*4.2.1. Gateway advertisement process*

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

#### *4.2.2. Assumptions*

The proposed TCA assumes the following.


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 connected.

#### *4.2.3. Phases of TCA*

The proposed TCA consists of the following five phases.

#### **a. Exchange of Information Between Nodes**

In the first exchange, each node broadcasts a HELLO message at maximum transmission power containing its node ID and the node position.

#### **b. Building the Maximum Power Neighbor Table (MPNT)**

From the information in the received HELLO messages, each node arranges its neighboring nodes in the ascending order of their distance. The result is the Maximum Power Neighbor Table (MPNT). Then, each node sends its MPNT along with its position and node ID to the gateway node using its control radio over the control channel.

Channel Assignment Using Topology Control Based on Power Control in Wireless Mesh Networks 59

**Figure 3.** Select x for less than x TCA

#### **c. Building the Direct Neighbor Table (DNT)**

For each node in the network, the gateway builds a Direct Neighbor Table (DNT). Based on information in the MPNT of node *v* and the MPNTs of its neighbors, if

(a) node *w* is in the MPNT of node *v,* and

(b) node *w* is closer to any other node *y* in the MPNT of node *w* than to node *v*, then gateway eliminates node *w* from the MPNT of node *v*.

If, after removing nodes from the MPNT of node *v*, the remaining number of nodes in the MPNT of node *v* is less than "*x*," then the gateway selects "*x*" nearest nodes as neighbors of node *v* which results in the DNT. However, if after removing nodes from the MPNT of node *v*, the remaining number of nodes is greater than or equal to "*x*," then the result is the DNT.

This algorithm is called *Select x for less than x* TCA where *x* is a positive integer. The *Select x for less than x* TCA ensures that each node has at least *x* neighbors, as shown in Figure 3.

#### **d. Converting into Bi-directional Links**

For each node in the network, the gateway converts the uni-directional links in the DNT of a node into bi-directional links. For each uni-directional link, this is done by adding a reverse link in the DNT of the neighboring node. This converts the DNT into Bi-directional DNT. This results in the Final Neighbor Table (FNT).

#### **e. Calculating the Minimum Power Required**

For each node in the network, the gateway calculates the minimum power, *P*min, required to reach each of the nodes in the FNT of a node, using the appropriate propagation model.

#### *4.2.4. Propagation models*

The free space model is used for short distances and the two ray ground reflection model is used for longer distances, depending on the value of the Euclidean distance in relation to the cross-over distance. The cross-over distance is calculated by [22]

$$Cross\\_over\\_dist = \frac{4\pi hhr}{\lambda},\tag{1}$$

where *ht* and *hr* are the antenna heights of the transmitter and receiver, respectively. If the distance between two nodes is less than the cross over distance, i.e. *d(u,v) < Cross\_over\_dist*, Free Space propagation model is used, whereas if *d(u,v) > Cross\_over\_dist*, Two-ray propagation model is used. The minimum power for the free-space propagation model is calculated by [22]

**Figure 3.** Select x for less than x TCA

From the information in the received HELLO messages, each node arranges its neighboring nodes in the ascending order of their distance. The result is the Maximum Power Neighbor Table (MPNT). Then, each node sends its MPNT along with its position and node ID to the

For each node in the network, the gateway builds a Direct Neighbor Table (DNT). Based on

If, after removing nodes from the MPNT of node *v*, the remaining number of nodes in the MPNT of node *v* is less than "*x*," then the gateway selects "*x*" nearest nodes as neighbors of node *v* which results in the DNT. However, if after removing nodes from the MPNT of node *v*, the remaining number of nodes is greater than or equal to "*x*," then the result is the DNT. This algorithm is called *Select x for less than x* TCA where *x* is a positive integer. The *Select x for less than x* TCA ensures that each node has at least *x* neighbors, as shown in Figure 3.

For each node in the network, the gateway converts the uni-directional links in the DNT of a node into bi-directional links. For each uni-directional link, this is done by adding a reverse link in the DNT of the neighboring node. This converts the DNT into Bi-directional DNT.

For each node in the network, the gateway calculates the minimum power, *P*min, required to reach each of the nodes in the FNT of a node, using the appropriate propagation model.

The free space model is used for short distances and the two ray ground reflection model is used for longer distances, depending on the value of the Euclidean distance in relation to the

<sup>4</sup> \_ \_ , *h ht r Cross over dist*

where *ht* and *hr* are the antenna heights of the transmitter and receiver, respectively. If the distance between two nodes is less than the cross over distance, i.e. *d(u,v) < Cross\_over\_dist*, Free Space propagation model is used, whereas if *d(u,v) > Cross\_over\_dist*, Two-ray propagation model is used. The minimum power for the free-space propagation model is

(1)

(b) node *w* is closer to any other node *y* in the MPNT of node *w* than to node *v*, then

**b. Building the Maximum Power Neighbor Table (MPNT)** 

gateway node using its control radio over the control channel.

gateway eliminates node *w* from the MPNT of node *v*.

cross-over distance. The cross-over distance is calculated by [22]

information in the MPNT of node *v* and the MPNTs of its neighbors, if

**c. Building the Direct Neighbor Table (DNT)** 

(a) node *w* is in the MPNT of node *v,* and

**d. Converting into Bi-directional Links** 

This results in the Final Neighbor Table (FNT). **e. Calculating the Minimum Power Required** 

*4.2.4. Propagation models* 

calculated by [22]

$$P\_{\min} = \frac{\text{RxThreshold} \left(4\pi d\right)^2}{\text{G}\_t \text{G}\_r \text{.} \text{.} \tag{2}$$

Channel Assignment Using Topology Control Based on Power Control in Wireless Mesh Networks 61

complete. Therefore, we propose an approximate algorithm for channel assignment. The proposed channel assignment algorithm, TICA, is shown in Figure 6 and has the following

In order to create the network connectivity graph with the aim of reducing the interference

All nodes send their MPNTs to the gateway using their control radio. Note that in order to send its MPNT to the gateway, each mesh node knows the next hop to reach the gateway using its control radio via the "gateway advertisement process." The gateway starts with the

between MRs, network topology is controlled using the topology control algorithm.

*Select 1 for less than 1* TCA and builds the FNTs of all nodes.

phases.

**Figure 4.** Two edges at distance-2 of each other [23]

**Figure 5.** Interference-range edge coloring

*4.3.2. Phases of TICA* 

**a. Topology Control** 

The minimum power for the two-ray propagation model is calculated by [22]

$$P\_{\min} = \frac{\text{Rx}\,\text{Threshold}(d)}{\text{G}\_{t}\,\text{G}\_{r}h\_{t}^{2}h\_{r}^{2}},$$

where *Gt* and *Gr* are transmitter and receiver antenna gains respectively, and *RxThresh* is power threshold required by radio interface of receiving node to correctly understand the message.

#### **4.3. Channel assignment algorithm**

#### *4.3.1. Interference-range edge coloring*

If *K* be the number of available colors (channels), then for *K* ≥ 4, the distance-2 edge coloring problem, also known as strong edge coloring problem, is NP-complete [23]. A distance-2 edge coloring of a graph *G* is an assignment of colors to edges so that any two edges within distance 2 of each other have distinct colors. Two edges of *G* are within distance 2 of each other if either they are adjacent, as shown in Figure 4a or there is some other edge that is adjacent to both of them, as shown in Figure 4b. The distance-2 edge coloring has been used in [24] for channel assignment, where the authors have described the interference model as two-hop interference model. In this model, two edges interfere with each other if they are within two-hop distance. In other words, two edges *e1* and *e2* cannot transmit simultaneously on the same channel if they are sharing a node or are adjacent to a common edge.

To minimize co-channel interference in a wireless mesh network, it is necessary to assign channels to links such that links within interference range of each other are assigned different channels (colors). This problem can be termed as *interference-range edge coloring*, and the corresponding interference model can be called *interference-range interference model*. In a grid topology where links are of equal length, the interference-range edge coloring is similar to distance-2 edge coloring, as shown in Figure 5a. The channel assigned to link *l*<sup>1</sup> cannot be assigned to links *l*2 and *l*3 as they are within the interference range of link *l*1. Note that *l*2 and *l*3 are also within two-hop distance of *l*1.

However, in a random topology where links are of different lengths due to the random nature of the topology, the interference-range edge coloring can be harder than distance-2 edge coloring as shown in Figure 5b. In this case, the channel assigned to link *l*1 cannot be assigned to links *l*2, *l*3 and *l*4 as they are within the interference range of link *l*1. Note that *l*2, *l*<sup>3</sup> and *l*4 are within three-hop distance of *l*1.

In the proposed network model, the number of available channels (colors) is 11 which means that *K* = 11. Based on its similarity to distance-2 edge coloring problem which is NPcomplete for *K* ≥ 4, the interference-range edge coloring problem is, therefore, also NP-

complete. Therefore, we propose an approximate algorithm for channel assignment. The proposed channel assignment algorithm, TICA, is shown in Figure 6 and has the following phases.

**Figure 4.** Two edges at distance-2 of each other [23]

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

min 2

The minimum power for the two-ray propagation model is calculated by [22]

message.

edge.

**4.3. Channel assignment algorithm** 

*4.3.1. Interference-range edge coloring* 

that *l*2 and *l*3 are also within two-hop distance of *l*1.

and *l*4 are within three-hop distance of *l*1.

*RxThresh d <sup>P</sup> G G*

min 2 2

where *Gt* and *Gr* are transmitter and receiver antenna gains respectively, and *RxThresh* is power threshold required by radio interface of receiving node to correctly understand the

If *K* be the number of available colors (channels), then for *K* ≥ 4, the distance-2 edge coloring problem, also known as strong edge coloring problem, is NP-complete [23]. A distance-2 edge coloring of a graph *G* is an assignment of colors to edges so that any two edges within distance 2 of each other have distinct colors. Two edges of *G* are within distance 2 of each other if either they are adjacent, as shown in Figure 4a or there is some other edge that is adjacent to both of them, as shown in Figure 4b. The distance-2 edge coloring has been used in [24] for channel assignment, where the authors have described the interference model as two-hop interference model. In this model, two edges interfere with each other if they are within two-hop distance. In other words, two edges *e1* and *e2* cannot transmit simultaneously on the same channel if they are sharing a node or are adjacent to a common

To minimize co-channel interference in a wireless mesh network, it is necessary to assign channels to links such that links within interference range of each other are assigned different channels (colors). This problem can be termed as *interference-range edge coloring*, and the corresponding interference model can be called *interference-range interference model*. In a grid topology where links are of equal length, the interference-range edge coloring is similar to distance-2 edge coloring, as shown in Figure 5a. The channel assigned to link *l*<sup>1</sup> cannot be assigned to links *l*2 and *l*3 as they are within the interference range of link *l*1. Note

However, in a random topology where links are of different lengths due to the random nature of the topology, the interference-range edge coloring can be harder than distance-2 edge coloring as shown in Figure 5b. In this case, the channel assigned to link *l*1 cannot be assigned to links *l*2, *l*3 and *l*4 as they are within the interference range of link *l*1. Note that *l*2, *l*<sup>3</sup>

In the proposed network model, the number of available channels (colors) is 11 which means that *K* = 11. Based on its similarity to distance-2 edge coloring problem which is NPcomplete for *K* ≥ 4, the interference-range edge coloring problem is, therefore, also NP-

*RxThresh d <sup>P</sup>*

*t r*

*t rt r*

2

(2)

*GGh h* (3)

(4 ) .

4

( ) ,

**Figure 5.** Interference-range edge coloring

#### *4.3.2. Phases of TICA*

#### **a. Topology Control**

In order to create the network connectivity graph with the aim of reducing the interference between MRs, network topology is controlled using the topology control algorithm.

All nodes send their MPNTs to the gateway using their control radio. Note that in order to send its MPNT to the gateway, each mesh node knows the next hop to reach the gateway using its control radio via the "gateway advertisement process." The gateway starts with the *Select 1 for less than 1* TCA and builds the FNTs of all nodes.

#### **b. Connectivity Graph**

Based on the FNTs of all nodes, the gateway builds the connectivity graph and checks the resulting network for connectivity. A topology is said to be connected if the gateway can reach any node in the connectivity graph directly or through intermediate hops.

If the resulting network is not connected, the gateway moves to the next higher TCA by incrementing *x* in the *Select x for less than x* TCA, builds the connectivity graph and checks the resulting network for connectivity. The gateway keeps on moving to a higher TCA until it finds that the network resulting from the connectivity graph is connected.

#### **c. Minimum Power-based Shortest Path Tree with a MND of Four**

After ensuring that the connectivity graph is connected, the gateway builds the Shortest Path Tree (SPT), using Dijkstra's algorithm [25], based on the connectivity graph. The metric for path selection is minimum power.

The node degree is defined as the number of TR neighbors of a node. The number of TR neighbors of a mesh router is bounded by the number of its radios and each node has four data radios. So, if any node in the shortest path tree has more than four links, the gateway selects those four links for that node which have the minimum weight and sets the weight of all other links to infinity. In other words, the gateway ensures that each node can have a maximum of four TR neighbors and builds a Minimum Power-based SPT (MPSPT) with a MND of 4 per node. The gateway checks the resulting MPSPT graph for connectivity. If the resulting MPSPT graph is not connected, the gateway moves to a higher TCA.

Once the MPSPT graph is determined, the gateway has to assign channels to links of the MPSPT. Now, the objective is to assign channels to the links of the MPSPT such that the interference between simultaneous transmissions on links operating on the same channel is minimized and the overall network throughput is maximized.

#### **d. Link Ranking**

In order to assign channels to the links of the MPSPT graph, each link is assigned a ranking by the gateway. The ranking associated with each link is derived from the number of nodes that use a link to reach the gateway node. If *l* is link and *n* is node using link *l* to reach the gateway, then rank of link *l*, i.e. *rl*, is given by

$$r\_l = \sum\_{n=1}^{N} I\_{n,l'} \tag{4}$$

Channel Assignment Using Topology Control Based on Power Control in Wireless Mesh Networks 63

The gateway assigns a channel to each link in the order of its rank, and it begins with assigning the 11 available non-overlapping channels to the 11 highest-ranked links such that Channel 1 is assigned to the highest-ranked link. For the 12th-ranked link and onwards, the gateway checks the channel assignment of all links within the interference range of both

Out of the 11 available channels, channels which are not assigned to any link within the interference range of both nodes that constitute the 12th-ranked link are termed as nonconflicting channels. If the gateway finds one or more non-conflicting channels, it assigns that channel from the unassigned non-conflicting channels to the 12th-ranked link which has

If the gateway cannot find any channel among the 11 available channels that is not assigned to any link within the interference range of both nodes that constitute the 12th-ranked link, it selects the least interfering channel and assigns it to that link. A Least Interfering Channel (LIC) is a channel which causes minimum interference within the interference range of both

In order to find out the LIC, the gateway builds the interference level (IL) for all the 11 channels. The LIC is the channel with the minimum IL, which means that assigning this

In order to build the IL for Channel One, the gateway finds all links within the interference range of each of the two nodes that constitute the 12th-ranked link that use Channel One, and calculates IL of each link based on its rank and distance from a node of the 12th-ranked link. It sums up individual ILs of all links that use Channel One within the interference range of each of the two nodes that constitute the 12th-ranked link, to find out total IL for

<sup>1</sup> ( ) , *<sup>m</sup>*

*m* is a link using channel *i* that is within the interference range of a node of the 12th-ranked link,

*m m*

*R d*

(5)

*i*

*d* is distance from a node of link *m* to a node of the 12th-ranked link, and α is the path loss exponent and is 2 or 4, depending on cross over distance.

where *i* is the channel that has value between 1 and 11,

*<sup>r</sup> IL*

channel to the 12th-ranked link would result in minimum interference in the network.

**e. Channel Assignment** 

nodes that constitute that link.

i. Non-conflicting Channel

the highest channel number.

ii. Least Interfering Channel

iii. Interference Level

Channel One. This is done by

*(IL)i* is interference level of channel *i*,

*R* is the maximum rank assigned to a link,

*r* is the rank of link *m*,

nodes that constitute the 12th-ranked link.

where *N* is the total number of nodes in the network. *In,l* is 1 if node *n* is using link *l* and 0 otherwise.

In the case of two or more links that have the same rank, the link whose power of the farthest node to the gateway is smaller is given priority in channel assignment. If there are some links that still have the same rank, the link with smaller node IDs is given priority in channel assignment.

#### **e. Channel Assignment**

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

reach any node in the connectivity graph directly or through intermediate hops.

**c. Minimum Power-based Shortest Path Tree with a MND of Four** 

resulting MPSPT graph is not connected, the gateway moves to a higher TCA.

minimized and the overall network throughput is maximized.

gateway, then rank of link *l*, i.e. *rl*, is given by

,

Based on the FNTs of all nodes, the gateway builds the connectivity graph and checks the resulting network for connectivity. A topology is said to be connected if the gateway can

If the resulting network is not connected, the gateway moves to the next higher TCA by incrementing *x* in the *Select x for less than x* TCA, builds the connectivity graph and checks the resulting network for connectivity. The gateway keeps on moving to a higher TCA until it finds that the network resulting from the connectivity graph is connected.

After ensuring that the connectivity graph is connected, the gateway builds the Shortest Path Tree (SPT), using Dijkstra's algorithm [25], based on the connectivity graph. The metric

The node degree is defined as the number of TR neighbors of a node. The number of TR neighbors of a mesh router is bounded by the number of its radios and each node has four data radios. So, if any node in the shortest path tree has more than four links, the gateway selects those four links for that node which have the minimum weight and sets the weight of all other links to infinity. In other words, the gateway ensures that each node can have a maximum of four TR neighbors and builds a Minimum Power-based SPT (MPSPT) with a MND of 4 per node. The gateway checks the resulting MPSPT graph for connectivity. If the

Once the MPSPT graph is determined, the gateway has to assign channels to links of the MPSPT. Now, the objective is to assign channels to the links of the MPSPT such that the interference between simultaneous transmissions on links operating on the same channel is

In order to assign channels to the links of the MPSPT graph, each link is assigned a ranking by the gateway. The ranking associated with each link is derived from the number of nodes that use a link to reach the gateway node. If *l* is link and *n* is node using link *l* to reach the

> 1 ,

where *N* is the total number of nodes in the network. *In,l* is 1 if node *n* is using link *l* and 0

In the case of two or more links that have the same rank, the link whose power of the farthest node to the gateway is smaller is given priority in channel assignment. If there are some links that still have the same rank, the link with smaller node IDs is given priority in

(4)

*N l nl n r I* 

**b. Connectivity Graph** 

for path selection is minimum power.

**d. Link Ranking** 

otherwise.

channel assignment.

The gateway assigns a channel to each link in the order of its rank, and it begins with assigning the 11 available non-overlapping channels to the 11 highest-ranked links such that Channel 1 is assigned to the highest-ranked link. For the 12th-ranked link and onwards, the gateway checks the channel assignment of all links within the interference range of both nodes that constitute that link.

i. Non-conflicting Channel

Out of the 11 available channels, channels which are not assigned to any link within the interference range of both nodes that constitute the 12th-ranked link are termed as nonconflicting channels. If the gateway finds one or more non-conflicting channels, it assigns that channel from the unassigned non-conflicting channels to the 12th-ranked link which has the highest channel number.

#### ii. Least Interfering Channel

If the gateway cannot find any channel among the 11 available channels that is not assigned to any link within the interference range of both nodes that constitute the 12th-ranked link, it selects the least interfering channel and assigns it to that link. A Least Interfering Channel (LIC) is a channel which causes minimum interference within the interference range of both nodes that constitute the 12th-ranked link.

#### iii. Interference Level

In order to find out the LIC, the gateway builds the interference level (IL) for all the 11 channels. The LIC is the channel with the minimum IL, which means that assigning this channel to the 12th-ranked link would result in minimum interference in the network.

In order to build the IL for Channel One, the gateway finds all links within the interference range of each of the two nodes that constitute the 12th-ranked link that use Channel One, and calculates IL of each link based on its rank and distance from a node of the 12th-ranked link. It sums up individual ILs of all links that use Channel One within the interference range of each of the two nodes that constitute the 12th-ranked link, to find out total IL for Channel One. This is done by

$$(IL)\_i = \sum\_{m} \left(\frac{r\_m}{R}\right) \left(\frac{1}{d\_m^{\alpha}}\right) \tag{5}$$

where *i* is the channel that has value between 1 and 11,

*(IL)i* is interference level of channel *i*,

*m* is a link using channel *i* that is within the interference range of a node of the 12th-ranked link, *r* is the rank of link *m*,

*R* is the maximum rank assigned to a link,

*d* is distance from a node of link *m* to a node of the 12th-ranked link, and

α is the path loss exponent and is 2 or 4, depending on cross over distance.

Channel Assignment Using Topology Control Based on Power Control in Wireless Mesh Networks 65

1 2 <sup>11</sup> min , ,........, . *LIC IL IL IL IL* (6)

If a link is emanating from either of the two nodes that constitute the 12th-ranked link and a

Using its control radio, the gateway then sends each mesh node the Channel Assignment and Routing Message (CARM). For each channel assigned to a mesh router, CARM contains the channel number and the neighbor node to communicate with, using this channel. The CARM also contains the next hop to reach the gateway for data traffic. Based on the channel assigned to a mesh router to communicate with a neighbor and its distance to that neighbor, the mesh router applies power control and adjusts its transmission power accordingly by

When a node fails, nodes in its sub-tree lose their connectivity to the gateway and hence, the Internet through the wired world. TICA supports automatic and fast failure recovery and reorganizes the network to bypass the failed node and to restore the connectivity. In case of node failure, the FRM of TICA, which is shown in Figure 7, is initiated by the gateway.

All nodes send periodic "keep-alive" messages to the gateway on the control channel using their control radios. The keep-alive message from a node tells the gateway that the node is active. If the gateway does not receive three consecutive keep-alives from a node z, then it concludes that node z has failed and is no longer active. The gateway then deletes the MPNT for this node and deletes node z from the MPNT of all its neighboring nodes. Note that the gateway has MPNTs of all nodes, as all nodes sent their MPNTs to the gateway during the setup phase. The gateway builds the FNTs for all nodes using the *Select x for less* 

Based on the FNTs of all nodes, the gateway builds the connectivity graph, the MPSPT with a MND of four, the link ranking for the links of the MPSPT and assigns the channels to all links of the MPSPT. The gateway then sends the new CARM to all nodes in the network on

In this section, the performance evaluation of the proposed channel assignment algorithm is provided. Different topologies used for performance evaluation are presented. The performance of the proposed channel assignment algorithm for MRMC WMNs is compared against a "Single-Radio Single-Channel" (SRSC) scheme and a "Common Channel

channel has been assigned to that link, then the IL for this channel is set to infinity.

The LIC is the channel with the minimum interference level and is selected by

Similarly, the gateway assigns channels to all the links of the MPSPT.

iv. Channel Assignment and Routing Message

using the appropriate propagation model.

*4.3.3. Failure recovery mechanism of TICA* 

*than x* TCA.

the control channel.

**5. Performance evaluation** 

**Figure 6.** Topology-controlled Interference-aware Channel-assignment Algorithm (TICA)

If a link is emanating from either of the two nodes that constitute the 12th-ranked link and a channel has been assigned to that link, then the IL for this channel is set to infinity.

The LIC is the channel with the minimum interference level and is selected by

$$\text{tr}\left(\text{IL}\right)\_{\text{LIC}} = \min\left[ \left( \text{IL}\right)\_1, \left( \text{IL}\right)\_2, \dots, \text{mm}\left( \text{IL}\right)\_{\text{11}}\right]. \tag{6}$$

Similarly, the gateway assigns channels to all the links of the MPSPT.

iv. Channel Assignment and Routing Message

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

If any node in the SPT has MND > 4

Connected

The gateway moves to the next higher TCA

If GW finds channels that are not assigned to any link within IR of a link

> Similarly, GW assigns channels to all links

**Figure 6.** Topology-controlled Interference-aware Channel-assignment Algorithm (TICA)

GW keeps four links with the

Not connected

GW checks resulting network for connectivity

> GW moves to a higher TCA

For 12th ranked link and higher, GW checks channel assignment within its IR neighborhood

GW selects a LIC

No

minimum weight Yes

GW runs TCA and builds connectivity graph

MRs send MPNT to GW

GW builds SPT based on minimum power

GW assigns 11 available channels to 11 highest ranked links

GW assigns channel with highest channel number

Yes

For each link in SPT, GW builds link ranking

Connected

GW checks resulting network for connectivity

No

Not connected

Using its control radio, the gateway then sends each mesh node the Channel Assignment and Routing Message (CARM). For each channel assigned to a mesh router, CARM contains the channel number and the neighbor node to communicate with, using this channel. The CARM also contains the next hop to reach the gateway for data traffic. Based on the channel assigned to a mesh router to communicate with a neighbor and its distance to that neighbor, the mesh router applies power control and adjusts its transmission power accordingly by using the appropriate propagation model.

### *4.3.3. Failure recovery mechanism of TICA*

When a node fails, nodes in its sub-tree lose their connectivity to the gateway and hence, the Internet through the wired world. TICA supports automatic and fast failure recovery and reorganizes the network to bypass the failed node and to restore the connectivity. In case of node failure, the FRM of TICA, which is shown in Figure 7, is initiated by the gateway.

All nodes send periodic "keep-alive" messages to the gateway on the control channel using their control radios. The keep-alive message from a node tells the gateway that the node is active. If the gateway does not receive three consecutive keep-alives from a node z, then it concludes that node z has failed and is no longer active. The gateway then deletes the MPNT for this node and deletes node z from the MPNT of all its neighboring nodes. Note that the gateway has MPNTs of all nodes, as all nodes sent their MPNTs to the gateway during the setup phase. The gateway builds the FNTs for all nodes using the *Select x for less than x* TCA.

Based on the FNTs of all nodes, the gateway builds the connectivity graph, the MPSPT with a MND of four, the link ranking for the links of the MPSPT and assigns the channels to all links of the MPSPT. The gateway then sends the new CARM to all nodes in the network on the control channel.
