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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 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 "Access Switches For Large ATM
Networks", Ser. No. 08/169,909, 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, 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 front 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 front 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 front 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 matter claimed in this
application relates to the total network including ATMUs, an ATM-CM, and
CCRs in each access switch; and an ATM transit crossconnect connected to
the CCRs for interconnecting the CCRs between access switches and for
directing cells between incoming and outgoing ATM streams without changing
the payload contents of each cell. Advantageously, such a network can be
used to interconnect a large number of PCM channels over an ATM network
which can simultaneously be used for transmitting broadband signals using
the very high capacity of the ATM transmission arrangement.
Advantageously, in such an arrangement permanent virtual paths can be
established through the ATM-CMs and the transit crossconnects for
composite cells 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 ATM-CM or transit
crossconnect and permanent virtual paths are deactivated only when there
is a substantial reduction in the need for the number of channels required
of permanent virtual paths between a source/destination pair of the ATM-CM
or transit crossconnect. The permanent virtual paths can also be
established for carrying conventional packetized voice (see FIG. 17) or
packetized data. Advantageously, virtual paths may be consolidated (see
FIG. 33) to insure high utilization of virtual paths even between
switching modules.
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 composi | | |
