APPARATUS AND METHOD FOR PROVIDING PROTECTION IN A PASSIVE
OPTICAL NETWORK
CROSS-REFERENCE TO RELATED APPLICATIONS
The present disclosure is related to U.S. Patent Application 13/033,379, entitled
Low-Energy Optical Network Architecture and filed on 23 Feb 201 1, the entire contents
of which are incorporated by reference herein.
TECHNICAL FIELD
The present invention relates generally to the field of communications networks,
and, more particularly, to apparatus and method for efficiently providing communication
protection and energy conservation for a communications network such as a GPON.
BACKGROUND
The following abbreviations are herewith defined, at least some of which are
referred to within the following description of the state-of-the-art and the present
invention.
APON ATM PON
ATM Asynchronous Transfer Mode
BER Bit Error Rate
BPON Broadband PON
CO Central Office
EPON Ethernet PON
GPON Gigabit PON
IEEE Institute of Electrical and Electronics Engineers
ITU International Telecommunication Union
ODN Optical Distribution Network
OLT Optical Line Terminal
ONT Optical Network Terminal
ONU Optical Network Unit
PIC Photonic Integrated Circuit
PON Passive Optical Network
SOA Semiconductor Optical Amplifier
SFP Small Form Factor Pluggable
VOA Variable Optical Attenuator
Note that the techniques or schemes described herein as existing or possible are presented
as background for the present invention, but no admission is made thereby that these
techniques and schemes were heretofore commercialized or known to others besides the
inventors.
Operators of large communications networks, who are sometimes referred to as
carriers or service providers, maintain widespread networks to handle many kinds of
traffic, for example Internet access or television programming. Telephone sendee may
also be provided. These large networks may be conceptually divided into the core
network and the access network or networks. The core networks carry large amounts of
digitally-encoded information over high-capacity cables or other transmission media.
Access networks are used by individual subscribers or other customers such as
institutions or businesses to reach the core network.
A PON (passive optical network) is one type of access network. PONS use fiber
optic cables to send light-energy signals carrying encoded information from the core
network to the premises of a subscriber or group of subscribers, such as a home,
apartment building or small business. The PON may in some cases reach only to a point
accessible to the customer by other means such as a copper wire or wireless connection,
although FTTH (fiber to the home) is becoming common. Wherever the demarcation
point, however, the subscriber may connect a single device to the PON or, more
commonly, have a network of their own that enables many devices to communicate with
the network via the PON.
PONs use standard multiplexing schemes to permit communications to and from
many different subscribers to be carried over one or a small number of cables, at least
until the point where the communication channel must diverge to reach each individual
subscriber premises. The transmission capacity of the PON is much lower than what is
available in the core network, although it remains adequate to service a great number of
subscribers.
PON standards have undergone a series of evolutions, for example APON,
BPON, and EPON, GPON (gigabit PON), the latter two being currently in widespread
use. Standards being developed include 10GEPON, xPON, and xGPON. Broadly
speaking, the present invention is applicable and useful in all or most of the foreseeable
evolutions of the basic PON concept.
A need exists to provide protection for the communications being handled by the
PON. In the sense used here, "protection" refers to a practice of ensuring that an
alternate communication path is available, where possible, in the event that a primary
communication path is lost or degrades to an unacceptable level of quality. It is highly
desirable, however, that this protection be provided as efficiently and cost-effectively as
possible so that it may be practically and cost-effectively implemented, even in existing
systems. These needs and other needs are addressed by the present invention.
SUMMARY
In one aspect, the present invention is an OLT (optical line terminal) for a PON
(passive optical network) including a plurality of primary ports, at least one protection
port, where each protection port is configuied to provide protection for a selected one of
the primary ports, and a network controller configured for selecting protection of a
primary port by the at least one protection port. The network controller resides, for
example, on an NT (network termination) module, wherein the network controller is
resident on the NT. In a preferred embodiment, OLT includes a plurality of LT (line
termination) cards, and the primary ports are distributed across the plurality of LT cards.
In this embodiment, at least one protection port is configured to protect a selected one of
a sub-set of the plurality of primary ports, wherein the subset of primary ports is resident
on the plurality of LT cards. Preferably, the subset of the plurality of primary ports
includes a single port on each of the plurality of LT cards.
The OLT of the present invention may be further characterized by a protection
port including an optical splitter for splitting a downstream signal for transmission on a
plurality of optical cables, where each optical cable associated with the protection of a
primary port. The OLT may also include an optical amplifier such as an SOA
(semiconductor optical amplifier) to amplify the downstream signal, and in this way
compensate partially or fully for the loss of splitting the downstream signal. The OLT
may further include an optical selector for selecting which optical cables of the plurality
of optical cables to disable. On the receive side, the OLT protection port may include an
optical combiner for combining upstream transmissions received from a plurality of
optical cables, where the optical cables are associated with the protection of a respective
primary port. The optical combiner is preferably a mode coupling receiver. In an
alternate embodiment an optical combiner and an optical amplifier to amplify the
upstream signal may be present, The present invention may be further characterized by a
network controller for selecting which optical cables of the plurality of optical cables to
disable may be present, and the network controller for the transmit side and the receive
side of one or more protection ports may be a single device. The network controller may,
for example, reside on an NT card in the OLT.
In another aspect, the present invention is a method for the protection of primary
ports of an OLT in a PON including detecting the failure of a communication channel
having an ODN optical splitter between a primary port and one or more CPE (customer
premises equipment) devices, disabling the primary port, switching the protection port to
communicate with the one or more CPE devices via the optical splitter, and routing
communications between the OLT and the one or more CPE devices through the
protection port. The method may further include determining whether a protection port
associated with the primary port is available prior to disabling the primary port, and
disabling the primary port only if a protection port is available.
In yet another aspect, the present invention can also be used as a method of
conserving power in an OLT including monitoring traffic flow through the OLT,
determining when the traffic flow has reached a threshold level, and routing traffic via a
protection port instead of a primary port. If the traffic from each of the ports on a
primary LT card is rerouted to a protection card, then the method may further include
powering down the primary LT card or placing it in a mode having reduced power
consumption. The protection ports of the protection card may use time-division sharing
or some other scheme to handle the traffic from a number of primary ports so that more
than one primary LT card may be powered down or placed in a reduced-power state in
this manner.
Additional aspects of the invention will be set forth, in part, in the detailed
description, figures and any claims which follow, and in part will be derived from the
detailed description, or can be learned by practice of the invention. It is to be understood
that both the foregoing general description and the following detailed description are
exemplary and explanatory only and are not restrictive of the invention as disclosed.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete understanding of the present invention may be obtained by
reference to the following detailed description when taken in conjunction with the
accompanying drawings wherein:
FIG. 1 is a simplified schematic diagram illustrating a typical PON in which an
embodiment of the present invention may be implemented;
FIG. 2 is a simplified schematic diagram illustrating a PON according to an
embodiment of the present invention;
FIG. 3 is a simplified schematic diagram illustrating an optical module according
to an embodiment of the present invention;
FIG. 4 is a flow diagram illustrating a method of providing PON protection
according to an embodiment of the present invention; and
FIG. 5 is a flow diagram illustrating a method of power conservation using PON
protection according to an embodiment of the present invention.
DETAILED DESCRIPTION
The present invention is directed at a manner of providing efficient
communication protection for optical communications networks. As mentioned above, a
PON typically provides a connection between a core network and individual subscribers.
FIG. 1 is a simplified schematic diagram illustrating a typical PON 100 in which an
embodiment of the present invention may be implemented. PON 100 extends from OLT
0 to the ONUs 140a through 140m. OLT 1 0 is typically located in the CO (central
office) of a carrier or service provider and is connected to the main or core part of the
carrier's network (not shown). Note that the PON 100 of FIG. 1 is simplified for
convenience; in a typical implementation, there may be a large number of OLTs.
Generally speaking, however, the layout depicted in FIG. 1 is representative across the
network.
OLT 120, like each of the OLTs in a typical deployment, serves a number of
ONUs, handling communication traffic both from the network in a downstream direction
and from the individual ONUs in an upstream direction. Shown in FIG. 1 are ONUs
140a through 140m. In many cases the ONU is located at the subscriber's premises and
is connected to a home gateway or router or similar equipment (not shown) owned or
provided by the subscriber.
OLT 120 itself is also simplified for convenience. In FIG. 1 OLT 120 includes an
LT (line terminal) module 5 and an NT (network terminal) module 110. Each of the
modules may be implemented on a separate card or printed circuit board. The NT
module 110 acts as an interface with the core network for upstream traffic and routes
downstream traffic to the appropriate LT module or modules for transmission to
subscribers. A single LT module 5 is depicted in FIG. 1. The LT module 5
interfaces with the subscriber lines. The communications between the NT module 0
and the LT module 5 are typically electronic signals, so the LT module 5 converts
electrical signals to optical signals in the downstream direction and received optical
signals into electrical signals in the upstream direction.
In the PON 100 of FIG. 1, a separate fiber optic cable is routed to each of the
subscriber ONUs 140a through 140m. These separate fibers do not, however, extend all
the way from the OLT 0. Instead the optical signals for ONUs 140a through 140m are
transmitted to a power splitter/combiner 130. The splitter/combiner 130 divides the
optical signal, which is then sent to each of the ONUs. Typically, only the content
intended for the respective subscriber is passed along by the ONU. Communications
from the ONUs are usually sent according to a schedule determined by the OLT 120, and
to directed splitter/combiner 30 for upstream transmission to LT module 15.
As might be expected, it is advantageous to place the splitter/combiner 30
relatively closer to the subscribers than to the CO to minimize the amount of fiber that is
needed for distribution to the end user. The splitter/combiner 130 may, for example,
reside (along with a number of other such devices) in an "outside plant" such as street
cabinet. It should be noted in this regard that the illustration of FIG. 1 is not to scale and
the splitter/combiner 130 is shown as centrally located only for clarity.
As mentioned above, there will typically be in the OLT a number of LT modules,
and they will often reside each on their own respective card. Each card can therefore be
removed and replaced separately, for example for maintenance or testing purposes. This
is more clearly illustrated in FIG. 2. FIG. 2 is a simplified schematic diagram illustrating
a PON 200 according to an embodiment of the present invention. Here, an OLT 220 is
shown to have five LT cards 211 through 215, each in communication with NT card 210.
In an actual implementation, of course, there could be more LT cards or fewer.
In the embodiment of FIG. 2, network controller 206 resides on NT card 210 and
is in communication with physical memory device 206. Network controller 206 may be
implemented in hardware or in the alternative in hardware executing software program
instructions stored for example on memory device 206. Network controller 206 controls
the function of various components of NT card, for example to effect the correct routing
of data traffic to the LT cards 2 1 through 215. It may also control the operation of
other components of OLT 220 including, for example, the optical switches illustrated in
FIG. 3.
In the embodiment of FIG. 2, each of the LT cards has a number of downstream
ports referred to as a through x in FIG. 2, although not all of the ports are shown. Each
port is associated with an ODN splitter/combiner in a fashion similar to that illustrated in
FIG. 1. For example, port 2 1l a of LT card 2 is in communication with ODN
splitter/combiner 231, port 212a of LT card 212 is in communication with ODN
splitter/combiner 232, and port 213a of LT card 213 is in communication with ODN
splitter/combiner 233, each by a respective fiber optic cable.
Also visible in FIG. 2 are ports 2 14a through 214x of LT card 2 4 and the
connections of ports 214a through 214c to ODN splitter/combiners 234, 235, and 236,
respectively. Port 214x (and any additional ports represented by ellipsis) are connected
in similar fashion. The same is true for the remaining ports of LT cards, 2 11 through
213. In this context, it is noted that in implementation not all ports of each card are
necessarily utilized, and there may be more or fewer cards present in a particular OLT.
In FIG. 2, exemplary ONUs 240 through 243 are also shown, and are served by
ODN splitter/combiner 234. Note that although four ONUs are shown, there could be
fewer in communication with splitter/combiner 234, though in most implementations
there will be more. Although not shown in FIG. 2, the remaining splitter/combiners are
similarly connected as is appropriate to the number of individual subscribers requiring
sendee.
In accordance with the present invention, ODN splitter/combiners 231 through
236 are not 1:m but 2:m (or, in the illustrated embodiment, 2:4). That is, each of the
illustrated ODN splitter/combiners has an additional fiber optic connection (shown as a
broken line) to OLT 220. In this embodiment, LT card 215 has been configured as a
protection card. For this reason, port 215a of LT card 215 has a fiber optic connection to
ODN combiner/splitters 23 1 through 234. Also shown in FIG. 2 are the connections
between port 215b and 215c to combiner/splitters 235 and 236, respectively. The
remaining connections from the ports of LT card 215 are similarly made although
omitted from FIG. 2 for clarity. Note that this arrangement is preferred, but other
arrangements of connections between the protection and primary cards are also possible.
It is noted that in this embodiment, the port 215a of LT card 215 provides
protection for primaiy ports 2 1a, 212a, 213a, and 214a. Similarly, protection port 215b
provides protection for primary port 214b, and for the "b" ports (not shown) of LT cards
2 through 213. The connection between protection port 215b and splitter combiner
235, which is also in communication with primaiy port 214b, is illustrated in FIG. 2.
Also illustrated is the connection between protection port 215c and ODN
splitter/combiner 236, which is also in communication with primaiy port 214c.
In operation, when a failure or unacceptable degradation of quality is detected at
a primary port, communications to and from the OLT 220 may be routed instead from the
corresponding protection port until the failure has been remedied. For example, if a
failure of communications between port 214a and splitter combiner 234 is detected, then
communication between OLT 220 and splitter combiner is shifted to protection port
215a. This process will be described in more detail below.
The apparatus of the present invention may also be employed for conserving
energy usage in the OLT even where an actual failure has not occurred. Note that herein
the LT card used is the same or similar to the LT card used only for failure protection,
and for convenience it will be referred to as a protection card regardless of its current
function.
As should be apparent, the protection scheme of this embodiment involves using
one (or in some cases more) of the LT cards as a protection card. As used herein, this
means that at least one of the ports on the protection card is used to provide a
communication path from the OLT to a plurality of splitters that are also connected to a
primary port h the preferred embodiment of FIG. 2, the protection card (LT card 15)
employs all its available ports for this purpose. Each protection port may of course be
used to protect any other primary port, but preferably each protection port protects
primary ports that are not the same LT card. Of course, under this scheme if not all of the
ports on the other LT cards are being utilized, some ports on the protection card may also
go unused.
It should be noted that in this embodiment of the present invention, each
protection port is configured to handle the communications associated with one primary
port at a time. Protecting multiple ports residing on different LT cards helps to reduce
the likelihood that protection for multiple ports will be required simultaneously. If a
given LT card is being replaced, for example, a single protection card may be sufficient
for protecting the communications the primary LT card's ports would normally handle.
The protection scheme of the present invention is therefore efficient to deploy, and may
often be implemented with only relatively-minor adjustments to existing equipment.
Note, however, that in some embodiments a single protection port may be allocated to
protection of a number of primary ports on a time-division or other basis.
In one embodiment, at least some of the protection fiber optic cables are routed
diversely from the OLT to their respective ODN splitter/combiner so that a local event
damaging one does not also damage the other.
In accordance with the present invention, the protection ports on the protection
card or module are in communication with a plurality of downstream devices such as
ODN splitter/combiners 23 1 through 236 shown in FIG. 2. In order to accomplish this,
each protection port includes an optical module configured according to the present
invention. FIG. 3 is a simplified schematic diagram illustrating an optical module 300
according to an embodiment of the present invention.
In this embodiment, optical module 300 includes a transmitter 310 for generating
an optical signal and including a light source such as LED or laser. In many
implementations, the transmitter 310 is similar or identical to the transmitters used in the
primary ports. Downstream of the transmitter 310 is an optical amplifier 315 for
amplifying the generated optical signal. This is to help ensure that signal from the
protection port is at or near the energy level of the primary port signal that it is intended
to replace, after having been split by splitter 320. This may not be required in all
implementations but is strongly preferred.
In the embodiment of FIG. 3, downstream of optical amplifier 315 is an optical
splitter 320 for disuibuting the signal among the separate protection fiber optic cables.
Note that while four such fibers are shown in FIG. 3, there could be any number within
the limitations of the splitter 320 (and combiner 340). In this embodiment, four pairs
(that is, transmit and receive ) of fibers are shown corresponding to the four LT cards
being protected in FIG. 2. Each pair provides protection for a selected one (or in some
cases more) out of a subset of the ports, for example the subset consisting of primary
ports 2 1la, 212a, 213a, and 214a. Again, however, the number of LT cards may vary, as
may the members of a subset protected by a given protection port.
In the embodiment of FIG. 3, it is not intended that the signal from transmitter
310 will be sent to more than one downstream splitter/combiner at a time, so a number of
optical switches 325a through 325d are provided, one on each fiber extending
downstream from splitter 320. The optical switches may, for example, may be
implemented by VOAs (variable optical attenuators) or MEMS (micro-electro
mechanical systems). Note that in other embodiments (not shown), other schemes may
be used, for example, using a wavelength-multiplex signal, for transmitting to more than
one down stream fiber. In such an embodiment, optical switches 325a through 325d may
still be present.
In the embodiment of FIG. 2, the optical switches 325a through 325d are
controlled by a network controller (for example, network controller 205 of FIG. 2) such
that the distributed optical signal is passed along only a selected one of the downstream
fibers. The network controller may be located on the LT card, the NT card, or at some
other location within the OLT. The network controller is implemented in hardware or
software executing on a hardware device. A table in a physical memory device (for
example memory 206 shown in FIG. 2) in communication with the network controller
may be used to register the state of each optical switch.
In the embodiment of FIG. 3, downstream of the optical switches are WDM
splitter/combiners respectively referred to as 330a through 330d. The purpose of the
WDM splitter/combiners is to permit optical signals in both the upstream and
downstream direction to be transmitted on a single fiber optic cable between the WDM
splitter/combiner and the ODN splitter/combiner (not shown in FIG. 3) that will
distribute the downstream signal to the ONUs {see FIGS. 1 and 2).
In this embodiment, in the upstream direction from WDM splitter/combiners 330a
through 330d are optical switches 335a through 335d, which may be operated by a
network controller (not shown in FIG. 3) to control which of the optical fibers collected
at combiner 340 will be allowed to pass a signal. A table in a physical memory device
(also not shown in FIG. 3) in communication with the network controller may be used to
register the state of each optical switch. When the signal arrives at combiner 340 it is
passed through optical amplifier 345 before reaching optical receiver 350. Here again,
the optical amplifier is not required but may be used to compensate partially or fully for
the loss due to the optical combiner. In another preferred embodiment, a receiver device
such as a mode coupling receiver may be used instead of the optical combiner and
amplifier arrangement of FIG. 3.
In a preferred embodiment, the optical module is implemented in a pluggable
optic module (for example an SFP) that is attached to the LT card, although the optical
selector may also be implemented in hardware on the LT card itself. Where a pluggable
module is used, there is an advantage that existing ports may be converted into protection
ports with relative ease.
FIG. 4 is a flow diagram illustrating a method 400 of providing PON protection
according to an embodiment of the present invention. At START it is presumed that the
components configured to perform the method are present and operable according to the
present invention. The process then begins when an OLT detects degradation (step 405)
in the communications at a primary port. This degradation may be a complete failure or
simply an attenuation of the communications below an acceptable quality (an excessive
BE , for example ). The detection may be performed by the OLT itself or received as a
message from another network entity.
In this embodiment, the non-working port is then disabled (step 410) such that no
further transmissions are sent from it. There may be signals received, but in most
implementations they are simply ignored until the port is re-activated. The protection
port corresponding to the disabled port is then determined (step 415). The optical
selector then selects the appropriate input/output pair (step 420). Referring to FIG. 3,
selecting the pair includes determining which fiber optic cables downstream of splitter
320 and combiner 340 should be used for the protection communications and setting the
optical switches 325a through 325d and 335a through 335d, as appropriate. A status
table in the OLT is updated (step 425) to indicate the status of each optical switch that
has been set.
In the embodiment of FIG. 4, once the optical switches have been set
appropriately, the NT card begins routing communication traffic (step 430) to the
protection port determined to correspond to the disabled port. Naturally, communication
received at the protection port will be handled as if they had been received at the primary
port. In this embodiment, the OLT then generates (step 435) a notification message to
alert the network operator. Traffic will continue to be routed through the protection port
until it is determined that the primary port is operational (step 440), at which time traffic
is directed to the primary port (step 445). Preferably, at this time the optical switches of
the protection port are all disabled (not separately shown) and left in that condition until
the protection port is needed. Naturally, of the protection port is being shared with
another primary port, the optical switches will be or remain set accordingly. In either
case, appropriate updates are made to the status table (step 425).
Note that method 400 is only one embodiment of the present invention and some
variation is possible. Operations may be added, for example, or in some embodiments
omitted. In addition, the operations of the method may be performed in any logically
consistent order. For example, the primary port may be disabled only after the protection
port has been determined and the appropriate downstream fibers selected.
In an alternate embodiment (not shown), a determination is also made that the
appropriate protection port is available. Under some circumstances, it may already be in
use. If it is not available, then a number of options are available. The process could
simply be abandoned, of course, although preferably the availability of the protection port
would be checked periodically. Alternately, the current settings could simply be over
ridden such that a given protection port is dedicated for the primary port that failed most
recently. In another embodiment, a time division sharing arrangement may be possible,
with the protection port handling the communications for respective primary ports at
assigned times.
Here it is also noted that the protection port may be used for other reasons than an
actual failure of the primary port. For example, the "failure" detection may be indicated
by the network operator so that maintenance may be performed or simply to route traffic
more efficiently. In one embodiment (not shown), in periods of light traffic the
protection ports may be used on a time-division basis to handle traffic for a number of
primary ports. In this embodiment the traffic may be monitored and traffic levels
compared to a threshold, so that a determination may be made as to when the protection
ports may advantageously be used in this manner. Once a determination is made,
protection port optical fiber pairs are selected and traffic rerouted as described above.
Ports and cards may be powered-down or placed on standby or sleep mode as
their traffic load is rerouted. This savings could be significant if the protection card is
able to handle traffic that would otherwise be handled by several other LT cards.
Naturally, when the protection ports and protection card are not in use, they may be
powered-down as well.
In another alternate embodiment (not shown), when traffic is low (for example at
night time or when only a relatively small number of ONUs are being served) all ODN
are connected to the protection LT card , which then functions as the active card. The
other LT cards are powered off or placed in a low-power stand-by state. When traffic
increases above a certain threshold, one or more primary LT cards can be powered on.
The switches in the protection SFP are now configured such that the traffic passing via
the corresponding ODNs is terminated in the primary LT card.
FIG. 5 is a flow diagram illustrating a method 500 of providing PON protection
according to an embodiment of the present invention. At START it is presumed that the
components configured to perform the method are present and operable according to the
present invention (see, for example, FIG. 2). The process then begins with monitoring
the traffic flow tlirough an OLT (step 505). A determination is then made as to whether a
traffic threshold has been reached (step 510). If not, the process simply returns to step
505 and monitoring continues. If it has been determined at step 510, however, that a
threshold has been reached then a new routing scheme is formulated (step 515).
In an alternate embodiment (not shown), a routing override message may be
received in the OLT. In other words, the formulation of a new routing scheme may be
performed for reasons unconnected to traffic monitoring. This message may have
originated at an operator-input device or may have come from a scheduler that enforces at
certain times a mandatory re-formulation of the routing scheme. In this alternate
embodiment, the override message may also include a mandatory routing scheme, in
which case the outcome of step is pre-determined.
Where no mandatory routing scheme is being enforced, the network controller
(see, for example, FIG. 2), determines the current state of each of the LT cards, including
the protection card. The traffic flow though each LT card may also be considered. If the
traffic flow is at a high level then in most implementations the primary LT cards (for
example LT cards 2 through 214 shown in FIG. 2) will remain active and the
protection LT card (for example LT card 215 shown in FIG. 2) will be powered down or
placed in a low-power standby state unless it is already being used (for example, if one or
more primary ports have failed). If the traffic flow is at a low level, on the other hand, a
routing scheme may be formulated such that all of the OLT traffic is handled by the
protection LT. In this case, the primary LT cards may be placed in a reduced-power state
(either powered down or placed on standby) but are available for protection in case one
or more the primary ports fail. For an intermediate traffic flow, the reformulation of step
515 may include having the protection LT handle traffic from one or more but not all of
the primary LT cards. this case, if a failure is experienced in one of the active primary
LT cards, the inactive LT cards may have to be returned to full power so that they can
handle their own traffic and the protection LT card is able to provide protection for the
failed port.
In the embodiment of FIG. 3, the network controller then executes the new
routing scheme (step 520). As with failure protection, this includes ensuring that traffic
is routed to the proper primary or protection port. Where a protection port is changing
function, optical switches in the protection port optical modules (see, for example, FIG.
3) may be used to enable or disable downstream fibers appropriately. The execution of
step 520 also includes adjusting the power state of affected LT cards. A schedule may
also have to be established for handling traffic from more than one primary port at a
protection port. A status table is updated (step 525) to reflect the new routing scheme an
the status of the protection port optical switches. The process then returns to step 505
and the traffic flow is monitored for further changes.
If the apparatus of the present invention is used for power saving, the number N
of primary LT cards may advantageously be dimensioned for the ratio of low traffic to
peak traffic (for example, if night time traffic is 25% of the peak traffic, N may be chosen
as 4). The scheme offers the same protection during low traffic hours as during peak
traffic hours, but the roles are reversed; the protection LT card is active and primary cards
1 to N are in a low power stand-by state. Switch-over from the protection LT card to the
primary LT cards can be controlled by monitoring the traffic flow until it reaches a
threshold, or switch-over can be scheduled during the day based on an average evolution
of the traffic (for example, using a day - night cycle).
It is also noted that re-ranging may have to occur whenever traffic is re-routed to
or from a protection port, especially if the protection fiber is routed differently than the
primary one.
Although multiple embodiments of the present invention have been illustrated in
the accompanying Drawings and described in the foregoing Detailed Description, it
should be understood that the present invention is not limited to the disclosed
embodiments, but is capable of numerous rearrangements, modifications and
substitutions without departing from the invention as set forth and defined by the
following claims.

CLAIMS:
1. An OLT (optical line terminal) for a PON (passive optical network),
comprising:
a plurality of primary ports;
at least one protection port, each protection port configured to provide
protection for a selected one of the primary ports; and
a network controller configured for selecting protection of a primary port
by the at least one protection port.
2. The OLT of claim 1, wherein the OLT comprises a plurality of LT (line
termination) cards and the plurality of primary ports are distributed on the plurality of LT
cards.
3. The OLT of claim 2, wherein the at least one at least one protection port is
configured to protect a selected one of a sub-set of the plurality of primary ports, wherein
the subset comprises primary ports respectively resident on the plurality of LT cards.
4. The OLT of claim 3, wherein the subset of the plurality of primary ports
comprises a single port on each of the plurality of LT cards.
5. The OLT of claim 1, wherein the at least one protection port comprises an
optical combiner for combining upstream transmissions received from a plurality of
optical cables, each optical cable associated with the protection of a primary port.
6. The OLT of claim 5, further comprising an optical amplifier to amplify the
upstream signal after combining.
7. A method of conserving power in a PON OLT comprising a plurality of
LT cards, the method comprising:
configuring at least one of the LT cards as a protection LT card, wherein at least
one of the ports of the protection LT card is in communication with a plurality of ODN
splitter combiners over a plurality of optical fibers;
monitoring the flow of traffic through the OLT;
determining whether the traffic flow has fallen below a traffic tlireshold level;
formulating, if it is determined that the traffic flow has fallen below the traffic
tlireshold level, a routing scheme routing at least some of the OLT traffic through the
protection LT card; and
executing the routing scheme.
8. The method of claim 7, further comprising placing at least one nonprotection
LT card in a reduced power state.
9. The method of claim 8, further comprising placing a plurality of nonprotection
LT cards in a reduced power state.
10. The method of claim 7, further comprising determining that the traffic
flow has risen above the threshold level and formulating a routing scheme routing the
OLT traffic through the non-protection LT cards.