Multiplex communications – Wide area network – Packet switching
Patent
1990-02-14
1992-03-31
Olms, Douglas W.
Multiplex communications
Wide area network
Packet switching
370 951, 455 54, 455 56, H04J 316, H04B 726
Patent
active
051014076
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to multiple access communication systems and a method of operating the same, especially such systems which allow communication of data between one or more base stations and a plurality of substations or mobiles.
Multiple access communication systems and in particular those employing mobile radio channel assignment are well known. Problems associated with such systems include difficulties in allowing the substations randomly to access the base station especially at busy periods. Simultaneous attempts at access by two or more substations can lead to a collision and mutilation of the data in question and subsequent attempts at access can cause an increasing number of collisions, further mutilation and ultimately instability of the whole system.
A polling system under which a substation communicates with a base station only on request by the base station can be secure and allow high capacity of use but suffers from the disadvantage of lengthy delays for any substations waiting to transmit.
Various attempts have been made to try to overcome the problems associated with wholly random access systems and with polling systems and these have centered on controlling the timing and/or manner of access of the base station by the substation through the use of prearranged protocols.
One of the first such protocols was termed "Pure Aloha" under which a substation transmits a request to the base station to transmit data and waits for an acknowledgment of the request. If none is forthcoming, the substation waits a random time before resubmitting the request.
A modified protocol was termed "Slotted Aloha" under which users of the system were allowed to transmit requests within a discrete timeslot. A related protocol was termed "Framed Aloha" under which the base station transmitted a message on a signalling channel to indicate which timeslots (arranged within "frames" containing a predetermined number of time slots) would be available to substations for making requests. However, all such systems generally were not particularly efficient under normal traffic conditions and became unstable under heavy traffic conditions.
A still further protocol termed "Dynamic Frame Length Aloha" attempted to take account of varying traffic conditions by varying the number of timeslots per frame on the basis of an evaluation of the level of use made of the previous frame. However, there were again problems with this protocol because of the limitations dictated by the system on the degree of variation of the number of slots in a frame and on the complexity of the systems employing the protocol.
DISCLOSURE OF THE INVENTION
The present invention is based on a protocol which can deal effectively with multi-access communication without resort to the complexity of a variable number of slots per frame and generally provides an efficient communication system.
In accordance with the invention, there is provided a multiple access communications system for use between a base and a plurality of substations and having a first channel for transmission of data from the base to the substations and a second channel for transmission of data from the substation to the base, wherein the system has, in use: be split into a plurality of groups, to a particular group, the number of substations in the groups, and of the groups and to make a judgment based on these numbers as to which group to associate itself with in order to next communicate with the base in the slot or slots assigned to that group.
On joining the system, substations may be assigned by the base to the groups on a rolling basis, but substations leaving the system during any particular period of operation are not automatically replaced by another substation joining the system. In this way, there tends to be a random, varying, number of substations in each group at a given time.
Alternatively, however, on joining the system, a substation may assign itself to a particular group by reference to the numbers of substations in each group.
In general operation of t
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P. J. Mabey et al., "UK Trunking System Signaling Standard-Protocol Aspects", The Journal of the Institution of Electronic and Radio Engineers, vol. 57, No. 3, May/Jun. 1987, IERE, pp. 119-124.
Harvey John D.
Whitehead Philip
Hitachi , Ltd.
Hsu Alpus H.
Nippon Hoso Kyokai
Olms Douglas W.
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