Note: Descriptions are shown in the official language in which they were submitted.
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COMMUNICATIONS USING HYBRID CIRCUIT-SWITCHED AND
PACKET-SWITCHED NETWORKS
BACKGROUND
The invention relates to communications using hybrid circuit-switched and
packet-switched networks.
A traditional telephone exchange configuration provides circuit connections
between remote locations. Many of the telecommunications networks currently
used
are synchronous digital networks. Digitized voice communications are
transmitted
synchronously over the networks at a fixed rate. Discrete time periods (time
slots) are
packed with the digital information for a particular call, and digital
information for
multiple calls can be packed sequentially to form a time division multiplexed
(TDM)
data stream. The connections may be provided, for example, using network
switches
having dedicated inter-switch connections. Because the number of inter-switch
connections is static, the number of incoming circuits that can be routed to
each
output port of the exchange also is static.
Situations may arise in which the demand for connections to a particular
location reaches its limit, while the demand for connections to another
location is
below its limit. In such cases, it would be advantageous to be able to
reconfigure the
network connections to allow more circuits to be connected to the location
having the
high demand. However, that is not possible in a system having dedicated inter-
switch
connections. In addition, systems using time division multiplexing are not
easily
scalable. As connections are added, the availability of tandem-layer switches
can
become quickly exhausted.
Packet-domain network architectures, such as asynchronous transfer mode
(ATM) networks, allow connections to be made between endpoints without
dedicated
inter-switch connections. Fixed-size packets of data, known as cells, are
transferred
between the ATM switches, which are packet switches that provide virtual
circuits
between the end points of a network. The virtual circuits may be reconfigured
depending upon data traffic volume. Hence, an ATM network can provide a more
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efficient way to connect end points in a network with rapidly changing
connectivity
requirements, such as a telephone system.
SUMMARY
According to one aspect, a method of communicating includes transporting
circuit-switched narrowband traffic over a packet network and delivering the
narrowband traffic from the packet network to a circuit-switched network. In
various
implementations, the connection through the packet network for the narrowband
traffic can be provided dynamically. Multiple circuit-switched narrowband
calls can
be multiplexed over a single connection in the packet network.
According to another aspect, a method of establishing a path for narrowband
traffic includes establishing a packet network connection between first and
second
interface points in one or more circuit-switched networks. The packet network
connection is associated with a narrowband circuit allocated to service the
narrowband traffic.
Various implementations may include one or more of the following features.
The packet network connection can include a switched virtual connection. An
identification code that uniquely identifies a narrowband circuit allocated to
service
the traffic can be forwarded between first and second gateways configured to
perform
adaptations between circuit-switched signals and packet-switched bearers. The
identification code can be forwarded from the first gateway to the second
gateway, for
example, using Signaling System 7 (SS7) messages. The identification code can
include a DSO circuit identification code.
The method also can include identifying a channel in the packet network
connection over which the narrowband traffic is to be sent. The narrowband
circuit
allocated to service the call can be associated with the channel in the packet
network
connection.
In another aspect, a method of communicating includes multiplexing multiple
circuit-switched narrowband calls over a single packet network connection and
releasing resources allocated to service a particular one of the circuit-
switched calls.
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For example, resources in the packet network servicing the particular circuit-
switched
call can be released once the call is terminated.
A communications system also is disclosed and can include at least one
circuit-switched network including first and second interface points, a packet
network,
and gateways coupled respectively to the interface points and the packet
network.
The gateways are configured to perfornz adaptations between circuit-switched
bearers
and packet-switched bearers. The system includes at least one controller
arranged to
provide call control signals to allow a packet network connection to be
established
between the first and second interface points and to allow the packet network
connection to be associated with a narrowband circuit allocated to service
narrowband
traffic that is to be transported across the first and second interface
points.
The techniques can be used with different types of packet networks.
Various implementations can include one or more of the following
advantages. Service providers can consolidate circuit-switched and variable
bit rate
data services over a single, broadband network. High-quality narrowband
services
can be delivered over packet and cell networks, such as ATM and Internet
Protocol
(IP). In particular, the system can support voice or other narrowband calls
over an
ATM or IP backbone network, while interfacing seamlessly with an existing SS7-
based public telephone network that uses circuit-switched technology. It also
can
provide better network utilization by ensuring that bandwidth is allocated
where it is
needed and not stranded in dedicated trunks as it is with circuit-switched
networks.
Other features and advantages will be readily apparent from the following
detailed description, the accompanying drawings and the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a telephone connection through a hybrid ATM
network and an associated signaling network.
FIG. 2 is a simplified block diagram of an exemplary media gateway.
FIG. 3 shows an exemplary conversion of a digital telephone signal bit stream
into ATM cells.
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FIG. 4 is a flow chart of a voice call set up process between TDM circuit
switches over an ATM network.
FIG. 5 is a signal flow diagram for a voice call set up process between TDM
circuit switches over an ATM network.
FIG. 6 is a signal flow diagram illustrating details of establishing a packet-
domain connection for the voice call set up process of FIG. 5.
FIG. 7 is a signal flow diagram for an alternative voice call set up process
between TDM circuit switches over an ATM network.
FIG. 8 is a signal flow diagram for a call set up process in which multiple
TDM calls share a single ATM connection.
FIG. 9 is a signal flow diagram for releasing a voice call over an ATM
connection.
DETAILED DESCRIPTION
A call control mechanism for carrying narrowband traffic, such as voice calls,
modem data or facsimile data, over an asynchronous transfer mode (ATM) or
other
packet network connection is described below. In particular, a switching
system is
described that supports narrowband calls over a packet backbone network, while
interfacing seamlessly with an existing SS7-based public telephone network
that uses
circuit-switched technology.
As shown in FIG. 1, a continuous call path is established starting with a
narrowband Signaling System 7 (SS7) call that originates, for example, in a
Public
Switched Telephone Network (PSTN) 102A. The path is established using a
virtual
circuit over an ATM network 101 and completes on the terminating side in a
narrowband circuit-switched SS7 call to the terminating subscriber through
another
circuit switched network 102B. The control mechanism interacts with the
circuit-
switched and packet-switched networks to correlate SS7 and ATM connections to
establish a single continuous information path.
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A large number of individual telephone circuits, such as DSO circuits, that
are
to be connected to the packet network 101 can be carried, for example, on
fiber optic
carriers 105 using time-division multiplexing (TDM) according to the Telcordia
Synchronous Optical Network (SONET) standards. The TDM carriers 105 are
coupled to access ports 116 (see FIG. 2) in media gateways 100A, 100B.
The gateways 100A, 1 OOB can adapt the TDM telephone line signals to
packet-based signals and vice-versa. Each gateway 100A, 100B can separate
incoming TDM signals into individual DSO signal streams. The TDM telephone
signals are circuit-switched, in other words, the bit stream can be divided
temporally
into individual DSO circuits. By contrast, in packet-based signals, the bit
stream can
be divided according to the destination address of each packet.
In one implementation, shown in FIG. 2, each gateway, such as the gateway
100A, includes a TDM switching matrix 117 that provides full switching
capabilities.
The switching matrices 117 permit the DSO circuits to be interconnected
flexibly with
narrowband channels appearing on the gateways. Echo cancellation and other
digital
signal processing functions can be performed in a digital signal processing
portion
118 of each gateway. The DSO streams are adapted by an ATM adaptation layer
120
into ATM cells. As shown in FIG. 3, the ATM adaptation layer 120 combines
incoming DSO signals from a particular carrier 105 into payloads 132 for ATM
cells
130. A header 134 is provided as part of each cell 130 and can be interpreted
by the
gateway to identify which call the ATM cell is associated with. After the
payload 132
of a cell 130 is loaded with data, the cell is inserted through the ATM ports
21 into the
ATM cell stream 135 that traverses an ATM network 101. Each gateway includes a
control section 119 that controls overall operation of the gateway. In one
implementation, the gateways 100A, 100B are implemented as Salix 7720 Class-
Independent Switches available from Tellabs Operations, Inc.
As illustrated in FIG. 1, each gateway 100A, 100B is connected to a respective
ATM end point switch 115. The connection between a gateway and an ATM end
point switch 115 and the connection between the ATM end point switch and the
ATM
network 101 are user-network interfaces (UNIs). Within the ATM network 101,
there
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are a number of ATM switches 110 which are interconnected by network-node
interfaces (NNIs).
A call control network 126, which forms part of an existing telephone system,
runs parallel to the voice network. The call control network 126 primarily
controls
telephone switching equipment to connect the originating and terminating ends
of a
telephone call using SS7 messages. A call controller 120A, 120B is coupled to
each
gateway 100A, 100B and provides an interface between the gateway and the call
control network 126. As discussed below, the exchange of call control signals
allows
the gateways 100A, 100B to establish a connection through the ATM network 101
to
enable the transmission of narrowband traffic between the end points.
As shown in FIGS. 4 and 5, to establish a voice connection, a user at the
originating end dials 210 a telephone number. A connection is established
through an
originating TDM circuit switch in the circuit switched network 102A, and the
call
controller 120A at the originating end receives 215 an SS7 initial address
message
(IAM) 150. The call controller 120A routes the call, in other words, it
identifies a call
controller 120B associated with a terminating DSO circuit in the circuit
switched
network 102B. Next, the call controller 102A sends 220 a connection control
message (CreateConn) 152 to the originating gateway 100A to initiate a
connection
through the ATM network 101. In response, the gateway 100A returns 225 an
acknowledgement message (CreateAck) 154 that includes a connection descriptor
("conndesc"). The connection descriptor includes an ATM address for the
gateway
100A as well as information that uniquely identifies the call. The information
that
uniquely identifies the call can identify a connection-related resource such
as the
narrowband circuit (e.g., DSO circuit) handling the call on the originating
side.
Next, the call controller 120A sends 230 an IAM message 156 to the
terminating call controller 120B. The message 156 includes the information
contained in the connection descriptor. Upon receiving the IAM message 156,
the
terminating call controller 120B routes the call. In other words, the
terminating call
controller 120B selects a TDM circuit on a particular gateway, such as the
gateway
100B, to handle the call. The call controller 120B then sends 235 a connection
control message (CreateConn) 158 to the terminating gateway 100B. The
CreateConn
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message 158 also includes the information contained in the connection
descriptor. In
response, the terminating gateway 100B establishes 240 a packet domain
connection
with the originating gateway 100A through the packet network 101.
Details of establishing a packet-domain connection between the gateways
100A, 100B through the ATM network 101 are illustrated in FIG. 6. ATM Setup
messages (e.g., UNI Setup and PNNI Setup messages) can be used to establish
the
packet-domain connection. For example, a UNI Setup message 160 is passed from
the terminating gateway 100B to a first ATM switch 115. PNNI or B-ISUP Setup
messages 162 are sent from one ATM switch 110, I 15 to the next ATM switch I
10,
I 0 I 15 in the network 101. Finally, a LJNI Setup message 164 is sent from
the last ATM
switch I 15 to the originating gateway I OOA. Each of the LJNI Setup and PNNI
Setup
messages 160, 162, 164 includes the information contained in the connection
descriptor so that each network element in the packet domain connection is
informed
of the connection identifier that uniquely identifies the narrowband voice
call. The
originating gateway 100A then associates 245 the packet-domain connection with
the
circuit-domain connection.
The gateways 100A, 100B and ATM switches 110, 115 also negotiate the
ATM routing headers that will be used between hops along the packet-domain
connection. Various LTNI connect messages 166, 174 and PNNI connect messages
170, as well as connect acknowledgement (Connect Ack) 168, 172, 176 messages
can
be used, as shown in FIG. 6. A control message (CreateAck) 178 then is sent by
the
terminating gateway 100B to the terminating call controller 120B to
acknowledge that
the packet-domain connection has been established for the voice call.
The terminating call controller 120B sends 250 a message to the originating
call controller 120A to acknowledge that a connection has been established.
The
terminating call controller 120B also sends 255 a message to the terminating
TDM
circuit switch in the circuit switched network 102B to establish a connection
to the
called party's telephone set. Standard SS7 signaling occurs 260 between the
terminating and originating ends to complete the voice call. The information
contained in the connection descriptor can, therefore, be used to permit the
implementation of a switching system that supports narrowband calls over an
ATM or
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other packet backbone network, while interfacing seamlessly with an existing
SS7-
based public telephone network that uses circuit-switched technology.
The foregoing technique can be used when different call controllers 120A,
120B are associated with the gateways 100A, 100B. However, in some cases, both
the originating and terminating gateways I OOA, 100B may share a common call
controller, such as the call controller 120A. In that case, a technique
similar to that
discussed above can be used with a single call controller performing the
functions of
both call controllers 120A, 120B. When the call controller 120A routes the
call after
receiving the IAM message 150 (FIG. 5), it selects the terminating TDM circuit
switch and the corresponding terminating gateway IOOB to handle the call.
Also,
when a single call controller 120A is involved, the IAM message 156 need not
be
used.
In the techniques described above, the packet-domain connection is
established in the upstream (or backward) direction, in other words, from the
terminating gateway 1 I OB to the originating gateway 110A. That technique is
particularly advantageous for minimizing the number of SS7 messages and,
therefore,
increasing the capacity of the call controllers to handle a greater number of
calls. On
the other hand, if the call setup crosses network boundaries, establishing the
packet-
domain connection in the upstream direction can limit the ability of the
originating
carrier to select the optimal route for the call. To allow the originating
carrier to
select an optimal route, the packet-domain connection can be established in
the
downstream (or forward) direction, in other words, from the originating
gateway
100A to the terminating gateway I OOB as illustrated in FIG. 7.
Referring to FIG. 7, when a subscriber at the originating end dials a
telephone
number, a connection is established through an originating TDM circuit switch
in the
circuit switched network 102A, and the call controller 120A at the originating
end
receives an IAM message 180. The call controller 120A routes the call, in
other
words, it identifies the call controller 120B associated with a terminating
DSO circuit
in the circuit switched network 102B. Next, the call controller 102A sends an
IAM
message 182 to the terminating call controller 120B requesting that a
connection be
established for the call.
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Upon receiving the IAM message 182, the terminating call controller 120B
routes the call. In other words, the terminating call controller 120B selects
a TDM
circuit on a particular gateway, such as the gateway 100B, to handle the call.
The call
controller 120B then sends a connection control message (CreateConn) 184 to
the
terminating gateway 100B to initiate the packet-domain connection. In
response, the
gateway 100B returns an acknowledgment message (CreateAck) 186 that includes a
connection descriptor ("conndesc"). In this case, the connection descriptor
includes
an ATM address for the terminating gateway 100B, as well as a connection
identifier
that uniquely identifies the connection-related resource (e.g., the DSO
circuit)
handling the call on the terminating side.
The call controller 120B then sends an SS7 facility (FAC) or other ISUP
message 188 to the originating call controller 120A. The message 188 includes
the
information contained in the connection descriptor. Upon receiving the IAM
message
188, the originating call controller 120A sends a connection control message
(CreateConn) 190 to the originating gateway 100A. 'The message 190 also
includes
the information contained in the connection descriptor. Next, the originating
gateway
100A establishes a packet-domain connection with the terminating gateway 100B
through the packet network 101. The details for establishing the packet-domain
connection in the ATM network 101 are similar to those illustrated in FIG. 6,
except
that the connection is established in the forward direction rather than in the
backward
direction. Once the packet-domain connection is established, a connection
control
message (CreateAck) 192 is sent from the gateway 100A to the call controller
120A.
As before, the information contained in the connection descriptor is used to
permit the
implementation of a connection that supports voice calls over an ATM backbone
network, while interfacing seamlessly with an existing SS7-based public
telephone
network that uses circuit-switched technology. Once the packet-domain
connection is
established, the terminating gateway 100B can associate the packet-domain
connection with the circuit-domain connection. Standard SS7 signaling can be
used
to complete the voice call. If both gateways 100A, 100B share a common call
controller, then the SS7 messages 182, 188 can be eliminated.
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'The techniques described above can be used in cases where one narrowband
call is assigned per ATM channel. In such situations, the setup messages for
the
packet-domain connection explicitly indicate which ATM virtual channel
connection
(VCC) to use, and the connection ID allows the gateway to associate the new
VCC
with the correct narrowband call. Where multiple TDM calls, however, are
multiplexed over the same ATM connection, three distinct items must be
identified:
the narrowband call, the ATM connection, and the particular channel within the
ATM
cell that is associated with the call. When a virtual channel connection does
not exist
between the gateways 100A, 100B for the narrowband call to be multiplexed
into, the
connection can be established as previously described with the additional use
of a
network call correlation identifier (NCCI) to identify the specific ATM VCC.
For
example, when the terminating gateway 100B establishes the packet-domain
connection through an ATM Setup message, it includes the NCCI which then is
communicated to the originating gateway 100A. Both gateways 100A, 100B store
the
NCCI and associate it with the specific ATM VCC.
For subsequent narrowband calls that are to be multiplexed onto the same
ATM VCC, ATM Setup messages are not required because the ATM connection
already has been established. Therefore, another mechanism is provided to
communicate to the originating gateway 100A which channel to use in the ATM
VCC. As shown in FIG. 8, establishing a voice call over the same ATM VCC
initially can proceed in the same manner as described previously with respect
to FIG.
5. In other words, the messages 152, 154, as well as the messages 154, 156 and
158
that contain the information for the connection descriptor (conndesc), are
sent as
previously described. Upon receiving the CreateConn message 158, the
terminating
gateway 100B recognizes the originating gateway 100A as the source of the call
and
determines that there already is an active VCC to the gateway 100A. The
terminating
gateway 100B then allocates a TDM circuit switch in the network 102B to be
used for
the current call. The gateway 100B also determines the NCCI of the ATM VCC to
be
used. Finally, the gateway selects an unused channel in the VCC and determines
the
channel identifier (CID).
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The NCCI and the CID are communicated to the originating gateway 100A.
To accomplish that, the termination gateway 100B returns a connection control
message (CreateAck) 300 to the terminating call controller 120B. The message
300
includes the information contained in the original connection descriptor
(conndesc),
as well as the NCCI, the CID and the ATM address for the terminating gateway
100B.
The call controller 120B then sends an FAC or other SS7 message 302 to the
originating call controller 120A. The message 302 forwards the information
contained in the connection descriptor (conndesc), as well as the NCCI, the
CID and
the ATM address for the terminating gateway 100B, to the call controller 120A.
The
call controller 120A sends a message (ModifyConn) 304 to the originating
gateway
100A. The message 304 also includes the information contained in the
connection
descriptor (conndesc), as well as the NCCI, the CID and the ATM address for
the
terminating gateway 100B. The gateway 100A uses that information to associate
the
previously allocated TDM circuit with the correct ATM VCC (as indicated by the
NCCI) and the correct channel within the VCC (as indicated by the CID).
Finally, the
gateway 100A sends a message (ModifyAck) 306 to the call controller 120A to
acknowledge the association of the circuit-switched connection with the
particular
channel in the ATM VCC.
FIG. 9 illustrates a process of releasing a call, for example, when the party
that
initiated the call hangs up its telephone set. The originating TDM circuit
switch sends
an SS7 release message (REL) 310 to the call controller 120A identifying which
DSO
circuit was handling the call. In response, the call controller 120A sends a
delete
message (DEL) 312 to the originating gateway 100A. The message 312 includes
the
information contained in the connection descriptor (conndesc), as well as the
NCCI
and the CID, to identify the call that is being terminated, the ATM VCC and
the
particular channel within the VCC. The gateway I OOA then returns a message
(Delete Ack) 3 I 6 to the originating call controller 120A to indicate that
the ATM
channel identified by the CID is now free and that the gateway no longer
contains an
association between the TDM circuit switch and the ATM channel used for the
call.
The call controller 120A recognizes that the TDM trunk previously used for the
call is
now idle and is available to be used for other telephone calls. Once the call
controller
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120A receives the Delete Ack message 316, it sends an SS7 release complete
message
(RLC) 318 to the originating TDM circuit switch.
In some cases, for example, if the call to be released is the last remaining
call
on the particular ATM VCC, the gateway 100A also may initiate UNI/NNI
signaling
314 to release the internal resources dedicated to the ATM connection. The
UNIlNNI
release message is relayed through the ATM network 101. Each ATM switch 110
receives the release message, releases the path for the connection, and
responds with a
release complete message.
The call controller 120A also sends an SS7 message (REL) 320 to the
I 0 terminating call controller 120B. The REL message 320 includes the
information
contained in the connection descriptor, as well as the NCCI and the CID, to
identify
the call that is being terminated, the ATM VCC and the particular channel
within the
VCC. Once the originating call controller 120A sends the REL message 320, it
recognizes that the packet trunk previously used for the call is idle and can
be used for
other calls.
When the terminating call controller 120B receives the REL message 320, it
sends a message (Delete) 322 to the terminating gateway 100B. The Delete
message
322 also includes the information contained in the connection descriptor, as
well as
the NCCI and the CID. In response, the gateway 100B returns a message (Delete
Ack) 324 to the terminating call controller 120B to indicate that the ATM
channel
identified by the CID is now free and that the gateway no longer contains an
association between the terminating TDM circuit switch and the ATM channel
used
for the call. The terminating call controller 120B recognizes that the packet
trunk
previously used for the call is now idle and can be used for other calls.
The terminating call controller 120B sends an SS7 release message (REL) 326
to the terminating ATM circuit switch and sends a release complete message
(RLC)
328 to the originating call controller 120A. The terminating TDM circuit
switch
returns an SS7 release complete message (RLC) 330 to the call controller 120B,
which recognizes that the terminating TDM trunk is now idle and can be used
for
other telephone calls. Release of the resources associated with the call is
completed.
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In some implementations, some of the messages, such as the CreateConn,
Create Ack, Delete, Delete Ack, Modify and Modify Ack, are Media Gateway
Control Protocol (MGCP) messages, although other protocols can be used as
well.
Although the foregoing implementations have been described with respect to
ATM networks, circuit-switched traffic can be routed over other packet-domain
networks, such as frame relay, Ethernet and Internet Protocol (IP) networks,
as well.
Various features of the system can be implemented in hardware, software, or a
combination of hardware and software. For example, some aspects of the system
can
be implemented in computer programs executing on programmable computers. Each
program can be implemented in a high level procedural or object-oriented
programming language to communicate with a computer system. Furthermore, each
such computer program can be stored on a storage medium, such as read-only-
memory (ROM) readable by a general or special purpose programmable computer,
for
configuring and operating the computer when the storage medium is read by the
computer to perform the functions described above.
Other implementations are within the scope of the claims.
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