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

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 performance gains due to high channel switching delay.

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

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 channels which eliminates the co-channel interference.

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

#### **3.3. Channel assignment problem**

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

scarce radio spectrum. So, the key issue in a MRMC WMN architecture is the channel assignment problem, which involves assigning a channel to each radio of a MR in a way that minimizes interference on any given channel and guarantees connectivity between the mesh nodes [12].

Channel Assignment Using Topology Control Based on Power Control in Wireless Mesh Networks 57

communicate for data transmission which implies that the Maximum Node Degree (MND) per node is four. The MND of 4 is selected in order to fully utilize the 11 available non-overlapping 802.11a frequency channels. Results have shown that with 12 available non-overlapping channels, the network throughput increases until a MND of four but saturates after that [11].

The proposed Topology Control Algorithm (TCA) controls the network topology by selecting the nearest neighbors for each node in the network. The objective of the proposed TCA is to build a connectivity graph with a small node degree to mitigate the co-channel interference and enhance spatial channel reuse as well as preserve network connectivity

Initially, the gateway broadcasts a "Hello" message, using its control radio on the control channel, announcing itself as the gateway. Each mesh node that receives this Hello message over its control radio broadcasts it again and in this way, this Hello message is flooded throughout the mesh network. The Hello message contains a hop-count field that is incremented at each hop during its broadcast. So, a mesh node may receive multiple copies of the Hello message over its control radio. However, distance of a mesh node from the gateway is the shortest path length (shortest hop count) of the Hello message received by a mesh node through its control radio over different paths. In this way, each mesh node

with the use of minimal power, as less transmit power translates to less interference.

Each node uses an omni-directional antenna for both transmission and reception.

TR for any pair of nodes to communicate directly is also the same.

The maximum transmission power is the same for all nodes and hence, the maximum

Note that all nodes start with the maximum transmission power, and that the initial topology graph created, when every node transmits with full power, is strongly

In the first exchange, each node broadcasts a HELLO message at maximum transmission

knows the next hop to reach the gateway using its control radio.

Each node is able to adjust its own transmission power.

The proposed TCA consists of the following five phases.

**a. Exchange of Information Between Nodes** 

power containing its node ID and the node position.

**4.2. Topology control algorithm** 

*4.2.1. Gateway advertisement process* 

The proposed TCA assumes the following.

Each node knows its location.

*4.2.2. Assumptions* 

connected.

*4.2.3. Phases of TCA* 

Given the connectivity graph, the main challenge for CAA is to assign a channel to each radio in a way that minimizes interference between MRs and ensures connectivity between them. In order to achieve these goals, the CAA should satisfy the following requirements.


At first glance, the problem of assigning channels to links in a mesh network appears to be a graph-coloring problem. However, standard graph-coloring algorithms cannot satisfy all of its constraints, and it is NP-hard to find an optimal channel assignment to maximize the overall network throughput [11]. Also, the channel assignment problem for mesh networks is similar to the list coloring problem, which is NP-complete [21].
