GB2061063A - Telecommunications exchange - Google Patents

Abstract

A telephone exchange consists of a number of so-called layers, each of which includes one or more line switching units (LSU), on which the lines served terminate, and a single directional switching unit (DSU) on which junctions terminate. Each LSU has access to every DSU and each switching unit has its own control equipment LCE, CCE. The inter-unit connections, include signalling paths, and may use different channels on TDM-PCM links. To set up an "own exchange" call, any one of the DSU's may be used, the choice depending on, for instance, current traffic considerations. Thus a call other than a tandem call when set up crosses one only of the directional switching units. <IMAGE>

Description

SPECIFICATION Telecommunications exchange This invention relates to an automatictelecommuni- cation exchange which is of the modular type.

The application of modularity in exchange design is useful as it simplifies the problems of extending an exchange when the numbers of subscribers and junctions served increases during the life of the exchange. Another aspect of modularity is that it may allow units to be used in exchanges differing somewhat from the exchange for which these units were designed. For instance, an exchange may then be extended by the addition of more modern versions ofthe units originally used.

According to the invention, there is provided an automatic telecommunication exchange, which includes line switching units each serving a number of the subscriber's lines served by the exchange, each said line switching unit having its own control equipment, directional switching units to which junctions incoming to and/or outgoing from the exchange are connected, each said directional switching unit having its own control equipment and being associated with one or a group of said line switching units and the directional switching units such that each said line switching unit has access to every one of the directional switching units, wherein to set up a call between two of the lines served by the exchange the line switching unit serving one of the lines is connected via one of said links to one of the directional switching units and that directional switching unit is connected via another of said links to the line switching unit serving the other line involved in that connection, the connection being set up under the control of the control equipment of that one of the directional switching unit, wherein for an incoming junction call the directional switching unit at which the junction terminates is connected via a said link to the line switching unit serving the wanted line, the connection being effected under the control of that directional switching unit's control equipment, and wherein for an outgoing junction call the line switching unit serving the calling line is connected via one of said links to a directional switching unit with access to a free outgoing junction in the desired direction the connection being effected under the control of that directional switching unit's control equipment.

With such an arrangement it is possible to have one exchange design which caters for a range of local exchange sizes, and to extend a small or medium-sized local exchange so that it becomes a large local exchange. From the structural aspect an exchange embodying our invention consists of two or more layers, each of which layers includes a central portion or core, referred to above as a directional switching unit, and one or more line switching units. These layers are arranged to cooperate to form a unified exchange, and in this respect it differs from some previously-proposed multi-unit exchange configurations.

To connect a subscriber to a junction, the speech path crosses one directional switching unit and one line switching unit, only one directional switching unit being used even when the subscriber and the junction involved are on different layers. Similarly, an "own exchange" call, i.e. a call between two subscriber's lines connected to the same exchange, only extends through one directional switching unit.

This use of only one directional switching unit to set up a call economises on the capacity of the switching units, but also, and more important, it economises the occupancy of the processors in the control equipment. This follows since the processor occupancy when a call involves two layers is not much greater than when one layer is involved.

The various switching units are interconnected for signalling and management information by signalling paths separate from the speech paths, the necessary information being conveyed as formal messages using message transmission systems in the switching units. Where the speech paths and the signalling paths are time division multiplex channels, it will be appreciated that these different paths may be conveyed by different time channels on the same physical paths. The basis of this signalling is that no switching unit ever makes any assumption as to the internal operations of another switching unit.

Thus the internal messages are at a functional level.

With the use of such a signalling philosophy the layers do not have to be identical, i.e. they can be of different design generations and can use different processors. Thus an existing exchange can be extended by a new layer of more modern type than the layers already present. Further, the lineswitching units and the directional switching units do not have to be co-located, nor do the directional switching units have to be co-located.

An embodiment of the invention will now be described with reference to the drawing, in which Figures 1 and 2 are simplified block diagrams of a two-layer exchange. Figure 1 shows some of the inter-unit and inter-layer signalling paths while Figure 2 shows some of the inter-unit and inter-layer speech paths. Both types of path are of course present and are functionally separate from each other: however, when the exchange is fully digital using time division multiplex the two sorts of paths may be conveyed over the same physical paths.

General description In Figures 1 and 2 each layer includes one directional switching unit, DSU1, DSU2, and one our a group of line switching units (or concentrator modules) of which LSU1 is one of the units of the layer including DSU1 and LSUx is one of the units of the layer including DSU2. Each DSU includes, inter alia, a switching network DSS, which can be a single or plural stage arrangement dependent on traffic, control equipment CCE and a message transmission system MTS. A line switching unit also has a switching network CS, which again may be single or plural stage, a message transmission system MTS and control equipment LCE. Each LSU also includes line circuits such as LC for the subscribers' lines it serves, and each DSU is connected to a number of incoming and outgoing junctions as shown in Figure 2.The signalling paths also include paths to other exchanges (see Figure 1), which are functionally separate from the junctions' speech paths.

Figure 1 shows part of the network of signalling paths between the switching units, and also some of the signalling paths to other exchanges, while Figure 2 similarly shows some of the speech paths. Note that for both the signalling and the speech paths, each LSU has access to every one of the DSU's, and that each DSU has signalling connections to every other DSU. Further, each DSU has access via respective speech paths to every one of the other DSU's these paths being for transit calls. In the present system the exchange is fully digital, with speech conveyed in time division multiplex (TDM) using pulse code modulation (PCM), the signalling also being digital and of the common channel type.

However, the multi-layer approach on which this invention is based is also usable in non-digital exchanges, although it is less advantageous in such cases.

Each DSU belongs to a particular layer, and is referred to as the parent DSU for the LSU or the LSU's of that layer and for all subscribers' lines connected to those LSU's. The parent DSU "manages" its dependent LSU's and stores centralised data which is permanently associated with the subscribers connected to those LSU's. However, as will be seen, it does not organise all calls to be set up for its lines.

In the LSU's used in the exchange described herein, there are eight PCM highways from the switch CS so that with the constraint that each LSU-DSU connection uses at least two PCM highways (for security), the exchange can have up to four layers. For more than four layers, longer CS switches are needed, with more than eight PCM highways.

Note that the arrangement of LSU-DSU highways can be flexible, subject to the above-mentioned need to have two such highways between an LSU and a DSU.

As can be seen, the DSU's are also inter-connected by PCM highways to provide both inter-layerjunc- tions (for tandem calls) and inter layer signalling facilities. These inter-layer junctions are needed when double switching is used, i.e. for tandem calls.

In the interest of security, there are at least two PCM highways between each pair of DSU's. These "inter DSU" highways can also be used if desired, for overflow connections in times of high traffic in respect of calls other than transit calls.

Figure 1 shows that each LSU and each DSU has signalling facilities - MTS's - which are used both for LSU - DSU (or DSU - LSU) signalling, and for signalling between each pairofDSU'sforthe exchange of signalling and management information. These signalling links are carried on PCM highways, usually via time-slot 16, and for security each MTS "signalling module" has to consist of a pair of MTS links on separate PCM highways. Thus the common-channel signalling links use the same physical paths through the system as do the speech links.

There are three types of speech path connections to be considered: (a) a subscriber is connected to another subscriber (b) a subscriber is connected to a junction (c) a junction is connected to another junction.

In case (a), only one DSU is involved in call setting, and this can be, but does not have to be, the caller's "parent" DSU. In case (b) also only one DSU is involved in call setting, but in this case it is the DSU for the junction involved. Finally, in case (c) two DSU's are involved, unless the two junctions are connected to the same DSU, in which case only that one DSU is involved. Thus, unless the inter DSU links are used for overflow connections, doubleswitching is only needed for a transit call.

As indicated above, for all normal own exchange calls, originating outgoing calls and terminating incoming calls, only one DSU is used, so that substantially all the call set up and release sequences can be handled by one DSU. For a normal own exchange call, any DSU can be chosen to do the central call handling, and for the above two types of junction calls the DSU used is the one to which the junction is connected. However, for a transit call, double switching, i.e. switching via two DSU's, is needed, unless both junctions involved are connected to the same DSU. For a transit call, the call is treated as two half-calls each handled by the DSU to which one of the junctions is connected.

For all originating calls, the DSU associated with the calling line deals with checking and up-dating the busy/free status of the call as stored in that DSU's memory, and with accessing stored semi-permanent data (e.g. class of service data) for that caller. These functions in the case of a terminating call are dealt with by the wanted subscriber's DSU. in the case of incoming or outgoing junctions the junction's DSU is responsible for all data for that junction.

One consequence of the above arrangement is that the control equipment of a DSU is so arranged that two such control equipments in different layers can be involved with the same call, and that those control equipments can send formal messages to each other.

We now consider the handling of the various types of call in more detail, the call handling methods described being those currently preferred, although other methods could be used.

Terminating incoming call In this case the DSU to which the incoming junction is connected performs all major call handling functions. It detects the incoming seizure, handles communication with the preceding exchange, and ensures that the junction's stored data are updated. It also deals with call set-up and call release communications with the LSU for the wanted subscriber. This governing DSU communicates with the wanted lines DSU (which may be the same DSU) to inform the control equipment thereof that there is an incoming call for that line and to obtain status information for that line. The parent DSU for the wanted line updates the subscriber's data, and in the interests of security it acts as a "watchdog" on the call so that it can take emergency action to protect the line in the event of failure in the governing DSU.However, the parent DSU does not participate in normal call connection or release, (unless of course it is also the DSU for the incoming junction) the governing DSU monitoring the call during conversation and taking such action as needed.

The control equipment of the governing DSU analyses the wanted directory number DN, and possibly also uses the incoming route identity, and from this determines whether it is a tandem call (for which see below) or a terminating call, in which case it obtains an equipment number EN. In the latter case, the governing DSU sends a terminatingincoming-seizure-request message to the wanted line's DSU, which message contains the wanted EN and a token to identify the call.

The wanted line's DSU (i.e. the parent DSU) checks the wanted line to see if it is free and if so marks it as busy. For a PBX group, this parent DSU initiates a PBX search to select a free circuit and get an "absolute" EN. The parent DSU also accesses the wanted line's data information, such as class of service, needed for normal call set-up. The parent DSU then sends to the governing DSU a terminatingincoming-seizure-reply message containing the wanted line's status information and in the case of a PBX group the "absolute" EN. This message also contains the call-identity token.

Assuming that the reply message indicates that the call can be set up, the governing DSU now sends an incoming seizure request message containing the wanted EN to the wanted line's LSU, which sends an incoming seizure reply message to the governing DSU to accept the call. The governing DSU and the LSU then set up the call in normal manner. The governing DSU monitors the call during conversation, and deals with any action needed, e.g. for register recall.

The governing DSU and the LSU monitor the call for an incoming clear signal or an on-hook indication from the wanted line, and co-operate directly for clear down and the appropriate release guards. After clear down, the governing DSU sends to the wanted line's DSU a release request message which contains the line's EN, so that the DSU now marks the line as free.

The governing DSU also keep the dynamic records of the call (as needed, for instance, for charging) for the duration of the call, and routes this information as needed. This call data transfer normally occurs at the end of the set up phase, and/or at the end of the call, to keep the inter-DSU message traffic low.

if the incoming junction and the wanted line are on the same layer, only one DSU is involved, the various messages then going directly to the relevant programmes to bring those programmes into use, instead of via the MTS arrangements.

Originating outgoing calls Here the governing DSU is the one for the junction to be used, but the caller's parent DSU analyses the digits received from the caller. Thus the caller's LSU initially communicates with his parent DSU until the latter has passed control to the DSU to which junctions for the appropriate outgoing route are connected. Thereafter the governing DSU communicates directly with the caller's LSU for call set up and release.

The caller's DSU updates the caller's records as needed, and is initially active as it does the DN analysis to determine the identities of the governing DSU and outgoing route. The parent DSU takes no further active part in the call when it has handed over to the governing DSU, but in the interests of security it acts as a "watchdog" for the call to take emergency action if needed. The governing DSU does all communication with the next exchange, and monitors the call during conversation and takes action as needed.

The LSU detects the caller's off hook, and receives digits therefrom. On detection of the off hook, the LSU sends an originating seizure message containing the caller's EN to the parent DSU, which checks its records to see if the caller is free and available, and if so marks the caller as busy. It then sends to the LSU a send-n-digit message which contains the calling EN and acts as an initial acceptance of the call. The LSU waits until it receives n digits (n being enough to identify an outgoing call and its route) from the caller, covered by a time-out, and then sends those n digits in block to the parent DSU.

Leading zeroes (as in the 01 code identifying the London Directory Area or in such codes as 0279, appropriate to the Harlow area) are sent to the parent DSU but are not counted as part of then digits. in this case the value of n is 3, to cater for special calls and the code digits of STD calls.

The caller's DSU analyses the digits it receives, and asks for more digits if necessary, to determine whether the call is terminating i.e. own exchange call (see below) or an outgoing call. In the latter case it derives the identities of the outgoing route and its DSU. The caller's parent DSU now sends to the governing DSU an originating-outgoing-seizurerequest message which contains a token to identify the call, the outgoing route identity, and all the routing digits of the DN which it has received.

On reception of this seizure request message the governing DSU seeks a free and available junction of the wanted route, and if it succeeds it sends an acceptance message to the caller's parent DSU. The governing DSU now updates records to mark the chosen junction as busy. If it fails it sends a message rejecting the call to the LSU, which either sends a busy signal to the call, or tries an alternative route, and potential governing DSU.

If it has found a suitable junction and accepted the call, the governing DSU takes over and the parent DSU has no further part in the call set-up. The governing DSU works directly with the caller's LSU for the call set up. First it sends an outgoing governing message to the LSU, advising the LSU that the DSU is now responsibleforthe call, and asks for the rest of the wanted DN. This message contains the caller's EN, and in response to it the LSU sends the rest of the DN to the DSU. The DSU now works with the LSU for call set-up, performs all communication with the following exchange, and sends relevant information, e.g. answer by the wanted party, to the LSU.

As for a terminating call, the governing DSU keep dynamic records for the call for transmission when desirable.

If the caller and the selected junctions are on the same layer, the situation is as for a terminating incoming call.

Own-exchange call Here any DSU can be selected for a call, e.g. on the basis of traffic equalisation, with the first part of the call handled as for an originating outgoing call and the latter part as for a terminating incoming call. The speech path thus crosses the caller's LSU, the selected DSU and the wanted line's LSU, so that dependent on the location of the calling and wanted lines, up to three DSU's may be involved.

On detection of the caller's off-hook, the LSU sends an originating-seizure message containing the caller's EN to its parent DSU. The latter checks records to seethatthe caller is free and available, and is so marks him as busy. The parent DSU then sends messages back to the LSU to solicit digits, as for an originating outgoing call. The caller's parent DSU does all digit analysis, asking the LSU for more digits as appropriate. This DSU determines what sort of call it is, and if it is an own-exchange call it obtains the called EN. This DSU selects which of the DSU's is to be the governing DSU - see the section on DN analysis.

After completion of the DN analysis, and having got the identity of the governing DSU and the called EN, the caller's parent DSU sends an own-exchangeseizure-request message to the governing DSU, which message includes the wanted EN and a call-identifying token. If the governing DSU can handlethe call, it sends a reply message to the caller's DSU to accept responsibility for the call. The governing DSU co-operates with the caller's LSU for the rest of the call, its first action being to send an own-exchange-governing message to the calling LSU telling the latter that it (i.e. the transmitting DSU) is the governing DSU.

The governing DSU sends a terminating-ownexchange-seizure message to the wanted line's DSU, as for a terminating incoming call. The wanted line's DSU takes appropriate action and returns to the governing DSU a reply message containing the relevant class of service and status information, with the "absolute" EN in the case of a call to a PBX group. The governing DSU now proceeds with the call connection phase, co-operating with the calling and wanted LSU's to set up the appropriate speech paths.

This governing DSU is responsible for call supervision and clear down, as for the other calls described above.

Depending on whether the calling and wanted lines are on the same layer, and on the algorithm used to select a governing DSU, one, two or three layers may be involved. This should not affect internal operations of the control equipments since only the routing of the messages is effected by the destination DSU: this follows from the partitioned nature of the processing by the control equipments CCE.

Tandem call This is treated as two half-calls, the DSU for the incoming junction being the governing DSU for the incoming half-call and "master" for the whole call, while the DSU for the outgoing junction is the governer for the outgoing half-call. The incoming half-call is basically the same as the first part of a terminating incoming call, and the outgoing half-call is as for the second part of an originating outgoing call. If the two junctions are on different layers, double switching is involved, but if they are on the same layer double switching is not involved.

The control equipment CCE of the incoming junction's DSU must know whether or not the junctions are on the same layer, since if they are, the CCE must suppress the functions appropriate to the connection or disconnections of the outgoing crossoffice speech path when setting up the outgoing half-call. Apart from such suppression, the internal operations of CCE are unaffected by whether the junctions are in the same or different layers.

The control equipment of the incoming governing DSU commences call handling, and analyses the wanted DN received from the preceding exchange, possibly also using the incoming route identity.

From this, the DSU determines whether the call is a terminating call or an outgoing one (i.e. a tandem call). In the latter case it also obtains the outgoing route plus the outgoing governing DSU identity.

On completion of the DN analysis, the incoming governing DSU sends to the outgoing governing DSU a tandem-outgoing-seizure-request message which includes the outgoing route identity and all the routing digits received by the incoming DSU.

When it receives this message, the outgoing DSU selects a free junction of the wanted route, and at this point the outgoing DSU checks to see whether or not it is also the incoming DSU. Assuming that the junctions are on different layers, the outgoing DSU sets up a cross-office path from the selected junction to an inter-layer link. It then sends to the incoming governing DSU a message containing the identity of the selected inter-layer link. if the junctions are on the same layer, the outgoing part of the DSU does not connect a cross-office path, and it sends a message to the incoming part of the DSU which contains the identity of the selected outgoing junction.

The incoming governing DSU now connects a speech path from the incoming junction to the inter-layer link or to the outgoing junction, as appropriate.

The incoming governing DSU is responsible for communication with the preceding exchange and for updating the records of the incoming junction, while the outgoing governing DSU deals with communication with the following exchange and for updating the records of the outgoing junction. If an inter-layer link is used each DSU updates the records for its end of that link. Meanwhile the incoming governing DSU is still receiving digits from the preceding exchange, and these it stores. At the appropriate stage of the call the outgoing DSU requests these digits, which the incoming DSU assembles into inter-DSU messages and these it sends to the outgoing DSU. The latter then sends the rest of the digits of the called DN to the following exchange.

All other signalling information received by either the incoming DSU from the preceding exchange, or by the outgoing DSU is analysed on reception and inter DSU messages sent to the other DSU as required.

Alternative routing Alternative routing may be needed for tandem calls, and possibly also for originating outgoing calls.

For a tandem call, the DSU to which the incoming junction is connected acts both as the incoming governing DSU and as the master DSU. After it determines the first-choice outgoing route, the DSU to which that route is connected has to select a free junction. However, if it cannot find a free outgoing junction of the wanted route, it sends a message back to the governing DSU to this effect, and rejects the call. The DSU to which the wanted route is connected clears any dynamic records it has used and takes no further action on that call. The governing DSU now offers the call to the DSU to which the second choice outgoing route is connected, and this third DSU now tests for a free junction of that route. Operation continues in this way, until it is "realised" that the call cannot be set up, in which case the appropriate action is taken.

Operation for an originating call is basically as for the tandem call: the caller's governing DSU takes the initial responsibility for the call, which it offers to the DSU to which the first choice route is connected.

This DSU accepts the call if it finds a suitable outgoing junction, and if not it rejects the call. In the latter case the caller's DSU offers the call to the DSU for the second-choice route, and so on.

The above assumes that a route is so defined that each route is specific to one layer, but there can be several routes to the same destination. Thus there may be several routes, i.e. sets of junctions, to a main area switching centre, one from each DSU of our exchanges.

General connect From the above description it will be seen that for all calls other than tandem calls, the connection as set up includes a single directional switching unit only, which leads to an economically arrangement of the switching network.

Claims (10)

1. An automatic telecommunication exchange, which includes line switching units each serving a number of the subscriber's lines served by the exchange, each said line switching unit having its own control equipment, directional switching units to which junctions incoming to and/or outgoing from the exchange are connected, each said directional switching unit having its own control equipment and being associated with one or a group of said line switching units, and inter-unit links extending between the line switching units and the directional switching units such that each said line switching unit has access to every one of the directional switching units, wherein to set up a call between two of the lines served by the exchange the line switching unit serving one of the lines is connected via one of said links to one of the directional switching units and that directional switching unit is connected via another of said links to the line switching unit serving the other line involved in that connection, the connection being set up under the control of the control equipment of that one of the directional switching units, wherein for an incoming junction call the directional switching unit at which the junction terminates is connected via a said link to the line switching unit serving the wanted line, the connection being effected under the control of that directional switching units' control equipment, and wherein for an outgoing junction call the line switching unit serving the calling line is connected via one of said links to a directional switching unit with access to a free outgoing junction in the desired direction the connection being effected under the control of that directional switching unit's control equipment.

2. An exchange as claimed in claim 1, wherein information relating to a subscriber's line, including its class of service and its free/busy state, is stored in storage equipment in the control equipment of the directional switching unit associated with that line's line switching unit, and wherein when a said line is involved in a connection that line's information in the said storage equipment is amended in accordance with the state of that line and information read from that storage equipment extracted for use in call setting and/or control.

3. An exchange as claimed in claim 2, wherein the directional switching unit in which a line's data is stored monitors all connections involving that line, whether or not set up under the control of that directional switching unit, and wherein if the directional switching unit handling a call develops a fault condition, the directional switching unit associated with the line, or with one of the lines, involved in the call assumes call control.

4. An exchange as claimed in claim 2 or 3 wherein when a connection is to be established between two lines served by the exchange and connected to line switching units associated with different directional switching units, call control is commenced by the control equipment of the directional switching unit associated with the caller's line switching unit, and wherein when the digital information from the caller identifies the line switching unit to which the wanted line is connected, any one of the directional switching units is selected to control the connection, the choice being based, for instance, on traffic considerations.

5. An exchange as claimed in claim 1, 2, 3, or 4, and wherein the links between the switching units include communication path links and signalling paths links, the two sorts of links being functionally separate.

6. An exchange as claimed in claim 5, wherein further communication path links are provided each interconnecting a pair of directional switching units, which further links are used for tandem calls, and wherein when a tandem call is set up between two junctions connected to the same directional switching unit the call is set up wholly within that directional switching unit.

7. An exchange as claimed in claim 6, and wherein the signalling path links are also provided between respective ones of the directional switching units.

8. An exchange as claimed in claims 5,6 or 7, wherein the connections are set up in time division multiplex manner using pulse code modulation to convey intelligence, and wherein different channels on the same time division multiplex highway are used to convey communication path intelligence and signalling path intelligence.

9. An exchange as claimed in any one of the preceding claims, and which includes a plurality of sub-assemblies each including one or a group of line switching units a directional switching unit and the links between those units, wherein the subassemblies are interconnected by links between respective ones of the switching units, and wherein to extend the exchange one or more of said subassemblies are added with additional links between the existing and the added sub-assemblies.

10. An automatic telecommunication exchange, substantially as described with reference to the accompanying drawing.