Multipoint Relaying

OLSR uses flooding of packets to diffuse topology information throughout the network. Flooding, in its simplest form, means that all nodes retransmits received packets. To avoid loops, a sequence number is usually carried in such packets. This sequence number is registered by receiving nodes to assure that a packet is only retransmitted once. If a node receives a packet with a sequence number lower or equal to the last registered retransmitted packet from the sender, the packet is not retransmitted. On wired networks other optimizations are usually added such as no retransmission on the interface on which a packet arrived. On a wireless multi-hop network however, it is essential that nodes retransmits packets on the same interface that it arrived, since this is the very nature of wireless multi-hop networks. This again causes every re-transmitter to actually receive a duplicate packet from every symmetric neighbor that re-transmits the packet. A wireless flooding scenario is depicted in 3.2. One can see that every transmission leads to a reception of the same packet. The originator of the flood could be any node in the figure.

The number of retransmissions using traditional flooding is $n - 1$ where n is the number of nodes in the network. In our case(figure 3.2) it will be 24. This flooding technique can clearly benefit from some sort of optimization.

Figure: Flooding a packet in a wireless multi-hop network. The arrows show all transmissions.
Figure: Flooding a packet in a wireless multi-hop network from the center node using MPRs(black). The arrows show all transmissions.

Andreas 2004-07-29