Patent GB-1323574-A (GB)
Title: Apparatus for operating a communications system via a satellite relay
IPC: H04Q11/00
Filed: 1970-07-13
Abstract: 1323574 Multiple access satellite communication systems; multiplex pulse signalling POST OFFICE 13 July 1970 [16 July 1969] 33883/70 Headings H4K and H4L A space satellite communication system employs demand assigned multiple access ground terminal stations which communicate with associated international trunk exchanges by way of a signal processing interface and which employ continuity tests in the backward direction for paths established over the satellite. The system is of the sort described in Specification 1,269,555 and has a common signalling channel over which each ground station is allotted a burst transmission time in which it broadcasts its demands and occupancy of channels in the system. One station of the system acts as a reference station to broadcast a burst containing a frame start reference code for all other stations and each station with reference to this start time regulates the transmission of its own burst so as to achieve a proper progression of burst signals through the satellite. As shown in Fig. 1, 4-wire trunks from the associated international exchange CT are interfaced at 101 by apparatus under the control of a signal processor 113 in a demand assignment and signal switching unit 102 which supervises channel selection by signalling over the common signalling channel CSC. Speech channels are established over channel units set up to operate on specific channels as instructed by codes from 102. To conserve satellite power supplies a voice detector 108 switches off the channel carrier in the absence of speed in the outgoing direction. Communication between the multiple access station of Fig. 1 and the earth station providing radio channels to the satellite is by way of an IF sub-system. Communication between the multiple access station and the associated international exchange is by way of a so-called interface unit 101 in which all data sent to and received from the exchange is buffered, the 4-wire full duplex voice circuits are connected through relay sets whereby splitting can be effected for connection of registers and the application of continuity tests in respect of each newly established satellite channel link in respect of which the multiple access station is the called station. As seen in Fig. 27 in a system in which the international trunk exchange employs individual circuit switching such as CCITT No. 5, each 4-wire circuit incoming from the exchange to the multiple access terminal is splittable in a trunk circuit group 211. A link switch 212 connects registers, senders, or continuity test circuits in either the forward or backward side of the split. Each such function has a buffer store and these stores over send/receive buses are serviced by the signal processor which accesses the buses by decoder 222 and a particular buffer by decoder 223. Trunk circuit switching orders are buffered direct by 218 and a buffer 217 is provided for processor control of the link switch 212. Signalling is by 2-out-of-6 voice frequency codes. Continuity is tested by sending a measured period of one voice frequency and connecting a receiver which checks the connection a ssatisfactory if a given fraction of the signal is received back. Continuity testing may be effected link-by-link or end-to-end according to the option of the station. As seen in Fig. 34, if the international exchange employs common channel signalling such as CCITT No. 6, the interface is simplified by the absence of registers and senders and the provision of direct conversion between the exchange and satellite common signalling channels. The link switch is still required, however, to effect the continuity test and line splitting is still required for this purpose. The system is served by a teleprinter 112, Fig. 1, by means of which a record is made of all call particulars and durations as well as the recording of system fault particulars as detected during normal fault and diagnostic routines. An all-station voice order wire facility may be employed by the dedication of a pair of system satellite channels. Synchronization of the common signalling channel burst times is effected by progression from a manual mode in which an operator uses a visual display to control burst transmission time to within the control range of automatic synchronizing circuits which operate first in a so-called wide-aperture mode which applies an immediate correction of transmission time to be within a tolerance of two bits and hands over to a normal mode in which corrections are limited to one bit either way at each check of synchronization. To allow for round-time transmission synchronization checks are applied only once in every sixth frame of common channel burst times. If synchronization deviates ouside the tolerance of 2 bits the system switches to manual mode. As shown in Fig. 4, reception of the reference synch code produces a pulse to mark the beginning of a frame and in the absence of synchronization this pulse is routed direct to set and start bit and burst counters 117, 120 which mark out the bit portions of successive bursts delivered from the satellite. A decoder 121 recognizes the station's own burst in order properly to calculate burst transmission time. If the received reference synch signal is received three times successively in consonance with operation of the counters 117, 120, an aperture of 3 bits width is established by circuit 115 which switches the route of the reference synch pulse to be by way of gate 116. This aperture is used to screen the system from spurious synch signals. A manual delay devic e 119 allows production of a pulse at any selected position in a frame of bursts, such a pulse being required to determine the transmission burst time of the station. In manual mode of operation the arbitrary pulse that happens to be produced by device 119 is applied to a 32-bit delay counter 129, Fig. 5, and after this delay (which must be compared with the 128 bits of a burst) a socalled vernier counter 127 is started to count at the bit rate. In manual mode, a bi-directional counter 126 is automatically set to its mid point count of 256 to determine at the instance of a comparator 128 the point in time at which vernier counter 127 reaches this count, this point in time being the time the station sends its own burst. From this first arbitrarily occurring send time the operator by use of a visual display observes the relative position of the station's burst when received back after a round trip, such burst comprising unmodulated carrier. By adjustment of the manual delay device 119 the burst transmission time is set to within 250 bits of being accurate this being within the range of the bi-directional counter 126 to adjust. Having attained this adjustment the operator switches to co-called wide aperture mode in which the pulse determined by manual delay device 119 continues to be applied to the 32-bit delay counter 129 of Fig. 5 this having the effect of shortening the transmitted burst which in this mode contains synchronizing codes and other data. Early and late gate generator circuits 123 determine the accuracy of the burst transmission time from an inspection of the burst when received back after its round trip. Reception is early if it is between 250 to one half bit before the correct time and it is late if it is one half to 250 bits after. In wide aperture mode the instants of reception are gated to a time comparator 125 which in accordance with the identified deviation sets the bi-directional counter 126 to give an accurate time for the station's transmission burst with a tolerance of 2 bits. If two successive bursts are received back without falling outside the tolerance of 2 bits synchronization is switched from the wide aperture to normal mode. In normal mode the delay counter 129 is withdrawn from serve so that the full length of burst is sent with full data content and the early and late gate generator circuits 123 at each check of synch can effect a correction of one bit this way or that as required. Once the frame reference synch aperture is established a " frame synch " lamp is lit. Once the normal synch mode is in operation a " burst synch " lamp is lit. Error checking is effected in respect of the 48 bit data word in each station burst by use of Bose-Chandhuri techniques. As the shortest word to encode in this technique is 55 bits long seven bits of the preceding synchronizing code are included in the checking procedures. The Specification includes algorithms for the burst synchronization process, Figs. 3a to 3c, not shown; for processes of channel seizure, supervision and release in a satellite connection between exchanges using CCITT No. 5 signalling Figs. 2a to 2g, not shown; and for various functions of the signal processor of a multiple access terminal, Figs. 14 to 24, not shown.