GB1566913A - Automatic telecommunication exchanges - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO AUTOMATIC TELECOMMUNICATION EXCHANGES (71) We, STANDARD TELE PHONES AND CABLES LIMITED, a British Company, of 190 Strand, London, W.C.2., England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to an automatic telecommunication exchange of the processor-controlled type.

Our rior application No. (Serial No.

1552812 40184/76 and No. (Serial No.

1552813 04854/78 describe telephone exchanges in which relatively simple modularisation concepts are applied, there being a single group exchange and a multi-group exchange described therein. In the present application we have extended the techniques used in the above applications to the provision of an improved method of switching for four-wire circuits, and to the handling of digital subscribers.

According to the present invention there is provided an automatic telecommunication exchange in which the subscribers' lines served include two-wire lines and four-wire lines, each of the latter having separate GO and RETURN paths, and junctions incoming to and/or outgoing from the exchange include four-wire junctions, each of the latter having separate GO and RETURN paths, in which the lines and junctions are served by a switching array served by one or more processors, in which the lines and junctions are connected to the outlets of the first stage of the switching array, in which the inlets to the last stage of the switching array are interconnected via links used for connections to be set up via the exchange, in which for a connection between four wire lines and/or junctions two connections are set up separately under the control of the appropriate processor or processors, each such connection using a different one of said links, and in which for a connection between a two-wire line or junction and a four-wire line or junction a connection is set up from the two-wire line or junction to a two-wire to four-wire conversion circuit such as a hybrid and two separate connections are set up from said conversion circuit to the four-wire line or junction.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a single-group telephone exchange of the type described in our abovementioned Application No. 40184/76, (Serial No. 1552812).

Figure 2 shows schematically a pluralgroup telephone exchange of the type described in our above-mentioned Application No. 40184/76, (Serial No. 1552812).

Figure 3 shows how a transit connection between two four-wire junctions is set up via an exchange of the type shown in Figure 2.

Figure 4 shows how a two-wire to fourwire connection is set up via an exchange of the type shown in Figure 2.

Figure 5 shows how "digital" subscribers are dealt with in an exchange such as that of Figure 2.

As in the above-mentioned applications, the "system architecture" is based on unitisation, the equipments associated with the various groups in a multi-group exchange being identical. In the exchanges described herein the basic unit is approximately 200 lines, plus peripherals such as VF receivers, MF receivers and/or senders, test access circuits, coin and fee check circuits and such other facilities circuits as are provided. The peripherals are spread over the groups in a multi-group exchange for even loading of the groups, with inter-group access as required.

Exchange control uses microprocessors, e.g. the INTEL 8080, the processors being duplicated per group and working in the load-sharing mode. Each line group is selfcontained in that it provides for directory number-equipment to number translation and class-of-service determination, plus call processing and route selection for a singlegroup exchange. In a multi-group exchange for inter-group connections and outgoing connections where selection digits may have to be translated, a further pair of processors also working in load-sharing mode is used.

These are known as back-end processors, by contrast with the per-group processors, which are known as front-end processors.

The single-group exchange, Figure 1, has two stages B and C of co-ordinate matrices of sealed-contact reed relays, with the lines and other peripherals connected as shown to B stage outlets. Links, one of which is used for each connection, are connected to the C stage inlets, each link being bi-directional, this allowing good freedom of route choice, and each link also being usable either as a through link or as a bridge link (i.e. one with power feed).

The control equipment includes processors PRA, PRB, which normally work in load-sharing mode, but if one fails the exchange still functions, albeit with a reduced grade of service. Associated with each processor are a line scanner LS1, LS2 and a link and peripheral scanner LPS1, LPS2. Note that some at least of the peripherals appear in both types of scanner.

There is also a marker MK for controlling path setting in the switching network.

In a multi-group exchange, Figure 2, each group is similar to Figure 1 except that the odd-numbered C switch inlets are each connected to one end of a link, the other ends of the links being connected via a further stage, D, of switches to evennumbered inlets of the C switches. These links are used for both inter-group and intra-group connections. The D switches are controlled by the back-end processors BPA, BPB, each associated with a D switch scanner DS1, DS2 to scan D switches for path choice. There is also a D switch marker DM1 for path setting via D switches. The links are, as in Figure 1, "universal" and reversible, so connection between C switch inlets can be sat up via a link and a D switch or vice-versa.

On call initiation. the call is detected via one of the line scanners LS1, LS2, serving the caller's group and the calling condition and caller's identity are reported to the processor PRA or PRB for that scanner. A signal is then sent to the scanner to remove the calling line from the scanning cycle, and the processor selects a free link for the call via the appropriate scanner, LPS1 or LPS2.

The identities of the caller and the link are applied to the group's marker MK, which selects and establishes a path through the B-C network between the calling line and the link. The caller now receives dial tone and sends the wanted number, which is assembled via the scanner LPS1 or LPS2 of the group from which it is passed to the processor. If the caller has a VF push button set, a condition recognisable from the caller's class of service, the processor selects a free push-button receiver, which is connected to the link seized for the call via the switching network. The wanted number after reception in the receiver is transferred to the processor.

When the wanted number has been assembled in the processor the latter causes the link to be connected to the called line or a junction with access thereto, in the same way as the caller was connected to that link.

In some cases, the initially-seized link may be dropped after a different link has been seized, as in the above-mentioned application.

Having described the basis of the exchanges shown in Figures 1 and 2, we now deal with the provisions according to the present invention whereby four-wire lines or junctions are handled. The following description with reference to Figures 3-6 assumes that the exchange is a multi-group one, but the operation is very much the same in the case of single-group exchanges, except that it is somewhat simpler.

The provision of facilities for four-wire connections enables the system's capabilities to be extended as compared with the exchanges described with reference to Figure 1 to deal with: (a) toll transit working using four-wire switching (b) connections from two-wire to fourwire, as in a connection from a two-wire junction or line to a four-wire trunk or junction or line.

(c) the mixed working of two-wire and four-wire circuits in the same switching network for all calls.

The switching network, B-C stages in a single group exchange or B-C-D stages in a multi-group exchange, which handles twowire connections is used to set up four-wire connections by setting up two paths between the devices to be interconnected. This mixes the two-wire and the four-wire traffic, thus avoiding the need to provide separate switching groups which is not efficient. As will be seen from Figures 3, 4 and 5, this merely involves a single type of peripheral circuit which provides four-wire to two-wire and two-wire to four-wire conversion using compromise balance in the hybrids or their equivalents. In addition, built-in gain can be provided to compensate for losses in the hybrids. This maintains system stability, and provides for the various types of connection without having to provide special links.

Figure 3 shows simple four-wire transit working, in which two four-wire junctions are interconnected via separate paths, using different links L1 and L2. These connections are set up under processor control as if they were separate connections. For this purpose such a four-wire junction has two appearances in the B stage of its group, one for its GO path and the other for its RETURN path, which appearances can be in the same or in different B switches. One of the connections which is set up is between the GO path of one junction and the RETURN path of the other junction, the other of the two connections being between the GO path of that other junction and the RETURN path of that one junction.

A connection between two four-wire subscribers' lines, or a connection between a four-wire line and a four-wire junction is set up in the same way as that just described for a transit connection between four-wire junctions. This would apply in particular in a special purpose or military installation where atl subscribers' lines are four-wire lines.

Figure 4 shows how a two-wire junction is connected to a four-wire junction. This involves connecting the two-wire junction via a switched path including a first link L3 to a two-wire to four-wire converter (hybrid) H. Then the converter H is connected via separate "GO" and "RETURN" paths to the four-wire junction, these two paths using separate links L4, L5. The provision of attenuation and amplification where necessary is indicated by the references to +3db associated with the converter H. Note that a similar technique is used to connect a two-wire line to a four-wire line, or a two-wire line to a four-wire junction, or a two-wire junction to a four-wire line.

Figure 5 shows how digital connections are set up, most of them being four-wire connections for digital communications. It will be appreciated that for most digital systems the transmission characteristics of reed relays are adequate, since tests have shown that reeds can handle signals in excess of 1MHz. This may lead to the Stone-type feeding bridges being preferred to the Hayes type, or alternatives for peripheral circuits which can handle the relatively high band-width of the digital stream, the links in this case being through links.

In the system shown some of the subscribers have digital sets in addition to audio sets, with the four-wire sets (usable either for digital representation of speech or for data) coupled to the exchange via coder/ decoders CODEC, digital transmission being assumed to use 64 kb/s. Each such line is connected to a normal line circuit LC and to a digital interface I/F, as shown. One or other of these is disabled depending on circumstances. With this arrangement, the subscriber can set up a connection as a two-wire audio subscriber using loop disconnect signalling. Then he has the ability to indicate his desire for digital transmissionsubscriber-controlled class of service - to set up the desired connection in four-wire manner via all-digital channels to a subscriber, junction or information source.Switch over from speech to data, or in the reverse direction, is much as in the present DATEL service, or as in the system of our ADplica- tion No. 929/75 (Serial No. 14620035. In a similar way, the provision of suitablyequipped terminals enables speech to be conveyed digitally, which improves security of communication.

Synchronisation is, in the case of calls incoming over the PCM junction shown at the bottom left of Figure 5 derived from the PCM MUX for subscriber to PCM junction connections. In the case of subscribersubscriber 64 kb/s connections the synchronisation is derived from the exchange. The synchronisation and power are sent on the balanced phantom, as can be seen from the insert to Figure 5, being extracted therefrom at the terminal. Thus a digital terminal is synchronised from the exchange or multiplex, with an interface to the multiplex to permit the necessary speed changes.

The subscriber's set provides the necessary capabilities for the function referred to: thus a two-wire line would cater for 64 kb/s by a hybrid at each end, in which case synchronisation is on the phantom to earth.

However, in Figure 5, a digital subscriber employs a four-wire line, and the exchange is instructed by such a subscriber when a digital connection is needed in the call preamble, e.g. by the use of the recall code plus a code indicating the requirement.

Note that in the exchange of Figure 5, normal speech calls would be set up in two-wire manner, as in the abovementioned Application, except for such differences as are caused by the use of digital transmission. As the penetration of digital facilities into the exchange increases, a cross-over point comes in the economicsbased comparision of space and time switching which favours time switching. When this occurs, or is about to occur, time digital groups are introduced, with mixing via the D switches.

WHAT WE CLAIM IS: 1. An automatic telecommunication exchange in which the subscribers' lines served include two-wire lines and four-wire lines, each of the latter having separate GO and RETURN paths, and junctions incoming to and/or outgoing from the exchange which include four-wire junctions, each of the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. without having to provide special links. Figure 3 shows simple four-wire transit working, in which two four-wire junctions are interconnected via separate paths, using different links L1 and L2. These connections are set up under processor control as if they were separate connections. For this purpose such a four-wire junction has two appearances in the B stage of its group, one for its GO path and the other for its RETURN path, which appearances can be in the same or in different B switches. One of the connections which is set up is between the GO path of one junction and the RETURN path of the other junction, the other of the two connections being between the GO path of that other junction and the RETURN path of that one junction. A connection between two four-wire subscribers' lines, or a connection between a four-wire line and a four-wire junction is set up in the same way as that just described for a transit connection between four-wire junctions. This would apply in particular in a special purpose or military installation where atl subscribers' lines are four-wire lines. Figure 4 shows how a two-wire junction is connected to a four-wire junction. This involves connecting the two-wire junction via a switched path including a first link L3 to a two-wire to four-wire converter (hybrid) H. Then the converter H is connected via separate "GO" and "RETURN" paths to the four-wire junction, these two paths using separate links L4, L5. The provision of attenuation and amplification where necessary is indicated by the references to +3db associated with the converter H. Note that a similar technique is used to connect a two-wire line to a four-wire line, or a two-wire line to a four-wire junction, or a two-wire junction to a four-wire line. Figure 5 shows how digital connections are set up, most of them being four-wire connections for digital communications. It will be appreciated that for most digital systems the transmission characteristics of reed relays are adequate, since tests have shown that reeds can handle signals in excess of 1MHz. This may lead to the Stone-type feeding bridges being preferred to the Hayes type, or alternatives for peripheral circuits which can handle the relatively high band-width of the digital stream, the links in this case being through links. In the system shown some of the subscribers have digital sets in addition to audio sets, with the four-wire sets (usable either for digital representation of speech or for data) coupled to the exchange via coder/ decoders CODEC, digital transmission being assumed to use 64 kb/s. Each such line is connected to a normal line circuit LC and to a digital interface I/F, as shown. One or other of these is disabled depending on circumstances. With this arrangement, the subscriber can set up a connection as a two-wire audio subscriber using loop disconnect signalling. Then he has the ability to indicate his desire for digital transmissionsubscriber-controlled class of service - to set up the desired connection in four-wire manner via all-digital channels to a subscriber, junction or information source.Switch over from speech to data, or in the reverse direction, is much as in the present DATEL service, or as in the system of our ADplica- tion No. 929/75 (Serial No. 14620035. In a similar way, the provision of suitablyequipped terminals enables speech to be conveyed digitally, which improves security of communication. Synchronisation is, in the case of calls incoming over the PCM junction shown at the bottom left of Figure 5 derived from the PCM MUX for subscriber to PCM junction connections. In the case of subscribersubscriber 64 kb/s connections the synchronisation is derived from the exchange. The synchronisation and power are sent on the balanced phantom, as can be seen from the insert to Figure 5, being extracted therefrom at the terminal. Thus a digital terminal is synchronised from the exchange or multiplex, with an interface to the multiplex to permit the necessary speed changes. The subscriber's set provides the necessary capabilities for the function referred to: thus a two-wire line would cater for 64 kb/s by a hybrid at each end, in which case synchronisation is on the phantom to earth. However, in Figure 5, a digital subscriber employs a four-wire line, and the exchange is instructed by such a subscriber when a digital connection is needed in the call preamble, e.g. by the use of the recall code plus a code indicating the requirement. Note that in the exchange of Figure 5, normal speech calls would be set up in two-wire manner, as in the abovementioned Application, except for such differences as are caused by the use of digital transmission. As the penetration of digital facilities into the exchange increases, a cross-over point comes in the economicsbased comparision of space and time switching which favours time switching. When this occurs, or is about to occur, time digital groups are introduced, with mixing via the D switches. WHAT WE CLAIM IS:

1. An automatic telecommunication exchange in which the subscribers' lines served include two-wire lines and four-wire lines, each of the latter having separate GO and RETURN paths, and junctions incoming to and/or outgoing from the exchange which include four-wire junctions, each of the

latter having separate GO and RETURN paths, in which the lines and junctions are served by a switching array served by one or more processors in which the lines and junctions are connected to the outlets of the first stage of the switching array, in which the inlets to the last stage of the switching array are interconnected via links used for connections to be set up via the exchange, in which for a connection between four wire lines and/or junctions two connections are set up separately under the control of the appropriate processor or processors, each such connection using a different one of said links, and in which for a connection between a two-wire line or junction and a four-wire line or junction a connection is set up from the two-wire line or junction to a two-wire to four-wire conversion circuit such as a hybrid and two separate connections are set up from said conversion circuit to the four-wire line or junction.

2. An exchange as claimed in claim 1, and in which the switching array consists of a single plural-stage switching network served by two processors operating in a load-sharing manner.

3. An exchange as claimed in claim 1, in which the switching array consists of a plurality of plural stage switching networks each of which networks serves a different one of a plurality of groups of lines and/or junctions, and in which the links connected to the inlets of the last stages of the switching networks include links each of which interconnects two different ones of said networks. which links are used for inter-group connections.

4. An exchange as claimed in claim 1, 2 or 3, and in which the subscriber's lines served include digital lines.

5. An automatic telecommunication exchange substantially as described with reference to Figures 3, 4, 5 or 6 of the accompanying drawings.