1. Field of the Invention
The present invention is related to media access protocol systems and more particularly to a protocol system which combines features of a table driven proportional access system and a slotted arbitration system.
2. Description of the Prior Art
The prior art media access protocols have included a strict time division multiple access protocol wherein each user is allocated a fixed equal period of time during which to transmit on the common bus, and table driven proportional access protocols wherein a specific time period is allocated for each user from a table which controls both the order and length of time period which each user is allowed to transmit on the common data bus.
The strict time division multiple access system provides a functioning access to a common bus by multiple users, but causes inefficiencies when various users do not use the time during their allocated access period. This inefficiency may not be a problem for slower systems or those which have a relatively small number of users, however this has become a problem when the time response for specific users becomes critical, or system capacity is approached.
The table driven proportional access system provides some improvement in that for a given node, the table may determine an arbitrary size and order for the transmission times. While this does at least allow some flexibility in the order of the transmissions and the duration of each transmission; the specific transmission allocations, once selected, are not changeable. In addition there is no provision for allowing a user other than the specific, table-designated user access to the bus during any given specific transmission period.
An alternative method of allocating time periods for multiple access to the bus system is a slotted arbitration system wherein the designated users are simply allocated a priority and attempt, and are granted, access to the system for each time period on a strict priority basis. This system does provide certain time efficiencies in that potential users who do not attempt access during a given time frame do not use up an allocated time slot. However, when the number of users varies from frame to frame the total frame length will vary causing a timing variation, or time jitter, from one frame to the next.