Anypath Routing for Reducing Latency in Multi-Channel Wireless Mesh Networks

Andreas Lavén

LICENTIATE THESIS | Karlstad University Studies | 2013:45

Computer Science

Faculty of Health, Science and Technology

LICENTIATE THESIS | Karlstad University Studies | 2013:45

Anypath Routing for Reducing Latency in Multi-Channel Wireless Mesh Networks

Andreas Lavén

urn:nbn:se:kau:diva-29359

Distribution:Karlstad University Faculty of Health, Science and TechnologyDepartment of Mathematics and Computer ScienceSE-651 88 Karlstad, Sweden+46 54 700 10 00

© The author

ISBN 978-91-7063-522-9

Print: Universitetstryckeriet, Karlstad 2013

ISSN 1403-8099

Karlstad University Studies | 2013:45

LICENTIATE THESIS

Andreas Lavén

Anypath Routing for Reducing Latency in Multi-Channel Wireless Mesh Networks

WWW.KAU.SE

iii

Anypath Routing for Reducing Latency in Multi-Channel Wireless Mesh NetworksANDREAS LAVÉNDepartment of Mathematics and Computer Science

AbstractIncreasing capacity in wireless mesh networks can be achieved by using multi-ple channels and radios. By using different channels, two nodes can send pack-ets at the same time without interfering with each other. To utilize diversityof available frequency, a channel assignment scheme is required. Hybrid chan-nel assignment is an interesting approach where at least one radio is tuned to afixed channel for receiving and the remaining interfaces switch their channelsdynamically in order to match the receiving channel at the receiving node.This provides full connectivity, but at the expense of introduced switchingcosts. Due to hardware limitations it is too costly to switch channels on a perpacket basis.

Instead, this thesis proposes an anypath routing and forwarding mecha-nism in order to allow each node along the route to select the best next hopneighbor on a per packet basis. The routing algorithm finds for each desti-nation a set of next hop candidates and the forwarding algorithm considersthe state of the channel switch operation when selecting a next hop candidate.Also, in order to allow latency-sensitive packets to be transmitted before otherpackets, latency-awareness has been introduced to distinguish e.g. VoIP flowsfrom FTP traffic.

The ideas have been implemented and tested using real-world experiments,and the results show a significant reduction in latency.

Keywords: Wireless Mesh Networks, Anypath Routing, Forwarding, VoIP

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AcknowledgementsFirst, I would like to thank my two supervisors, Andreas Kassler and AnnaBrunstrom, for giving me this opportunity and for all their advices along thejourney. Second, I would like to thank Peter Dely and Marcel Cavalcanti deCastro for all the help that I was given in the beginning of my research. Fi-nally, I would like to thank all my remaining friends from the time at Karlstaduniversity.

Linköping, September 19, 2013 Andreas Lavén

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ContentsList of Appended Papers ix

INTRODUCTORY SUMMARY 1

1 Introduction 3

2 Background 32.1 Wireless Mesh Networks (WMNs) . . . . . . . . . . . . . . . . . . 4

2.1.1 Application Areas . . . . . . . . . . . . . . . . . . . . . . . . 42.1.2 Multi-Channel Multi-Radio WMNs . . . . . . . . . . . . 5

2.2 Routing in WMNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.2.1 Routing Protocols . . . . . . . . . . . . . . . . . . . . . . . . 62.2.2 Routing Metrics . . . . . . . . . . . . . . . . . . . . . . . . . 82.2.3 The Anypath Paradigm . . . . . . . . . . . . . . . . . . . . . 11

3 Challenges, Research Questions and Contributions 113.1 Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.2 Research Questions and Contributions . . . . . . . . . . . . . . . . 12

4 Research Methodology 14

5 Summary of Appended Papers 14

6 Conclusions 16

PAPER IMulti-Channel Anypath Routing in Wireless Mesh Net-works 19

1 Introduction 21

2 Multi-Channel Anypath Routing 232.1 Routing and Forwarding in Hybrid Mesh Networks . . . . . . . 232.2 Routing Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.3 Next Hop Candidate Selection and Forwarding Algorithm . . 272.4 Implementation Details . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3 Evaluation 283.1 Scenario I: Small topology with extra hop . . . . . . . . . . . . . . 303.2 Scenario II: Impact of Path Diversity . . . . . . . . . . . . . . . . . 30

4 Conclusions 32

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PAPER IIPerformance Evaluation of the Anypath Routing and For-warding Mechanism AP-OLSR 34

1 Introduction 37

2 AP-OLSR 392.1 Anypath Routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392.2 Anypath Forwarding . . . . . . . . . . . . . . . . . . . . . . . . . . . 412.3 Implementation Details . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3 Evaluation 423.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.2 Single Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.3 1 Gateway, 3 Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.4 2 Gateways, 3 Flows Each . . . . . . . . . . . . . . . . . . . . . . . . 493.5 Crossing Traffic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4 Conclusions 53

PAPER IIILatency Aware Anypath Routing and Channel Schedul-ing for Multi-Radio Wireless Mesh Networks 55

1 Introduction 57

2 Design and Implementation 592.1 Channel Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . 592.2 Routing and Forwarding . . . . . . . . . . . . . . . . . . . . . . . . . 592.3 Queuing and Channel Scheduling . . . . . . . . . . . . . . . . . . . 602.4 Implementation Details . . . . . . . . . . . . . . . . . . . . . . . . . . 60

3 Evaluation 603.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613.2 Scenario 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623.3 Scenario 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 633.4 Scenario 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

4 Conclusions 66

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List of Appended PapersI. Andreas Lavén and Andreas J. Kassler. Multi-Channel Anypath Rout-

ing in Wireless Mesh Networks. In IEEE Globecom 2010 Workshopon Heterogeneous, Multi-hop Wireless and Mobile Networks, Miami,FL, USA, 2010.

II. Andreas Lavén, Andreas J. Kassler and Anna Brunstrom. PerformanceEvaluation of the Anypath Routing and Forwarding Mechanism AP-OLSR. In The 16th ACM/IEEE International Conference on Model-ing, Analysis and Simulation of Wireless and Mobile Systems (MSWiM’13), Barcelona, Spain, 2013.

III. Andreas Lavén, Andreas J. Kassler and Anna Brunstrom. Latency AwareAnypath Routing and Channel Scheduling for Multi-Radio WirelessMesh Networks. Under submission.

Comments on my ParticipationPaper I I was responsible for design, implementation and the experimentalevaluation. Also, I am the main author.

Paper II I was responsible for design, implementation and the experimentalevaluation. Also, I am the main author.

Paper III I was responsible for design, implementation and the experimen-tal evaluation. Also, I am the main author.

Introductory Summary

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1 IntroductionWireless mesh networks (WMNs) have been proposed as a mechanism to pro-vide alternative broadband wireless Internet access. WMNs consist of severalsmall routers relaying packets wirelessly towards the destination or the Inter-net. The wireless deployment allows to build a dense access network at a lowcost and the mesh topology makes the network reliable [1].

In order to send information through the network, routing is required forthe nodes to detect each other and to find the best paths. Typical routing pro-tocols for WMNs use a single path to send packets from source to destination.This path is used as long as it is available and shows a good enough metric. Asa result, short term variations on link quality are not considered.

The capacity of a WMN can be increased by equipping the nodes withmultiple radios [2]. This allows simultaneous transmissions by using diversefrequency bands. A channel assignment is required to determine which chan-nel to be tuned to each radio. Hybrid channel assignment is an interesting ap-proach where at least one radio is tuned to a fixed channel for receiving and theremaining interfaces switch their channels dynamically in order to match thereceiving channel at the receiving node [3]. This avoids multi-channel deaf-ness problems, but imposes large and variable delay due to the accumulatedswitching latency over multiple hops. If the required channel for a packet tobe transmitted is not tuned, then the packet has to wait in a queue until thechannel switch has been performed.

A large packet delay reduces the performance greatly for latency-sensitivetraffic. An example of latency-sensitive traffic is Voice over IP (VoIP), whichis widely utilized among users in today’s Internet. Reducing the latency isimportant for WMNs to be a part of the future wireless Internet.

This thesis investigates how anypath routing can reduce the latency in hy-brid multi-radio multi-channel WMNs. A routing mechanism following theanypath paradigm, where each node along the path selects the next hop on aper packet basis, has been designed to reduce the switching latency. In compar-ison to single path routing, anypath routing allows the actual channel switch-ing state to be considered. Also, in order to allow latency-sensitive packets tobe transmitted before other packets, latency-awareness has been introduced todistinguish e.g. VoIP flows from FTP. The ideas have been implemented andtested using real-world experiments. The results show a significant reductionin end-to-end latency in hybrid WMNs.

2 BackgroundThis section explains fundamental elements of this thesis. First, WMNs willbe described in order to understand the environment of this thesis. Second,routing in WMNs will be introduced by explaining different routing approach-es, link cost metrics and the anypath paradigm.

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2.1 Wireless Mesh Networks (WMNs)This thesis strives to decrease the latency in multi-channel multi-radio WMNs.WMNs are composed of radio nodes that form a mesh topology. Thanks to itswireless deployment a dense access network can be set up at a low cost, bothin terms of time and money. WMNs can be used to provide Internet accessby connecting at least one of the nodes to the Internet. Due to their low cost,WMNs are considered as an interesting architectural candidate for the futurewireless Internet.

WMNs are dynamically self-organized and self-configured, meaning thatWMN nodes automatically establish and maintain connectivity among them-selves. Also, thanks to these capabilities, a WMN is easy to expand by deploy-ing new nodes whenever needed. The size of a WMN may vary from a fewnodes to the coverage of an entire city.

The nodes in a WMN relay packets among each other using wireless links.Usually, the nodes operate in unlicensed bands such as the ISM band at 2.4 GHzand the U-NII at 5 GHz. This means that interference is common, both fromexternal WLAN devices but also from other appliances such as microwaveovens [4]. In order to further improve the flexibility of a WMN, a mesh nodecan be equipped with multiple wireless interfaces. These wireless interfacesmay be built on the same or different wireless access technologies.

The multi-hop mesh environment requires a routing protocol in order todetermine how to relay the packets towards their destinations. The mini-mum hop-count is typically used as a performance metric in wired networks.However, minimum hop-count is a poor choice in WMNs due to the varyingquality of the links. Instead, link quality, link capacity and channel diversityare beneficial to consider.

Another difference from wired networks is the restricted capacity of aWMN due to interference (both internal and external), the number of avail-able frequencies, nodes employing power control et cetera. In order to in-crease the capacity of a WMN, multiple channels can be used to allow simulta-neous transmissions on different radios. Also performance in terms of latencyis reduced over wireless links due to less bandwidth.

Although the challenges of WMNs are well known and have been inves-tigated since a long time, the area is still under research and development. In2009, IEEE 802.11n was released. This includes MIMO, which uses multipleantennas to coherently resolve information allowing higher data rates. An-other interesting idea is described in [5], where Cipriano et al implementsthe HARQ process from 3GPP LTE in order to reduce the average numberof transmissions in WMNs. However, the focus in this thesis lies on anypathrouting which is exploited in a layer 2.5 forwarding algorithm. Hence, theproposed design can be applied on all lower layer technologies.

2.1.1 Application Areas

Due to the low cost, rapid deployment and flexible structure, WMNs haveseveral application areas, including:

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* Broadband home networking: Many houses have dead zones, i.e. areaswith no service coverage. Mesh routers can be deployed in order toeliminate this.

* Enterprise networking: Enterprises can gain much from using WMNs.In addition to a lower cost compared to wired access points, WMNsalso improve robustness and allow an easy expansion. WMNs can alsobe preferable in order to avoid damage to the buildings.

* Metropolitan area networks: WMNs are used by several municipalitiesand user communities. An example is FunkFeuer [6], an experimentalnetwork that provides free Internet access in several cities in Austria.

* Emergency Network: Thanks to its rapid deployment, WMNs can beused by military forces or emergency services in field operations.

2.1.2 Multi-Channel Multi-Radio WMNs

A single-radio network is one where each node has one radio only. Each radiomust be tuned to the same channel in order for the network to be connected.A network is connected only if all nodes can reach one another, possibly overmultiple hops. Because all radios transmit on the same channel, radios withinrange may not transmit simultaneously. Otherwise, the signals will interferewith each other and the receiving nodes will not be able to decode. This iscalled a collision. As a result, a retransmission is required which results inthroughput degradation.

By equipping the nodes with multiple radios the capacity increases as mul-tiple channels can be used to allow simultaneous transmissions [2]. In multi-radio multi-channel WMNs a channel assignment protocol is important to de-termine which channel to be assigned to each radio. The channel assignmenthas a great impact on the performance, such as the throughput. In additionto attempting to maximize the performance of the network, a channel assign-ment algorithm must also maintain the connectivity.

Channel assignments are classified as static, dynamic or hybrid. Static ap-proaches assign channels for permanent use, or for a long duration of timesuch as minutes or hours. The latter is sometimes referred to as semi-dynamicchannel assignment. In dynamic approaches channels are assigned more fre-quently, giving the ability to cope with external interference [7] and changesin traffic demand [8]. Hybrid channel assignment is an interesting approachwhere both static and dynamic assignment are used, on different interfaces. Inthis approach, multiple channels can be used, yet allowing full connectivity.Full connectivity means that all nodes, within range, can communicate witheach other. This avoids multi-channel deafness problems, meaning that twonodes cannot communicate with each other due to no common channel.

Net-X Net-X [3] is an example of a hybrid channel assignment scheme. Inorder to achieve full connectivity, Net-X uses multiple radios of which at leastone is tuned to a fixed channel for receiving. The remaining interfaces switch

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their channels dynamically in order to match the receiving channel at the re-ceiving node. For each transmitting interface, Net-X maintains a separate setof channel queues.

However, due to hardware limitations it is too costly to switch channels ona per packet basis. A queuing component inserts each packet into the appro-priate channel queue and a round-robin scheduler is run on each interface toselect the next channel to serve. Once an interface is tuned to a specific chan-nel, it will stay there for some time (e.g. 20 ms). This introduces a switchingcost when packets are to be transmitted on a different channel. With morechannels available, this may lead to excessive latency which has a negative im-pact on e.g. VoIP performance. Reducing this switching cost over multiplehop paths is the main objective of this thesis.

2.2 Routing in WMNsRouting is about detecting nodes and finding the best path between them.Compared to wired networks, routing in WMNs must cope with unstruc-tured networks, dynamically changing topologies and an unreliable medium.In this subsection, different strategies and routing metrics will be discussedand the anypath routing paradigm will be explained. Anypath routing uses aset of next hops, utilized in this thesis to minimize the switching cost intro-duced by the hybrid channel assignment.

2.2.1 Routing Protocols

A routing protocol specifies how the routers detect each other and what in-formation to disseminate. This information is typically used in a routing al-gorithm to select the best path between the nodes.

Routing protocols are typically classified as proactive or reactive. A proac-tive routing protocol finds a path to each node in the network so that whena packet is to be sent the forwarding neighbor is already known. The nega-tive aspect of being proactive is that power and network resources are usedeven for possibly unused paths. A reactive routing protocol finds a path ondemand by flooding the network with route request messages. This leads tohigh latency in route finding and excessive flooding may be a problem.

OLSR Optimized Link State Routing protocol (OLSR) [9] is a proactiverouting protocol, which means the connection setup delay is minimized atthe expense of the heavier control traffic load. OLSR uses Hello messages fordetection and Topology Control messages to disseminate link state informa-tion throughout the network.

A key concept of OLSR is multi-point relays (MPRs). MPRs are selectedto forwards broadcast messages during the flooding process. Compared toclassical flooding mechanism, where every node retransmits each message, thistechnique substantially reduces the message overhead.

Criticism has been raised because OLSR does not include sensing of linkquality, a link is assumed to be up if a number of Hello messages has been

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received recently. Also, power and network resources are used to propagateinformation on possibly unused routes.

Multimetric OLSR Multimetric OLSR [10] is an extension of Olsrd [11]to allow nodes to distribute information on several metrics for a link, such asa link quality metric and which channel that is used.

AODV Ad hoc On-Demand Distance Vector (AODV) [12] is an example ofa reactive routing protocol. When a node has a packet to send to a destinationit broadcasts a route request message, which is forwarded by the other nodes.All nodes remember from which neighbor the route request was forwardedin order to give the route information when a reply is received. A reply issent backwards when the route request reaches a node that has a route to thedestination. This is either the destination or a node that has a route to thedestination in its route cache. If several replies reach the node which generatedthe route request, the node uses the route that has the least number of hops.Each route request has a sequence number in order to not repeat route requestswhich have already been forwarded.

The advantages of AODV are that routes are established on demand andno extra traffic is created for communication along existing links. Also, dis-tance vector routing does not require much memory or calculation. However,since the routes are found on demand, more time is required to establish a con-nection.

B.A.T.M.A.N. Better Approach To Mobile Adhoc Networking (B.A.T.M.A.N.) is an interesting strategy for routing in large WMNs. The key ideaseliminate the need of total knowledge of the network in every node, while stillbeing proactive. Instead of each node having information on each link to usewhen running its routing algorithm, B.A.T.M.A.N. uses originator-messageswhich are disseminated through the network. The originator-messages con-tain information on the source and a sequence number. When a node re-ceives an originator-message it remembers which neighbor it got it from. Ifa node receives originator-messages from a source node via several neighbors,the number of received messages is used to measure the quality of the path.Also, the sequence number of the message can be used to determine whichpath is fastest.

An important aspect of B.A.T.M.A.N. is that the originator-messages tra-verse the network in the opposite direction from a packet sent towards thesource. Hence, it is not sure that the neighbor which delivered the mostoriginator-messages will be the best forwarder. For example, when using Net-X (see Section 2.1.2) a link most often has different channels for the differentdirections.

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2.2.2 Routing Metrics

Routing metrics are used to determine the best path. A routing metric consistsof a value representing a property of a route. Typically, the routing metricvalue of each link is aggregated and the route with the lowest total is selectedas the best route.

Hop count Hop count is a simple metric where the number of links on apath is counted. However, hop count is unlikely to provide the best perfor-mance in a wireless network where quality of links may vary heavily. Also,when trying to achieve the minimized number of hops the distance of eachhop maximizes. This is likely to lead to minimized signal strength and maxi-mized packet loss ratio [13].

ETX Expected Transmission Count (ETX) [13] considers the quality of alink. Instead of counting each link as 1, the ETX value represents the numberof times a packet, on average, is required to be transmitted until it is received.A retransmission is required when a packet is lost, e.g. due to interferencefrom another transmission. By using ETX as routing metric, the packet willbe transmitted on the path that is believed to need the minimum number oftransmissions.

The ETX value is calculated using the following equation.

ET X =1

d f ∗ dr(1)

where d f is the forward delivery ratio and dr is the reverse delivery ratio,both measured using dedicated link probe packets. The product of d f anddr is the expected probability that a transmission is successfully received andacknowledged.

ETT Expected Transmission Time (ETT) [14] is based on ETX, but consid-ers also the capacity of the links. The capacity denotes the quality and the bitrate of the link, that is how many packets that can be received within a timespan.

The capacity is measured using packet-pair technique. Two probes, onesmall followed by a larger, are sent back-to-back on the given link and theinter-arrival time is measured by the receiving node. The small packet is sentonly to start a timer at the receiving node, which makes only the size of thelarger packet relevant. The time consumed to transmit the larger packet is re-ported back to the probing node and the bit rate is estimated by the followingequation.

B = SL/ min1¶i¶n

di (2)

where B is the link capacity, SL is the size of the large probe packet, di is adelay sample and n is a predefined number of delay samples.

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Once the bit rate is known, the ETT value can be calculated using thefollowing equation.

ET T = ET X ∗ S/B (3)

where S is a generic packet size, normally the one used for probing, measuredin bytes. B is the link capacity, which is measured using packet-pair technique.ET X is the expected transmission count.

The main benefit of the ETT metric is that it provides information on thecapacity of the links. In a wireless network, where the quality often variesbetween different links, this information can be very beneficial when select-ing routes. On the other hand, probing means more consumption of powerand network resources. However, the ETT value can be calculated using apredefined bit rate as well. This will give only an estimated value, and cannothandle dynamically changing link capacities.

Channel Switching Cost The channel switching cost metric [15]measuresthe increase in delay a packet experiences on account of channel switching.When using a hybrid channel assignment, such as Net-X (see section 2.1.2),the channel switching time has a great impact on the latency. The channelswitching cost metric estimates how long time it will take until the requiredchannel is tuned by calculating the probability for a channel switch. In casethe node is able to transmit the packet on a fixed interface the channel switch-ing cost is zero.

The probability that a channel switch will be required for a channel k isestimated as follows.

Ps (k) =

∑

i∈C ,i 6=k Ui

1000000(4)

where i is a channel, k is the channel to switch to, C is the set of availablechannels and Ui is the number of microseconds the channel i has been usedthe last second. The given probability is then used to calculate the switchingcost.

C = Ps (k) ∗ d (5)

where C is the switching cost, Ps (k) is the probability that a channelswitch is required and d is the delay of a switch, which depends on hardwareand the configured time for which an interface stays at a channel once tuned.

The benefit of this metric is that it captures the time consumed while apacket is waiting for a given channel to be tuned. However, the switchingcost metric alone is not sufficient. Instead, the switching cost value must becombined with another metric which increases the complexity.

Multi-Channel Routing (MCR) MCR [15] is a path metric proposed formulti-channel wireless networks. MCR combines the ETT metric and theswitching cost into a single path cost. In addition to ETT and the switchingcost, MCR considers the number of links along the path that use a common

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channel. MCR aims to minimize the maximum number of links on any chan-nel.

MCR is calculated by the following equation.

M C R= (1−β) ∗l∑

i=1

(ET Ti + SCi )+β ∗ max1≤ j≤c

(∑

∀k ,ck= j

ET Tk ) (6)

Where β is is a weight between 0 and 1, l is the number of links on thepath, i is a link, ET Ti is the ETT cost of link i , SCi is the switching cost oflink i , j is a channel, c is the total number of available channels, k is a linkand ck is the channel on link k.

The advantage of this metric is the combination of the switching cost andthe ETT metric. However, MCR assumes that all links along a route that usea common channel interfere with each other. This means that MCR does notconsider if two links that share a common channel are located within range ornot.

Context-based Routing Like the MCR metric, context-based routing [16]considers the impact that the links have on each other. Typically, routingprotocols aggregate the cost of each link, where each cost is estimated indi-vidually. When new lower layer technologies are incorporated it becomes ap-parent that this does not capture all factors. Context-based routing allow thecost of a link to depend on previous links along the route. Unlike MCR, thecontext-based routing is a generic approach which can be used for any lowerlayer technology. In its proposal paper, context-based routing shows improve-ments in both multi-radio multi-channel networks and single-radio networksequipped with network coding.

The multi-radio approach determines the path cost by the following for-mula:

SI M (P )¬ (1−β)∑

k

(ET T (ek ))+βmaxk

E SI (ek |Pk−1), (7)

where ek is the k-th edge along the path P , β is a weighting factor between0 and 1, ET T is the expected transmission time and E SI is the estimatedservice interval at the bottleneck link. ESI reflects how fast the route can sendpackets in the absence of contending traffic, and calculated by the followingformula:

ESI (ek |Pk−1)¬ ET T (ek )+∑

j<k

p j k ET T (e j ), (8)

where P j k is a binary number that reflects whether e j and ek interfere. This isan improvement compared to MCR, but interference from other flows is notconsidered.

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2.2.3 The Anypath Paradigm

The term anypath routing was introduced to routing in 2006 by Zhong etal. [17] and Dubois-Ferrière et al. [18] based on an opportunistic routingapproach called ExOR proposed by Biswas et al. [19] in 2003. The key ideaof ExOR is to select each hop on a packet’s route after the transmission ofthat hop. This is made possible by taking advantage of the broadcast natureof wireless transmissions. The decision of a node to forward the packet ismade after learning the set of nodes which actually received the packet. Thisreduces the number of retransmissions necessary when a transmission fails inclassic wireless network routing. Therefore, ExOR allows usage of long radiolinks, which would be avoided by traditional routing due to high loss rates.However, this approach was criticized by Zhong et al. [17] due to the extentof interference caused by the transmissions of per-packet acknowledgementmessages (ACKs). Despite giving the receiving nodes reserved acknowledge-ment slots, it is not rare that the ACKs cause collisions (especially in networksunder heavy traffic loads) [20].

Instead, Zhong et al. [17] proposed an approach with a smaller set offorwarding candidates. By selecting a few good candidates the number of ACKtransmissions can be reduced. Dubois-Ferrière et al. [18] focused on findingthe optimum size of the candidate set. A large candidate set increases theprobability of at least one node receiving the packet, but it also increases theprobability of a packet diverging from the shortest-path route.

The major benefit of anypath routing is the per packet possibility of se-lecting a next hop candidate. This allows the nodes to consider frequentlychanging properties, such as the current channel at a transmitting interface.However, anypath routing requires a more complex routing algorithm in or-der to find a set of next hop candidates.

In this thesis, the anypath paradigm is used to decrease the switching costin multi-channel networks. An anypath routing algorithm is designed to findmultiple next hop candidates, each listening on a unique channel. A forward-ing algorithm is designed to select the best candidate on a per packet basis,considering the state of the channel switch operation in order to minimizethe latency.

3 Challenges, Research Questions and Contribu-tions

3.1 ChallengesA hybrid channel assignment, such as Net-X (see section 2.1.2), allows for mul-tiple channels in order to increase the capacity without multi-channel deafnessproblems. This is achieved by utilizing multiple radios. However, high delayis imposed due to channel switching, which accumulates over multiple hops.This reduces the performance greatly on latency-sensitive traffic, such as voiceover IP (VoIP).

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A:140D:100

B:36

C:64

(a) Topology

Single Path Routing

Multi-Channel Anypath Routing

10 20 30 40 50 60 70 80 90 100

Time (ms)

Sw

itch

ing

de

lay (

ms)

50

0

25

10

Channel switch

36 64 36 64 36

(b) Switching delay at node A

Figure 1: Switch delay example

The goal of this thesis is to provide low latency communication in hybridWMNs. The challenge lies in designing an anypath routing mechanism formulti-radio multi-channel WMNs to reduce the channel switching delay. Sup-pose a node A in a topology given in Fig. 1(a) is sending packets to node D .The routing protocol has selected node B as the next hop, but there is alreadyan additional flow from node A to node C . Therefore, node A needs to switchbetween channels 36 and 64. In single path routing, all packets that are tobe transmitted while the interface is tuned to channel 64 must wait. How-ever, in anypath routing they can be sent immediately using node C as thenext hop. Fig. 1(b) illustrates the switching delay for both single path routingand anypath routing. The figure shows that packets experience up to 25 msof switching delay when single path routing is used, compared to zero withanypath routing.

The design of the anypath routing mechanism involves an algorithm thatfinds multiple paths and a forwarding algorithm in order to utilize the nexthop candidates to minimize the overall delay. When the routing algorithmfinds to each destination multiple next hops listening on different channels,then the forwarding algorithm may consider the current channel in order toavoid switching delay. How this is designed is described in the following sec-tion.

3.2 Research Questions and ContributionsThese are the research questions addressed in this licentiate thesis:

Question 1: How can the anypath paradigm be utilized to reduce the channelswitching latency in WMNs with hybrid channel assignment?In Paper I, the idea of a set of forwarding candidates is used to design arouting algorithm. In order to reduce the switching latency, the algo-rithm identifies for each destination a set of multiple next hops, whichare reachable via different channels. This allows the nodes along thepath to select, on a per packet basis, the channel with the smallest cost.

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The implementation of the routing algorithm is based on OLSR (seeSection 2.2.1). In OLSR, link information is distributed throughout thenetwork. It can be configured so that each node has complete knowl-edge of the network. Hence, the routing algorithm can be modified tofind multiple paths since all links are already known by the node. InAODV, a route reply is sent back to the requesting node. If a node re-ceives a route reply from multiple neighbors it forwards the one withthe lowest cost. In anypath routing, the cost of multiple nodes may de-pend on each other. E. g. if node A finds an alternative path, via nodeB , the switching cost may be reduced and so will the total cost. If nodeB finds out it can use node A as an alternative next hop, and thereforelower its cost, these two nodes will get a lower cost for each reply. Thisleads to an excessive amount of route replies, which makes AODV apoor choice for anypath routing.

Question 2: How should a forwarding algorithm be designed in order to mini-mize the latency?

The local node has a very precise view on the state of its channel switchoperation. In Paper I, this information is used, together with the pathcost given from the routing algorithm, to estimate which path has cur-rently the smallest cost.

Also, a diversity factor α ≥ 1 is introduced to determine the maximumnumber of allowed hops. A small α will not provide many opportuni-ties for packets to reduce switching delay, but a large α value may leadto very long routes. Paper I and Paper II investigate the performance fordifferent values of α.

Using extra hops was shown to decrease the channel switching cost.However, by adding extra hops the packets will consume more trans-mission resources. This was shown to significantly reduce the perfor-mance when anypath forwarding was used on all flows in heavily loadednetworks. Therefore, latency-awareness was added in Paper III in or-der to use the anypath forwarding mechanism only for latency-sensitiveflows. In order to identify a latency-sensitive packet the type of servicefield in the IP header was used. This mechanism may also be beneficialfor TCP traffic to avoid reordering caused by anypath routing.

Thanks to the ability to rapidly change forwarding nodes on a per packetbasis, the anypath routing mechanism was shown in Paper II to give themost gain in the scenarios where flows cross one another.

Question 3: How can priority queues reduce the latency further and what is theimpact on co-existing non-latency-sensitive flows?

Paper III introduces latency-awareness to prioritize all packets from alatency-sensitive flow, such as VoIP. The latency of VoIP flows was shownto be reduced significantly by always serving the priority packets of thecurrent channel first. Also, when a channel switch is to be performed,only channels that have priority packets are considered.

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The throughput of co-existing non-latency-sensitive flows, such as anFTP flow, was shown to remain unchanged. The priority packets aresent earlier, but no extra load is introduced. Therefore, no additionaltransmission resources are used.

4 Research MethodologyIn computer science there are two common research methods; analytical andexperimental. In analytical research, hypotheses are tested by modeling prob-lems with mathematical descriptions. This gives a quick insight into the over-all behavior, but typically embrace many simplifications.

When an analytical model becomes too complex or requires too manysimplifications then experimental methods are usually used. Experimentalmethods either model the problems as simulations or perform real measure-ments. Simulation allows a controlled environment and parameters to easilybe modified; two properties that are beneficial in order to separate differenteffects. However, improper initial conditions or too many simplifications canlead to inaccurate results and therefore incorrect conclusions may be drawn.

Real-world experiments have the lowest level of abstraction since they arerun on operational systems. A drawback, however, is the difficulty of per-forming tests. With real-world experiments comes a high cost in terms ofequipment and implementation. Also, because of unpredictability and un-certainty of the environment, evaluations are very time-consuming until ahypothesis can be accepted with a high confidence.

In this thesis, the KAUMesh test bed [21]was used in order to achieve real-world results. KAUMesh is a multi-radio multi-channel network deployed atthe Karlstad University Campus. The mesh nodes are based on the CambriaGW2358-4 platform operating with Linux 2.6.35.7. The limitation lies in thenumber of nodes. There are currently 20 nodes, of which 9 are placed in amesh topology.

5 Summary of Appended Papers

Paper I – Multi-Channel Anypath Routing in Wireless MeshNetworksThis paper proposes a novel routing mechanism, called AP-OLSR, for hybridmulti-radio multi-channel mesh networks based on the anypath paradigm.The routing layer maintains for each destination at each hop a set of candi-date forwarders, where each neighbor is tuned to a different channel. Thiswas implemented by modifying the Dijkstra shortest path algorithm withinMultimetric OLSR (see section 2.2.1). The path cost is calculated by aggregat-ing the ETT cost of each link and the average switching cost of each node (seesection 2.2.2).

A layer 2.5 forwarding and scheduling algorithm decides for each packet

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individually, which actual neighbor to forward the packet to, in order to min-imize the overall delay. The algorithm adds the path cost given by the routingalgorithm with the time left until the required channel will be tuned. Thecandidate with the lowest total cost is selected.

In order to avoid switching cost, a variable α is introduced in order to pos-sibly allow additional hops. The source node multiplies the number of hopsrequired by the average best path with α and puts this value in the IP header asa maximum allowed number of hops. Each node along the path reduces thisvalue by 1, and forwards the packet to a candidate that can reach the destina-tion with the remaining hops.

The routing and forwarding mechanism was implemented and tested inthe KAUMesh test bed. The evaluation shows significant reduction in end-to-end latency due to the lower switching cost and exploitation of path diversity,even if path diversity may lead to longer routes. However, the evaluation con-tains only two scenarios to provide a proof of concept. Also, an issue in thenetwork interface card driver caused packet losses when switching channel.

Paper II – Performance Evaluation of the Anypath Routingand Forwarding Mechanism AP-OLSRPaper II presents a thorough evaluation of AP-OLSR, proposed in Paper I.The evaluation includes four different scenarios to analyze the performanceof AP-OLSR and compare it with standard single path routing. The scenariosinvolve single flows, multiple random flows to both one and two gateways,and also flows crossing each other. Each scenario is run with both light andheavy load. Also, the driver and software architecture have been changed toavoid the packet loss due to channel switches.

The results show that AP-OLSR significantly decreases the latency as longas the network is not congested and, typically, the latency is lower when manyextra hops are allowed. However, by adding extra hops the packets will con-sume more transmission resources. This extra load worsens the performancein an already congested network.

The greatest gain from AP-OLSR is given when there exist flows crossingeach other. Thanks to its ability to rapidly change forwarding nodes on a perpacket basis, AP-OLSR is very well suited for these scenarios. The limitationsof this paper lie in the evaluation which contains only one topology with fewnodes. Also, the impact of different channel assignments is not evaluated indetail.

Paper III – Latency Aware Anypath Routing and ChannelScheduling for Multi-Radio Wireless Mesh NetworksIn Paper III, latency-awareness has been added to AP-OLSR. The key ideais that latency-sensitive flows, such as VoIP, which typically have few andsmall packets are forwarded using anypath forwarding, while TCP traffic isforwarded using normal single path routing. Hence, transmission resources

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can be used more economically.In order to increase the VoIP experience further, priority queues have been

added to allow latency-sensitive packets to be transmitted before other pack-ets. Also the channel scheduling is modified. Typically, when multiple chan-nel queues have packets a round-robin scheduler determines which channel tobe served next. In the proposed priority queue mechanism, only channels thathave priority packets are considered.

The anypath forwarding and the priority queues reduce the latency indisjoint parts of the system. The anypath forwarding reduces the switchingcost and the priority queues reduces the queuing delay. Hence, the anypathforwarding and the priority queues improve the performance individually,as well as combined. An evaluation performed in the KAUMesh test beddemonstrates that the latency is significantly reduced. E.g. in one scenario theamount of arrived VoIP packets within the time limit for VoIP was increasedfrom approximately 60 % up to 100 %. However, as in the previous papers,the evaluation is limited to one topology consisting of 9 nodes only.

6 ConclusionsIn this thesis, the anypath paradigm has been investigated in order to reducethe channel switch overhead in hybrid multi-radio multi-channel WMNs. Arouting algorithm has been designed to identify for each destination at eachhop a set of candidate forwarders, where each candidate listens to a differentchannel. Also, a forwarding algorithm was designed to utilize the forwardingcandidate set in order to select, for each packet, the currently best candidate.

The forwarding algorithm was designed to allow additional hops in orderto avoid switching cost by exploiting path diversity. However, a longer pathuses additional transmission resources. In order to use them more econom-ically, latency-awareness was added to allow the anypath forwarding mecha-nism to be applied only for latency-sensitive flows.

The anypath routing and forwarding algorithms have been implemented,together with a scheduling mechanism which allow packets from latency-sensitive flows to be transmitted before others, and evaluated in a real-worldtest bed. The results showed significant reductions in latency due to mini-mized channel switching delay and prioritized queuing. E.g. in one scenariothe amount of arrived VoIP packets within the time limit for VoIP (150 msone-way transmission time recommended by the ITU-T RecommendationG.114 [22]) was increased from approximately 60 % up to 100 %.

References[1] I.F. Akyildiz and Xudong Wang, ”A survey on wireless mesh networks,”

Communications Magazine, IEEE, vol.43, Issue 9, pp. 23-30, 2005.

[2] J. Li, C. Blake, D. S. J. De Couto, H. I. Lee and R. Morris, ”Capacity ofad hoc wireless networks,” Proc. of 7th ACM MOBICOM, 2001.

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[3] P. Kyasanur, C. Chereddi, and N. H. Vaidya, ”Net-X: System eXtensionsfor Supporting Multiple Channels, Multiple Interfaces, and Other Inter-face Capabilities,” Tech. Rep., University of Illinois at Urbana-Champaign,2006.

[4] A. Kamerman, A and N. Erkocevic, ”Microwave oven interference onwireless LANs operating in the 2.4 GHz ISM band,” PIMRC ’97., The 8thIEEE International Symposium on , vol.3, no., pp.1221,1227 vol.3, 1997.

[5] A.M. Cipriano, P. Agostini, A. Blad and R. Knopp, ”Cooperative com-munications with HARQ in a wireless mesh network based on 3GPPLTE,” in Proceedings of the 20th European Signal Processing Conference(EUSIPCO), 2012.

[6] FunkFeuer, Available: http://www.funkfeuer.at/ [Accessed May,2013].

[7] K. N. Ramachandran and E. M. Belding and K. C. Almeroth and M. M.Buddhikot, ”Interference-Aware Channel Assignment in Multi-RadioWireless Mesh Networks,” Proc. of INFOCOM, 2006.

[8] M. Alicherry, R. Bhatia, Li Erran Li, ”Joint Channel Assignment andRouting for Throughput. Optimization in Multi-radio Wireless MeshNetworks,” Proc. of ACM MOBICOM, 2005.

[9] T. Clausen and P. Jacquet, ”Optimized Link State Routing Protocol(OLSR),” RFC 3626, Internet Engineering Task Force, 2003.

[10] J. A. A. Lavén, P. Hjärtquist, ”Multimetric OLSR and ETT,” in 5thOLSR Interop & Workshop, 2009.

[11] UniK OLSR daemon software, Available: http://www.olsr.org/ [Ac-cessed May, 2013].

[12] C. Perkins, E. Belding-Royer and S. Das, ”Ad hoc On-Demand DistanceVector (AODV) Routing,” RFC 3561, Internet Engineering Task Force,2003.

[13] D.S.J. De Couto, D. Aguayo, J. Bicket, R. Morris, ”A high-throughputpath metric for multi-hop wireless routing,” Wireless Networks, v 11, n4, pp. 419-34, 2005.

[14] P.M. Esposito, M. Campista, I.M. Moraes, L. Costa, O. Duarte, M.G.Rubinstein, ”Implementing the expected transmission time metric forOLSR wireless mesh networks,” in 2008 1st IFIP Wireless Days, pp. 5,2008.

[15] P. Kyasanur, ”Multichannel wireless networks: capacity and protocols,”Ph.D. dissertation, University of Illinois at Urbana-Champaign, IL, USA,2006.

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[16] S. Das, Y. Wu, R. Chandra and C. Hu, ”Context Based Routing: Tech-nique, Applications and Experience,” in Networked Design Systems & Im-plementation (NSDI), 2008.

[17] Z. Zhong, J. Wang, S. Nelakuditi, and G.-H. Lu, ”On Selection of Can-didates for Opportunistic AnyPath Forwarding,” ACM SIGMOBILE Mo-bile Comp. and Comm. Review, vol. 10, no. 4, pp. 1-2, 2006.

[18] H. Dubois-Ferrière, ”Anypath Routing,” Ph.D. dissertation, Ecole Poly-technique Fédérale de Lausanne, Lausanne, Switzerland, 2006.

[19] S. Biswas and R. Morris, ”Opportunistic routing in multi-hop wirelessnetworks,” in HotNets-II, 2003.

[20] K. Zeng, ”Opportunistic Routing in Multihop Wireless Networks: Ca-pacity, Energy Efficiency, and Security,” Ph.D. dissertation, WorcesterPolytechnic Institute, MA, USA, 2008.

[21] KAUMesh,http://www.kau.se/en/kaumesh/ [Accessed: April, 2013].

[22] ”One-way transmission time,” ITU-T Recommendation G.114, 1996.

urn:nbn:se:kau:diva-29359

Anypath Routing for Reducing Latency in Multi-Channel Wireless Mesh Networks

Increasing capacity in wireless mesh networks can be achieved by using multiple channels and radios. By using different channels, two nodes can send packets at the same time without interfering with each other. To utilize diversity of available frequency, a channel assignment scheme is required. Hybrid channel assignment is an interesting approach where at least one radio is tuned to a fixed channel for receiving and the remaining interfaces switch their channels dynamically in order to match the receiving channel at the receiving node. This provides full connectivity, but at the expense of introduced switching costs. Due to hardware limitations it is too costly to switch channels on a per packet basis.

Instead, this thesis proposes an anypath routing and forwarding mechanism that considers the state of the channel switch operation in order to allow each node along the route to select the best next hop neighbor on a per packet basis. Also, in order to allow latency-sensitive packets to be transmitted before other packets, latency-awareness has been introduced to distinguish e.g. VoIP flows from FTP traffic.

The ideas have been implemented and tested using real-world experiments, and the results show a significant reduction in latency.

LICENTIATE THESIS | Karlstad University Studies | 2013:45

ISSN 1403-8099

ISBN 978-91-7063-522-9