*2.2.2. Dynamic channel assignment schemes*

52 Wireless Mesh Networks – Efficient Link Scheduling, Channel Assignment and Network Planning Strategies

Channel Assignment (CA) in a multi-radio WMN environment consists of assigning channels to the radios in order to achieve efficient channel utilization (i.e. minimize cochannel interference) and, simultaneously, to guarantee an adequate level of connectivity. The problem of optimally assigning channels in an arbitrary mesh topology has been proven to be NP-hard, based on its mapping to a graph-coloring problem [11]. Therefore, channel assignment schemes employ heuristic techniques to assign channels to radios belonging to mesh nodes. A taxonomical classification of various CA schemes for wireless mesh

Fixed assignment schemes assign channels to radios either permanently, or for intervals that are long with respect to the radio switching time. Such schemes can be further subdivided

In CCA scheme [13], the radios of each node are all assigned the same set of channels. For example, if each node has two radios, then the same two channels are used at every node. The main benefit is that the connectivity of the network is the same as that of a single channel approach, while the use of multiple channels increases network throughput. However, it does not take into account the effect of interference on the channel assignment

In the VCA class of schemes, the radios of different nodes are assigned different sets of channels. However, the assignment of channels may lead to network partitions and/or topology changes, which may increase the length of routes between mesh nodes. Therefore, in such a scheme, channel assignment needs to be carried out carefully. The VCA approach is discussed in more detail by presenting algorithms that belong to this

C-HYA, a centralized channel assignment algorithm for multi-radio multi-channel WMNs, was proposed in [11]. Assuming that the offered traffic load is known, this algorithm assigns channels ensuring network connectivity and satisfying the bandwidth limitations of each

MesTiC, a fixed algorithm for centralized CA, was proposed in [14], and visits nodes once in the decreasing order of their rank. The rank of each node is computed on the basis of its

ii. Mesh-based Traffic and interference-aware Channel-assignment (MesTiC)

link-traffic characteristics, topological properties and number of radios on a node.

**2.2. Channel assignment schemes** 

*2.2.1. Fixed channel assignment schemes* 

**a. Common Channel Assignment (CCA)** 

**b. Varying Channel Assignment (VCA)** 

i. Centralized Hyacinth (C-HYA)

networks is as follows [12].

into two types.

in a WMN.

category.

link.

In dynamic CA schemes, any radio can be assigned any channel but additionally, radios can frequently switch from one channel to another. Therefore, when nodes need to communicate with each other in such a scheme, a coordination mechanism is required to ensure that they are on a common channel.

### **a. Multi-channel Medium Access Control (MMAC)**

MMAC [15] [16] is a link-layer multi-channel protocol for nodes with a single network interface. A node equipped with a single interface can only listen to one channel at a time. Therefore, in order to use multiple channels, the interface has to be switched between channels.

When nodes require to switch channels, a pair of nodes need to listen on the same channel at the time of communication and a channel coordination method is necessary, which is not required in TICA.

## **b. Distributed Hyacinth (D-HYA)**

D-HYA, a dynamic and distributed channel assignment algorithm proposed in [17], can adapt to traffic load dynamically. The algorithm builds on a spanning tree network topology. The gateway node is the root of the spanning tree, and every mesh node belongs to that tree. Based on per-channel total load information, a WMN node determines the set of channels that are least used in its vicinity. As nodes higher up in the spanning trees need more relay bandwidth, they are given a higher priority in channel assignment. The priority of a WMN node is equal to its hop distance from the gateway.

The CA schemes, such as C-HYA, MesTiC and D-HYA, require the traffic load to be known before assigning channels, whereas TICA requires no such knowledge for channel assignment.
