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There is no explicit support for VBR. VBR uses the normal reservation mechanism, but is given priority over ABR in the reservation scheduling algorithm
It is assumed the call admission control and traffic policing are sufficient to keep the VBR traffic within the agreed traffic contract
We assume that the majority of the traffic load will be from bursty sources, either VBR or ABR. Typically there will be a small number of mobiles within each pico-cell.
The MAC protocol is actually more efficient with bursts of cells. If there are only a small number of users transmitting single cells then the utilisation is relatively poor
For this reason cell-rate pacing in which blocks are transmitted by evenly distributing the cells is not the most efficient way of using the radio bandwidth. Instead in the up-stream direction t is more efficient to move the cell-rate pacing away from the mobile and implement it in the base-station. In the down-stream direction it may make sense to delay queueing of cells until a block has arrived.
CBR reservations are made at connection setup time as part of the call admission control functionality. CBR transmissions do not require a reservation request, but are granted periodically giving the required bandwidth of the source.
The CBR reservations are made using a fixed length reservation frame which is traversed every n ms. It contains n slots, each slot identifies the mobile which owns the reservation and which correspond to a reservation of a single cell. Each CBR VC can therefore be guaranteed a bandwidth from 1 cell every n ms = n kbit/s to some proportion of the total bandwidth.
In a situation where the distributed clock is available to base-stations the CBR reservation frame can be synchronised to this source. The mobile maintains its own clock which is synchronised to the base CBR reservation frame.
At first sight it might not seem practical to use ARQ for CBR traffic due to the delay involved with re-transmission. In practise however, it is possible to have a relatively large number of repeats for a practical CDV. This is really only practical if the repeat mechanism is implemented in hardware and a failed transmission is detected early.
The bandwidth for CBR repeats is that which would be allocated to ABR. The priority order is fixed CBR reservations- those determine by the CBR reservation frame, CBR repeats, VBR and then ABR.