Wimax standards and Security The Wimax

FIGURE 8.9
Subnets in a 802.16 mesh network. The backbone network uses two subnets, one with high QoS (centralized scheduling) and one with low QoS (distributed scheduling). The wireless terminals connect to mesh nodes over the wireless access subnet. Mesh nodes use network address translation to forward packets from the access network to the backbone network. The base station also uses NAT when forwarding packets from the mesh backbone to the Internet.

access network uses the same block of private IP addresses in the range 192.168.2.0/24 [16]. The mesh nodes use network address translation (NAT) [17] to allow WTs to access the mesh backbone.

In the backbone, mesh routers use a range of private addresses differ- ent from the range in the access networks. We assume that mesh routers are assigned addresses in the range 10.0.0.0/8 with the assignment of IP addresses closely matching the assignment of mesh IDs. We set the last 16 bits of the IP address to the mesh ID and keep bits 8–15 of the addresses for subnet classification. For example, address 10.1.0.4 corresponds to the address of mesh router 4 on subnet 1. Since the base station acts as a POP for the network, it also provides NAT services for the packets traversing the mesh backbone. We assign each mesh router to multiple subnets to simplify how QoS is enforced in the network (Figure 8.9). We use a subnet providing a low QoS and a subnet providing a high QoS; however, a number of subnets with different QoS may be larger if necessary. We use the source marking model of QoS [8], where WTs mark the level of service they require in the type of service (TOS) field of their outgoing IP packets [8,18]. Mesh routers examine the TOS field of packets coming from their access network and, depending on the QoS specified, either forward the packet over the high QoS subnet or the low QoS subnet. This way, all per packet QoS decisions are made at the edge of the mesh backbone and the forwarding engine on each mesh router is simplified. The QoS classification of packets is done by the routing module,
before the convergence sublayer, which we describe next.

3.