Method of and Apparatus for Switching Service Information Units in a TDM System

The present invention relates to a method of switching at least one unit of service information referring to a special service to a number of subscribers whose telephone sets are connectable to a time division multiplex system in order to transfer during allotted time slots digital information words which previously have been stored periodically and by groups. Furthermore the invention relates to apparatus for executing the method.

Within a group of rural telephone networks and especially within a network of a town, the subscribers of the network are often offered a varying number of special services in the form of service information obtained from voice recording device. The voice recording devices are called by and connected to, respectively, the subscribers in a normal way and provide information such as news, weather reports and references due to a disturbance which affects the requested establishment of an arbitrary telephone connection. The voice recording devices or service machines are hereby considered as subscribers with special problems.

The most important problem when planning the network is that each service machine is connected simultaneously to a number of ordinary subscribers. For this reason, heretofore the respective exchange was provided with a special group selector stage which was arranged between the service machines and the other selector stages of the exchange, whereby the terminals of the special stage had generally been distributed with a certain proportion per service based on its average traffic load. However, an individual special service can be exposed to heavy demand, for example the news service during political crises. The demands do not exclusively lead to busy signals when the distribution lines allotted to the respective service are completely occupied, but create traffic chaos extending the entire network. In addition, the peak demands for one special service very rarely coincide with those of other special services and therefore the ordinary telephone traffic should bear a capacity decrease without increase of the risk for congestion.

In view of the above it is apparent there are large administration expenses if there is provided a conventional exchange which switches analog information signals, with an arrangement also intended for analog signals, by which the switching system between the service machines and the network varies dynamically according to need. In addition in a stored program controlled exchange, the realization of such an arrangement demands extra hardware and extra software.

An object of the present invention is to produce automatically the mentioned dynamic switching of the service machines by means of a practically and economically insignificant expenditure. In particular there is used an extra equipment intended for digital signals which in principle is already present if the network is digital so called IST (Integrated Switching Transmission) system in which the telephone connections are established by means of digital exchanges as known, for example, by the U.S. Pat. No. 3,864,525.

The invention, the characteristics of which appear from the claims will hereinafter be explained with reference to the accompanying drawing which shows such parts of a IST-network which participate in switching a special service to the subscribers of the network.

Between the subscribers AB of the network and the voice recording devices, for example tape recorders TR.sub.1 to TR.sub.s, information converted according to a time division multiplex method is switched by a digital exchange DEX. In the drawing the network is assumed to include s special services the informative voice messages of which are transferred, according to the a well-known method by an analog-digital-converter AD, by time division multiplex arrangements TDM and by at least one link, continously to the exchange. A not shown variant of the service machine is a memory scanned by a time division multiplex arrangement, in which memory the respective service information is stored as a sequence of digital information words.

In a homogenous time division multiplex system, n information words are transferred on each of t transmission links, and each of the words is allotted a time slot within a frame, so the system contains t.multidot.n information channels, each of them defined through a link number t.sub.x and its associated channel number n.sub.x. A combined time division multiplex system is obtained if its information channels are divided into groups t.sub.i .multidot.n.sub.i , each of which refers to a homogenous system. A transfer of the channels of an incoming system .SIGMA.t.sub.i .multidot.n.sub.i to the channels of an outgoing system .SIGMA.k.sub.i .multidot.m.sub.i results in a change of the multiplex size without congestion, if .SIGMA.t.sub.i .multidot.n.sub.i = .SIGMA.k.sub.i .multidot.m.sub.i, with transfer between two systems being such that it is unalterably determined that an incoming channel defined by a link number t.sub.x and a channel number n.sub.x interchanges into an outgoing channel defined by link number k.sub.x and channel number m.sub.x. If .SIGMA.t.sub.i .multidot.n.sub.i <.SIGMA.k.sub.i .multidot.m.sub.i is valid for the systems, a redundancy is obtained for outgoing channels, which redundance usually is needed in order to avoid congestion in exchanges including several selector stages. By a switching between two systems is understood that the information in an arbitrary channel on an arbitrary incoming link is switched by at least one interchange in time and at least one interchange in space to an arbitrary channel on an arbitrary outgoing link. In general, a digital exchange comprises at least one time stage in order to carry out the interchanges in time and at least one space stage in order to carry out the interchanges in space. In a time stage the information words are moved freely from incoming to outgoing time slots, but it is unchangeably determined how imcoming links are connected to outgoing links. In a space stage the information words are moved freely from incoming to outgoing links but the time relations are unchangeably determined.

In order to switch, according to the invention, the service information of the service machines to calling subscribers one of the time stages TS of the exchange is used having a time multiplex system MUX1 wherein incoming information words are switched to an outgoing time multiplex system MUX2. The embodiment shown in the drawing assumes that the outgoing system MUX2 is homogeneous and comprises m channels on each of k links, so that an arbitrary outgoing channel is defined by a link number 1 .ltoreq. k.sub.x .ltoreq. k and its associated channel number 1 .ltoreq. m.sub.x .ltoreq. m. The incoming system MUX1 is assumed to be combined and to comprise t links with the link numbers 1 .ltoreq. t.sub.x .ltoreq. t. The link with the number t.sub.x = 1 has s channels with the numbers 1 .ltoreq. s.sub.x .ltoreq. s and each of the other links with the numbers 2 .ltoreq. t.sub.x .ltoreq. t has n channels with the channel numbers 1 .ltoreq. n.sub.x .ltoreq. n.

The fixed or space relationships of the time stage TS are of that kind that the information words of two incoming links leave the stage on a determined allotted outgoing link, for example the links with the numbers t.sub.x = 1 and t.sub.x = 2 are assigned to the link with the number k.sub.x = 1, so that t = 2.k. Furthermore, it is assumed that s channels transfer each one of the s special services and that m = 2.n and that s < n.

Consequently, in the embodiment shown in the drawing, interchange in time is combined with a changing of the multiplex size and there is a redundancy concerning the outgoing link with the number k.sub.x = 1. To introduce this redundancy is not absolutely necessary but, as it will appear later, the congestion risk is diminished in connection with the switching of the voice message services.

Other not-shown variants, in order to connect service machines to the time stage, associate each machine with its own one channel connection or form as well the incoming time multiplex system homogenously with n channels on all the links, whereby the redundancy is kept because on the link used for the service machines only s < n channels are occupied with constant numbers.

The time stage TS comprises word memories the writing inlets and the reading outlets of which are connected to the incoming and outgoing time multiplex system MUX1 and MUX2, respectively. Each of the incoming channels of the system MUX1 is allotted to one of the word memories which are arranged by groups. In the drawing there is shown that each of the service words sw.sub.1 to sw.sub.s incoming on the link with the number t.sub.x = 1 and the informattion words iw.sub.1 to iw.sub.n incoming on the link with the number t.sub.x = 2 are stored in a word memory, forming a memory group G1 which is allotted to the outgoing link with the number k.sub.x = 1. The means for regularly and cyclically addressing the word memories to receive words for writing by means of timing pulses for the combined time multiplex division system MUX1 is not shown because it is accomplished by well-known techniques. Similarly, the well known means for avoiding the coincidence of writing and reading in the word memories is not shown.

Furthermore, the time stage TS comprises control memories CM.sub.1 to CM.sub.k each of which being associated with an outgoing link to store addresses, in order to, by means of address decoders DEC.sub.1 to DEC.sub.k, access the reading of a word memory of the memory groups G.sub.1 to G.sub.k associated with the respective outgoing link. Every control memory which, by means of one of the scanners SC.sub.1 to SC.sub.k is scanned for reading by determined timing pulses belonging to the outgoing time multiplex division system MUX2 has a storage capacity of m addresses corresponding to the number of outgoing time slots. Thus, an information word iw.sub.x accessed for reading by means of the address a.sub.x is transferred on the respective outgoing link during the time slot which is allotted to the channel with the number m.sub.x and, during which the address a.sub.x is read from the respective control memory. A control unit CU of the exchange carries out the writing of addresses in the control memories: the writing is based on switching orders received by calls according to well known techniques.

When a special service is called, for example the one associated with the service words sw.sub.1 which are accessible by the address a.sub.1 in the memory group G.sub.1, the control unit CU selects an idle channel on a link with the number k.sub.x = 1 associated with this group, for example the one with the number m.sub.x = m, and writes the address in the respective location in the control memory CM.sub.1. The same address a.sub.1 is written in response to further calls for this service in an arbitrary idle location in the control memory CM.sub.1. On condition that the word memories storing the service words are of the non-destructive read-out type, i.e. there is no erasure of the contents in connection with reading, the described embodiment gives the opportunity of switching one special service to a maximum of m subscribers but, in that case, there will be a congestion for switching all the rest of the service words and the channels transferred on the link with the number t.sub.x = 2. The above-introduced redundancy of the link with the number k.sub.x = 1, i.e., set n < m, lessens the risk of congestion.

If it must be avoided that one special service blocks all the rest of the channels which use the same outgoing link, a logical arrangement is provided in the control unit which stops the respective address registrations in the control memory at a given maximum even though there may still be some idle locations.

As a conclusion some modifications are mentioned concerning the distribution of the service words in the outgoing time division multiplex system. One modification is that a memory group only consists of word memories storing service information, in an extreme case only one service word. Another modification is that several memory groups comprise word memories storing both service information and ordinary information words.

In all the modifications using memory groups in which only one word memory storing service information is included, the possibility of adapting the traffic of the special services in a dynamic distribution among themselves is lost. Nevertheless, a modification should be mentioned, according to which each of the memory groups comprise one word memory to store a service information which informs about a fault situation which has arisen due to, for example, a break down in the links incoming to the respective memory group. The control unit comprises in this case an address converter which upon such fault situation converts all called ordinary addresses to the address which accesses reading the service information from the memory group.

The hereby described method of switching special services is not limited to exchanges provided with IST-networks but is also applicable in a conventional analog network. In this case a separate time stage is arranged as an extra equipment with a memory group which consists of word memories for storing service information. The reading of the group is controlled by a control memory, a scanner and a control unit completely according to the above description. As further extra equipment, a digital-to-analog converter is needed, by which the link outgoing from the memory group is connected to analog selector stages of the exchange through as many analog distribution lines as there are time slots in the outgoing time division multiplex system.