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A Survey: Routing Metrics for Wireless Mesh Networks


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  • 1. A Survey: Routing Metrics for Wireless Mesh Networks Bharat Kumar Addagada, Vineeth Kisara and Kiran Desai May 5, 2009 1 Abstract This paper provides a comprehensive survey of various routing metrics that have been proposed for wireless mesh networks and two new routing metrics have been proposed. Avoidance of congestion at nodes, high throughput, low levels of interference, isotonicity, ecient utilization of channel capacity, minimal transmission delay are some of the desired characteristics of a good routing metric for wireless mesh networks. We compare the existing routing metrics based on the above characteristics and propose two new routing metrics that meet the above considerations. 2 Introduction Wireless Mesh Networks (WMNs) are an emerging technology and are making signicant progress in the eld of wireless networks in recent years. Mesh networks are capable of rapid deployment and reconguration and this gives them advantages like low up-front cost, easy network maintenance, robustness, and reliable service coverage. Typically WMNs consist of mesh routers and mesh clients where each node can operate both as host and router. Mesh routers generally have minimal mobility in a mesh network and form the backbone of WMNs. The clients could be either stationary or mobile and can form self organized ad hoc networks which can access services by relaying requests to wireless backbone network. Mesh routers are generally equipped with multiple wireless interfaces to improve exibility while mesh clients usually have only a single wireless interface. Based on functionality of nodes, WMNs can be classied into three categories: Infrastructure backbone, client backbone and hybrid WMNs. In Infrastructure WMNs, the mesh routers form a mesh of self-conguring and self-healing links among themselves and provide an infrastructure for the clients that connect to them. The network consists of access links to the end-users and mesh relay links between mesh routers to form the packet transport backbone. This type of network enables integration of WMNs with existing wireless networks, through gateway/bridge functionalities in mesh routers. In Client WMNs, client nodes form peer-to-peer mesh network among themselves. The client nodes perform routing and conguration as well as providing wireless access to end user applications. Hybrid WMNs are a combination of both the above types of WMNs and are expected to be the best choice in the next generation WMNs. Based on their unique characteristics, WMNs have a wide range of applications. The WMNs provide support for applications that are not possible with other existing wireless networks such as cellular networks, wireless sensor networks, ad hoc networks etc. The potential applications include wireless broadband services, commu- nity networking, instant surveillance systems, high speed metropolitan area networks, intelligent transportation systems, transient networks in convention centers, disaster recovery and back-haul service for large-scale wireless sensor networks. Routing protocols are at the heart of Wireless Mesh Networks and control the formation, conguration and maintenance of topology of the network. Owing to their common features, routing protocols developed for ad-hoc networks are applicable for WMNs. Some of the commonly used routing protocols in WMNs are Dy- namic Source Routing (DSR) and Ad-hoc On-demand distance vector (AODV) routing. However, design of new routing protocols for WMNs is still an active research topic as new performance metrics need to be discovered and utilized to improve performance of routing protocols. They need to address issues of scalability and the dierence in requirements for power eciency and mobility in Ad-hoc and WMNs. An optimal routing protocol for WMNs must capture the following features: Performance Metrics/Routing Metrics, Fault Tolerance, Load 1
  • 2. Balancing and Adaptive support for both Mesh clients and routers. Recently, a lot of research eort has been focused on multi-radio wireless mesh networks. Due to the rela- tively low cost of commodity wireless hardware such as radio interfaces based on IEEE 802.11 standards, it is now feasible to include multiple radios on a single node. By operating these interfaces on orthogonal channels, the capacity of a Mesh Router can be signicantly increased, and overcomes the limitation of half duplex oper- ation of single-radio nodes. However, routing protocols must be designed to take advantage of the availability of multiple interfaces eciently. In our survey, we focus on design of good routing metric for routing protocols in WMNs. In WMNs, a routing protocol provides one or more network paths over which packets can be routed to the destination. The routing protocol computes such paths to meet criteria such as minimum delay, maximum data rate, minimum path length etc. A routing metric that accurately captures quality of network links and thus aids in meeting such criteria is central to computation of good quality paths. We have surveyed multiple routing metrics that have been proposed for routing protocols in WMNs and have compared them based on their salient features, advantages and limitations and nally propose two routing metrics that address most of the requirements and limitations we discovered from the survey. Rest of our document is organized as follows: i. We outline the characteristics of a good routing metric ii. Brief overview of routing protocols used in WMNs iii. Survey report on each of the routing metric we have studied in our work iv. We present the two new routing metrics we have proposed in this survey v. Finally, we cover conclusion and scope for future work. 3 Characteristics of Routing Metrics As mentioned earlier, a good routing metric must accurately capture the quality of network links and aid in computation of good quality paths. Key components that can be utilized to compose a routing metric for mesh networks are: Number of hops, Link Capacity, Link Quality and Channel Diversity. Below we describe some of the desirable characteristics of a good routing metric for WMNs, that is, some of the criteria routing metric needs to address[14]. A. Interference: Interference in a mesh network can be of three types Intra-ow Interference: Intra-ow interference occurs when the radios of two or more links of a single path or ow operate on the same channel and can be reduced by increasing channel diversity. i.e. by selecting non-overlapping channels for adjacent hops of a path. Interference range of a node is typically bigger than a single hop and hence links on same channel in a multi-hop path can still interfere with each other and not just restricted to immediate neighbors. Inter-Flow Interference: Inter-ow interference is the interference caused by other ows that are operating on the same channels and are competing for the medium. Inter-ow interference is harder to control than intra-ow interference, due to the involvement of multiple ows and routes. External Interference: External interference occurs when a link experiences interference outside of the con- trol of any node in the network. Here, we have two kinds of external interference: Controlled Interference, where other nodes external to the network use networking technologies that overlap with those used by the network, and Uncontrolled Interference, which is caused by is caused by any other source of radio signals emitted in the same frequency range, but not participating in the same MAC protocol. B. Locality of Information: Some metrics require information such as channels used on previous hops of a path, or other metrics observed on other nodes of the networks, such as packet delivery rate or noise levels. This non-local information can be part of routing metric and can be used to make more optimal routing decisions. C. Load Balancing: The ability of a metric to balance load and provide fairer usage of the network's distributed resources. This is a very important consideration especially when there is concentration of trac at the Internet Gateways in mesh networks. 2
  • 3. D. Agility: The agility of a metric refers to its ability to respond quickly and eciently to changes in the network in terms of topology or load. In order for a metric to be considered agile, the rate at which measure- ments are taken should be higher than the rate of change in the network. E. Isotonicity: The isotonic property of a routing metric means that a metric should ensure that the order of weights of two paths is preserved if they are appended or prexed by a common third path. Isotonicity is the necessary and sucient condition of a routing metric for the existence of ecient algorithms to nd minimal weight paths, such as Bellman-Ford[1] or Dijkstra's algorithm. F. Throughput: In general, a metric should be able to select routes with greater throughput consistently. 4 Routing Protocols for Wireless Mesh Networks In order to have a better understanding of the routing metrics, in this section, we describe three dierent routing protocols in which routing metrics are incorporated in wireless mesh networks to nd best possible paths[15]. A. Destination Source-Routing Protocol (DSR) DSR[2] is an on-demand routing protocol that is based on concept of source routing. In source routing algorithm, each data packet contains complete routing information to reach its destination. Nodes are required to maintain route caches that contain source routes of which the node is aware. There are two major phases in DSR; the route discovery and route maintenance. For route discovery, the source node broadcasts a route request message which contains the address of the destination, along with source nodes address and a unique identication number. Every node which receives this packet checks if it has route information to destination. If not, it appends its own address to route record of the packet and forwards the packet to its neighbors. A route reply is generated if the route request reaches either the destination itself or an intermediate node which has route information to the destination. DSR has route cache to maintain route information to the destination. Route maintenance is done through the use of route error packets and acknowledgments. Main disadvantage of DSR is it has increased trac overhead as it contains complete route information in each of its data packet. This degrades DSRs routing performance. B. Destination Sequence Distance Vector Routing Protocol (DSDV) DSDV[3] is a proactive unicast routing protocol based on classical Bellman-Ford[1] routing mechanism. Every node in the network has a routing table which contains information on all possible destinations within the network. Sequence numbers are used to distinguish stale routes from fresh ones. To maintain consistency, routing table updates are periodically transmitted throughout the network. If two updates have same sequence number, the path with smaller metric is used in order to optimize the path. DSDV protocol only supports bi-directional links. C. Ad-hoc On-demand Distance Vector Routing Protocol (AODV) AODV[4] is a reactive on-demand routing protocol which builds on both DSR and DSDV. AODV is an improve- ment on DSDV as it minimizes the number of required broadcasts by creating routes on demand basis. It is also an improvement on DSR as a node only needs to maintain routing information about the source and destination as well as next hop, thereby largely cuts back the trac overhead. The process of route discovery is similar to DSR. Route request (RREQ) packets are broadcasted for route discovery while route reply (RREP) packets are used when active routes towards destination are found. HELLO messages are broadcasted periodically from each node to its neighbors, informing them about their existence. 5 Routing Metrics A. Hop Count: Hop count is the traditional routing metric used in most of the common routing protocols like AODV, DSR, DSDV designed for multi-hop wireless networks. It nds paths with shortest number of hops 3
  • 4. Advantages : 1. In scenarios of high mobility, hop-count can outperform other load dependent metrics. This is mostly a result of the metric's agility. 2. It is also a metric with high stability and further has the isotonicity property, which allows minimum weight paths to be found eciently. Drawbacks : 1. This metric treats all links in the network to be alike. 2. It does not account for link load, load capacity, channel diversity and interference experienced by the links. 3. It can often result in paths which have high loss ratio and poor performance. 4. It may choose paths with low throughput and poor medium utilization, as slower links take more time to send packets. B. ETX: Expected Transmission count ETX[5] is dened as the number of transmissions required to successfully deliver a packet over a wireless link. The ETX of a path is dened as sum of ETX of each link along the path. Pf be the packet loss probability in forward direction and Pr be the packet loss probability in the reverse direction. Let P denote the probability that the packet transmission from node x to node y in a link is not successful.P = (1−P f ).(1 − P r ). The Expected number of transmissions to successfully deliver a packet in 1 hop can be ∞ expressed as ET X = x k=1 xp (1 − p) x-1 = 1−p .ETX is measured in link of a real network by ET X = Df 1 r , 1 ∗D where Df is the forward delivery ratio (1 − P f ), D r is the reverse delivery ratio (1 − P r ). The delivery ratios Df and Dr are measured by broadcasting dedicated link probe packets of a xed size every average period (a typical value is 1 second) from each node to its neighbors. Advantages : 1. ETX is based on delivery ratios, which directly aects throughput and accounts for the eects of link loss ratios and asymmetry in the loss ratio in both directions of each link. 2. It favors paths with higher throughput and lower number of hops as longer paths have lower throughput due to intra-ow interference. 3. ETX deals with inter-ow interference indirectly. As ETX measures link-layer losses, the links with a high level of interference will have a higher packet loss rate and therefore higher ETX value. 4. ETX is isotonic and therefore allows ecient calculation of minimum weight and loop-free paths. Drawbacks : 1. It is a routing metric for single-channel multihop wireless network. 2. It only captures link loss ratio ignoring the interference experienced by the links which has a signicant impact on the link quality and the data rate at which packets are transmitted over each link. 3. It does not consider dierences in transmission rates. 4. As the transmission rate of probe packets is typically low, it does not accurately reect loss rate of actual trac. 5. It does not give any information on the eective link share. 6. As it does not consider load of the link, it will route through heavily loaded nodes leading to unbalanced resource usage. 7. ETX does not discriminate between same channel paths and channel-diverse paths. So, it makes no attempt to minimize intra ow interference. 8. In highly mobile single radio environments, ETX shows poor agility due to long time window over which it is obtained. C. ETT: The Expected transmission time(ETT)[6] metric is an extension of ETX which considers dierent link routes or capacities. ETT is expected time to successfully transmit a packet at the MAC layer and is dened for a S single link as ET T = ET T ∗ B , S denotes the average size of packet and B denotes current link bandwidth. ETT path metric is obtained by adding up all the ETT values of individual links in the path. Advantages : 4
  • 5. 1.It can increase the throughput of path by measuring the link capacities and would increase the overall performance of the network 2. ETT is isotonic Drawbacks : 1. ETT retains many disadvantages of ETX. 2. ETT does not consider link load explicitly due to which it cannot avoid routing trac through already heavily loaded nodes and links. 3. ETT is not designed for multiradio networks so it does not minimize intra-ow interference. D. WCETT: Weighted Cumulative Expected transmission Time(WCETT)[7] is an extension over ETT. The WCETT metric of a path p is dened as follows: W CET Tp = (1 − α) ∗ ET T + α ∗ maxi≤j≤k Xj i p Xj is the sum of ETT values of links that are on channel j in a system that has orthogonal channels and α is a tunable parameter within the bounds 0 ≤ α ≤ 1, that allows controlling preference over path lengths versus channel diversity. The rst term is summation of the individual link ETTs, and therefore favors shorter and high quality paths.The second term in the equation is summation of ETT of all links of a given channel and takes maximum over all channels. So, this gives higher value for a path with larger number of links operating on same channel i.e it favors channel diversity and low intra ow interference. Advantages : 1. WCETT eectively considers intra-ow interference into account and selects channel diversied paths. 2. It retains all the advantages of ETT except isotonicity. 3. It manages to improve the performance of multi-radio, multi-rate wireless networks when compared to simpler metrics such as ETT, ETX and hop count. 4. The two weighted components tuned by α of WCETT substitutes the simple summation of ETT and attempt to strike a balance between throughput and delay. Drawbacks : 1.WCETT simply considers the number of links operating on the same channel and their respective ETTs but does not consider the relative location of these links. It assumes all links of a path operating on same channel interfere which can lead to selection of non-optimal paths. 2. Because of the second term, WCETT is not isotonic. If a metric is not isotonic, then it is very dicult to use with link state routing protocols. 3. WCETT does not explicitly consider the eect of interow interference. Due to this, it may establish routes which suer from high levels of interference. 4. This metric suers from same limitations as ETX/ETT by not estimating the eective link share. F. MIC: Metric of interference and channel switching (MIC)[8] is designed to support load balanced routing and to consider intra-ow and inter-ow interference, in addition to being isotonic. MIC for a path p is dened as follows: 1 M IC(p) = IRUl + CSCi N ∗ min(ET T )link i p nodei p whereN is the number of nodes in the network and min(ETT) is the smallest ETT in the network. The two components of MIC, IRU (Interference-aware Resource Usage) and CSC (Channel Switching Cost) are dened as follows. IRUl = ET Tl ∗ Nl 5
  • 6. CSCi = w1 If CH(prev(i)) = CH(i) CSCi = w2 If CH(prev(i)) = CH(i) ,0 ≤ w1 ≤ w2 Nl is the set of neighbors that interfere with transmissions on line i. CH(i) represents channel assigned for node i's transmission and prev(i) represents the previous hop of node i along path p. Advantages : 1. MIC takes both interow interference, intra-ow interference and it can be made isotonic if it is decom- posed into virtual nodes while applying minimum weight path nding algorithms such as Dijkstra's algorithm. Drawbacks : 1. The overhead required to maintain update information of the ETT for each link can signicantly aect the network performance depending on trac loads. 2. This metric assumes that all links located in the collision domain of a particular link contributes to same level of interference and counts the amount of interference on a link only by the position of interfering nodes no matter whether they are involved in any transmission simultaneously with that link or not. 3.The second component CSC captures intra-ow interference only in two consecutive links. G. LAETT: The two main goals of LAETT[9] is to provide a path which satises the bandwidth request of the ow and to leave room for future requests by balancing the load across the network. It combines wireless access character- istics and load estimates. It consists of an adaption of ETT metric S ET Tij = ET Xij ∗ Bij ET Xij =Expected transmission count on link(i,j) S =Packet size Bij =Eective bit rate Bi Bij = γij Bi =Transmission rate of node i γij= Link quality factor γij = 1 when the link of good quality when the transmission quality degrades, γij increases and Bij decreases. To consider load balancing, remaining capacity (RCi) on each node is introduced and it is given by N RCi = Bi − fik γik k=1 fik are the transmission rates of the Ni current ows that traverse node i. The cost of a ow on remaining capacity is weighted by factor γik : good quality transmissions use less resources than bad quality ones. The packet pair algorithm can be used to estimate the available bandwidth on a link which provides γij . We dene LAET Tij by: S LAET Tij = ET Xij ∗ RCi +RCj 2γij The second factor captures the remaining capacity at both end nodes. When two paths have same cumula- tive weight in terms of ETX, LAETT metric favors the one with the most remaining capacity. Advantages : 1. LAETT is a load aware isotonic routing scheme that uses weighted shortest path routing to balance the load across the network. 2. It captures link quality and trac load. Drawbacks : 1. It does not consider intraow interference and does not explicitly consider interow interference. 6
  • 7. H. EETT: Exclusive Expected Transmission Time(EETT)[10] is a novel interference aware routing metric which selects multi-channel routes with least interference to maximize end to end throughput. It is used to give better evaluation of a multichannel path. For any given l, Interference set (IS) is dened as the set of links that interfere with it. A links interference set also includes the link itself. The link l's EETT is denes as EET Tl = ET Ti linki IS(l) IS(l)= Interference set of link l. The path weight is dened as the sum of EETT's of all links on the path. Advantages : 1. As this metric builds over ETT, it has all the advantages of ETT. 2. It eectively considers intraow interference and indirectly considers inter-ow interference 3. EETT is isotonic. Drawbacks: 1. EETT of link l represents the busy degree of the channel used by link l. It is the worst case estimation of transmission time for passing link l. I. ILA: Interference Load Aware(ILA)[12] metric is built over MIC metric. It is composed of two components: Metric of channel interference(MTI) and channel switching cost(CSC). CSC component is same as that in MIC metric. MTI metric is dened as follows M T Ii (C) = ET Tij (C) ∗ AILij (C), Nl (C) = 0 M T Ii (C) = ET Tij (C), Nl (C) = 0 AILij= Average load of neighbors that may interfere with transmission between nodes i and j over channel C. ILij (C) AILij (C) = NL l (C) N , NL (C) = Ni (C) Nj (C). ILij (C) = (Interfering load) is the load of interfering neighbors. Nl (C) = Set of interfering nodes of neighbors i and j The path weight function is as follows ILA(p) = α∗ M T Ii + CSCi linki p nodei p To balance the dierence of magnitude of the two components (MTI and CSC), scaling factor αis applied to MTI metric αis given as 1 = min(ET T ) ∗ min(AIL), Nl (C) = 0 α 1 = min(ET T ), Nl (C) = 0 α where min(ETT) and min(AIL) is the smallest ETT and average load in the network.The important imple- mentation issue of the metric is estimation of load of interfering neighbors[11]. 7
  • 8. Advantages : 1. This metric addresses the aforementioned limitations of existing metrics such as hop count, ETX, ETT, WCETT, MIC for Wireless mesh networks. 2. This routing metric nds paths with less congestion, low level of interference, low packet drop ratio and high data rate. 3. ILA calculates interow interference by considering the amount of trac generated by interfering neigh- bors which is drawback of MIC. Drawbacks : 1. The second component CSC captures intra-ow interference only in two consecutive links. J. iAWARE: iAWARE[13] is the rst routing metric for multiradio WMN to factor in both interow and intraow interference and characterized by the physical interface model. The iAWARE metric is dened as follows: n iAW ARE(p) = (1 − α) ∗ iAW AREi + α ∗ max1≤j≤k Xj i=1 Xj is same as in WCETT. The iAWARE value of a link j is dened as follows: ET Tj iAW AREj = IRj IRj =Interference ratio for a link j is the value between two nodes u and v. It is dened as follows. IRj = min(IRj (u), IRj (V )) Interference ratio(IR) value for a link j is the value between two nodes u and v. It is dened as follows. SIN Ri (u) IRi (u) = SN Ri (U ) SIN Ri (u) is the signal to interference noise ratio and SN Ri (u) is the signal to noise ratio at node u for link i. Advantages : 1.iAWARE captures the eect of variation in link loss ratio, dierences in transmission rate as well as inter ow and intra ow interference. 2.iAWARE retains many of the properties of WCETT with the exception of its handling of interow inter- ference measurements. It directly measures the average interference generated by neighboring nodes. 3. The introduction of SINR is a great breakthrough for interow interference routing compared with other ETX based metric like MIC, ETX, WCETT etc. Drawbacks : 1. iAWARE is a non-isotonic routing metric. 2. When a link has higher IRj that ET Tj , the iAW AREj metric will have a lower value. This will result in the iAW AREj metric choosing a path with lower ETT but higher interference. The drawback of this metric is that it gives more weightage to ETT compared to interference of the link. 6 Proposed Routing Metrics A. Mod-iAWARE: Mod-iAWARE is a new metric which is based on the existing routing metrics such as iAWARE, LAETT and EETT. The path metric of Mod-iAWARE is dened as follows: n M od − iAW ARE(P ) = M od − iAW AREij link(i,j)=1 8
  • 9. The link metric EET Tij M od − iAW AREij = IRij EET Tij =Exclusive expected transmission time of a link ij IRij =Interference Ratio EET Tij = link(i,j) IS(i,j) LAET Tij , LAET Tij = Load aware expected transmission time IS(i,j) = Interference set of link(i,j) (mentioned in EETT section) Explanation : In Mod-iAWARE metric, we rst calculate the LAETT values of all the links in the path. This LAETT value considers the link quality, remaining capacity and packet size into consideration. Now, EETT value of each link is calculated. For any link in the path, the EETT value of the link is summation of all the LAETT values of links which are in the interference set (IS) of this link. A links interference set also includes the link itself. EETT of a link l represents the busy degree of the channel used by link l. It is a worst case estimation of transmission time for passing link l. If there are more neighboring links on the sane channel with link l, link l may have to wait for a longer period to do the transmission on that channel. As a result,a path with larger EETT indicates that it has more severe interference and needs more time to nish the transmission over all links within the path. In essence, a better channel distribution over a path results in less intra-ow interference. Hence EETT can accurately reect the optimality of channel distribution on a path. The interference Ratio(IRj) calculates the inter-ow and external interference. Advantages : 1.Unlike the iAWARE metric, Mod-iAWARE does not have an individual summation component that cap- tures intra ow interference. As explained above, EETT considers intra ow interference in Mod-iAWARE and hence is isotonic. 2. In the absence of interference (IRj=1), iAWAREij is equal to ETTij and the metric will capture the link loss ratio and packet transmission rate of linkij. But when a link has higher IRij than ETTij, the iAWAREij metric will have a lower value. This will result in iAWAREij metric choosing a path with lower ETT but higher interference. The drawback of this metric is that it gives more weightage to ETT compared to interference of the link. Mod-iAWARE overcomes this drawback. Mod-iAWAREij is the ratio of EETTij and IRij. EETT considers intra ow as well as inter ow interference. So, if a link has high interference, not only IR value but also EETT value will be high. Thus Mod-iAWARE selects path with less interference. 3. It even considers load balancing as it uses LAETT metric which takes care of load balancing. 4. It has all advantages of iAWARE, EETT and LAETT routing metrics. Drawbacks : 1. There may be considerable amount of trac overhead to calculate the link metric; it needs knowledge of interference set. 2. Inter ow interference is calculated twice, both in EETTij and IRij. This might be redundant. 3. This metric does not consider the cost of channel switching delay. We do not consider this routing characteristic because if a path has less intra ow interference, it should have dierent channels on all the links in the path. So, this path has high channel switching cost. Now, consider a path which has low channel switching delay. This path may have links on same channels which may increase intra ow interference. For this reason, we consider channel switching delay not to be an eective routing metric. B.Mod-ILA This metric is a modied version of ILA with inputs from metrics such as LAETT and EETT. This metric addresses the limitations of existing metrics such as hop count, ETT, ETX, WCETT, MIC etc for WMN. This metric nds paths with less congestion, low level of interference, low packet drop ratio and high data rate.The path metric of Mod-ILA is as follows M od − ILA(p) = M T Iij linkij p 9
  • 10. where M T Iij =Metric of interference on link ij. MTI metric is dened as follows M T Iij (C) = EET Tij (C) ∗ AILij (C), Nl (C) = 0 M T Iij (C) = EET Tij (C), Nl (C) = 0 AILij (C) is the average load of the neighbors that may interfere with the transmission between nodes i and j over channel C and it is given as NlILij (C) AILij (C) = Nl (C) Nl (C) = Ni (C) Nj (C) ILij, Interfering load is the load of the interfering neighbor. Nl is the set of interfering neighbors of nodes i and j. EET Tij = LAET Tij link(i,j) IS(i,j) LAET Tij =load aware expected transmission time. IS(i,j) = interference set of links(i,j) LAET Tij is same as in Mod-iAWARE. Explanation : In Mod-ILA metric, rst the LAETT values of all the links in the path are calculated. This LAETT value takes the link quality, remaining capacity and packet size into consideration. Next EETT value of each link is calculated similar to the one mentioned in Mod-iAWARE metric. EETT does better channel distribution over a path which results in less intra ow interference. MTI value which is the product of EETTij(C) and AILij(C) considers interow interference on the link. The calculation of AILij of a link is mentioned in ILA section of the paper. It is aimed at decreasing the packet delay due to the load of neighboring nodes. In this way, the Mod-ILA metric eectively considers intraow, inter-ow and external interference along with packet loss ratio, high data rate,congestion. Advantages : 1. Unlike the ILA metric, Mod-ILA does not have an individual summation component that captures intra ow interference. As explained above, EETT considers intra ow interference in Mod-ILA and hence is isotonic. 2. In ILA metric, the second component CSC captures intra ow interference only in two consecutive links. But Mod-ILA overcomes this drawback. Instead of the second term, it uses EETT which eectively considers interference on all the links in the path that interfere with each other. 3. Mod-ILA considers load balancing as it uses LAETT metric which addresses load balancing. 4. It has all the advantages of ILA, ETT and LAETT routing metrics. Drawbacks : 1. There may be considerable amount of trac overhead. To calculate the link metric, it needs the knowledge of interference set (IS) etc. 2. This metric also considers inter ow interference twice i.e in EETTij as well as in AILij. So, this may lead to redundant computation of inter ow interference. 3. The method of computing interference load(ILij) is not eectively mentioned in the ILA metric. 4. This metric also does not consider channel switching delay. Below is a table that summarizes characteristics of all the routing metrics we have studied so far including Mod-iAWARE and Mod-ILA. 10
  • 11. Hop ETX ETT WCETT MIC ILA iAWARE EETT LAETT Mod-ILA Mod-iAWARE Intra X X X » » » » » X » » Inter X » * »* »* »# » » »* X » » External X X X X » » » » X » » Load bal X X X X X X X X » » » Agility » X X X X X X X X X X Isotonic » » » X »$ X X » » » » * - Indirectly , #- Not Accurate, $ -Complex, 7 Conclusion In this survey, we have performed a comprehensive analysis of the various routing metrics that have been proposed for routing protocols in Wireless Mesh Networks. We have considered criteria such as Interference, Load Balancing, Isotonicity, Throughput, Link Quality etc in evaluation of each routing metric. We found that each routing metric has limitation(s) in not meeting one or more criteria. Based on the limitations, we have proposed two new routing metrics which are built using the existing routing metrics and that addresses most of the criteria for a good routing metric. As future work, we propose to implement both Mod-ILA and Mod- iAWARE metrics using network simulators such as ns2 software and evaluate their performance in comparison with some of the routing metrics we have considered in this survey. We also propose to work on including channel switching delay cost in design of the new routing metrics. 8 References [1] Bellman ford algorithm, [2] David B. Johnson and David A. Maltz, Dynamic Source Routing in Ad Hoc Wireless Networks. [3] Charles E.Perkins and Pravin Bhagwat ,Highly Dynamic Destinaation-Sequenced Distance -Vector Rout- ing (DSDV) for Mobile Computers. [4] Charles E. Perkins and Elizabeth Royer,Ad-Hoc On-Demand Distance Vector Routing. [5] Douglas S. J. De Couto ,Daniel Aguayo ,John Bicket and Robert Morris , A HighThroughput Path Metric for MultiHop Wireless Routing [6] Richard Draves ,Jitendra Padhye and Brian Zill,Routing in Multi-Radio, Multi-Hop Wireless Mesh Networks , in ACM Mobicom,2004. [7] Richard Draves ,Jitendra Padhye and Brian Zill,Routing in Multi-Radio, Multi-Hop Wireless Mesh Networks , in ACM Mobicom,2004. [8] Yaling Yang,Jun Wang and Robin Kravets, Designing Routing Metrics for Mesh Networks. [9] Hervé Aïache, Laure Lebrun, Vania Conan and Stéphane Rousseau, LAETT A load dependent metric for balancing Internet trac in Wireless Mesh Networks [10] Weirong Jiang, Shuping Liu, Yun Zhu and Zhiming Zhang,Optimizing Routing Metrics for Large-Scale Multi-Radio Mesh Networks [11] Hossam Hassanein and Audrey Zhou,Routing with Load Balancing in Wireless Ad hoc Networks [12] Devu Manikantan Shila and Tricha Anjali,Load-aware Trac Engineering for Mesh Networks [13] Anand Prabhu Subramanian ,Milind M. Buddhikot and Scott Miller,Interference Aware Routing in Multi-Radio Wireless Mesh Networks [14] Jonathan Guerin, Marius Portmann and Asad Pirzada, Routing Metrics for multi radio Wireless Mesh Networks, ATNAC 2007. [15] Hui Liu, Wei huang, Xu Zhou and X.H.Wang, A Comprehensive Comparision of Routing Metrics for Wireless Mesh Networks. 11