Virtual subnets protocol

The Virtual Subnet Protocol (VSP) breaks up a large body of nodes into smaller logical groups calledsubnets. It then applies a hierarchical addressing scheme to these subnets. A novel routing scheme is then employed to enable broadcasting within subnets and limited broadcasting between subnets. The virtual subnet-addressing scheme is somewhat reminiscent of that used in ATM.

In this method, network nodes are assigned addresses depending on their current physical connectivity. We assume that the network is segmented into physical subnets containing mobile nodes. Each node in the network is assigned a unique address con- structed of two parts: one part is a subnet address allocated to the entire subnet (subnet id) and the other part is an address that is unique within the node’s subnet (node id).

Each node in this topology is affiliated with nodes whose address differs only in one digit; that is, node x1.x0 is affiliated with nodes x1.x0 and x1.x0. Thus, every node is affiliated with every node within its subnet, as well as one node in every other subnet. These cross-linked affiliations are the building blocks of thead hoc network.

Each node in the network is affiliated with a physical subnet (the local nodes all sharing the same subnet id) and a virtual subnet (the nodes all sharing the same node id). Nodes that are members of a physical subnet (subnet id) are within close proximity in a local geographic area. Nodes that are members of a virtual subnet (node id) form a regional network (i.e., beyond a local area). All nodes within a physical subnet have the same subnet id, while all nodes within a virtual subnet have the same node id.

A node becomes a member of a physical subnet by acquiring the first available address (with the lowest node id) in that subnet. Once a node becomes affiliated with a specific physical subnet, it automatically becomes a member of a virtual subnet defined by the node id in its address. As long as a node remains within hearing distance of its subnet neighbors, it will keep its current physical subnet affiliation and its address.

When a node moves to a new location in which it cannot establish a connection with its previous physical subnet’s members, it will drop its previous address and join a new physical subnet.

In the simple case in which the destination node is within two hops of the source node, packets traverse one network address digit at a time in fixed order. For example, when the source node address is 13.33 and the destination node address is 11.36, the packet would follow the route: 13.33 to 13.36 to 11.36. In this case, routing requires at most two hops.

PROBLEMS TO CHAPTER 9 179

In general, the network will be arranged such that more than two hops are necessary from source to destination. In this case, the routing is performed in two phases. In the first phase, routing is performed only in the physical subnet. Packets are routed to the node belonging to the same virtual subnet as the destination. Using the same example as above, Phase 1 consists of routing packets from 13.33 to 13.36.

In Phase 2, packets are routed between virtual subnets. Adjustments of transmission frequencies, transmission power, and/or directional antennae to facilitate logical network connections are needed. It is assumed that all nodes are capable of reaching neighboring physical subnets when required to do so.

The VSP is a method to optimize throughput when multiple frequencies and/or spa- tial reuse is possible, on the condition that nodes are close together relative to their transmitter range.

9.3 SUMMARY

Routing protocols forad hoc networks can be divided into two categories: table-driven and on-demand routing, on the basis of when and how the routes are discovered. In table-driven routing protocols, consistent and up-to-date routing information to all nodes is maintained at each node, whereas in on-demand routing, the routes are created only when desired by the source host.

In table-driven routing protocols, each node maintains one or more tables containing routing information with every other node in the network. All nodes update these tables so as to maintain a consistent and up-to-date view of the network. When the network topology changes, the nodes propagate update messages throughout the network in order to maintain a consistent and up-to-date routing information about the whole network. These routing protocols differ in the method by which the information regarding topology changes is distributed across the network and in the number of necessary routing-related tables.

In on-demand routing protocols, all up-to-date routes are not maintained at every node; instead the routes are created as and when they are required. When source wants to send a packet to destination, it invokes the route discovery mechanisms to find the path to the destination. The route remains valid till the destination is reachable or until the route is no longer needed.

PROBLEMS TO CHAPTER 9

Routing protocols in mobile and wireless networks

Learning objectives

After completing this chapter, you are able to

• demonstrate an understanding of routing protocols in mobile and wireless networks;

• explain table-driven routing protocols; and

Practice problems

9.1: What is anad hoc network?

9.2: What are the two categories of routing protocols forad hoc networks? 9.3: What are the functions of table-driven routing protocols?

9.4: What are the functions of on-demand routing protocols? Practice problem solutions

9.1: In an ad hoc network, all nodes are mobile and can be connected dynamically in an arbitrary manner. All nodes of the network behave as routers and take part in discovery and maintenance of routes to other nodes in the network. Mobile nodes change their network location and link status on a regular basis. New nodes may unexpectedly join the network or existing nodes may leave or be turned off. Ad hoc routing protocols must minimize the time required to converge after the topol- ogy changes. A low convergence time is more critical in ad hoc networks because temporary routing loops can result in packets being transmitted in circles, further consuming valuable bandwidth.

9.2: Routing protocols for ad hoc networks can be divided into two categories: table- driven and on-demand routing on the basis of when and how the routes are discov- ered. In table-driven routing protocols, consistent and up-to-date routing information to all nodes is maintained at each node, whereas in on-demand routing, the routes are created only when desired by the source host.

9.3: In table-driven routing protocols, each node maintains one or more tables containing routing information with every other node in the network. All nodes update these tables so as to maintain a consistent and up-to-date view of the network. When the network topology changes, the nodes propagate update messages throughout the network in order to maintain a consistent and up-to-date routing information about the entire network. These routing protocols differ in the method by which the topology changes information is distributed across the network and in the number of necessary routing-related tables.

9.4: In on-demand routing protocols, all up-to-date routes are not maintained at every node; instead the routes are created as and when they are required. When source wants to send a packet to destination, it invokes the route discovery mechanisms to find the path to the destination. The route remains valid till the destination is reachable or until the route is no longer needed.

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Handoff in mobile