Comparative Study of AODV Route Repair Mechanism with Impact on Node Mobility and

Traffic Load in Wireless Mesh Networks

1,2Suhazlan Suhaimi, 2Kamaruddin Mamat, 3S.R Azzuhri

1Sultan Idris Educational University, 2MARA University of Technology, 3University of Malaya,

Abstract—Wireless Mesh Networks (WMNs) have become one of the popular key technology and solution for the new wireless implementation especially for wireless end user. One of the widely used routing protocols in WMNs is Ad-Hoc On-Demand Distance Vector (AODV) protocols. In this paper, we investigate the performance of AODV in situations of link failures due to mobility of nodes and consequent route recovery mechanism using ns2 simulators. We defined a different set of threshold for routing protocols to do local repair based on the link breaks location with respect to the total number of hops from source to destination. Packet Delivery Ratio (PDR) and Normalized Routing Load (NRL) or overhead were used as performance metrics in our simulations. For link-failure detection, HELLO message and Link Layer Feedback (LLF) were used separately in the simulations. Simulations were done by varying the number of source-destination pair and speed of the mobile node.

Keywords: Mesh , Wireless, network, mobile

I. INTRODUCTION

Wireless technology has become one of the important aspects of networking technology nowadays. There was a significance advanced in the whole wireless networking technology, resulting in several different kind of wireless networking ‘segments’. One of the most important technologies is Wireless Mesh Network. A node in WMNs can be simple receive send node, and can also function as router and can relay message to its neighbour. With its advantageous features of low cost, high scalability, easy maintenance and reliable services, WMNs are a natural solution to the broadband communication demands in rural communities where wired-backbone infrastructure is virtually non-existent. WMNs typically consist of access points (AP), wireless mesh routers and clients [23].

In recent years, several wireless routing protocols have been designed to provide communication in a wireless ad-hoc environment. These routing protocols have to adapt quickly to the frequent change in the ad-hoc network topology. Routing protocols for wireless mesh networks can be classified into three categories. The first category is table driven (proactive) routing protocols such as DSDV [1] and OLSR [2]. The second category is on demand (Reactive) routing protocols such as AODV [3], DSR [4], DYMO [5] and TORA [6]. The third category is hybrid (Reactive and Proactive) routing protocol such as SHARP [7],

CHAMELON [8] and WRP [9]. There have also been works [10, 11, 12] that compared the performances of these routing protocols, and their results show that while some protocols may work well in one network scenario, other protocols may perform better in another network scenario.

In proactive routing protocols, route is maintained actively in neighbour nodes and to all possible destinations. Changes in any part of the network will floods the update messages throughout the whole network in order to maintain the consistent network vies which suffer the heavy bandwidth utilization. Reactive routing protocols such as on the other hand creating routes when only it’s needed by the source node (minimize bandwidth resources) and it’s maintained as long as it’s ‘desired’ by the source node. When a link failure occurs in an active route, reactive routing protocols require an error notification to be sent to nodes that were using the broken link. Network with limited diameters, these error messages can be propagated back to the source node relatively quickly and repairing action can be performed faster. However, network with larger diameter and longer path lengths, error messages may have to propagate for more hops to reach the source which may take the repairing action longer and be delayed. This will lead to an ineffective performance in more stressful scenarios. This is where the local repair of route in reactive protocols might come in handy to address this problem. In this paper, we choose to evaluate different route repair mechanism in AODV protocols since it’s a well known reactive routing protocol.

The rest of this paper is organized as follows; section II presents a motivation of our work and section III describes an overview of standard AODV operation and route maintenance. Section IV discussed the related previous work on local repair approach in wireless mesh network. Proposed modification on AODV protocol is briefly explained in section V together with the discussions on the simulations results. The last and final section will brings out the conclusion and the future work.

II. MOTIVATION

One of the important aspects in wireless network research is to investigate the link breakage resulting from different sources such as node mobility, signal interference, congestion, fading environment and packets collision.

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Mobility produces and actual route breaks while other sources produce ‘temporary’ or partial route breaks [13]. Routing protocols such as AODV depend upon feedback from various mechanisms to detect a failed link. Most commonly, routing protocols use failure notification from the MAC layer called Link layer Feedback (LLF) but they may also use HELLO message or passive ACK. Either way, if link break is detected, routing protocols have one of the following options to deal with such link error:

- Nothing will be done and wait for the timeout.

After the timeout (no positive acknowledgement from the destination), source will start a new route discovery cycle.

- Propagate RERR message to the source node and re-initiate route discovery for that destination if a route is still desired.

- Perform a local recovery scheme or try an alternate route from an existing route cache.

Several studies on locally recovered route have already

being proposed to address the efficiency of source repairing in reactive routing protocols such as in [14], [15], [16], [17], [18]. This paper evaluates and compares the AODV’s behaviour when several different route recovery mechanisms are applied in AODV to see the impact when a link breaks occurs in an active route. For the comparative study described here, the modules of AODV recovery routes were separated and evaluate apart. Performance evaluations are based on different mobility, traffic load level and different number of source-destination pairs.

The main contribution of this paper is to provide a comparative analysis using a different route repair mechanism in AODV that later can be used to make it more adaptive to different kind of scenario. Instead of using the standard AODV for route repairing, we can see how some of the route repair mechanism has better performance than others when it comes to different degree of mobility, traffic load and source-destination pair.

III. AD-HOC ON DEMAND DISTANCE VECTOR (AODV) PROTOCOL

A. Overview

AODV [3] is a classic distance vector routing algorithm, and a successor DSDV protocols. AODV only has one entry per destination and provides loop free routes in case of link breakage. AODV uses a broadcast Route Request (RREQ) discovery algorithm and a unicast Route Reply (RREP) message. When a source node desires to send a message to some destination node and does not already have a valid route to the destination, it initiates a path discovery process to locate the other node. It will broadcast the RREQ to its neighbour and its neighbour will re-broadcast the RREQ message so-on until it finds the destination or node that have a valid route to destination. AODV protocol utilizes a sequence numbers to ensure that all routes contain the most recent route information (fresh enough route). Each node maintains its own sequence number. Intermediate nodes that

received a RREQ message will update their routing table and record the address of which it first receive the RREQ in order to establish the reverse route entry. Once the destination node or intermediate node that have fresh enough route to destination, the node will reply with RREP back to the neighbours that send RREQ message before. After the source node receives the RREP message and the route is verified as valid route, the data packet transmission will begin.

B. AODV Local Repair

Whenever the route breaks or expired, effected node will send a Route Error Message (RERR) to the affected upstream neighbours, until up to the source node. The process of route request will be repeated until the new route has been found. Alternatively, AODV protocol can initiate local repair mechanism when a link failure happens on an active route [22]. To repair the link failure, the upstream node broadcasts RREQ packet after increasing the destination sequence number. The TTL value used in RREQ packet is set to the following value:

TTL = Max (MIN_REPAIR_TTL, 0.5 * #hops) +

LOCAL_ADD_TTL (1)

Where:

MIN_REPAIR_TTL: the last known hop count from the upstream node of the failure to the destination.

#hops: the number of hops from the upstream node of the failure to the source of the currently undeliverable packet.

LOCAL_ADD_TTL: constant value

After the upstream node broadcasts the RREQ packet, it waits the discovery period to receive RREP packets in response to the RREQ packet. When the destination or an intermediate node that has a fresh route to the destination receives the RREQ packet, a RREP packet will be forwarded towards the upstream node. If discovery period finished and the upstream node didn't receive a RREP for that destination, it transmits a RERR message for that destination to the source. On the other hand, if the upstream node receives one or more RREP packets during the discovery period, it first compares the hop count of the new route with the value in the hop count field of the invalid route table entry for that destination. In the case of the hop count of the newly determined route to the destination is greater than the hop count of the previously known route, the upstream node transmits a RERR message for that destination towards the source, with 'N' bit set. Finally, the upstream node updates its route table entry for that destination.

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IV. RELATED WORK

In wireless mesh network, link can easily be

broken due to the dynamic and mobile nature of wireless network. It will prone to more link failures when the degree of mobility increases, especially in reactive routing protocols that use broadcast message to discover routes to the destination. In some cases, when link failure happens at an intermediate link, it’s more beneficial to discover new route locally without resorting to end-to-end route discovery. Some of the approaches of the local repair mechanism have been proposed in [14], [15], [16], [17] and [18].

Youn et al [14] propose a new local repair scheme using promiscuous mode, which can be categorized into two parts, adaptive promiscuous mode and quick local repair scheme. The adaptive promiscuous mode repeats the switching processes between promiscuous mode and non-promiscuous mode. The proposed scheme adopts promiscuous mode such that each node keep monitoring the overheard packets from which the routing information about the route path in adjacent nodes can be obtained. However, it suffers from excessive energy used and reduced network efficiency.

AODV with Local Repair Trials (AODVLRT) [15] is a modification of local repair in AODV. Instead of single trial by broadcasting RREQ packet with TTL from equation (1) like original AODV, AODVLRT will do more than one trial to find a repair in route. When route failure happens in AODVLRT, the upstream node increments the destination sequence number by one and then initiates it first local repair trial by broadcasting RREQ packet with TTL = LR_TTL_START, which is set to be equal to 2. The upstream node that initiates the route repair will wait for the RREPs packet within the discovery period. If it fails to receive any RREPs during that period, it increments its TTL with LR_TTL_INCREMENT (equal to 2) and compare the resulted TTL with threshold limit of the TTL (denoted as LR_TTL_THRESHOLD) allowed to do local repair. If it’s lower than threshold value, the process will be repeated, and if equal to threshold, then the upstream node make its final trial by broadcasting RREQ packet with TTL = LR_TTL_MAX which is the worst case for AODVLRT.

In AODV with Backup Routing (AODV-BR) [18], nodes create an alternate route by overhearing route reply message from its neighbours. Whenever a node detect a broken routes, it will broadcast to all its neighbours in a hope for that at least one of it neighbours have a valid route to destination. At the same time, node will also send a RERR message to the source to initiate a route rediscovery. Instead of using alternate route (if any), new route is reconstructed from source to find an optimal route and to maintain the freshness of a route that reflects the current state of the network. This method successfully increased the packet delivery ratio of a network, but suffers a duplicate route transmission, overhead and stale routes. Moreover, the scheme is unscalable when a node overhears a large number of RREP messages under situations of heavy traffic.

Another method of local recovery protocol is Bypass Routing proposed by Sengul et all [17]. It’s a local recovery protocol that aims to reduce the route request flood triggered by broken routes through localizing the reaction to route failures using on-demand local recovery and a novel cache invalidation scheme. The proposed mechanism uses link state information to find a patch between one of the neighbours and a node along the route to the destination. The proposed scheme is suitable for source routing since it has cached of complete route information that are stored for each route entry. If a node breaks, it will first search for available bypass route that connects the node with any of the downstream nodes of the broken route (cached information). If no available bypass route available, broken node will initiate a local recovery to its neighbours in a hope that one of them had a valid route to the destination. If no available local repair, bypass routing will be the same as DSR but with additional overhead and delay. It also does not propose any solution on the stale route problem in DSR.

Bypass-AODV proposed in [16] uses a specific strategy of cross layer MAC-interaction to identify mobility related packet loss then triggers a routing layer to start local repair, which allow upstream node of a broken link to do the repair by setting up a bypass between it and the downstream node via an alternative node. By restricting the bypass to a very small topological radius, routings overheads are minimized considerably. Other proposed studies aim to change AODV protocol to make it more efficient in relation to performance metrics such as throughput or overhead by utilizing the information from the neighbours to prevent breaks or provide spare routes [19], using formation of clusters [20] or creating a path densely populated with nodes [21]. Although the above studies have proposed new schemes for route recovery of AODV algorithm in case of broken links, none of them made a comparative study between the mechanism already implemented in the original AODV RFC [3] and possible impacts on different degrees of mobility, traffic load and number of source-destination pairs.

The following section briefly explains the AODV’s local recovery mechanism implemented in this paper, performance evaluation comparison and results discussion.

V. RESULTS & DISCUSSIONS

A. Simulation Environment

Current implementation of AODV RFC [3] stated that whenever a link breaks in active route, the node upstream of that break MAY choose to repair the link locally if the destination was no farther than MAX_REPAIR_TTL, which is give in formula below:

MAX_REPAIR_TTL = 0.3 * NET_DIAMETER (2)

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where NET_DIAMETER value is set to 30 hops.

In this paper we have set a threshold for the affected node to decide when wills it performed local repair based on the link breaks location with respect to the total hop count from source to destination. It ranges between 0% of the length (forcing affecting node to do local repair no matter where it breaks) to 100% of total path length (always do a route repair form the source).

For the simulations performed in this work, we use ns2 [24] version 2.33. The simulation environment has an area of 1500m x 300m, containing 50 mobile nodes moving around randomly within the area. Static source and destination are located at both end of the area, as shown in figure 1. We used a single and 5 source-destination pair with Constant Bit Rate (CBR) traffic pattern. We opted for UDP traffic because it’s not connection oriented. The simulation parameters are shown in table 1.

Figure 1: Source and destination node location (single-pair case)

Parameter Value Route Repair Mechanism (based on location of link

breaks)

From 0% (always do local repair), 25%, 50%, 75% and 100% (always repair from the source) of total

path length Node Speed (Average) 10m/s and 20m/s

Nodes Number 50 nodes Simulated Time 900 seconds

802.11 MAC Rate 11Mbps Transmission Range 250 meters

Traffic Encoding Type UDP (Constant Bit Rate)Source Sending Rate 16Kbps and 32 Kbps

Mobility Model Random WaypointSource-Destination Pairs 1 and 5 Link Failure Detection HELLO message and Link

Layer feedback (LLF)

Table 1: Simulation parameters

The following is two important performance metrics being used in this simulation:

Packet Delivery Ratio (PDR): The ratio of data packets delivered to the destinations to those generated by the CBR sources

Normalized Routing Load (NRL): The number of routing packets transmitted per data packet delivered at the destination (normalized overhead).

B. Results & Discussions

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Figure 1 shows the PDR and overhead results for 16Kbps single source destination pair. AODV-HELLO used HELLO messages to detect link break while AODV-LL used a link layer feedback (LLF) to detect route failure. From the results, it’s found that always do local repair have the highest PDR compare to other method. AODV-LL gives the best performance with slightly better PDR and lower overhead than AODV-HELLO. This is because in lower traffic load, possibility of packet collision is low and route

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can be converge faster if it can be repaired locally without having to send additional control packet, hence the overhead will be lower.

When a sending rate of the source is increase from 16Kbps to 32Kbps, we can see slightly a degrade performance of network with single source-destination pair. This is shown in figure 2, where PDR drop below 90% for both AODV-HELLO and AODV-LL. Nevertheless, always do local repair still give the best performance in terms of PDR, but have a highest overhead since it have to generate a much higher control packets because of the higher traffic.

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On the other hand, both source rates of 16Kbps and 32Kbps for 5 source-destination pair have a lowest PDR and highest overhead (worst performance) since more collision and spurious link failure will not scale well if a node being forced to do local repair no matter where it breaks (figure 3 and 4). It will become worst for higher traffic load. From the results in Figure 3 and 4, we can get an optimum

performance in terms of the highest PDR if node is locally repaired if link breaks occurred at the halfway (between 50 percent from the total path length). From Figure 3 and 4, after the optimum length of local repair, PDR will drop a little bit and there is also slightly increase in overhead as well.

For all cases, AODV-LL outperformed AODV-HELLO for both PDR and overhead since Link layer feedback (LLF) can detect a link break faster than Hello message. Furthermore, using HELLO message will increase overhead since HELLO message is one of the control packet (RREP with TTL = 1).

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VI. CONCLUSION & FUTURE WORK

It can be conclude that, for a low traffic load and lower number of source-destination pair, always do local repairs

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give the best performance in terms of both PDR and overhead value. If the traffic load and number of source-destination pair increases, the optimum performance in term of highest PDR and lower overhead can be obtained if local

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repair is performed when the link breaks occur at halfway of the total hop count from source to destination. Simulations result also shows that LLF is a better mechanism to be used for link break detection. For future works, more through simulation must be done in order to get an accurate result. Other adaptive parameter like ACTIVE ROUTE TIMEOUT, LOCAL_REPAIR_WAIT_TIME and many more in AODV protocol can be experimentally tuned in order to get an optimum result in mobility and high traffic volume environment. Understanding the LLF mechanism in MAC layer also is one of the contributing factors toward a formal methodology of adaptive optimization.

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