Routing Protocols for Wireless Ad Hoc Networks Hybrid routing protocols Theofanis Kilinkaridis

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1 Routing Protools for Wireless Ad Ho Networks Hyrid routing protools Theofanis Kilinkaridis Astrat This paper presents a partiular group of routing protools that aim to omine the advantages of proative and reative protools. Hyrid protools try to exploit the advantages of oth these shemes and to provide etter routing solutions for moile ad ho networks. The partiular paper is more foused on their main onepts as well as on their performane. It analyzes five hyrid protools and presents their advantages and disadvantages. 1. Introdution Routing protools generally fall in two ategories: Proative protools and Reative protools. Proative protools estalish network route regardless of the demand for suh routes. While these protools periodially exhange routing information, they an provide routes quikly after soure s request. Reative protools, on the other hand, disover routes only when it s needed. While this on-demand operation may result in route disovery delay, the protool overhead is muh lower. An optimal routing protool depends on network harateristis whih may hange dynamially. A stati network where routes to all nodes are equal is more suitale for a proative protool. A dynami network however, where a few moile nodes are the most popular destinations might use a reative protool. Hyrid routing protools find the optimal mix of proative route dissemination and reative route disovery. 2. Hyrid Routing Protools Usually in hyrid protools nodes are grouped into zones, ased on their loation in the network. Inside these zones, proative sheme is used for routing while the reative one is used for the ommuniation etween zones. In this paper the following protools are presented: Core Extration Distriuted Ad Ho Routing Protool(CEDAR), Zone Routing Protool(ZRP), Zone-ased Hierarhial Link State Routing Protool(ZHLS), Preferred Link-ased Routing Protool(PLBR) and Optimized Link State Routing Protool(OSLR). 2.1 Core Extration Distriuted Ad Ho Routing Protool Core Extration Distriuted Ad Ho Routing(CEDAR) protool [1] [2] estalishes a ore of the network whih is used for effiient delivery of the transmitted pakets. Route estalishment uses reative routing sheme and is performed y ore nodes. CEDAR onsists of three main phases: Core extration, link state propagation and route omputation. During the ore extration a set of nodes is dynamially eleted to form the ore of the network. During the link state propagation the andwidth availaility information of stale high andwidth links is propagated to ore nodes, while information regarding low andwidth and unstale links is kept loal. Finally, during the last phase, the ore path is used to estalish a route from the soure to the destination. In CEDAR there is at least on ore node every three hops. The path etween two ore nodes is alled virtual path. Here, the seletion of ore nodes is done y distriuted algorithm. Core nodes are used to perform the paket transmission over the network in uniast mode. In order to ahieve this transmissions effiiently, eah ore node has to know its neighoring ore nodes. If the ore node moves away, all the nodes that were attahed to it have to find a new 1

2 ore node. Also in CEDAR, every node piks up a node within a distane not greater than one hop from it, as its dominator. During the route estalishment, a ore path has to e found from the soure to the destination and then QoS requirements have to e satisfied for the seletion of the path. First the soure heks the loal topology whether the destination is in the loal topology tale of soure s ore node. If it is, the path is estalished diretly. If it s not, the soure initiates a RouteRequest message whih is roadasted to all the ore nodes of the network. Eah of the ore nodes that reeive the message forwards it to its neighoring ore nodes if the destination is not among its ore memers. If it is among one of its ore memers, this ore node originates a reply message to the soure ore node. So the ore path is estalished. Let s see the Figure 1, where the soure is the node 1 and the destination is the node 15. The ore nodes are 3, 5, 11, 12 and 13. Node 5 is the dominator of nodes 1 and 6 and node 12 is the dominator of node 15. The aovementioned proedure is followed till one ore node finds the destination node among its ore memers. In this ase, ore node 12 finds it and replies to the ore node 5. One the path etween the nodes is estalished the last one to e heked is whether the required andwidth for the transmission is availale on this path. If its not availale, the ore path will e rejeted. Figure 1: Route estalishment in CEDAR As for the link reak ase, CEDAR deals with it quite effiiently. When a link reak ours, the node after whih the reak ourred(on the path from soure to destination) sends a notifiation of failure and egins to find a new path from it to the destination. The partiular node rejets every reeived paket till the moment it finds a new path to the destination. Meanwhile, as the soure reeives the notifiation message, it stops to transmit and tries to find a new route to the destination. If the new route is found y either of these two nodes, a new path from the soure to the destination is estalished. The main advantage of the CEDAR is that it utilizes the use of ore nodes to satisfy oth routing estalishment and QoS path omputation. It is also very flexile in ase of link reak restoring the onnetivity of the network quikly and effiiently. Its main disadvantage however, is that the route estalishment and omputation is relied on ore nodes. Due to the moility of ad ho networks, ore nodes may move. This movement affets the performane of the protool. 2

3 2.2 Zone Routing Protool As we know oth proative and reative protools have their advantages and disadvantages. Zone Routing Protool (ZRP) [1] [3] is a protool whih omines the advantages of oth approahes into a hyrid sheme. Its key feature is that it uses proative routing sheme within a node s loal neighorhood and reative sheme for ommuniation etween these neighorhoods. These loal neighorhoods are alled zones. An intra-zone routing protool (IARP) is used in the zone, where a partiular node estalishes a proative routing. However, routing eyond the zone is managed y a reative protool alled inter-zone routing protool (IERP). Eah zone of a given node is a suset of the network. Eah node may e in more than one zone and eah zone may e of a different size. The size of the zone is determined y the zone radius r, where r is the numer of hops to the perimeter of the zone. Figure 2 shows an example of routing zone with r = 2. Figure 2: routing zone of the node A with r = 2. With zone radius = 2, the nodes B, C, D, E, F, G, H, I and J omprise A s zone. Nodes C, G and I are interior nodes, while nodes B, D, E, F, H and J are alled peripheral nodes. Eah node maintains the information aout the routes to its zone nodes using proative routing sheme y exhanging periodi route update pakets. IARP is used for ommuniation etween a node and its zone s nodes. Sine the loal neighorhood is dynami with a lot of topology hanges, the IARP, as mentioned aove, is a proative, tale-driven protool. Due to its proative nature, the route seletion within the zone is very effiient y having availale all the possile routes to the loal destinations(zone nodes). IERP however, is responsile for route disovery to the nodes whih are not within the routing zone. In other words, IERP takes advantage of the known loal topology of a node s zone and using a reative routing sheme, estalishes ommuniation with nodes in other zones. When a node s wants to send a paket to the node d, it first heks whether the destination node is within its zone. If it is, the soure node delivers the paket diretly to it. Otherwise, it orderasts the RouteRequest to its peripheral nodes. More partiularly, this proedure is held y a protool alled the Borderast Resolution Protool (BRP) whih is used for optimizing the effiieny of the in-zone ommuniation. BRP is responsile for sending the RouteRequest message to all the peripheral nodes. Unlike IARP and IERP, BRP is more like a paket delivery servie and not a routing protool. If any of these peripheral nodes finds the destination node in its own routing zone, it sends a RouteReply ak to the soure node s 3

4 indiating the path. Otherwise, it orderasts the RouteRequest to the peripheral nodes of its zone. This proess is ontinued till the destination node d is loated. The nodes that find the destination node in their routing zone, initiate the sending of the RouteReply pakets to the soure. During the spread of the RouteRequest message, eah node appends its address to it whih an e used later for estalishing the path for RouteReply message, sent ak to the soure. When an intermediate node, detets a roken link in the path, it hooses another alternative path to ypass the roken link. This proess is alled loal path reonfiguration and during it, a path update message is sent to the sender to inform it aout the link failure. After that a new, su-optimal path is estalished etween the soure and the destination nodes. In order to estalish an optimal path, the soure node reinitiates the path finding proess. Generally, omining the advantages of oth proative and reative routing shemes, ZRP provides etter solutions than oth of them. Compared to them, it redues the ontrol traffi produed y periodi flooding of routing information pakets(proative sheme). It also redues the wastage of andwidth and ontrol overhead ompared to reative shemes where RouteRequest flooding mehanisms are used. The main disadvantage however of ZRP, is the large overlapping of routing zones. 2.3 Zone-ased Hierarhial Link State Routing Protool Zone-ased Hierarhial Link State (ZHLS) [1] [4], is a hyrid hierarhial routing protool that uses the geographial loation information of the nodes to divide the network into non-overlapping zones. ZHLS is ased on node ID and zone ID approah whereas eah node knows only the node onnetivity within its zone and the zone onnetivity of the whole network. A low level, within a zone, onnetivity is alled node-level topology while the high level zone onnetivity is alled zone-level topology. The node-level topologial information is distriuted only to the nodes of a partiular zone while the zone-level information is distriuted to all nodes of the network. No luster heads are defined in this protool. Routing is estalished ased on zone ID and node ID of the destination. No path ontaining the nodes etween the soure and the destination is required. Therefore, no link reak ould ause any prolem to the delivery of the information. In ZHLS, a node knows its physial loation(node ID) y loation tehniques like GPS. Using this information it an determine its zone ID y mapping its node ID to zone map. The zone map is done during the network design phase. Two types of link state pakets(lsp) are defined in ZHLS. A node LSP of a partiular node ontains a list of its onneted neighors. A zone LSP ontains a list of its onneted zones. At this point, having its node ID and zone ID, eah node an start the intra-zone lustering and then the inter-zone lustering proedures to uild its routing tales. In intra-zone lustering, eah node roadasts a link request. The nodes reply with a link response that ontains their node ID and their zone ID. After all link responses, every node generates a node LSP that ontains the node IDs of its neighors of the same zone, and the zone IDs of its neighors of different zones. 4

5 5 g 4 d a e f h i Soure a d e f Node LSP,,d,4 a,e a,3 a,f,2 e,2 J Figure 3: Node-level topology Figure 4: Node LSPs in zone 1 Then, every node roadasts its LSP to the nodes all over its zone. As a result, a list of node LSPs like in Figure 4 an e generated in every node and eah node knows now the node-level topology of its zone. The Shortest Path algorithm is used now for uilding intra-zone routing tales. Figure 5 shows the intra-zone routing tale of node a. Destination d e f Next node d g Figure 5: Intra-zone routing tale of node a. After the initial roadast of a link request from every node, some nodes reeive also responses from nodes whose loation is in neighor zones. These nodes are alled gateway nodes through whih the ommuniation with the neighor zones an e estalished. As it is mentioned aove, node LSPs ontain the zone IDs of the neighoring zones. So eah node knows whih zones are onneted to its zone. Just after reeiving all nodes LSPs, eah node of the same zone generates the same zone LSP. Then the gateway nodes roadast the zone LSP all over the network. As a result a list of zone LSPs is maintained y every node whih now knows the zone-level topology of the network. Finally the inter-zone routing tales are uilt in every node. If a soure node s wants to ommuniate with the destination node d, s heks whether d is loated in its zone. If it is, the paket is sent to the destination node ased on intra-zone routing tale. If it is not, then s initiates a loation request paket whih ontains soure s and destination s information. The loation request paket is sent to every zone. A gateway node of eah zone will reeive the paket and hek whether the requested node elongs to its zone. The gateway node to whih zone the destination node elongs will initiate a loation response paket indiating the zone information of d. After knowing the d s zone ID, node s will ommuniate with node d, ased on its inter-zone routing tale. 5

6 The main advantage of ZHLS against ZRP is that it does not have any overlapping zones. Also the fat that the zone-level topology information is distriuted to all nodes and that there is no luster head, redues the traffi and avoids single point of failure. The main disadvantage of the protool is additional traffi produed y the reation and maintaining of the zone-level topology. 3. Routing Protools with Effiient Flooding Mehanisms The main mehanism of reative protools is flooding the network with RouteRequest pakets in order to estalish a path from the soure node to the destination. These on-demand protools inrease the traffi overhead and derease the already limited, availale andwidth. A provision of effiient flooding mehanisms however, ould redue the overhead aused y different routing shemes. Two protools will e disussed further: Preferred Link-ased Routing Protool(PLBR) and Optimized Link State routing protool(olsr). 3.1 Preferred Link-ased Routing Protool The Preferred Link-ased Routing(PLBR) [1] [5] is a reative routing protool whih applies partiular mehanisms to avoid just flooding the network with RouteRequest pakets. It provides a mehanism where not all the nodes are allowed to forward the reeived RouteRequest paket. Hene, it redues the ontrol overhead. Here, every node maintains a tale if its neighors(nt) and of its neighor s neighors(nnt). A node selets always a suset of nodes from its neighor list(nl). This suset is alled the preferred list(pl). The determination of the PL is ased on nodes or links harateristis and will e disussed later. The RouteRequest paket ontains the PL of the sender and is roadasted to its neighors. The ones that are in the PL will forward the RouteRequest paket. The others will disard it. The size of the PL is K, where K is the maximum numer of nodes omprising the PL. PL is always a suset of PLT where PLT is the preferred list tale maintained y every node and whih ontains all neighor nodes in order of preferene. Any hange in neighors topology is maintained y sending a eaon that updates the tales. PLBR onsists of three different phases: route estalishment, route seletion and route maintenane. During the first phase of route estalishment, the soure node s is trying to estalish a route to the destination node d. If the node d is in soure s NNT, the route is estalished diretly and the information is forwarded to the destination. If it is not, the soure initiates a RouteRequest paket whih ontains: Soure node s address: SoureID Destination node s address: DestinationID Unique sequene numer: SeqNum, whih prevents the forwarding of multiple opies of the same RouteRequest paket reeived from different neighors. Traversed Path: TP, whih indiates the nodes that have een visited so far y the paket. Preferred List: PL whih, as already mentioned, is list of nodes determined y the sender node as eligile to forward the RouteRequest paket. Time to Live(TTL) field: a dereasing value whih determines the allowed duration of paket existene in the network. Every time, efore forwarding the RouteRequest paket, the node that reeives the RouteRequest paket updates its PLT and inserts in the PL field of the paket its own PL(whih ontains the first K nodes from its PLT). The PL of the reeived paket is always replaed y the one of the reeiver. The RouteRequest paket is always roadasted to all node s neighors. However, not all these nodes are onsidered to e eligile to forward the paket further. In order to e eligile 6

7 a reeiver node has to e in the PL of the reeived paket, not to have forwarded the same paket again and the TTL value must e greater than zero. If the node satisfies all these requirements, it an forward the paket to all its neighor nodes. Before roadasting the paker however, the node heks whether the destination node is in its NT or NTT. If it is, it uniasts the paket diretly to the neighor whih an e the destination node or an just have the destination node as a neighor in its NT. Otherwise, the paket will e roadasted with new PL and new parameters. As soon as the destination node is deteted, the route seletion proedure is followed. As it is ovious multiple RouteRequest pakets an reah the destination node. The est route is seleted ased on the shortest path, the least delay path or the most stale path. After reeiving the first RouteRequest paket, the destination node starts a timer whih indiates for how long it will e reeiving RouteRequest pakets from the network. After the expiration of the timer, no more RouteRequest pakets are reeived y the destination node and the route seletion phase an start. Finally in order to deal with link reaks, PLBR uses a partiular mehanism to ahieve optimal route maintenane. PLBR uses a quik route repair mehanism to ypass the roken link using information aout the next two hops from NNT. But how the preferred list(pl) is onstruted? Every time the node reeives RouteRequest paket, it has to determine its own preferred list of eligile nodes and to replae the reeived with the new one. Two different algorithms have een proposed for the seletion of eligile nodes. The first one is alled Neighor Degree-ased Preferred Link Algorithm and is ased on neighor nodes degree information. The seond one is alled Weight-ased Preferred Link algorithm. Here, the preferred list is alulated ased on the staility of the links indiated y the weight. Neighor Degree-ased Preferred Link Algorithm is ased on neighor nodes degree. Degree of a node is the numer of its neighors. The algorithm is exeuted for one node and takes under onsideration parameters that have to do with the degree of its neighor nodes. After the exeution of the algorithm, two lists are reated: INL and EXL The Inlude List(INL) ontains a set of neighor nodes whih are onsidered as reahale for forwarding the RouteRequest paket. EXL is the Exlude List whih ontains the set of neighor nodes whih are onsidered as unreahale. On the other hand, Weight-ased Preferred Link algorithm is ased on the weight given to a node, whih in turn is ased on its neighors temporal and spatial staility. Temporal staility has to do with a partiular time period in whih it is ommuniating with its neighors, while spatial staility has to do with the distane estimated from the strength of the signals reeived from its neighors. In oth algorithms a preferred list tale PLT is reated with nodes eing set in order of preferene. After this step, a node an hoose K first nodes from PLT and to reate its preferred list. The effiient flooding mehanism used in oth PLBR and WBPL redues the routing ontrol overhead and provides etter solutions than the other reative protools. A flooding effiient protool has higher salaility and dereases the network ollisions. However, oth PLBR and WBPL are muh more omputationally omplex than the other reative protools. 3.2 Optimized Link State Routing Protool The Optimized Link State Routing (OLSR) protool [1] [6] is a proative routing protool and is ased on the link state algorithm. It employs periodi information exhange keeping every node updated aout the topology of the network. OLSR is an optimization of a pure link state protool as it redues the size of information messages and the numer of retransmissions of these messages throughout the network. Its asi onept is the use of multipoint relaying tehnique to flood the network with ontrol messages. It an provide routes to the destination nodes in an effiient and quik manner. The partiular protool is more effiient in large and dense ad ho networks. 7

8 The idea of the multipoint relay is to redue the retransmissions and generally minimize pakets flooding in the network. Eah node in the network selets a set of its neighors whih are supposed to retransmit the pakets reeived from it. This set of seleted nodes, whih is suset of node s neighors, is alled the multipoint relays(mpr) of that node. Any neighor that is not in node s MPR, reeives the paket ut does not retransmit it. Similarly, eah node maintains a suset of its neighors whih is alled the MRP Seletors of the node. MRP Seletors is the set of node s neighors that have seleted the node as a multipoint relay. Thus, every roadast message that is oming from the MRP Seletors is supposed to e retransmitted y reeiver node. Eah node selets its own set of multipoint relays. Node s MPRset has to e onfigured in suh way to ontain the minimal suset of neighor nodes though whih the node an otain aess to all of its two-hop neighors. Sine eah node selets its MPRset independently, it knows the topology of its two-hop neighorhood. The MPRset is re-alulated only when a hange is deteted in the neighorhood: idiretional link reak or idiretional link appearane. Also, the MPRset is realulated when a hange in the two-hop neighor set is deteted. Multipoint relays are seleted among the one hop neighors with a idiretional link. In order to deide whih neighor nodes an e in the MPRset, eah node roadasts periodially HELLO messages whih ontain the information aout its neighors and their link status. These HELLO messages are reeived y all one-hop neighor nodes ut are not retransmitted to other nodes. A HELLO message originated from one node ontains a list of neighors with whih the node has idiretional link and a list of neighors from whih the node has reeived HELLO message ut their link is not yet onfirmed as idiretional. OLSR supposes i-diretional links and so the onnetivity must e heked in oth diretions. Every node maintains a topology tale aout the network topology in order to estalish routes to destination nodes. That s why every node roadasts topology ontrol (TC) messages that ontain topology information. Every TC message ontains the MPRSeletor set of every node. A node reords information aout the multipoint relays of other nodes in this topology tale. An entry in the topology tale onsists of an address of a potential destination node whih is the MPRSeletor and the address of a last-hop, whih is the node that originated the TC message. So using the topology tale a node an reah every other node in the network. The main advantage of OSLR against other proative protools is the redued numer of roadasts, due to the use of MPRs, whih leads to redued ontrol overhead. One of its main disadvantages is the overlapping MPRsets and MPRSeletors whih leads to dupliate messages. Conlusion In this paper few hyrid routing protools were presented. In most of the ases, hyrid solutions provide redued ontrol traffi overhead and redued routing message size. Hyrid protools provide flexile mehanisms for handling link reaks and restore quikly the onnetivity of the network. However, they have quite few disadvantages. Some of them are sujet to single point of failure, while others require additional traffi for reation and maintaining of their topology information. Sometimes they are just very omplex omputationally. 8

9 Referenes [1] C. Siva Ram Murthy and B.S. Manoj. Ad Ho Wireless Networks: Arhitetures and Protools. [2] R. Sivakumar, P. Sinha, and V. Bharghavan. CEDAR: A Core-Extration DistriutedAd Ho Routing Algorithm. IEEE journal on seleted areas in ommuniations, Vol. 17, No. 8, August1999. [3] Niklas Beijar. Zone Routing Protool(ZRP). Networking Laoratory, Helsinki University of Tehnology, Finland. [4] Mario Joa-Ng and I-Tai Lu. A GPS-ased Peer-to-Peer Hierarhial Link State Routing For Moile Ad Ho Networks. Polytehni University, IEEE, VTC2000. [5] R. S. Sisodia, Karthigeyan. I, B. S. Manoj, and C. Siva Ram Murthy. A Preferred Link Based Multiast Protool for Wireless Moile Ad ho Networks. Indian Institute of Tehnology, 2003 IEEE. [6] P. Jaquet, P. Muhlethaler, T. Clausen, A. Laouti, A. Qayynum and L. Viennot. Optimizes link state routing protool for ad ho networks. Hiperom Projet, Frane, 2001 IEEE. 9