WDM PON with non tunable legacy ONUs
The present document relates t o optical transmission systems. In
particular, the present document relates t o a system and method
enabling the coexistence of tunable and non-tunable Optical Network
Units (ONUs) in a passive optical network (PON), e.g. a wavelength
division multiplexing (WDM) PON.
For a WDM PON, tunable lasers are used within the ONUs and the Optical
Line Terminal (OLT) t o adjust the transmitter wavelength t o the
passband of a particular optical filter. Non-tunable legacy lasers are
typically not usable in such a WDM PON, because their wavelength is
uncontrolled and will therefore drift due t o temperature changes.
Consequently, the non-tunable lasers of legacy ONUs installed at
customer premises cannot be used in WDM PONs. In the present
document, methods and systems for enabling the use of non-tunable
lasers, i.e. the use of un-controlled transmitters, within WDM PONs are
described.
According t o an aspect a media access controller for a passive optical
network is described. The passive optical network (PON) comprises a
first optical line terminal (OLT) with a receiver for a first wavelength
range. The first wavelength range may be a first WDM channel of a WDM
PON. Furthermore, the PON may comprise a second optical line terminal
with a receiver for a second wavelength range. The second wavelength
range may be a second WDM channel of a WDM PON. The second
wavelength range may be adjacent t o the first wavelength range. In
particular, the first and second WDM channels may be adjacent, i.e.
neighboring, WDM channels in a WDM PON. In addition, the PON may
comprise an optical network unit (ONU), e.g. a non-tunable ONU, with a
transmitter having a transmitter wavelength which drifts between the
first and the second wavelength range. As such, the transmitter
wavelength may take on any values from the first and/or the second
wavelength ranges.
The media access controller may be configured to assign the optical
network unit to the first and the second optical line terminal, such that
an optical burst transmitted by the optical network unit is received by
the first optical line terminal and the second optical line terminal. In
other words, the controller may be configured to assign the ONU to both
neighboring OLTs at the same time, thereby allowing the ONU to
transmit an optical burst which is received by both neighboring OLTs at
the same time. The assigning of the ONU may comprise reserving or
attributing a corresponding time slot on a respective uplink from the
optical network unit to the first and the second optical line terminal.
This means that the optical burst would be transmitted concurrently
within a time slot of the uplink of the first OLT and within a time slot of
the uplink of the second OLT.
The media access controller may be configured to determine a first
quality of the optical burst received by the first optical line terminal,
and /or determine a second quality of the optical burst received by the
second optical line terminal. The first and/or second quality may be a
bit error rate of the data transmitted by the ONU within the optical
burst. In particular, the first and/or second quality may be determined
based on a bit parity or bit interface parity and/or forward error
correction code and /or received signal strength indicator of the data
comprised within the optical burst received by the first and/or second
optical line terminal.
The media access controller may be configured to select a first portion
of the optical burst received by the first optical line terminal based on
the first quality, and/or to select a second portion of the optical burst
received by the second optical line terminal based on the second
quality. In particular, the controller may be configured to identify a first
portion of the optical burst received by the first optical line terminal
which has a higher quality than the corresponding portion of the optical
burst received by the second optical line terminal. In a similar manner,
the controller may be configured to identify a second portion of the
optical burst received by the second optical line terminal which has a
higher quality than the corresponding portion of the optical burst
received by the first optical line terminal. The first and the second
portions may make up the total of the optical burst. Subsequently, the
controller may be configured to determine a reconstructed optical burst
from the first and second portions.
The media access controller may be configured to determine an estimate
of the transmitter wavelength based on the first and second quality. In
particular, the controller may be configured to determine that the
transmitter wavelength lies within a transition or border range between
the first and the second wavelength ranges. The controller may
determine the estimate by comparing a ratio of the first over the second
quality to a ratio of a center wavelength of the first wavelength range
over a center frequency of the second wavelength range.
Alternatively or in addition, the media access controller may be
configured to determine a temperature of the optical network unit. For
this purpose the ONU may comprise a temperature sensor and the media
access controller may be configured to retrieve temperature data from
the temperature sensor of the ONU. The knowledge about the
temperature of the ONU, possibly combined with information regarding
the first and/or second quality, may enable the media access controller
to determine an estimate of the transmitter wavelength.
Having the above knowledge about the transmitter wavelength, the
media access controller may be configured t o modify a length of the
optical burst, in response to the determined estimate of the transmitter
wavelength. The length of the optical burst may be increased by
inserting a preamble to the optical burst. The modification of the length
of the optical burst may be used to control a temperature increase of
the ONU during the transmission of the optical burst. By controlling the
temperature increase, the transmitter wavelength may be controlled. By
way of example, it may be determined that the estimated transmitter
wavelength lies within the border range. By modifying the length of the
optical burst, the transmitter wavelength may be controlled such that is
drifts outside the border range.
The media access controller may be configured to determine that the
estimate of the transmitter wavelength lies at a predetermined
wavelength distance from the second wavelength range. As a result from
such determining, the controller may terminate the assignment of the
optical network unit to the second optical line terminal. In other words,
the controller may be configured to terminate the double assignment
two the first and the second OLT, if it determines that the transmitter
wavelengths lies sufficiently far way from the second wavelength range.
As such, the controller may be configured to make use of double
assignment only if the transmitter wavelength lies in a transition range
or border range between the first and the second wavelength range.
According to a further aspect a passive optical network (PON) is
described. As discussed above, the PON may comprise a first optical line
terminal with a receiver for a first wavelength range; and/or a second
optical line terminal with a receiver for a second wavelength range,
adjacent to the first wavelength range; and/or an optical network unit
with a transmitter having a transmitter wavelength which drifts between
the first and the second wavelength range. Furthermore, the PON may
comprise a media access controller according to any of the aspects
outlined in the present document.
In particular, the PON may be a WDM PON and the first and second
wavelength ranges may be wavelength division multiplex channels of a
wavelength division multiplex passive optical network. In particular, the
first and second wavelength ranges may have a width of 50GHz. The
passive optical network may be a wavelength set division multiplex
passive optical network.
The passive optical network may comprise a first passband filter for the
first wavelength range; and a second passband filter for the second
wavelength range. The first and second passband filters may be designed
to isolate the first wavelength range from the second wavelength range,
such that an attenuation of the optical burst in either the first
wavelength range or the second wavelength range is lower than a
predetermined value. In other words, the first and second passband
filters may be designed such that the optical burst is attenuated by less
than the predetermined value for any transmitter wavelengths lying
within the first and second wavelength ranges. At the same time, the
first passband filter may provide an isolation from the second
wavelength range, and the second passband filter may provide an
isolation from the first wavelength range. The predetermined value may
be a certain value exceeding the passband attenuation, i.e. the
attenuation of the optical signal within the passband of the first and/or
second passband filters. By way of example, the predetermined value
may be the passband attenuation plus an additional attenuation of 3dB.
According to another aspect, a method for operating a non -tunable
optical network unit in a passive optical network is described. As
outlined above, the passive optical network may comprise a first optical
line terminal with a receiver for a first wavelength range; a second
optical line terminal with a receiver for a second wavelength range,
adjacent t o the first wavelength range; and a optical network unit with a
transmitter having a transmitter wavelength which drifts between the
first and the second wavelength range. The method may comprise
assigning the optical network unit t o the first and the second optical line
terminal, such that an optical burst transmitted by the optical network
unit is received by the first optical line terminal and the second optical
line terminal.
According t o a further aspect, a software program is described. The
software program may be stored on a computer-readable medium (which
may be tangible or otherwise non-transitory) as instructions that are
adapted for execution on a processor and for performing the aspects and
features outlined in the present document when carried out on a
computing device.
According t o another aspect, a storage medium comprising a software
program is described. The storage medium may be memory (e.g. RAM,
ROM, etc.), optical media, magnetic media and the like. The software
program may be adapted for execution on a processor and for
performing the aspects and features outlined in the present document
when carried out on a computing device.
According t o a further aspect, a computer program product is described.
The computer program product may comprise executable instructions for
performing the aspects and features outlined in the present document
when executed on a computing device.
It should be noted that the methods and systems including its preferred
embodiments as outlined in the present patent application may be used
stand-alone or in combination with the other methods and systems
disclosed in this document. Furthermore, all aspects of the methods and
systems outlined in the present patent application may be arbitrarily
combined. In particular, the features of the claims may be combined
with one another in an arbitrary manner.
The claimed subject-matter is explained below in an exemplary manner
with reference t o the accompanying drawings, wherein
Fig. 1 illustrates an example PON network;
Fig. 2 illustrates an example WDM PON network using a cyclic
wavelength division multiplexer;
Fig. 3 illustrates an example WDM grid; and
Fig. 4 illustrates example optical bursts received by a plurality of OLTs.
PON is typically a point-to-multipoint, fiber t o the premises network
architecture in which unpowered passive optical splitters are used t o
enable a single optical fiber t o serve multiple premises, typically 32 up
t o 128. A PON comprises an Optical Line Termination or Terminal (OLT)
at the service provider's central office and a number of Optical Network
Units (ONUs) or Optical Network Terminals (ONT) near end users. A PON
configuration reduces the amount of fiber and central office equipment
required compared with point-to-point (PTP) architectures.
Downstream signals in PON are broadcast t o each premise sharing a
single feeder fiber. Upstream signals are combined using a Media Access
Control (MAC) protocol based on Time Division Multiple Access (TDMA).
The OLTs configure the served ONTs or ONUs in order t o provide time
slot assignments for upstream communication.
Fig. 1 illustrates an example PON network 100 with Optical Network
Units (ONU) or Optical Network Terminals (ONT) 101 providing a User
Network Interface (UNI). The ONUs 101 are connected t o the Optical
Distribution Network (ODN) 102 which may be implemented by an optical
splitter / combiner. Via an Optical Trunk Line (OTL) 103, e.g. an optical
fiber, the ONUs 101 are connected t o the Optical Line Terminal (OLT)
104. As outlined above, the OLT 104 receives time multiplexed optical
bursts on the upstream link from a plurality of ONUs 101 .
Wavelength Division Multiplexing PON (WDM- PON) may be used for
increasing the capacity of PON systems. The multiple wavelengths of a
WDM-PON can be used t o separate individual or groups of Optical
Network Units (ONUs) into several virtual PONs co-existing on the same
physical infrastructure. Typically, one wavelength of the WDM system is
used for the downstream communication from a central office OLT
(optical line terminal) t o one or more ONUs, and another wavelength of
the WDM system is used for the upstream communication from the one
or more ONUs t o the OLT. The downstream and upstream communication
may be performed on the same or on separate fibers.
Fig. 2 illustrates an example WDM PON network 200 comprising a
plurality of ONUs 2 11, 2 12, 2 13, 214, 2 15, 2 16 and a plurality of OLTs.
Fig. 2 illustrates the upstream situation with a plurality of receivers 201 ,
202, 203, 204 of a plurality of corresponding OLTs. The downstream
traffic is handled by a WDM transmitter 205 at the central office (CO).
The WDM PON network further comprises an optical feeder (e.g. an
optical fiber) 222 and a power splitting optical distribution network or a
remote node (RN) 221 (e.g. an optical power splitter).
The example WDM PON network 200 of Fig. 2 comprises a cyclic
wavelength division multiplexer 205, which allows the grouping of the
different WDM wavelengths into sets of wavelength, wherein the
wavelengths within the sets are spaced by a multiple of the underlying
WDM grid (e.g. by a multiple of a 50GHz grid). In the illustrated
example, a 1 t o 4 cyclic wavelength multiplexer 205 is shown, thereby
yielding sets of wavelengths which are spaced by four times the
underlying WDM grid (e.g. by 200 GHz). The use of cyclic wavelength
multiplexers 205 may be beneficial when using heater tunable DFB
(distributed feedback) lasers within the ONUs 2 11, 2 16.
The present document addresses the particular issues of coexistence of
legacy non-tunable ONUs, e.g. an XG PON ONU, with tunable ONUs, e.g.
a heater tunable XG PON ONU, i n a Wavelength Division Multiplex PON.
The methods and systems outlined herein are particularly beneficial i n
the context of wavelength set division multiplex (WSDM) PON networks,
but can be used for WDM PONs in general.
The following description will be based on the WSDM PON shown in Fig. 2,
i.e. based on a WSDM PON using four wavelength sets. It should be
noted, however, that the systems and methods outlined here may be
applicable t o any variant of WDM PON.
The tunable ONUs 2 11,.. .,21 6 within a WSDM PON 200 are typically
assigned t o a particular channel within the WDM system, i.e. t o a
particular wavelength. The particular channel or the particular
wavelength is assigned t o a particular OLT which controls a particular
PON within the WDM PON. In other words, a tunable ONU 2 11, . ..,216 is
assigned t o a particular PON controlled by a particular OLT.
In contrary t o tunable ONUs, legacy non-tuned ONUs tend t o drift across
different channels. In other words, non-tuned ONUs change their laser
frequency or wavelength and therefore transmit in varying WDM
channels. The change of the wavelength may be due t o temperature
changes of the laser during the transmission of an optical burst. A typical
laser changes its laser wavelength with a gradient of about 0.08nm/K,
i.e. the laser wavelength increases by about 0.08nm for every K (Kelvin)
of temperature increase. Consequently, a non-tunable ONU may change
a WDM channel even during the transmission of a single optical burst,
during which the laser heats up. This drifting of the transmission
wavelength of non-tunable ONUs may lead t o the interference of a nontunable
ONU with the one or more tunable ONUs assigned t o a particular
WDM channel.
Fig. 3 illustrates an example WDM channel grid 300. The heater tuned
ONUs 2 11, ... ,21 6 may be tuned t o the center of a passband 301 , 302,
303, 304 of the WDM channel grid 300. As outlined above, legacy non
heater tuned ONUs will typically drift across the four wavelength sets of
the WSDM PON 200. As outlined above, a MAC (media access control) is
used per channel, in order t o control the traffic on the uplink of a
particular WDM channel. In other words, a particular OLT uses a MAC t o
control the assignment of time slots on the uplink t o the ONUs of the
particular PON. By way of example, the MAC may assign the different
time slots of the particular WDM channel t o a plurality of ONUs which
transmit on the uplink t o the OLT. In order t o perform its control task,
the MAC has t o be aware of the ONUs which want t o transmit data on the
particular WDM channel. Due t o the fact, that non-tunable ONUs have
changing laser wavelengths, they typically cannot be assigned t o a
particular WDM channel. Consequently, the MAC of the particular
channel cannot be reliably enabled t o assign time slots t o the nontunable
ONU.
In order t o overcome the above problem, i t is suggested t o make use of a
so called Super-MAC protocol which coordinates the access t o a plurality
of WDM channels of a WDM PON. In particular, instead of only controlling
the upstream traffic t o one particular OLT (associated with one
particular WDM channel), the Super-MAC is configured t o control the
upstream traffic t o a plurality of OLTs (associated with a plurality of
WDM channels, respectively).
In case of the WSDM PON shown i n Fig. 2, a Super-MAC may take care of
the administration of legacy (non-tunable) ONUs t o the 4 wavelength
sets. In a first step, the Super-MAC may assign the ONU-ID of the non
tunable ONU t o the current OLT (or t o the current wavelength set). In
such a situation, the upstream traffic is controlled by the MAC of the
current OLT. However, during the drift process the non tunable ONU may
drift t o the edge of the passband of the current WDM channel and/or
enter the passband of a neighboring WDM channel. In such a transitory
situation, the Super-MAC may assign the non tunable ONU to two OLTs.
Both OLTs, i.e. both PONs, will then reserve a timeslot for the legacy
ONU t o transmit upstream. In other words, the Super-MAC will control
the transition of a non -tunable ONU between two PONs by requesting the
MACs of the two PONs t o both reserve resources on the uplink for the
non-tunable ONU. Furthermore, the Super-MAC may coordinate the
resource reservation process t o ensure that the MACs of both PONs
reserve aligned time slots on which the non -tunable ONU can transmit.
By using such a Super MAC, i t can be ensured that the non-tunable ONU
does not interfere with the transmission of ONUs in adjacent PONs, i.e.
in adjacent WDM channels. As long as the number of legacy non-tunable
ONUs within a WDM PON is relatively low, the usable bandwidth will not
be strongly affected by the double assignment of non-tunable ONUs t o
two adjacent PONs.
As discussed above, the drifting of non-tunable ONUs across different
WDM channels may be addressed by a Super-MAC which performs the
dynamic bandwidth allocation (DBA) of the four wavelength sets of the
WSDM PON 200 shown i n Fig. 2. More generally, the Super-MAC may
perform the DBA t o the different WDM channels (or the different PONs)
of a WDM PON. An uncontrolled ONU that enters the crossover region
between two WDM channels will have t o be assigned t o both wavelength
sets. This crossover region is typically provided by the passband filters of
the two WDM channels. The passbands 301 , 302, 303, 304 of such filters
are illustrated in Fig. 3. As a result of assigning the non-tunable ONU t o
two WDM channels (i.e. t o two wavelength sets of the WSDM PON) both
wavelength sets reserve a time slot for the non-tunable ONU and the
signal will be received in both receivers, when the uncontrolled ONU is
allowed t o send upstream.
It has already been indicated that the OLTs typically make use of
passband filters i n order t o clearly delimit the adjacent WDM channels
from one another. This is illustrated in Fig. 3, where the passband range
305 of filter 302 and the transition range 306 of filter 302 are shown. It
can be seen that within the transition range 306, the attenuation of an
incoming signal continuously increases as the wavelength of the incoming
signal increases. At the same time, i t can be seen that the transition
range 306 of the neighboring passband filter 303 overlaps with the
transition range 306 of filter 302. Consequently, the attenuation of an
incoming signal in the neighboring WDM channel (provided by OLT #3)
continuously decreases as the wavelength of the signal increases.
Typical passband filters 302, 303 are designed, in order t o have a
relatively steep transition range 306. In other words, the filters 302, 303
are designed t o ensure a strong selectivity between neighboring WDM
channels, i.e. filters 302, 303 with little t o no overlap within the
transition range 306 are used. As outlined above, the wavelength of an
upstream signal of a non-tunable ONU may drift into the transition range
306 between the passband filters 302, 303 of two adjacent OLTs (OLT #2
and OLT #3). A strong selectivity of the passband filters 302, 303 will
lead t o a strong attenuation of the upstream signal of the non-tunable
ONU at the transition between OLT #2 and OLT #3. This means that even
though the non-tunable ONU may transmit on both PONs, neither of the
two PONs (i.e. neither of OLT #2 and OLT #3) may be able t o reliably
receive the upstream signal.
In view of the above, i t is suggested t o design the passband filters 301 ,
302, 303, 304 of the WDM PON such that the maximum attenuation
incurred by an upstream signal in the transition range 306 between two
adjacent WDM channels is below a predetermined value in either one of
the two adjacent WDM channels. The predetermined value may be 3dB.
As a consequence, by drifting from one OLT t o the other, the worst
attenuation incurred by an upstream signal is when the wavelength is at
the crossing of the edges. At this wavelength at the crossing of the edges
an additional attenuation of 3 dB will occur.
In other words, i t is proposed t o design WDM filters (or cyclic WDM
filters) with overlapping passbands. By this design, any wavelength can
be received in one of the (four) OLT receivers with a max. additional
attenuation of e.g. 3 dB. As a drawback of this design, the isolation
requirements between the (four) WDM channels may be more
challenging. However, the isolation requirements can typically be met by
adjusting the controlled (i.e. tunable) ONUs t o the center of the
passband of filters 302, 303.
As outlined above, a Super-MAC may be used t o control the assignment
of non-tunable ONUs t o one or more of the OLTs. The Super-MAC may be
configured t o monitor the received signal quality of a signal received at
one or more OLTs from a non -tunable ONU. The received signal quality,
notably the bit error rate (BER) of the received signal, may be estimated
by analyzing the bit interface parity (BIP) and/or forward error
correction (FEC) code and/or the received signal strength indicator
(RSSI) of the received signal (i.e. of the received optical burst). These
indicators may provide the Super-MAC with a fast estimate of the BER of
the received signal.
By analyzing BIP or FEC, the Super-MAC can decide which OLT receives
the better quality of the legacy ONUs upstream message. The Super-MAC
may use the quality information t o determine if the non-tunable ONU
transmits on a wavelength which i s close t o or within a first WDM
channel or which is rather close t o or within a second WDM channel.
Consequently, the Super-MAC can estimate the approximate wavelength
of the transmitting non-tunable ONU.
Furthermore, the Super-MAC can select data from all the OLTs which
receive the upstream data of the non-tunable ONU. If the legacy ONU
sends i n a filter cross point between two adjacent OLTs, the cross point
penalty can be reduced by comparing the two FEC protected data
streams in the OLTs and by selecting the data stream that has no or a
minimum of uncorrectable errors. By doing this, the penalty due t o the
increased attenuation at the cross point between two filters 302, 303
may be reduced t o 2 or 2.5dB (in case of a cross point attenuation of
3dB).
As such, the Super-MAC may be configured t o compare and optimize the
quality of the signals received from the non-tunable ONU in the crossing
region 306 between two adjacent OLTs. The quality of the received
signal (BER) can be judged by fast RSSI, BIP or FEC parity. As the signal is
received on two paths (i.e. i n two WDM channels), the Super-MAC can
reconstruct the received signal by combining error-free portions
received at both OLTs (i.e. OLT #2 and OLT #3). This i s shown in Fig. 4
which illustrates an optical burst 400 received by a first OLT (e.g.
OLT#2) and the corresponding optical burst 410 received by a second
(neighboring) OLT (e.g. OLT#3). The two optical bursts 400, 410
correspond t o a single optical burst which has been transmitted by the
non-tunable ONU. The non-tunable ONU has been assigned t o the first
and the neighboring second OLT.
The optical burst 400 comprises payload data and a plurality of FEC
codewords 401 , 402, 403 corresponding t o respective portions of the
payload data. The burst 410 comprises payload data and a plurality of
FEC codewords 4 11, 412, 4 13 corresponding t o the same respective
portions of the payload data as optical burst 400. However, due t o
transmission errors on the first and second PON (i.e. on the first and
second WDM channel), the received payload data and the FEC codewords
may be different.
The Super-MAC may verify the FEC data 401 , 402, 403 of the first optical
burst 400 and the FEC data 4 11, 412, 4 13 of the second optical burst
410. Based on this verification, the Super-MAC may select error- free
portions of the payload data. By way of example, the Super-MAC may
select the payload data corresponding t o FEC codewords 401 and 403
from the first optical burst 400, and the payload data corresponding t o
FEC codewords 402 from the second optical burst 410. This is illustrated
by the arrows in Fig. 4. By doing this, an error-free or at least an errorreduced
optical burst 420 may be generated which comprises payload
data from both received optical bursts 400, 410 corresponding t o the FEC
codewords 421 , 422, 423. As a result of the combination of the two
received optical bursts 400, 410, a part of the additional penalty caused
by the filter attenuation at the cross point (e.g. the attenuation of 3 dB)
can be compensated.
As outlined above, the Super-MAC may be configured t o determine the
approximate wavelength of the non-tunable ONU. This may be done by
analyzing the quality of the signals received from the non-tunable ONU
within two neighboring WDM channels. As has been discussed, the
upstream signal of a non-tunable ONU typically incurs attenuation, if the
wavelength of the upstream signal is within the transition range 306
between adjacent WDM channels. This attenuation may be reduced by
an appropriate design of the passband filters 302, 303 of the WDM
channels. Nevertheless, it is desirable that this transition range 306 is
not used by the uncontrolled ONU for a long time. In other words, it is
desirable t o provide a method for operating the uncontrolled ONU only
for a very short time within the crossing region 306. In particular, this is
desirable in order t o ensure a reasonable BER of the upstream traffic of
the non-tunable ONU. Furthermore, this is desirable because outside the
transition range 306, the non-tunable ONU may be assigned to only a
single WDM channel, thereby reducing the bandwidth required by the
non-tunable ONU.
In a typical wavelength grid of 50 GHz, the crossing region 306 will have
a width of approx 5-1 0GHz. As indicated above, this range 306 should not
be used by the uncontrolled ONU for a long time. This can be
accomplished by using the self heating property of the uncontrolled
ONUs laser chip during the burst. By way of example, if the wavelength
has t o be kept low, the burst should be short t o keep the lasers
temperature low. If the wavelength should be increased t o pass the
crossing region 306, the burst should be long t o heat up the laser chip.
The gradient of the wavelength / temperature function is about
0.08nm/K. In other words, it is proposed t o control the length of an
optical burst transmitted by a non-tunable ONU, in order t o control the
wavelength of the optical burst transmitted by the non-tunable ONU. By
way of example, the length of an optical burst may be increased by a
special preamble with a high content of " 1" . In yet other words, due t o
self heating of the laser with the non-tunable ONU, there is a certain
wavelength drift over the length of the transmission of an optical burst.
Therefore a „tuning" of the burst length may be used t o pass the
crossing range 306.
In addition, the bias current of the non-tunable ONU may be changed, in
order t o pass the critical part of the transition range 306 (which may
have a width of less than 5GHz) quickly by applying an additional bias
current. The gradient of the frequency / bias current function is about
500MHz/mA.
In the present document, methods and systems for operating nontunable
ONUs within a WDM PON have been described. The proposed
methods and systems allow the reuse of legacy (non-tunable) ONUs in
the context of WDM PON using tunable ONUs. A Super-MAC is described
which allows the assignment of a non-tunable ONU t o one or more WDM
channels. For this purpose, the Super-MAC may be configured t o track
the transmitting wavelength of the non-tunable ONU. Furthermore, the
Super-MAC may be configured t o control (at least partly) the
transmitting wavelength of the non-tunable ONU, e.g. through the
modification of the length of transmitted optical bursts. In addition, it is
outlined how an appropriate filter design may improve the performance
of non-tunable ONUs in a WDM PON.
It should be noted that the description and drawings merely illustrate
the principles of the proposed methods and systems. It will thus be
appreciated that those skilled in the art will be able t o devise various
arrangements that, although not explicitly described or shown herein,
embody the principles of the invention and are included within its spirit
and scope. Furthermore, all examples recited herein are principally
intended expressly t o be only for pedagogical purposes t o aid the reader
in understanding the principles of the proposed methods and systems
and the concepts contributed by the inventors t o furthering the art, and
are t o be construed as being without limitation t o such specifically
recited examples and conditions. Moreover, all statements herein
reciting principles, aspects, and embodiments of the invention, as well
as specific examples thereof, are intended t o encompass equivalents
thereof.
Furthermore, it should be noted that steps of various above-described
methods and components of described systems can be performed by
programmed computers. Herein, some embodiments are also intended t o
cover program storage devices, e.g., digital data storage media, which
are machine or computer readable and encode machine-executable or
computer-executable programs of instructions, wherein said instructions
perform some or all of the steps of said above-described methods. The
program storage devices may be, e.g., digital memories, magnetic
storage media such as a magnetic disks and magnetic tapes, hard drives,
or optically readable digital data storage media. The embodiments are
also intended t o cover computers programmed t o perform said steps of
the above-described methods.
In addition, it should be noted that the functions of the various elements
described in the present patent document may be provided through the
use of dedicated hardware as well as hardware capable of executing
software in association with appropriate software. When provided by a
processor, the functions may be provided by a single dedicated
processor, by a single shared processor, or by a plurality of individual
processors, some of which may be shared. Moreover, explicit use of the
term "processor" or "controller" should not be construed t o refer
exclusively t o hardware capable of executing software, and may
implicitly include, without limitation, digital signal processor (DSP)
hardware, network processor, application specific integrated circuit
(ASIC), field programmable gate array (FPGA), read only memory (ROM)
for storing software, random access memory (RAM), and non volatile
storage. Other hardware, conventional and/or custom, may also be
included.
Finally, it should be noted that any block diagrams herein represent
conceptual views of illustrative circuitry embodying the principles of the
invention. Similarly, it will be appreciated that any flow charts, flow
diagrams, state transition diagrams, pseudo code, and the like represent
various processes which may be substantially represented in computer
readable medium and so executed by a computer or processor, whether
or not such computer or processor is explicitly shown.
Claims
1) A media access controller for a passive optical network (200),
wherein
- the passive optical network (200) comprises a first optical line
terminal (201 ) with a receiver for a first wavelength range; a
second optical line terminal (202) with a receiver for a second
wavelength range, adjacent to the first wavelength range; and
an optical network unit (101 ) with a transmitter having a
transmitter wavelength which drifts between the first and the
second wavelength range; and
- the media access controller is adapted to assign the optical
network unit ( 10 1) to the first (201 ) and the second (202)
optical line terminal, such that an optical burst transmitted by
the optical network unit (101 ) is received by the first optical
line terminal (201 ) and the second optical line terminal (202).
2) The media access controller of claim 1, wherein assigning comprises
- reserving a corresponding time slot on a respective uplink from
the optical network unit (101 ) to the first (201 ) and the second
(202) optical line terminal.
3) The media access controller of any previous claim, further adapted to
- determine a first quality of the optical burst received by the
first optical line terminal (201 ); and
- determine a second quality of the optical burst received by the
second optical line terminal (202).
4) The media access controller of claim 3, wherein the first and/or
second quality is a bit error rate.
5) The media access controller of any of claims 3 to 4, wherein
- the first and /or second quality is determined based on a bit
parity and /or forward error correction code and /or received
signal strength indicator of the optical burst received by the
first (201 ) and/or second (202) optical line terminal.
6) The media access controller of any of claims 3 to 5, further adapted
to
- select a first portion of the optical burst received by the first
optical line terminal (201 ) based on the first quality;
- select a second portion of the optical burst received by the
second optical line terminal (202) based on the second quality;
and
- determine a reconstructed optical burst from the first and
second portions.
7) The media access controller of any of claims 3 to 6, further adapted
to
- determine an estimate of the transmitter wavelength based on
the first and second quality.
8) The media access controller of claim 7, further adapted to
- modify a length of the optical burst, in response to the
determined estimate of the transmitter wavelength.
9) The media access controller of claim 8, wherein the length of the
optical burst is increased by inserting a preamble to the optical
burst.
10)The media access controller of any of claims 7 to 9, further adapted
to
- determine that the estimate of the transmitter wavelength lies
at a predetermined wavelength distance from the second
wavelength range; and
- terminate the assignment of the optical network unit (101 ) to
the second optical line terminal (202).
11)A passive optical network (200) comprising
- a first optical line terminal (201 ) with a receiver for a first
wavelength range;
- a second optical line terminal (202) with a receiver for a
second wavelength range, adjacent to the first wavelength
range;
- an optical network unit (101 ) with a transmitter having a
transmitter wavelength which drifts between the first and the
second wavelength range; and
- a media access controller according to any of claims 1 to 10.
12)The passive optical network (200) of claim 11, further comprising
- a first passband filter (302) for the first wavelength range;
- a second passband filter (303) for the second wavelength
range; wherein the first (302) and second (303) filters are
designed to isolate the first wavelength range from the second
wavelength range, such that an attenuation of the optical
burst in either the first wavelength range or the second
wavelength range is lower than a predetermined value.
13)The passive optical network (200) of claim 12, wherein the
predetermined value is a passband attenuation plus 3dB.
14)The passive optical network (200) of any of claims 11 to 13, wherein
- the first and second wavelength ranges are wavelength division
multiplex channels of a wavelength division multiplex passive
optical network; and /or
- the optical network unit is a non -tunable optical network unit;
and/or
- the passive optical network (200) is a wavelength set division
multiplex passive optical network (200); and /or
- the first and second wavelength ranges have a width of 50GHz.
)A method for operating a non-tunable optical network unit (101 ) in a
passive optical network (200), wherein the passive optical network
(200) comprises a first optical line terminal (201 ) with a receiver for
a first wavelength range; a second optical line terminal (202) with a
receiver for a second wavelength range, adjacent to the first
wavelength range; and the optical network unit (101 ) with a
transmitter having a transmitter wavelength which drifts between
the first and the second wavelength range; and wherein the method
comprises:
- assigning the optical network unit (101 ) to the first (201 ) and
the second (202) optical line terminal, such that an optical
burst transmitted by the optical network unit (101 ) is received
by the first optical line terminal (201 ) and the second optical
line terminal (202).