GB2128445A - Multi-exchange telecommunication networks - Google Patents

Abstract

A telecommunications network including a plurality of exchanges connected by links in a configuration such that calls may be routed directly between exchanges (A and B) serving respective groups of subscribers if a free link is available or indirectly through a tandem exchange (G) of the network. Central processors (such as 10a) are provided to serve the exchanges fed with data from scanners (14a, 15a, 16a) as to the state (free or busy) of links between exchange (A), subscribers, and other exchanges (B and G) as well as data, preferably via a common channel signalling system (17a) and high speed data links, from other processors associated with these exchanges (B and G). The data is stored in memories (11a, 12a, 13a). When a directly routed call clears down, the processor (10a) interrogates the memories (11a, 12a, 13a) to determine whether there is a subsisting indirectly routed call which could be re-routed along a direct path between exchanges (A, B) serving the respective groups of subscribers concerned to reduce the blocking probability in respect of other calls which have to be routed through the tandem exchange (G). <IMAGE>

Description

SPECIFICATION Multi-exchange telecommunication networks This invention relates to multi-exchange telecommunication networks and methods of operating same.

A multi-exchange telecommunication network to which the present invention is applicable (hereinafter called a multi-exchange telecommunication network of the kind specified) comprises a plurality of telecommunication exchanges being at least three in number and in practice normally a substantially larger number, connected to each other by links in a configuration such that between at least certain of the exchanges communication can take place between them directly and also indirectly through one or more further exchanges (herein called tandem exchanges).

The exchanges may be local exchanges which are connected to subscribers stations, or they may be higher order exchanges.

The term "local exchange" is used herein to denote any of the exchanges at which a call is received from a subscriber's station connected to such exchange, and any of the exchangers to which such a call is routed and from which it is transmitted to another subscriber's station connected to that exchange.

The term "subcriber's station" is used herein to denote any apparatus which in operation forms the source or forms the destination of a call, whether connected directly to a local exchange or connected to a local exchange through the intermediary of a private branch exchange, or whether situated at the local exchange itself.

The term "higher order exchanges" is used herein to denote exchanges which are connected wholly or primarily to other exchanges, as distinct from being connected to subscribers stations and include tandem exchanges, group switching centres, and trunk exchanges.

The term "tandem exchange" is used herein to denote an exchange connected to a plurality of other exchanges (local or tandem) and incorporating switch means settable to transfer calls selectively between these other exchanges.

The term "link" is used generically to include connection through any transmission medium, e.g. electrically conductive such as lines or electromagnetic waves.

Communication between certain pairs of the local exchanges is possible along a direct path by way of a link which connects these exchanges directly, or alternatively along an indirect path by way of one or more tandem exchanges, The ability of the network to handle a call is dependent upon a link establishing the direct path or the links and tandem exchanges establishing the indirect path being free in the sense that any such link is not already busy (i.e. in use for the purpose of handling an earlier call). If none of the direct or indirect paths are free then the network is said to be blocked with respect to calls. The proportion of calls lost through blocking is termed the grade of service.

The grade of service can, of course, be improved by increasing the number of links between any two local exchanges, whether such links establish direct or indirect paths of communication, but exchanges are necessarily often widely spaced physically sometimes by many miles and the provision of additionai links, and possibly additional tandem exchanges, or tandem exchanges affording a greater number of switch paths between the terminals of the exchange, entails very substantial cost.

The object of the present invention is to enable an improved grade of service to be achieved more economically, without the provision of any additional links or to obtain a similar grade of service with fewer links.

According to one aspect of the present invention resides in the provision in or for a multi-exchange telecommunication network of the kind specified affording between pairs of the exchanges a plurality of paths presenting different degrees of probability as regards the use of any one of such path blocking calls between other pairs of exchanges, of the combination of:: a. setting-up means responsive to an incoming call incident at one of the exchanges to set up the required call to another one of the exchanges, b. path selector means included in or associated with the setting-up means to select, from the available free paths between said exchanges one which has the lowest blocking probability for calls between other pairs of exchanges, c. memory means for storing data defining the paths of the calls which are set-up, d. sensing means responsive to the clearance of any of such set-up calls to activate the selector means and setting-up means to reselect and re-set-up any call already set-up along a path having a higher blocking probability for calls between other pairs of exchanges than that pertaining to the cleared path, and without noticeable interruption of communication.

In one form of the invention the selector means would be arranged to select links connecting exchanges directly in preference to links connecting the same exchanges indirectly.

Thus, the sensing means may be arranged to interrogate the memory means and to respond to activate the selector means and setting-up means only when there has been clearance of a call set up along a direct path between exchanges while another call already set-up between these exchanges has been established along an indirect path. The operation of the selector means and setting up means then brings about re-routing of the already set-up call along the then available direct path.

Again, however, it is contemplated that there may be cases where two indirect paths between exchanges afford different degress of blocking probability with respect to calls between another pair of exchanges and in this case it would of course be within the scope of the invention, as broadly defined above, for the setting-up means and selector means, when re-activated, to reroute an already set-up call from an indirect path having a greater probability of blocking to another indirect path which has a lesser probability of blocking for calls to other exchanges.

The combination comprising memory means, setting up means, selector means, and sensing means may be provided at each of some or all of the exchanges which are connected to each other by direct links and which are also connected to each other indirectly via tandem exchanges. The memory means may provide for storage of data as to the identity of the link or links carrying the call.

Each memory means (pertaining to a partciular exchange) stores the destination of each call that is set up over any link to a tandem exchange in association with the identity of the link. The identity of the link defines the identity of the next exchange to which the call is routed. The minimum data which is required to define the destination of the call is the identity of the terminating exchange and the number on that exchange of the called subscriber.

Each of said exchanges may be provided with a processor means for controlling operation of the setting-up and path selector means, and memory means, and the sensing means in the required sequence.

As a possible alternative, processor means, and memory means may be provided which are common to a group comprising some or all of the exchanges of the network (and situated at a station coincident with one of these exchanges while being remote from the others of the group or remote from all of them), and may be connected operatively with the remotely positioned exchanges to receive route data therefrom and to supply control signals to the setting-up and path selector means at such exchanges via common channel data signalling links.

From a further aspect the invention resides in a method of operating a multi-exchange telecommunication network of the kind specified comprising the steps of a. setting up calls between exchanges in a mode such that each call is routed along that one of the available free paths having the lowest blocking probability for calls between other pairs of exchanges, b. storing in a memory means data defining the paths in use between exchanges of the network, c. interrogating the memory means at least as often as there is clearance of a call between the exchanges, d. re-setting-up along the cleared path any already set-up call between the exchanges subsisting along a path having a higher blocking probability for calls between other pairs of exchanges than that pertaining to the cleared path but without noticeably interrupting communication.

One preferred method of carrying out this method is that upon clearance of a call between the exchanges routed along a path including a direct link between these exchanges, any already set-up call between the same exchanges and routed along an indirect path is reset-up along the direct path.

The invention will now be described by way of example with reference to the accompanying drawings wherein Figure 1 illustrates a fragment of a telecommunication network showing alternative paths between certain of the exchanges and to which the present invention is applicable; Figure 2 illustrates diagrammatically a simple network comprising three local exchanges and a tandem exchange to which the present invention is applicable and which is shown primarily for the purpose of explaining the principles of the present invention; Figure 3 is a schematic circuit diagram of one embodiment of the combination of setting up means, path selector means, memory means and sensing means required to be provided at the exchanges of a telecommunication network at which the invention is to be put into operation;; Figure 4 illustrates how the emboidment of Figure 3 would be incorporated in part of the network shown in Figure 1; and Figure 5 is a schematic circuit diagram of an alternatively embodiment to that of Figure 4.

A multi-exchange telecommunication network is normally design to enable a communication path to be established from an originating source (subscriber's station at which a call is initiated) to a destination (subscriber's station at which the call is received). A fragment of a typical network is in the form of a hierarchy as illustrated in Figure 1 in which subscribers stations, of which only certain are shown, a,, a2, an, b1, h2 bnt are connected to respective local exchanges A to F. The local exchanges in turn are connected to group switching centres G and H and at successively higher levels in the hierarchy the group switching centres are connected to trunk transit exchanges J, K and L.

To handle large numbers of calls it is economic to provide direct paths between certain of the exchanges and this is illustrated in Figure 1 where, for example, direct paths are provided between local exchanges A, B and E, F and also between group switching centres G, H (which are tandem exchanges) and again between J and K.

In the network illustrated in Figure 1 it will be evident that if several calls were subsisting from A to B some of these could be along the direction path A to B and others along the indirect path A, G, B. However, it would not be possible for a call to be established between C and B if all links on path G, B were busy. Hence with respect to calls for B originating at subscribers stations connected to C or at C itself, the network would be blocked.

In a small network illustrated in Figure 2 there are three local exchanges A, B, C each connected to subscribers stations (not shown) and each of these exchanges is of a form such that it cannot act as a tandem exchange, i.e. it can receive calls from any of the subscribers stations connected thereto and route them onwards but exchange A, for example, cannot route a call received from exchange B onwards to exchange C.

Exchange Ton the other hand is a tandem exchange and can route calls received from any one of the exchanges A, B, C to another one of these exchanges.

For simplicity only one link in each direction, and only those links which are relevant for calls from exchange B to exchange C are shown (thus incoming links to B and outgoing links from C are omitted, but links between A and T in both directions are shown since each of these links is relevant).

The circuits afforded by the network of Figure 2 can handle up to five simultaneous calls.

The table below shows the busy connections.

The states in which it is not possible to make a further connection between B and C are marked by X in column 4.

State No. of No. calls Busy connections 0 0 None 1 1 BA 2 BC 3 AC 4 BTA 5 BTC 6 ATC 7 2 BA,BC 8 BA, AC 9 BA, BTA 10 BA,BTC 11 BA,ATC 12 BC, AC 13 BC,BTA X 14 BC,BTC X 15 BC,ATC X 16 AC,BTA 17 AC,BTC 18 AC,ATC 19 BTA,ATC 20 3 BA,BC,AC 21 BA,BC,BTA X 22 BA,BC,BTC X 23 BA,BC,ATC X 24 BA,AC,BTA 25 BA,AC,BTC 26 BA, AC, ATC 27 BA,BTA,ATC 28 BC,AC,BTA X 29 BC,AC,BTC X 30 BC,AC,ATC X 31 BC,BTA,ATC X 32 AC,BTA,ATC 33 4 BA, BC, AC, BTA X 34 BA, BC, AC, ATC X 35 BA, BC, AC, BTC X 36 BA,BC,BTA,ATC X State No. of No. calls Busy connections 37 BA,BC,BTC,ATC X 38 BA,AC,BTA,ATC X 39 BC,AC,BTA,ATC X 40 5 BA,BC,AC,BTA,ATC X In some of these states, there are already two connections between B and C.These are states 1 4, 22, 29, 35 and 37. However, in others of these states, there is only one connection between B and C. These are states 13, 15, 21, 23, 28, 30, 31,33, 34, 36, 39 and 40. In states 21,30, 33, 34, 36, 39 and 40, all outgoing links from B are busy or all incoming links to C are busy. Thus, any further connection is prevented by congestion.

However, in states 13, 15, 23, 28 and 31 there is a free link outgoing from B and one incoming to C; thus a further connection is prevented by blocking.

In the blocking states 13, 15, 23 and 28 there is one connection via the tandem exchange which could be made over a direct link since it is free. In state 31 , there are two such connections. In a network with normal automatic alternative routing, these states would not be reached from a state having fewer connections, since a direct link would be selected instead of the route via the tandem exchange. For example, there would be transition from state 7 to 20 instead of to state 23. However, the blocking states can be reached as a result of calls clearing down. For example, state 23 follows state 34 when the connection over link AC is cleared.

These blocking states 13, 15, 23 and 28 can be avoided completely if existing connections may be rearranged by replacing connections via the tandem exchange by direct connections when the latter become free. Thus, a transition from state 34 to 23 would be immediately followed by replacing connection ATC by AC, resulting in state 20. Similarly, connections can be rearranged to replace state 13 by 7, to replace 15 by 12 and to replace states 28 and 31 by state 20. In this way, it is always possible to make two connections between B and C, unless all the outgoing links from B or all the incoming links to C are already busy.

Exchanges such as A, B, G shown in Figure 1, and the exchanges A, B, C, shown in Figure 2 require the provision of additional facilities to enable the invention to be put into practice. A basic illustration of these faciliites is shown in Figure 3 taking, by way of example, exchange A of Figure 1.

Exchange A comprises a switching network having terminals to which are connected subscriber's links a .... an and terminals to which are connected outgoing links to exchanges G and B and incoming links also for exchanges G and B. The switching network provides for the establishment of connections as follows:- (a) between one subscriber's link and another; (b) between a subscriber's link and a link outgoing to another exchange G or B; (c) between a link incoming from another exchange G or B and a subscriber's link; (d) between a link incoming from one exchange G and a link outgoing to another exchange B.

For effecting the selection and setting-up of any particular connection through the switching network, there is provided in association therewith a processor 1 Oa. In practice more than one processor 1 Oa would be provided to ensure continuity of service in the event of a fault, only one such processor being used at any given time.

The processor 1 Oa obtains information as to the state of the links through sensing means in the form of scanning units 14a, 15a, 1 6a of which the first 1 4a provides data as to the state of the subscribers links, the second 1 5a provides data as to the state of the outgoing links to other exchanges G and B, and the third 1 6a provides data as to the state of the incoming links from these other exchanges G and B. Further, the processor 1 0a recieves data from processors corresponding to 10a and associated with other exchanges, e.g.G and B, by way of a common channel signalling system indicated at unit 1 7a, itself connected by high speed data links to corresponding common channel signalling system units associated operatively with the other exchanges.

Memory means comprising units 11 a, 1 2a, 1 3a is provided to store data required by the processor for setting up and re-setting calls.

In operation, clearing down of a call set up as in any of the cases mentioned in sub-paragraphs (a) to (d) above is sensed by the appropriate scanner unit and causes the processor 1 Oa to interrogate the memory units 11 a, 1 2a, 1 3a for the purpose of determining whether there is a subsisting call which could beneficially make use of the link rendered free by the cleared down call in the sense that this link would set up a call along a path having lower probability of blocking with respect to other calls than subsists in respect of that call.

In that event, after scanning of the memory means, the process generates instructions to the switching network of exchange A to reroute the subsisting call making use of the links rendered free by the cleared call. This re-routing can be effected without noticeable interruption to the communication because the operation of the electronic units is incorporated in the switching network and the processor 1 0a is effected in a sufficiently short interval to be virtually undetectable by the subscribers.

In the system shown in Figure 3, the common channel signalling system 1 7a and its high speed links is not essential to the re-setting of a call subsisting along an indirect path to a direct path when the link defining the latter has cleared down, but its provision does significantly reduce time during which interruption of communication occurs and therefore improves the risk of such interruption being noticed by the user.

Considering firstly operation of the system, assuming no provision of the common channel signalling system and high speed links, in relation to a call already set up from a subscriber a1 to a subscriber b, (serviced by exchange B not shown in Figure 3) along an indirect path a" A, G, B, h the sequence of re-setting is as follows::- (i) processor 1 Oa detects by means of scanners 1 5a, 1 6a that a link A, B has cleared; (ii) processor 1 Oa scans the memories 11 a, 12a, 13a and reads that the call a1, A, G, B b, subsists (including desination b,); (iii) processor 1 Oa ciears the A, G linkfrom a, producing clearance also of the link B to b1; (iv) processor 1 0a sets up a1 A, B; (v) processor 1 Ob at exchange B senses the clearance of the link G, B and the setting of a1 A, B; (vi) the processor 1 Oa transfer the data ascertained in (ii) via the link A, B to the corresponding unit of exchange B;; (vii) processor 1 Ob completes the setting up of a1 A, Bh1.

If the common channel signalling system 1 7a and its high speed data links are provided then the scanners 1 Sa, 1 6a may be dispensed with or provided as described. The following represents a typical sequence.

(i) the processor 1 Oa detects by means of scanners 15a, 16a that a link A, B has cleared. If 1 Sa, 1 6a are not provided, detection is effected by 1 0a scanning the memory units in which data as to the state of ink A, B is stored; (ii) the processor 1 Oa interrogates the memory units and reads that the call at A, G, B b, subsists (including the destination b,); (iii) the processor 1 Oa transfers the data ascertained in (ii) via unit 1 7a and its high speed links to the corresponding unit at exchange B and also the identity of the cleared A, B link; (iv) concurrently with (iii) the processor 1 Oa clears the link G, B;; (v) the processor 1 Oa and the corresponding processor at B then act concurrently. The processor 1 0a sets up a, A, B. The processor 1 Ob sets up a connection in B from the A, B link (identified in (iii) to b,).

Thus the provision of the common channel signalling system entails two concurrent operations (iii) and (iv) followed by two concurrent operations in (v), whereas without its provision there are four sequential operations (iv), (v), (vi) and (vii) as above described.

Figure 4 illustrates, with a somewhat greater degree of elaboration, the application of the system illustrated in Figure 3 to exchanges A, B and G of the circuit shown in Figure 1.

Corresponding reference bearing suffixes a, b, g etc. are applied to functionally corresponding units.

As shown in Figure 5, the invention may also be put into effect in respect of a group of exchanges A, B, G in a multi-exchange telecommunication network, or possibly all of the exchanges, where these are situated at appropriately close distances to each other by utilising a processor such as 10 common to all of the exchanges of the group. Memory means such as the units 11, 12, 13 and a common channel signalling system 1 7 would be provided adjacent to, and connected to, the processor, with complementary units 1 7a, 1 7b, 1 7g at each exchange A, B, G.

In such a system the memory means adjacent to and connected to the processor would require sources of data derived from the exchanges common to the group. This could be achieved by retaining the scanning units to feed this data direct to the memory means via the common channel signalling system. Thus for a group of n exchanges there would be one processor and one memory means.

It may however be more convenient in some cases to provide a processor and associated memory means at each exchange of the group and in addition to provide a further processor and associated memory means common to the group for performing certain of the functions which would otherwise require to be handled by the exchange processors and memory means. In this case a group of n exchanges would effectively be provided with n+1 processors and memory means.

It will, of course, be understood that in applying the circuitry shown in Figure 3 only to tandem exchanges, the subscribers links a1, .... an and associated scanning unit 1 4a would not be required. In the case of a tandem exchange, the functions referred to in subparagraph (a), (b) and (c) above would not be required to be carried out.

Similarly, where the exchange is a local exchange, the function referred to in subparagraph (d) above would not be required.

Claims (17)

Claims

1. In or for a multi-exchange telecommunication network of the kind specified affording between pairs of the exchanges a plurality of paths presenting different degrees of probability as regards the use of any one of such paths blocking calls between other pairs of exchanges, the combination of: a. setting-up means responsive to an originating call incident at one of the exchanges to set-up the required call to another one of the exchanges, b. path selectornveans included in or associated with the setting-up means to select from the available free paths between said exchanges one which has the lowest blocking probability for calls between other pairs of exchanges, c. memory means for storing data defining the paths of the calls which are set-up, d. sensing means responsive to the clearance of any of such set-up calls to activate the selector means and setting-up means to reselect the reset-up any call already set-up along a path having a higher blocking probability for calls between other pairs of exchanges than that pertaining to the cleared path, and without noticeable interruption of communication.

2. The combination claimed in claim 1 wherein the selector means is arranged to select links connecting exchanges directly in preference to links connecting the same exchanges indirectly.

3. The combination as claimed in claim 2 wherein the sensing means is arranged to interrogate the memory means and to response to activate the selector means and setting-up means only when there has been clearance of a call set up along a direct path between exchanges while another call already set-up between these exchanges has been established along an indirect path.

4. The combination as claimed in any one of the preceding claims wherein the setting up means, selector means, memory means and sensing means is provided at each of some or all of the exchanges which are connected to each other both by direct links and which are also connected to each other indiectly via tandem exchanges.

5. The combination as claimed in any one of the preceding claims wherein the memory means provides for storage of data as - to the identity of the terminating exchange and the identity of the next exchange (if different from the terminating exchange) to which the call is already routed.

6. The combination as claimed in any one of the preceding claims wherein there is provided a processor means for controlling operation of the setting-up and path selector means, memory means, and the sensing means in the required sequence.

7. The combination as claimed in claim 6 further including a data transmission means for transferring data via the processor means at each exchange from the sensing means and/or memory at such exchange to the memory means at one or more of the other exchanges of the system.

8. The combination as claimed in claim 6 wherein processor means, and associated memory--means are common to a group comprising some or all of the exchanges of the network, and are connected operatively therewith through data transmission means to receive route data therefrom and supply control signals to the setting-up and path selector means at such exchanges via said data transmission means.

9. 9. The combination according to Claim 8 wherein the route data is derived from the memory means and processors individual to respective ones of the exchanges.

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1-0. A method of operating a multi-exchange telecommunication network of the kind specified comprising the steps of a. setting up calls between exchanges in a mode such that each call is routed along that one of the available free paths having the lowest blocking probability for calls between other pairs of exchanges, b. storing in a memory means data defining the paths in use between exchanges of the network, c. interrogating the memory means at least as often as there is clearance of a call between the exchanges and having a lower blocking probability for calls between other exchanges, d. re-setting-up along the cleared path any already set-up call between the exchanges subsisting along a path having a higher blocking probability for calls between other pairs of exchanges than that pertaining to the cleared path but without noticeably interrupting communication.

11. A method according to claim 10 wherein, upon clearance of a call between exchanges routed along a direct path, any already set-up call between the same exchanges and routed along an indirect path is set-up along the direct path.

1 2. A multi-exchange telecommunication network of the kind specified having any novel feature or novel combination of features herein disclosed.

13. A method of operating a multi-exchange telecommunication network of the kind specified comprising any novel step or any novel combination of steps herein disclosed.

14. In or for a multi-exchange telecommunication network the combination of means substantially as hereinbefore described with reference to and as illustrated by Figure 3 of the accompanying drawings.

1 5. In or for a multi-exchange telecommunication network the combination of means substantially as hereinbefore described with reference to and as illustrated by Figure 4 of the accompanying drawings.

16. In or for a multi-exchange telecommunication network the combination of means substantially as hereinbefore described with reference to and as illustrated by Figure 5 of the accompanying drawings.

17. A method of operating a multi-exchange telecommunication network substantially as hereinbefore described with reference to and as illustrated by any of Figures 3,4 and 5 of the accompanying drawings.