*2.1.3. Taxonomy of topology control schemes*

These schemes are mainly divided into two types [10].

#### **a. Homogeneous**

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

presents the conclusions, along with some directions for future work.

**2. Background** 

**2.1. Topology control schemes** 

*2.1.1. Topology control in multi-radio WMNs* 

connectivity with the use of minimum power.

many aspects of the operation of the network [7].

**a. Effect of TC on the Performance of the Network** 

ability to deliver a packet to its destination [8].

The power level affects the throughput capacity of the network [9].

*2.1.2. Effect of topology control* 

the end-to-end delay.

whole network.

The results of simulations for performance evaluation of the proposed CAA based on throughput analysis of a 36-node mesh network are presented in this section. Section 6

This section discusses the effects of topology control on the operation of a network, gives the taxonomy of the topology control schemes for multihop wireless networks, and presents some related well-known topology control algorithms. The section also contains a taxonomical classification of the channel assignment schemes for wireless mesh networks, and discusses some related well-known channel assignment algorithms for each class.

The importance of Topology Control (TC) lies in the fact that it affects network spatial reuse and hence the traffic carrying capacity. Choosing a large transmit power results in excessive interference, while choosing a small transmit power results in a disconnected network [5]. Using TC through transmission power control, the network connectivity and hence the network topology is affected, interference levels are mitigated, which reduces the co-channel

The connectivity graph in a multi-radio WMN is determined through topology control. So, the problem of TC in multi-radio WMNs involves the selection of transmission power for each radio interface of each mesh node in the network, so as to maintain the network

The problem of TC is complex, since the choice of the transmit power fundamentally affects

TC via transmission power control has a multi-dimensional effect on the performance of the

 The transmit power levels determine the performance of medium access control, since the spatial channel reuse depends on the number of other nodes within the interference range. The choices of power levels affect the connectivity of the network, and consequently the

Power control affects the network topology which affects the number of hops, and thus

interference, and the opportunity of spatial channel re-use is enhanced [6].

This is the basic type of TC, as all the nodes are assumed to use the same transmitting range. So, the topology control problem reduces to determining the minimum value of transmission range that ensures network connectivity. This minimum transmission range is also called the Critical Transmitting Range (CTR).

#### **b. Non-homogeneous**

In this type of TC, nodes are allowed to choose different transmitting ranges, provided they do not exceed the maximum range. Depending on the type of information that is used to compute the topology, non-homogeneous topology control is further classified into three categories.

i. Location-based schemes

In such schemes, exact node positions are known. If this information is used by a centralized authority to compute a set of transmitting range assignments which optimizes a certain measure such as the energy cost, this is the case of the Range Assignment Problem and its variants. The Local Minimum Spanning Tree (LMST) algorithm [5] and the Enhanced Local Minimum Shortest-Path Tree (ELMST) algorithm [6] are examples of location-based topology control schemes.

#### ii. Direction-based schemes

In such schemes, it is assumed that nodes do not know their position, but they can estimate the relative direction of each of their neighbors.

iii. Neighbor-based schemes

In such schemes, nodes are assumed to know only the ID of the neighbors, and are able to order them according to some criterion such as link quality.

Channel Assignment Using Topology Control Based on Power Control in Wireless Mesh Networks 53

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

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

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

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

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

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

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

*2.2.2. Dynamic channel assignment schemes* 

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

of a WMN node is equal to its hop distance from the gateway.

are on a common channel.

channels.

required in TICA.

assignment.

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

*2.2.3. Hybrid channel assignment schemes* 

be switched dynamically among channels.

designated as "switchable interfaces."

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

#### **2.2. Channel assignment schemes**

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 networks is as follows [12].
