TDMA frame structure:
frame and burst formats for TDMA system
Fixed assignment is recommended for networks involved in routing large volumes of traffic between a small numbers of stations of high capacity. On-demand assignment provides better utilisation of the satellite network in the case of a large number of stations of low capacity per access with large variations in demand. Each station can thus benefit occasionally from a greater capacity than that which it would have in the case of a fixed assignment. Management of the assignment implies a connection set-up time of the order of a second. When the connection is required for several minutes, this set-up time is of no consequence. The choice of a demand assignment technique must, therefore, take the following aspects into account:
—specifications on the user side: traffic density, number of destinations, blocking probability;
—the gain resulting from the operation of demand assignment: this involves comparing the increase of revenue resulting from a higher traffic throughput for a given blocking probability with the increased expense involved in the installation of equipment to manage demand assignment;
—the choice between centralised and distributed management.
The set-up time of the connection can be a decisive factor for some types of traffic, such as data exchanged between data processing systems. The traffic generated in communication between computers, or between computers and computer terminals, is characterised by a large variation in 284 Multiple Access the duration of messages and the interarrival time between messages. Furthermore, the user often imposes a clause concerning transmission time which can be short compared with the time interval between messages. Under these conditions, the set-up time of a connection can exceed the utilisation time and this corresponds to inefficient use of the network. On the other hand, the set-up time of the connection can lead to an unacceptable transmission time.
TDMA Superframe:
An SS-TDMA network can operate with fixed or demand assignment. With demand assignment, variations of capacity allocated to stations are obtained by variations of burst length as with TDMA. Variation of station burst length is accompanied by variation of the position of bursts of other stations, and consequently a change in the assignment of other bursts (burst time plan change). For changes that involve a sequence of switch states, the new sequence of switch states must be loaded into the distribution control unit (DCU) memory by means of a dedicated link (which can be the telecommand link). The assignment changes occur at the start of a superframe consisting of an integer number of TDMA frames in order to guarantee synchronisation of the change among all stations and the satellite.
Below table shows the distinction between the centralised control and distributed control of demand assigned (DA-FDMA):
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