One of the key design issues in satellite communication is how to efficiently allocate transponder channels. Uplink channel is shared by all the ground stations in the footprint of a satellite, as shown in Fig.

Uplink frequency is shared, and downlink signal is broadcasted

The round robin and contention-based medium access control schemes have been found to be suitable for local area networks. But most of the schemes are unsuitable for communication satellite medium used in wide area networks. Apart from the nature of traffic, unique features of the satellite channels are to be taken into consideration for designing suitable medium access control protocol for them. The most important feature of the satellite channels is their long up-and-down prorogation delay, which is about one fourth of a second. The second most important feature of the satellite channels is that, after about one fourth a second a station has ceased transmission, it knows whether the transmission was successful or suffered a collision. These two features along with the nature of traffic, whether bursty or streamed are the determining factors for the designing of medium access control schemes.

As more than half a second is necessary to get response of a poll, polling scheme is inefficient for satellite channels. The CSMA-based schemes are also impractical because of long propagation delay; whatever a station senses now was actually going on about on quarter of a second ago.

For a satellite system with a limited number of ground stations and all of them having continuous traffic, it makes sense to use FDM or TDM. In FDM, each transponder channel is divided into disjoint subchannels at different frequencies, with guard bands to reduce interference between adjacent channels. In TDM, the channel is divided into slots, which are grouped into frames. Each slot is allocated to each of the ground stations for transmission.

But, in situations where the number of ground stations is large, and stations have bursty nature of traffic, both TDM and FDM are inefficient because of poor utilization of the slots and subchannels, respectively. A third category of medium access scheme, known as reservation has been invented for efficient utilization of satellite channels. In all the reservation schemes, a fixed frame length is used, which is divided into a number of time slots. For a particular station, slots in the future frames are reserved in some dynamic fashion, using ALOHA or S-ALOHA. The schemes differ primarily in the manner the reservations are made and released using either a distributed or a centralised policy as discussed in the following subsections.

Contention-free protocols:

Fixed assignment protocols using FDMA or TDMA: Allocation of channel assignment is static; suitable when number of stations is small. These provide deterministic delay, which is important in real-time applications. Demand assignment protocols: Suitable when the traffic pattern is random and unpredictable. Efficiency is improved by using reservation based on demand. The reservation process can be implicit or explicit.

      TDMA MAC technique

Random access protocols:

·         Pure ALOHA

·         Selective-reject ALOHA

·         Slotted ALOHA

·         Reservation Protocols

·         Reservation ALOHA (R-ALOHA)

·         Hybrid of random access and reservation protocols

·         Designed to have the advantages of both random access and TDMA

Distributed Protocols

A large number of reservation schemes have been proposed. A few representative schemes are briefly outlined below. It is assumed that there are n stations and m slots per frame.

R-ALOHA:

The simplest of the schemes, proposed by Crowther et al (1973) is known as R-ALOHA. As illustrated in Fig 5.10.18, the scheme assumes that the number of stations is larger than the number of slots (n>m) and with time the number of active stations is varying dynamically. A station wishing to transmit one or more packets of data monitors the slots in the current frame. The station contends for a slot in the next frame, which is either free or contains a collision in current frame. Successful transmission in a slot serves as a reservation for the corresponding slot in the next frame and the station can send long stream of data by repeated use of that slot position in the subsequent frames. The scheme behaves like a fixed assignment TDMA when the stations send long streams of data. On the other hand, if most of the traffic is bursty, the scheme behaves like the slotted ALOHA. In fact, the performance can be worse than S-ALOHA.

R-ALOHA based MAC technique