A new routing protocol for the reconfigurable wireless networks

In this paper, we propose a new routing protocol, the Zone Routing Protocol (ZRP), for the Reconfigurable Wireless Networks, a large scale, highly mobile ad-hoc networking environment. The novelty of the ZRP protocol is that it is applicable to large flat-routed networks. Furthermore, through the use of the zone radius parameter, the scheme exhibits adjustable hybrid behavior of proactive and reactive routing schemes. We evaluate the performance of the protocol, showing the reduction in the number of control messages, as compared with other reactive schemes, such as flooding. INTRODUCTION Recently, there has been an increased interest in ad-hoc networking [1]. In general, ad-hoc networks are network architecture that can be rapidly deployed, without preexistence of any fixed infrastructure. A special case of ad-hoc networks, the Reconfigurable Wireless Networks (RWN), was previously introduced [2,3] to emphasize a number of special characteristics of the RWN communication environment: 3⁄4 large network coverage; large network radius, net r , 3⁄4 large number of network nodes, and 3⁄4 large range of nodal velocities (from stationary to highly mobile). In particular, the topology of the RWN is quite frequently changing, while self-adapting to the connectivity and propagation conditions and to the traffic and mobility patterns. Examples of the use of the RWNs are: • military (tactical) communication for fast establishment of communication infrastructure during deployment of forces in a foreign (hostile) terrain • rescue missions for communication in areas without adequate wireless coverage • national security for communication in times of national crisis, when the existing communication infrastructure is non-operational due to a natural disasters or a global war • law enforcement similar to tactical communication 1 For example, the maximal nodal velocity is such that the lifetime of a link can be between hundreds of milliseconds to few seconds only. • commercial use for setting up communication in exhibitions, conferences, or sale presentations • education for operation of virtual classrooms • sensor networks for communication between intelligent sensors (e.g., MEMS) mounted on mobile platforms. Basically, there are two approaching in providing ad-hoc network connectivity: flat-routed or hierarchical network architectures. An example of a flat-routed network is shown in Figure 1 and of a two-tiered hierarFigure 1: A flat-routed ad-hoc network chical network in Figure 2. In flat-routed networks, all the nodes are “equal” and the packet routing is done based on peer-to-peer connections, restricted only by the propagation conditions. In hierarchical networks, there are at least two tiers; on the lower tier, nodes in geographical proximity create peer-to-peer networks. In each one of these lower-tier networks, at least one node is designated to serve as a "gateway” to the higher tier. These “gateway” nodes create the highertier network, which usually requires more powerful transmitters/receivers. Although routing between nodes that belong to the same lower-tier network is based on peer-to-peer routing, routing between nodes that belong to different lower-tier networks is through the gateway nodes. Figure 2: A two-tiered ad-hoc network tier-1 network tier-2 network tier-1 network tier-1 network tier-1 network cluster cluster head We will omit here the comparison of the two architectures. Nevertheless, we note that the flat-routed networks are more suitable for the highly versatile communication environment as the RWN-s. The reason is that the maintenance of the hierarchies (and the associated cluster heads) is too costly in network resources when the lifetime of the links is quite short. Thus, we chose to concentrate on the flat-routed network architecture in our study of the routing protocols for the RWN. PREVIOUS AND RELATED WORK The currently available routing protocols are inadequate for the RWN. The main problem is that they do not support either fast-changeable network architecture or that they do not scale well with the size of the network (number of nodes). Surprisingly, these shortcomings are present even in some routing protocols that were proposed for ad-hoc networks. More specifically, the challenge stems from the fact that, on one hand, in-order to route packets in a network, the network topology needs to be known to the traversed nodes. On the other hand, in a RWN, this topology may change quite often. Also, the number of nodes may be very large. Thus, the cost of updates is quite high, in contradiction with the fact that updates are expensive in the wireless communication environment. Furthermore, as the number of network nodes may be large, the potential number of destinations is also large, requiring large and frequent exchange of data (e.g., routes, routes updates, or routing tables) between network nodes. The wired Internet uses routing protocols based on topological broadcast, such as the OSPF [4]. These protocols are not suitable for the RWN due to the relatively large bandwidth required for update messages. In the past, routing in multi-hop packet radio networks was based on shortest-path routing algorithms [5], such as Distributed Bellman-Ford (DBF) algorithm. These algorithms suffer from very slow convergence (the “counting to infinity” problem). Besides, DBF-like algorithms incur large update message penalty. Protocols that attempted to cure some of the shortcoming of DFB, such as DestinationSequenced Distance-Vector Routing (DSDV) [6], were proposed and studied. Nevertheless, synchronization problems and extra processing overhead are common in these protocols. Other protocols that rely on the information from the predecessor of the shortest path solve the slow convergence problem of DBF (e.g., [7]). However, the processing requirements of these protocols may be quite high, because of the way they process the update messages. Use of dynamic source routing protocol, which utilizes flooding to discover a route to a destination, is described in [8]. A number of optimization techniques, such as route caching are also presented that reduce the route determination/maintenance overhead. In a highly dynamic environment, such as the RWN is, this type of protocols lead to a large delay and the techniques to reduce overhead may not perform well. A query-reply based routing protocol has been introduced recently in [9]. Practical implementation of this protocol in the RWN-s can lead, however, to high communication requirements. A new distance-vector routing protocol for packet radio networks (WRP) is presented in [10]. Upon change in the network topology, WRP relies on communicating the change to its neighbors, which effectively propagates throughout the whole network. The salient advantage of WRP is the considerable reduction in the probability of loops in the calculated routes. The main disadvantage of WRP for the RWN is in the fact that routing nodes constantly maintain full routing information in each network node, which was obtained at relatively high cost in wireless resources In [11], routing is based on temporary addresses assigned to nodes. These addresses are concatenation of the node’s addresses on a physical and a virtual networks. However, routing requires full connectivity among all the physical network nodes. Furthermore, the routing may not be optimal, as it is based on addresses, which may not be related to the geographical locations, producing a long path for communication between two close-by nodes. The above routing protocols can be classified either as proactive or as reactive. Proactive protocols attempt to continuously evaluate the routes within the network, so that when a packet needs to be forwarded, the route is already known and can be immediately used. Reactive protocols, on the other hand, invoke the route determination procedures on demand only. Thus, when a route is needed, some sort of global search procedure is employed. The advantage of the proactive schemes is that, once a route is requested, there is little delay until route is determined. In reactive protocols, because route information may not be available at the time a routing request is received, the delay to determine a route can be quite significant. Because of this long delay, pure reactive routing protocols may not be applicable to realtime communication. However, pure proactive schemes are likewise not appropriate for the RWN environment, as they continuously use large portion of the network capacity to keep the routing information current. Since in an RWN nodes move quite fast, and as the changes may be more frequent than the routing requests, most of this routing information is never used! This results in an excessive waste of the network capacity. What is needed is a protocol that, on one hand, initiates the route-determination procedure on-demand, but with limited cost of the global search. The introduced here routing protocol, which is based on the notion of routing zones, incurs very low overhead in route determination. It requires maintaining a small amount of routing information in each node. There is no overhead of wireless resources to maintain routing information of inactive routes. Moreover, it identifies multiple routes with no looping problems. The ZONE ROUTING PROTOCOL (ZRP) Our approach to routing in the RWN is based on the notion of a routing zone, which is defined for each node and includes the nodes whose distance (e.g., in hops) is at most some predefined number. This distance is referred to here as the zone radius, zone r . Each node is required to know the topology of the network within its routing zone only and nodes are updated about topological changes only within their routing zone. Thus, even though a network can be quite large, the updates are only locally propagated. Since for radius greater than 1 the routing zones heavily overlap, the routing tends to be extremely robust. The rout