Patent GB-1530633-A (GB)
Title: Digital switch apparatus for a communication network
IPC: H04B7/185
Filed: 1975-12-29
Abstract: 1530633 Automatic exchange systems INTERNATIONAL BUSINESS MACHINES CORP 29 Dec 1975 [30 Dec 1974 (5) 26 June 1975] 52963/75 Heading H4K An extendable TDM exchange is provided for interconnecting up to 4 line groups comprising duplex telephone and data links and up to 4 high speeed digital TDM trunks on which voice activity compression (VAC) is utilized. Local, toll and tandem trunk switching is provided. Speech is switched in adaptively companded # modulation form. The duplex telephone links may extend to PBXs. The exchange constitutes a branch exchange of the private telecommunications system disclosed in USA Specification No. 4,009,343. Switching network, Figure 1: The basic exchange unit comprises one line group of 96 duplex links together with an associated line interface unit LIU and buffer stores LGM, a time slot interchange module comprising 192 memory locations, and one high speed digital trunk pair IT, OT with its corresponding buffer stores TGM. A common control unit CCU is provided, and the exchange can be extended by adding further exchange units, the CCU being capable of handling 4 such units. The network provides a non-blocking interconnection facility, between any two duplex lines, between any duplex line and a channel on the digital trunk pair, and between any two digital trunk channels. Line Interface Process Unit (LIU), Figures 1, 2 provides A/D conversion and multiplex/demultiplex facilities between the 96 duplex lines and a pair of 96 channel intermediate highways respectively extending to and from 192 location line group buffer stores LGIM and LGOM (Figure 2). A series of eight speech samples are converted into an eight bit byte using adaptively companded # modulation and the LIU in addition produces a one bit indication of speech activity in the byte i.e. if any of the eight samples in a byte shows speech activity, the activity bit is mode I. If echo, noise or zero activity is detected for all samples the bit is made 0. An activity bit is likewise allocated to data bytes. The LIU also informs the CCU as to which channels on the intermediate highways are allocated to data and which are allocated to speech. This information is used in connection with the VAC procedures used in the digital trunks. The speech or data bytes together with their respective activity bits are passed via the incoming intermediate highway in serial bit form to an incoming line group buffer store LGIM comprising two sections of 96 locations. During one frame information from the intermediate highway is written into one section whilst the information written in in the previous frame is read out in bit parallel form to locations in the time slot interchange memory SIM. All the buffer stores are separated as two sections in the same manner. The connection from the SIM memory to the interface takes place in reverse manner via line group outgoing buffer store LGOM. The time slot interchange memory SIM comprises up to four modules of 192, 12 bit locations. In the fully extended form the 768 locations are divided into two sections. Each location stores for the corresponding channel the activity bit (VAC bit), the eight bits of the information byte and two parity bits, the other bit position (bit position one) being reserved. The SIM memory operates on a 6 ms. slot interchange frame of 48 channels. During odd channel times information is written into the first section of Î 96 locations of the SIM from the LGIM buffer stores and during the even channel times the second section is loaded from the incoming trunk buffer stores TGIM. Under the control of a connection memory ICM information is time switched from the second section to the outgoing line group and trunk group buffer stores LGOM, TGOM during odd time channels and from the first section to the outgoing buffer stores during the even time channels. During the SIM interchange frame 24 bytes are transferred in each direction for each connection. Trunk buffer stores; Voice activity compression: The outgoing trunk buffer stores TGOM each comprise two 96 location sections and each location stores 24 sequential bytes of information associated with a respective connection. The outgoing 48 channel digital trunk OT, however, only has 46 192 bit channels per frame of 6 ms. available for connections, a first channel being used for signalling and a second channel for transmission of a 96 bit VAC mask. At the end of each 6 ms. frame, a new set of 24 bytes will have been stored in one section of the outgoing trunk buffer store and for each group of 24 bytes an activity bit is stored, the bit value being 1 if any of the 24 bytes is accompanied by an activity bit I and being 0 if all 24 bytes show zero activity. Only byte groups showing activity are considered for transmission and if the number of such groups is less than 47 all active groups are transmitted. If more than 46 groups are active data is given preference over speech. Further when choosing which speech groups to transmit, continuing activity is given preference over new activity. The same condition applies if it is necessary to make a selection among data groups (>46 channels carrying active data). The above selection, fully described in the specification, takes place during channel 0 of the trunk frame, i.e. dunng the signalling channel, and at the same time a new VAC mask is prepared for transmission in channel 1. The mask has 96 bit positions corresponding to the 96 byte groups of trunk buffer store TGOM, and each bit position is given the value 1 or 0 in dependence on whether the corresponding group is to be transmitted or not. Checking bits are added to the basic VAC mask to bring the number of bits up to 192. The incoming trunk buffer stores TGIM each comprise two sections of only 46, 24 byte, locations. The received VAC mask is used to determine to which locations of the SIM memory to switch the 46 active channels. Noise is inserted in the SIM memory locations of inactive channels. The Digital Trunks operate at 1À544 MBS transmitting 48 192 bit channels per 6 ms. with each channel separated by a synchronization bit. The first 40 synchronization bits alternate 1010... and the last 8 sync. bits are used for multi-access purposes (see above referenced Applications). Byte and frame synchronization between trunks and the exchange are described. All timing originates from a single clock phase locked to incoming data on one of the trunks.