Chapter: Data Networks
with contributions by René van der Vleuten
Controlling the Retransmission back-off
The aloha protocol can become unstable is packets collide and their retransmissions collide again and again.
|See the collision resolution animation and the exercise.|
An adhoc approach: How to select a good Pr?
In a network, the packet arrival probability P0
and the retransmission backoff probability Pr
determine the stability. The operator may not have a direct influence of the arrival process of new packets, but he may dicate a certain the retransmission
behavior for all terminals.
For mobile channels, the range of the parameters where bistability occurs is relatively small: the transition through this area from a single (low-backlog) equilibrium into saturation occurs for relatively small increments of the packet generation probability P0. In contrast to this, since the bifurcation curve of the mobile net is almost parallel to the Pr-axis, a change in the retransmission probability will have less effect on the stability of the net. For a packet generation probability up to P0 = 0.002, even a persistent retransmission schedule (Pr = 1) leads to a stable network, at least in theory. In practice, high retransmission probabilities may be used, but certainly not larger than say 0.8.
Simulations indicated that reducing the probability of retransmission Pr has a positive effect on the network performance in saturated networks (increasing throughput, decreasing backlog and delay), but a negative effect in stable, unsaturated networks (lower throughput, increasing backlog and delay). In bistable nets, appropriate reduction of Pr can remove bistability, but this measure may not sufficiently relieve the backlog and packet delay: The curve suggests that a relatively drastic reduction of Pr may be required. Reducing Pr in a bistable network can result in a stable network, but with relatively high backlog.
By reducing the probability of retransmission, the effective time each terminal spends in the origination mode also reduces, which indirectly resulted in a low input traffic load. This would suggest that mobile channels might as well be managed by directly controlling the input traffic, for instance by limiting the number of terminals N that are allowed to be signed on simultaneously.
There exists an optimal retransmission backoff control method for a controlled ALOHA system. It achieves maximal throughput, automatically adapts to changes in average traffic intensity or the number of active stations in the system, and applies to both pure and slotted ALOHA systems.