Patent GB-1355048-A (GB)
Title: Switching centre for a data network
IPC: H04L12/52
Filed: 1970-08-20
Abstract: 1355048 Digital switching centres STANDARD TELEPHONES & CABLES Ltd 13 Aug 1971 [20 Aug 1970] 40097/70 Heading H4P A switching centre for a synchronous digital data network including terminal equipments for a plurality of pairs of incoming and outgoing data transmission channels respectively comprises, a first storage-device having for each pair a dedicated storage location, divided into at least two portions one of which is used to store an address of a different storage location in which an incoming data character is to be stored, the or another portion being used to store an incoming data character to which a different storage location is dedicated. A second storage device has a plurality of storage locations dynamically allocated for the receipt of address characters from an incoming channel whereby an address of a dedicated storage location associated with a different incoming transmission means can be assembled to enable transfer of subsequent data characters direct from the incoming channel from which the address characters were recevied to the second portion of the dedicated storage location identified by that address. Sequentially and cyclically the terminal equipments are connected with the first storage device whereby an incoming character may be inserted in the storage location specified by the address held in the location dedicated to the incoming terminal equipment via which the character is received, and a character already held in a storage location may be extracted from that location and transferred to an outgoing terminal equipment to which the storage location holding that character is dedicated, said incoming and outgoing terminal equipments being associated with one pair of incoming and outgoing transmission means. Each dedicated storage location may include a third portion which is used to store functional information specifying a mode of utilization of the contents of that location whereby the address contained in the second portion of the location is used to indicate either another dedicated storage location in the first storage device for storage of an incoming data character or a dynamically allocated storage location in the second storage device for the receipt of an address character. The functional information may alternatively specify a mode of utilization of the contents of the location whereby the address is used to indicate a different storage location from which character is to be extracted for transfer via outgoing terminal equipment to which the location containing the functional information is dedicated. Each dedicated storage location may include a portion which is used to store signalling supervision information. The switching centre (DSC) which may operate at more than one bit rate as applied to a character TDM system may have means for stacking characters into blocks at one or more rates for transmission and unstacking for distribution among subscribers. The data structure described has a ten bit envelope comprising eight data bits, a synchronizing or framing bit carrying a regular pattern, and a status bit which in one state identifies the envelope and in its other state indicates a network signal or an empty envelope, distinction being made by the remaining contents of the envelope. Data is transferred from incoming channels via input register 15 through store 10 and to outgoing channels via output register 16 over input and output highways 17,18. Channels are terminated at switches 19, 20 connected with highways 17, 18 under the control of the switch address control 12. Similarly addressed data block trunk links to and from other centres or large computer processing units (CPU) are connected through terminals LC and input/ output highways 21, 22 to store also directed by control 12 connected to processor 13. Each incoming and associated outgoing channel has a dedicated 32 bit word in store 12 and as each coming character is scanned the corresponding address of a dedicated word is formed and its contents extracted. Organization of store 10 is shown in Fig. 1 and the format of the dedicated word illustrated in Fig. 2 (not shown). The first 16 bits are used to address character positions in the switch store, the 17th to specify ¢ character positions: its function is to specify where the character incoming on the associated channel is to be placed when a call is in progress. Bits 18-24 are used to set control conditions and the remaining 8 bits are allocated to characters to determine the output on a corresponding channel. Status characters specify the control signals between terminal and centre in each direction. Protection against invalid characters may be provided by a counter used to define the character proceeding to the processor via a queue and to inhibit further transfer which guards against (n - 1) spurious characters for the case where the nth character is input, but not against an incorrect but valid nth the signal character. An incorrect nth character is stated to be a rare occurrence but parity bits provide additional protection. An alternative method providing greater security is based on a transfer of the first two signal characters to the processor which compares the two and takes action if they are identical or if not initiates a transfer of two more characters. In this method the first character may be eliminated and characters 2 and 3 transferred to reduce processing load. A further method uses the above mentioned counter bits to store an incoming signal character in abbreviated form for comparison with the next character before transfer to signal queue, the basic operation of which is described with reference to Fig. 3 (not shown).