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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, now U.S. Pat. No. 5,345,446;
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, now U.S. Pat. No. 5,365,524;
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, now U.S. Pat. No. 5,327,421;
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, now U.S. Pat. No. 5,345,445; 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", Ser. No. 08/169,913, filed Dec. 20, 1993; filed on an even date herewith,
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", Ser. No. 08/170,550, filed Dec. 20, 1993, now U.S. Pat. No. 5,390,175, filed
on an even date herewith and
Thomas L. Hiller, Ronald A. Spanke, John J. Stanaway, Jr., Alex L. Wierzbicki, and Meyer J. Zola entitled "Intra-Switch Communications In Narrow Band ATM Networks", Ser. No. 08/169,915, filed Dec. 20, 1993, filed on an even date herewith,
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 carded 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 claimed in this application relates to the access switch including at least one ATMU, an ATM-CM and at least one CCR.
Advantageously, such an access switch can gather traffic from a large number of sources and bundle this traffic into composite cells carrying PCM samples for a plurality of communications wherein each of the composite cells at the output of an access
switch contains traffic destined for a single destination access switch. The access switch can also be used advantageously for carrying conventional packetized voice (see FIG. 12) or packetized data from and to the switches and modules that are part of
or are connected to the access switch.
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 carrier 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 and 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 carded 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 carried 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 herein refers both to voice signals transmitted by PCM and to data (including
facsimile (FAX) and video) transmitted over PCM channels.)
Throughout this specification, there are frequent discussions of activation of permanent virtual circuits and permanent virtual paths. A permanent virtual path, comprising a plurality of permanent virtual circuits, is provisioned by including
its identification in the memory of an ATMU, ATM-CM, CCR or ATM crossconnect. The memory is used for switching cells having the virtual path identification as their header, in the ATM-CM and ATM crossconnect and for identifying the cells to the ATMU and
CCR for their operations. The virtual circuit information is retained in the ATMU and CCR in order to steer PCM samples between the correct cells or between the correct cells and correct PCM stream and position in that stream. A virtual path is
activated only when actually used to carry communications. The number of provisioned permanent virtual paths can therefore be much greater than the number of activated permanent virtual paths. A permanent virtual circuit is similarly activated when the
specific circuit (or channel), of an activated permanent virtual path, is used. If a cell of a virtual path is full and another channel is needed, then another, previously provisioned, permanent virtual path is activated to carry that channel over a
virtual circuit of the newly activated permanent virtual path. Note that this is inconsistent with standard ATM terminology, but such terminology does not have to deal with composite cells for carrying a plurality of communications. Applicants have
chosen to call the byte or segment position of a cell for carrying each such communication a virtual circuit since it carries, essentially, a circuit switched communication.
The access switch and transmission/crossconnect network can also be used for switching and transporting conventional packetized voice as illustrated in the cell of FIG. 17. The disadvantage of conventional packetized voice is that it incurs
additional packetizing delay and it requires expensive interface circuits to assemble, store and transmit the 48 samples of speech that are conveyed in each cell.
The CCR units of the access switches are connected to entities which gather outgoing traffic and which distribute incoming traffic. The ATM-CM, to which a CCR is connected, switches individual cells of its input and output ATM streams between
CCRs and Asynchronous Transfer Mode Interface Units (ATMUs), described further below. This unit distributes intra-cell traffic to a plurality of switch modules (SMs) of a 5ESS switch or distributes such traffic to one or more stand-alone switches. It
may be desirable in the future to connect switch modules of more than one 5ESS switch to an ATMU.
Much of the traffic carried through the ATM signal transmission/crossconnect network 10 is CBR traffic wherein the individual CBR cells in each 125 .mu.s frame are switched to a destination CCR unit. A CCR unit is used advantageously for
switching toll voice traffic, because the access to a transit crossconnect for switching ATM cells provides the CCR with the ability to access a large number of different destination CCRs with different cells. The provision of permanent virtual paths
(PVPs) to these destination CCRs and the relatively low rate of activation and deactivation of these PVPs allows for a relatively low rate of changes of paths in the network 10. The routing pattern | | |
