG. 13B, the actual laser turn-on time
1311 and turn-off time 1312 are both less than 512 ns. Hence, in
the data burst overhead can be reduced compared with that shown in
FIG. 13A. Note that, even when the actual laser turn-on and
turn-off times are less than the default values, the dead zone
remains the same if overlapping occurs.
[0075] In one embodiment of the present invention, an ONU
communicates to an OLT its actual laser turn-on and turn-off times
in a registration message when the OLT initially registers the
ONU.
[0076] The foregoing descriptions of embodiments of the present
invention have been presented for purposes of illustration and
description only. They are not intended to be exhaustive or to
limit the present invention to the forms disclosed. Accordingly,
many modifications and variations will be apparent to practitioners
skilled in the art. Additionally, the above disclosure is not
intended to limit the present invention. The scope of the present
invention is defined by the appended claims.
* * * * *