Multiplex communications – Pathfinding or routing – Combined circuit switching and packet switching
Reexamination Certificate
1999-08-10
2001-08-28
Chin, Wellington (Department: 2664)
Multiplex communications
Pathfinding or routing
Combined circuit switching and packet switching
C370S395430, C370S410000, C370S467000, C379S220010, C379S230000
Reexamination Certificate
active
06282191
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates broadly to telecommunications. More particularly, the invention relates to methods and apparatus for redirecting common channel signalling system
7
(SS7) signalling messages through the asynchronous transfer mode (ATM) network.
2. State of the Art
Perhaps the most awaited, and now fastest growing technology in the field of telecommunications in the 1990's is known as Asynchronous Transfer Mode (ATM) technology. ATM is providing a mechanism for removing performance limitations of local area networks (LANs) and wide area networks (WANs) and providing bandwidth on the order of gigabits/second. Because ATM cells can carry many different kinds of data across a single backbone network, the ATM technology provides a unitary mechanism for broadband end-to-end telecommunications traffic.
Although the ATM network was originally conceived to eventually handle all types of data including ordinary narrowband voice telephone calls, its initial application was almost exclusively for the transport of broadband data transmissions. Initially, ATM technology was expensive to implement and existing narrowband TDM (time division multiplexing) technology was perfectly adequate for ordinary voice telephone calls. As ATM technology has been more and more accepted in recent years, there is now more incentive to migrate all communications traffic (including narrowband voice calls) onto the ATM network. The incentive to migrate all voice circuits onto the ATM network has been amplified by the popularity of the Internet. Existing narrowband telephone technology was designed to handle telephone calls which average four minutes in length. Today, with millions of people using narrowband voice circuits to connect via modem to the Internet, the average voice circuit call length has grown to well over twenty minutes. Thus, as “voice calls” become longer in length, the existing narrowband telephone technology becomes less adequate.
One challenge in migrating narrowband voice calls to the ATM network is that the signalling/addressing system used in the narrowband network for provisioning trunk lines is different from the signalling/addressing system used in the broadband ATM network for provisioning virtual circuits. The international standard signalling/addressing system for narrowband circuits is known as the Common Channel Signalling System No. 7 (“SS7”, “Signalling System 7”, or “C7”).
Prior art
FIG. 1
illustrates how narrowband SS7 signalling operates. The public switched telephone network (PSTN)
10
includes a plurality of switches, e.g.
12
,
14
,
16
,
18
which are interconnected via trunk lines
20
,
22
,
24
,
26
,
28
,
30
. Generally speaking, switches are located at central offices (COs) and at other locations determined by the length and nature of the trunk lines connecting the switches. Some of the switches in COs are directly connected to subscribers (class 4/5 switches). Other switches (tandem switches) couple switches to other switches. As shown in
FIG. 1
switch
12
is connected to a subscriber line
32
and switch
18
is connected to a subscriber line
34
. When the subscriber at subscriber line
32
calls the subscriber at subscriber line
34
, at least two switches (
12
and
18
) will be involved in the connection. Depending on the level of congestion in the network
10
, the call may need to be routed from switch
12
to switch
14
and/or switch
16
before reaching switch
18
. The switches which originate and terminate the call are referred to as service switching points (SSPs). Network traffic between SSPs may be routed via a packet switch called a Signal Transfer Point (STP).
In order for a call to be correctly setup, managed, and torn down, all of the switches on the network must be able to communicate with each other. According to the SS7 protocol, each switch is coupled to the SS7 network. In
FIG. 1
, the SS7 network is shown as
36
. In particular, each switch is connected to the SS7 network by a single clear channel link, typically a DS0 or 64 k link.
FIG. 1
shown links
38
,
40
,
42
, and
44
coupling switches
12
,
14
,
16
,
18
, respectively to the SS7 network. According to the SS7 standard, the SS7 network may also include one or more centralized databases (not shown) referred to as Service Control Points (SCPs). In order for a call to be completed, it may be necessary for the originating SSP to consult an SCP in order to obtain routing information.
The hardware and software of the SS7 protocol are divided into functional abstractions called “levels” which map loosely to the Open Systems Interconnect (OSI) 7-layer model defined by the International Standards Organization (ISO). Prior art
FIG. 2
illustrates the OSI reference model and the SS7 Protocol Stack.
The Message Transfer Part (MTP) is divided into three levels. The lowest level, MTP Level
1
, is equivalent to the OSI Physical Layer. MTP Level
1
defines the physical, electrical, and functional characteristics of the digital signaling link. Physical interfaces defined include E-1 (2048 kb/s; thirty-two 64 kb/s channels), DS-1 (1544 kb/s; twenty-four 64 kb/s channels), V.35 (64 kb/s), DS-0 (64 kb/s), and DS-0A (56 kb/s). MTP Level
2
ensures accurate end-to-end transmission of a message across a signaling link. Level
2
implements flow control, message sequence validation, and error checking. When an error occurs on a signaling link, the message (or set of messages) is retransmitted. MTP Level
2
is equivalent to the OSI Data Link Layer. MTP Level
3
provides message routing between signaling points in the SS7 network. Each node in the SS7 network has a point code. Routing messages include the originating point code (OPC) as well as the destination point code (DPC). MTP Level
3
re-routes traffic away from failed links and signaling points and controls traffic when congestion occurs. MTP Level
3
is equivalent to the OSI Network Layer.
An SS7 message (carried in MTP Level
2
) is called a signal unit (SU). There are three types of signal units: fill in signal units (FISU), link status signal units (LSSU), and message signal units (MSU). The FISUs are transmitted continuously unless other SUs are present. The LSSUs are used to control link alignment and to indicate the status of a signalling point, e.g. to signal an outage. The MSUs carry all call control, database query and response, network management, and network maintenance data.
The ISDN User Part (ISUP) defines the protocol used to set-up, manage, and release trunk circuits that carry voice and data between terminating line exchanges (e.g., between a calling party and a called party). ISUP is used for both ISDN and non-ISDN calls. However, calls that originate and terminate at the same switch do not use ISUP signaling. The basic messages used to setup and teardown a connection between switches include the CIC (circuit identification code). The CIC indicates the trunk circuit reserved by the originating switch to carry the call. The CIC is followed by one of the following message types: IAM (initial address message), ACM (address complete message), ANM (answer message), and REL (release message).
In some parts of the world (e.g., China, Brazil), the Telephone User Part (TUP) is used to support basic call setup and tear-down. TUP handles analog circuits only. In most countries, ISUP has replaced TUP for call management.
The Signaling Connection Control Part (SCCP) provides connectionless and connection-oriented network services and global title translation (GTT) capabilities above MTP Level
3
. A global title is an address (e.g., a dialed 800 number, calling card number, or mobile subscriber identification number) which is translated by SCCP into a destination point code and subsystem number. A subsystem number uniquely identifies an application at the destination signaling point. SCCP is used as the transport layer for TCAP-based services.
The Transaction Capabilities Applications Part (TCAP) supports the exchange of non-circuit related data between ap
Cassella Pasquale
Chen Xing
Cumberton John
McLoughlin Michael
Ramanathan Ramanathan
Chin Wellington
Gallagher Thomas A
General Datacomm, Inc.
Gordon David P.
Jacobson David S.
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