Robin Manet

Dept. of Electronics & Communication,Govt. Engg. College,Thrissur.

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ABSTRACT In the recent years communication technology and services have advanced. Mobility has become very important, as people want to communicate anytime from and to anywhere. In the areas where there is little or no infrastructure is available or the existing wireless infrastructure is expensive and inconvenient to use, Mobile Ad hoc NETworks, called MANETs, are becoming useful. They are going to become integral part of next generation mobile services. A MANET is a collection of wireless nodes that can dynamically form a network to exchange information without using any pre-existing fixed network infrastructure. The special features of MANET bring this technology great opportunities together with severe challenges. The military tactical and other security-sensitive operations are still the main applications of ad hoc networks, although there is a trend to adopt ad hoc networks for commercial uses due to their unique properties. However, they face a number of problems. In this paper, we describes the fundamental problems of ad hoc networking by giving its related research background including the concept, features, status, and applications of MANET. Some of the technical challenges MANET poses are also presented based on which the paper points out the related kernel barrier. Some of the key research issues for ad hoc networking technology are discussed in detail that are expected to promote the development and accelerate the commercial applications of the MANET technology.

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1. INTRODUCTION During the last decade, advances in both hardware and software techniques have resulted in mobile hosts and wireless networking common and miscellaneous. Generally there are two distinct approaches for enabling wireless mobile units to communicate with each other:

1) Infrastructured:Wireless mobile networks have traditionally been based on the cellular concept and relied on good infrastructure support, in which mobile devices communicate with access points like base stations connected to the fixed network infrastructure. Typical examples of this kind of wireless networks are GSM, UMTS, WLL, WLAN, etc.

2) Infrastructureless:As to infrastructureless approach, the mobile wireless network is commonly known as a mobile ad hoc network (MANET) [1, 2]. A MANET is a collection of wireless nodes that can dynamically form a network to exchange information without using any pre-existing fixed network infrastructure. It has many important applications, because in many contexts information exchange between mobile units cannot rely on any fixed network infrastructure, but on rapid configuration of a wireless connections on-the-fly. Wireless ad hoc networks themselves are an independent, wide area of research and applications, instead of being only just a complement of the cellular system. Seminar Report 2004 Network

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In this paper, we describes the fundamental problems of ad hoc networking by giving its related research background including the concept, features, status, and applications of MANET. Some of the technical challenges MANET poses are also presented based on which the paper points out the related kernel barrier. Some of the key research issues for adhoc networking technology are discussed in detail that are expected to promote the development and accelerate the commercial applications of the MANET technology.

The paper is structured as follows. In Section II, the background information related to ad hoc wireless networks is introduced; including the MANET concept, difference between wireless LAN and MANET, features, current research status, and some of its applications. The working and various important protocols related to MANET are presented in section III. The various challenges related to the implementation of MANET are given in section IV. Section V mainly discusses the key research issues of MANET with the emphasis on network layer routing strategies.

Finally, we summarize the paper by

conclusions in Section VI. Main references are included as section VII.

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2. RELATED BACKGROUND 2.1 MANET Concept A mobile ad hoc network is a collection of wireless nodes that can dynamically be set up anywhere and anytime without using any pre-existing network infrastructure. It is an autonomous system in which mobile hosts connected by wireless links are free to move randomly and often act as routers at the same time.

The topology of such networks is likely highly dynamic because each network node can freely move and no pre-installed base stations exist. Due to the limited wireless transmission range of each node, data packets then may be forwarded along multi-hops. Seminar Report 2004 Network

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2.2Difference between MANET and WLAN MANETs are dynamically created and maintained by the individual nodes comprising the network. They do not require a pre-existing architecture for communication purposes and do not rely on any type of wired infrastructure; in an ad hoc network all communication occurs through a wireless median. MANET comprises a special subset of wireless networks since they do not require the existence of a centralized message-passing device. Simple wireless networks require the existence of access points or static base stations (BS), which are responsible for routing messages to and from mobile nodes (MNs) within the specified transmission area. Ad hoc networks, on the other hand, do not require the existence of any device other than two or more MNs willing to cooperatively form a network. Instead of relying on a wired BS to coordinate the flow of messages to each MN, the individual MNs form their own network and forward packets to and from each other. This adaptive behavior allows a network to be quickly formed even under the most adverse conditions. Other characteristics of ad hoc networks include “team collaboration of a large number of MN units, limited bandwidth, the need for supporting multimedia real time traffic and low latency access to distributed resources (e.g. distributed database access for situation awareness in the battlefield)” (Hong et al., 1999). Two different architectures exist for an ad hoc network: flat and hierarchical (Haas, 1997). Flat networks are the simplest because all MNs are “equal”. Flat networks require each MN to participate in the forwarding and receiving of packets depending on the implemented routing scheme. Hierarchical networks use a tiered approach and consist of two or more tiers. The bottom layer Seminar Report 2004 Network

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consists of MNs grouped into smaller networks. A single member from each of these groups acts as a gateway to the next higher level. Together, the gateway MNs create the next higher tier. When an MN belonging to group A wants to

interact with another MN located in the same group,routing is the same as in a flat ad hoc network. However, if an MN in group A wants to communicate with another MN in group B, more advanced routing techniques incorporating the higher tiers must be implemented. For the purposes of this thesis, further reference to ad hoc networks assumes a flat architecture.

Figure shows the examples of both infrastructured and infrastructureless ad hoc wireless networks.

(a) Infrastructure-based wireless network

(b) Ad hoc wireless network

Infrastructured and infrastructureless wireless networks Seminar Report 2004 Network

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The traffic types in ad hoc networks are quite different from those in an infrastructured wireless network [3], including:

1) Peer-to-Peer:Communication between two nodes which are within one hop. Network traffic (Bps) is usually consistent.

2) Remote-to-Remote:Communication between two nodes beyond a single hop but which maintain a stable route between them. This may be the result of several nodes staying within communication range of each other in a single area or possibly moving as a group. The traffic is similar to standard network traffic.

3) Dynamic Traffic:This occurs when nodes are dynamic and moving around. Routes must be reconstructed. This results in a poor connectivity and network activity in short bursts.

2.3 MANET Features MANET has the following features:

1) Autonomous terminal:In MANET, each mobile terminal is an autonomous node, which may function as both a host and a router. In other words, besides the basic processing

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ability as a host, the mobile nodes can also perform switching functions as a router. So usually endpoints and switches are indistinguishable in MANET.

2) Distributed operation:Since there is no background network for the central control of the network operations, the control and management of the network is distributed among the terminals. The nodes involved in a MANET should collaborate amongst themselves and each node acts as a relay as needed, to implement functions e.g. security and routing.

3) Multihop routing:Basic types of ad hoc routing algorithms can be single-hop and multihop, based on different link layer attributes and routing protocols. Single-hop MANET is simpler than multihop in terms of structure and implementation, with the cost of lesser functionality and applicability. When delivering data packets from a source to its destination out of the direct wireless transmission range, the packets should be forwarded via one or more intermediate nodes.

4) Dynamic network topology:Since the nodes are mobile, the network topology may change rapidly and unpredictably and the connectivity among the terminals may vary with time. MANET should adapt to the traffic and propagation conditions as well as the mobility patterns of the mobile network nodes. The mobile nodes in the network dynamically establish routing among themselves as they move about, forming their own network on the fly. Moreover, a user in the MANET may not only operate within the ad hoc network, but may require access to a public fixed network (e.g. Internet). Seminar Report 2004 Network

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Topology change in ad hoc networks: nodes A, B, C, D, E, and F constitute an ad hoc network. The circle represents the radio range of node A. The network initially has the topology in (a). When node D moves out of the radio range of A, the network topology changes to the one in (b).

5) Fluctuating link capacity:The nature of high bit-error rates of wireless connection might be more profound in a MANET. One end-to-end path can be shared by several sessions. The channel over which the terminals communicate is subject to noise, fading, and interference, and has less bandwidth than a wired network. In some scenarios, the path between any pair of users can traverse multiple wireless links and the link themselves can be heterogeneous.

6) Light-weight terminals:In most cases, the MANET nodes are mobile devices with less CPU processing capability, small memory size, and low power storage. Such devices Seminar Report 2004 MANET:-The Art of Networking without a Network

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need optimized algorithms and mechanisms that implement the computing and communicating functions.

2.4 MANET Status Ad hoc networking is not a new concept. As a technology for dynamic wireless networks, it has been deployed in military since 1970s. Commercial interest in such networks has recently grown due to the advances in wireless communications. A new working group for MANET has been formed within the Internet Engineering Task Force (IETF) [2], aiming to investigate and develop candidate standard Internet routing support for mobile, wireless IP autonomous segments and develop a framework for running IP based protocols in ad hoc networks. The recent IEEE standard 802.11 [4] has increased the research interest in the field. Many international conferences and workshops have been held by e.g. IEEE and ACM. For instance, MobiHoc (The ACM Symposium on Mobile Ad Hoc Networking & Computing) has been one of the most important conferences of ACM SIGMOBILE (Special Interest Group on Mobility of Systems, Users, Data and Computing). Research in the area of ad hoc networking is receiving more attention from academia, industry, and government. Since these networks pose many complex issues, there are many open problems for research and significant contributions.

2.5 MANET Applications With the increase of portable devices as well as progress in wireless communication, ad hoc networking is gaining importance with the increasing number of widespread applications. Ad hoc networking can be applied anywhere where there is little or no communication infrastructure or the existing Seminar Report 2004 Network

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infrastructure is expensive or inconvenient to use. Ad hoc networking allows the devices to maintain connections to the network as well as easily adding and removing devices to and from the network. The set of applications for MANETs is diverse, ranging from large-scale, mobile, highly dynamic networks, to small, static networks that are constrained by power sources. Besides the legacy applications that move from traditional infrastructured environment into the ad hoc context, a great deal of new services can and will be generated for the new environment. Typical applications include:

1) Military battlefield:Military equipment now routinely contains some sort of computer equipment. Ad hoc networking would allow the military to take advantage of commonplace network technology to maintain an information network between the soldiers, vehicles, and military information head quarters. The basic techniques of ad hoc network came from this field.

2) Commercial sector:Ad hoc can be used in emergency/rescue operations for disaster relief efforts, e.g. in fire, flood, or earthquake. Emergency rescue operations must take place where non-existing or damaged communications infrastructure and rapid deployment of a communication network is needed. Information is relayed from one rescue team member to another over a small handheld. Other commercial scenarios include e.g. ship-to-ship ad hoc mobile communication, law enforcement, etc.

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3) Local level:Ad hoc networks can autonomously link an instant and temporary multimedia network using notebook computers or palmtop computers to spread and share information among participants at a e.g. conference or classroom. Another appropriate local level application might be in home networks where devices can communicate directly to exchange information. Similarly in other civilian environments like taxicab, sports stadium, boat and small aircraft, mobile ad hoc communications will have many applications.

4) Personal Area Network (PAN):Short-range MANET can simplify the intercommunication between various mobile devices (such as a PDA, a laptop, and a cellular phone). Tedious wired cables are replaced with wireless connections. Such an ad hoc network can also extend the access to the Internet or other networks by mechanisms e.g. Wireless LAN (WLAN), GPRS, and UMTS. The PAN is potentially a promising application field of MANET in the future pervasive computing context.

3. How MANETs Work A Mobile Ad hoc NETwork (MANET) [1], [2] is a network architecture that can be rapidly deployed without relying on existing fixed wireless network infrastructure. This means that the network nodes should be able to communicate to each other even if no static infrastructure, such as backbone Seminar Report 2004 Network

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network, base stations, and centralized network management function are available. Under these situations a node itself provides these functions. The transmission range of a node is limited to a circular region around the node, whose radius depends on the transmitted power, receiver sensitivity and propagation loss model. If the destination node is not in the transmission range of the source node, then the mobile ad hoc network works like a multi hop network with one or more node acting as routing node. All the active nodes of the MANET need to transmit a hello message at regular intervals, to indicate their presence. Other nodes, in the transmission range of a node, can use that node as next hop to forward their packets toward the destination. The selection of hello interval is an important parameter for mobile ad hoc network. For on-demand routing protocols it is taken as 1second and it varies from 1sec to 5sec for table driven-routing protocols [3]. In the Mobile Ad hoc Network, nodes are communicating on ad hoc basis. Till now it has not been standardized.

Figure shows a MANET connected to the Internet. A wired/wireless gateway provides the Internet connection to a MANET. Since the gateway is fixed, the mobility of the network is restricted around the gateway. A series of gateways may provide a free movement to the MANETs.

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MANET has its own limitations in terms of coverage area, bandwidth, scarce battery power, scalability and security. The major problem a MANET faces is its requirement of large number of nodes in a given area for a scalable network. It requires 50 nodes in 1000m*1000m terrain and 100 nodes in 1500m*1500m area for the proper operation of the network. Since the MANET is highly dynamic and there is no centralized control, the existing shortest path routing algorithms [4], and adaptive shortest path algorithms [5] are not suitable for it. Some routing algorithms [6], which have been developed for such environment are Destination-Sequence Distance Vector (DSDV) routing protocol [7], Wireless Routing Protocol (WRP) [8], Dynamic Source Routing (DSR) [9], Associativity based routing protocol, Clustered based routing protocol, Signal Stability Routing and Ad hoc On-demand Distance Vector (AODV)[10] Routing protocol. The applicable MAC protocols are Multiple Access Collision Avoidance (MACA) [11]. Carrier Sense Multiple Access-

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Collision Avoidance (CSMA/CA)[1] and IEEE 802.11 Distributed Coordination Function (DCF)[12].

3.1 ROLE OF MAC AND ROUTING PROTOCOLS IN MANET:In the MANETs, since the medium is common, simultaneous communication will collide. A suitable MAC layer protocol avoids the collision. The transmission of unicast packet is preceded by a Request-to-Send/Clear-toSend (RTS/CTS) exchange that reserves the channel for transmission of the data packets. Routing protocols are used to set up and maintain the route between the source and destination by means of Route-Request/Route-reply (RREQ/RREP) packet exchange. Route-Error (RERR) packet is used to detect link/route failure.

3.1.1 MAC Protocols The traditional carriers sense multiple Accesses/Collision detection (CSMA/CD) has the probability of collision during vulnerable period, which is equal to the one-way propagation delay. The CSMA/CA is designed to avoid collision of data packets. It uses small control packets before data packets to avoid collision. In CSMA/CA when a station wants to send data to another node, it first sends a short Request To Send (RTS) packet to the destination. The receiver responds with a Clear To Send (CTS) packet. After receiving the CTS packet the source can send its queued data packet. In the paper we have considered following two MAC protocols.

a) Multi Access Collision Avoidance (MACA):MACA is well suited for MANET. MACA uses packet sensing multiple access (PSMA), along with RTS and CTS. In MACA carrier sensing is provided only for data packet transmission, not for control packet. So RTS packet Seminar Report 2004 Network

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duration is the vulnerable period for MACA protocol and collision can occurs in this period. These RTS packets are small in compare to data packet so loss is also small, and collision can further reduced by introducing randomized exponential back off.

Back Off Interval:When a node wishes to transmit a packet, it first waits until the channel is idle. Once channel becomes idle, the node waits for a randomly chosen duration before attempting to transmit. When transmitting a packet, nodes choose a backoff interval in the range [0,cw] where cw is contention window. Nodes which wants to send a data starts count down the backoff interval when medium is idle and the count-down is suspended if medium becomes busy. When backoff interval reaches 0, transmit RTS. The time spent counting down backoff intervals is a part of MAC overhead. Choosing a large cw leads to large backoff intervals and can result in larger overhead. Choosing a small cw leads to a larger number of collisions (when two nodes count down to 0 simultaneously). Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage contention is needed.

b) IEEE 802.11 DCF:The IEEE 802.11 DCF uses the collision avoidance before RTS transmission and ACK transmitted by the receiver ensures successful reception of the data packet. The addition of collision avoidance, to control packet exchange, aids in the prevention of control packet collision. The protocol is suited for highly loaded network. IEEE 802.11 DCF also provides feedback when next hope is Seminar Report 2004 Network

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unreachable by means of negative ACK, thus reduces MAC layer and routing loads considerably.

Binary Exponential Backoff in IEEE 802.11 DCF Here the contention window cw is chosen dynamically depending on collision occurrence. When a node fails to receive CTS in response to its RTS, it increases the contention window. For example it is doubled (up to an upper bound).When a node successfully completes a data transfer, it restores cw to Cwmin. Or in other words cw follows a sawtooth curve

3.1.2 Routing Protocols One of the most important aspects of classifying ad hoc routing protocols is whether or not the nodes of the network should keep track of route to all possible destinations. Protocols, that keep track of routes for all destinations before the communication to start, are called proactive or table- driven routing protocols. On the other hand, the protocols that acquire routing information only when it is actually needed is called reactive or on-demand routing protocols. The various proactive routing protocols are Dynamic DestinationSequenced Distance Vector (DSDV) routing, Wireless Routing Protocols (WRP), Global State Routing (GSR), Fisheye State Routing (FSR), and Hierarchical State Routing (HSR) etc. The various on-demand routing protocols are Cluster-Based Routing (CBR), Ad hoc On-demand Distance Vector (AODV) routing, Dynamic Source Routing (DSR), Temporally Ordered Routing Algorithm (TORA), Associativity-Based Routing (ABR), and Signal Stability Routing (SSR) protocol etc. In this paper following two on-demand and two table-driven protocols have been studied.

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1) Table-driven Routing protocols In table-driven routing protocols all possible routes for each destinations are stored and maintained at every node, even before communication to start. The DSDV and the WRP are two table-driven routing protocols considered in this paper.

a) Destination-Sequence Distance Vector (DSDV) Protocol :In DSDV all possible routes for each destination are stored in routing table. The routing table is transmitted periodically and also if significant change has occurred in routing information. A sequence number is used to distinguish stale route from new one. The route with higher sequence number is newer. Destination-Sequence Distance Vector protocol works as follows i)Each node maintains a routing table which stores -next hop towards each destination -a cost metric for the path to each destination -a destination sequence number that is created by the destination itself -Sequence numbers used to avoid formation of loops.

ii)Each node periodically forwards the routing table to its neighbors -Each node increments and appends its sequence number when sending its local routing table. -This sequence number will be attached to route entries created for this node. Assume that node X receives routing information from Y about a route to node Z.

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Let S(X) and S(Y) denote the destination sequence number for node Z as stored at node X, and as sent by node Y with its routing table to node X, respectively. Node X takes the following steps: -If S(X) > S(Y), then X ignores the routing information received from Y

-If S(X) = S(Y), and cost of going through Y is smaller than the route known to X, then X sets Y as the next hop to Z

-If S(X) < S(Y), then X sets Y as the next hop to Z, and S(X) is updated to equal S(Y)

b) Wireless Routing Protocol (WRP) :In WRP, distance of each destination, possible routes and next hops are stored in the routing table at each node. The routing information is broadcast only if there is a significant change in the routing information. If there is no change in routing table, an idle Hello message is used to ensure connectivity. This reduces the routing load of the network.

2) On demand routing protocols In the on-demand routing protocols, a route is established only if it is required. The DSR and the AODV are two on-demand routing protocols considered in this paper.

a) Dynamic Source Routing (DSR) protocol:DSR is an improved version of DSDV. It minimizes the number of broadcasts by creating route on demand. To find out a route, source;S Seminar Report 2004 Network

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broadcasts RREQ(Route REQuest) to all of its neighbours, which then rebroadcast it to their neighbours. Before rebroadcasting the packet each node will append its own identifier to the packet header. Packet numbers are used such that a node will retransmit the packet only once. The process continues till the destination;D is reached. The reverse path is used by the destination to send RREP(Route REPly) to the source. RREP includes the route from source to destination through which RREQ was received by destination node. Node S on receiving RREP, caches the route included in the RREP. When node S sends a data packet to D, the entire route is included in the packet header so this routing is called source routing. Intermediate nodes use the source route included in a packet to determine to whom the packet should be forwarded. RERR(Route ERRor )is used to indicate any link failure during data packet transfer. On receiving the RERR the source starts another route request by transmitting RREQ.

Dynamic Source Routing can be optimized by using route caches. The route caches work as given below. Assume that node S wants to communicate with node D and A,B,C,E,F,G,H,I,J,K,L,M, etc are the other nodes present in the network.

i) Each node caches a new route it learns by any means ii) When node S finds route [S,E,F,J,D] to node D, node S also learns route [S,E,F] to node F iii) When node K receives Route Request [S,C,G] destined for node, node K learns route [K,G,C,S] to node S iv) When node F forwards Route Reply RREP [S,E,F,J,D], node F learns route [F,J,D] to node D v) When node E forwards Data [S,E,F,J,D] it learns route [E,F,J,D] to node D Seminar Report 2004 Network

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vi) A node may also learn a route when it overhears Data packets The main uses of route caching are, i) When node S learns that a route to node D is broken, it uses another route from its local cache, if such a route to D exists in its cache. Otherwise, node S initiates route discovery by sending a route request

ii)Node X on receiving a Route Request for some node D can send a Route Reply if node X knows a route to node D

iii)Thus use of route cache -can speed up route discovery -can reduce propagation of route requests While using route caches we should be careful because i) Stale caches can adversely affect performance ii) With passage of time and host mobility, cached routes may become invalid iii) A sender host may try several stale routes (obtained from local cache, or replied from cache by other nodes), before finding a good route The main advantage of dynamic source routing is its simplicity. Routes are maintained only between nodes who need to communicate, thus it reduces the overhead of route maintenance. The disadvantages of dynamic source routing are 1) Packet header size grows with route length due to source routing 2) Flood of route requests may potentially reach all nodes in the network

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3) Care must be taken to avoid collisions between route requests propagated by neighboring nodes -insertion of random delays before forwarding RREQ 4) Increased contention if too many route replies come back due to nodes replying using their local cache -Route Reply Storm problem -Reply storm may be eased by preventing a node from sending RREP if it hears another RREP with a shorter route 5) An intermediate node may send Route Reply using a stale cached route, thus polluting other caches

6) This problem can be eased if some mechanism to purge (potentially) invalid cached routes is incorporated.

7) For some proposals for cache invalidation, -Static timeouts -Adaptive timeouts based on link stability

b) Ad Hoc On Demand Distance Vector (AODV) routing protocol:AODV routing protocol is designed specially for Ad-Hoc network and provides quick and efficient route establishment, with minimal control overhead and minimal route acquisition latency. The route recovery of AODV is similar to that of DSR. In addition to that it stores the previous routing information

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for time out duration. If stored route is not used within the time-out period, it is eliminated from the table. The protocol is suitable for a highly mobile network.

Comparison of AODV and DSR DSR includes source routes in packet headers. It results in large packet headers when the route is very long. Resulting large headers can sometimes degrade performance particularly when the data contents of a packet are small. AODV attempts to improve on DSR by maintaining routing tables at the nodes, so that data packets do not have to contain routes. AODV retains the desirable feature of DSR that routes are maintained only between nodes which need to communicate.

The various steps in AODV are

i) Route Requests (RREQ) are forwarded in a manner similar to DSR.

ii)When a node re-broadcasts a Route Request, it sets up a reverse path pointing towards the source that is AODV assumes symmetric (bi-directional) links.

iii)When the intended destination receives a Route Request, it replies by sending a Route Reply.

iv)Route Reply travels along the reverse path set-up when Route Request is forwarded.

v)On receiving the RREP the sourge will send the data packet to the destination similar to that in DSR.

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vi) An intermediate node (not the destination) may also send a Route Reply (RREP) provided that it knows a more recent path than the one previously known to sender S.

vii)To determine whether the path known to an intermediate node is more recent, destination sequence numbers are used.

viii)The likelihood that an intermediate node will send a Route Reply when using AODV not as high as DSR.A new Route Request by node S for a destination is assigned a higher destination sequence number. An intermediate node which knows a route, but with a smaller sequence number, cannot send Route Reply.

Timeouts in AODV A routing table entry maintaining a reverse path is purged after a timeout interval and the timeout should be long enough to allow RREP to come back A routing table entry maintaining a forward path is purged if not used for a active_route_timeout interval so that if no is data being sent using a particular routing table entry, that entry will be deleted from the routing table (even if the route may actually still be valid)

Link Failure Reporting in AODV The various steps in reporting link failure in AODV are,

i) A neighbour of node X is considered active for a routing table entry if the neighbour sent a packet within active_route_timeout interval which was forwarded using that entry. Seminar Report 2004 Network

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ii)When the next hop link in a routing table entry breaks, all active neighbors are informed.

iii)Link failures are propagated by means of Route Error messages, which also update destination sequence numbers.

iv)When node X is unable to forward packet P (from node S to node D) on link (X,Y), it generates a RERR message.

v)Node X increments the destination sequence number for D cached at node X.

vi)The incremented sequence number N is included in the RERR.

vii)When node S receives the RERR, it initiates a new route discovery for D using destination sequence number at least as large as N.

vii)When node D receives the route request with destination sequence number N, node D will set its sequence number to N, unless it is already larger than N.

Link Failure detection in AODV Neighboring nodes periodically exchange hello message. Absence of hello message is used as an indication of link failure. Alternatively, failure to receive several MAC-level acknowledgement may be used as an indication of link failure.

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Why Sequence Numbers in AODV i)To avoid using old/broken routes and to determine which route is newer.

ii)To prevent formation of loops as explained below.

-Assume that A does not know about failure of link C-D because RERR sent by C is lost -Now C performs a route discovery for D. Node A receives the RREQ (say, via -path C-E-A) -Node A will reply since A knows a route to D via node B -Results in a loop (for instance, C-E-A-B-C )

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Summary: AODV i)Routes need not be included in packet headers

ii)Nodes maintain routing tables containing entries only for routes that are in active use

iii)At most one next-hop per destination maintained at each node in AODV where as in DSR may maintain several routes for a single destination

iv)Unused routes expire even if topology does not change

4. CHALLENGES AND KERNEL BARRIER 4.1 MANET Challenges:Regardless of the attractive applications, the features of MANET introduce several challenges that must be studied carefully before a wide commercial deployment can be expected. These include:

1) Routing:Since the topology of the network is constantly changing, the issue of routing packets between any pair of nodes becomes a challenging task. Most protocols should be based on reactive routing instead of proactive. Multicast routing is another challenge because the multicast tree is no longer static due to the random movement of nodes within the network. Routes between nodes may

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potentially contain multiple hops, which is more complex than the single hop communication.

2) Security and Reliability:In addition to the common vulnerabilities of wireless connection, an ad hoc network has its particular security problems due to e.g. nasty neighbour relaying packets. The feature of distributed operation requires different schemes of authentication and key management. Further, wireless link characteristics introduce also reliability problems, because of the limited wireless transmission range, the broadcast nature of the wireless medium (e.g. hidden terminal problem), mobility-induced packet losses, and data transmission errors.

3) Quality of Service (QoS):Providing different quality of service levels in a constantly changing environment will be a challenge. The inherent stochastic feature of communications quality in a MANET makes it difficult to offer fixed guarantees on the services offered to a device. An adaptive QoS must be implemented over the traditional resource reservation to support the multimedia services.

4) Internetworking In addition to the communication within an ad hoc network, internetworking between MANET and fixed networks (mainly IP based) is often expected in many cases. The coexistence of routing protocols in such a mobile device is a challenge for the harmonious mobility management.

5) Power Consumption For

most

of

the

light-weight

mobile

terminals,

the

communication-related functions should be optimized for lean power consumption. Conservation of power and power-aware routing must be taken into consideration.

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4.2 Kernel Barrier It has been widely recognized that routing strategy is the most important research problem among others. To determine viable routing paths and deliver messages in a decentralized environment where network topology fluctuates is far less than a well-defined problem. New models are needed to describe the mobile ad hoc feature of the target wireless networks, while new algorithms are required to safely and efficiently route information to mobile destination in order to support different types of multimedia applications. Factors such as variable wireless link quality, propagation path loss, fading, multi-user interference, power expended, and topological changes become relevant issues that add more difficulties and complexities to the routing protocol design. Many routing protocols have been proposed with the form of IETF working documents of both Internet Drafts and Request For Comments (RFC) [2]. Numerous projects related to different aspects of MANET are employed by academics and institutes all over the world, with individual standards being presented occasionally in literatures [5-9]. They serve the purpose of demonstrating the functionality and performance of ad hoc routing with comparatively simple protocols, whereas very few of them can be regarded to really fulfill the requirements of a real application scenario. There are still many relative aspects to be deeply researched before the wide deployment of the commercial ad hoc systems.

5. KEY RESEARCH ISSUES This section analyses key Research issues concerning MANET network layer routing strategies, including four selected key problems in MANET: X-cast routing, security & reliability, QoS, and interworking with outside IP Seminar Report 2004 Network

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networks. These issues are currently main challenges of ad hoc wireless networks. The lack of robust solutions to these problems prevents MANET from wide commercial deployment.

5.1 X-cast Routing Algorithms:As in the infrastructured wireless networks, all kinds of X-cast communication schemes should be supported in an ad hoc mobile environment. These include unicast, anycast, multicast, and broadcast. MANET also brings new X-cast modes into communications, e.g. geocast [10] and content-based. In particular, multicast is desirable to support multiparty wireless communications [11]. Since the multicast tree is no longer static (i.e. its topology is subject to change over time), the multicast routing protocol must be able to cope with mobility, including multicast membership dynamics (e.g., leave and join). In a multihop ad hoc context, the routing problem becomes more complex because of the mobility of both hosts and routers. The random movement of the nodes and the uncertainty of path quality render the traditional routing protocols impractical. Trade-off between reactive and proactive schemes in terms of latency and overhead of route discovery and maintenance are to be considered depending on different traffic and mobility patterns. Issues to be taken into account include routing discovery and flooding, caching, data delivery, locationaided and power-aware, broadcast storm issue, route request and reverse path.

5.2 QoS Supporting Model:Just like in wired networks, QoS protocols can be used to prioritize data within ad hoc networks in order to reserve better connections for high data rate applications while still maintaining enough bandwidth for lower bit rate communication. The support of multimedia services will most likely be required Seminar Report 2004 Network

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within and throughout the MANET, for which different QoS classes (e.g. voice, video, audio, web, and data stream) are needed to facilitate the use of multimedia applications. In such a stochastic changing environment involving dynamic nodes, hidden terminals, and fluctuating link characteristics, supporting end-to-end QoS at different levels will be a great challenge that requires in-depth investigation [12]. An adaptive QoS must be implemented over the traditional plain resource reservation to support the multimedia services. Special emphasis should be put on achieving a new QoS model for MANETs by taking into account the ad hoc features of the target networks: dynamic node roles, data flow granularity, traffic profile, etc.

5.3 Security, Reliability, and Availability Schemes:Security, reliability, and availability are three crucial aspect of MANET, especially in security-sensitive applications. Since ad hoc relies on wireless communication medium, it is important to deploy a security protocol to protect the privacy of transmissions. The requirements regarding confidentiality, integrity, and availability are the same as for any other public communication networks. However, the implementation schemes of key management, authentication, and authorization are quite different because there is no aid of a trusted third-party certification authority to create trusted relationships by exchanging private/public keys [13]. Different types of threats and attacks against routing in MANET should be analyzed leading to the requirement of ad hoc routing security, and advanced solutions are needed for the secure routing of MANET. Wireless communication is subject to many types of problems due to interference and poor signals. As for reliability and availability issues, besides low level error masking and recovery mechanisms (i.e. link layer error detection and correction coding), special attention should be paid to studying fault-tolerant Seminar Report 2004 Network

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routing algorithm. In multihop ad hoc wireless networks, there exists an inherent attribute of redundant routing paths between nodes. Exploiting this property, it’s possible to provide a fault-tolerant routing scheme [14], for increasing the reliability and security of the target routing algorithm. Since overhead occurs in this reliable-increasing algorithm, research should also study the tradeoff between performance and reliability in order to calculate the most efficient solution.

5.4 Internetworking Mechanisms:To integrate the two mobility management schemes in the domains of both traditional infrastructured wireless networks and the new mobile ad hoc networks is an important issue. The mobility mode of an ad hoc network is quite different from that of infrastructured networks. In infrastructured networks only the nodes (terminals) at the very edges (the last hop) of fixed networks are moving, whereas an ad hoc network can be completely mobile, since a device can serve both as router and host at the same time. Consequently, in an ad hoc network mobility is handled directly by the routing algorithm. In many cases, device accesses both within the ad hoc network and to public networks (e.g. the Internet) can be expected to form a universal communication scenario. In other words, a terminal in an ad hoc wireless network is able to connect to nodes outside the MANET while being itself also accessible by external nodes. The interworking between ad hoc and fixed networks is necessary. In particular, the coexistence and cooperation with the public IP based wireless networks is necessary to many contexts. The Mobile IP protocol for MANET should be deeply studied in order to give nodes in ad hoc networks the ability of accessing the Internet and other IP based networks to take advantage of the services of Mobile IP.

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6. CONCLUSIONS This seminar describes the fundamental issues and analyses key research problems of MANET. Firstly, the background information of MANET are introduced, including the MANET concept, features, current status, and application areas. Important MAC and routing protocols are also described. Then the main challenges of MANET are discussed that lead to the analysis of relevant kernel barrier. Finally, four key network layer research issues of MANET routing strategies are described in detail. The novel and advanced solutions to these issues are necessary to fulfil the requirements of wide commercial deployment of MANET. Mobile ad hoc networking is one of the most important and essential technologies that support future pervasive computing scenario. The special characters of MANET bring this technology great opportunities together with severe challenges. Currently MANET is becoming more and more interesting research topic and there are many research projects employed by academic and companies all over the world. Various interesting issues are investigated that cover all aspects of ad hoc wireless networks. Meanwhile, many routing protocols designed for ad hoc networks have been proposed as Internet Draft and RFC of IETF. MANETs can be exploited in a wide area of applications, from military, emergency rescue, law enforcement, commercial, to local and personal contexts.

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7. REFERENCES [1] M. Frodigh, P. Johansson, and P. Larsson. “Wireless ad hoc networking: the art of networking without a network,” Ericsson Review, No.4, 2000, pp. 248-263. [2] IETF Working Group: Mobile Adhoc Networks (manet). http://www.ietf.org/html.charters/manet-charter.html. [3] Ad Hoc Networking Extended Research Project. Online Project. http://triton.cc.gatech.edu/ubicomp/505. [4] IEEE 802.11 Working Group. http://www.manta.ieee.org/groups/802/11/. [5] E.M. Royer and C.K. Toh, “A review of current routing protocols for ad hoc mobile wireless networks,” IEEE Personal Communications, 1999, 6(2), pp. 46-55. [6] S.R. Das, R. Castaneda, and J. Yan, “Simulation-based performance evaluation of routing protocols for mobile ad hoc networks,” Mobile Networks and Applications, 2000, 5, pp. 179-189. [7] S.-J. Lee, M. Gerla, and C.-K. Toh, “A simulation study of table-driven and on-demand routing protocols for mobile ad-hoc networks,” IEEE Network, 1999, 13(4), pp. 48-54. [8] M. Joa-Ng and I.-T. Lu, “A peer-to-peer zone-based two-level link state routing for mobile ad hoc networks,”. IEEE Journal on Selected Areas in Communications, 1999, 17(8), pp. 1415-1425. [9] L. Ji, M. Ishibashi, and M.S. Corson, “An approach to mobile ad hoc network protocol kernel design,” In Proceedings of IEEE WCNC’99, New Orleans, LA, Sep. 1999, pp. 1303-1307.

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[10] Y.-B. Ko and N. H. Vaidya, “Geocasting in mobile ad hoc networks: Location-based multicast algorithms,”. Technical Report TR-98-018, Texas A&M University, Sep. 1998.

[11] M. Gerla, C.-C. Chiang, and L. Zhang, “Tree multicast strategies in mobile, multihop wireless networks,” ACM/Baltzer Mobile Networks and Applications, speical issue on Mobile Ad Hoc Networking, 1999, 4(3), pp. 193-207. [12] S. Chakrabarti and A. Mishra, “QoS issues in ad hoc wireless networks,” IEEE Communications Magazine, 2001, 39(2), pp. 142–148. [13] L. Zhou and Z. J. Haas, “Securing ad hoc networks,” IEEE Network Journal, 1999, 13(6), pp. 24-30. [14] E. Pagnani and G. P. Rossi, “Providing reliable and fault tolerant broadcast delivery in mobile ad-hoc networks,” Mobile Networks and Applications, 1999, 5(4), pp. 175-192.

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