Routing Protocols In Mobile Wimax Environment Computer Science
Today's broadband Internet connections are restricted to wireline infrastructure using DSL, T1 or cable-modem based connection. However, these wireline infrastructures are considerably more expensive and time consuming to deploy than a wireless one. Moreover, in rural areas and developing countries, providers are unwilling to install the necessary equipment (optical fiber or copper-wire or other infrastructures) for broadband services expecting low profit. Broadband Wireless Access (BWA) has emerged as a promising solution for "last mile" access technology to provide high speed connections. IEEE 802.16 standard for BWA and its associated industry consortium, Worldwide Interoperability for Microwave Access (WiMAX) forum promise to offer high data rate over large areas to a large number of users where broadband is unavailable. This is the first industry wide standard that can be used for fixed wireless access with substantially higher bandwidth than most cellular networks , . Development of this standard facilitates low cost equipment, ensure interoperability, and reduce investment risk for operators. In the recent years, IEEE 802.16 working group has developed a number of standards for WiMAX. The first standard IEEE 802.16 was published in 2001 and focused on the frequency range between 10 and 66 GHz and required line-of-sight (LOS) propagation between the sender and the receiver . This reduces multipath distortion, thereby increases communication efficiency. Theoretically IEEE 802.16 can provide single channel data rates up to 75 Mbps on both the uplink and downlink. Providers could use multiple IEEE 802.16 channels for a single transmission to provide bandwidths of up to 350 Mbps . However, because of LOS transmission, cost-effective deployment is not possible. Consequently, several versions came with new features and techniques. IEEE 802.16-2004, has been developed to expand the scope to licensed and license-exempt bands from 2 to 11 GHz. IEEE 802.16-2004 specifies the air interface, including the Media Access Control (MAC) of wireless access for fixed operation in metropolitan area networks. Support for portable/mobile devices is considered in IEEE 802.16e standard, which is published in December 2005. WiMAX networks consist of a central radio Base Station (BS) and a number of Subscriber Stations (SSs). In Mobile WiMAX network, BS (which is fixed) is connected to public network and can handle multiple sectors simultaneously and SSs are mobile.
A number of wireless routing protocols are already designed to provide communication in wireless environment, such as AODV, OLSR, DSDV, ZRP, LAR, LANMAR, STAR, DYMO etc. Performance comparison among some set of routing protocols are already performed by the researchers such as among PAODV, AODV, CBRP, DSR, and DSDV , among DSDV, DSR, AODV, and TORA , among SPF, EXBF, DSDV, TORA, DSR, and AODV , among DSR and AODV , among STAR, AODV and DSR , among AMRoute, ODMRP, AMRIS and CAMP , among DSR, CBT and AODV , among DSDV, OLSR and AODV  and many more. These performance comparisons are carried out for ad-hoc networks but none for Mobile WiMAX. For this reason, evaluating the performance of wireless routing protocols in Mobile WiMAX environment is still an active research area and in this paper we study and compare the performance of AODV, DSR, OLSR and ZRP routing protocols.
For performing the simulation, we assume that each of the subscriber station maintain routing table for its own network, so that it can send data directly to the destination without the help of base station. However, if one subscriber station has to send data to a station located in another network, it must send data through the base station and vice versa.Wireless Routing ProtocolsAd-hoc On-demand Distance Vector Routing Protocol (AODV)
Ad-hoc On-demand distance vector (AODV)  is another variant of classical distance vector routing algorithm, based on DSDV  and DSR . It shares DSR's on-demand characteristics hence discovers routes whenever it is needed via a similar route discovery process. However, AODV adopts traditional routing tables; one entry per destination which is in contrast to DSR that maintains multiple route cache entries for each destination. The initial design of AODV is undertaken after the experience with DSDV routing algorithm. Like DSDV, AODV provides loop free routes while repairing link breakages but unlike DSDV, it doesn't require global periodic routing advertisements.
Apart from reducing the number of broadcast resulting from a link break, AODV also has other significant features. Whenever a route is available from source to destination, it does not add any overhead to the packets. However, route discovery process is only initiated when routes are not used and/or they expired and consequently discarded. This strategy reduces the effects of stale routes as well as the need for route maintenance for unused routes. Another distinguishing feature of AODV is the ability to provide unicast, multicast and broadcast communication.
AODV uses a broadcast route discovery algorithm and then the unicast route reply massage. The following sections explain these mechanisms in more detail.A .1 Route Discovery
When a node wants to send a packet to some destination node and does not locate a valid route in its routing table for that destination, it initiates a route discovery process. Source node broadcasts a route request (RREQ) packet to its neighbors, which then forwards the request to their neighbors and so on. Fig. 1 indicates the broadcast of RREQ across the network.
Fig. 1 Propagation of RREQ throughout the network
Fig. 2 Reply of RREP towards the network
To control network-wide broadcasts of RREQ packets, the source node use an expanding ring search technique. In this technique, source node starts searching the destination using some initial time to live (TTL) value. If no reply is received within the discovery period, TTL value incremented by an increment value. This process will continue until the threshold value is reached.
When an intermediate node forwards the RREQ, it records the address of the neighbor from which first packet of the broadcast is received, thereby establishing a reverse path. When the RREQ is received by a node that is either the destination node or an intermediate node with a fresh enough route to the destination, it replies by unicasting the route reply (RREP) towards the source node. As the RREP is routed back along the reverse path, intermediate nodes along this path set up forward path entries to the destination in its route table and when the RREP reaches the source node, a route from source to the destination established. Fig. 2 indicates the path of the RREP from the destination node to the source node.A .2 Route Maintenance
A route established between source and destination pair is maintained as long as needed by the source. If the source node moves during an active session, it can reinitiate route discovery to establish a new route to destination. However, if the destination or some intermediate node moves, the node upstream of the break remove the routing entry and send route error (RERR) message to the affected active upstream neighbors. These nodes in turn propagate the RERR to their precursor nodes, and so on until the source node is reached. The affected source node may then choose to either stop sending data or reinitiate route discovery for that destination by sending out a new RREQ message.Dynamic Source Routing (DSR)
The Dynamic Source Routing (DSR)  is one of the purest examples of an on-demand routing protocol that is based on the concept of source routing. It is designed specially for use in multihop ad hoc networks of mobile nodes. It allows the network to be completely self-organizing and self-configuring and does not need any existing network infrastructure or administration. DSR uses no periodic routing messages like AODV, thereby reduces network bandwidth overhead, conserves battery power and avoids large routing updates. Instead DSR needs support from the MAC layer to identify link failure.
DSR is composed of the two mechanisms of Route Discovery and Route Maintenance, which work together to allow nodes to discover and maintain source routes to arbitrary destinations in the network. The following sections explain these mechanisms in more details.
Fig. 3 Propagation of route request message across the networkB .1 Route Discovery
When a mobile node has a packet to send to some destination, it first checks its route cache to determine whether it already has a route to the destination. If it has an unexpired route, it will use this route to send the packet to the destination. On the other hand, if the cache does not have such a route, it initiates route discovery by broadcasting a route request packet.
Each node receiving the route request packet searches throughout its route cache for a route to the intended destination. If no route is found in the cache, it adds its own address to the route record of the packet and then forwards the packet to its neighbors. This request propagates through the network until either the destination or an intermediate node with a route to destination is reached. Fig. 3 demonstrates the formation of the route record as the route request propagates through the network.
Whenever route request reaches either to the destination itself or to an intermediate node which has a route to the destination, a route reply is unicasted back to its originator. Fig. 4 illustrates the path of the RREP from the destination node to the source nodeB .2 Route Maintenance
In DSR, route is maintained through the use of route error packets and acknowledgments. When a packet with source route is originated or forwarded, each node transmitting the packet is responsible for confirming that the packet has been received by the next hop. The packet is retransmitted until the conformation of receipt is received. If the packet is
Fig. 4 Propagation of route reply message towards the source
transmitted by a node the maximum number of times and yet no receipt information is received, this node returns a route error message to the source of the packet. When this route error packet is received, the hop in error is removed from the host's route cache and all routes containing the hop are truncated at that point.Optimized Link State Routing (OLSR)
The Optimized Link State Routing (OLSR)  protocol inherits the stability of the pure link state algorithm and is an optimization over the classical link state protocol, adopted for mobile ad hoc networks. It is proactive in nature and has the advantage of having routes immediately available when needed. The key concept used in this protocol is that of multipoint relays (MPRs). MPRs are selected set of nodes in its neighbor, which forward broadcast messages during the flooding process. OLSR reduces the size of control packet by declaring only a subset of links with its neighbors who are its multipoint relay selectors and only the multipoint relays of a node retransmit its broadcast messages. Hence, the protocol does not generate extra control traffic in response to link failures and additions. The following section describes the functionality of OLSR in details.C .1 Neighbor Sensing
For detecting the neighbor, each node periodically broadcasts its HELLO messages, which contains the information of the neighbors and their link status. The protocol only selects direct and bidirectional links, so that the problem of packet transfer over unidirectional links is avoided. HELLO messages are received by all one-hop neighbors, but they are not relayed further. These messages permit each node to learn the knowledge of its neighbors up to two hopes and help performing the selection of its multipoint relays.C .2
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