Multiplex communications – Pathfinding or routing – Combined circuit switching and packet switching
Reexamination Certificate
2001-02-28
2002-05-14
Vu, Huy D. (Department: 2664)
Multiplex communications
Pathfinding or routing
Combined circuit switching and packet switching
Reexamination Certificate
active
06389011
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a telecommunications architecture. More particularly, the present invention relates to tandem switching systems for use within a public switched telephone network (PSTN). The present invention enables voice trunking over an asynchronous transfer mode (ATM) network by replacing tandem switches with a distributed virtual tandem switching system that includes a high speed ATM network. The replacement is virtual because as far as the end offices are concerned, the ATM-based distributed virtual tandem switching system is functionally equivalent to the traditional time division multiplexed (TDM) tandem switching system.
2. Background Information
Within the public switched telephone network (PSTN), an originating caller communicates with a destination by establishing a connection between an end office serving the originating caller and an end office serving the destination.
FIG. 1
shows the architecture of the current PSTN. In today's PSTN, end office switches
10
are connected to each other via tandem trunk groups
12
, direct trunk groups
14
, or both tandem trunk groups
12
and direct trunk groups
14
. Each trunk within a trunk group is typically a digital service level
0
(DS
0
) (i.e., 64 kilobits per second) communication line that transmits between the end offices
10
in a time division multiplexed (TDM) manner. When an end office utilizes a direct trunk group
14
, the connection between the end offices
10
is without any intermediaries. When an end/central office
10
utilizes a tandem trunk group
12
, the connection between end offices
10
is via a tandem switch
16
.
The tandem switch or office
16
is an intermediate switch or connection, between an originating telephone call location and the final destination of the call, which passes the call along. Tandem switches are often utilized to handle overflow calls. That is, when all paths are busy on a primary route, e.g., the direct interoffice trunk group
14
between the originating and destination end offices
10
, alternative routes through the tandem switch
16
handle the overflow call volume. The tandem switch
16
can also function as a physical path to non-directly-connected offices in addition to functioning as an overflow path for directly connected offices. If the overflow route through the tandem switch
16
becomes full, an alternate final route may be provided. The alternate final route is via another end office
10
, thus employing two interoffice trunk groups
14
.
Signaling is needed within the PSTN to establish a connection (i.e., setup a telephone call) between a calling party and a destination. The signaling enables line acquisition and sets up call routing, in addition to performing other functions. The signaling can be transmitted through a channel common with the voice data (in-band signaling) or can be transmitted through a dedicated channel (out of band signaling). The dominant signaling protocol currently in use today is transmitted via the dedicated channel and is called Signaling System 7 (SS7).
A conventional connection setup between two end offices
20
,
22
in a tandem network is now described with reference to
FIGS. 2 and 3
. When a calling party
19
(e.g., 235-1111) dials a telephone number (e.g., 676-2222), the originating central office
20
interprets the dialed digits and routes the call to either a direct interoffice trunk group
14
between end offices
20
,
22
or a pair of tandem office trunk groups
12
and the corresponding tandem switch
16
between end offices
20
,
22
. Assuming the pair of tandem office trunk groups
12
and the corresponding tandem switch
16
is utilized, a trunk from each of the trunk groups
12
needs to be selected and reserved by signaling within the SS7 network. Thus, necessary information is transmitted from the originating end office
20
to its associated signaling transfer point
18
. Although only a single signaling transfer point is shown in the figures, a network typically includes many signaling transfer points. Thus, each signaling transfer point
18
transfers signals from one signaling link to another signaling link in the SS7 network that transports SS7 messages.
The transmitted information is in the form of an ISUP (ISDN user part) message. It contains a unique point code, which uniquely identifies each end office, corresponding to the originating end office (originating point code (OPC)) and the destination (destination point code (DPC)). Because the message must first go to the tandem office
16
, the ISUP message contains the destination point code of the tandem office. The message also contains a circuit identification code (CIC) that corresponds to the physical circuit that will be employed to transport the data. Thus, interoffice trunks are identified by originating point code (OPC), destination point code (DPC), and circuit identification code (CIC).
As shown in the example illustrated in
FIG. 3
, initially an ISUP message is sent containing a DPC equal to
246
1
2
, an OPC equal to
246
1
1
, and a CIC equal to
22
. Consequently, a circuit will be setup between the originating end office
20
and the tandem office
16
. The tandem switch
16
receives the SS7 message and determines from the called number, which is embedded in the protocol, where to route the call, i.e., the appropriate destination end office
22
. Then, via the SS7 network, the call is setup between the tandem switch
16
and the appropriate terminating office
22
in a similar manner. Thus, because the tandem office
16
needs to transport the data to the destination end office
22
, the tandem office
16
sends an ISUP message to the signaling transfer point
18
, including the destination end office=s destination point code, i.e.,
246
1
3
, the tandem office's origination point code, i.e.,
246
1
2
, and the circuit identification code corresponding to the circuit between the tandem office
16
and the destination office
20
, e.g., circuit
7
. After this ISUP message is sent to the signaling transfer point
18
, the signaling transfer point
18
forwards the ISUP message to the destination end office
22
in order to setup the connection between the tandem office
16
and the destination office
22
, thus reserving the circuit. The terminating central office switch
22
receives the SS7 message and determines where to terminate the call by interpreting the called number embedded in the protocol.
A call flow scenario is now described with reference to
FIG. 2. A
caller
19
dials the telephone number of a destination
23
. The first end office
20
(end office A) collects the digits of the called number and checks routing tables to determine to which end office
22
the dialed telephone number belongs. Then the originating end office
20
finds a direct trunk group
14
between itself and the end office owning the dialed telephone number. Subsequently, the originating end office finds an idle trunk within the trunk group
14
. The originating end office
20
selects and reserves the idle trunk of the trunk group
14
and initiates an SS7 IAM (initial address message) message containing the following: signaling transfer point routing address of the destination end office; the calling telephone number; the called telephone number, and the trunk ID (CIC) for the selected trunk of the trunk group.
The signaling transfer point
18
receives the IAM message and forwards it to the destination end office
22
. The destination end office
22
then receives the IAM message and uses the CIC information to reserve the selected trunk within the trunk group
14
. The destination end office
20
(end office B) then checks the called telephone number
23
for on-hook and feature support and holds the line, assuming the dialed telephone number is on hook. The destination end office
22
then applies a ring to the line and ring tone to the selected trunk in the trunk group
14
. Next, the destination end office
22
connects the dialed telephone number line
Allen, Jr. George C.
Bi Haifeng
Partridge Steven R.
Sigarto Samuel
Stephenson Richard W.
Greenblum & Bernstein P.L.C.
Harper Kevin C.
SBC Technology Resources Inc.
Vu Huy D.
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