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Interference-Aware Channel Assignments with Seamless Multi-Channel Monitoring in Wireless Mesh Networks Sunghun Kim School of Engineering Information and Communications Univ. Daejeon, Korea

Jongsub Cha RFID/USN Research Division ETRI Daejeon, Korea

Abstract— The wireless mesh networks (WMNs) are statically deployed on heterogeneous areas and are operating in open wireless media, and thus it coexists with other networks operating on the same frequency with the same or different radio access technology (RAT). The WMNs experience two types of interferences according to the source of interference. Coexisted networks with WMN induce the interference called external interference and nodes in WMN experience interference each other, which is called internal interference. Most of existing protocols strive for dealing with internal interference. But the increased external interferences can degrade the performance of WMN significantly. To resolve this coexistence problem, we propose three channel assignment schemes for hybrid multichannel protocol (HMCP) by capturing the states of each channel and selecting its operating channel. We also devise a seamless multi-channel monitoring method to recognize channel states without performance degradation. Intensive simulation results demonstrate that our proposed channel assignment schemes outperform existing HMCP’s channel assignment scheme in terms of aggregate throughput, delay, and fairness.

I. I NTRODUCTION Wireless mesh networks (WMNs) based on IEEE 802.11 are increasingly getting attention especially in last-mile broadband and back-haul services. The typical WMN has a static topology for providing stable wireless back-haul service. Although the WMN is well appreciated as the emerging and promising networks, it still has several challenging issues such as capacity limitation and heterogeneous networks coexistence problem, as reported in [1]. In particular, the capacity problem in WMNs can be alleviated by equipping the mesh nodes with multiple radios tuned to non-overlapping channels. Many multi-channel protocols have been proposed to boost the capacity of the networks, i.e., MMAC [2], CAM-MAC [3], Dynamic Channel Assignment MAC (DCA) [4] and Hybrid Multi-Channel Protocol (HMCP) [5]. Both MMAC and CAM-MAC use only single radio to transmit data by exploiting non-overlapping multi-channel. That is, available channels are separated into dedicated control channel and multiple data channels. More specifically, one data channel is first selected among available data channels via dedicated channel’s negotiation. After that, each node transmits data by tuned negotiated data channel. On the other hand, the

Joongsoo Ma School of Engineering Information and Communications Univ. Daejeon, Korea

DCA uses two types of radios to transmit data: control radio tuned on control channel to negotiate the data channel and data radio via negotiated data channel. Its performance is critically affected by the condition of control channel. If the control channel is highly loaded, the probability of successful negotiation severely drops. Even if the available channels are idle, the throughput is low due to the negotiation failure via control channel. Besides, the DCA is vulnerable to even single failure of the control channel. As a result, the network is prone to shut down. In HMCP, each node uses two radios to communicate with its neighbor nodes. One is a fixed radio to receive data from neighbor nodes via its own fixed channel. The other is a switchable radio for transmitting data to neighbor nodes via neighbor nodes’ fixed channel. The HMCP node maintains the table for neighbors’ fixed channel information and exploits it to lookup the fixed channel of the destined node of the packet. Thus, the fixed channel assignment of HMCP determines the network performance. Even with no dedicated control channel, the performance of network is not affected by the condition of control channel and also each node can utilize the all available channels as data channel. As a result, the performance of HMCP is superior to others mentioned above. Typically, the WMNs are statically deployed in heterogeneous areas and operate in open wireless media such as Industrial, Scientific and Medical (ISM) band. These characteristics make wireless links of WMNs experiencing significant quality fluctuations and performance degradation [7]. This is because WMNs coexist with other networks operating in the same frequency with the same or different radio access technology (RAT). In other words, the WMNs experience two types of interferences depending on the source of interference: external interference and internal interference. Fig. 2 illustrates these interferences. Most of existing multi-channel protocols strive for dealing with internal interference. With no exception, the HMCP considered only internal interference. But external interferences from co-located networks cause the significant performance degradation. To resolve such coexistence problem, we need to capture the state of each channel and utilize channel states

This research was supported by the MKE(Ministry of Knowledge Economy), Korea, under the ITRC support program(IITA-2008-C1090-0801-0015)

Fixed Radio with colored fixed channel C H 2 Node A

 
  
  

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Radio usage with fixed channel in HMCP

information to select WMNs’ operating channel. During the channel monitoring period for capturing channel states, the network performance should not be degraded. The purpose of our work is to boost network performance of multi-channel multi-radio WMN with the coexisted networks under consideration. Toward this goal, we propose three distributed channel assignment schemes under external and internal interference environment. To prevent the performance degradation during monitoring channels, we devise seamless multi-channel monitoring technique to capture multi-channel states in coexistent network environment. The rest of this paper is organized as follows. In Section II, we describe the system model and problem statement for channel assignment and multi-channel monitoring for WMN with the coexisted networks. The proposed channel assignment schemes for the HMCP are provided in Section III. In Section IV, the indispensable technique to assign operating channel and the seamless multi-channel monitoring method are described. Then, the performance evaluated for the proposed channel assignment schemes is given in Section V and finally conclusions are made in Section VI. II. S YSTEM M ODEL AND P ROBLEM S TATEMENT As mentioned previously, each node in HMCP uses two radios to communicate with its neighbor nodes: fixed radio tuned its fixed channel for receiving data and switchable radio tuned neighbor’s fixed channel for sending data. As shown in Fig. 1, if node B has a packet to node C, node B finds the fixed channel of node C and transmits the packet through switchable radio tuned node C’s fixed channel. The table for neighbor node’s fixed channel information is maintained by periodically fixed channel assignment, and the HELLO packet broadcast. This HELLO packet contains the newly selected fixed channel information. Thus, the fixed channel assignment of HMCP is very important because this fixed channel assignment governs the performance of HMCP. Here we consider two kinds of networks. One is HMCP network, and the other is co-located networks with same RAT such as WLANs, different service provider’s networks, as shown in Fig. 2. In this figure, the internal interference occurs between HMCP links which are denoted by coloredsolid links, and the external interference happens between HMCP links (colored-solid line links) and co-located network links (dotted-line links). Under this network environment, we model the coexistence problem and devise interferenceaware channel assignment schemes. We assume the co-located



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