Ann. Telecommun. (2010) 65:247 262 DOI 10.1007/s12243-009-0130-1 EFT: a hgh throughput routng metrc for IEEE 802.11s wreless mesh networks Md. Sharful Islam Muhammad Mahbub Alam Md. Abdul Hamd Choong Seon Hong Sungwon Lee Receved: 13 March 2009 / Accepted: 4 September 2009 / Publshed onlne: 25 September 2009 Insttut TELECOM and Sprnger-Verlag 2009 Abstract In ths paper, we present a throughputmaxmzng routng metrc, referred to as expected forwardng tme (EFT), for IEEE 802.11s-based wreless mesh networks. Our study reveals that most of the exstng routng metrcs select the paths wth mnmum aggregate transmsson tme of a packet. However, we show by analyses that, due to the shared nature of the wreless medum, other factors, such as transmsson tme of the contendng nodes and ther denstes and loads, also affect the performance of routng metrcs. We therefore frst dentfy the factors that hnder the forwardng tme of a packet. Furthermore, we add a new dmenson to our metrc by ntroducng traffc prorty nto our routng metrc desgn, whch, to the best of our knowledge, s completely unaddressed by exstng studes. We also show how EFT can be ncorporated nto the hybrd wreless mesh protocol (HWMP), the Md. S. Islam M. M. Alam C. S. Hong (B) S. Lee Department of Computer Engneerng, Kyung Hee Unversty, 1 Seocheon, Gheung, Yongn, Gyeongg 449-701, Korea e-mal: cshong@khu.ac.kr Md. S. Islam e-mal: sharf@networkng.khu.ac.kr M. M. Alam e-mal: mahbub@networkng.khu.ac.kr Md. A. Hamd Department of Informaton and Communcatons Engneerng, Hankuk Unversty of Foregn Studes, 89 Wangsan-r, Mohyun-myon, Cheon-Gu, Yongn-s, Kyongg-do 449-791, Korea e-mal: hamd@hufs.ac.kr S. Lee e-mal: drsungwon@khu.ac.kr path selecton protocol used n the IEEE 802.11s draft standard. Fnally, we study the performance of EFT through smulatons under dfferent network scenaros. Smulaton results show that EFT outperforms other routng metrcs n terms of average network throughput, end-to-end delay, and packet loss rate. Keywords Wreless mesh networks Routng metrc Medum access tme Hybrd wreless mesh protocol 802.11s 1 Introducton Wreless mesh networks (WMNs) have revolutonzed the way wreless access s avalable to end users. Due to ther favorable characterstcs, such as self-organzaton, self-confguraton, self-healng, easy mantenance, hgh scalablty, and relable servces, WMNs have ganed acceptance from both ndustry and academa as a cost-effectve approach to support hghspeed last-mle connectvty and ubqutous broadband access [1 3]. A schematc example of WMN archtecture s presented n Fg. 1 that ncludes four dfferent enttes or network elements: mesh pont portal (MPP), mesh access pont (MAP), mesh pont (MP), and legacy staton (STA) [4]. An MPP s a gateway node that connects the WMN to a wred nfrastructure, possbly to the Internet or local networks. There can be multple MPPs deployed n a WMN. An MP s responsble for relayng traffc to other MPs or MAPs, whereas an MAP performs both the functons of an MP and an access pont. The archtecture of WMN has some unque characterstcs (.e., use of a shared wreless medum, staton-
248 Ann. Telecommun. (2010) 65:247 262 Mesh Pont Portal (MPP) Mesh Pont (MP) Mesh Access Pont (MAP) Legacy Staton (STA) Mesh Lnk Legacy Lnk Fg. 1 Archtecture of a typcal WMN Internet ary wreless nodes, heterogeneous traffc patterns, both access pont and relay functonaltes) that dfferentate a WMN from other networks and demand revstng of exstng routng protocols for use n WMN. The current draft D2.02 of IEEE 802.11s [4] has defned a pathselecton protocol (ths term s used to dfferentate t from layer-3 routng) for WMN and termed as hybrd wreless mesh protocol. Hybrd wreless mesh protocol (HWMP) can operate n both proactve and reactve modes. The proactve mode s used when nodes are prmarly statc and traffc flows manly to/from the Internet va the MPP to/from MAPs or MPs (nter-mesh traffc). In contrast, the on-demand mode s used manly for traffc between MPs 1 wthn a mesh network (ntramesh traffc). Both proactve and on-demand modes select paths usng the Artme metrc, whch takes nto account the dfferent data rates of dfferent lnks but does not consder the effect of packet loss due to nterferng transmssons/collsons from contendng MPs and the traffc loads of contendng neghbors. In ths study, we focus on desgnng a new routng metrc that, together wth HWMP, can be used for path selecton n IEEE 802.11s-based wreless mesh networks. A routng metrc can be defned as a parameter, value, or weght assocated wth a lnk (termed a lnk metrc) or a path (termed a path metrc). A routng protocol makes the routng decson based on the routng metrc, and thus, t plays a crtcal role n determnng the performance of a routng protocol. Desgnng a routng metrc for WMNs requres consderaton of WMNs unque archtecture and the assocated wreless networkng envronment. Unlke tradtonal wreless local area networks (WLANs), whch have a centralzed structure, WMNs have no herarchy. To create a wreless mesh network, neghborhood MPs must overlap. MPs can operate autonomously, are capable of forwardng traffc, and can 1 Hereafter, unless mentoned explctly, the term MP s used for bothmpsandmaps. be a fnal source or destnaton. Therefore, redundant paths between MP and MPP or MP and MP can exst n a sngle WMN doman. Our goal s to desgn a routng metrc that leverages the routng protocol to fnd the best path among the avalable paths for a specfc flow. Exstng routng metrcs usually select paths that requre the mnmum aggregate transmsson tme (whch does not reflect the actual end-to-end delay) for a packet n a path and, thus, select the lnks wth hgh data and success rates. However, a hgh-throughput path s one that can delver a packet wth the shortest end-toend delay. Hence, a routng metrc needs to consder factors other than the tme that a packet uses the wreless medum (successful or unsuccessful transmsson). However, for better utlzaton of the medum, network traffc need to be spread over the network, and thus, a routng metrc needs to balance the network load to the avalable paths. Furthermore, the presence of hghprorty packets mght starve packets of low-prorty flows, whch a routng metrc needs to consder. All the factors addressed above motvate us to desgn a routng metrc that consders the forwardng tme of a packet n a node. We defne the forwardng tme of a packet as the tme that a node requres to successfully delver t to the next hop. In ths paper, we propose a new routng metrc, expected forwardng tme (EFT), whch selects the path that has the lowest end-to-end forwardng delay. The man contrbutons of our work are as follows. (1) Through detaled analyses and results, we frst determne the factors that domnantly affect the EFT of a packet, and then, we extract the parameters that estmate EFT wth low cost effectvely. (2) We desgn the proposed routng metrc based on extracted parameters and propose methods for low-cost estmaton of the parameters. (3) We show how EFT can be ncorporated wth IEEE 802.11s s HWMP for path selecton. (4) Fnally, through smulatons, we show that our proposed metrc outperforms exstng routng metrcs n terms of average network throughput, end-to-end delay, and packet loss-rate. The remander of ths paper s organzed as follows: We dscuss the major routng metrcs desgned for wreless mesh networks n Secton 2. InSecton3, we present the detaled desgn of the proposed routng metrc, and n Secton 4, we nvestgate the performance of our proposed metrc. Fnally, we present our conclusons n Secton 5. 2 Routng metrcs for WMNs In ths secton, we present and analyze some good examples of routng metrcs specfcally desgned for wre-
Ann. Telecommun. (2010) 65:247 262 249 less mesh networks. Comprehensve surveys of WMN routng metrcs can be found n [5, 6]. 2.1 Expected transmsson count Expected transmsson count (ETX) [7] s one of the frst routng metrcs desgned for wreless mesh networks. It s a lnk metrc that estmates the number of transmsson attempts (ncludng retransmssons) requred for a successful transmsson on a partcular wreless lnk. The ETX of a lnk s defned by Eq. 1 and the weght of a path s determned by summaton of ETX values of all lnks along the path. ETX can be calculated as 1 ETX =, (1) d f d r where d f and d r denote the delvery rato n the forward and reverse drectons, respectvely. Though the computaton of ETX s smple and captures the effect of packet loss rates, ths routng metrc has several lmtatons. ETX does not consder the mpact of varyng transmsson rates of dfferent wreless lnks and szes of data packets. Another lmtaton of ETX s the use of an actve broadcast-based probng scheme to measure the packet loss rates. Ths does not reflect the lnk qualty accurately, snce probng packets are small and broadcast packets use lower data rates than those of actual data packets [8]. ETX also does not consder the mpact of ntra-flow and nter-flow nterference. 2 2.2 Weghted cumulatve expected transmsson tme Draves et al. [9] proposed the weghted cumulatve expected transmsson tme (WCETT) as a path metrc for routng n mult-rado mult-channel WMNs. Frst, they proposed the expected transmsson tme (ETT) metrc to address the ssue of varyng data rates of dfferent wreless lnks. They calculated the tme requred to transmt a packet of sze S on a lnk wth a data rate B (raw data rate) usng S/B and obtaned the bandwdthadjusted ETX or ETT usng Eq. 2. Thus, the ETT of a lnk s the duraton of tme a node uses the medum to successfully delver a packet to the next hop. ETT of the -th lnk s defned by ETT = ETX S B, (2) 2 Intra-flow nterference occurs when nodes n a sngle path attempt to transmt packets of the same flow and nterfere wth each other. Inter-flow nterference s the nterference suffered among concurrent flows. where B s the data rate of the -th lnk. Note that nether ETT nor ETX consders the presence of multple channels. Furthermore, ETT characterzes the transmsson tme n the absence of nterference. Therefore, to fnd paths wth less ntra-flow nterference and channel dversty, the authors n [9] proposed WCETT, whch s defned by WCETT = (1 β) n =1 ETT + β max 1 j k X j, (3) where X j s the summaton of ETT of the lnks n a path p operatng on channel j, k s the number of orthogonal channels avalable, and 0 β 1 s a tunable parameter. Note that the frst component of WCETT defnes the end-to-end delay experenced n a partcular path, whle the second component accounts for channel dversty and fnds the path wth less ntraflow nterference. However, WCETT does not consder the lnk qualty and traffc loads of the contendng nodes. Moreover, WCETT does not explctly consder the effect of nter-flow nterference. Therefore, n the presence of multple flows n the network, t may end up fndng paths through more dense areas where congeston s more lkely and overall network throughput degrades. 2.3 Artme The Artme metrc s the default routng metrc specfed n the draft 2.02 of IEEE 802.11s [4]. Ths metrc defnes the amount of channel resources (C a ) consumed by transmttng the frame over a partcular lnk and s calculated as C a = [ O + S t r ] [ 1 1 e f ], (4) where O and S t are constants that defne channel access overhead and number of bts n the test frame, respectvely. The nput parameters r and e f are the data rate n megabts per second and the frame error rate for a test frame, respectvely. The rate r represents the data rate at whch a node would transmt a frame of standard sze S t based on current condtons, and ts estmaton s dependent on local mplementaton of rate adaptaton. The frame error rate e f s the probablty that, when a frame of standard sze S t s transmtted at the current transmsson rate r, the frame s corrupted due to transmsson error. The path metrc s the sum of metrc values of all lnks n the path. Ths metrc only takes the transmsson rate and transmsson error rate nto consderaton. In realty, the frame error rate due to transmsson error does not reflect the actual lnk qualty of a wreless lnk. A closer
250 Ann. Telecommun. (2010) 65:247 262 look at the Artme metrc reveals that t s analogous to the ETT metrc, where the frst part of Eq. 4 reflects the transmsson tme and the second part measures the number of retransmssons requred, lke ETX. Thus, Artme, lke ETT, by not addressng the behavor of contendng nodes, can route traffc to congested areas of the network because lnks wth a hgher data rate wll always be gven hgher prorty. 2.4 Metrc of nterference and channel-swtchng In [10], the authors presented a new metrc that ncorporates both ntra-flow and nter-flow nterference. The metrc of nterference and channel-swtchng (MIC) for apathp s defned as follows: MIC(p) = 1 N mn(ett) lnk l p IRU l + node p CSC, (5) where N s the total number of nodes n the network and mn(ett) represents the smallest ETT n the network. IRU l represents the nterference-aware resource usage and CSC accounts for the channel swtchng cost. However, MIC has some major drawbacks n terms of mplementaton. Frst, t assumes that all the nodes located n the collson doman of a partcular lnk contrbute to the level of nterference, rrespectve of whether those nodes are actually generatng nterferng traffc or not. Second, t requres up-to-date nformaton regardng the ETT of each lnk ths requres sgnfcant overhead and may degrade the overall network performance. 3 Proposed routng metrc 3.1 Problem descrpton and motvaton In wreless mesh networks, multple paths between any source destnaton par usually exst. End-to-end delay experenced by a packet on dfferent paths mght be dfferent because of the shared nature of wreless lnks. Obvously, the path that wll take less tme to delver a packet from a source to a destnaton s the better choce to route the packet. Our goal s to desgn a routng metrc that can estmate the end-to-end delay experenced by a packet for the avalable paths, to enable the routng protocol to select the best path. If all the packets can be delvered wth mnmum delays, the overall network throughput wll ncrease. In the followng, we dscuss the factors that affect the forwardng tme of a packet n a node. Transmsson rate: Traffc routed va a lnk wth a hgher transmsson rate take less transmsson tme and should outperform lnks wth lower transmsson rates. Furthermore, due to the shared nature of the wreless medum, the transmsson rates of neghbors (.e., contendng nodes) also affect the forwardng tme. Ths s because a node has to wat for the medum to be free whle neghbors keep the medum busy. Success rate: Ths represents the number of MAC layer transmsson attempts requred for a successful transmsson. A lossy wreless lnk or a lnk that experences more collsons results n multple transmssons of a sngle packet on that lnk. Ths has a negatve mpact on the routng metrc value. Contendng neghbors and ther loads: The number of contendng/nterferng nodes and ther traffc loads n the neghborhood of a forwardng node has a large mpact on the performance of WMNs. Furthermore, low overhead avalablty of ths nformaton s a challenge for WMNs. Load awareness: Ths represents the traffc load of the forwardng nodes n a path. If the forwardng nodes are loaded, ther queues buld up quckly and the queueng delays of the packets ncrease. In contrast, selectng a lghtly loaded path balances the network load. Traffc prorty: Wreless mesh networks are supposed to provde QoS to the flows (for example, EDCA [11] s used n the MAC layer) by supportng dfferent classes of traffc. Therefore, packets have dfferent prortes dependng on the traffc class of the flows, and consequently, the forwardng tme of a packet vares accordng to the traffc class. A low-prorty packet mght be starved n the presence of hgh-prorty packets. A longer path or relatvely worse path mght produce better throughput for low-prorty packets. Moreover, dstrbutng the hgh-prorty packets n dfferent paths mght also balance the load n the network. Most of the routng metrcs proposed n the lterature and dscussed n Secton 2 (for example, ETT, MIC, and Artme) determne end-to-end delay by summng up the value of ETT of the lnks n a path, as proposed by Draves et al. [9]. However, ETT does not address all the factors dscussed above. More specfcally, the medum access tme depends not only on the number of retransmssons (or transmssons), but also on the tme requred for each transmsson attempt. For example, a forwardng node wth more contendng
Ann. Telecommun. (2010) 65:247 262 251 nodes experences more nterruptons durng the backoff process, whch results n a greater medum access delay. Further, a contendng node wth low channel qualty (or low transmsson rate) or packets of larger sze mght force a node to freeze ts backoff counter for a longer perod. Moreover, a low-prorty packet mght starve for a long tme, because of the presence of several hgh-prorty packets. As a result, a path wth the mnmum expected delay for that traffc class s the best path for the packet, even though ths mght not be the best path n terms of transmsson rate and success rate,.e., ETT. Ths also ensures load-balancng n the networks. Moreover, ETT mght drect all the packets of the network towards a sngle path (.e., the best path), leadng to ncreased congeston and contenton. As a result, the queung delay of packets may ncrease and the network throughput may decrease due to under-utlzaton of network resources, f lghtly loaded parallel paths exst wth comparatvely low qualty. Furthermore, due to ncreased congeston and contenton, a better path wll turn nto a worse path and ETT wll choose a new path. But, due to the nature of ETT, t wll forward all the packets to that new better path and the new path wll n turn become worse. The man challenge n desgnng a routng metrc s therefore to fnd the parameters that ncorporate the aforementoned factors; furthermore, the routng metrc should take nto account the avalablty of the value of each parameter wth mnmum overheads. 3.2 Desgn consderatons As mentoned prevously, our goal s to desgn a routng metrc based on the EFT of a packet n a node. In ths secton, we explore and justfy the parameters that affect the forwardng tme of a packet. 3.2.1 Number of retransmssons and transmsson rate The forwardng tme of a packet for any outgong lnk depends on the number of transmsson attempts requred to successfully delver a packet for ths lnk and the transmsson rate of the lnk. Lke ETT [9] and Artme [4], EFT also consders both of these parameters. Furthermore, the sze of the contenton wndow (.e., number of backoff slots) vares wth the number of retransmssons. 3.2.2 Multple traffc classes IEEE 802.11s-based WMNs allow multple classes of traffc (such as background, best-effort, vdeo, voce, etc.) to co-exst, and thus, the medum access delay of a packet depends on the traffc class of the packet. If a hgh-qualty path s domnantly accessed by the hgh-prorty flows, the transmsson of a low-prorty packet mght not defreeze ts backoff counter due to consecutve transmssons of hgh-prorty flows, and hence, ts defer tme may become very long. Therefore, the forwardng tme of a packet s severely affected by the defer tme, especally for low-prorty flows. As a result, ncluson of the defer tme n the forwardng tme wll select the path wth the shortest delay for a partcular traffc class. Such a path mght not have the best ETT or ETX. However, ths wll select the best path for that traffc class and balance the load of the network for dfferent traffc classes. The medum access delay of the packets of the j-th traffc class, denoted as d j a, s gven by d j a = M 1 =0 ] M [d jd + d jb () 1 + =1 d j c, (6) where M s the maxmum retransmsson lmt, d j d and dc j are the defer tme and unsuccessful transmsson tme for the j-th traffc class, respectvely, and d j b () s the backoff tme of a packet of the j-th traffc class at the -th transmsson attempt. Note that each nterrupton durng the backoff process s followed by a defer tme. The defer tme for the j-th traffc class s gven by E[d j d ]= { AIFS[ j]+e[h] E[T], j = 1 AIFS[ j], j = 2, 3, (7) where AIFS[ j] ( j = 1, 2, 3, and a hgher value of j ndcates hgh prorty) s the arbtrary nter frame space accordng to the 802.11e [11], E[H] s the expected number of nterruptons by neghbors wth hgherprorty packets n a sngle defer tme and E[T] s the expected nterrupton tme (see Appendx A for more detals). The analytcal results n Fg. 2 show the mpact of defer tme on the average medum access delay for dfferent traffc classes. As shown n the fgure, a node wth low-prorty traffc experences hgher backoff delay n the presence of hgh-prorty traffc. Further, the medum access delay ncreases as the number of hgh-prorty contendng nodes ncreases due to the ncrease of defer tme. Thus, the qualty of a lnk for a partcular traffc class s affected not only by the conventonal parameters (lke ETX, ETT), but also by the contendng hgh-prorty traffc.
252 Ann. Telecommun. (2010) 65:247 262 Fg. 2 Average medum access delay for dfferent traffc classes due to the varaton n defer tme. Each node uses the same data rate and packets of same sze Fg. 3 Impact of number of contendng nodes on the average backoff delay. Each node uses the same data rate and packets of same sze 3.2.3 Neghbor densty and loads A packet forwarded n a path wth lghtly loaded and/or a fewer numbers of contendng nodes experences less delay. However, t s not trval to obtan the nformaton about contendng neghbors. Exstng routng metrcs (for example, MIC [10]) count the number of contendng nodes and cannot dfferentate between neghbors and contendng nodes. Ths s because a neghbor s queue mght be empty, and hence, such a node wll not contend for the lnk. Further, the loads of the neghbors are updated by perodc explct messages [12], whch ncur a huge control overhead. We use a novel method to mplctly measure the number of contendng nodes and ther loads. In CSMA/CA networks, nodes go through a backoff procedure before accessng the medum. A node s forced to freeze ts backoff counter upon hearng a transmsson from the neghbors. The number of nterruptons n a transmsson attempt depends on the number of contendng nodes and the probablty at whch the nodes attempt to access the medum, where the load of a contendng node determnes the probablty at whch t accesses the medum. A node experences more medum-access delay f t has a dense neghborhood wth heavly loaded nodes. The expected backoff delay n the -th transmsson attempt, E[d j b ()], canbe calculated as an nterrupton. Therefore, the expected backoff delay, E[d j b ], can be calculated as M E[d j b ]= p s 1 (1 p s ).E[d j 1 (1 p s ) M b ()], (9) =0 where p s s the probablty of success of a transmsson. Appendx B detals the dervaton of Eqs. 8 and 9. Fgure 3 shows that the average backoff delay of a node ncreases wth an ncrease n the number of contendng nodes, because an ncreased number of contendng nodes results n more nterruptons. In contrast, a better success rate requres fewer retransmssons, and hence, the backoff delay decreases wth ncreasng success rates. Fgure 4 shows the average backoff delay of a node wth dfferent offered loads. As the load of a neghbor of a node ncreases, the neghbor attempts to access the medum wth hgher probablty, and hence, the backoff E[d j j b ()] =E[w ] d s + E[b j ] (d b + d j d ), (8) where E[w j ] s the expected number of dle slots for the j-th traffc class and d s s the duraton of a generc slot, E[b j ] s the expected number of nterruptons for the j-th traffc class, and d b s the expected duraton of Fg. 4 Impact of loads of contendng nodes on the average backoff delay. Each node uses the same data rate and packets of same sze
Ann. Telecommun. (2010) 65:247 262 253 delay of the node ncreases wth ncreasng neghbors loads. Therefore, a node experences less backoff delay wth lghtly loaded contendng nodes. A closer look at Eq. 8 ndcates that the backoff delay s determned domnantly by the number of nterruptons n a sngle transmsson attempt. Therefore, the expected number of nterruptons mplctly determnes the densty and load of the neghborhood of a node. 3.2.4 Transmsson rate and packet sze of neghbors The medum access delay (.e., backoff delay) depends not only on the densty and loads of the contendng nodes (as mentoned n Secton 3.2.3), but also on the transmsson tme (.e., the busy tme n Eq. 8) of each nterrupton. To the best of our knowledge, none of the exstng routng metrcs address ths ssue. The transmsson tme of a neghbor (.e., the duraton of an nterrupton) depends on the channel qualty and the packet sze of the contendng nodes. If a contendng node experences bad channel qualty (and, hence, transmts at a lower rate) or transmts a very large packet, the transmttng node mght have to wat for a longer perod of tme n a freezng state. However, the avalablty of neghbors transmsson rates and packet sze at any node requres perodc control packet exchange among neghbors, whch ncurs control overheads and mght not be feasble for a rapdly changng wreless envronment. Fgure 5 shows the mpact of the transmsson rate and packet sze of the neghborng nodes on the backoff delay of a transmttng node. The medum-access delay of a node ncreases f ts neghbors transmt at a lower rate or they transmt larger data packets. The reason for ths s that a node needs to stay n the freezng Fg. 5 Impact of transmsson rate and packet sze of the contendng nodes on the average backoff delay. Number of contendng nodes and ther transmsson probabltes are fxed state for longer perods n both cases. Ths strongly suggests that the neghbors transmsson rates and packet sze should be ncluded n the routng metrc; these factors are largely gnored n exstng routng metrcs. Therefore, we nclude the expected freezng tme of an nterrupton n our routng metrc. 3.2.5 Queueng delay of forwardng nodes The queung delay s the amount of tme a packet spends watng n the transmtter s queue before t gets the chance of transmsson. Therefore, f a packet s scheduled to forward through a node that already has enough packets n the transmsson queue, t wll have to wat untl other packets n the queue fnsh ther transmsson successfully. If we consder only servce tme as the routng metrc, most of the packets wll be forwarded on paths wth less servce tme. Ths wll ncrease the queue sze and, eventually, the queung delay of a packet. However, f the same packet s transmtted through a lghtly loaded node, t wll experence less queung delay, and the end-to-end delay wll decrease. Therefore, traffc should be splt n such a manner that load-balancng can be acheved n the forwardng nodes. Ths motvates us to ncorporate the queung delay of a packet of a partcular traffc class n our proposed routng metrc. 3.3 EFT: the proposed metrc In ths secton, we explan the proposed routng metrc. The routng metrc assgns a weght value, whch represents the requred EFT of a packet to successfully delver t from a node to the next hop. In Secton 3.2,we already explored the parameters that ncur delays to a packet n a node. The EFT of a packet s the sum of these delays. The assocated delays are narrated brefly n the followng: The transmsson tme of a packet n a sngle transmsson attempt s the duraton wheren the packet uses the wreless medum. The transmsson tme depends on the data rate and the packet sze. The dle tme s the duraton of the backoff slots that a node requres to access the wreless medum. Ths also ncreases wth the ncreasng number of retransmssons. The watng tme due to traffc prorty s represented by the defer tme and s the duratons at the begnnng of a transmsson and before resumng the backoff after each nterrupton. The freezng tme represents the contendng neghbors and ther loads. The freezng tme n each
254 Ann. Telecommun. (2010) 65:247 262 nterrupton s the sum of the defer tmes and the duraton of the nterruptons (.e., the duraton of the transmsson of the neghbor). The number of nterruptons (.e., transmsson by the contendng neghbors) also affects the freezng tme. The queueng delay of a packet s the tme that a packet wats n the queue before t starts any transmsson attempt. A successful transmsson of a packet mght requre a number of transmsson attempts. The EFT of a packet s the sum of the tmes that a packet requres both successful and unsuccessful transmsson attempts. Let ET j denote the expected tme requred for a packet of the j-th traffc class n the -th transmsson attempt. Thus, ET j s the sum of aforementoned delays and s gven by j E[w ET j = d j d + ] d s + k=0 E[b j ] k=0 [d b + d j d ]+d t + d j q (10a) = E[w j ] d s + E[b j ] d b + (1 + E[b j ]) d j d + d t + d q, (10b) where d t s the transmsson tme of a packet and dq j s the queueng delay of a packet of the j-th traffc class. For a successfully delvered packet, the EFT s the sum of the expected tme n each transmsson attempt. If the number of the transmsson attempts (.e., ETX) requred for a successfully delvered packet s M, the EFT of a packet of the j-th traffc class for the l-th lnk, EFT j l, s gven by E[M] EFT j l = ET j (11) =1 Combnng Eqs. 10b and 11, we have the followng: EFT j l = E[M] E[w j ] d s + E[M] E[b j }{{} ] d b + }{{} Expected Idle tme Expected freezng tme E[M] (1 + E[b j ]) d j d + E[M] d t + E[d }{{}}{{} q j }{{ ]. } Expected defer tme ETT Queue delay (12) In Eq. 12, the frst term ndcates expected duraton of the dle slots that a node needs to wat durng backoff; ths value s domnant when a node requres a large number of retransmssons. The second term ndcates two factors mentoned n Secton 3.2: (1) the number of nterruptons durng the medum access perod, whch provdes an ndcaton of the number of contendng nodes and ther loads, and (2) the channel qualty (data rate) and the packet sze of the contendng nodes. The thrd term ndcates the defer tme for a packet, and t has a sgnfcant nfluence on the flow of lower traffc classes. The fourth term ndcates the ETT metrc mentoned n [9], and fnally, the ffth term ndcates the average queueng delay of a packet of a partcular traffc class. Now, the path metrc referred to as cumulatve expected forwardng tme (CEFT) for a flow of the j-th traffc class for a path of h hops can be gven by CEFT j = h l=1 EFT j l. (13) Note that CEFT s an addtve metrc, and as the number of lnks n a path ncreases, the value of CEFT ncreases. Among multple paths between a partcular source destnaton par, the path havng the lowest CEFT wll be the best path. 3.4 Implementaton ssues 3.4.1 Estmaton of the requred parameters To calculate the value of EFT, a forwardng node needs to estmate the values of the parameters presented n Eq. 12. The value of d t depends on the selected rate of a lnk, and we assume that a rate adaptaton mechansm that selects the rate for each lnk s runnng. There are many rate adaptaton technques avalable n the lterature (for example, [13 17]). However, we opt to use the automatc rate fallback (ARF) [13] technque to estmate the data rate because of ts ease of mplementaton and wde acceptance by dfferent WLAN vendors. Therefore, we assume that each forwardng node knows the value of E[M] and d t for each of the outgong lnks. Each node measures the number of nterruptons n a sngle transmsson attempt and the duraton of the nterruptons for each traffc class. We use exponentally weghted movng average (EWMA) to estmate the expected value of the parameters gven by E[b j j j (t)] =α b (t) + (1 α) E[b (t 1)] (14a) E[d j b (t)] =β d j b (t) + (1 β) E[d j b (t 1)], (14b) where α and β are the tunng parameters to smooth the estmated value. Followng the same procedure, the average queueng delay s estmated by measurng the average queue sze and queue servce tme for each traffc class. Accordngly, each node estmates the defer tme of each traffc class.
Ann. Telecommun. (2010) 65:247 262 255 3.4.2 Incorporatng EFT n HWMP The HWMP s the default path selecton protocol defned n draft 2.02 of IEEE 802.11s. As specfed n [4] and [18], only a sngle actve routng protocol and a correspondng path selecton metrc should be used n a sngle WMN. HWMP uses the Artme metrc as the default routng metrc. Instead, we ncorporate EFT as the routng metrc to be used n HWMP. In the current draft, the metrc dentfer value of the Artme metrc s set to 0, whereas values 1 254 are reserved for future use. We therefore use the value 1 for EFT n the actve path selecton metrc dentfer feld. Ths nformaton s embedded n the mesh confguraton element that s used to advertse mesh servces. Whenever an MP establshes a lnk wth another MP, t uses peer lnk open and peer lnk confrm frames. The mesh confguraton element s contaned on these frames and also n beacon frames transmtted by the MPs. Thus, all the MPs can be notfed about the default routng protocol (HWMP) and the metrc (EFT) they are gong to use. HWMP s a blend of on-demand (reactve) and proactve routng mechansms. In the on-demand mode, a source MP broadcasts a path request (PREQ) message requestng a route to the destnaton wth the EFT feld ntalzed to zero. Note that an MP may receve multple copes of the same PREQ from a source through dfferent paths. After recevng a PREQ, an ntermedate MP creates a path to the source or updates ts current path f the PREQ contans a greater sequence number, or the sequence number s the same as n the current path and the PREQ offers a better EFT value than the current path. If a new path s created or an update occurs, the PREQ s then re-broadcasted wth an updated metrc feld that reflects the CEFT of the path to the source. The destnaton MP uncasts a path reply (PREP) message after creatng or updatng a path to the source. In contrast, the proactve tree-buldng mode can be executed n two ways to let the MPs n the WMN create a path wth the root or portal MP (MPP). Frst, n the proactve PREQ mechansm, the root MP perodcally broadcasts a proactve PREQ message wth ncreasng sequence number, destnaton address set to all 1s, and the EFT value ntalzed to zero. After recevng a proactve PREQ, each MP creates or updates ts forwardng nformaton to the root MP, updates the EFT value and hop-count of PREQ, and retransmts the updated PREQ. A proactve PREP from an MP establshes the path from the root MP to that MP. In the case of the proactve RANN mechansm, the root MP starts to perodcally broadcast a root announcement (RANN) message that propagates the metrc nformaton across the network. Upon recepton of a RANN message, an MP that wants to create or refresh a path to the root MP sends a uncast PREQ to the root MP. The root MP then uncasts a PREP n response to each PREQ. The uncast PREQ creates the reverse path from the root MP to the orgnatng MP, whle the PREP creates the forward path from the MP to the root MP. 3.5 Dscussons Some mnor detals of the proposed routng metrc are worth mentonng. Frst s the computaton cost of the metrc, whch ncludes the tme complexty and the message overhead. Each of the parameters used n the metrc contrbutes lttle computaton cost and may or may not requre message overhead. All the exstng mechansms need to determne the expected number of retransmssons and the data rate, and ths depends on the specfc algorthm used to measure the parameters. We have used ARF, whch does not need any extra message to exchange among the neghbors and only needs to count the number of retransmssons based on local data and accordngly, selects the desred data rate. The other parameters used n the metrc are locally calculated by the nodes and, thus, do not need to exchange any control message. Furthermore, each node uses a local varable to store the value of the parameters, and the computatonal cost assocated wth measurng, updatng, and averagng the values s neglgble. Second s the energy consumpton for the overheads assocated wth the metrc computaton. Snce the computatonal cost of the metrc s neglgble and t requres no extra control message (depends on the rate selecton mechansm), the metrc s assumed to be energy effcent. Furthermore, the mesh nodes are supposed to be statc and are not energy-constraned [1]. Thus, energy consumpton mght not be so crtcal for the mesh networks. Thrd s the stablty of the proposed metrc and the performance of the routng protocol. EFT s a dynamc metrc and ts value changes wth the change of the network states. When EFT s used wth an on-demand routng protocol, the changed metrc value only affects the selecton of new paths and the exstng paths reman unchanged unless any of these paths s broken. Therefore, we assume that ncorporatng EFT wth an on-demand routng protocol provdes stable network operatons. In contrast, a dynamc (or network-statusaware) metrc mght affect the stablty when used wth a proactve routng protocol. Therefore, the operaton of a proactve routng protocol wth a dynamc
256 Ann. Telecommun. (2010) 65:247 262 metrc requres careful handlng. To combat that, EFT smoothes the abrupt changes of the metrc value by usng EWMA to estmate the value of each parameter. Besdes, HWMP can ncrease the stablty by ncreasng the duraton of the route update ntervals. Moreover, we have restrcted HWMP to enforce a route change only f a new path provdes 20% mprovement n the metrc value. We have found n the smulaton that the above-mentoned methods provde route stablty for most of the smulaton runs, and the mproved results found n the smulaton justfy that. Fnally, we have valdated the performance of EFT only by smulatons. Consderng the smlartes of the EFT and Artme metrcs, we beleve that EFT can be mplemented wth HWMP. The performance of EFT n a real mplementaton wll further justfy the effectveness of the metrc, and our future plan s to mplement ths wth an enhancement of the routng protocol. 4 Performance evaluaton We have done extensve smulatons to evaluate the performance of EFT and compare t wth the well known exstng routng metrcs, namely, ETX, ETT, Artme, and MIC, usng ns-2 [19]. To measure and compare the results, we have used a mesh network wth 50 statonary mesh routers (MP) randomly deployed n an area of 1,500 1,500 m. We have used both ntramesh flows and flows that have an end-pont outsde the WMN (.e., nter-mesh flow). We have consdered a packet sze of 1,024 bytes. The transmsson range s set to 250 m and the nterference range (carrer sense range) s set to 550 m. We consder UDP to be the transport layer and we assume that all flows generate data at a constant rate. The sources of the flows are randomly chosen for both ntra- and nter-mesh traffc, whereas the destnaton s the gateway node for ntermesh traffc. The on-demand mode of HWMP s used for ntra-mesh traffc and the proactve mode s used for nter-mesh traffc. Each smulaton run has been executed 15 tmes, and the average results are plotted n the graphs. The error bars n the graphs parallel to the y-axs ndcate the varatons of the obtaned results from the presented average values and, thus, show the mnmum and maxmum values obtaned among the smulaton runs. For each smulaton run, the actve nodes are randomly selected (as long as the number of nodes s less than 50), and the source node for each of the flows s also randomly selected. Thus, the varatons n the obtaned results manly occur due to the randomness of the topology. We have consdered the followng performance metrcs: (1) average network throughput the sum of the sze of the total data packets receved by the destnatons (MPP, n the case of the outgong nter-mesh traffc) per unt tme, (2) average end-to-end delay the average delay experenced by all successfully delvered packets (for nter-mesh traffc, delay s measured only wthn the mesh network), and (3) packet loss rate ths ndcates the rato of the packets that are lost n the path to the number of packets generated by the sources. Smulaton results for dfferent network scenaros are shown to demonstrate the mpact of network sze, traffc load, varable lnk qualty, and traffc classes on the performance metrcs. 4.1 Scenaro 1: mpact of network sze In ths scenaro, we have assumed that 20 flows exst n the network wth a sendng rate of 5 pkts/s. To determne the mpact of network sze, we gradually ncrease the number of nodes n the network from ten to 50 nodes. We consder the case of a mult-rado, mult-channel envronment, where each mesh node s equpped wth two 802.11b rados and each rado operates on one of the three avalable channels. As shown n Fg. 6, EFT outperforms other exstng metrcs n terms of average network throughput wth an ncreasng number of nodes. The average network throughput acheved usng ETT and Artme metrcs are almost dentcal, as both consder the lnk wth hgher data rate as the better one and do not consder the effect of nterference. MIC, on the other hand, performs slghtly better than the aforementoned metrcs as t chooses the next hop based on the number of neghbors. EFT outperforms MIC as t consders the actual number of Fg. 6 Average network throughput for mult-rado, multchannel node wth IEEE 802.11b nterface. Each flow mantans a rate of 5 pkts/s
Ann. Telecommun. (2010) 65:247 262 257 Fg. 7 CDF of average network throughput, where CDF s defned as the probablty that throughput s less than or equal to a gven value nterferng nodes and ther loads by ncorporatng the total forwardng delay. As the number of nodes and ther loads ncrease, the EFT values of nodes wth more contendng nodes ncrease, whch allows EFT to choose paths around less-congested areas. Ths also ndcates that EFT acheves load-balancng by makng effcent use of the medum. We also analyze the smulaton results usng the help of cumulatve dstrbuton functon (CDF), where CDF plots for throughput n Fg. 7 show that, for EFT, the throughput s above 4.5 Mbps 80% of the tme, whereas for MIC and ETT, the throughput s hgher than 2.8 and 2 Mbps, respectvely, 80% of the tme. Fgure 8 demonstrates that EFT outperforms all the other metrcs n terms of average end-to-end delay. Unlke the exstng metrcs, where n most cases only the transmsson tme of a packet s consdered, EFT ncludes every possble delay n a node. Therefore, the end-to-end delays of the packets n EFT are mnmal Fg. 9 CDF of end-to-end delay, where CDF s defned as the probablty that delay s less than or equal to a gven value compared to the other metrcs. The CDF plot n Fg. 9 also shows that, for 90% of the tme, the end-to-end delay for EFT s less than 50 ms, whereas t s much hgher for ETX, ETT, Artme, and MIC metrcs. Fgure 10 shows that the average packet loss rate acheved usng EFT s lower than that of other metrcs. In the ntal stage, when the number of nodes s less (.e., network connectvty s low), packet loss rates are hgher for most of the metrcs as the number of avalable paths s less. As the number of nodes ncreases, avalable paths also ncrease and packet loss rates tend to decrease. However, due to the effcent dstrbuton of the traffc and the fact that EFT prefers paths through less congested network areas, t acheves better results n terms of packet loss rate. 4.2 Scenaro 2: mpact of traffc load In ths scenaro, we have assumed that the network sze s fxed and there are a fxed number of flows n Fg. 8 Average end-to-end delay for 20 actve flows wth varyng numbers of nodes Fg. 10 Average packet loss rate for varyng numbers of nodes wth 20 flows
258 Ann. Telecommun. (2010) 65:247 262 Fg. 11 Average network throughput for dfferent traffc loads of the flows Fg. 13 Average packet loss rate wth ncreasng loads of the actve flows the network. There are 50 nodes n the network, and 20 flows are generatng data wth randomly selected sources and destnatons. We gradually ncrease the data generaton rates of the flows from 5 to 30 pkts/s to measure the mpact of traffc load on the routng metrcs. Note that the nodes are operated n mult-rado and mult-channel mode. Fgures 11, 12, and13 show the average network throughput, end-to-end delay, and packet loss rates, respectvely, for dfferent metrcs under varous network loads. Most exstng routng metrcs only consder lnk qualty, and thus, the network throughput does not ncrease at the expected rate wth ncreasng loads. As shown n Fg. 11, for ETX, ETT, and Artme metrcs, the network throughput ncreases very slowly wth ncreasng loads, because all these metrcs forward the packets toward the best path. In contrast, MIC selects the path based on the number of neghbors of Fg. 12 Average end-to-end delay wth ncreasng loads of the actve flows the forwardng node and, hence, acheves a slghtly better network throughput than others. However, MIC cannot dfferentate a neghbor and a contendng node and, therefore, cannot estmate the ultmate load of the neghbors. On the contrary, EFT selects paths consderng the number of contendng nodes and ther loads (.e., the expected number of nterruptons n each transmsson attempt) and, therefore, outperforms all the exstng metrcs n terms of acheved throughput. Note that EFT balances the loads of the network by drectng the traffc toward the lghtly loaded zone of the network, and hence, t spreads the traffc over the network. Fgure 12 shows the average end-to-end delays for dfferent metrcs. EFT performs better n terms of average end-to-end delay than ETX, ETT, Artme, and MIC. Because ETT and Artme only consder the data rates of lnks and prefer lnks that have hgher data rates, both these metrcs tend to forward all packets to the same path, whch results n network congeston. Also, MIC does not balance the traffc load over the network nodes and, thus, creates congested regons. Therefore, both queung delay and medum access delay are greater when these routng metrcs are used, whch have negatve effects on the average end-to-end delay. In contrast, the average end-to-end delay for a packet that uses EFT s less, as ths metrc chooses paths wth less medum access, transmsson, and queung delays (.e., consders all the factors that mpact the forwardng tme of a packet). Fgure 13 shows the average packet loss rate of the network wth ncreasng traffc loads. In general, as the traffc load ncreases, the packet loss rate of all the metrcs tends to ncrease. Due to ther nablty to address load balancng, most of the packets tend to
Ann. Telecommun. (2010) 65:247 262 259 choose paths wth hgh data rate when ETT, Artme, or MIC metrcs are used. Ths results n buffer drops n the ntermedate nodes. Moreover, as ETT and Artme do not address the effects of nterference, packets are also lost due to nterference or collson from contendng nodes. In contrast, EFT prefers paths n less congested regons of the network, and packets experence only low to medum contenton. Therefore, the packet loss rate when usng EFT s lower than that when usng the other metrcs. 4.3 Scenaro 3: mpact of lnk qualty We consder a smple scenaro n Fg. 14 to show that the qualty of a lnk depends not only on the transmttng node s data rate, but also on the data rates of the nodes n ts neghborhood; a lower data rate contrbutes negatvely to the overall throughput. As shown n the fgure, nodes A, B, D, E, andf transmt data at a rate of 54 Mbps, whereas node C transmts data at 48 Mbps and G uses the lowest rate of 6 Mbps. We assume that all statons are tuned to a sngle channel and that the transmttng power of nodes s reduced so that nodes can hear transmsson only from ther one-hop neghbors. There are three exstng UDP flows from nodes G F, A C, andb C. We start a new flow from node D E. Based on ther propertes, the routng metrcs mght choose any of the paths from D C E and D F E. Fgures 15 and 16 show the average throughput and end-to-end delay acheved by the new flow, respectvely. It s evdent that EFT acheves a sgnfcantly hgher throughput than ETX, ETT, Artme, and MIC, as EFT always chooses the best throughput path D C E. Ths s because the transmsson from G to F at a lower data rate keeps node F busy for a longer perod of tme and the medum access tme n node Fg. 15 Average throughput of a new flow for dfferent routng metrcs n scenaro 3 F ncreases. Ths makes path D F E a lower throughput path than D C E; by ncorporatng the transmsson rate and packet sze of the contendng nodes (.e., the expected busy perod of the contendng nodes), EFT dentfes ths fact. However, because of the hgh data rate, ETT and Artme wll more often choose the low-throughput path D F E. Based on the packet loss probablty of the lnks, ETX mght swtch between the two avalable paths. Interestngly, MIC n ths scenaro acheves a lower throughput as t always prefers D F E as the better path because node F has fewer nterferng neghbors than node C. For all these reasons, the average end-to-end delay acheved by EFT s also lower than that acheved usng the other metrcs. Fgure 16 shows that the average endto-end delay of EFT s 62% lower than that of ETX, 54 mbps 54 mbps A D 48 mbps 54 mbps 6 mbps C F G 54 mbps B 54 mbps E Fg. 14 Impact of contendng nodes data rates and packet szes n path selecton of a new flow Fg. 16 Average end-to-end delay of a new flow for dfferent routng metrcs n scenaro 3
260 Ann. Telecommun. (2010) 65:247 262 prortes, paths are not selected for partcular classes. Therefore, low-prorty traffc starves n the MAC layer for gettng access to the medum. Furthermore, the lowprorty traffc contends wth the hgh-prorty traffc and reduces the throughput and ncreases the delay of the hgh-prorty traffc. In contrast, EFT forwards the traffc by consderng the best path for a partcular traffc class. Thus, for most cases, the low-prorty packets avod the paths used by the hgh-prorty packets, resultng n shorter delays and hgh throughput. Ths also reduces the loads and contentons of the hghprorty traffc, and they acheve better performance. Fg. 17 Average throughput of flows wth dfferent traffc prortes 59% lower than that of ETT, 56% lower than that of Artme, and 65% lower than that of MIC. 4.4 Scenaro 4: mpact of traffc prorty We now nvestgate the mpact of traffc prorty on path selecton. We make use of a network comprsng 50 nodes and 20 flows. Among the flows, ten flows are hgh-prorty flows whle the remanng ten flows are low-prorty flows. Fgures 17 and 18 show the average throughput and end-to-end delay acheved by the dfferent traffc classes usng dfferent metrcs, respectvely. As shown n the fgures, the hgh-prorty traffc not only acheves better performance usng EFT than other metrcs, but the low-prorty traffc also acheves a very hgh throughput and low delay. Because the exstng metrcs forward traffc wthout consderng ther 5 Conclusons In ths paper, we have presented a new routng metrc, EFT, that addresses all factors (transmsson rate, success rate, contentng neghbors and ther loads, loadawareness, and traffc prorty) that affect the forwardng tme of a packet n a node. EFT chooses the path that has mnmum end-to-end delay, whch, along wth transmsson delay, also ncludes medum access and queung delays. EFT can capture the effect of the traffc loads of neghbors and chooses paths through less congested areas of the network and balances traffc loads among network nodes. The EFT metrc treats packets of dfferent traffc classes dfferently and selects paths sutable for the partcular traffc class. The proposed metrc s ncorporated wth IEEE 802.11s s HWMP routng protocol, and smulaton results demonstrate that t performs sgnfcantly better than the exstng routng metrcs. Acknowledgement Ths work was supported by the IT R&D program of MKE/IITA [2009-F-016-02, CASFI]. Dr. Choong Seon Hong s the correspondng author. Appendx A. Defer tme wth traffc prorty Fg. 18 Average end-to-end delay of flows wth dfferent traffc prortes For smplcty of analyss, we consder only three classes of traffc, and accordng to [11], for j = 2, 3, defer tme (.e., AIFS[j]) s equal to the duraton of DIFS and s constant. However, for low-prorty traffc (.e., j = 1), AIFS[1] =DIFS + d s. Because a node wth hghprorty traffc can ntate a transmsson durng the extra slot of the low-prorty traffc, a node wth lowprorty traffc cannot defreeze ts backoff counter. Thus, d 1 d depends on the number of nodes assocated wth the hgher-prorty traffc and the probablty at