GB2201564A - Telephone exchange systems - Google Patents

Abstract

Where existing and relatively modern exchanges such as TXE4A (or TXE4) are used in a network which includes more modern exchanges such as System X, it is desirable to up-date the older exchanges to improve the speed and quality of communications during set-up. This includes providing them with common channel signalling for use during call set up, while retaining methods used in the older exchanges for call supervision. <IMAGE>

Description

TELEPHONE EXCHANGE SYSTEMS The present invention relates to automatic telephone exchange systems, and especially to signalling arrangements in such systems.

According to the invention there is provided an automatic telephone exchange system, which includes a plurality of similar analogue telephone exchanges, in which when a connection is set up between two of said analogue exchanges speech is conveyed therebetween in analogue or digital manner and signalling is effected therebetween in common channel signalling (CCS) manner, and in which said CCS is only used during the establishment of a connection which involves a pair of said analogue exchanges while call supervision is effected in the manner normally used for connections involving lines served by such exchanges.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which Figs. 1 and 2 show, for explanatory purposes, known arrangements for signalling between an analogue exchange and a digital exchange, and Figs. 3 and 4 show arrangements according to the invention for signalling between two analogue exchanges.

Fig. 1 shows in highly schematic form how an analogue exchange, such as one of the TXEA or TXE4/A type, and a digital exchange, such as one of the System X type, are interconnected, with special reference to the signalling between those exchanges.

When a single channel is used for a number of speech channels between two telephone exchanges as mentioned above, each speech circuit is transmitted as a 64Kbit/sec channel of a 2048Kbit/sec bearer to the associated digital exchange. Each such speech circuit is produced by sampling the speech waveform at 8Khz/sec, and then expressing each sample thus produced by an 8 bit binary digital combination. The bit stream forming the 2048Kbit/sec bearer is produced by a standard primary multiplexer, with its associated signalling equipment.

Such a system has 32 digital channels, which is used to support 30 speech channels, one synchronising channel, usually time slot 0 (TSO), and one signalling channel, usually time slot 16 (TS16). The line signalling for such a system is conveyed as continuous signals in TS16. This slot is used in effect as a sub-multiplex with 30 channels, each associated respectively with one of the speech channels. This is a special form of common channel signalling, known as channel associated signalling (CAS).

Before the introduction of what we herein regard as t true common channel signalling (CCS), the line signalling information is transmitted via To16 of the 2048Kbit/sec bit stream. Thus, see Fig. 1, the signalling origination at the main control unit 1 of the TXE4A exchange and passes via an outgoing junction circuit 2 to a PCM circuit 3, where it is encoded into TS16 for transmission to the System X exchange. At the System X exchange it is received in the switching network 4 from which it passes to a TS16 extraction circuit 5.

The latter extracts the signalling for use as appropriate in the System X exchange.

Fig. 2 refers to the condition when true CCS, as explained below is introduced. When CCS is introduced, the 2048Kbit/sec bit stream is passed through a channel extractor inserter (CXI), in which the line signalling information is extracted from TS16 and used by the associated CCS facility (CCF). The equivalent information is sent out as messages to the message transmission sub-system (MTS), with each message "labelled" to identify the call to which it relates.

In Fig. 2, the PCM equipment passes the bit stream to line via the CXI 6, which extracts what is present in TS16 and passes it to the CCF7. This is controlled from the MCU1 via a set-up call handler 8, which passes to CCF the call identity information referred to above. The CCF6 assembles the messages, and sends them via CXI6 to line. Hence the signalling data passes via PCM3, CXI6, to CCF7, from which it is sent back by CCF to CXI as labelled messages. These pass as before to the DSS block 4 from which they pass to the MTS block 9. Thus the signalling is conveyed from CCF to MTS, so these units are shown as connected by dotted lines - labelled CCS (logicial) link.Thus we have the signalling conveyed by what is in effect a virtual link which is logically separate from the speech path, even though it may follow the same physical path.

Thus TS16 is used in a manner which bears some resemblance to that in which it is used for CAS, but more efficiently. Thus a single CCS link can support a much larger number of speech circuits than is the case for the CAS approach discussed above. Since, as indicated, the system is message based with each message labelled with a routing label, CCS messages can follow totally different paths from the speech path. In such case the signalling is normally switched through a signalling transfer point (STP) using a technique known as quasi-associated signalling (QAS). This is described in our Patent No.

2144601B (G. Harland 1).

We now consider the case arrangements which embody? the present invention, where two like exchanges, in this case of the TXE4A type have to be interconnected, again with the use of CCS. This use of CCS is desirable since, as already mentioned, the existing exchanges, in this case assumed to be of the TXE4A type, have to co-exist with more modern exchanges such as System X exchanges. Furthermore, CCS is faster than some other signalling systems, even those of fully electronic systems.

When two similar exchanges are connected together it is possible to exploit the fact that their "architectures" are similar to reduce some of the complexity associated with the conversion to CCS. Thus when the use of CCS is to enhance TXE4A (or TXE4), it is possible to exploit the advantages of CCS without having to convert the analogue circuitry to digital working.

CCS is performed on these circuits while the call is being set up, but the system relies on conventional line supervision once call set up is complete. That is, the supervision is based on monitoring the looped or unlooped states of the lines in the connection, plus any signals (such as answer) which may occur.

Thus in Fig. 3 we see the case in which the speech is conveyed in analogue manner between the junction circuits 11, 12. At each exchange, the junction circuit is connected during call set up via an MCU 13, and a set-up call handler 14 to the common channel processor 15. With the CCS actually conveyed over the same physical path as the speech, we have a virtual link between the two CCFsj as indicated by the double broken lines. The system is also regarded as having a virtual CXI16 associated with each CCF.

Where the two like exchanges, TXE4As in this case, are connected for speech digitally we get the selection represented in Fig. 4. This only differs'from Fig. 3 in that PCM terminations such as 17 are present, and the PCM-PCM link uses digital line supervision. This is effected by the conversion of the analogue link signalling conditions into 4 bit signalling codes transmitted in timeslot 16 as described in the third paragraph on page 2.

CCS under these circumstances can be conveyed as quasi-associated signalling, but with suitable interfaces could use an independent 64Kbit/sec bearer.

Where it is conveyed with quasi-associated signalling, each message is a labelled message, sent over an independent path via intermediate STP. Note, however, that if the circuits between TXE4A exchanges have already been converted into PCM, the interfaces at each end of the connection are still analogue.

Analogue CCS is inherently simpler than its digital equivalent, which can be exploited in three ways: (a) as no digital conversion is performed on line signalling, no CXI is needed for the extraction of line signals, and the cost per installed circuit is greatly reduced; (b) the processing power per call is less, since no processing of the CXI is needed, and no call processing is needed once call set up is complete; (c) supervision of point-to-point connections is marginally faster and can occur independently of call set up.

If CCS communication between two TXE4A exchanges is lost, a facility of "fall-back" to conventional address signalling is used, which provides more security against loss of calls than for a digital CCS version.

Finally we give a few general comments regarding the system described herein.

lthough the 'logical' link referred to follows the same physical path through the network, it is still separate from the speech path a) In space because it is only passed through one of the many PCM systems linking the two exchanges.

b) In tire within the above PCM systems because it is only transmitted during Timeslot 16.

Quasi-associated signalling is only a special case in respect of-the signalling path being separated as (a) from ALL of the PCM systems or analogue junctions linking the two exchanges.

The advantages of CCS are not just compatibility but speed and communication capabilities.

Once CCS is introduced to a part of the network, calls through that network can be set up in milliseconds rather than seconds and other information (such as Calling Line Identification) can be conveyed between exchanges. The phrase that summarises the above is that TXE4A or TXE4 thus 'looks like' a new network exchange.

Claims (4)

1. An automatic telephone exchange system, which includes a plurality of similar analogue telephone exchanges, in which when a connection is set up between two of said analogue exchanges speech is conveyed therebetween in analogue or digital manner and signalling is effected therebetween in common channel signalling (CCS) manner, and in which said CCS is only used during the establishment of a connection which involves a pair of said analogue exchanges while call supervision is effected in the manner normally used for connections involving lines served by such exchanges.

2. A system as claimed in claim 1, in which speech is conveyed between a said pair of exchanges in analogue manner so that the speech passes directly between the junction circuits of the two exchanges, and in which the signalling is controlled by digital processors at each said exchange.

3. A system as claimed in claim 1, in which speech is conveyed between the exchanges digitally and in PCM manner.

4. An automatic telephone . exchange system, substantially as described with reference to Fig. 3 or Fig. 4 of the accompanying drawings.