A Dynamic Multi-Channel MAC for Ad Hoc LAN

Applying single channel ad hoc MAC protocol (such as 802.11b) in multichannel environment (such as the ISM band) has two potential problems: connectivity and load balancing. Connectivity problem refers to the fact that stations in one channel cannot connect to those in the other channels. As resource is partitioned into multiple independent channels, w hen the load of two channels are different, resource in the underloaded channel cannot be used to offload the overloaded channels, and thus, reduces the overall system performance. The use of m ultichannel protocols where each station is capable of accessing to more than one channel is a way of solving the two problems. This paper proposes a multichannel MAC protocol for that purpose. The preliminary result shows that the objective is achievabl e while, at the same time, maintain good performance. A. INTRODUCTION Ad hoc network does not have a predefined topology. Multiple stations pool together and communicate without assistance of any centralized control. The ad hoc nature makes network anywhere possible and adds convinence for local group communications. Due to the lack of centralized control, the size of an ad hoc LAN varies. Currently, the most popular ad hoc MAC implemented is the 802.11b [1] which utilizes CSMA/CA and channel reservation via Network Allocation Vector (NAV). IEEE 802.11b is a single-channel MAC protocol which operates in the ISM band. For CSMA type MAC protocol, the system throughput decreases as the network size increases due to collisions and longer backoff periods. Our simulation shows that the same effect occurs even in a single cell environment where all stations are in the transmission range of each other. The ISM band is a typical multichannel environment. In the band, spread spectrum is used for channellization. Each channel is identified by a code. Depends on the physical layer properties, a code refers to either a direct sequency spreading code or a frequency hopping pattern. In either case, multiple channels are allowed to co-exist in the same band. If more than one channel is available and the network size is large, it is logical to allocate the stations to operate in different channels in order to improve system throughput. This is acceptable if communication is confined to the stations in the same channel. Otherwise, special equipment will be needed to bridge the stations in different channels to allow cross-channel communicaton. However, in ad hoc networks, such equipment is usually …