New MAC protocol currently under development.

Distributed reservation protocol designed to support bursty traffic where the allocation of bandwidth must be rapidly changed between mobiles which are themselves moving between base-stations.

Operates in presence of an unreliable channel and supports antenna diversity.

The protocol uses ALOHA requests for bursy traffic, which is expected to provide the majority of the traffic load on the network.

CBR reservations are made at connection setup time and do not require a reservation request.

The MAC protocol uses a frame, typically of 10-20 cell times in length.

MAC frame minimises the turn-round time between transmit and receive which is typically one of the irreducible times which the physical layer imposes. In this scheme the up-stream and down-stream tranmissions are grouped - requiring only 2 turn-round times per MAC frame.

MAC frame provides structure in which reservations are made and cells are acknowledged

The system can operate either with variable frame length. In this mode the frame only contains cells if transmissions are available. In the typical case in which there are a small number of sources and the proportion of up-stream and down-stream traffic changes rapidly this provides the most efficient use of bandwidth.

In order to improve the utilisation in a cellular environment the frame structure can also be fixed. This means that base-station transmissions can be synchronised so that interference between mobiles can be much more tightly controlled.

As yet it is not clear without further experimentation in which circumstances fixed or variable MAC frames will be most useful.

Each cell transmission is acknowledged in the immediaterly following frame. As I will outline this is important for the error protection of real-time traffic.

The different ATM traffic classes are supported using a priority mechanism. Three traffic priorities are used combined with the reservation mechanism and variable numbers of repeat attempts to provide the required services.