To make the most efficient use of the TDMA frame for a satellite communications network, the assignment of each local station's TDMA burst duration is based on a statistical assessment of the demand by local voice and data ports. Occasionally the actual voice and data port demand at a local station exceeds that station's assigned burst duration so that some of the messages which are ready for transmission cannot be transmitted and may be frozen-out. To avoid or minimize the effects of message freeze-out, the various types of messages are categorized into a hierarchy of priorities for transmission. Messages are divided into four basic types, control signaling messages which cannot be interrupted without loss of system control, high speed data messages whose flow cannot be interrupted without the destruction of data and the need to retransmit them, low speed data messages whose transmission can be deferred for short periods, and voice messages which can be frozen-out to a limited extent without destroying the meaning of the aggregate voice signals.
The subject invention deals with the architecture and operation of a digital switch at each local station which enables the messages in each priority level to be assembled in a separate respective threaded list for transmission, with the highest priority messages in the beginning of the burst and the lowest priority at the end of the burst. Thus, if a statistically greater number of messages occur than can be transmitted within the assigned burst period, only the lowest priority messages cannot be sent. A voice port prioritization mechanism is disclosed which increases the priority of voice ports as a function of the duration of their talkspurts, with those voice ports having talkspurts of longer duration being assigned to threaded lists which will be transmitted earlier in the TDMA burst.