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CROSS-REFERENCE TO RELATED APPLICATION
This application is related to the applications of:
Thomas L. Hiller, James J. Phelan, and Meyer J. Zola entitled "Establishing
Telecommunications Call Paths In Broadband Communication Networks", Ser.
No. 07/972,789, filed Nov. 6, 1992;
Thomas L. Hiller, James J. Phelan, and Meyer J. Zola entitled "Establishing
Telecommunications Call Paths Between Clustered Switching Entities", Ser.
No. 07/972,787, filed Nov. 6, 1992;
Thomas L. Hiller, James J. Phelan, and Meyer J. Zola entitled "Apparatus
For Interfacing Between Telecommunications Call Signals And Broadband
Signals", Ser. No. 07/972,786, filed Nov. 6, 1992;
Thomas L. Hiller, James J. Phelan, and Meyer J. Zola entitled "Establishing
Telecommunications Calls In A Broadband Network", Ser. No. 07/972,788,
filed Nov. 6, 1992; and
Thomas L. Hiller, Ronald A. Spanke, John J. Stanaway, Jr., Alex L.
Wierzbicki, and Meyer J. Zola entitled "ATM Networks For Narrow Band
Communications", Ser. No. 08/170,549, filed Dec. 20, 1993,
Thomas L. Hiller, Ronald A. Spanke, John J. Stanaway, Jr., Alex L.
Wierzbicki, and Meyer J. Zola entitled "ATM Distribution Networks For
Narrow Band Communications", filed on an even date herewith, Ser. No.
08/169,913, filed Dec. 20, 1993,
Thomas L. Hiller, Ronald A. Spanke, John J. Stanaway, Jr., Alex L.
Wierzbicki, and Meyer J. Zola entitled "Access Switches For Large ATM
Networks", filed on an even date herewith, Ser. No. 08/169,909, filed Dec.
20, 1993,
Thomas L. Hiller, Ronald A. Spanke, John J. Stanaway, Jr., Alex L.
Wierzbicki, and Meyer J. Zola entitled "Inter-Cell Switching Unit For
Narrow Band ATM Networks", filed on an even date herewith and, Ser. No.
08/170,550, filed Dec. 20, 1993,
which applications are assigned to the assignee of the present application.
TECHNICAL FIELD
This invention relates to arrangements for establishing digital
telecommunications connections, and more specifically, for establishing
such connections using broadband networks and switching systems.
Problem
The provision of large quantities of toll telephone service continues to be
expensive. While there have been major breakthroughs in the cost of
provisioning of high capacity transmission systems using fiber optics, the
access to such fiber optic systems and the switching of signals on such
fiber optic systems continues to be costly. Further, the cost of switching
the signals from one channel to another in a tandem-toll switch remains
high.
Further, there has been an increasing need for a very large
telecommunications switching system or its equivalent. In the past, this
need has been partially met by the use of smaller switching systems
interconnected by moderate capacity tandem switching systems. In the case
of a switching system such as AT&T's 5ESS.RTM. switch, a fairly large
system has been devised using switching modules of substantial capacity
interconnected by a time multiplexed switch. None of the available
solutions, however, have resulted in an economically satisfactory solution
to the need for a very large switching system or cluster of systems for
simultaneously handling substantial quantities of telephone traffic, low
speed telecommunications data traffic, and high speed telecommunications
data traffic.
A new standard has been implemented for transmitting combinations of
broadband and narrower band, packet and circuit signals over broadband
facilities. This standard, the Asynchronous Transfer Mode (ATM) standard,
packs data into frames each frame comprising a plurality of cells, each
cell being 53 bytes long, the 53 bytes consisting of a 5 byte header and a
48 byte payload. When a 125 microsecond ATM frame is transmitted, each of
the cells may be headed for a separate destination, the destination being
identified in the cell header. ATM standards under consideration specify
how to pack 48 voice samples from one speech channel into a cell and to
transmit this cell across an ATM network. However, this incurs an
undesirable 48 sample (6 millisecond) delay for filling the cell with the
samples, and requires a large amount of storage for the samples prior to
their transmission and after their reception. No sound proposal has been
made public for the economic use of ATM for transmitting voice signals
from an ingress node to a network to a multiplicity of egress nodes of the
network without incurring this delay. While the ATM standard is finding
increasing use in broadband networks, especially those using fiber optic
transmission facilities, no economic solution has been made public to the
problem of designing a communications network for transmitting a large
multiplicity of narrowband voice signals from any of a multiplicity of
ingress nodes of the network to any of a multiplicity of egress nodes of
the network using the ATM standard without incurring this delay. Further,
no sound economic proposal has been made public for the economic use of
ATM for achieving a very high capacity low delay large switching system or
a large highly interconnected cluster of smaller switching systems.
The above problems are partly solved in accordance with the teachings of a
prior patent application, Ser. No. 07/972,789, and its related
applications, Ser. Nos. 07/972,787, 07/972,786, and 07/972,788. According
to the teachings of those applications, signals from a plurality of pulse
code modulated (PCM) channels on a plurality of PCM lines, each channel
for one telecommunications call, are switched by ATM interface units
(ATMUs) onto ATM signal outputs such that each of the calls destined for a
common switching module or independent switching system are packed into a
single composite ATM or ATM-like cell; the ATM signals containing such
composite cells are transmitted to and from an ATM Communication module
(ATM-CM) (called a common broadband platform in the cited application)
which is used for switching ATM cells; such composite cells are
transmitted at a repetition rate that is the same or a sub-multiple of the
repetition rate of the PCM signals that represent the voice signals; the
cells are transmitted over constant bit rate (CBR) permanent virtual
circuits (PVC) from an ingress switching module or system to the ATM-CM of
that node or access switch, thence, to an egress switching module or
switching system or to the ATM-CM of another access switch. This solution
avoids the 6 millisecond delay because only one sample from a given call
is placed into a composite cell. Permanent virtual paths are provisioned
as the traffic between a particular ingress and egress module changes, but
such paths need to be activated or deactivated only when an additional
group (the group size being determined by the number of voice channels
that are transmitted in each cell) is needed or can be released.
A problem with this partial solution is that there is insufficient traffic
to justify permanent virtual paths among the large plurality of ATMUs in a
network having many switching modules. Further, the partial solution has
the disadvantage that the addition of a single switching module to any
access switch of a network requires that all access switches be informed,
thus presenting a significant operational and administrative problem.
Solution
In a departure from the prior art, described in this application, a
composite cell remap (CCR) unit is introduced between ATM cell switching
stages. The access switch is enhanced to include a plurality of CCR units,
each of which converts between cells on an ATM signal received from an
ATM-CM, each cell being from one of a plurality of source switches or
switch modules and destined for that CCR unit, to cells each containing
communications destined for a common destination CCR; this is accomplished
by switching the individual channels within each cell of the input ATM
signal to cells headed for the appropriate destination CCR in the output
ATM signal of the CCR. A transit crossconnect is interposed between CCRs
of different access switches, to switch cells of incoming ATM streams to
different outgoing ATM streams. This transit crossconnect is equivalent to
the network of crossconnects used today in long distance synchronous voice
networks. Advantageously, using this kind of an arrangement, a transit
crossconnect, in conjunction with a plurality of CCRs in an access switch,
can be used efficiently to interconnect the links of a permanent virtual
path between two access switches, and can link modules of different access
switches even when the traffic between the modules is well below the
capacity of a single cell.
In accordance with one specific embodiment, each composite ATM cell between
CCRs carries one byte of each of up to 48 voice communications, and the
composite cells of the CBR PVCs are transmitted at a rate of one cell per
125 microseconds (.mu.s). Advantageously, such an arrangement simplifies
the interface to existing PCM systems.
Virtual paths in the ATMU are assigned to gather traffic that is input to
the ATMU into cells such that each cell goes only to one CCR or ATMU
destination. For intra-access switch traffic the cells go to the ATMU
connected to the destination switch module. For inter-access switch
traffic, the cells go to a CCR and are then transmitted over an ATM link
to a transit crossconnect or directly to another access switch. The access
switch that contains the destination CCR, or for intra-access switch
traffic, the destination ATMU, determines the choice of which inputs are
assigned to which cells. In the case of cells destined for a CCR, if the
CCR is directly connected to another access switch, a situation which may
occur when either there is a great deal of traffic between two access
switches or when networks are initially small (i.e., few access switches),
then the traffic that can be placed in a cell is any traffic destined for
the terminating access switch. For the more general case, in which the CCR
that is the destination of the cell is connected to a transit
crossconnect, the cell from the ATMU may contain traffic for any of the
access switches connected to the transit crossconnect; the CCR will serve
to segregate traffic for each of the possible destination access switches
of the connected transit crossconnect into different cells. The transit
crossconnect routes each cell to the appropriate destination access
switch. There, another CCR unit will segregate traffic for each ATMU of
that access switch into different cells. Each cell is then routed through
the destination access switch ATM-CM to the correct ATMU for that cell,
and thence to the destination switching module.
The assignment of traffic to cells, i.e., the assignment of traffic to the
virtual paths of the ATMU, is further modified by considerations of
reliability so that, whenever possible, traffic between access switches
may be switched by a network of crossconnects, e.g., via two different
transit crossconnects. If the amount of traffic between two access
switches is small, then it may normally be carried over one of the two
transit crossconnects and in case of system failure may then be switched
to a CCR connected to the second transit crossconnect interconnecting the
two access switches.
The assignment of traffic to output cells within the CCR is relatively
straightforward. If the CCR is connected to a transit crossconnect, each
output cell contains traffic destined for a single destination CCR of a
destination access switch. Because the destination access switch also
contains a CCR unit and because of the full interconnectivity of the
ATM-CM in the destination access switch, there is no need to segregate
traffic destined for switching by different ATMUs in the destination
access switch.
All of the units described herein carry traffic in both directions. In
general, only the outgoing direction is described in detail. Those skilled
in the prior art will readily understand the adaptations necessary for the
reverse direction of transmission and switching.
While the preferred embodiment uses ATM, the principles of applicants'
invention apply to any packet network. Implementation is simplified by the
constant length packet size of the ATM cells and the use of ATM standard
parts will make implementation of applicants' invention economical. The
equivalent of an ATMU and a CCR in a more general packet network are units
which can pack and unpack the contents of the packets over the period of a
frame of a multiplexed periodic communication signal such as PCM or
PCM-like (e.g., adaptive PCM) signals. The equivalent of a CCR in a more
general packet network is a composite packet remap (CPR) unit. The
equivalent of an ATM crossconnect or network of crossconnects is a packet
switching fabric unit, or network of packet switching fabric units.
While the above principles are used in all five related applications being
submitted concurrently, the particular subject matter claimed in this
application relates to the transportation within the access switch of
supplementary control data along with the PCM samples when a connection is
within the access switch. In the specific embodiment, the supplementary
control data is a supervisory signal referred to as an E-bit, the name
used for this signal in the 5ESS switch of the preferred embodiment.
Instead of simply switching individual PCM samples, an extended segment is
switched for all samples pertaining to a particular call, the extended
segment containing an 8 bit PCM sample and the E-bit. The same basic
principles can be used for transporting any other control information
which it is important to retain with the call signal data in any other
switching system. In accordance with the principles of this invention a
segment (in contrast to a byte or other entity containing only a
communication signal such as a PCM sample) is switched through the access
switch for all communications that use the composite cells described
herein. If the segment is transmitted to a CCR, the CCR discards the
control information portion and derives its output cells only from the
communication signal, e.g., the PCM sample. If the composite cell is
internally switched within the access switch by the ATM-CM back to the
source or another ATMU, the segment is retained. The ATMU operates on the
segments in performing its composite cell creation function. The ATM-CM
simply switches the cells received from an ATMU to an output, i.e., an
ATMU or a CCR. Advantageously, this ,arrangement permits control
information required within an access switch for intra-access switch calls
to be transported along with the communication signal in composite cells.
BRIEF DESCRIPTION OF THE DRAWING
FIGS. 1-4 are high-level network configuration diagrams whose configuration
is in conformance with the principles of this invention;
FIG. 5 is a functional diagram of central stages of a network, an ATM
distribution network, designed in conformance with the principles of
applicants' invention;
FIG. 6 is a block diagram showing a network of interconnected access
switching networks;
FIG. 7 illustrates the connections to the access switches of such a
network;
FIG. 8 illustrates one 125 .mu.s frame of ATM cells (a glossary of
abbreviations is found at the end of the Detailed Description) comprising
a plurality of CBR and a plurality of CBR cells; the constant bit rate
(CBR) cells carrying voice channels are sent every 125 .mu.s;
FIG. 9 illustrates an ATM segment including a CBR cell and a variable bit
rate (VBR) cell;
FIG. 10 illustrates one CBR cell for carrying traffic that is initiated
from both of the two access switches communicating via the CBR cell;
FIG. 11 illustrates the Synchronous Optical Network (SONET)/ATM signal
transmission/crossconnect network of FIG. 6;
FIG. 12 illustrates the access switch structure and the position of a
composite cell remap (CCR) function relative to the communication network;
FIG. 13 presents an overview of how cells are transmitted from a source
switch module to a destination switch module;
FIG. 14 illustrates a single destination composite cell format of the type
which would be used for communications between switching modules of one
ATM-CM switch;
FIG. 15 illustrates a multiple destination composite cell (MDCC) format of
a type used for communicating between a switching module and a composite
cell remap unit;
FIG. 16 illustrates the format of a cell used for communicating between CCR
blocks;
FIG. 17 illustrates a tandem destination cell arranged to communicate with
a packetized voice destination;
FIG. 18 illustrates the mapping performed between incoming time slots to an
ATMU and an SDCC/MDCC (Single Destination Composite Cell/Multiple
Destination Composite Cell) cell;
FIG. 19 illustrates the remapping function performed in the CCR;
FIG. 20 illustrates the overall mapping process from source Switch Module
(SM) to destination SM for communications using a composite cell remap
function;
FIG. 21 illustrates the signaling used between ATM switches and between ATM
and between toll and local exchange carder switches;
FIG. 22 is a block diagram of an ATM switch for supporting both broadband
and narrow band ATM communications in conformance with applicants'
invention;
FIG. 23 is a block diagram of an access switch of FIG. 6;
FIG. 24 is a block diagram of an Asynchronous Transfer Mode Interface Unit
(ATMU) for interfacing between PCM signals and ATM signals;
FIGS. 25-29 illustrate various blocks of the ATMU;
FIG. 30 illustrates the control complex of an ATM Communications Module
ATM-CM unit for switching ATM cells;
FIGS. 31-33 are flow diagrams illustrating the processes of selecting a
channel for a communication activating permanent virtual circuits and
combining traffic of partially loaded permanent virtual circuits;
FIGS. 34-37 illustrate the operation of a call at the originating end;
FIGS. 38-41 illustrate the operation of a call at the terminating end;
FIGS. 42-45 are block diagrams of four versions of a CCR; and
FIG. 46 is a block diagram of an address generator for a CCR.
Overview
The telecommunications network described herein uses, packets, and in the
preferred embodiment ATM cells, to switch PCM sources supplied from local
switches. According to the principles of this invention, which can be
followed in FIG. 12 for the purposes of this overview, the network not
only uses ATM signals as a broadband means of transporting PCM signals and
of switching different PCM streams to different destinations through the
use of an ATM switch fabric capable of switching different cells to
different ATM destinations, but also includes facilities for switching
different PCM samples within a cell to other cells. The result is that PCM
streams, each containing communications destined for a large number of
different destinations, can have their contents rearranged and switched
among a plurality of ATM streams each containing cells destined for
different destinations. ATM cell switches are described in Journal of High
Speed Networks, Vol. 1, No. 3, pp. 193-279, 1992.
In accordance with the principles of this invention, input PCM streams from
one or more switching modules 510 are transmitted to an ATM interface unit
(ATMU) 540 of FIG. 23. In the specific embodiment these PCM streams are
sent on Network Control and Timing (NCT) links and are generated by a
switching module of a 5ESS switch which has inputs (not shown in FIG. 12)
from local switches and from telephone stations connected to the switching
module. The 5ESS.RTM. switch and switching module are extensively
described in the AT&T Technical Journal, Volume 64, No. 6, July/August
1985, pp. 1303-1564. The ATMU takes PCM streams from a plurality of such
switching modules arid generates ATM streams each containing cells, where
for each cell all of the PCM samples are destined for a common composite
cell remap (CCR) unit 4000 (FIG. 42) of the source access switch 1. ATM
streams from a plurality of ATMUs are transmitted to an ATM Communications
Module (ATM-CM) 550, comprising an ATM switch fabric which switches cells
from that plurality of ATM streams into a plurality of CCR units.
(Remember that each cell of an ATMU output is destined for a particular
CCR and contains only PCM samples for that CCR.)
The output of the CCR is a single ATM stream which is transmitted to a
transit ATM crossconnect 600 (FIG. 11) which, like the ATM-CM, switches
cells among different ATM streams, but keeps the payload content Of each
cell intact. The CCR performs the function of re-packaging the cells of
its input stream so that each cell in its output stream contains PCM
samples destined for a single CCR unit at a destination access switch. The
transit crossconnect can then take the inputs from a plurality of CCRs and
gather those cells destined for a CCR in a particular destination access
switch into a single ATM stream for transmission to that destination
access switch CCR. The transit crossconnect switches the cells in its
input ATM streams but does not switch parts of the payload data of one
cell to two or more different cells.
At the destination access switch, the process is simply reversed. The CCR
gathers samples destined for a common ATMU into cells and transmits its
output stream to an ATM-CM. The ATM-CM sends the cells received from a
plurality of CCRs of the destination access switch and destined for a
particular ATMU to that ATMU. That ATMU then forms PCM streams to its
connected switching modules from the contents of the ATM streams that it
has received. The ATMU can take individual PCM samples from within the
cells of its input ATM streams and direct these samples to different PCM
output streams. In this preferred embodiment, the ATMU generates and
receives a plurality of ATM streams, while the CCR operates on only one
ATM stream; clearly this is a matter of design choice and the ATMU can be
arranged to interface with a single ATM stream and/or the CCR can be
arranged to interface with a plurality of ATM streams.
For the case in which the access switch acts as a tandem switch between
local offices connected thereto or an intraoffice call between lines
directly connected to switching modules of the access switch, the ATM-CM
acts to connect output streams of an ATMU to input streams of the same or
another ATMU. For compatibility reasons it is desirable in this case that
additional information associated with each PCM sample, in this case a
supervisory indicator or E-bit, be switched through the ATMU in both
directions. The ATMU in order to provide for this capability in the
preferred embodiment deals with 9 bit segments, each segment including an
8 bit PCM byte and an E-bit associated therewith and 42 of these 9 bit
segments make up the payload of each ATM cell in the ATMU. For
convenience, this is done whether or not the cell is connected to a CCR
for further switching and subsequent transportation to another access
switch. Advantageously, using this arrangement the E-bit which is
transported over NCT links connecting a switching module to an ATMU is
thereby retained in all connections which do not leave the access switch.
Clearly, this is a matter of design choice and the ATMU and CCR could be
designed to work with different size segments and the number of segments
adjusted accordingly to fit into a 48 byte ATM payload. While in the
preferred embodiment, only the E-bit is switched through in an
intra-access switch connection, it is obvious that additional bits can be
switched through using the same principles. In some applications, other
bits may be transmitted on a sampling basis or can be transmitted via
signaling messages.
FIGS. 1-5 present a high-level overview of telecommunications networks
designed in conformance with applicants' invention. In all cases, in the
illustrative embodiment, the inputs are PCM streams and the outputs are
PCM streams.
FIG. 1 is based on the teachings of the prior patent application Ser. No.
07/972,789. It shows the treatment of an intra-access switch call. A PCM
stream from, for example a 5ESS switch enters an ATMU where composite
cells are generated, each composite cell containing PCM samples for
destinations served by one ATMU. The cells are switched in an ATM cell
switch, the ATM-CM 550, where inputs from a plurality of ATMUs 540 are
switched to a plurality of outputs to ATMUs 540. The ATM-CM does not
change the payload within each cell, but simply switches individual cells
on one ATM input stream to one of a plurality of ATM output streams. These
output streams are received in ATMU units 540 acting, in this case, as ATM
composite cell receivers and the individual PCM samples of each received
cell are distributed to appropriate positions within the correct PCM
output stream.
FIG. 2 illustrates the case of a two-stage network using composite cells.
This is a network for switching PCM samples on a plurality of PCM input
streams on one access switch to PCM samples on a plurality of PCM output
streams on a different access switch, wherein the two access switches are
directly interconnected via a link carrying one or more ATM streams. The
PCM input streams enter a composite cell generation ATMU 540 which
assembles cells all of which are destined for the same CCR, a CCR which,
in this case, is directly connected to a single destination access switch.
The ATMU 540 of the first (top) access switch effectively generates
composite cells destined for the second access switch. These composite
cells are transmitted over one or more ATM streams to ATM-CM 550 where
cells from a plurality of ATMUs entering the ATM-CM are combined into a
single ATM stream for CCR 4000, which is connected to the second access
switch. The CCR remaps the PCM samples within the cells such that all PCM
samples within the same output cell are destined for the same ATMU in the
destination access switch. In the second (bottom) access switch, the
output of the CCR 4000 from the first access switch can be directly
connected to an ATM-CM 550, which separates the cells destined for each of
the ATMUs 540 of the second access switch. Each of these latter ATMUs then
converts its input ATM streams into PCM streams in essentially the way
previously described for the composite cell reception in FIG. 1. A problem
arises in that the switch containing the CCR needs to know about the ATMUs
of the other access switch. One solution to this problem is to exchange
data messages between the access switches. Another is to equip only
incoming CCR portions in each access switch. It is also possible to equip
CCRs in both access switches.
FIG. 3 illustrates the network configuration most likely to be encountered
and the one that is described most fully in this specification. It
illustrates the case of two access switches communicating via a transit
crossconnect. In the first (top) access switch, the ATMU 540 acts as a
composite cell generation unit which generates individual cells, each of
which contain samples headed for a common CCR unit 4000 of the first
access switch. The ATM-CM of the first access switch assembles all cells
destined for that CCR into a single ATM stream which it transmits to CCR
4000. CCR 4000 maps the PCM samples from different incoming cells,
destined for a single access switch, to outgoing cells each containing
only PCM samples destined for one access switch. The output of CCR 4000 is
transmitted to a transit crossconnect 600 which collects all cells
destined for a common destination access switch into an ATM stream and
transmits that ATM stream to the CCR 4000 of the second (bottom)
(destination) access switch. The switching function executed by ATM
crossconnect 600 is the same as the switching function executed by ATM-CM
550. In the destination access switch, the CCR remaps cells so that each
cell contains PCM samples destined for a single composite cell reception
ATMU 540. The output stream of CCR 4000 is sent to ATM-CM 550 of the
destination access switch, where cells destined for a common ATMU are
gathered into ATM streams for transmission to that ATMU. The ATMU then
receives its input ATM streams and distributes the individual PCM samples
contained in the stream to its PCM outputs. While FIG. 3 shows only a
single ATM cell switch (transit crossconnect), in practice a network of
such crossconnects is likely to be used.
Finally, FIG. 4 illustrates a connection wherein the path through the
transit network interconnecting the source and destination access switches
includes two transit crossconnects connecting through a CCR. The actions
in the source (top) access switch are essentially the same as described
with respect to FIG. 3, except that in this case, the output of CCR 4000
of the source access switch may include cells which contain samples
destined for a plurality of destination access switches. Presumably, this
would be a case in which there is low traffic to each of the destination
access switches served by this type of connection. First, the fight ATM
transit crossconnect 600 receives inputs from a plurality of access
switches and transmits those cells destined for reswitching to transit CCR
4000 interconnecting the two ATM transit crossconnects. In transit CCR
4000, samples destined for a common destination access switch are
assembled into cells to be switched by the second (left) transit
crossconnect of the transit network to that destination access switch. A
second (left) ATM transit crossconnect then receives inputs from CCR 4000,
as well as direct inputs from other access switches, and generates output
ATM streams, each stream containing only cells with PCM samples destined
for a common destination (bottom) access switch (as well as output streams
destined for the ATM inter-transit crossconnect CCR 4000 of the same type
discussed as being received by the second ATM transit crossconnect). In
the destination access switch, the same types of operations are performed
as in the destination access switch of FIG. 3. The middle CCR receives
special control signals from the overall network control or, following
exchanges of messages between the two access switches, receives control
signals from one of the two, to establish virtual paths and circuits using
that CCR. The configuration of FIG. 4 can also have connections between
the two transit crossconnects 600 in order to allow some of the CCR output
cells to return to a crossconnect that may also be connected to a
destination access switch. (Such a connection is shown as connection 7 in
FIG. 11.) As previously stated for FIG. 3, the transit crossconnects may
each represent an interconnected network of such crossconnect units.
While FIGS. 1-4 and many other diagrams of this specification show only a
single direction of flow of information, it is understood that a
comparable opposite flow of information is simultaneously taking place in
essentially the same way. Each ATMU, for example, acts both as a composite
cell generator and as a composite cell receiver. Similarly, each CCR
performs its remap function in both directions.
FIG. 5 presents another view of the functions of the CCR and the ATM
crossconnect. The input to the CCR comprises cells, each of which may have
samples destined for a plurality of the CCRs at the output of an ATM
crossconnect 600. The input CCRs gather these samples into cells, each of
which contains samples destined only for a single output CCR. These cells
are placed on an output ATM stream which enters the ATM crossconnect 600.
The ATM crossconnect 600 then switches individual cells from all the
incoming ATM streams to outgoing ATM streams such that the outgoing ATM
streams each contain cells only destined for the particular CCR to which
the output stream is connected. The output CCR then takes individual
cells, each of which may contain samples for a plurality of the ATMUs of
the access switch of which the output CCR is a part, and creates cells
each of which contain samples destined for only one of these ATMUs. These
cells are then subsequently switched in the ATM-CM of the output access
switch to the appropriate ATMU of the output access switch. An ATM-CM 550
is basically an ATM crossconnect, enhanced to provide interfaces with
other units as shown in FIG. 22. A distribution "network" consisting of a
plurality of CCR units interconnected by one or more crossconnects is a
very useful element for switching narrowband signals carried by ATM
streams. In the preferred embodiment, such a distribution network accepts
as its inputs ATM signals containing cells carrying PCM channels destined
for any access switch accessible via the transit crossconnect connected to
the output of the CCR (and in the case of a CCR directly linked to another
access switch, the access switch to which the latter CCR is connected).
The contents of the input cells to these CCRs are switched into cells each
of which contains communications destined for a common access switch. The
output cells of these CCRs are then switched in the transit crossconnect
to the ATM streams connected to that destination CCR where the ATM signal
is switched into cells each of which are destined for the same ATMU in the
destination access switch. Advantageously, the combination of the CCRs and
the transit crossconnect yields a distribution network of enormous
capacity with low blockage for serving a large number of very high
capacity access switches. In theory, the CCRs can be associated with
either the access switches or the transit network. In the preferred
embodiment, the control information required by the CCR comes naturally
from the control of the access switch, thereby making a co-location of
access switch and CCR more natural.
Advantageously, in such an arrangement permanent virtual paths can be
established through the transit crossconnects for composite cells each
carrying individual PCM samples for a plurality of PCM channels. Such
permanent virtual paths can be pre-provisioned and need be activated only
when an additional group of channels is required for a particular
source/destination couplet of the transit crossconnect and are deactivated
only when there is a substantial reduction in the need for the number of
channels required of permanent virtual paths between such a
source/destination pair of the transit crossconnect.
General Description
This General Description first presents an overview of all of the diagrams
and is followed by a detailed description of special characteristics of
elements of these diagrams for implementing applicants' invention.
FIG. 6 is a block diagram showing a plurality of interconnected access
switching systems of a network. A group of access switches 1 are connected
to an ATM signal transmission/crossconnect network 10 in accordance with
the principles of this invention. Such a network is a network of ATM
crossconnects for interconnecting a plurality of Composite Cell Remapping
(CCR) units and for switching cells of the ATM streams generated by the
connected access switches. Each access switch contains a CCR that receives
an ATM signal from the ATM-CM of the access switch, comprising a plurality
of Constant Bit Rate (CBR) composite cells each carrying signals destined
for the network, i.e., for a CCR connected to the network, and distributes
the individual PCM channel signals carded in each input cell to a CBR
composite output cell having a common access switch as a destination. The
composite output cell is then switched in the network of ATM crossconnects
to a CCR of that destination access switch. The access switch contains an
ATM interface unit (ATMU) for forming composite cells from PCM samples of
inputs to the access switch and sending the composite cells on ATM signals
to an ATM Communications Module (ATM-CM) which switches the individual
cells of the ATM signal from the ATMU to the correct CCR of the access
switch. Each composite cell of the output of an ATMU is destined for a
single CCR of the access switch or for the same or for inter-access switch
communicating for another ATMU of the access switch. Constant bit rate
(CBR) cells are used to carry PCM voice traffic, and variable bit rate
(VBR) cells are used to carry packetized data. (The term PCM as used | | |
