yte synchronisation transmissions only system signalling & control Figure 52.2 Typical frame periods and traffic time slots for TDM/TDMA systems At the CS, low bit rate circuits are time division multiplexed 52.2.2 Channel access (TDM) into a relatively high aggregate data stream which is then transmitted as a modulated signal at one frequency F1 in a con- A management or control time slot in the up-link and down-link tinuous mode from a wide beamwidth sector antenna. Each roof top frame periods (named time slot zero, for instance, in Ballance, radio terminal (or outstation), equipped with a directional parabo- 1984) is generally used for tasks such as time slot request/allocation, loid or cassegrain antenna, receives this signal and extracts from it ranging, error messages and system updates. The number of time the data addressed to it. Return transmissions from all identical OSs slots determines the number of connections possible at any one occur at a second frequency 2 Low bit rate customer data are time. For private wire or leased circuits, the time slot allocation is formatted at the same aggregate data rate of the CS for transmission fixed or pre-assigned. On the other hand, time slots may be assigned at regular intervals as interleaved bursts with adequate guard time as and when they are required on demand. In this case two methods between adjacent bursts. Thus the OSs access the CS in a time of channel access are generally in use, polling and slotted ALOHA division multiple access (TDMA) mode. random request (see Chapter 13). In polling method, the CS continu- The down-link (CS to OSs) modulated signal carries data continu- ously polls each os to determine if a time slot allocation is required. ously at the system aggregate rate. In the up-link direction (OSs to Where a large number of OSs is involved, such a technique requires CS) burst transmissions are repeated periodically once every 'time an unacceptably long time for polling. In ALOHA, time slot alloca- frame' period. tion requests are again made du