GB2185657A - Automatic telecommunication system - Google Patents

Abstract

Digital multiplex local lines (e.g. from ISPBX or an ISDN multiplexer) to a relatively modern analogue exchange (e.g. TXE4 or TXE4A) (6) are connected to a multiplex unit (5) which separates out the voice channels which terminate on the analogue exchange (6) and converts them to analogue form. Then they are applied to line circuits on the analogue exchange and the calls set up therethrough in normal manner. Digital channels on the same local line not stopping at the analogue exchange are routed to other multiplex units (9, 10) so that they bypass the analogue exchange. Some junctions from the analogue exchange go via suitable conversion (11) to digital junctions. The analogue exchange also serves analogue lines and junctions. The digital lines also pass via channel extraction and insertion units (2, 12, 13) to extract and insert as appropriate signalling messages conveyed in channel 16 of the multiplex junctions. These units are connected to a local area network (4) which also interconnect the analogue exchange control units, as in Application No. 2143403A (M. B. Kelly 1). <IMAGE>

Description

SPECIFICATION Automatic telecommunication system This invention reiates to automatic telecommunication systems, and especially to such systems where access is needed for digital local lines to non-digital exchanges.

Such a requirement exists in Great Britain with special reference to a system which includes both relatively modern analogue exchanges such as TXE4 and TXE4A, with an increasing number of digital exchanges, such as System X exchanges.

However, the invention to be described herein is applicable to systems using other types of analogue exchanges and digital exchanges.

An object of the invention is to provide a telecommunication exchange in which digital access is provided for in an economical manner.

According to the invention, there is provided an automatic telecommunication system, in which an analogue telephone exchange has to interface with digital local lines, in which the said lines incoming to the exchange each convey in TDM-PCM form different channels some of which are voice channels over which voice signals are conveyed in digital format, which voice channels are directed to the analogue exchange, and some of which channels are digital channels not directed to the analogue exchange, in which a said incoming digital local lines is coupled to a first multiplex unit which separates out the voice channels from the digital channels, in which the voice channels are connected via digital to analogue and analogue to digital conversion means to line circuits on the analogue exchange, the calls to which intelligence received over said voice channels relate being set up via the analogue exchange in the manner appropriate to that analogue exchange, in which the separated-out digital channels extend to second multiplex units of types appropriate to the intelligence conveyed in those channels, which second multiplex units give access to digital junctions leaving the exchange, so that said separated out digital channels by-pass the analogue exchange, in which some at least of the analogue junctions which leave the analogue exchange are connected via further conversion means to third multiplex units which in turn give access to multiplex digital junctions outgoing from the exchange, and in which the analogue exchange also has analogue subscribers lines and junctions terminated on it.

An embodiment of the invention will now be described with reference to the accompanying highly schematic block drawing.

With a relatively large number of modern analogue exchanges in the network, digital access to the analogue telephone network and packet switched system (pss) is a reasonable and economical alternative to full ISDN (Integrated Services Digital Network) access. Primary rate access, either from PABXs or from multiplexers, is often attractive compared with 144 kbit's direct access to local exchanges. The present arrangement takes advantage of both of these trends and is a natural extension of existing TXE4/A enhancement arrangements, such as those described in our Application No. 2144601A(G. Harland 15) and No.

2159367A (D. A. Weir- G. Harland 7S16).

The accompanying drawing shows the present arrangement, in which each 2 Mbitls local line such as 1 terminates on a PCM primary multiplex equipment via a CXI (Channel Extraction- Insertion) card 2. Such a local line extends to an ISPBX (Integrated Services PBX) orto an ISDN multiplexer. As used herein, the CX12 diverts time slot 16 (both directions) to a CCF (Common Channel Facility), in this case the DASS-CCF 3. The abbreviation DASS refers to the British Telecom Digital Recess Signalling System. In this case, the CCF 3 has software dedicated to the BT protocol DASS 2 (or later an internationally agreed version), instead of Signalling System No.7.The DASS CCF accesses the other enhancement subsystems via the LAN (Local Area Network) 4 as in the system of our Application No. 2143403A (M.B. Kelly 1).

Such a 2 Mbit/s local line carries a number of 64 kb/s channels in TDM-PCM manner. In a 32 channel system channel 1 is used for synchronisation and channel 16 (mentioned above) for signalling, in common channel manner. Any desired proportion of the 64 kbit/s channels on the local line are provided with A-D and D-A conversion in the PCM MUX 5, so as to interface to normal subscriber line circuits to provide telephony service. The analogue exchange in this case uses a TXE4A switching network 6, to which there are also connected analogue subscribers' lines and junctions. The remainder of the channels provided by the 2Mb/s local line are connected as 64 kbit/s digital circuits such as 7,8 to other primary multiplex equipments such as 9,10.Some can be connected to Kilostream multiplexers, e.g. to provide digital private circuits and/or direct PSS access. Others go to junction primary multiplexers, e.g. 10, to provide access to a digital exchange such as a Digital Principal Local Exchange (DPLE) for circuit switched digital service or switched access to PSS.

The junction multiplexers such as 10,11 collect together the digital channels from a number of local line multiplexers. If necessary voice junctions to/ from the TXE4/A switch could also be mixed in, instead of separately as shown for MUX 11. The 2 Mbit/s junction lines to DPLE's are terminated on the MUX via a CXI 12 or 13, as normal for a CCS equipped TXE4/A exchange. The supporting CCF 14, which in this case serves the two CXl's 12 or 13 handles Signalling System No. 7 as normal.

However, since the digital channels are effectively 'straight through', it may be preferable for the CCF to use DASS 2 to the DPLE. In this case, it could be the same CCF as that which supports digital access 2 Mbit/s lines, which is not able to support voice junctions also.

The digital access multiplexer 5 only requires simple signalling units to interface to the TXE4/A line circuits (not shown). They are simply required to loop the pair, to create a calling condition on originating voice calls, and to trip the ringing on terminating calls. A simple type of incoming loopdisconnect signalling unit is thus suitable. Earth calling is probably not necessary, since contention is sorted out by the DASS CCF.

It is possible with such a system to do away with dedicated DDI (Direct Dialling Into an associated PABX) lines, since the DASS signalling handles the transfer of address information to the PABX. The resultant cost saving, since no expensive Outgoing Junction (OGJ) relay sets are required, helps to pay for the PCM multiplex equipment which is not required in a digital local exchange.

Operation on voice calls would be very similar to enhanced TXE4/A CCS operation, as described briefly in certain of the above-mentioned applications and a paper entitled TXE4 Switching Network, by G. Harland and J. P. Ronayne, Electrical Communication. Vol. 51, No.4, 1976, pp 223--228.

Receipt of an Initial Service Request Message (ISRM) in the DASS CCF 3 causes that CCF to allocate to that message's call a channel connected to a TXE4/A line circuit. The CCF also causes the PCM MUX 5 to apply a calling loop, which is detected in the Cyclic Store of the TXE4/A, and broadcast to Main Control Units (MCUs) such as 15 as for a normal subscriber calling condition. The allocated MCU detects the calling condition and, on seeing the digital access class of service, refers to the DASS CCF 3 via Set-up Call Handler (SCH) 16 and the LAN 4.

Called address information from the DASS CCF 3 causes the call to be set up in the normal way. This follows normal TXE4-type operation, as described, for instance, in the above mentioned paper.

Terminating voice calls are set up to a free line circuit via a bridge link in the normal way, although it may be necessary for the MCU such as 15 to refer to the DASS CCF 3 to resolve contention on bothway channels. On detection of the digital access class of service, the MCU passes call set up information, including any DDI address, to the DASS CCF 3 for transmission to the subscriber in Channel Seized (CS) and incoming Call Indication (icy) messages.

The DASS CCF 3 then causes the MUXto trip the ringing from the bridge link, either immediately on seizure or, possibly, on called subscriber answer (CSA). The latter allows call logging to be performed as normal but the sustained ringing voltage might not be acceptable to the MUX. The former requires the DASS CCF 3 to advise a suitably primed Call Monitoring Handler (CMH) 17 of the arrival of the Call Connected Message (CCM).

Digital call attempts do not involve an MCU, but the DASS CCF 3 has to convert DASS messages into a form which a SS No.7 CCF could understand; and vice versa. Alternatively, a DASS CCF has to reassemble DASS messages from the several local access lines onto time slot 16 of a 2 Mbit/s line to the DPLE; and vice versa.

If necessary, the DASS CCF such as 3 for the local line can hold call records for digital connections and pass them to the Charge Determination Facility (CDF) 18 for call logging purposes. Alternatively, call logging might be left to the parent DPLE.

Numbering is inevitably a problem where only the voice calls are switched by the TXE4/A exchange. channels should carry the same directory number.

Since the voice channels terminate on the TXE4/A switch, the directory numbers should be within the number range of that exchange. This requires the DPLE to analyse the Service Indicator Code on terminating calls and to route voice calls over normal junctions to the TXE4/A exchange. Digital calls would need to be routed to the appropriate channels in those 2 Mbit/s junction systems which carry the dedicated digital channels. The DPLE would therefore have to hold a table of all digital access subscribers on TXE4/A exchanges served by that DPLE.

An alternative, which would not be strictly in accordance with the ISDN standards, would be to allocate separate directory numbers (DNs) to voice channels and digital channels. Such a voice channel would have a TDE4/A DN, while the associated digital channels would have a DN appropriate to the parent DPLE. A variation on this theme is to allocate a separate block of numbers from the TXE4/A number range, especially for digital channels. This could be easily recognised by the DPLE, for special attention.

In the longer term, it may be possible to consider switching of channels within the multiplexer, under a sub-system control. This would place the final selection function within enhanced TXE4/A but would still require intelligent routing in the DPLE to select an appropriate 2 Mbit's junction system.

The arrangement described above requires little or no hardware development beyond that already being done for TXE4/A enhancement. Since 2 Mbit/s access uses the normal CCITT primary multiplex standards, the present CXI works without difficulty.

A normal CCF replica (or several replicas if necessary) would support the access CXls as normal but using appropriate DASS software.

The current generation of PCM primary multiplex equipments allows a maximum of only 6 channels to be made available in 64 kbit/s digital form, the remainder being available only as analogue voice channels which could be terminated on the TXE4/A switch. This provision of digital channels could well be adequate for many PABX access lines but would not be sufficient for remote (or co-located) multiplexer access, where only B-channel from each 80 kbit/s or 144 kbit/s basic access may be required for voice traffic. Latest designs of MUX should relieve this constraint.

The analogue interface to the TXE4/A is normal subscriber line circuits but they have a special class of service in the Cyclic Store. As indicated earlier, normal loop-disconnect incoming junction signalling units can be used in the MUX, assuming that they would not malfunction when faced with ringing voltage from the TXE4 orTXE4/A bridge link.

The simplest 2-wire version available would be used.

The DASS CCF is required to support DASS 2 controlling software, including the ability (not used in SS No. 7) to select a channel on originating call attempts. Note that strictly speaking ISDN access does not allow channels to be dedicated to voice or to make such a selection based on the Service Indicator Code in the ISRM.

The DASS CCF should require only simple software to control the PCM primary MUX signalling units. Application of the loop condition at the appropriate time on both originating and terminating calls, is the basic requirement.

Detection of the answer reversal on originating calls will also be necessary. Blocking of channels by Pwire control will be a desirable ancilliaryfunction.

The DASS CCF will be required to communicate with the appropriate SS No.7 CCF, in addition to communication with the MCU via the SCH as is currently required of CCFs. It may also be required to hold call records for digital calls and to communicate directly with the CDF.

Claims (4)

1. An automatic telecommunication system, in which an analogue telephone exchange has to interface with digital local lines, in which the said lines incoming to the exchange each convey in TDM-PCM form different channels some of which are voice channels over which voice signals are conveyed in digital format, which voice channels are directed to the analogue exchange, and some of which channels are digital channels not directed to the analogue exchange, in which a said incoming digital local line is coupled to a first multiplex unit which separates out the voice channels from the digital channels, in which the voice channels are connected via digital to analogue and analogue to digital conversion means to line circuits on the analogue exchange, the calls to which intelligence received over said voice channels relate being set up via the analogue exchange in the manner appropriate to that analogue exchange, in which the separated-out digital channels extend to second multiplex units of types appropriate to the intelligence conveyed in those channels, which second multiplex units give access to digital junctions leaving the exchange, so that said separated out digital channels by-pass the analogue exchange, in which some at least of the analogue junctions which leave the analogue exchange are connected via further conversion means to third multiplex units which in turn give access to multiplex digital junctions outgoing from the exchange, and in which the analogue exchange also has analogue subscribers lines and junctions terminated on it.

2. A system as claimed in claim 1, in which some at least of said digital lines are each connected to the appropriate said multiplex unit via a channel extraction and insertion (CXI) unit, which extracts from the incoming bit stream signalling messages conveyed in a dedicated one of the TDM channels, said CXI unit also inserting such signalling messages into said dedicated channel for the outgoing bit stream.

3. A system as claimed in claim 2, in which the CXI units for said multiplex units are connected via common channel facilities (CCF) units to a local area network (LAN) which interconnect the control units of the analogue exchange.

4. A system as claimed in claim 1,2 or 3, and in which the analogue telephone exchange is of the TXE4 or TXE4A type.