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Demonstration of Channel Assignment in a Wireless Metropolitan MESHNetwork ∗

Nikolaos Petroulakis, Manolis Delakis, Manolis Genetzakis, Theodoros Dionysiou,Stefanos Papadakis and Vasilios A. Siris

Institute of Computer Sciences, Foundation for Research and Technology-Hellas (FORTH)P.O. Box 1385, GR 711 10 Heraklion, Crete, Greece

Abstract

We demonstrate the channel assignment in a metropoli-tan wireless multi-radio mesh network with directional an-tennas. A channel assignment approach is presented usingsuitable algorithms with key components the interferencemodel, the link ordering and the channel selection metric.We have implemented and evaluated the proposed channelassignment in an actual metropolitan mesh network whichcovers an area of 60km2 in the city of Heraklion and con-sists of 14 nodes, among which seven are core nodes withup to four 802.11a wireless interfaces each, and one formanagement and monitoring. Finally, we developed a Javaapplication to demonstrate the channel assignment processin a metropolitan mesh network in a graphical environment.

1 Introduction

Wireless multi-radio mesh networks have the potentialto provide ubiquitous and ultra high-speed broadband ac-cess in urban and rural areas, to both fixed and mobile users,with low operation and management costs. Such mesh net-works can achieve significantly higher performance com-pared to single-radio single-channel mesh networks, by ex-ploiting spatial diversity through multiple radio interfaceslocated in mesh nodes, each operating in different channels,and directional antennas.

Channel assignment in a wireless multi-radio mesh net-work influences its overall performance, since it determinesthe level of interference between links internal to the meshnetwork (intra-network interference), but also the interfer-ence from external sources. An important motivation forthis work, reported in this demo and in [1], was to per-form automated channel assignment in an experimental

∗This work was supported in part by the European Commission in the7th Framework Programme through project EU-MESH (Enhanced, Ubiq-uitous, and Dependable Broadband Access using MESH Networks), ICT-215320, http://www.eu-mesh.eu

metropolitan wireless multi-radio mesh network we havedeployed in the city of Heraklion [2]. We demonstrate chan-nel assignment process in a graphical environment by a de-veloped java application.

2 Multi-Point Link Conflict Graph

In this section we discuss the multi-point link conflictgraph (MPLCG), which can effectively model interferencein wireless mesh networks with a known wireless interfacecommunication graph. A vertex in the MPLCG represents amulti-point communication link, i.e., a set of interfaces thatcommunicate with each other, hence should be assigned thesame channel, Figure 1. Unlike the typical conflict graphwhere a vertex corresponds to a link between two nodes ina mesh network, in the the multi-point link conflict graph avertex is a set of two or more interfaces belonging to differ-ent nodes, which are connected in a point-to-point, point-to-multi-point, or multi-point to-multi-point manner, Fig-ure 1 [1].

Figure 1. Three types of links between meshnodes with directional antennas

As noted above, an edge between two vertices in theMPLCG indicates that the two corresponding links interferewith each other, hence they cannot be assigned channels in-dependently [3].

In this demo, we will assume that links interfere (hencethere is an edge between the corresponding vertices in the

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multi-point link conflict graph), if wireless interfaces be-longing to the two links are located in the same mesh node.Additionally, we will assume that interfering links are as-signed channels with a one channel separation, i.e. there isone channel between the channels assigned to the two links.

3 Channel Assignment Procedure

The Channel Assignment procedure has three compo-nents: the interference model, the link ordering and thechannel selection metric, Figure 2. The pseudo-code forthe channel assignment procedure used in this demo whenthe multi-point link conflict graph is used for modelling in-terference is shown in Figure 3. Firstly, the algorithm startsputting all the vertices of the multi-point link conflict graphin a list V (line 1). Then the vertices are categorized basedon the order of choice that has been defined (e.g fixed or-dering). Line 6 in the algorithm considers for a link υ onlychannels that have a one channel separation from channelsthat have already been assigned to other links for whichthere is an edge with link υ in the multi-point link conflictgraph. Among these channels, the one with the best metric(one-way SNR, two-way SNR, round-trip delay) is selectedin line 7 [1].

Figure 2. The three components of ChannelAssignment procedure

Figure 3. Channel Assignment Algorithm us-ing multi-point link conflict graph interfer-ence model

4 Wireless Metropolitan Mesh Network

The metropolitan mesh network, that was used as atest-bed to evaluate proposed channel assignment proce-dure, covers an area of approximately 60km2 in the cityof Heraklion-Crete and currently contains 14 nodes, amongwhich seven are core mesh nodes with up to four 802.11awireless interfaces each, and one for management and mon-itoring. Each wireless interface is assigned a static IP ad-dress. The mesh network is connected to a fixed networkthrough three nodes (FORTH M1 & M2 and K4-UoC), Fig-ure 4.

Figure 4. Heraklion Metropolitan Mesh Net-work

The wireless interface communication graph and themulti-point link conflict graph for the core links of themetropolitan test-bed is shown in Figure 4. Note that thecurrent topology does not contain a multi-point connection,since the number of links is small [2].

5 Demonstration of Channel Assignmentprocedure

There was the need to develop an application to demon-strate channel assignment procedure in a metropolitan wire-less mesh network. For this reason we developed a playerapplication in Java that can be used for different 802.11 pro-tocols , topologies, number of nodes and links.

The application manages three main types of input:

• Map Info - map figure and map coordinates,

• Node List - containing name, coordinates, type andnumber of interfaces of each node

• Link List - a list of network links including connectednodes, available excluded and assigned channels)

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Figure 5. Structure of Channel AssignmentPlayer

The main purpose of the player is to demonstrate thechannel assignment in a metropolitan mesh network. Thereis a number of files that are correlated and comprise thestructure of the player, Figure 5. The node list is inserted inthe player and the nodes are depicted upon the inserted mapby converting the geographical coordinates into pixel coor-dinates, storing simultaneous the imported node info. Thesystem is dynamic, which means that if we change the map,the nodes will be placed into new positions correspondingto the new map. Then the link-list file, which includes theavailable, the excluded and the assigned channels, is parsedinto the player. The order of the assigned channel per linkis inside the file and depends on the assumed link order-ing. The three different metrics (one-way SNR, two-waySNR and round-trip delay) assigned similar channels corre-sponding on the assumed link ordering. The selected linkorder of the demo was based on the distance from the fixedgetaway using the two-way SNR metric.

At the beginning all the links are red. When the playerscans a link, its color becomes yellow. The applicationchecks for the best available channel, removing the ex-cluded ones from the list, and assigns which channel givesthe best results on the chosen metric. A table at the bot-tom of the player displays the link info such as the link dis-tance, the excluded the available and the assigned channelsin every scanning link. To preview the end of the scanningprocedure and the assigned channel, the color of the link be-comes green. The channel assignment procedure ends whenall links have an assigned channel, Figure 6.

6 Conclusions

We have demonstrated a new conflict graph that is appro-priate for wireless multi-radio mesh networks with direc-tional antennas and a channel assignment procedure whichwas evaluated in channel assignment in an actual Metropoli-

Figure 6. Channel Assignment Player

tan Mesh network which covers 60km2 in the city ofHeraklion-Crete. The experimental results show that theproposed channel assignment procedure achieves better per-formance, in terms of average packet delay, that is veryclose to a lower bound of average packet delay and signif-icantly better than a channel-unaware channel assignmentprocedure. Finally, we developed a Java application demon-strating, in a dynamic graphical environment, the channelassignment procedure.

References

[1] M. Delakis and V. A. Siris, ”Channel Assignment ina Metropolitan Wireless Multi-Radio Mesh Network”,In Proc. of 5th International Conference on BroadbandCommunications, Networks and Systems (Broadnets),London, Sep. 2008.

[2] M. Delakis, K. Mathioudakis, N. Petroulakis,V. A. Siris, ”Experiences and Investigations withHeraklion MESH: An experimental MetropolitanMulti-Radio Mesh Network”. In Proc. of 4th Int’lConference on Testbeds and Research Infrastructuresfor the Development of Networks & Communities(TRIDENTCOM), Innsbruck, Austria, March 2008.

[3] V. Angelakis, S. Papadakis, N. Kossifidis, V. A. Siris,A. Traganitis, ”The Effect of Using Directional Anten-nas on Adjacent Channel Interference in 802.11a: Mod-eling and Experience With an Outdoors Testbed”, 4thInternational workshop on Wireless Network Measure-ments (WiNMee), Berlin, Germany, March 2008.

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