U.S. patent application number 09/756739 was filed with the patent office on 2002-01-31 for architecture repartitioning to simplify outside-plant component of fiber-based access system.
Invention is credited to Ellis, Donald Russell, Graves, Alan Frank, Morris, Todd Douglas.
Application Number | 20020012138 09/756739 |
Document ID | / |
Family ID | 22002123 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020012138 |
Kind Code |
A1 |
Graves, Alan Frank ; et
al. |
January 31, 2002 |
Architecture repartitioning to simplify outside-plant component of
fiber-based access system
Abstract
An improved access system for use in a Fiber-In-The-Loop (FITL)
communications network is disclosed. The access system comprises a
host digital terminal (HDT) and a plurality of subtending optical
network units (ONUs). The digital signal processing (DSP) functions
traditionally executed by line interface units (LIUs) within the
ONUs are migrated to the HDT, rendering the individual ONUs
simpler, cheaper and more reliable. This is made possible by the
provision in each ONU of an oversampling codec for sampling (and
conversion) of upstream and downstream data at a very high bit
rate. The large bandwidths of the data communicated between the
ONUs and the HDT are easily handled by the fiber optic medium
therebetween.
Inventors: |
Graves, Alan Frank; (Kanata,
CA) ; Morris, Todd Douglas; (Kanata, CA) ;
Ellis, Donald Russell; (Ottawa, CA) |
Correspondence
Address: |
WINSTON HSU
SF. 389, FU-HO ROAD
YUNGHO CITY, TAIPEI
TW
|
Family ID: |
22002123 |
Appl. No.: |
09/756739 |
Filed: |
January 10, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09756739 |
Jan 10, 2001 |
|
|
|
09056096 |
Apr 7, 1998 |
|
|
|
6198558 |
|
|
|
|
Current U.S.
Class: |
398/66 ;
398/139 |
Current CPC
Class: |
H04Q 11/0478 20130101;
H04L 2012/567 20130101; H04Q 11/0062 20130101; Y10S 370/907
20130101; H04Q 11/0071 20130101; H04L 2012/5605 20130101; H04Q
11/0067 20130101 |
Class at
Publication: |
359/118 ;
359/152 |
International
Class: |
H04B 010/20; H04J
014/00; H04B 010/00 |
Claims
We claim:
1. An optical network unit (ONU) for enabling communication between
a plurality of subscriber loops and a host digital terminal (HDT),
comprising: a plurality of substantially identical line interface
units (LIUs) for respectively interfacing to the plurality of
subscriber loops and each having an oversampling codec; an optical
transceiver for connection to the optical fiber; and a
bidirectional multiplexer connected between the optical transceiver
and the plurality of LIUs.
2. An ONU according to claim 1, wherein at least one LIU further
comprises a decimator and an interpolator placed between the codec
and the multiplexer, wherein the decimator decreases the rate of
data flowing to the multiplexer and the interpolator increases the
rate of data flowing to the codec.
3. An ONU according to claim 1, further comprising an ONU control
processor connected between the transceiver and the multiplexer for
interpreting control instructions received from the HDT and for
sending status information to the HDT.
4. An ONU according to claim 2, wherein each LIU further comprises
a ringing generator, loop status detector and analog front end for
generating and interpreting currents and voltages on the associated
subscriber loop.
5. An ONU according to claim 4, wherein the ringing generator and
loop status detector on each LIU are connected to the
multiplexer.
6. An ONU according to claim 4, wherein the ringing generator and
loop status detector on each LIU are connected to the ONU control
processor.
7. An ONU according to claim 4, wherein the analog front end on
each LIU interfaces to a copper twisted pair subscriber loop.
8. An ONU according to claim 4, wherein the analog front end on
each LIU interfaces to a coaxial cable subscriber loop.
Description
[0001] This application is a division of application Ser. No.
09/056,096, filed Apr. 7, 1998 and hereby incorporated by reference
herein.
FIELD OF THE INVENTION
[0002] The present invention is directed to communication network
access architectures and particularly relates to reducing the
complexity of Optical Network Units (ONUs) in a Fiber-In-The-Loop
(FITL) architecture by repartitioning some of the functionality to
other elements of the network.
BACKGROUND OF THE INVENTION
[0003] In order to provide a communications network with the
capability to accommodate current and future high bandwidth
(broadband) services, optical fiber is being extended deeper into
the network, towards the end user. The final link to homes or
businesses in present-day systems is often still part of the
installed distribution infrastructure, comprised mainly of twisted
pairs of copper wire arranged in a topology of distribution cables
and drop lines. For high-bandwidth applications, signal loss along
a twisted pair increases with frequency and so the length of the
twisted pairs must be kept small, leading to deeper penetration of
the fiber.
[0004] In fact, it is known that the loss in decibels is
nonlinearly related to the frequency of measurement (raised to the
power 0.5 to 0.7, depending on the frequency and the type of cable)
and hence a cable with a loss of, for example, 20 dB at 1 MHZ would
have a loss of at least 28 dB at 2 MHZ, and at least 40 dB at 4
MHZ. Moreover, the signal loss in a twisted pair is also
proportional to its length. It has been found that if the twisted
pair is intercepted at a distance close enough to the end user so
that high bit rates (on the order of 25 Megabits per second (Mbps))
can be successfully delivered, then, depending upon the complexity
of the loop transmission equipment, the loop must be shortened so
as to have a length of at most approximately 500 to 3,000 feet.
[0005] This upper bound on loop length has led to the development
of new access architectures, known in the art as
Fiber-To-The-Cabinet (FTTCab), Fiber-To-The-Neighbourhood (FTTN),
Fiber-To-The-Curb (FTTC) or Fiber-To-The-Building (FTTB), all
generically referred to as Fiber-In-The-Loop (FITL). The FTTC
architecture has been the method of choice when considering the
delivery of broadband services to a residential area consisting of
single-family dwellings.
[0006] Traditional FITL implementations provide a system in which a
Host Digital Terminal (HDT) controls the FITL network and is
located at, say, a central office. The HDT is connected on one side
to core network resources and on another side (the "access side")
to a series of dependent Optical Network Units (ONUs) via a
fiber-based link in the form of a Passive Optical Network (PON), a
Synchronous Optical Network (SONET) ring or a number of
point-to-point links. Finally, the ONUs communicate bidirectional
data with the individual end users along the final (short)
stretches of copper.
[0007] At such short maximum loop lengths of only a few hundred
feet, the number of subscribers that can be served by a single ONU
is rather limited. Therefore, the ONU must be small, simple and
inexpensive for the service provider to buy and install so that its
initial cost can be borne by the revenues from the small number of
subscribers that the ONU serves. Furthermore, having only a small
group of subscribers served by any one ONU requires that a very
large number of ONUs be deployed to create a ubiquitous access
network. This demands that the ONUs, once installed, be
individually very cheap to maintain while allowing for future
changes in subscriber service requirements. Since the ONUs are
placed deep in the "outside plant", any requirement which causes
these ONUs to be visited, either for repair purposes or for
provisioning different subscriber services (by changing line card
functionality), will result in a system that is too costly to
operate.
[0008] Conventional prior art FITL architectures, FTTC in
particular, have adopted the approach of installing shelves or
frames of equipment, including service-specific line cards, in a
protective housing on the curbside. Such ONUs are large, complex
and require regular visits, in order both to modify services by
changing line card types and to repair the units, since more
complex ONUs are more likely to fail. Hence, the cost of deploying
an array of service-specific line cards is prohibitively high in
terms of capital cost (complex electronics, large cabinets) and
also in terms of operating costs due to the need to visit the ONU
so as to implement a service type change by replacing the line card
type. Furthermore, installing cabinet-mounted equipment is often
complicated by the unavailability of acceptable locations in
residential areas. This becomes more critical as the loop length is
shortened and ONU size is reduced to the point where ONUs are
installed within subdivisions and not at their edges.
[0009] An alternative prior art approach consists of replacing the
service-specific line cards with (somewhat more expensive)
service-independent line cards that can be configured in software.
These are primarily based upon the use of wideband analog front-end
loop drivers, oversampling codecs, bit-rate-reduction (decimator)
blocks and digital filtering components, also known as Digital
Signal Processor Application-Specific Integrated Circuits (DSP
ASICs). This combination of functions allows the service-specific
functions of the line card to be implemented in software, which can
be downloaded to the ONU from the HDT, thereby eliminating the need
to visit the ONU to change the service type delivered to a
subscriber.
[0010] This solution, also referred to as Service-Adaptive Access
(SAA), has been adopted by Nortel in the development of its S/DMS
Access Node, which can be deployed in a FTTC or FTTCab
configuration. The ONU, also called an RDT (Remote Digital
Terminal), consists of an array of service-dependent line cards, or
alternatively service-independent line cards based upon on-card DSP
processing and each using a DSP dedicated to that card, or possibly
(in order to control cost) a mix of both types of line cards, in
addition to common equipment for multiplexing the digitized
signals, a control processor and an optoelectronic transceiver. The
number of different line card types can be reduced by replacing
some or all of the standard POTS (Plain Old Telephone Service)
cards with SAA line cards.
[0011] When data flows from the subscriber into the ONU, (known as
the "upstream" path), the S/DMS Access Node samples the input
analog signal arriving on the twisted pair and puts it into a
standard digital format prior to transmission from the ONU to the
HDT. In the opposite ("downstream") direction, the ONU converts,
for example, .mu.-law-encoded digital voice data into an analog
format for delivery to a user's home. Unfortunately, the deployment
of such ONUs, each comprising a set of service-independent line
cards, has several serious drawbacks in the context of a FITL
system with deep fiber penetration:
[0012] 1) Cost
[0013] The DSP-based line card has a larger power consumption,
complexity and failure rate, which translates into significantly
higher system cost;
[0014] 2) Size
[0015] The size of the ONUs has increased, making it more difficult
to install them in locations close to the end user;
[0016] 3) Complex software download
[0017] The ONU and access system at the HDT have to provide a
high-integrity software download/verification path which requires a
processor in each ONU for monitoring download integrity;
[0018] 4) Initial servicing
[0019] The functionality of the individual line cards is such that
the ONU must be visited each time a new subscriber is to be
accommodated. The SAA cards do not allow "future-proofing", i.e. it
is not possible to connect every loop to a line card (regardless of
whether or not that loop was expected to go into service
immediately) and then to remotely provision, or "initialize", that
loop;
[0020] 5) Efficiency
[0021] The DSP is placed on the line card and as such is dedicated
to a single loop. Furthermore, it has to be dimensioned for the
most stringent expected processing demands that can be encountered
in the loop. In combination, this leads to the number of
high-performance DSPs deployed being equal to the number of lines
served. Thus for many service types, including low-bandwidth POTS
(the most common), each DSP may be operating at a fraction of its
full capacity. However, this spare capacity cannot be shared across
other loops, leading to an effective increase in power consumption
and total system cost.
[0022] It is important to note that reducing the size of the ONU by
reducing the number of DSP-based SAA line cards per ONU does little
in the way of mitigating the above disadvantages. In fact,
partitioning the equipment into smaller ONUs with lower line counts
per ONU results in an increased overall complexity since the
simplification achieved on a per-ONU basis is more than offset by
the increased number of ONUs that have to be deployed. As the ONU
line count falls, the overall complexity of the ONU population
required to serve a particular area or group of subscribers rises
and has deleterious consequences on the mean-time-between-failures
(MTBF) of the ONU population, requiring a higher degree of
maintenance activity. This translates into more frequent on-site
visits ("truck rolls") by the repair crew and requires more
travelling to the increased number of ONU sites.
SUMMARY OF THE INVENTION
[0023] It is an object of the present invention to obviate or
mitigate one or more disadvantages of the prior art.
[0024] The invention may be summarized according to a broad aspect
as an optical network unit (ONU) for enabling communication between
a plurality of subscriber loops and a host digital terminal (HDT),
comprising a plurality of substantially identical line interface
units (LIUs) for respectively interfacing to the plurality of
subscriber loops and each having an oversampling codec; an optical
transceiver for connection to the optical fiber; and a
bidirectional multiplexer connected between the optical transceiver
and the plurality of LIUs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The present invention will be described with reference to
the following drawings, in which:
[0026] FIG. 1A is a block diagram illustrating a prior art FITL
communications network;
[0027] FIG. 1B is a block diagram showing a FITL communications
network constructed in accordance with the present invention,
including an exemplary inventive HDT and ONU;
[0028] FIG. 2A shows an exemplary data structure on the downstream
fiber link of the prior art network of FIG. 1A;
[0029] FIG. 2B illustrates upstream data flow on the fiber link of
the prior art network of FIG. 1A;
[0030] FIG. 3A shows an exemplary data structure on the downstream
fiber link of the inventive network of FIG. 1B;
[0031] FIG. 3B illustrates upstream data flow on the fiber link of
the inventive network of FIG. 1B; and
[0032] FIGS. 4A, 4B and 4C are detailed block diagrams illustrating
three different embodiments of part of the HDT of FIG. 1B in
accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0033] Before the invention is described in detail the structure
and function of the conventional prior art system of FIG. 1A will
be described.
[0034] With reference to FIG. 1A, a fiber-based access system
intended to provide FTTCab, FTTC or FTTB as part of a
communications network consists of two main types of components, an
HDT 1 and a plurality of ONUs 2 (only one of which is shown). Each
ONU 2 has a plurality of Line Interface Units (LIUs) 3, 27
connected to a bidirectional optical fiber distribution cable 4 via
an intervening mux (multiplexer-demultiplexer) 5, a PON out station
(PON-OS) 28, and an optical transceiver 6.
[0035] A number of different ONUs in the same vicinity are grouped
together by virtue of their associated distribution cables being
joined together at a passive optical splitter 30 which is connected
directly by means of an optical fiber umbilical 4a to a transceiver
16 of the HDT 1. There may be a plurality of groups of ONUS, each
group being connected to the HDT through a respective optical fiber
umbilical and transceiver. Prior art configurations for the fiber
link between the HDT and the multiple ONUs include the PON
configuration shown in FIG. 1A, a point-to-point connection between
the HDT and each ONU, as well as ring configurations with an
optical transport ring passing from the HDT through each of the
ONUs in turn and returning to the HDT.
[0036] The HDT 1 further comprises a digital switch matrix 17
connected to the transceivers 16, in addition to an operations,
administration and maintenance (OAM) processor 18, a control
processor 19 and a signalling processor 20, each of which are also
connected to the digital switch matrix 17. The OAM processor 18
includes a communication port 200 by which it can receive control,
provisioning and configuration instructions from the management
layer of the core network 23 as well as return the access system
operational and maintenance status to the network management
system. Finally, a plurality of transceiver blocks 21 are connected
between the switch matrix 17 and the core network 23.
[0037] Turning now to the structure of the ONU, each LIU 3 is
connected on one side by a bidirectional signal path 23 to the mux
5 and on the other side to a respective subscriber loop 7 which is
commonly a copper twisted pair. The LIU 3 performs the function of
bidirectional communication of signals with the subscriber
equipment in the appropriate analog format (e.g., 4 kHz voice for
POTS, 2B1Q line coded signals for ISDN--Integrated Service Digital
Network) over the intervening twisted pair 7; the insertion of
suitable loop currents by an Analog Front End (AFE) 8; and the
superimposition of a ringing signal when required (and its rapid
removal when the line conditions change to those of an "off-hook"
phone) via a ringing generator 9. The LIU 3 includes a loop status
detector 10 to detect when the phone or other service is activated
(this may include detecting modem tones or changes in d.c. (direct
current) or a.c. (alternating current) conditions on the loop
7.
[0038] The LIU 3 usually includes a wideband digital one-bit
delta-sigma oversampling codec 11 able to provide adequate
bandwidth and quantizing noise performance when converting signals
between the analog and digital domains, a decimator 12D which
removes some of the excess upstream bandwidth from the oversampling
codec 11, and an inverse decimator (or "interpolator") 12ID for
converting downstream words into a high-rate bit stream. The
multi-bit words are fed into (read from) a service-specific
processor 14 implemented as a digital signal processing (DSP)
engine which converts the upstream (downstream) oversampled and
decimated data on the subscriber side 22 of the DSP 14 to (from) a
standard format data stream on the core network side 23 of the DSP
14. For instance, data arriving from the subscriber may be
converted, in stages, from a 4 kHz analog POTS signal on the loop 7
into an analog voice waveform (free of d.c. loop signalling) at the
output 24 of the AFE 8, then into a 1 Mbps one-bit delta-sigma
encoded bit stream at the output 25 of codec 11, subsequently into
32 kHz.times.20 bits/word linearly encoded samples at the output 22
of the decimator, and finally into an 8-bit .mu.-law pulse code
modulation (PCM) signal at the output 23 of DSP 14.
[0039] Typically, a service-specific Service Application Software
(SAS) is downloaded from the HDT 1 under instructions from an OAM
manager via the OAM processor 18 located in the HDT 1, and stored
in a service-specific SAS Random Access Memory (RAM) 15 associated
with the DSP 14. Each LIU 3 interfaces with one physical path to
one subscriber, such that if a subscriber has two twisted pair
drops to the subscriber's premises, then two LIUs, and hence two
DSPs, are required.
[0040] As an alternative to the oversampling codec, decimator,
service-specific processor and SAS downloaded to the SAS RAM 15, a
simple, fixed functional block such as a .mu.-law (or A-law) PCM
codec or an ISDN 2B1Q line driver/receiver and formatting block can
be used. In these cases the LIU 3 would take on a fixed function
and it would be necessary to visit the remote site of the ONU to
physically change the LIU type in order to change the services
delivered. This is both costly and time-consuming because the LIU
is usually located in a small cabinet in an outside-plant location,
and technical staff have to find the location of the ONU and drive
to it before they can physically change the appropriate LIU.
[0041] An ONU 2 is implemented by assembly of an array of LIUs 3 in
a card cage (or its equivalent) along with additional circuit packs
for common equipment such as the mux 5, the PON-OS 28, the optical
transceiver 6 and an ONU control processor 26 which receives and
transmits ONU control commands from and to the HDT 1. The Loop
Status Detector 10 and Loop Status Processor 13 of the LIU 3
communicate loop-specific status and processing commands from the
ONU control processor 26 to the ringing generator 9. Not shown is a
control link from the ONU control processor to the codec 11 for
controlling its output and sampling rates.
[0042] The mux 5 may be implemented using time slots or packets.
For this discussion, time division multiplexed (TDM) time slots
will be assumed. The mux 5 has to accommodate differing final
processed bandwidths on its signal paths 23 from each of the LIUs 3
and hence has to be programmable in bandwidth per port on its
access (subscriber) side. For instance, a POTS circuit would occupy
64 kbps and hence would require one 8-bit word (time slot) every
125 .mu.s (the standard frame period for TDM) for the information
path. On the other hand, an ISDN circuit runs at 144 kbps, thus
requiring three 8-bit time slots every 125 .mu.s.
[0043] In addition, a form of signalling and control path between
the HDT and ONU is required. This can be achieved in one of many
known forms, such as common channel signalling with multiplexed
signalling messages from all line cards flowing in a single
signalling channel, channel associated signalling or even embedded
tone signalling or bit-robbing.
[0044] The fiber optic links 4, 4a support a bidirectional
transmission path over one or two fibers. Either two fibers with
unidirectional operation of each fiber could be used, or
alternatively optical signals could be propagated in both
directions down a single fiber with optical carriers being of a
different wavelength in each direction.
[0045] In the direction from the HDT 1 to the ONU 2, the basic
partitioning of the transmitted bandwidth from the HDT to each ONU
is carried out by known means such as assembling the traffic
information into a subframe of packets, cells or sequences of time
slots. The subframe can also comprise control information as well
as the ONU address. An example of a prior art format at the input
to ONU 2 is shown in FIG. 2A. Each 125 .mu.s frame N sent down the
umbilical 4A comprises a plurality of subframes, each of which is
addressed to a specific ONU. The subframe for ONU #3 consists of an
ONU address synchronisation field, a control field, a common
channel multiplexed signalling field and a traffic field comprising
T eight-bit time slots for the transmission of data.
[0046] The traffic, signalling and control fields, are multiplexed
in one of many well known ways. One method is to allocate several
time slots to the address field, then the first of two timeslots
after the address field to a signalling channel and the other to a
control channel. The signalling channel carries loop status
information and instructions to and from a specific line card
interface in a multiplexed format (e.g. Common Channel Signalling
or Multiplexed Channel-Associated Signalling). The control channel
carries ONU control information including SAS downloads as well as
OAM status information.
[0047] The remainder of the payload time slots are used for
multiplexed traffic data, which is in one or more 64 kb/s, 8-bit
bytes (assuming a conventional 125 .mu.s frame rate). Each service
payload is in its final format as required at the access/core
network interface. In the illustrated example, POTS occupies 1 time
slot, ISDN takes up 3 time slots and DS-1 occupies 25 timeslots,
while the total number of traffic time slots is T=29. The
demarcation boundaries between each subframe can be changed as long
as the sum of the lengths of all packets, cells or sequences of
timeslots does not exceed the frame length.
[0048] In the direction from the ONU 2 to the HDT 1, each ONU
transmits a burst of data, timed so that, when combined by the
splitter 30, the bursts of data from all the ONUs form a train of
incoming bursts at the HDT end as shown in FIG. 2B. The
transmission protocol operates in TDM mode with HDT synchronization
of ONU burst timing to avoid burst collision, which would otherwise
result in one ONU overwriting another ONU's data in the upstream
path. In this way, transmission path delay from each ONU can be
measured. Pairs of upstream bursts on the umbilical are separated
by "guard bands" to allow tolerance on the burst control loop. The
structure of the individual subframes travelling in either
direction is the same, although the inter-subframe assembly methods
are different.
[0049] In the HDT 1, the switch matrix 17 accepts TDM frames from
transceiver 16 and, according to a mapping controlled by the
control processor 19, routes the individual time slots in each
frame towards the appropriate transceiver 21. Similarly, the switch
matrix 17 accepts downstream data from the transceivers 21,
subdivides the data into traffic time slots that constitute a
particular subframe that is routed to the appropriate ONU. This
switch "fabric" also acts as a conduit to connect ONU signalling
and control paths to the signalling, control and OAM processors 20,
19, 18.
[0050] The signalling processor 20 formats the signals from the
ONUs into a standard protocol (e.g., TR-303) to feed the network
interfaces 21 (and vice versa), and formats the signalling messages
to pass on subscriber-generated and access-generated messages to
the core network 23 (and vice versa).
[0051] The control processor 19 controls the overall operation of
the HDT and subtending ONUs, based on system status inputs and
inputs from the OAM processor 18 and signalling processor 20. For
instance, the control processor 19 will manage the cross-connection
map for the HDT switch matrix 17.
[0052] It is noted that a key feature of the prior art system is
the transmittal of fully formatted data across the fiber 4, 4a. The
ONU 2 is responsible for producing an analog version of an
oversampled digital signal based on a received downstream flow of,
say, mu-law-encoded voice data. Similarly, the ONU 2 oversamples
its subscriber input and formats it for upstream use by the HDT 1.
Clearly, the benefit of this technique lies in the bandwidth
savings achieved by transmitting fully formatted data across the
PON. However, the complexity of such ONUs leads to the previously
discussed disadvantages in the areas of cost, size, software
download complexity, initial servicing and efficiency.
[0053] It would instead be more desirable to place complex
processing functions in the HDT 1, by transmitting "raw"
(unformatted) data across the PON. This is particularly feasible in
today's era of fiber optic bandwidth abundance. Accordingly, the
present invention is now described with reference to FIG. 1B, in
which an inventive fiber-based access system intended to provide
FITL (especially FTTC) comprises an HDT 101 and a plurality of ONUs
102 (only one of which is shown). Each ONU 102 consists of an array
of LIUs 103, 127 along with a bidirectional mux 105, an ONU control
processor 126, as well as a PON-OS 128 and an optoelectronic
transceiver 6. As in the prior art, the mux 105 is of the TDM type,
comprising ports that are programmable so as to allot a selectable
number of time slots (and hence, bandwidth) to each LIU in both
directions of communication.
[0054] The mux 105 is connected to an oversampling codec 111 in
each LIU 103 by a downstream line 153 and an upstream line 125. Not
shown is a control link from the ONU control processor to the codec
for controlling its output and sampling rates. The codec 111
preferably comprises complementary one-bit sigma-delta
analog-to-digital and digital-to-analog converters, and is
connected to a wideband AFE, which interfaces directly with an
analog drop line 7 leading to and from a subscriber. Preferably,
the link from the fiber at the curb to the subscriber is formed by
copper twisted pairs, although coaxial cable may be accommodated by
the use of a suitable AFE 8.
[0055] Each LIU further comprises a ringing generator 9 and a loop
status detector 10, which are connected to each other by line 147,
to the AFE 8 by respective lines 145, 146 and to the mux by
respective lines 133, 134. The ringing generator 9 adds a ringing
signal to the line under control from signal 133 received from the
mux 105, and removes it when the loop status detector 10 determines
that the line is in the off-hook position. The loop status detector
10 also provides a digital rendition of the analog line voltage on
signal 134 connected to the mux 105. It is to be understood that
the ringing generator 9 and loop status detector 10 may be
connected directly to the control processor 126 instead of to the
mux 105. Moreover, the mux 105 may itself be connected to the ONU
control processor 126.
[0056] Electrical communication between the mux 105 and the PON-OS
128 can be effected using a bidirectional link 135 or two
unidirectional links. The ONU control processor 126 is connected to
the PON-OS 128 by a bidirectional signal link 123. The transceiver
6 serves to transform the (multiplexed) electronic data into an
optical signal destined for the HDT, and to convert an optical
signal from the HDT into electronic data used by the mux 105. The
optical signals in both directions preferably originate from, and
are combined onto, a single fiber optic cable 4.
[0057] Multiple optical fibers come together at a passive optical
splitter 30, which in the upstream direction adds the optical
energy on each fiber and sends the resultant signal along an
umbilical link 4a to the HDT, and in the downstream direction
splits the downstream optical signal on the fiber umbilical 4a into
a number of identical optical signals travelling along respective
individual fibers 4.
[0058] The HDT interfaces with the umbilicals (4a as well as others
not shown) by means of respective optoelectronic transceivers 16
connected to a digital switch matrix 117. The switch matrix is a
conventional TDM digital switch with traffic data entered into
sequential locations in a large data memory at a given fixed frame
rate, and the same data read out again in a sequence controlled by
a connection memory. The sequencing is controlled via a control
link (not shown) by a control processor 119 in the HDT. The control
processor 119 is preferably also connected to a loop status
processor 113, which performs functions such as decoding a
telephone number dialled by the subscriber based on the sampled
digital line voltage transmitted from the loop status detector 10
in each LIU 103.
[0059] The HDT 101 further comprises a second switch matrix 131,
also a conventional TDM digital switch controlled by the control
processor 119, which is connected to a plurality of transceivers 21
that interface with the core network (not shown). Also connected to
switch matrix 131 are a signalling processor 20 and an OAM
processor 118. As in the prior art, the signalling processor 20
formats outgoing data so that it is in the proper signalling format
(e.g., TR-303) used by the core network, and vice versa. The OAM
processor 118 provides the core network with status information via
a link 200; this link also serves to relay instructions for
configuring the mux 105 in the ONUs 102. The control processor
controls the overall operation of the HDT and subtending ONUs,
based on inputs from the OAM processor 118 and the signalling
processor 20, as well as system status inputs.
[0060] The switch matrices 117, 131 are connected by a
bidirectional "hair pin" connection 132 and also through sets of
DSPs. The connections are shown in greater detail in FIG. 4B. The
first bank of processors consists of a plurality of DSPs 114X, Y, Z
that process respective demultiplexed upstream signals 160X, Y, Z
and produce respective signals 170X, Y, Z that are routed by switch
matrix 131. Decimators 130X, Y, Z respectively intercept the
upstream signals 160X, Y, Z so that the associated DSPs are fed
fixed-length words of data at a certain speed instead of an
oversampled bit stream at a higher rate, as output by the codec in
a given LIU.
[0061] The second set of processors joining the switch matrices
117, 131 is a plurality of DSPs 114A, B, C which process signals
161A, B, C arriving from switch matrix 131, forming signals 163A,
B, C. The DSPs 114A, B, C are connected to respective interpolators
129A, B, C, which create respective high-rate bit streams 164A, B,
C that are routed by switch matrix 117.
[0062] Each DSP 114X, Y, Z and 114A, B, C is preprogrammed by
application and data files stored in respective SAS RAMs 115X, Y, Z
and 115A, B, C to execute a conversion algorithm that converts
digital data from one format to another. The actual number of DSPs,
decimators and interpolators required will depend on total system
requirements.
[0063] The hair pin connection 132 serves to interconnect the two
switch matrices 117, 131, should it be necessary to implement a
complex conversion algorithm involving multiple processing steps
executed by traversing the DSPs several times in sequence.
[0064] From the above, it can be seen that the structure of the
inventive system differs from that of the prior art in that the
ONUs have been simplified by migrating the DSP functionality to the
HDT. As a result, instead of transmitting fully formatted data
across the PON, only "raw" (unformatted) data at high bit rates is
exchanged between the HDT 101 and ONU 102 (and others not shown)
along the fibers 4, 4a. The high data rates required are easily
achievable using commonly available optical fibers.
[0065] It is helpful to first describe the format of data
travelling downstream from the HDT on the fiber 4a with reference
to FIG. 3A, which illustrates how a downstream frame F of 125/M
.mu.s (microseconds) is divided into subframes SF1-SF5 destined for
respective ONUs. The value of 125 .mu.s is the standard length of a
frame in the public switched telephone network (PSTN) and M is the
factor by which this frame length is reduced, usually 1, 8, 12, 16,
24, 25 or 32. As will be shown hereunder, M is used in determining
the so-called bandwidth granularity (BG), which is a measure of the
resolution in bandwidth deliverable across the PON.
[0066] The relative size of a subframe, expressed as the number of
BG units required to provide enough transport capacity for the
corresponding ONU, may differ from one ONU to another. Considering
a particular subframe SF3, it is shown as divided into four fields:
an ONU address and synchronization field, a control field, a
signalling field and a traffic field. There may also be residual
(or spare) bandwidth that is available on the fiber 104 but
unexploited by the ONUs, which is shown for the purpose of
illustration as occupying a subframe SF6, although in reality the
fields of this subframe do not carry useful information.
[0067] At the basic physical transport layer the address, control,
signalling and traffic fields (or "channels"), are preferably time
slots populated with bits and dedicated to transmitting certain
classes of information from the HDT to the ONU. The address field
in each subframe identifies the ONU for which the traffic is
destined. The signalling field preferably carries instructions
(such as ringing generator control) to a specific LIU in a known
multiplexed format. The control field provides OAM status
information and instructions to configure the mux 105, thereby to
allocate a certain bandwidth to each LIU according to the
service-dependent bandwidth needs for that LIU. The control channel
in the downstream subframes also provides control of the codec
sampling and output rates in each LIU, as well as precise timing
instructions for the transmittal of bursts of upstream data.
[0068] The traffic field is divided into a multitude of (in this
case, twenty-nine) time slots T1-T29 of "P" bits each. The BG can
be defined as the bandwidth offered by the transmission of one time
slot per frame, and is dependent on the number of bits per time
slot ("P") and on the above-identified frame size reduction factor
("M"). In mathematical terms, 1 BG = ( # BITS / TIME SLOT ) ( #
SECONDS / FRAME ) = P ( 125 s M ) = 8 .times. P .times. M kbps
.
[0069] The number of time slots occupied by an LIU in a subframe is
dependent on "M", "P" and the required bandwidth by the LIU. It is
useful to set P.times.M=64 (yielding a BG of 512 kbps) when the
oversampled data is required to be sent at data rates that are
multiples of 0.5 Mbps. Nonetheless, the bandwidth granularity is an
arbitrary but fixed design parameter that can be designed to
accommodate a different base multiple of bandwidth used in the
system.
[0070] The traffic time slots are arranged into a known number (in
this case, fifteen) of groups G1-G15, each group providing
downstream data to a respective LIU. The number of time slots
required per group is selectable and will depend on the bandwidth
granularity and on the type of service provided.
[0071] These same time slots are used in the analogous construction
of upstream subframes transmitted by the ONU 102 to the HDT 101.
The mux 105 forms a subframe that is subdivided into groups of time
slots, whereby a group is associated with a specific LIU and is
allotted a number of time slots that is dependent on the BG and on
the required upstream bandwidth. Upon command from the HDT, an ONU
transmits its fully constructed upstream subframe on a
once-per-frame basis, although the subframes arriving from various
ONUs are not contiguous, but instead arrive separated by guard
bands.
[0072] The flow of downstream and upstream data between the core
network and a subscriber, passing through the inventive access
system, is now considered with reference to FIGS. 1B and 4B. It is
particularly useful to contemplate two exemplary scenarios, denoted
A and B. Scenario A deals with the situation in which the core
network sends and receives multiplexed channels of 8-bit mu-law PCM
voice data that are connected through the HDT and ONUs to analog
subscriber loops that send and receive analog POTS signals.
Scenario B treats the situation in which a Frame Relay (or similar
packetized) service carried across an ATM core network is delivered
to and from an end user as a Frame Relay service over a DS-1 (1.544
Mbps) twisted pair link.
[0073] In downstream scenario A, switch matrix 131 routes the
multiplexed channels of 8-bit mu-law encoded voice samples
(arriving in a standard network format) to DSP 114A after
reformatting is done by the signalling processor 20. DSP 114A is
dedicated to producing a stream 163A of, say, 20-bit linearly
encoded samples at 32 kHz from the 8-bit mu-law encoded data. In
the prior art, this exact same function would be performed by a
dedicated DSP within each destination LIU. In contrast, DSP 114A in
the present invention processes multiple channels destined for
corresponding LIUs, and is thus effectively shared by a number of
different LIUs. The data stream 163A passes through interpolator
129A so as to enter switch matrix 117 as a high-rate bit stream
164A, typically on the order of 1 Mbps per channel. This data is in
a generic data format, as it simply requires digital-to-analog
conversion by the codec in the destination LIU.
[0074] Switch matrix 117 also accepts the other high rate data
streams 164B,C produced by the respective DSPs 114B, C, and
arranges the data into groups, subframes and frames according to
destination LIU, ONU and PON in the manner described earlier. The
optical downstream signal exiting the HDT, which may have a data
rate on the order of several hundred Mbps, is converted to
electronic format by the transceiver 6 and subsequently fed to the
PON-OS 128.
[0075] At the PON-OS 128, the address field in each subframe is
checked in order to determine whether the current ONU is the
intended recipient of that subframe. Only the subframes intended
for that particular ONU are output on link 135 to the mux 105. For
each LIU 103, the mux 105 outputs, by a process of demultiplexing,
the proper traffic time slots on the link 153 to the codec 111,
along with control information for the ringing generator 9 on link
133. In addition, the PON-OS 128 provides control information to
the ONU control processor 126 via link 123; alternatively, this
information may be delivered from the mux 105.
[0076] Within each LIU, the codec 111 then converts the high-rate
bit stream on its network-side link 153 into an analog POTS
waveform, and the AFE 8 adds appropriate ringing voltages and loop
currents. As discussed earlier, the AFE is also responsible for
removing the ringing voltage when an off-hook condition is
detected, and may interface to a variety of loop termination media,
such as copper twisted pair or coaxial cable.
[0077] Considering now the upstream path in scenario A, the AFE 8
will prepare the analog POTS signal for sampling by the
oversampling codec 111 at around 1 MHz. The oversampled data 152 is
fed to the mux 105, where a suitable number of time slots in a
subframe are allotted to this stream. Also, the mux 105 will
partially fill the control and signalling fields with the status of
the analog line received from the loop status detector 10 via path
134. The address field will indicate the source ONU.
[0078] The mux 105 then assembles the time slots from each LIU, as
well as all of the information in the remaining fields, forming a
subframe, and sends it to the PON-OS 128. The PON-OS waits for the
go-ahead from the ONU control processor 126 before sending the
subframe onto the fiber 4 via the transceiver 6. The ONU control
processor 126 receives this timing information from the HDT in the
control field of the downstream subframes. Each ONU sharing the
same fiber umbilical 4a is cyclically instructed to send its burst
of data, resulting in a "train" 400 of subframes SF1-3 as shown in
FIG. 3B. Any consecutive pair of bursts is separated by a short
time span 402 of variable length during which no transmission
occurs, called a guard band. This is designed to account for the
delay in instructing one ONU to transmit while ensuring that the
previous ONU has ceased transmission.
[0079] The train 400 of data containing the oversampled POTS signal
of upstream scenario A arrives at switch matrix 117 of the HDT 101
through transceiver 16. The corresponding traffic time slots are
extracted and routed via decimator 130X to a DSP 114X which
converts the oversampled decimated data arriving from the
subscriber to 8-bit mu-law data. DSP 114X will likely be assigned
the task of converting multiple upstream data channels from
oversampled decimated format into mu-law format. The output 170X of
DSP 114X subsequently passes through switch matrix 131, where it is
routed towards its possibly multiple destinations elsewhere in the
network via transceivers 21. The signalling processor 20
appropriately formats the outgoing signals prior to optoelectronic
conversion by transceivers 21.
[0080] In downstream scenario B, ATM cells arriving from the core
network and carrying the Frame Relay service are routed by switch
matrix 131 to a first DSP 114B. DSP 114B is dedicated to the
process of reassembling segments of Frame Relay packets contained
in the ATM cell stream into pure Frame Relay packets. This
reassembly portion of a so-called segmentation and reassembly (SAR)
process is achieved by removing the ATM envelope around the Frame
Relay packet segments in the payload of each ATM cell and
reassembling those segments into Frame Relay packets.
[0081] However, the output 166 of DSP 114B is still not in a
suitable format for delivery to the customer (who is expecting to
receive line coded 1.544 Mbps DS-1 data). Therefore, the output
163B of DSP 114B is rerouted to the input of another DSP processor
114C by switch matrix 117, hair pin connection 132 and switch
matrix 131. DSP 114C is empowered with the insertion of Frame Relay
packets into the payload of a 1.544 Mbps DS-1. DSP 114C also
formats the digital signal with the required line code, yielding
data stream 163C.
[0082] Data stream 163C is subsequently passed through an
interpolator 129C to yield a very high rate oversampled bit stream
164C, having a data rate on the order of 20 Mbps and requiring, for
example, 40 time slots at a bandwidth granularity of 512 kbps per
slot. The bit stream 164C is multiplexed by switch matrix 117 and
delivered to the appropriate codec 111 of the destination ONU in
the manner described above. At the codec 111, the oversampled line
coded DS-1 data is converted into an analog waveform, although the
data per se is still in digital format, being encoded in the
various voltage level durations and changes characteristic to the
line code in use.
[0083] It is to be noted that bit stream 164C in this downstream
scenario B is in the same universal oversampled format as bit
stream 164A previously considered in downstream scenario A
(although its rate is higher). In fact, the commonness of the data
format communicated between the HDT and the ONUs (and vice versa)
is an important property of the present invention. The rates, on
the other hand, will depend on the service being offered, and the
output or sampling rate of the codecs can be controlled via the
downstream control channel, as previously discussed.
[0084] It is also noteworthy that interpolation is not applied at
the output 163B of DSP 114B since this data requires further
processing by DSP 114C. This does not imply that an interpolator
should be absent at the output of DSP 114B, but rather that all
interpolators 129A, B, C be preferably equipped with "bypass mode"
functionality (i.e., OUTPUT=INPUT), so that data which is hair
pinned several times is interpolated only after having gone through
the final DSP prior to delivery to the subscriber.
[0085] In upstream scenario B, the digital DS-1 signal sent by the
subscriber along the loop 7 undergoes frequency selective loss,
accumulates noise and suffers from other impairments as it is
propagated along the twisted pair drop. By the time the
subscriber-emitted signal reaches the AFE 8, regeneration is
required to recover the original digital data from the distorted
analog waveform. In the prior art, this regeneration is performed
in the LIU proper. In contrast, the codec 111 in the inventive
system simply oversamples the data at around 20 MHz as if it were a
wideband analog input signal. In other words, the codec 111
"blindly" oversamples the signal and performs no data recovery,
leaving the data in the common, high-bandwidth digital format.
[0086] The mux 105 inserts the oversampled bit stream into the time
slots preassigned to that LIU, subsequently creating a subframe
which is sent to the HDT via the PON-OS 128 and transceiver 6 using
the upstream burst transmission procedure described above. Clearly,
the inventive system trades bandwidth efficiency for simplicity of
operation and economy of construction.
[0087] At the HDT, oversampled DS-1 data arrives at a transceiver
16, and is subsequently routed to a first DSP 114Y which is
programmed to recover the 1.544 Mbps bit stream from the
oversampled version of the distorted line coded signal. This known
regeneration process is achieved by a combination of frequency
equalization, noise filtering and the application of a clocked
decision threshold. The output 170Y of DSP 114Y is then routed to
the input of a second DSP 114Z via switch matrix 131, hairpin
connection 132 and switch matrix 117.
[0088] The second DSP 114Z removes the DS-1 header and plainly
outputs the payload in the form of Frame Relay packets which had
been contained in the original DS-1 stream. The output 170Z of DSP
114Z is once again "hair pinned" back to a third DSP (not shown)
which segments the Frame Relay packets into ATM cells by applying
the segmentation portion of the SAR process described above.
Finally, the ATM data is ready to be sent to its destination
through switch matrix 131 and a transceiver 21. Analogous to
interpolation in the downstream case, decimation performed in the
HDT occurs only once, i.e., at the input to the first DSP in line
for processing subscriber-generated data.
[0089] Typical oversampling and decimating rates for several common
service types are illustrated in the following table:
1 Oversampled Service Bit Rate Oversampled and Decimated Bit Rate
POTS 1-2 Mbps 32 kHz .times. 20 bits/word = 640 kbps Foreign 1-2
Mbps 32 kHz .times. 20 bits/word = 640 kbps Exchange ISDN 2-10 Mbps
160 kHz .times. 10 bits/word = 1.6 Mbps DS-1 20-40 Mbps 1.5 MHZ
.times. 10 bits/word = 15 Mbps
[0090] Incidentally, it is also interesting to consider the
requirements of the switch matrices 117, 131 in view of the above
rates. It is noted that the throughput of a prior art switch matrix
17 would determined by the aggregate fully formatted data capacity
to and from all of the PONs connected to that switch matrix,
whereas inventive switch matrix 117 is sized to carry the aggregate
of all the oversampled data to and from the ONUs in addition to all
of the data that is "hair pinned", resulting in the requirement for
a much larger data memory when using a standard 125-.mu.s frame
length. However, if the frame length is shortened to match the
larger channel bandwidths of the oversampled signals, the memory
requirement is reduced since less data arrives per frame. The value
of M discussed above can thus be chosen to alleviate the
requirements on switch matrix 117 by setting a convenient operating
frame rate.
[0091] The digital switch matrix 131 has somewhat lesser
requirements in that it handles data exiting the DSPs in a
finalized format while also handling higher-bandwidth data
"hair-pinned" back to the access side switch matrix 117. However,
no data need travel through switch matrix 131 in non-decimated
form. Switch matrix 131 would thus be chosen as having a frame rate
of standard length, i.e., 125 .mu.s. Alternatively, several
switches may be concatenated in the case where a high amount of
"hair-pinning" is expected, one switch operating, for example, on a
short frame with another one operating on a 125-.mu.s frame.
[0092] It is important to note that relocation of digital signal
processing tasks from the ONU to the HDT results in a cheaper,
simpler, more efficient and more reliable ONU for deployment deep
into the network. On the HDT side, considerable gains in DSP
efficiency are also realized. For example, although individual
processors are dedicated to a particular task, say conversion of
mu-law PCM to linearly encoded samples, a single DSP can be used to
perform the task at hand on a number of different data streams.
These streams may be destined for completely different ports on the
network, such as LIUs on different ONUs in different PONs. Whereas
the number of processors required in the prior art was equal to the
number of LIUs, the inventive system permits the use of a pool of
DSP resources that can be shared across many LIUs. Since not all
tasks require the same amount of processing, the HDT need concern
itself with total DSP processing power, but not with a particular
number of DSPs. Moreover, the DSPs themselves may offer varying
degrees of processing ability, and need not be sized to accommodate
the worst-case scenario of data conversion, as was formerly the
case.
[0093] As an illustration of the DSP savings that can be achieved
by the present invention, it is worthwhile to consider, for
instance, a bank of 16 DSPs each capable of handling either 24
simultaneous mu-law-to-POTS conversions, 6 ISDN-to-POTS conversions
or 1 DS-1-to-POTS conversion. If there exists a downstream service
requirement for 192 POTS lines, 24 ISDN lines and 2 DS-1 lines,
then the following setup of DSPs would be able to accommodate the
service mix:
8 DSPs.times.24 POTS lines/DSP->192 POTS LIUs serviced
4 DSPs.times.6 ISDN lines/DSP->24 ISDN LIUs serviced
2 DSP.times.1 DS-1 lines/DSP->2 DS-1 LIUs serviced
[0094] Clearly, a total of 218 LIUs can be accommodated by a mere
16 DSPs sized to handle DS-1-to-POTS conversion. This is minute
compared to the 218 DSPs of at least the same power (i.e., not
counting combinations of services) that would be required in a
prior art approach based on service-independent line cards.
[0095] Notwithstanding the benefits of the inventive system given
the artificial service mix assumed above, the following more
detailed analysis of realistic loading conditions will reveal that
in a typical service mix, the usage of a shared set of DSP blocks
indeed allows each DSP to be more optimally loaded. For instance,
if a DSP is capable of processing "m" lines of service type A, "n"
lines of service type B and "p" lines of service type C, then, on a
system with a total need to service "w" LIUs, the total DSP count
for full service across the entire system is w/m+w/n+w/p. In other
words, with DSPs in the HDT that are dedicated to a particular type
of processing, one must stock up enough DSPs to cover any and all
of the three worst cases. Clearly, DSP savings are achieved
when
(w/m+w/n+w/p)<w, or
(1/m+1/n+1/p)<1.
[0096] Depending on the processing power of the DSPs in the HDT,
this may require fewer resources than the prior art.
[0097] However, the advantages of centralizing the DSP resources
become indisputable in the event that more than 3 lines of service
on average (i.e., across all service types) can be processed in a
DSP. Then m, n and p are all greater than 3 and the above
inequality is satisfied, resulting in DSP savings due to
"centralization" of DSP resources. Typical numbers for modern DSPs
processing POTS, ISDN and DS-1 are even more encouraging, and are
on the order of 24 POTS/DSP, 6 ISDN/DSP, 2.5 DS-1/DSP, yielding
(1/m+1/n+1/p)=0.6083.
[0098] The analysis may be extended one step further by applying
known practical traffic mix requirement limits into the process of
dimensioning the DSPs. For instance, if only a certain maximum
percentage (e.g., 10%) of lines will ever need DS-1 service and
another maximum percentage of lines (e.g., 25%) will ever need ISDN
service at one time (without knowing which lines are occupied by
what service), then the above inequality becomes
{fraction (1/24)}*100% (POTS could be used 100% of the time)
+1/6*25% (ISDN is used at most 25% of the time)
+1/2.5*10% (DS-1 is used at most 10% of the time)
0.1233<1
[0099] for almost an order of magnitude savings (8.11:1) in the
number of DSPs required.
[0100] On top of the added capacity, a further advantage of the
present invention is that the DSPs are found in a centralized
environment, which reduces the cost of provisioning and
dimensioning the DSPs to meet future traffic demands. Moreover, the
DSPs are flexible and their respective RAMs are reprogrammable by
the control processor 119, either through a control bus 183 as
illustrated in FIG. 4B or through one of the switch matrices 117,
131, thereby providing the ability to track the evolving demands of
the network.
[0101] The control processor 119 in the HDT can also play a vital
role in reducing the bandwidth taken up by the various LIUs,
particularly in the case of ISDN and DS-1 services. For instance,
an on-hook (unused) POTS line takes up very little bandwidth, as
does an unused DS-1 video conference line (i.e., the far end modem
at the customer premises is in a quiescent mode), since the only
requirement on that DS-1 loop is to detect the start up of the DS-1
Customer Premises Equipment. The control processor 119 can thus
lower the sampling and output rates of the oversampling codecs and
decimators on LIUs which are in an on-hook or quiescent condition
to values much below that which the LIUs would require for an
active delivery of POTS or DS-1 services.
[0102] Hence, assuming a service mix of 80% POTS at 640 kbps, 10%
ISDN at 1.6 Mbps, and 10% DS-1 at 15 Mbps (all data rates are
oversampled and decimated), and further assuming an average
off-hook (in use) duty cycle of 25% along with 80% bandwidth
reduction during on-hook (out of use) periods for both POTS and
DS-1, then the average bandwidth per loop would be on the order
of:
[(640 kbps*25%)+(0.2*640 kbps*75%)]*80%
+[(1.6 Mbps*100%)]*10%
+[(15 Mbps*25%)+(0.2*15 Mbps*75%)]*10%
=964.8 kbps per loop
[0103] This would allow up to 621 subscribers to be accessed with a
single 600-Mbps PON, corresponding to the installation of up to
sixteen 38-line ONUs or eight 77-line ONUs. A single fiber
umbilical can thus serve a distribution area with over 600
customers, which is the norm for current North American
telecommunications company serving areas.
[0104] The preceding example has assumed that decimated data are
transmitted across the PON. This is achieved by an alternate
embodiment of the present invention, in which the decimation and
inverse functions are kept in the LIUs. Thus, considering the
upstream path, a decimator would be placed between the codec 111
and the mux 105 instead of in the HDT. Optionally, decimators could
be placed in both locations, whereby each upstream signal path
would comprise one fully functional decimator and another operating
in bypass mode. Clearly, analogous arrangements apply to the
interpolators in the downstream path.
[0105] In another variant of the present invention, the
functionality of the loop status processor 113 would be placed in
each LIU 103, 127. Specifically, the loop status detector 10 may
feed its signal 134 directly to the ONU control processor 126 or to
an intermediate loop status processing block. The ONU control
processor would perform the control functions of determining the
condition of the line or decoding the dialled digits, relaying this
information to the HDT via the upstream control channel. Similarly,
the ringing generator 9 may be controlled from the ONU control
processor 126, thus further liberating the mux 105, which is left
with the task of simply routing the data to and from the LIUs.
[0106] It is also to be understood that many alternate embodiments
of the present invention exist in which the processing chain in the
HDT is configured differently than in FIG. 4B. Such is the case in
FIG. 4A, wherein a single high-capacity switch matrix 195 replaces
the switch matrices 117, 131 of FIG. 4B. In this case, hair pinning
does not require a link external to the switch matrix. Instead,
data both from the ONUs and from the core network are continuously
routed to the DSP bank and back through the switch matrix 195 until
the required number of processing operations have been
performed.
[0107] There may also be a 125-.mu.s framed switch matrix 193
present at the core network side connected to the signalling
processor which provides grooming of the frames leaving or entering
the HDT at a 125 .mu.s frame rate. In all other respects, the HDT
is identical to that of FIG. 4B.
[0108] Yet another example of an inventive HDT partitions the
short-frame switch matrices of FIG. 4B into two, resulting in four
STS switches 117U, 117D, 131U, 131D as shown in FIG. 4C. In this
case, two hair pin connections 132U, 132D are required, one for
each direction travelled by the data. The signalling processor 20
now provides independent grooming of the frames in both the
downstream and upstream paths. However, there is no fundamental
difference in operation of the embodiment illustrated in FIG. 4C
with respect to what has already been described with reference to
FIG. 4B.
[0109] Numerous other modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims, the invention may be practised otherwise than as
specifically described herein.
* * * * *