Telecommunications network and method

Abstract: A method and apparatus for routing traffic in a circuit switched network. A call between two nodes interconnected by a direct link is first offered to the direct route, and if that is blocked it is offered to a currently nominated two-link altern­ ative route beween the two nodes. If that route is busy, the call is lost, and a randomly chosen two-link route is assigned to be the new current nominated alternative route. The strategy is particularly effective because it is simple, available routes are quickly located and once one available route is found, that same route is used for rerouting further calls until it is full. Trunk reservation protection is applied on alternative routes.

Description: The present invention relates to telecommunications networks and methods. In particular it relates to routing traffic in a circuit-switched network with full or multiple interconnection. With the introduction of new switching technologies in recent years, it is no longer necessary for telephone traffic (or voice and data traffic on an integrated network) to be routed along a fixed origin-destination route. One of the early steps towards more flexible routing was the introduction of automatic alternative routing (AAR), which allows traffic on overloaded direct routes to be diverted via alternative, pre-assigned routes. In AAR the alternative routes from a given source node to a given destination node are predetermined and are in fixed ranking so that for each call blocked on the direct route the control equipment will always offer the first choice alternative route, the second choice alternative route, etc until all of the alternative routes are tested. In the AAR disclosed in U.S. Patent No. 3,394,231, the first choice and final (second) choice alternative routes are two-link routes and if they are blocked the control equipment is further arranged to offer a two-link route from the source node to the intermediate node of the final choice alternative route. The introduction of stored program controlled (SPC) exchanges and common channel signalling have opened up the possibility of increased flexibility. A number of routing strategies have been put forward. In a strategy proposed by Bell-Northern Research, calls are routed directly if possible; if not they are routed on an alternative two link path recommended by a central processor. The central processor chooses the recommended path probabilistically with a probability proportional to the number of free circuits on each route after subtracting a reserved number of circuits (a trunk reservation parameter). Each exchange is linked to the central processor and transmits data on availability to the central processor every 5 to 10 seconds. For large networks, the amount of information received by the central processor each 5 - 10 second period is large. The central processor decides on alternative routes for calls on the basis of information gathered on average about 5 seconds ago. Another similar system, proposed by Forestier and Lottin, avoids the use of a central processor. Exchanges communicate with one another to find the least congested alternative route for traffic. Again, there is delay in gathering this information and the system is complex. U.S. Patent No. 3,536,842 discloses a telephone network in which each trunk has an associated flip flop GB which is set when the called party answers and is reset at the start of the next call. The flip flops GB are cyclically interrogated and their states written to a trunk history memory. Each call is attempted in accordance with a fixed sequence of routes to the destination office, and if the recorded states in the trunk history memory for any of the trunks of the route currently being offered is "reset", then a corresponding blocking relay is operated to simulate all the trunks on that route being busy, and the marker proceeds in known manner through the routing sequence. Thus even under heavy traffic conditions the marker will always start at the first choice route in the routing table to hunt for an idle route. The trunk history memory does not change the routing table nor affect the sequence in which the marker progresses through the sequence. The simulation of all trunks busy prevents transmission of call digits on an outgoing idle trunk of a route which has failed in a remote link or switch (node). It is known from "Feedback Methods for Calls Allocation on the Crossed Traffic Routing" by B Hennion; ITC-9, Int. Teletraffic Congr., Oct. 1979, session 61, paper 4, to route calls to a destination via two routes of equal hierarchical status. In the disclosed method, calls are offered to the first route until it is blocked and then calls are offered to the second route until it is blocked, whereupon calls are again offered to the first route, and so on. US Patent No. 4,345,116 discloses the generation of respective routing tables of preplanned routes, and the use of additional real-time routes, which effectively extends the routing tables. These real-time routes are subject to trunk reservation restrictions, and once allocated are not changed. It is known from "Routing Techniques used in Computer Communication Networks" by M Schwartz and T E Stern; IEEE Transactions on Communications, Vol. COM-28, No. 4, April 1980, to generate routing tables in a packet switching network on the basis of a least cost algorithm. The tables are changed only when a line or node changes its operational status, e.g. changes from In Service to Out Of Service, and vice-versa. Further work, largely theoretical, proposes various decentralised routing strategies, referred to as learning automata. In one of the simplest of them, the probabilities of success for a call along each possible path are updated after every call has been processed. If there are r routes from exchange i to exchange j, the r routes will be attempted in a random order with changing probabilities. Many such schemes may be more sophisticated than this. The amount of processing capacity required for these strategies can be relatively large, particularly for the more sophisticated ones. According to a first aspect of the present invention there is provided a method of routing in a circuit-switched network comprising a plurality of nodes and wherein each node has stored therein for each destination node, a respective first list containing the identity of an initially-allocated route to the respective destination node, the method comprising, for a call from an origin node to a destination node, offering the call to the initially-allocated route of the first list, the method being characterised by, whenever a call is blocked on an offered route of the first list, selecting an entry of a respective second list stored in the origin node and containing the identities of a plurality of alternative routes to the respective destination node, the first list being a subset of the second list, and copying the selected entry of the second list into the first list at the entry corresponding to the blocked offered alternative route. According to a second aspect of the present invention there is provided apparatus for routing traffic through a circuit-switched network comprising a plurality of processors at respective nodes, each node having stored therein, for each destination node, a respective first list containing the identity of an initiallyallocated route and a respective second list containing the identity of an alternative route to the respective destination node, each processor being adapted to offer a call between its associated respective node and a destination node call to the initially-allocated route of the first list and, in the event that the initially-allocated route is blocked, to offer the call to the alternative route of the second list, characterised in that, in use, each respective second list contains the identities of a plurality of alternative routes to the respective destination node, and each processor is adapted to select, in the event that the initially-allocated route is blocked, an entry of the respective second list, and to copy the selected entry of the second list into the first list at the entry corresponding to the blocked offered alternative route. According to a third aspect of the present invention there is provided a method of routing in a circuit-switched network comprising a plurality of nodes and wherein each node has stored therein, for each destination node, a respective first list containing the identity of at least one preassigned route to the respective destination node and a respective second list containing the identity of at least one alternative route to the respective destination node, the method comprising, for a call from an origin node to a destination node, offering the call to the at least one preassigned route of the first list and, in the event that the call is blocked on the at least one preassigned route, offering the call to an alternative route of the second list, the method being characterised by, in the event that the offered alternative route is blocked, selecting an entry of a respective third list stored in the origin node and containing the identities of a plurality of alternative routes to the respective destination node, the second list being a subset of the third list, and copying the selected entry of the third list into the second list at the entry corresponding to the blocked offered alternative route. According to a fourth aspect of the present invention there is provided apparatus for routing traffic through a circuit-switched network comprising a plurality of processors at respective nodes, each node having stored therein, for each destination node, a respective first list containing the identity of at least one preassigned route to the respective destination node and a respective second list containing the identity of at least one alternative route to the respective destination node, each processor being adapted to offer a call between its respective node and a destination node to a preassigned route of the first list and, in the event that the call is blocked on the at least one preassigned route of the first list, to offer the call to an alternative route of the second list, the apparatus being characterised in that: each node comprises a respective third list containing the identities of a plurality of alternative routes to the respective destination node, and each processor is adapted to select, in the event that the offered alternative route is blocked, an entry of the respective third list, and to copy the selected entry of the third list into the second list at the entry corresponding to the blocked offered alternative route. Herein, a route which is stored in the first list of the first and second aspects of the present invention is also referred to as a current nominated route, and a route which is stored in the second list of the third and fourth aspects of the present invention is also referred to as a current nominated alternative route. In accordance with the first aspect of the present invention there is provided in a preferred embodiment a simple and effective routing scheme in which calls are directed to a current nominated route, this current nominated route being used until it is no longer available. Similarly, in accordance with the third aspect of the present invention there is provided in a preferred embodiment a simple and effective routing scheme in which blocked calls on a first choice route are directed to a current nominated alternative route, this current nominated alternative route being used until it is no longer available. The current nominated route, or the current nominated alternative route, as the case may be, is then changed in a way which is random in the sense that the search technique does not restrict unduly the patterns of routing that can emerge, taking into account the various alternative routes used by different source-destination pairs. In particular, it is desirable that the search technique used should not bias the routing pattern that emerges towards any predefined routing pattern and/or away from any pattern which might under certain network operating conditions be desirable. Thus a choice may be made among the various available alternative routes according to, for example, a stochastic, pseudo random, or cyclic mechanism. A number of preferred embodiments of the invention will now be described with reference to the accompanying drawings in which:- Figure 1 is a schematic diagram of a fully interconnected five node network with link capacities labelled; Figures 2 and 3 are schematic diagrams of the same network with two possible patterns of unidirectional offered traffic; Figure 4 is a schematic diagram of an alternative network which is not fully interconnected; and Figure 5 is a schematic diagram showing the routing of international calls. Referring to Figure 1, suppose the exchange at node 1 has a call for node 2, but the direct route between the two nodes is busy. The exchange at node 1 maintains a record (set), which constitutes a third list of the third and fourth aspects of the present invention, of the three alternative two-link routes that are available, via nodes 3, 4 and 5 respectively, and maintains a record, which constitutes a second list of the third and fourth aspects of the present invention, of which of these three alternative routes is the current nominated alternative route (this will normally be the last used alternative route, unless the previous overflowing call was blocked). Trunk reservation is used on each link. Without trunk reservation, a link could become busy with alternatively routed calls, causing some calls which would otherwise be directly routed along the link to be re-routed over two links (or blocked), which is undesirable on cost and other grounds. Trunk reservation is a widely used technique and will not be described in detail here. The exchange at node 1 then checks whether the current nominated alternative route between nodes 1 and 2 has any available circuits, after subtracting the trunk reservation parameters for the two links from the actual number of free circuits for those links. For example, the current nominated route may be via node 5. The capacity of the link between nodes 1 and 5 is 125 circuits and its trunk reservation parameter (TRP) is, say, 10. Node 1 stores these figures and also the number of circuits currently in use. The link between nodes 1 and 5 is available for alternative routing if the sum of the circuits in use and the TRP is less than 125. In order to find whether the second link between node 5 and node 2 is available, node 1 sends a message to node 5. Node 5 includes processing means which holds details of the capacity of the link between nodes 5 and 2 (100 circuits), the TRP (10) of this link, and the number of busy circuits on the link. Again, the link is available if the capacity exceeds the number of busy circuits and the TRP. A message is transmitted by node 5 to node 1 to indicate whether or not the link is available. This message need be only a single bit in length. If both links are available, the call is routed via node 5. For as long as there are available circuits on the route via node 5, this alternative route is chosen. In a preferred embodiment of the invention, if the current nominated alternative route is not available for a call then the call is lost, and the next time a call overflows from link (1,2) it is offered either to any one of the two-link routes via nodes 3, 4 and 5, or, if desired, to one of such two-link routes, but not the one which has just overflowed (i.e. the call is offered only to one of the two-link routes via node 3 or node 4). The choice among the three alternative routes (i.e. the replacement nomination for the unavailable current nominated alternative route) may be made in accordance with any desired criteria and need not be weighted in accordance with a history record of each alternative route. The newly chosen alternative route is then nominated as the current alternative route, to which subsequent calls not capable of direct routing are offered. In an alternative embodiment of the invention, if the current nominated alternative route is not available for a call, the same call is offered to a new alternative route, and again the current alternative route is updated. This method has been found to be highly successful and it has three advantages over prior art systems. First, the strategy for selecting alternative routes is a very simple one which can be easily implemented and uses only a small amount of processor time. Secondly, there is no significant delay between checking whether a route is available and assigning a call to the route so the information used is always up-to-date. Thirdly, suitable routes are pinpointed and once they have been used successfully once, they continue to be used whenever calls cannot be routed on direct routes, until they are full (allowing for trunk reservation). Thus, it is necessary to find a new alternative route relatively infrequently. That the results are so satisfactory from such a simple system is very surprising; the trend has been to develop more and more sophisticated strategies, for example so as to choose the best possible route each time re-routing is necessary. Development and implementation costs are high for such strategies and yet performance of the simple system described above has been found generally to be at least as good. The above example involved a very small network of five nodes; in most applications there will be far more, but the method may be operated in the same way. In a fully interconnected network with N nodes, a call may arise between nodes i and j (i, j = 1,2, ...,N, i≠j). The call is offered first to the direct link (i,j), but if all the circuits on this link are in use then the call is offered to the current alternative route, through node k(i,j). The alternatively routed call is only accepted if doing so leaves at least t(i,k(i,j)) circuits free on link (i,k(i,j)) and t(k(i,j),j) circuits free on link (k(i,j),j). If the call is accepted on the alternative route, the indicator k(i,j) is left unaltered and calls continue to be offered to this route whenever alternative routing is necessary. If the call is not accepted on the alternative route, then the call is lost and the indicator k(i,j) is reset to a value chosen at random from the set {1,2,...,N} - {i,j}. To illustrate the high performance achieved by the scheme described above, consider the network of Figures 1 to 3 where the number of nodes N is 5. The trunk reservation parameter is 10 for all links, i.e. t(i,j) = 10, for all pairs (i,j). These parameters are chosen so that if direct traffic alone would receive a grade of service of 3.75% from a link then the grade of service received by direct traffic when that link is used as part of alternative routes is no worse than 7.5%. Figure 2 shows a possible offered traffic pattern for the network of Figure 1. Note that the traffic offered to links (1,2) and (4,5) is higher than can reasonably be accommodated by direct routing over those links. However, by comparing the offered traffic with the capacities indicated in Figure 1 and allowing for a TRP of 10, it can be seen that there is spare capacity along the routes 1 - 3 - 2 and 4 - 1 - 5. If this mismatch of offered traffic and capacity could be predicted in advance then it would be possible to designate 1 - 3 - 2 and 4 - 1 - 5 as alternative routes for overflow traffic from links (1,2) and (4,5) respectively. The aim of the present dynamic routing method is automatically to seek out such beneficial patterns of carried traffic as offered traffic, and possibly capacity, varies. Table 1 shows values calculated in a computer simulation of the network of Figure 1 for grades of service received by the various offered traffic streams and the patterns of carried traffic using the method outlined above under the offered traffic pattern shown in Figure 2. Table 1 shows that 8% of the traffic between nodes 1 and 2 is routed via node 3, and 9% of the traffic between nodes 4 and 5 is routed via node 1. The method in accordance with the invention has effectively found and utilised the spare capacity along the alternative routes 1 - 3 - 2 and 4 - 1 - 5. Source destination pair % of calls routed directly % of calls routed via tandem nodes 1 - 5 % of calls lost 1 2 3 4 5 1 - 2 84 - - 8 2 2 5 1 - 3 100 - 0 - 0 0 0 1 - 4 100 - 0 0 - 0 0 1 - 5 100 - 0 0 0 - 0 2 - 3 100 0 - - 0 0 0 2 - 4 97 0 - 1 - 0 2 2 - 5 98 0 - 1 0 - 2 3 - 4 98 1 0 - - 0 1 3 - 5 98 1 0 - 0 - 1 4 - 5 85 9 0 1 - - 5 Overall blocking = 1.9% Table II gives the corresponding estimates for the offered traffic pattern shown in Figure 3. In this case useful extra capacity has been found between nodes 3 and 4, via node 1, which carries 9% of the traffic between nodes 3 and 4. Source destination pair % of calls routed directly % of calls routed via tandem nodes 1 - 5 % of calls lost 1 2 3 4 5 1 - 2 98 - - 2 0 0 0 1 - 3 100 - 0 - 0 0 0 1 - 4 100 - 0 0 - 0 0 1 - 5 100 - 0 0 0 - 0 2 - 3 100 0 - - 0 0 0 2 - 4 97 1 - 0 - 0 2 2 - 5 97 1 - 1 0 - 2 3 - 4 83 9 2 - - 1 5 3 - 5 98 1 0 - 0 - 1 4 - 5 98 2 0 0 - - 1 Overall blocking = 1.2% (Percentages within the tables are estimated to the nearest per cent. Thus 100 indicates a percentage value above 99.5%, while 0 indicates a percentage value below 0.5%). The above estimates illustrate the scheme's performance for a very simple network and for just two particular patterns of offered traffic; dynamic routing as described in this example can perform similarly for a wide range of asymmetric and symmetric networks and over a wide range of overload conditions. In the computer simulation used to generate Tables 2 and 3, the search for a new alternative route between two nodes when the current nominated alternative route has failed on the previous call takes place stochastically through all alternatives. Once a suitable alternative route has been found it is used repeatedly for re-routing so that routes used successfully are re-used. Moreover, for the network as a whole, the random reset procedure searches stochastically through the space of alternative routing patterns with a bias towards especially beneficial patterns of carried traffic. It is believed that the fact that such a simple scheme can perform so well is related to features of the transient response of large circuit-switched networks. In many networks, it may be satisfactory for all routes to have an equal probability of being chosen. However, in other networks there is preferably a bias towards or against certain routes. For example, certain routes may be especially likely to have spare capacity, or some alternative routes may be forbidden. Probabilities for the alternative routes can be chosen accordingly. In the above example, each time that the current alternative route for a particular source-destination pair was reset, the choice of a new current alternative route was made randomly or stochastically. It will be noted however that, even if each individual choice is made by a pseudo-random or cyclic mechanism, the overall routing pattern (i.e. the entire collection of current alternative routes used by all the source-destination pairs in the network) will still be random. This is because even when a cyclic list is used, the movement through the list is randomly driven by the call arrival processes, which are of their nature stochastic. In some networks, one particular link may have much larger offered traffic and capacity than associated links. This can create problems if there is a large overflow from the high capacity link, say link (i,j), since, for example, an intermediate or tandem node selected for rerouting as described above may have to deal with large numbers of calls in quick succession. To overcome this in a further preferred embodiment of the invention, a sub-set of s possible two-link alternative routes is chosen from the N alternative routes available. These s two-link alternative routes each constitutes a current alternative route, and the s current alternative routes are used in cyclic fashion. A small number s (s < N) of indicators (k 1 , k 2 ,...,k s ) is used, together with a pointer r ← {1,2,...,s}. If a call is blocked on the direct link (i,j) it is rerouted on the alternative route through node k r , if that route is available taking into account trunk reservation. If that route is not available the call is lost and the indicator k r is replaced by a value chosen from the set {1,2,...,N} - {i,j}. Whether or not the alternative route through node k r is available the pointer r is updated to r(mod s) + 1. Thus if a sequence of calls is blocked on the direct link (i,j) they are offered, in cyclic fashion, to the alternative routes through nodes (k 1 ,k 2 ,...,k s ). When an alternative route within the cycle is found unavailable its place within the preferred subset of tandem nodes (k 1 ,k 2 ,...,k s ) is taken by another route through an alternative tandem node, and the cycle continues. Note that the cycle may include two or more incidents of a particular route so some routes may carry more than 1/s of the overflow traffic. The method has been described for a network which is fully-interconnected, but it can readily be adapted for a network which is not fully-interconnected. Two simple alternative adaptations are as follows:- (i) to implement the method as described with a missing link treated as a link of capacity zero. Traffic for a source-destination pair corresponding to such a link would automatically be offered to a two-link route - the one used previously if the last such attempt had been successful, and a randomly chosen two-link route otherwise. (ii) to replace a missing link by a virtual link, constructed by reserving a number of circuits on one or more two-link paths. This would result in a network of virtual links which is fully-interconnected. In certain circumstances, for example if a local exchange is interconnected with two main exchanges, there will be two or more direct routes for calls from an exchange. In accordance with a further preferred embodiment of the invention, if one direct route is blocked the call is offered to each other direct route. The order in which the call is offered to the direct routes may be determined in accordance with any desired criteria, for example a predefined order may be utilised. Only if all direct routes are blocked is an alternative indirect route selected. Figure 4 illustrates the use of the method in accordance with the invention in such a network. In Figure 4, node A is the origin node, and node L the destination node. As shown in Figure 4, nodes A, B, C, and D are trunk exchanges, and node L is a local exchange, having a respective processor P. Node L is connected to two trunk exchanges B and C. Calls from trunk exchange A to local exchange L therefore have two preferred main network routes, via nodes B and C. If these are both busy, a third choice is used via the current nominated alternative (i.e. indirect) route. In the embodiment illustrated the currently nominated alternative route is via tandem node D. A routing table is held at node A, which holds values defining the sequence in which calls are offered to the available choices of route. Table III is an example of a suitable routing table showing the sequence of preferred routes, and trunk reservation parameters at node A for calls to node L. The fixed choice section of the table constitutes a first list of the third and fourth aspects of the present invention, and the dynamic choice section of the table constitutes a second list of the third and fourth aspects of the present invention. Each call offered from node A to node L first attempts connection via node B. Because this is the most preferred route, the trunk reservation parameter is 0. If the route via node B is not available, connection via node C is attempted. In the example illustrated, the trunk reservation parameter on route 2 via node C is 3, indicating that this route is slightly less preferred than route 1. If connection is not possible via either of preferred routes 1 or 2, an alternative route is attempted, via the current nominated alternative route, (currently via node D in Table III). From node D, only the two direct routes are used, via nodes B and C respectively, the route via node B being tried first. It will of course be understood that Figure 4 illustrates only a small part of a large network comprising many nodes, for example nodes E, F, etc., as illustrated in Table IV, which is a dynamic choice list constituting a third list of the third and fourth aspects of the present invention. Dynamic choice list . . . D ← E F . . . If a call attempted via node D fails due to congestion, then node D is replaced in the second list by a new tandem node (node E in the example illustrated) selected from the dynamic choice list. In the embodiment illustrated in Table IV, the choice of the next node to try is made cyclically, simply by stepping through the subsequent available tandem nodes defining the various alternative routes in sequence. In alternative embodiments however choice as to the next node to select from those available may be made by a truly random or pseudo-random method, or by choosing to step through the dynamic choice list in some other predefined sequence. An example of this arises when it is desired to utilise the same dynamic choice list for a number of destination nodes from a given source node. In such a case, it may be sensible to allocate a different pointer to the dynamic choice list in accordance with different rules. For example, if the number of items in the dynamic choice list is a prime number, it may be desirable to arrange for each pointer to step through the list with a different increment. Figure 5 illustrates an alternative aspect of the invention, which is applicable to the routing of international calls from a trunk exchange T to a foreign destination F. The trunk exchange is connected to a number of possible international gateway exchanges I, each of which has a route to the foreign countries. Each of T, F, and I comprises an exchange having a respective processor P. In this case, there exists no direct or preferred route from the trunk exchange to the foreign destination. This case may be likened to the case illustrated in Figure 1, but in which the direct route between the source and destination nodes has a capacity of 0. In this aspect of the invention, the last used route through a particular gateway is the current nominated route, and continues to be used, until that route becomes unavailable. At this point, another gateway is selected of the five available. The selection may be carried out according to any desired criteria, for example at random, cyclically, or according to fixed weighting criteria, such as cost criteria. The new route is made the current nominated route, and future calls to the foreign destination are routed via that route, until that route in turn becomes unavailable. In a preferred embodiment of this alternative aspect of the invention, a call is lost if circuits are not free on the current nominated route, i.e. that selected from the first list. In alternative embodiments however, a call blocked on the current nominated route may be offered to one or more further alternative routes, the current nominated route being changed whenever the current nominated route is unable to accept a call. In practice we have found that in many networks, there is little advantage to the network in offering calls to further alternative routes, and the additional processing necessary results usually in an undesirable sacrifice of simplicity. Thus, it is generally preferable for efficient network operation for a call at a particular source node n to a given destination node to be lost if the first same destination node, the availability of any direct route to the destination node is checked (or each of the direct routes if the call can be directly routed via two or more alternative nodes). If a direct route to the destination node is not available, the current nominated alternative route is tried. If, because a call has failed, it is decided to change the current nominated alternative route, then a selection is preferably made amongst all the available alternatives, including the route offered to the previous call (which was blocked). Alternatively, the route which was unavailable for the last call can be excluded from the search. Trunk reservation is advantageous as, for example, it ensures that direct links are unlikely to become blocked with alternatively routed calls, thus forcing calls which could otherwise be routed directly on the links to be routed on alternative two link routes. However, it is possible to use different methods to achieve similar results, although again simplicity may be sacrificed. It will of course be appreciated that a wide range of other specific implementations may be realised, within the scope of the appended claims. In particular it will be understood that the term circuit switched network as used herein is intended to mean any network in which calls are allocated at their outset to a particular route, and the term is intended to include within its scope networks comprising virtual circuits, for example networks in which no fixed bandwidth is allocated to each call on its chosen route, for the duration of the call, but the route is itself fixed. It will be understood also that the method in accordance with the invention may be used not only in the routing of calls for which the origin and destination nodes indicated above correspond respectively with the exchanges of the calling and originating subscribers, but also in cases where either or both of the origin and destination nodes are simply intermediate nodes on a more complex overall route.

Claims: A method of routing in a circuit-switched network comprising a plurality of nodes and wherein each node has stored therein, for each destination node, a respective first list containing the identity of at least one preassigned route to the respective destination node and a respective second list containing the identity of at least one alternative route to the respective destination node, the method comprising, for a call from an origin node to a destination node, offering the call to the at least one preassigned route of the first list and, in the event that the call is blocked on the at least one preassigned route, offering the call to an alternative route of the second list, the method being characterised by, in the event that the offered alternative route is blocked, selecting an entry of a respective third list stored in the origin node and containing the identities of a plurality of alternative routes to the respective destination node, the second list being a subset of the third list, and copying the selected entry of the third list into the second list at the entry corresponding to the blocked offered alternative route. A method as claimed in claim 1, wherein, in the event that the first offered alternative route is blocked, no other alternative route is offered and the call thus becomes abandoned. A method as claimed in claim 1, wherein, in the event that the first offered alternative route is blocked, after the selected entry of the third list has been copied into the second list the call is again offered to an alternative route of the second list. A method as claimed in any one of the preceding claims, wherein the second list at the origin node has a plurality of entries each containing the identity of an alternative route to the destination node, and each successive call to the destination node which is blocked on the at least one preassigned route of the first list is offered to the alternative route of a respective cyclically selected entry of the second list. A method as claimed in any one of the preceding claims, wherein the selection from the third list is made such as to give equal weight to each of the alternative routes. A method as claimed in claim 5, wherein the selection from the third list is made by a stochastic, pseudo-random or cyclic mechanism. A method as claimed in claim 6, when claim 5 is dependent on any one of claims 1 to 3, wherein the selected entry of the third list constitutes the associated second list, and the steps of selecting and copying are together constituted by the selecting step. A method as claimed in claim 6, wherein successive selections from the third list are made by stepping sequentially through the entries of the third list. A method as claimed in either claim 6 or claim 7, wherein the respective third lists for a plurality of destination nodes are constituted by a single third list and the respective selections from the single third list are made by stepping through the list with different respective step sizes. A method as claimed in any one of claims 1 to 4, wherein the selection from the third list is made on the basis as to which route is likely to be available. A method as claimed in any one of the preceding claims, including the step of preventing the selection of the blocked offered alternative route from the third list. A method of routing in a circuit-switched network comprising a plurality of nodes and wherein each node has stored therein for each destination node, a respective first list containing the identity of an initially-allocated route to the respective destination node, the method comprising, for a call from an origin node to a destination node, offering the call to the initially-allocated route of the first list, the method being characterised by, whenever a call is blocked on an offered route of the first list, selecting an entry of a respective second list stored in the origin node and containing the identities of a plurality of alternative routes to the respective destination node, the first list being a subset of the second list, and copying the selected entry of the second list into the first list at the entry corresponding to the blocked offered alternative route. A method as claimed in claim 12, comprising, in the event that the offered initially-allocated route is blocked and the first selected entry of the second list has been copied into the first list, offering the call to the current route of the first list. A method as claimed in claim 13, comprising, in the event that the second offered alternative route is blocked and a second selected entry of the second list is copied into the first list, offering the blocked call only once more to the current route of the first list. A method as claimed in claim 12, wherein the first list has a plurality of entries each containing the identity of an initially-allocated route to the respective destination node, and, in the event that an offered route is blocked, the call is offered again to a route of the first list. A method as claimed in claim 15, wherein, in the event that a second offered route is blocked, the call is abandoned. A method as claimed in claim 15, wherein, in the event that a third offered route is blocked, the call is abandoned. A method as claimed in any one of claims 15 to 17, wherein each successive call is offered to the route of a respective cyclically selected entry of the first list. A method as claimed in any one of claims 12 to 18, wherein successive selections from the second list are made by stepping cyclically through the entries of the second list. A method as claimed in claim 19, wherein the respective second lists for a plurality of destination nodes are constituted by a single second list and the respective selections from the single second list are made by stepping through the list with different respective step sizes. A method as claimed in either claim 19 or claim 20, when claim 19 is dependent on any one of claims 12 to 14, wherein the selected entry of the second list constitutes the first list, and the steps of selecting and copying are together constituted by the selecting step. A method as claimed in any one of claims 12 to 21, including the step of preventing the selection of the blocked offered route from the second list. Apparatus for routing traffic through a circuit-switched network comprising a plurality of processors at respective nodes, each node having stored therein, for each destination node, a respective first list containing the identity of at least one preassigned route to the respective destination node and a respective second list containing the identity of at least one alternative route to the respective destination node, each processor being adapted to offer a call between its respective node and a destination node to a preassigned route of the first list and, in the event that the call is blocked on the at least one preassigned route of the first list, to offer the call to an alternative route of the second list, the apparatus being characterised in that: each node comprises a respective third list containing the identities of a plurality of alternative routes to the respective destination node, and each processor is adapted to select, in the event that the offered alternative route is blocked, an entry of the respective third list, and to copy the selected entry of the third list into the second list at the entry corresponding to the blocked offered alternative route. Apparatus as claimed in claim 23, wherein each processor is adapted to abandon a call for which a first offered alternative route is blocked. Apparatus as claimed in either claim 23 or claim 24, wherein the selected entry of the third list constitutes the associated second list, and the selecting and copying performed by the processor are together constituted by the selecting of the entry. Apparatus as claimed in either claim 23 or claim 24, wherein each processor is adapted to select from the plurality of alternative routes of its associated third list a subset comprising a plurality of alternative routes, to store said subset in its associated second list, to offer successive calls which are blocked on the at least one preassigned route to respective cyclically selected members of the subset, and, if a respective member of the subset to which a call is offered is blocked, to copy the selected entry of the third list into the second list at the entry corresponding to said respective member. Apparatus as claimed in any one of claims 23 to 26, wherein each processor is adapted to make successive selections from its associated third list by stepping cyclically through said plurality of entries of the third list. Apparatus as claimed in claim 27, wherein at a node the respective third lists for a plurality of destination nodes are constituted by a single third list and the processor is adapted to make respective selections from the single third list by stepping through the list with different respective step sizes. Apparatus as claimed in any one of claims 23 to 28, wherein the processor is adapted to prevent the selection of the blocked offered alternative route from the third list. Apparatus for routing traffic through a circuit-switched network comprising a plurality of processors at respective nodes, each node having stored therein, for each destination node, a respective first list containing the identity of an initially-allocated route and a respective second list containing the identity of an alternative route to the respective destination node, each processor being adapted to offer a call between its associated respective node and a destination node to the initially-allocated route of the first list and, in the event that the initially-allocated route is blocked, to offer the call to the alternative route of the second list, characterised in that, in use, each respective second list contains the identities of a plurality of alternative routes to the respective destination node, and each processor is adapted to select, in the event that the initially-allocated route is blocked, an entry of the respective second list, and to copy the selected entry of the second list into the first list at the entry corresponding to the blocked offered alternative route. Apparatus as claimed in claim 30, wherein each first list contains a plurality of initially-allocated routes to the respective destination node, and each processor is adapted to abandon a call for which a second offered route is blocked. Apparatus as claimed in either claim 30 or claim 31, wherein the selected entry of the second list constitutes the first list, and the selecting and copying performed by the processor are together constituted by the selecting of the entry. Apparatus as claimed in any one of claims 30 to 32 wherein each respective first list contains, in use, the identities of a plurality of initially-allocated routes, each processor is adapted to create initially each associated first list by selecting a respective subset of alternative routes from the plurality of alternative routes in the respective second list and storing the respective subset in the respective first list to constitute the plurality of initially allocated routes, and to offer each successive call to a destination node to a respective cyclically selected member of the subset in the respective first list. Apparatus as claimed in any one of claims 30 to 33, wherein each processor is adapted to make successive selections from its associated second list by stepping cyclically through the entries of the second list. Apparatus as claimed in claim 34, wherein at a node the respective second lists for a plurality of destination nodes are constituted by a single second list and the processor is adapted to make respective selections from the single second list by stepping through the list with different respective step sizes. Apparatus as claimed in any one of claims 30 to 35, wherein the processor is adapted to prevent the selection of the blocked offered route from the second list.

Inventors: GIBBENS RICHARD JOHN, KELLY FRANCIS PATRICK, KEY PETER BERNARD, STACEY ROGER RICHARD, TURTON PAUL ANTHONY, WHITEHEAD MARTIN JOHN, GIBBENS, RICHARD JOHN, KELLY, FRANCIS PATRICK, KEY, PETER BERNARD, STACEY, ROGER RICHARD, TURTON, PAUL ANTHONY, WHITEHEAD, MARTIN JOHN
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1986
Main classification: H04Q3
IPC codes: H04M3, H04Q3