Dynamic call routing system

Abstract: Network congestions can largely be combated by preventing a congested point being approached right back at that network location where a new connection is initiated. To combat network congestions, at certain (important) points (A) at which new connections are initiated, the chosen (intermediate or final) destinations are automatically subjected to an investigation as to whether the chosen destination is or is not available at that instant. In a dynamic filter (F), destinations which appear to be unavailable are recorded for a certain, optionally variable, relaxation time in order thereby to hold back calls intended for such congested destinations for said relaxation time.

Description: A. BACKGROUND OF THE INVENTION The invention relates to a transmission system comprising a transmission network for the transmission of transmission signals between peripheral systems connected to said transmission network, in which, prior to such a transmission of transmission signals between a first peripheral system and a second peripheral system, said first peripheral system emits a call code signal to the transmission network, which call code signal comprises a code indicating said second peripheral signal, after which the transmission network or the second peripheral system sends back a 'positive' or 'negative' status code signal, indicating whether said second peripheral system is, or is not, accessible or available, respectively. Such a system is known from "PROCEEDINGS OF THE FIRST INTERNATIONAL CONFERENCE ON INTELLIGENT NETWORKS", 14 March 1989, Bordeaux (FR), pages 28-30, BEZEMER ET AL: "Controlling grades of services in integrated networks". This document discloses on pages 28-30 a dynamic call gapping method. According to this method a list for "hard-to-reach codes" is filled automatically, with numbers that experience failures. During a relaxation time for each number in the list, all calls to that number are prohibited. The use of such a list is based on the insight that, given a particular total network capacity, serious congestions can largely be combated by preventing a congested point being approached at the source itself, i.e. the network location at which a new connection is initiated. The said peripheral systems may, for example, be subscribers, exchanges or other switching systems, or even (subsidiary) transmission networks. In this last case, the total transmission system is therefore formed by several networks connected together. This list for "hard-to-reach codes" is a measure which is taken against network congestion in a transmission system as referred to above. The concept of congestion (congested etc.) will be understood below as meaning both the inaccessibility of a particular destination, for example as a consequence of the lack of adequate means, such as transmission channels, in a particular section of the transmission network, and the unavailability of a particular destination, for example as a consequence of the destination (second peripheral system) being busy, out-of-action or damaged. In this connection, congestion is therefore caused both by the called (second) peripheral system itself not being available, and by the transmission network via which the peripheral system to be called cannot be accessed. Incidentally, (in)accessibility can also be understood as meaning (un)availability, since in this case a peripheral system which is, for example, unavailable is in fact also inaccessible to the calling peripheral system, i.e. no information exchange can take place. For said reason, the concept of (in)accessibility will also include the concept of (un)availability. Hitherto it has been customary in a (large) transmission system when more or less structural congestion has been detected in a section of the network, to subject the traffic handling in such a congested network section manually (by an operator) or semi-automatically (by logging equipment) to a further analysis in order to be able to take appropriate measures such as rerouting particular traffic flows. B. SUMMARY OF THE INVENTION It is an object of the invention, inter alia, to provide a transimssion system as described in the preamble which is more flexible. The transmission system according to the invention is characterised in that at least one filter device is connected to the first peripheral system and to the transmission network, which filter device comprises means for recording and for storing, during a relaxation time, comparison code signals corresponding to said call code signals emitted by the first peripheral system or to a portion of such a call code signal, in response to which the transmission network has sent back a 'negative' status code signal indicating 'inaccessible' or the called second peripheral system has sent back a 'negative' status code signal indicating 'unavailable', which filter device furthermore comprises means for comparing a call code signal emitted by said first peripheral system with the comparison code signals then recorded and for not transmitting said call code signal in the event of apparent correspondence between said call code signal or a portion thereof and one of said comparison code signals during said comparison, but for transmitting said call code signal in the event of apparent noncorrespondence between said call code signal or a portion thereof and one of said comparison code signals during said comparison, whereby the filter device is adapted for adjusting the relaxation time. Due to the at least one filter device which records and stores, during the relaxation time, the comparison code signals corresponding to said call code signals emitted by the first peripheral system or to a portion of such a call code signal, in response to which the transmission network has sent back a 'negative' status code signal indicating 'inaccessible' or the called second peripheral system has sent back a 'negative' status code signal indicating 'unavailable', and which filter device compares the call code signal emitted by said first peripheral system with the comparison code signals then recorded, in response to which in the event of the apparent correspondence said call code signal is not transmitted and in the event of apparent noncorrespondence said call code signal is transmitted, and which filter device can adjust the relaxation time, a telecommunication system is provided which is very flexible. The invention is based on the insight, inter alia, that there should be a relationship between the list for "hard-to-reach codes" (being full or empty) and the congestion (causing large or small problems). According to the invention, at various (important) points in the transmission system at which new connections are initiated, the chosen destinations are therefore automatically subjected to an investigation, viz. as to whether the destination selected in a call is, or is not, accessible or available at that instant ('locking condition investigation'). Such an initiation point and such a destination may both be regarded as peripheral system for the subsidiary network situated in between. The filter device, by means of which the call code signals are investigated (filtered) for the possible selection of a congested destination (second peripheral system) is not static in nature, but dynamic in the sense that the contents of the filter are automatically filled with the codes of congested destinations under the control of the call codes and/or the status code signals received back in response thereto and in the sense that the relaxation time, during which comparison signals are recorded and stored, can be adapted by the filter device. Said codes are either derived directly from the call code signals or from the status code signals, if the latter - as will usually be the case - not only indicate the accessibility/availability of the called peripheral system but also indicate which second peripheral signal the (failed) call was intended for. The comparison codes with which the filter is filled can also be formed by error location codes sent back from the transmission network which can be sent concomitantly with the 'negative' status code signals in the event of the inaccessibility of a called destination as a consequence of a congestion at a network location indicated by said error location code. In particular in a large network it is not readily possible in practice to store congestions which have been detected and the result thereof for the entire network in each dynamic filter. For said reason an embodiment of the telecommunication system according to the invention is characterised by a multiplicity of the said filter devices, each filter device relating to a section of the transmission network, and also by a filter selection device connected, on the one hand, to the first peripheral system and, on the other hand, to each of said filter devices and comprising means for receiving the call code signals emitted by the first peripheral system and, on the basis of such a call code signal, selecting and connecting, to the first peripheral system, the filter device which relates to said section of the transmission network to which the second peripheral system indicated by the call code is connected. For each call, it is therefore determined for which portion of the network the call is destined, after which the call is investigated for a possible congested status of the destination by the (sub)filter assigned to said network portion. Because use is made of subfilters for each subnetwork, the processing time can be kept within acceptable limits. With respect to the relaxation time ('reset' time) - which has to ensure that the filter does not 'clog up' - there are several options. One of the options in this connection is that the relaxation time (t) assigned to a particular comparison code signal is made the longer, the greater the number of call code signals emitted by the first peripheral system (A) and corresponding to said comparison code signal is. The relaxation time is therefore made the longer, the greater the call pressure for said (congested) destination is. A second option is that the relaxation time (t) assigned to a particular comparison code signal is made the longer, the greater the number of 'negative' status code signals received back after the emission of call code signals corresponding to said comparison code signal from the transmission network (T) or from the second peripheral system (B) is. The relaxation time is restarted, for example, after further reception of 'negative' status code signals (for example as a consequence of sluggishness of the network) after the filter has already been activated as a result of receiving a (first) 'negative' status code signal, as a result of which the total relaxation time is thus increased. A third option is that the relaxation time (t) assigned to a particular comparison code signal is terminated after the receipt of a 'positive' status code signal, indicating that said second peripheral system is available again, from the second peripheral system (B), indicated by the call code signal corresponding to said comparison code signal. In this case, provision is therefore made that the filter is immediately 'reset' as soon as a congestion logged in the filter is over. Under these circumstances it is possible to give the relaxation time a particular value initially and to stop the relaxation time as soon as the second peripheral system is again accessible or available according to a 'positive' status code signal. It is equally possible to terminate the relaxation time exclusively (that is to say, without 'presetting') on receiving back the 'positive' status code signal. In relation to the recording of congested destinations (ie. the codes thereof) in the filter, this can be done, according to the invention, as soon as it has been signalled (once) that a called destination is congested, but it is also possible, according to the invention, only to record a congested destination in the filter after said destination has been called a number of times (for example, two or three times) and 'unavailable' has been received as status code signal in response thereto. Said 'number of times' may, for that matter, be in the absolute sense or, for example, the number of times for each shorter or longer time period, in other words the short- or long-term average. It may also be another statistical value of the number of times, for example a 'leaky bucket' value thereof. The invention further relates to a filter device for use in a transmission system comprising a transmission network for the transmission of transmission signals between peripheral systems connected to said transmission network, in which transmission system, prior to such a transmission of transmission signals between a first peripheral system and a second peripheral system, said first peripheral system emits a call code signal to the transmission network, which call code signal comprises a code indicating said second peripheral signal, after which the transmission network or the second peripheral system sends back a 'positive' or 'negative' status code signal, indicating whether said second peripheral system is, or is not, accessible or available, respectively. It is a further object of the invention, inter alia, to provide a filter device as described above which makes the transmission system more flexible. The filter device according to the invention is characterised in that the filter device is connectable to the first peripheral system and to the transmission network, which filter device comprises means for recording and for storing, during a relaxation time, comparison code signals corresponding to said call code signals emitted by the first peripheral system or to a portion of such a call code signal, in response to which the transmission network has sent back a 'negative' status code signal indicating 'inaccessible' or the called second peripheral system has sent back a 'negative' status code signal indicating 'unavailable', which filter device furthermore comprises means for comparing a call code signal emitted by said first peripheral system with the comparison code signals then recorded and for not transmitting said call code signal in the event of apparent correspondence between said call code signal or a portion thereof and one of said comparison code signals during said comparison, but for transmitting said call code signal in the event of apparent noncorrespondence between said call code signal or a portion thereof and one of said comparison code signals during said comparison, whereby the filter device is adapted for adjusting the relaxation time. Due to the filter device which records and stores, during the relaxation time, the comparison code signals corresponding to said call code signals emitted by the first peripheral system or to a portion of such a call code signal, in response to which the transmission network has sent back a 'negative' status code signal indicating 'inaccessible' or the called second peripheral system has sent back a 'negative' status code signal indicating 'unavailable', and which filter device compares the call code signal emitted by said first peripheral system with the comparison code signals then recorded, in response to which in the event of the apparent correspondence said call code signal is not transmitted and in the event of apparent noncorrespondence said call code signal is transmitted, and which filter device can adjust the relaxation time, a telecommunication system is provided which is very flexible. From "IEEE MILITARY COMMUNICATIONS CONFERENCE", vol. 1, 19 October 1987, Washington, US, pages 7-12, L.F. Gifford: "Adaptive routing and traffic control in damaged circuit switched networks" it is known to provide adaptive routing in damaged circuit switched networks at the hand of a failure detection algorithm which uses local information instead of global information. The transmission system according to the invention and the filter device according to the invention are not known from this reference. Further, there is no incentive in this reference pointing into the direction of the invention. From EP-A-0 291 815 it is known to provide a method for limiting the traffic flow based on incoming call attempts in a telecommunication system, by releasing a percentage proportion of the incoming call attempts for further processing or subjecting a percentage proportion of the incoming call attempts to a limiting measure for avoiding congestion as determined by the traffic flow. The incoming call attemps are cyclically counted by a counter with adjustable counting volume and, for each counting cycle, on the one hand, in each case at least one first adjustment information item causing an increase in the traffic flow by a respective increment is formed and, on the other hand, in each case at least one counter control information item causing a reduction in the counting volume by a respective count is formed until a minimum value predetermined for the counting volume is reached. In the event of negative results of call attempts released for further processing, on the one hand, in each case a second adjustment information item provided for reducing the traffic flow by a respective decrement is formed, and, on the other hand, in each case at least one resetting information item provided for resetting the counting volume to a predetermined maximum value is formed. The transmission system according to the invention and the filter device according to the invention are not known from this reference. Further, there is no incentive in this reference pointing into the direction of the invention. C. REFERENCES "PROCEEDINGS OF THE FIRST INTERNATIONAL CONFERENCE ON INTELLIGENT NETWORKS", 14 March 1989, Bordeaux (FR), pages 28-30, BEZEMER ET AL: "Controlling grades of services in integrated networks" "IEEE MILITARY COMMUNICATIONS CONFERENCE", vol. 1, 19 October 1987, Washington, US, pages 7-12, L.F. Gifford: "Adaptive routing and traffic control in damaged circuit switched networks" EP-A-0 291 815 D. EXEMPLARY EMBODIMENTS 1. Description of figures Figure 1 shows a first exemplary embodiment of the invention, in which a first peripheral system is able to make contact with a number of second peripheral systems (of which four are shown) via a filter device comprising a selection device and three subfilters and via a network comprising three subnetworks. Figure 2 shows an exemplary embodiment of the filter device shown in Figure 1; the filter device is filled from the call code signal side. Figure 3 shows an exemplary embodiment corresponding to the exemplary embodiment of Figure 2, in which the filter device is filled from the status code signals. Figure 4 shows a transmission system according to the invention in which the said peripheral systems are formed by (subsidiary) networks - for example a section of the public transmission network, a public telecommunication exchange or a PABX - which can be connected to each other via a (subsidiary) network and in which the said filter devices are incorporated between said (subsidiary) networks. 2. Operation In the exemplary embodiment shown in Figure 1, a periphal system A is connected to a transmission network T via a filter F. Connected to said network T is a (large) number of peripheral systems (B), of which four, B1..4, are shown. The peripheral systems may be formed by subscribers, or small or large subscriber exchanges (PABXs). In dealing with this exemplary embodiment, reference will be made, for the sake of convenience, to 'the subscribers' A, B1, B2,... From the point of view of subscriber A, the network T is (notionally) split into three (subsidiary) networks T1, T2 and T3. All the calls sent out from A pass through the filter (F) in which they are investigated for their destination: if a destination B1, B2,..., represented by a call code signal emitted by A is known to be congested (damaged, busy, out-of-action), the call code signal is held back by the filter. If the destination is not known to be congested, the call code signal is transmitted. When the transmitted call code signal arrives at its destination, for example B2, the latter indicates, by means of a status answerback signal to A, whether the connection can or cannot be implemented for the exchange of speech or data. If, for example, the called subscriber B2 is busy at the instant of the call, the status signal will indicate, by means of a status code signal, that B2 is not available. When such a status code signal is received at the A end, a representation of B2 is recorded in the filter F and stored for a certain (relaxation) time. If a call to B2 is now initiated again from A within the relaxation time (this frequently occurs in practice, particularly if A and B2 both have an exchange function), such a call is held back by the filter F. Said situation is continued until the relaxation time has elapsed. If many calls destined for B2 appear during the relaxation time (and these are therefore all stopped by the filter), it may, according to a preferred embodiment of the invention, be sensible to prolong the relaxation time in accordance with said number of calls. The great advantage of holding back calls in this way which are destined for a subscriber who is known to be unavailable is that the network T does not receive any (avoidable) calls for processing which ultimately do not result in a connection. As a result, the network T is not unnecessarily loaded, as a result of which, for example, (internal) network congestion occurs less rapidly. In addition, the network is not unnecessarily loaded with calls which do not ultimately result in (billable) connections. In the present exemplary embodiment, the filter F comprises a selector SEL and three (sub)filters F1..3. The selector SEL determines on the basis of the call code signal emitted by A the section, T1, T2 or T3, of the network T to which the subscriber B to be called is connected. If said subscriber (B2) is connected to section T2, the call code signal is passed via subfilter F2 to B2. If it appears there that B2 was found to be congested during the previous call and the relaxation time has not yet elapsed, the call to B2 is stopped by F2; if the call code of B2 is not present in F2, the call code signal is then transmitted. Figure 2 shows a possible implementation of the filter F in Figure 1. The filter F is formed by a switching device SWTCH to which the call code signals originating from A are applied and which does, or does not, connect through said signals to the downstream network T. The switching device SWTCH is controlled by means of a control signal s from a comparator COMP to which the call code signal 'call' and a series of n comparison code signals 'comp' are applied. Said comparator COMP compares the applied 'call' and 'comp' signals. In the event of correspondence, the comparator COMP delivers to the switching device SWTCH a control signal s having a value which brings about the result that the call code signal 'call' is not switched through by the switching device SWTCH; in the event of lack of correspondence between the call code signal 'call' and one of the n comparison code signals 'comp', the control signal s acquires a value which brings about the result that the switching device SWTCH does in fact switch the call code signal through to the network T. The series of comparison code signals 'comp' is generated by a comparison code signal generator GEN. The generator GEN shown in Figure 2 is formed by a memory device MEM and a control device CONT which controls the memory device with the aid of a control code signal c. The two most important tasks of the control device CONT are, on one hand, causing the contents of the memory MEM to be read out in the event of a call from A and causing a call code to be written into said memory if it is found that the destination (B) indicated by said call code is not available. The memory device MEM comprises three memories, indicated by T1..3 in which the call codes of the destinations B in the network portions T1..3 which are unavailable are recorded. After A sends out a call code signal intended for a particular destination B, said code signal 'call' is applied to the comparator COMP and also to the control device CONT which determines, for example on the basis of the first two or three code elements (bytes) of said code signal, the section of the network, T1, T2 or T3, for which the call is destined (to which section the subscriber B to be called is connected), for example the network section T2. The contents of the submemory 'T2' are then read out ('R'), the codes recorded therein at that instant being transmitted as comparison code signals 'comp' to the comparator COMP, which compares them with the call code signal. It is pointed out that for the purpose of a correct 'timing', it will be necessary to incorporate memory buffers in the circuit on either side of the comparator COMP for the purpose of temporarily storing the codes 'call' and 'comp'; said buffers are, however, not shown in the figure for the sake of clarity. The same applies to the control device CONT, to which the applied signals 'call' and 'stat' (to be discussed below) will also have to be temporarily buffered. If it is found, after comparison in the comparator COMP, that the call code does not occur in the (sub)memory MEM(T2), the call code signal 'call' is sent through to the network T(2) by the switching device SWTCH, after which it arrives at its destination B(2). If said destination is available, this is signalled by means of a 'positive' status code signal 'stat' which is sent back to the calling subscriber A, after which the connection between A and B is implemented. If, however, the called subscriber B is not available (for example busy as a result of a connection to another subscriber), a 'negative' status code signal ('unavailable') is sent back. On receipt of such a 'negative' status code signal - the status code signal 'stat' is applied to the control device CONT - the control device CONT is activated in order to write ('W') the call code signal to which the negative status code signal 'stat' relates into the memory device MEM. In the event of a subsequent call from A, said last call code, which was after all destined for a destination found to be 'unavailable' will therefore also be used as comparison code 'comp'. Each of the comparison codes recorded in the memory device MEM remains there only for a certain time, the so-called relaxation time, after the expiry of which the relevant code is erased. After the expiry of the relaxation time of a comparison code written into the memory device, the respective destination B does not, of course, have to be available again. In that case, after said destination B has been called from A and B has (again) sent back a 'negative' status code signal, the call code for the unavailable destination is again written into the memory device MEM. The relaxation time may be permanently set, but it is also possible to prolong it in the event of a large number of calls to the unavailable destination. This prevents the network being loaded with a relatively large number of 'stray' calls after the relaxation time has elapsed. Because it always takes some time until the status code signal is received back after a call has been emitted to a particular destination, it may happen that, after it has been found as a result of receiving a 'negative' status code signal that the respective destination is congested, still more 'negative' status code signals are received, viz. as a response to call code signals which were still under way in the network. Preferably, the relaxation time will be 'reset' on receiving each of said 'negative' status code signals still received, as a result of which the total relaxation time is therefore prolonged. Conversely, it is also possible to stop the relaxation time (or at least shorten it) as soon as it is found, by means of a 'positive' status signal, that the respective destination B has become available. This prevents calls from not being transmitted (by SWTCH) in fact wrongly while the destination to be called is in fact available. Both the mechanisms mentioned above for influencing the length of the relaxation time can be operative at the same time. Finally, the filter F can be so designed that the switching device SWTCH does in fact transmit a call code signal, for example, the first time a corresponding comparison code occurs in the memory device MEM, when the call code signal should not be transmitted by SWTCH, but does not do so a second and subsequent time. The threshold should also be capable of being set at a greater number of calls than two. This variant prevents call code signals not being transmitted while their destination is nevertheless available once again when they reach it; this situation may occur in relatively sluggish signalling networks, as a consequence of which the adjustment of the filter F, viz. the 'resetting' of the contents of the memory device MEM, lags too far behind the actual state. However, to design this variant it is necessary to incorporate a counting device with memory and comparison means between the comparison device COMP and the switch device SWTCH, which counting device then counts the number of times that a corresponding comparison code has been found for a call code and which activates the switching device SWTCH, by means of the control signal s, so as not to transmit the call code signal as soon as the number of counted cases of correspondence between said call code and a comparison code goes above a set threshold. Figure 3 shows the same exemplary embodiment of the filter device as shown in Figure 2, but the filter F, i.e. the memory device MEM is filled from the status code signals received back insofar as the latter are 'negative'. The status code signals (stat) comprise, in said exemplary embodiment, a code, corresponding to the call code (call), which represent the called second peripheral systems B. It is also possible that the ('negative') status code signals comprise a code of the intermediate destination in the transmission network T, at which said network T has detected that the called second peripheral system B was not accessible, in other words an error location code. In the exemplary embodiment shown in this figure, the comparison codes recorded in the memory device MEM are therefore always formed by a section of the ('negative') status code signal representing the called destination (not accessible or available) or intermediate destination (for example, in the telephone number 050-8, the first section (050) represents an intermediate destination, viz. the exchange of the town of Groningen and the second section (8) the final destination, viz. the PABX of PTT Nederland. Just as in the preceding exemplary embodiment, all the call code signals (call) emitted by the first peripheral system A are compared with the comparison codes recorded in the memory device MEM (but now therefore derived from the 'negative' status code signals). With respect to the sequence of the comparison codes recorded in the (sub)filters F, it is pointed out that these will usually be recorded in time sequence, but the codes chronologically recorded in this way will preferably have to remain written in in their sequence sorted according to time of 'arrival'. In that way, the result is achieved that, provided the relaxation time of all the codes is equally long (!), only the comparison code written in earliest among the comparison codes written in will need to be monitored since the relaxation time of said code will have approached its reset instance the most closely. An advantage is therefore a simple monitoring of the relaxation times; a disadvantage is that the variable relaxation time options as indicated above cannot readily be used in this case. Figure 4 shows a transmission system comprising several transmission networks A, T, B1, B2, B3 and B4, which are connected to each other and form a large transmission network. The networks A and B1..4 can be regarded as peripheral systems of the network T. Incorporated between each of the networks is a filter device F which brings about the result that calls from a 'peripheral' network A, B1..3 to a destination network A, B1..3 do not reach said transmission network T, and therefore do not reach the intended destination network either, in the event of (known) congestion in the respective destination network A, B1..3 or in the network T situated in between. A filter F is incorporated not only between A and T, but also between B1 and T, B2 and T, B3 and T and B4 and T. Each of said networks B1, B2, B3 and B4 can, after all, function not only as a destination for calls, but also as a source of calls. Each call is therefore passed via a filter F from each network A, B1, B2, B3 and B4. Calls which are received from other networks are applied to the 'destination' network via a 'bypass' b. The dynamic call filters F, which all function in the same way as was described above in relation to the filter F between A and T, may be incorporated not only between the networks B1 and T, B2 and T, B3 and T and B4 and T, but also mutually between the networks A, B1, B2, B3 and B4.

Claims: Transmission system comprising a transmission network (T) for the transmission of transmission signals between peripheral systems (A,B1,B2,B3,B4) connected to said transmission network (T), in which, prior to such a transmission of transmission signals between a first peripheral system (A) and a second peripheral system (B1,B2,B3,B4), said first peripheral system (A) emits a call code signal (call) to the transmission network (T), which call code signal (call) comprises a code indicating said second peripheral signal (B1,B2,B3,B4), after which the transmission network (T) or the second peripheral system (B1,B2,B3,B4) sends back a 'positive' or 'negative' status code signal, indicating whether said second peripheral system (B1,B2,B3,B4) is, or is not, accessible or available, respectively, characterised in that at least one filter device (F) is connected to the first peripheral system (A) and to the transmission network (T), which filter device (F) comprises means for recording (CONT) and for storing (MEM), during a relaxation time, comparison code signals (comp) corresponding to said call code signals (call) emitted by the first peripheral system (A) or to a portion of such a call code signal (call), in response to which the transmission network (T) has sent back a 'negative' status code signal indicating 'inaccessible' or the called second peripheral system (B1,B2,B3,B4) has sent back a 'negative' status code signal indicating 'unavailable', which filter device (F) furthermore comprises means (COMP) for comparing a call code signal (call) emitted by said first peripheral system (A) with the comparison code signals (comp) then recorded and for not transmitting said call code signal (call) in the event of apparent correspondence between said call code signal (call) or a portion thereof and one of said comparison code signals (comp) during said comparison, but for transmitting said call code signal (call) in the event of apparent noncorrespondence between said call code signal (call) or a portion thereof and one of said comparison code signals (comp) during said comparison, whereby the filter device (F) is adapted for adjusting the relaxation time. Transmission system according to Claim 1, characterised by a multiplicity of the said filter devices (F1,F2,F3), each filter device (F1,F2,F3) relating to a section (T1,T2,T3) of the transmission network (T), and also by a filter selection device (SEL) connected, on the one hand, to the first peripheral system (A) and, on the other hand, to each of said filter devices (F1,F2,F3) and comprising means for receiving the call code signals (call) emitted by the first peripheral system (A) and, on the basis of such a call code signal (call), selecting and connecting, to the first peripheral system (A), the filter device (F1,F2,F3) which relates to said section of the transmission network (T1,T2,T3) to which the second peripheral system (B1,B2,B3,B4) indicated by the call code signal (call) is connected. Transmission system according to Claim 1 or 2, characterised in that the relaxation time (t) assigned to a particular comparison code signal (comp) is made the longer, the greater the number of call code signals (call) emitted by the first peripheral system (A) and corresponding to said comparison code signal (comp) is. Transmission system according to Claim 1 or 2, characterised in that the relaxation time (t) assigned to a particular comparison code signal (comp) is made longer, the greater the number of 'negative' status code signals received back, after the emission of call code signals (call) corresponding to said comparison code signal (comp), from the transmission network (T) or from the second peripheral system (B1,B2,B3,B4) is. Transmission system according to Claim 1 or 2, characterised in that the relaxation time (t) assigned to a particular comparison code signal (comp) is terminated after the receipt of a 'positive' status code signal, indicating that the second peripheral system (B1,B2,B3,B4) is available again, from the second peripheral system (B1,B2,B3,B4), indicated by the call code signal (call) corresponding to said comparison code signal (comp). Transmission system according to Claim 1 or 2, characterised in that at least one filter device (F,F1,F2,F3) records comparison code signals (comp) which correspond to call code signals (call) emitted by the first peripheral system (A), in response to which a 'negative' status code indicating 'inaccessible' or 'unavailable' has been received back a particular number of times from the transmission network (T) or from the called second peripheral systems (B1,B2,B3,B4) during a certain detection time. Transmission system according to Claim 6, characterised in that the said detection time is equal to the said relaxation time (t). Transmission system according to Claim 1 or 2, characterised in that the status code signals sent back contain not only an indication relating to the accessibility or availability of the called second peripheral system (B1,B2,B3,B4), but also a code corresponding to the respective call code signal (call), and in that said code is recorded in at least one filter device (F,F1,F2,F3) as comparison code in the event of the inaccessibility or unavailability of the called second peripheral system (B1,B2,B3,B4). Transmission system according to Claim 1 or 2, characterised in that the status code signals sent back contain not only an indication relating to the accessibility or availability of the called second peripheral system but also, in the event of a 'negative' status code signal, an error location code indicating the location in the transmission network (T) or second peripheral system (B1,B2,B3,B4), from which the 'negative' status code signal is sent back, and in that said error location code is recorded in at least one filter device (F,F1,F2,F3) as comparison code. Transmission system according to Claim 1 or 2, characterised in that the comparison code signals (comp) are stored in at least one filter device (F,F1,F2,F3) in their chronological sequence of recording in said filter device (F,F1,F2,F3). Transmission system according to Claim 1 or 2, characterised in that at least one filter device (F,F1,F2,F3) is provided with a switching device (SWTCH) connected to the first peripheral system (A) and to the transmission network (T) and comprising means for transmitting, or not transmitting, a call code signal (call) from the first peripheral system (A) to the transmission network (T) under the control of a switching control signal (s); a comparison code signal generator (GEN) which comprises the means for recording (CONT) as comparison code signals (comp) and storing (MEM), during a relaxation time (t) comparison codes which correspond to said call codes sent to said second peripheral systems (B1,B2,B3,B4) or to a section of such a call code, in response to which a 'negative' status code signal indicating 'inaccessible' or 'unavailable' has been received back from the transmission network (T) or from the called second peripheral system (B1,B2,B3,B4), which comparison code signal generator (GEN) furthermore comprises means for generating the then recorded comparison code signals (comp) when a call code signal (call) is emitted by the first peripheral system (A); a comparison device connected to the first peripheral system (A), to the comparison code signal generator (GEN) and to the switching device (SWTCH), and comprising the means (COMP) for comparing the call code signal (call) originating from the first peripheral system (A) with the comparison code signals (comp) generated by the comparison code signal generator (GEN), and also means for generating the said switch control signal (s) with a value such that the call code signal (call) originating from the first peripheral system (A) is not transmitted by the switching device (SWTCH) if the call code signal (call) corresponds fully or partially to one of said comparison code signals (comp) and that said call code signal (call) is in fact transmitted if the call code signal (call) does not correspond to one of the comparison code signals (comp). Filter device (F) for use in a transmission system comprising a transmission network (T) for the transmission of transmission signals between peripheral systems (A,B1,B2,B3,B4) connected to said transmission network (T), in which transmission system, prior to such a transmission of transmission signals between a first peripheral system (A) and a second peripheral system (B1,B2,B3,B4), said first peripheral system (A) emits a call code signal (call) to the transmission network (T), which call code signal (call) comprises a code indicating said second peripheral signal (B1,B2,B3,B4), after which the transmission network (T) or the second peripheral system (B1,B2,B3,B4) sends back a 'positive' or 'negative' status code signal, indicating whether said second peripheral system (B1,B2,B3,B4) is, or is not, accessible or available, respectively, characterised in that the filter device (F) is connectable to the first peripheral system (A) and to the transmission network (T), which filter device (F) comprises means (MEM) for recording and for storing, during a relaxation time, comparison code signals (comp) corresponding to said call code signals (call) emitted by the first peripheral system (A) or to a portion of such a call code signal (call), in response to which the transmission network (T) has sent back a 'negative' status code signal indicating 'inaccessible' or the called second peripheral system (B1,B2,B3,B4) has sent back a 'negative' status code signal indicating 'unavailable', which filter device (F) furthermore comprises means (COMP) for comparing a call code signal (call) emitted by said first peripheral system (A) with the comparison code signals (comp) then recorded and for not transmitting said call code signal (call) in the event of apparent correspondence between said call code signal (call) or a portion thereof and one of said comparison code signals (comp) during said comparison, but for transmitting said call code signal (call) in the event of apparent noncorrespondence between said call code signal or a portion thereof and one of said comparison code signals (call) during said comparison, whereby the filter device (F) is adapted for adjusting the relaxation time. Filter device (F) according to Claim 12, characterised by a multiplicity of the said filter devices (F1,F2,F3), each filter device (F1,F2,F3) relating to a section (T1,T2,T3) of the transmission network (T), and also by a filter selection device (SEL) connectable, on the one hand, to the first peripheral system (A) and connected, on the other hand, to each of said filter devices (F1,F2,F3) and comprising means for receiving the call code signals (call) emitted by the first peripheral system (A) and, on the basis of such a call code signal (call), selecting and connecting, to the first peripheral system (A), the filter device (F1,F2,F3) which relates to said section of the transmission network (T1,T2,T3) to which the second peripheral system (B1,B2,B3,B4) indicated by the call code signal (call) is connected. Filter device (F) according to Claim 12 or 13, characterised in that at least one filter device (F,F1,F2,F3) is provided with a switching device (SWTCH) connectable to the first peripheral system (A) and to the transmission network (T) and comprising means for transmitting, or not transmitting, a call code signal (call) from the first peripheral system (A) to the transmission network (T) under the control of a switching control signal (s); a comparison code signal generator (GEN) which comprises the means for recording (CONT) as comparison code signals (comp) and storing (MEM), during a relaxation time (t) comparison codes which correspond to said call codes sent to said second peripheral systems (B1,B2,B3,B4) or to a section of such a call code, in response to which a 'negative' status code signal indicating 'inaccessible' or 'unavailable' has been received back from the transmission network (T) or from the called second peripheral system (B1,B2,B3,B4), which comparison code signal generator (GEN) furthermore comprises means for generating the then recorded comparison code signals (comp) when a call code signal (call) is emitted by the first peripheral system (A); a comparison device connectable to the first peripheral system (A), and connected to the comparison code signal generator (GEN) and to the switching device (SWTCH), and comprising the means (COMP) for comparing the call code signal (call) originating from the first peripheral system (A) with the comparison code signals (comp) generated by the comparison code signal generator (GEN), and also means for generating the said switch control signal (s) with a value such that the call code signal (call) originating from the first peripheral system (A) is not transmitted by the switching device (SWTCH) if the call code signal (call) corresponds fully or partially to one of said comparison code signals (comp) and that said call code signal (call) is in fact transmitted if the call code signal (call) does not correspond to one of the comparison code signals (comp).

Inventors: BEZEMER DANIEL BOLTE, HOFSTEE JAN HENK EELSE, BEZEMER, DANIEL BOLTE, HOFSTEE, JAN HENK EELSE
Applicants: NEDERLAND PTT, KONINKLIJKE PTT NEDERLAND N.V.
Application year: 1991
Main classification: H04Q3
IPC codes: H04Q3