A review of current routing protocols for ad hoc mobile wireless networks

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  • 1. AbstractAn ad hoc mobile network is a collection of mobile nodes that are dynamically and arbitrarily located in such a manner that the interconnections between nodes are capable of changing on a continual basis. In order to facilitate communication within the network, a routing protocol is used to discover routes between nodes. The primary goal of such an ad hoc network routing protocol is correct and efficient route establishmentbetween a pair of nodes so that messages may be delivered in a timely manner. Route construction should be done with a minimum of overhead and bandwidth consumption. This article examines routing protocols for ad hoc networks and evaluates these protocols based on a given set ofparameters. The article provides an overview of eight different protocols by presenting their characteristics and functionality, and then provides a comparison and discussion of their respective merits and drawbacks. A Review of Current Routing Protocols for Ad Hoc Mobile Wireless Networks Elizabeth M. Royer, University of California, Santa Barbara Chai-Keong Toh, Georgia Institute of Technology S ince their emergence in the1970s, wireless networks have become increasingly popular inthe computing industry. This is particularly true within the Existing Ad Hoc Routing Protocols Since the advent of Defense Advanced Research Projectspast decade, which has seen wireless networks being adapted Agency (DARPA) packet radio networks in the early 1970sto enable mobility. There are currently two variations of [1], numerous protocols have been developed for ad hocmobile wireless networks. The first is known as the infrastruc- mobile networks. Such protocols must deal with the typicaltured network (i.e., a network with fixed and wired gateways). limitations of these networks, which include high powerThe bridges for these networks are known as base stations. A consumption, low bandwidth, and high error rates. Asmobile unit within these networks connects to, and communi- shown in Fig. 1, these routing protocols may generally becates with, the nearest base station that is within its communi- categorized as:cation radius. As the mobile travels out of range of one base • Table-drivenstation and into the range of another, a “handoff” occurs from • Source-initiated (demand-driven)the old base station to the new, and the mobile is able to con- Solid lines in this figure represent direct descendants, whiletinue communication seamlessly throughout the network. Typ- dotted lines depict logical descendants. Despite being designedical applications of this type of network include office wireless for the same type of underlying network, the characteristics oflocal area networks (WLANs). each of these protocols are quite distinct. The following sec- The second type of mobile wireless network is the infras- tions describe the protocols and categorize them according totructureless mobile network, commonly known as an ad hoc their characteristics.network. Infrastructureless networks have no fixed routers; allnodes are capable of movement and can be connected dynami- Table-Driven Routing Protocolscally in an arbitrary manner. Nodes of these networks function Table-driven routing protocols attempt to maintain consistent,as routers which discover and maintain routes to other nodes up-to-date routing information from each node to every otherin the network. Example applications of ad hoc networks are node in the network. These protocols require each node toemergency search-and-rescue operations, meetings or conven- maintain one or more tables to store routing information, andtions in which persons wish to quickly share information, and they respond to changes in network topology by propagatingdata acquisition operations in inhospitable terrain. updates throughout the network in order to maintain a consis- This article examines routing protocols designed for these tent network view. The areas in which they differ are thead hoc networks by first describing the operation of each of number of necessary routing-related tables and the methodsthe protocols and then comparing their various characteris- by which changes in network structure are broadcast. The fol-tics. The remainder of the article is organized as follows. lowing sections discuss some of the existing table-driven adThe next section presents a discussion of two subdivisions of hoc routing protocols.ad hoc routing protocols. Another section discusses currenttable-driven protocols, while a later section describes those Destination-Sequenced Distance-Vector Routing — The Des-protocols which are classified as on-demand. The article tination-Sequenced Distance-Vector Routing protocolthen presents qualitative comparisons of table-driven proto- (DSDV) described in [2] is a table-driven algorithm based oncols, followed by demand-driven protocols, and finally a gen- the classical Bellman-Ford routing mechanism [3]. Theeral comparison of table-driven and on-demand protocols. improvements made to the Bellman-Ford algorithm includeApplications and challenges facing ad hoc mobile wireless freedom from loops in routing tables.networks are discussed; and finally, the last section con- Every mobile node in the network maintains a routingcludes the article. table in which all of the possible destinations within the net-46 1070-9916/99/$10.00 © 1999 IEEE IEEE Personal Communications • April 1999
  • 2. Ad hoc routing protocolswork and the number of hops to each destination are record-ed. Each entry is marked with a sequence number assigned bythe destination node. The sequence numbers enable the Source-initiated Table-driven on-demandmobile nodes to distinguish stale routes from new ones, there-by avoiding the formation of routing loops. Routing tableupdates are periodically transmitted throughout the networkin order to maintain table consistency. To help alleviate the DSDV WRP AODV DSR LMR ABRpotentially large amount of network traffic that such updatescan generate, route updates can employ two possible types of CGSR TORA SSRpackets. The first is known as a full dump. This type of packetcarries all available routing information and can require mul- s Figure 1. Categorization of ad hoc routing protocols.tiple network protocol data units (NPDUs). During periods ofoccasional movement, these packets are transmitted infre-quently. Smaller incremental packets are used to relay only rithm within the cluster. The disadvantage of having a clus-that information which has changed since the last full dump. ter head scheme is that frequent cluster head changes canEach of these broadcasts should fit into a standard-size adversely affect routing protocol performance since nodesNPDU, thereby decreasing the amount of traffic generated. are busy in cluster head selection rather than packet relay-The mobile nodes maintain an additional table where they ing. Hence, instead of invoking cluster head reselectionstore the data sent in the incremental routing information every time the cluster membership changes, a Least Clusterpackets. Change (LCC) clustering algorithm is introduced. Using New route broadcasts contain the address of the destina- LCC, cluster heads only change when two cluster headstion, the number of hops to reach the destination, the come into contact, or when a node moves out of contact ofsequence number of the information received regarding the all other cluster heads.destination, as well as a new sequence number unique to the CGSR uses DSDV as the underlying routing scheme, andbroadcast [2]. The route labeled with the most recent hence has much of the same overhead as DSDV. However, itsequence number is always used. In the event that two modifies DSDV by using a hierarchical cluster-head-to-gate-updates have the same sequence number, the route with the way routing approach to route traffic from source to destina-smaller metric is used in order to optimize (shorten) the tion. Gateway nodes are nodes that are within communicationpath. Mobiles also keep track of the settling time of routes, range of two or more cluster heads. A packet sent by a nodeor the weighted average time that routes to a destination will is first routed to its cluster head, and then the packet is rout-fluctuate before the route with the best metric is received ed from the cluster head to a gateway to another cluster head,(see [2]). By delaying the broadcast of a routing update by and so on until the cluster head of the destination node isthe length of the settling time, mobiles can reduce network reached. The packet is then transmitted to the destination.traffic and optimize routes by eliminating those broadcasts Figure 2 illustrates an example of this routing scheme. Usingthat would occur if a better route was discovered in the very this method, each node must keep a “cluster member table”near future. where it stores the destination cluster head for each mobile node in the network. These cluster member tables are broad-Clusterhead Gateway Switch Routing — The Clusterhead cast by each node periodically using the DSDV algorithm.Gateway Switch Routing (CGSR) protocol differs from the Nodes update their cluster member tables on reception ofprevious protocol in the type of addressing and network such a table from a neighbor.organization scheme employed. Instead of a “flat” network, In addition to the cluster member table, each nodeCGSR is a clustered multihop mobile wireless network with must also maintain a routing table which is used to deter-several heuristic routing schemes [4]. The authors state that mine the next hop in order to reach the destination. Onby having a cluster head controlling a group of ad hoc receiving a packet, a node will consult its cluster membernodes, a framework for code separation (among clusters), table and routing table to determine the nearest clusterchannel access, routing, and bandwidth allocation can be head along the route to the destination. Next, the nodeachieved. A cluster head selection algorithm is utilized to will check its routing table to determine the next hop usedelect a node as the cluster head using a distributed algo- to reach the selected cluster head. It then transmits the packet to this node. The Wireless Routing Protocol — The Wireless Routing Pro- tocol (WRP) described in [5] is a table-based protocol with 5 the goal of maintaining routing information among all nodes in the network. Each node in the network is responsible for maintaining four tables: 4 • Distance table 1 3 • Routing table 6 • Link-cost table 8 • Message retransmission list (MRL) table 2 Each entry of the MRL contains the sequence number of the 7 update message, a retransmission counter, an acknowledg- ment-required flag vector with one entry per neighbor, and a list of updates sent in the update message. The MRL records Node which updates in an update message need to be retransmit- Gateway Cluster head ted and which neighbors should acknowledge the retransmis- sion [5].s Figure 2. CGSR: routing from node 1 to node 8. Mobiles inform each other of link changes through the useIEEE Personal Communications • April 1999 47
  • 3. of update messages. An update message is sent only between tain the most recent route information. Each node maintainsneighboring nodes and contains a list of updates (the destina- its own sequence number, as well as a broadcast ID. Thetion, the distance to the destination, and the predecessor of broadcast ID is incremented for every RREQ the node initi-the destination), as well as a list of responses indicating ates, and together with the node’s IP address, uniquely identi-which mobiles should acknowledge (ACK) the update. fies an RREQ. Along with its own sequence number and theMobiles send update messages after processing updates from broadcast ID, the source node includes in the RREQ theneighbors or detecting a change in a link to a neighbor. In most recent sequence number it has for the destination. Inter-the event of the loss of a link between two nodes, the nodes mediate nodes can reply to the RREQ only if they have asend update messages to their neighbors. The neighbors then route to the destination whose corresponding destinationmodify their distance table entries and check for new possible sequence number is greater than or equal to that contained inpaths through other nodes. Any new paths are relayed back the RREQ.to the original nodes so that they can update their tables During the process of forwarding the RREQ, intermediateaccordingly. nodes record in their route tables the address of the neighbor Nodes learn of the existence of their neighbors from the from which the first copy of the broadcast packet is received,receipt of acknowledgments and other messages. If a node is thereby establishing a reverse path. If additional copies of thenot sending messages, it must send a hello message within a same RREQ are later received, these packets are discarded.specified time period to ensure connectivity. Otherwise, the Once the RREQ reaches the destination or an intermediatelack of messages from the node indicates the failure of that node with a fresh enough route, the destination/intermediatelink; this may cause a false alarm. When a mobile receives a node responds by unicasting a route reply (RREP) packethello message from a new node, that new node is added to the back to the neighbor from which it first received the RREQmobile’s routing table, and the mobile sends the new node a (Fig. 3b). As the RREP is routed back along the reverse path,copy of its routing table information. nodes along this path set up forward route entries in their Part of the novelty of WRP stems from the way in which route tables which point to the node from which the RREPit achieves loop freedom. In WRP, routing nodes communi- came. These forward route entries indicate the active forwardcate the distance and second-to-last hop information for route. Associated with each route entry is a route timer whicheach destination in the wireless networks. WRP belongs to will cause the deletion of the entry if it is not used within thethe class of path-finding algorithms with an important excep- specified lifetime. Because the RREP is forwarded along thetion. It avoids the “count-to-infinity” problem [6] by forcing path established by the RREQ, AODV only supports the useeach node to perform consistency checks of predecessor of symmetric links.information reported by all its neighbors. This ultimately Routes are maintained as follows. If a source node moves,(although not instantaneously) eliminates looping situations it is able to reinitiate the route discovery protocol to find aand provides faster route convergence when a link failure new route to the destination. If a node along the route moves,event occurs. its upstream neighbor notices the move and propagates a link failure notification message (an RREP with infinite metric) to Source-Initiated On-Demand Routing each of its active upstream neighbors to inform them of theA different approach from table-driven routing is source-initi- erasure of that part of the route [7]. These nodes in turnated on-demand routing. This type of routing creates routesonly when desired by the source node. When a node requiresa route to a destination, it initiates a route discovery process N2within the network. This process is completed once a route isfound or all possible route permutations have been examined. N5 N8 DestinationOnce a route has been established, it is maintained by a routemaintenance procedure until either the destination becomesinaccessible along every path from the source or until the Source N1route is no longer desired. N7 N4Ad Hoc On-Demand Distance Vector Routing — The Ad HocOn-Demand Distance Vector (AODV) routing protocoldescribed in [7] builds on the DSDV algorithm previously N3 N6described. AODV is an improvement on DSDV because ittypically minimizes the number of required broadcasts by cre- (a) Propagation of the RREQating routes on a demand basis, as opposed to maintaining acomplete list of routes as in the DSDV algorithm. The authorsof AODV classify it as a pure on-demand route acquisition sys- N2tem, since nodes that are not on a selected path do not main- N5 Destination N8tain routing information or participate in routing tableexchanges [7]. When a source node desires to send a message to some Source N1destination node and does not already have a valid route to N7that destination, it initiates a path discovery process to locate N4the other node. It broadcasts a route request (RREQ) packetto its neighbors, which then forward the request to theirneighbors, and so on, until either the destination or an inter- N3 N6mediate node with a “fresh enough” route to the destinationis located. Figure 3a illustrates the propagation of the broad- (b) Path of the RREP to the sourcecast RREQs across the network. AODV utilizes destinationsequence numbers to ensure all routes are loop-free and con- s Figure 3. AODV route discovery.48 IEEE Personal Communications • April 1999
  • 4. N1-N2 N1-N2-N5-N8 N2 N2 N1-N2-N5 Destination N1-N2-N5-N8 Destination N5 N8 N5 N8 N1 N1-N2-N5-N8 N1-N3-N4-N7 Source N1 N1-N3-N4 Source N1 N7 N7 N4 N1-N3-N4 N4 N1 N1-N3-N4-N6 N1-N3 N3 N3 N6 N6 N1-N3-N4 (a) Building of the route record during route discovery (b) Propagation of the route reply with the route records Figure 4. Creation of the route record in DSR.propagate the link failure notification to their upstream neigh- is the destination, it places the route record contained in thebors, and so on until the source node is reached. The source route request into the route reply. If the responding node isnode may then choose to reinitiate route discovery for that an intermediate node, it will append its cached route to thedestination if a route is still desired. route record and then generate the route reply. To return the An additional aspect of the protocol is the use of hello route reply, the responding node must have a route to themessages, periodic local broadcasts by a node to inform each initiator. If it has a route to the initiator in its route cache, itmobile node of other nodes in its neighborhood. Hello mes- may use that route. Otherwise, if symmetric links are sup-sages can be used to maintain the local connectivity of a ported, the node may reverse the route in the route record. Ifnode. However, the use of hello messages is not required. symmetric links are not supported, the node may initiate itsNodes listen for retransmission of data packets to ensure that own route discovery and piggyback the route reply on thethe next hop is still within reach. If such a retransmission is new route request. Figure 4b shows the transmission of thenot heard, the node may use any one of a number of tech- route reply with its associated route record back to theniques, including the reception of hello messages, to deter- source node.mine whether the next hop is within communication range. Route maintenance is accomplished through the use ofThe hello messages may list the other nodes from which a route error packets and acknowledgments. Route error packetsmobile has heard, thereby yielding greater knowledge of net- are generated at a node when the data link layer encounters awork connectivity. fatal transmission problem. When a route error packet is received, the hop in error is removed from the node’s routeDynamic Source Routing — The Dynamic Source Routing cache and all routes containing the hop are truncated at that(DSR) protocol presented in [8] is an on-demand routing point. In addition to route error messages, acknowledgmentsprotocol that is based on the concept of source routing. are used to verify the correct operation of the route links.Mobile nodes are required to maintain route caches that Such acknowledgments include passive acknowledgments,contain the source routes of which the mobile is aware. where a mobile is able to hear the next hop forwarding theEntries in the route cache are continually updated as new packet along the route.routes are learned. The protocol consists of two major phases: route discovery Temporally Ordered Routing Algorithm — The Temporallyand route maintenance. When a mobile node has a packet to Ordered Routing Algorithm (TORA) is a highly adaptivesend to some destination, it first consults its route cache to loop-free distributed routing algorithm based on the conceptdetermine whether it already has a route to the destination. If of link reversal [10]. TORA is proposed to operate in a high-it has an unexpired route to the destination, it will use this ly dynamic mobile networking environment. It is source-initi-route to send the packet. On the other hand, if the node does ated and provides multiple routes for any desirednot have such a route, it initiates route discovery by broad- source/destination pair. The key design concept of TORA iscasting a route request packet. This route request contains the the localization of control messages to a very small set ofaddress of the destination, along with the source node’s nodes near the occurrence of a topological change. Toaddress and a unique identification number. Each node accomplish this, nodes need to maintain routing informationreceiving the packet checks whether it knows of a route to the about adjacent (one-hop) nodes. The protocol performs threedestination. If it does not, it adds its own address to the route basic functions:record of the packet and then forwards the packet along its • Route creationoutgoing links. To limit the number of route requests propa- • Route maintenancegated on the outgoing links of a node, a mobile only forwards • Route erasurethe route request if the request has not yet been seen by the During the route creation and maintenance phases,mobile and if the mobile’s address does not already appear in nodes use a “height” metric to establish a directed acyclicthe route record. graph (DAG) rooted at the destination. Thereafter, links A route reply is generated when the route request reaches are assigned a direction (upstream or downstream) basedeither the destination itself, or an intermediate node which on the relative height metric of neighboring nodes, ascontains in its route cache an unexpired route to the destina- shown in Fig. 5a. This process of establishing a DAG istion [9]. By the time the packet reaches either the destination similar to the query/reply process proposed in Lightweightor such an intermediate node, it contains a route record Mobile Routing (LMR) [11]. In times of node mobility theyielding the sequence of hops taken. Figure 4a illustrates the DAG route is broken, and route maintenance is necessaryformation of the route record as the route request propagates to reestablish a DAG rooted at the same destination. Asthrough the network. If the node generating the route reply shown in Fig. 5b, upon failure of the last downstream link,IEEE Personal Communications • April 1999 49
  • 5. a node generates a new reference level which results in the TORA’s route erasure phase essentially involves flooding apropagation of that reference level by neighboring nodes, broadcast clear packet (CLR) throughout the network toeffectively coordinating a structured reaction to the failure. erase invalid routes.Links are reversed to reflect the change in adapting to the In TORA there is a potential for oscillations to occur,new reference level. This has the same effect as reversing especially when multiple sets of coordinating nodes are con-the direction of one or more links when a node has no currently detecting partitions, erasing routes, and buildingdownstream links. new routes based on each other. Because TORA uses inter- Timing is an important factor for TORA because the nodal coordination, its instability problem is similar to the“height” metric is dependent on the logical time of a link fail- “count-to-infinity” problem in distance-vector routing proto-ure; TORA assumes that all nodes have synchronized clocks cols, except that such oscillations are temporary and route(accomplished via an external time source such as the Global convergence will ultimately occur.Positioning System). TORA’s metric is a quintuple comprisingfive elements, namely: Associativity-Based Routing — A totally different approach• Logical time of a link failure in mobile routing is proposed in [12]. The Associativity-• The unique ID of the node that defined the new reference Based Routing (ABR) protocol is free from loops, deadlock, level and packet duplicates, and defines a new routing metric for• A reflection indicator bit ad hoc mobile networks. This metric is known as the degree• A propagation ordering parameter of association stability. In ABR, a route is selected based on• The unique ID of the node the degree of association stability of mobile nodes. Each The first three elements collectively represent the refer- node periodically generates a beacon to signify its existence.ence level. A new reference level is defined each time a When received by neighboring nodes, this beacon causesnode loses its last downstream link due to a link failure. their associativity tables to be updated. For each beacon received, the associativity tick of the current node with respect to the beaconing node is incremented. Association stability is defined by connection stability of one node with Source respect to another node over time and space. A high degree of association stability may indicate a low state of node mobility, while a low degree may indicate a high state of node mobility. Associativity ticks are reset when the neigh- bors of a node or the node itself move out of proximity. A fundamental objective of ABR is to derive longer-lived routes for ad hoc mobile networks. The three phases of ABR are: • Route discovery • Route reconstruction (RRC) Destination • Route deletion The route discovery phase is accomplished by a broad- Ad hoc node cast query and await-reply (BQ-REPLY) cycle. A node desiring a route broadcasts a BQ message in search of Height metric mobiles that have a route to the destination. All nodes receiving the query (that are not the destination) append (a) their addresses and their associativity ticks with their neigh- bors along with QoS information to the query packet. A successor node erases its upstream node neighbors’ associa- B B tivity tick entries and retains only the entry concerned with C C itself and its upstream node. In this way, each resultant packet arriving at the destination will contain the associativ- A A D D ity ticks of the nodes along the route to the destination. The G G destination is then able to select the best route by examin- E E ing the associativity ticks along each of the paths. When (1) F (2) F multiple paths have the same overall degree of association stability, the route with the minimum number of hops is selected. The destination then sends a REPLY packet back B B C C to the source along this path. Nodes propagating the REPLY mark their routes as valid. All other routes remain A A D inactive, and the possibility of duplicate packets arriving at D G G the destination is avoided. E E RRC may consist of partial route discovery, invalid route F F erasure, valid route updates, and new route discovery, depend- (3) (4) ing on which node(s) along the route move. Movement by the source results in a new BQ-REPLY process, as shown in Fig. (b) Link reversal Link failure 6a. The RN[1] message is a route notification used to erase the route entries associated with downstream nodes. When the destination moves, the immediate upstream node erasess Figure 5. a) Route creation (showing link direction assign- its route and determines if the node is still reachable by a ment); b) route maintenance (showing the link reversal phe- localized query (LQ[H]) process, where H refers to the hop nomenon) in TORA. count from the upstream node to the destination (Fig. 6b). If50 IEEE Personal Communications • April 1999
  • 6. SRC DESTthe destination receives the LQ packet, it BQ DESTREPLYs with the best partial route; other- SRC LQ[H]wise, the initiating node times out and the SRCprocess backtracks to the next upstream H=3 DESTnode. Here an RN[0] message is sent to thenext upstream node to erase the invalid RN[0]route and inform this node that it should RN[1] RN[0]invoke the LQ[H] process. If this processresults in backtracking more than halfway tothe source, the LQ process is discontinued (a) Route maintenance (b) Route maintenance for a source move for a destination moveand a new BQ process is initiated at thesource. s Figure 6. Route maintenance for source and destination movement in ABR. When a discovered route is no longerdesired, the source node initiates a routedelete (RD) broadcast so that all nodes along the route The DRP is responsible for the maintenance of the Sig-update their routing tables. The RD message is propagated by nal Stability Table (SST) and Routing Table (RT). Thea full broadcast, as opposed to a directed broadcast, because SST records the signal strength of neighboring nodes,the source node may not be aware of any route node changes which is obtained by periodic beacons from the link layerthat occurred during RRCs. of each neighboring node. Signal strength may be recorded as either a strong or weak channel. All transmissions areSignal Stability Routing — Another on-demand protocol is received by, and processed in, the DRP. After updating allthe Signal Stability-Based Adaptive Routing protocol (SSR) appropriate table entries, the DRP passes a received packetpresented in [13]. Unlike the algorithms described so far, SSR to the SRP.selects routes based on the signal strength between nodes and The SRP processes packets by passing the packet up thea node’s location stability. This route selection criteria has the stack if it is the intended receiver or looking up the destina-effect of choosing routes that have “stronger” connectivity. tion in the RT and then forwarding the packet if it is not. IfSSR can be divided into two cooperative protocols: the no entry is found in the RT for the destination, a route-searchDynamic Routing Protocol (DRP) and the Static Routing process is initiated to find a route. Route requests are propa-Protocol (SRP). gated throughout the network, but are only forwarded to the next hop if they are received over strong channels and have not been Parameters DSDV CGSR WRP previously processed (to prevent looping). The destination chooses Time complexity (link addition/failure) O(d) O(d) O(h) the first arriving route-search pack- et to send back because it is most Communication complexity (link addition/failure) O(x = N) O(x = N) O(x = N) probable that the packet arrived over the shortest and/or least con- Routing philosophy Flat Hierarchical Flat* gested path. The DRP then revers- Loop-free Yes Yes Yes, but not es the selected route and sends a instantaneous route-reply message back to the ini- tiator. The DRP of the nodes along Multicast capability No No** No the path update their RTs accord- ingly. Number of required tables Two Two Four Route-search packets arriving at Frequency of update transmissions Periodically Periodically Periodically the destination have necessarily and as needed and as needed chosen the path of strongest signal stability, since the packets are Updates transmitted to Neighbors Neighbors and Neighbors dropped at a node if they have cluster head arrived over a weak channel. If Utilizes sequence numbers Yes Yes Yes there is no route-reply message received at the source within a spe- Utilizes hello messages Yes No Yes cific timeout period, the source changes the PREF field in the Critical nodes No Yes No header to indicate that weak chan- (cluster head) nels are acceptable, since these may Routing metric Shortest Path Shortest Path Shortest Path be the only links over which the packet can be propagated. Abbreviations: When a failed link is detected N = Number of nodes in the network within the network, the intermedi- d = Network diameter ate nodes send an error message to h = Height of routing tree the source indicating which channel x = Number of nodes affected by a topological change has failed. The source then initiates another route-search process to * While WRP uses flat addressing, it can be used hierarchically [15]. find a new path to the destination. ** The protocol itself currently does not support multicast; however, there is a separate protocol described in [16], which runs on top of CGSR and provides multicast capability. The source also sends an erase mes- sage to notify all nodes of the bro-s Table 1. Comparisons of the characteristics of table-driven routing protocols. ken link.IEEE Personal Communications • April 1999 51
  • 7. Comparisons the underlying routing protocol. Routing in CGSR occurs over cluster heads and gateways. A cluster head table is nec-The following sections provide comparisons of the previously essary in addition to the routing table. One advantage ofdescribed routing algorithms. The next section compares CGSR is that several heuristic methods can be employed totable-driven protocols, and another section compares on- improve the protocol’s performance. These methods includedemand protocols. A later section presents a discussion of priority token scheduling, gateway code scheduling, andthe two classes of algorithms. In Tables 1 and 2, time com- path reservation [4].plexity is defined as the number of steps needed to perform a The WRP protocol differs from the other protocols inprotocol operation, and communication complexity is the several ways. WRP requires each node to maintain fournumber of messages needed to perform a protocol operation routing tables. This can lead to substantial memory require-[11, 14]. Also, the values for these metrics represent worst- ments, especially when the number of nodes in the networkcase behavior. is large. Furthermore, the WRP protocol requires the use of hello packets whenever there are no recent packet trans- Table-Driven Protocols missions from a given node. The hello packets consumeThe discussion here is based on Table 1. As stated earlier, bandwidth and disallow a node to enter sleep mode. How-DSDV routing is essentially a modification of the basic Bell- ever, although it belongs to the class of path-finding algo-man-Ford routing algorithm. The modifications include the rithms, WRP has an advantage over other path-findingguarantee of loop-free routes and a simple route update algorithms because it avoids the problem of creating tem-protocol. While only providing one path to any given desti- porary routing loops that these algorithms have through thenation, DSDV selects the shortest path based on the num- verification of predecessor information, as described in anber of hops to the destination. DSDV provides two types of earlier section.update messages, one of which is significantly smaller than Having discussed the operation and characteristics ofthe other. The smaller update message can be used for each of the existing table-driven routing protocols, it isincremental updates so that the entire routing table need important to highlight the differences. During link failures,not be transmitted for every change in network topology. WRP has lower time complexity than DSDV since it onlyHowever, DSDV is inefficient because of the requirement informs neighboring nodes about link status changes. Dur-of periodic update transmissions, regardless of the number ing link additions, hello messages are used as a presenceof changes in the network topology. This effectively limits indicator such that the routing table entry can be updated.the number of nodes that can connect to the network since Again, this only affects neighboring nodes. In CGSR,the overhead grows as O(n 2 ). In CGSR, DSDV is used as because routing performance is dependent on the status of Performance parameters AODV DSR TORA ABR SSR Time complexity (initialization) O(2d) O(2d) O(2d) O(d + z) O(d + z) Time complexity (postfailure) O(2d) O(2d) or 0* O(2d) O(l + z) O(l + z) Communication complexity (initialization) O(2N) O(2N) O(2N) O(N + y) O(N + y) Communication complexity (postfailure) O(2N) O(2N) O(2x) O(x + y) O(x + y) Routing philosophy Flat Flat Flat Flat Flat Loop-free Yes Yes Yes Yes Yes Multicast capability Yes No No** No No Beaconing requirements No No No Yes Yes Multiple route possibilities No Yes Yes No No Routes maintained in Route table Route cache Route table Route table Route table Utilizes route cache/table expiration timers Yes No No No No Route reconfiguration methodology Erase route; Erase route; Link reversal; Localized Erase route; notify source notify source route repair broadcast query notify source Routing metric Freshest and Shortest path Shortest path Associativity and Associativity and shortest path shortest path and stability others*** Abbreviations: l = Diameter of the affected network segment y = Total number of nodes forming the directed path where the REPLY packet transits z = Diameter of the directed path where the REPLY packet transits * Cache hit. ** Like CGSR, TORA also does not support multicast; however, there is a separate protocol, LAM [18], which runs on top of ORA and provides multicast capability. *** ABR also uses the route relaying load and cumulative forwarding delay as routing metrics.s Table 2. Comparisons of the characteristics of source-initiated on-demand ad hoc routing protocols.52 IEEE Personal Communications • April 1999
  • 8. specific nodes (cluster head, gateway, or normal nodes), of the novelty of TORA stems from its creation of DAGs totime complexity of a link failure associated with a cluster aid route establishment. One of the advantages of TORA ishead is higher than DSDV, given the additional time need- its support for multiple routes. TORA and DSR are the onlyed to perform cluster head reselection. Similarly, this on-demand protocols considered here which retain multipleapplies to the case of link additions associated with the clus- route possibilities for a single source/destination pair. Routeter head. There is no gateway selection in CGSR since each reconstruction is not necessary until all known routes to anode declares it is a gateway node to its neighbors if it is destination are deemed invalid, and hence bandwidth canresponding to multiple radio codes. If a gateway node potentially be conserved because of the necessity for fewermoves out of range, the routing protocol is responsible for route rebuildings. Another advantage of TORA is its sup-routing the packet to another gateway. port for multicast. Although, unlike AODV, TORA does not In terms of communication complexity, since DSDV, incorporate multicast into its basic operation, it functions asCGSR, and WRP use distance vector shortest-path routing as the underlying protocol for the Lightweight Adaptive Multi-the underlying routing protocol, they all have the same degree cast Algorithm (LAM), and together the two protocols pro-of complexity during link failures and additions. vide multicast capability [18]. TORA’s reliance on synchronized clocks, while a novel idea, inherently limits its Source-Initiated On-Demand Routing Protocols applicability. If a node does not have GPS or some otherTable 2 presents a comparison of AODV, DSR, TORA, external time source, it cannot use the algorithm. Addition-ABR, and SSR. ally, if the external time source fails, the algorithm will cease AODV employs a route discovery procedure similar to to operate. Furthermore, route rebuilding in TORA may notDSR; however, there are a couple of important distinctions. occur as quickly as in the other algorithms due to the poten-The most notable of these is that the overhead of DSR is tial for oscillations during this period. This can lead topotentially larger than that of AODV since each DSR packet potentially lengthy delays while waiting for the new routes tomust carry full routing information, whereas in AODV pack- be determined.ets need only contain the destination address. Similarly, the ABR is a compromise between broadcast and point-to-route replies in DSR are larger because they contain the point routing, and uses the connection-oriented packet for-address of every node along the route, whereas in AODV warding approach. Route selection is primarily based on theroute replies need only carry the destination IP address and aggregated associativity ticks of nodes along the path. Hence,sequence number. Also, the memory overhead may be slight- although the resulting path does not necessarily result in thely greater in DSR because of the need to remember full smallest possible number of hops, the path tends to be longer-routes, as opposed to only next hop information in AODV. A lived than other routes. A long-lived route requires fewerfurther advantage of AODV is its support for multicast [17]. route reconstructions and therefore yields higher throughput.None of the other algorithms considered in this article cur- Another benefit of ABR is that, like the other protocols, it isrently incorporate multicast communication. On the down- guaranteed to be free of packet duplicates. The reason is thatside, AODV requires symmetric links between nodes, and only the best route is marked valid, while all other possiblehence cannot utilize routes with asymmetric links. In this routes remain passive. ABR, however, relies on the fact thataspect DSR is superior, since it does not require the use of each node is beaconing periodically. The beaconing intervalsuch links and can utilize asymmetric links when symmetric must be short enough to accurately reflect the spatial, tempo-links are not available. ral, and connectivity state of the mobile hosts. This beaconing The DSR algorithm is intended for networks in which requirement may result in additional power consumption.the mobiles move at moderate speed with respect to packet However, experimental results obtained in [19] reveal that thetransmission latency [8]. Assumptions the algorithm makes inclusion of periodic beaconing has a minute influence on thefor operation are that the network diameter is relatively overall battery power consumption. Unlike DSR, ABR doessmall and that the mobile nodes can enable a promiscuous not utilize route caches.receive mode, whereby every received packet is delivered to The SSR algorithm is a logical descendant of ABR. It uti-the network driver software without filtering by destination lizes a new technique of selecting routes based on the signaladdress. An advantage of DSR over some of the other on- strength and location stability of nodes along the path. As indemand protocols is that DSR does not make use of period- ABR, while the paths selected by this algorithm are not neces-ic routing advertisements, thereby saving bandwidth and sarily shortest in hop count, they do tend to be more stablereducing power consumption. Hence, the protocol does not and longer-lived, resulting in fewer route reconstructions. Oneincur any overhead when there are no changes in network of the major drawbacks of the SSR protocol is that, unlike intopology. Additionally, DSR allows nodes to keep multiple AODV and DSR, intermediate nodes cannot reply to routeroutes to a destination in their cache. Hence, when a link requests sent toward a destination; this results in potentiallyon a route is broken, the source node can check its cache long delays before a route can be discovered. Additionally,for another valid route. If such a route is found, route when a link failure occurs along a path, the route discoveryreconstruction does not need to be reinvoked. In this case, algorithm must be reinvoked from the source to find a newroute recovery is faster than in many of the other on- path to the destination. No attempt is made to use partialdemand protocols. However, if there are no additional route recovery (unlike ABR), that is, to allow intermediateroutes to the destination in the source node’s cache, route nodes to attempt to rebuild the route themselves. AODV anddiscovery must be reinitiated, as in AODV, if the route is DSR also do not specify intermediate node rebuilding. Whilestill required. On the other hand, because of the small this may lead to longer route reconstruction times since linkdiameter assumption and the source routing requirement, failures cannot be resolved locally without the intervention ofDSR is not scalable to large networks. Furthermore, as pre- the source node, the attempt and failure of an intermediateviously stated, the need to place the entire route in both node to rebuild a route will cause a longer delay than if theroute replies and data packets causes greater control over- source node had attempted the rebuilding as soon as the bro-head than in AODV. ken link was noticed. Thus, it remains to be seen whether TORA is a “link reversal” algorithm that is best suited intermediate node route rebuilding is more optimal thanfor networks with large dense populations of nodes [10]. Part source node route rebuilding.IEEE Personal Communications • April 1999 53
  • 9. Parameters On-demand Table-driven Availability of routing Available when needed Always available regardless of interact on a commonly shared information need whiteboard. Ad hoc mobile com- Routing philosophy Flat Mostly flat, except for CGSR munication is particularly useful in relaying information (status, situa- Periodic route updates Not required Required tion awareness, etc.) via data, video, and/or voice from one rescue team Coping with mobility Use localized route discovery Inform other nodes to achieve as in ABR and SSR a consistent routing table member to another over a small handheld or wearable wireless Signaling traffic generated Grows with increasing mobility Greater than that of on- device. Again, this applies to law of active routes (as in ABR) demand routing enforcement personnel as well. Current challenges for ad hoc Quality of service support Few can support QoS, although Mainly shortest path as the wireless networks include: most support shortest path QoS metric •Multicast [21]s Table 3. Overall comparisons of on-demand versus table-driven routing protocols. •QoS support •Power-aware routing [22] •Location-aided routing [23] As mentioned above, multicast Table-Driven vs. On-Demand Routing is desirable to support multiparty wireless communications.As discussed earlier, the table-driven ad hoc routing approach Since the multicast tree is no longer static (i.e., its topologyis similar to the connectionless approach of forwarding pack- is subject to change over time), the multicast routing proto-ets, with no regard to when and how frequently such routes col must be able to cope with mobility, including multicastare desired. It relies on an underlying routing table update membership dynamics (e.g., leave and join). In terms ofmechanism that involves the constant propagation of routing QoS, it is inadequate to consider QoS merely at the networkinformation. This is not the case, however, for on-demand level without considering the underlying media access con-routing protocols. When a node using an on-demand protocol trol layer [24]. Again, given the problems associated with thedesires a route to a new destination, it will have to wait until dynamics of nodes, hidden terminals, and fluctuating linksuch a route can be discovered. On the other hand, because characteristics, supporting end-to-end QoS is a nontrivialrouting information is constantly propagated and maintained issue that requires in-depth investigation. Currently, there isin table-driven routing protocols, a route to every other node a trend toward an adaptive QoS approach instead of thein the ad hoc network is always available, regardless of “plain” resource reservation method with hard QoS guaran-whether or not it is needed. This feature, although useful for tees. Another important factor is the limited power supply indatagram traffic, incurs substantial signaling traffic and power handheld devices, which can seriously prohibit packet for-consumption. Since both bandwidth and battery power are warding in an ad hoc mobile environment. Hence, routingscarce resources in mobile computers, this becomes a serious traffic based on nodes’ power metrics is one way to distin-limitation. Table 3 lists some of the basic differences between guish routes that are more long-lived than others. Finally,the two classes of algorithms. instead of using beaconing or broadcast search, location- Another consideration is whether a flat or hierarchical aided routing uses positioning information to define associ-addressing scheme should be used. All of the protocols con- ated regions so that the routing is spatially oriented andsidered here, except for CGSR, use a flat addressing scheme. limited. This is analogous to associativity-oriented andIn [20] a discussion of the two addressing schemes is present- restricted broadcast in ABR.ed. While flat addressing may be less complicated and easier Current ad hoc routing approaches have introduced severalto use, there are doubts as to its scalability. new paradigms, such as exploiting user demand, and the use of location, power, and association parameters. Adaptivity and Applications and Challenges self-configuration are key features of these approaches. How- ever, flexibility is also important. A flexible ad hoc routingAkin to packet radio networks, ad hoc wireless networks have protocol could responsively invoke table-driven and/or on-an important role to play in military applications. Soldiers demand approaches based on situations and communicationequipped with multimode mobile communicators can now requirements. The “toggle” between these two approachescommunicate in an ad hoc manner without the need for fixed may not be trivial since concerned nodes must be “in sync”wireless base stations. In addition, small vehicular devices with the toggling. Coexistence of both approaches may alsoequipped with audio sensors and cameras can be deployed at exist in spatially clustered ad hoc groups, with intraclustertargeted regions to collect important location and environ- employing the table-driven approach and intercluster employ-mental information which will be communicated back to a ing the demand-driven approach or vice versa. Further work isprocessing node via ad hoc mobile communications. Ship-to- necessary to investigate the feasibility and performance ofship ad hoc mobile communication is also desirable since it hybrid ad hoc routing approaches. Lastly, in addition to theprovides alternate communication paths without reliance on above, further research in the areas of media access control,ground- or space-based communication infrastructures. security, service discovery, and Internet protocol operability is Commercial scenarios for ad hoc wireless networks include: required before the potential of ad hoc mobile networking can• Conferences/meetings/lectures be realized.• Emergency services• Law enforcement Conclusion People today attend meetings and conferences with theirlaptops, palmtops, and notebooks. It is therefore attractive to In this article we provide descriptions of several routinghave instant network formation, in addition to file and infor- schemes proposed for ad hoc mobile networks. We alsomation sharing without the presence of fixed base stations and provide a classification of these schemes according to thesystems administrators. A presenter can multicast slides and routing strategy (i.e., table-driven and on-demand). Weaudio to intended recipients. Attendees can ask questions and have presented a comparison of these two categories of54 IEEE Personal Communications • April 1999
  • 10. routing protocols, highlighting their features, differences, [15] S. Murthy and J. J. Garcia-Luna-Aceves, “Loop-Free Internet Routing Using Hierarchical Routing Trees,” Proc. INFOCOM ’97, Apr. 7–11, 1997.and characteristics. Finally, we have identified possible [16] C.-C. Chiang, M. Gerla, and L. Zhang, “Adaptive Shared Tree Multicast inapplications and challenges facing ad hoc mobile wireless Mobile Wireless Networks,” Proc. GLOBECOM ’98, Nov. 1998, pp. 1817–22.networks. While it is not clear that any particular algorithm [17] C. E. Perkins and E. M. Royer, “Ad Hoc On Demand Distance Vectoror class of algorithm is the best for all scenarios, each pro- (AODV) Routing,” IETF Internet draft, draft-ietf-manet-aodv-02.txt, Nov. 1998 (work in progress).tocol has definite advantages and disadvantages, and is well [18] L. Ji and M. S. Corson, “A Lightweight Adaptive Multicast Algorithm,”suited for certain situations. The field of ad hoc mobile Proc. GLOBECOM ’98, Nov. 1998, pp. 1036–42.networks is rapidly growing and changing, and while there [19] C.-K. Toh and George Lin, “Implementing Associativity-Based Routingare still many challenges that need to be met, it is likely for Ad Hoc Mobile Wireless Networks,” Unpublished article, Mar. 1998. [20] D. Baker et al., “Flat vs. Hierarchical Network Control Architecture,”that such networks will see widespread use within the next ARO/DARPA Wksp. Mobile Ad-Hoc Networking; http://www.isr.umd.few years. edu/Courses/Workshops/MANET/program.html, Mar. 1997. [21] M. Gerla, C.-C. Chiang, and L. Zhang, “Tree Multicast Strategies in Acknowledgments Mobile, Multihop Wireless Networks,” ACM/Baltzer Mobile Networks and Apps. J., 1998.We would like to thank C.-C. Chiang, J. J. Garcia-Luna- [22] S. Singh, M. Woo, and C. S. Raghavendra, “Power-Aware Routing inAceves, David Maltz, Vincent Park, and Charles Perkins for Mobile Ad Hoc Networks,” Proc. ACM/IEEE MOBICOM ’98, Oct. 1998.their assistance in ensuring accurate descriptions of the opera- [23] Y. B. Ko and N. H. Vaidya, “Location-Aided Routing (LAR) in Mobile Adtion of their protocols. Hoc Networks,” Proc. ACM/IEEE MOBICOM ’98, Oct. 1998. [24] C. R. Lin and M. Gerla, “MACA/PR: An Asynchronous Multimedia Multi- References hop Wireless Network,” Proc. IEEE INFOCOM ’97, Mar. 1997.[1] J. Jubin and J. Tornow, “The DARPA Packet Radio Network Protocols,” Proc. IEEE, vol. 75, no. 1, 1987, pp. 21–32.[2] C. E. Perkins and P. Bhagwat, “Highly Dynamic Destination-Sequenced Biographies Distance-Vector Routing (DSDV) for Mobile Computers,” Comp. Com- ELIZABAETH M. ROYER (eroyer@alpha.ece.ucsb.edu) received her M.S. degree mun. Rev., Oct. 1994, pp. 234–44. in electrical and computer engineering from the University of California,[3] L. R. Ford Jr. and D. R. Fulkerson, Flows in Networks, Princeton Univ. Santa Barbara in December 1997, and her B.S. degrees in computer science Press, 1962. and applied mathematics from Florida State University in April 1996. She is[4] C.-C. Chiang, “Routing in Clustered Multihop, Mobile Wireless Networks with currently a Ph.D. student in the Electrical and Computer Engineering Fading Channel,” Proc. IEEE SICON ’97, Apr. 1997, pp. 197–211. Department at the University of California, Santa Barbara. At UCSB she is a[5] S. Murthy and J. J. Garcia-Luna-Aceves, “An Efficient Routing Protocol for member of the Computer Networks and Distributed Systems Laboratory. Wireless Networks,” ACM Mobile Networks and App. J., Special Issue on Her research interests include routing and multicast for ad hoc mobile net- Routing in Mobile Communication Networks, Oct. 1996, pp. 183–97. works. She is the recipient of a National Science Foundation Graduate Fel-[6] A. S. Tanenbaum, Computer Networks, 3rd ed., Ch. 5, Englewood Cliffs, lowship and a University of California Doctoral Scholar Fellowship. NJ: Prentice Hall, 1996, pp. 357–58.[7] C. E. Perkins and E. M. Royer, “Ad-hoc On-Demand Distance Vector Routing,” CHAI-KEONG TOH [M] (C-K.Toh@acm.org) received his Diploma in electronics Proc. 2nd IEEE Wksp. Mobile Comp. Sys. and Apps., Feb. 1999, pp. 90–100. and communication engineering with a Certificate of Merit award from the[8] D. B. Johnson and D. A. Maltz, “Dynamic Source Routing in Ad-Hoc Singapore Polytechnic in 1986, his B.Eng. degree in electronics engineering Wireless Networks,” Mobile Computing, T. Imielinski and H. Korth, Eds., with first class honors from the University of Manchester Institute of Science Kluwer, 1996, pp. 153–81. and Technology (UMIST) in 1991, and his Ph.D. degree in computer science[9] J. Broch, D. B. Johnson, and D. A. Maltz, “The Dynamic Source Routing from the Computer Laboratory, University of Cambridge, England in 1996. He Protocol for Mobile Ad Hoc Networks,” IETF Internet draft, draft-ietf- founded and chaired the Mobile Special Interest Group (Mobile SIG) from manet-dsr-01.txt, Dec. 1998 (work in progress). 1994 to 1996. Before joining Cambridge University, he was a network special-[10] V. D. Park and M. S. Corson, “A Highly Adaptive Distributed Routing ist, R&D engineer and technical staff member. At Cambridge, he was an Hon- Algorithm for Mobile Wireless Networks,” Proc. INFOCOM ’97, Apr. 1997. orary Cambridge Commonwealth Trust Scholar and a King’s College[11] M. S. Corson and A. Ephremides, “A Distributed Routing Algorithm for Cambridge Research Scholar. He authored the book Wireless ATM and Ad Hoc Mobile Wireless Networks,” ACM/Baltzer Wireless Networks J., vol. 1, Networks: Protocols and Architectures (Kluwer, 1996). He serves on the edito- no. 1, Feb. 1995, pp. 61–81. rial board for ACM Mobile Computing and Communications Review, IEEE Net-[12] C-K. Toh, “A Novel Distributed Routing Protocol To Support Ad-Hoc work, and Personal Technologies Journal (Springer Verlag). He is a member of Mobile Computing,” Proc. 1996 IEEE 15th Annual Int’l. Phoenix Conf. IEE, USENIX, ACM, Sigma Xi Honor Society, New York Academy of Science, Comp. and Commun., Mar. 1996, pp. 480–86. American Association for the Advancement of Science (AAAS), and is a Fellow[13] R. Dube et al., “Signal Stability based Adaptive Routing (SSA) for Ad- of the Cambridge Philosophical Society and Cambridge Commonwealth Soci- Hoc Mobile Networks,” IEEE Pers. Commun., Feb. 1997, pp. 36–45. ety. He is currently an assistant professor with the School of Electrical and[14] C-K. Toh, “Associativity-Based Routing for Ad-Hoc Mobile Networks,” Computer Engineering at the Georgia Institute of Technology, Atlanta, direct- Wireless Pers. Commun., vol. 4, no. 2, Mar. 1997, pp. 1–36. ing the Mobile Multimedia and High Speed Networking Laboratory.IEEE Personal Communications • April 1999 55