*2.2.3. Hybrid channel assignment schemes*

Hybrid channel assignment schemes combine both static and dynamic assignment properties by applying a fixed assignment for some radios and a dynamic assignment for other radios. The fixed radios can be assigned dedicated channels while the other radios can be switched dynamically among channels.

#### **a. Hybrid Multi-Channel Protocol (HMCP)**

HMCP [18] [19] is a link-layer multi-channel protocol for nodes with multiple radio interfaces. Out of the available interfaces at each node, X interfaces are assigned statically to X channels, and these interfaces are designated as "fixed interfaces." The fixed interfaces stay on the specified channels for long durations of time. The remaining interfaces can frequently switch between any of the remaining channels, based on the data traffic, and are designated as "switchable interfaces."

A co-ordination protocol is required to decide what channel to assign to the fixed interface, and also for enabling neighbors of a node X to know about the channels used by fixed interface of node X. Time synchronization and coordination between mesh nodes which is required in HMCP is not needed in TICA.

Channel Assignment Using Topology Control Based on Power Control in Wireless Mesh Networks 55

A Single-Radio Single-Channel (SRSC) mesh architecture suffers from hidden and exposed terminal problems. Assigning orthogonal channels to the MRs within the interference range can help alleviate the hidden and exposed terminal problems, and assist in improving the overall capacity of the network. However, considering the traffic characteristics in a WMN, frequent channel switching may be required to communicate with neighboring nodes. In such scenarios, single-radio multi-channel MAC may not provide any significant

The use of a multi-channel approach using multiple radios overcomes the problems encountered in the previous architectures [1]. Two or more radios are employed for the backhaul link. The uplink and downlink backhaul radios operate at non-overlapping

As each mesh router can be equipped with multiple radios, fixed channel assignment to these radios is a more viable solution. Efficient and intelligent channel assignment schemes

In a typical WMN, the total number of radios is much higher than the number of available channels. Thus, many links between the mesh routers operate on the same set of channels and interference among transmissions on these channels decreases their utilization. Therefore, minimizing the effect of interference is required for the efficient reuse of the

**Figure 2.** Hidden and exposed terminal problems [1]

**3.2. Wireless mesh network architectures** 

*3.2.1. Single-radio single-channel mesh network* 

performance gains due to high channel switching delay.

channels which eliminates the co-channel interference.

have to be designed, as the number of channels is limited.

**3.3. Channel assignment problem** 

*3.2.2. Multi-radio multi-channel mesh network* 

#### **b. Breadth First Search - Channel Assignment (BFS-CA)**

BFS-CA [20] is a centralized, interference-aware algorithm aimed at improving the capacity of the WMN backbone and at minimizing interference. This algorithm is based on an extension to the conflict graph concept called the Multi-radio Conflict Graph (MCG) where the vertices in the MCG represent edges between radios instead of edges between mesh routers.

BFS-CA requires certain number of MRs with certain number of radio interfaces to be placed at certain hops from the gateway, whereas TICA simply requires all MRs to have four data radios, does not require any careful router placement strategy, and works with any placement of routers as verified by a comprehensive performance evaluation.
