Patent GB-1457930-A (GB)
Title: Digital message switching and transmitting system
IPC: H04L12/54
Filed: 1974-08-22
Abstract: 1457930 Automatic exchange systems; packet switching INTERNATIONAL STANDARD ELECTRIC CORP 22 Aug 1974 [29 Aug 1973] 36945/74 Headings H4K and H4P In a packet switching system comprising a plurality of switching units, each packet is headed by bytes indicating how the packet is to be switched in each unit and the units each have means for recognizing the byte relevant to the switching in the respective unit. The packets may be concerned with speech, telegraphic or data transmission or may relate to maintenance or supervisory functions. Speech may be coded using PCM or delta modulation. Four wire connections are used for speech whereas two wire connections are used for telegraphy and data. Fig. 4 shows one such switching unit for switching packets between 16 incoming highways E0-E15 and 16 outgoing highways S0-S15. Stored program control may be used. Referring to Fig. 3, a 16 byte frame is used on these highways, each byte comprising 8 bits. A packet comprises 256 bytes and the beginning of a packet (byte DP) is not synchronized with the start of a frame. The function of the input stages ME0-ME15 of the switching unit is to separate the packet from the incoming data stream, to regenerate at least the heading bytes DP-CH which are transmitted in a redundancy code, set to zero certain of the redundant bits, and to cause the packet to be stored in a 256 byte input shift register store. Concerning the regeneration, the heading bytes DP-CH comprise a first group of 4 bits carrying information and a redundant second group of four bits which enable the first 4 bits to be checked and regenerated. Bytes L1, L2 indicate the total number of bytes subject to checking and regeneration, which number may or may not include the information bytes. During the regeneration process the second group of 4 bits of byte DP are set to zero. Once the whole packet has been set up in an input store, a 3 wire path is established via a crosspoint network 23 to an intermediate storage unit J0-Jn, one wire being used for the byte by byte transfer of the packet, a second wire for synchronization signals, and the third wire for inter-unit control signals between the input and intermediate storage units. The subsequent treatment of the packet then depends on its status as indicated by the first four bits of byte DP, i.e. whether it is a service, telegraphic (including data), telephonic or spot check (maintenance) message. Initially, however, for all packets byte CS, which is incremented by 1 each time the packet passes through a switching unit, is examined so as to determine which of the switching instruction bytes S1- S3 &c. is relevant to this unit. Having determined the corresponding outgoing highway from the relevant byte, an attempt is made to access the corresponding outgoing storage unit MS0-MS15. If the unit is free, the packet is transferred thereto via switching stage 27 and is finally outputted to the corresponding highway. If the outgoing storage unit is not immediately available, the subsequent operation depends on the type of message. For a service message the immediate storage unit waits until the outgoing storage unit becomes free and then transfers the packet. For a telegraph message, the packet is dumped into a buffer storage unit MT0-MTn, from where it is eventually transferred back to an intermediate storage unit. A telephonic message is also dumped into a buffer storage unit, but in this case the second four bits of byte DP are incremented by 1 each time dumping occurs. When an intermediate unit receives from buffer storage, a telephonic packet with the second four bits of byte DP equal to 15, the packet is completely cancelled. A maintenance message is treated in the same manner as a telephonic message except that the packet is not cancelled after being buffered 15 times. When the packet is finally transferred to the output storage unit the CS byte is incremented by 2 and the value of the second four bits of byte DP is read and is written in a predetermined byte position following the heading bytes. For a detailed description the reader is directed to the Specification and Figs. 5-13 (not shown). Co-pending Application 26932/74 is referred to in relation to the exchange of information between the various storage units (Fig. 5, not shown). It is worth noting that the various storage units each include a plurality of shift register stores for packets so that several packets can be processed at the same time, i.e. one can be being written into a store as another packet is being read out &c. Fig. 6 (not shown) illustrates a circuit for ensuring that double seizure of stores by storage unit control circuitry cannot take place. The service packets referred to above may be messages sent from one processor to another, e.g. when determining that a called subscriber station is free before permitting the caller to start transmission. The packets relating to a particular connection are subsequently identified by the value of byte CH.