The IEEE 802.16 standard defines two possible network topologies (see Figure 3.6):
PMP (Point-to-Multipoint) topology (see Figure 3.5);
Figure 3.5: PMP topology
Figure 3.6: Mesh topology. The BS is no longer the centre of the topology, as in the classical PMP mode
Mesh topology or Mesh mode (see Figure 3.6).
The main difference between the two modes is the following: in the PMP mode, traffic may take place only between a BS and its SSs, while in the Mesh mode the traffic can be routed through other SSs until the BS and can even take place only between SSs. PMP is a centralised topology where the BS is the centre of the system while in Mesh topology it is not. The elements of a Mesh network are called nodes, e.g. a Mesh SS is a node.
In Mesh topology, each station can create its own communication with any other station in the network and is then not restricted to communicate only with the BS. Thus, a major advantage of the Mesh mode is that the reach of a BS can be much greater, depending on the number of hops, until the most distant SS. On the other hand, using the Mesh mode brings up the now thoroughly studied research topic of ad hoc (no fixed infrastructure) networks routing.
When authorised to a Mesh network, a candidate SS node receives a 16-bit Node ID (IDentifier) upon a request to an SS identified as the Mesh BS. The Node ID is the basis of node identification. The Node ID is transferred in the Mesh subheader of a generic MAC frame in both unicast and broadcast messages (see Chapter 8 for frame formats).
First WiMAX network deployments are planned to follow mainly PMP topology. Mesh topology is not yet part of a WiMAX certification profile (September 2006). It has been reported that some manufacturers are planning to include the Mesh feature in their products, even before Mesh is in a certification profile.