Rate-Based Channel Assignment Algorithm for Multi-Channel Multi-Rate Wireless Mesh Networks

Sok-Hyong Kim and Young-Joo Suh

Department of Computer Science & Engineering

Pohang University of Science & Technology (POSTECH), Pohang, Korea

IEEE GLOBECOM 2008

Outline Introduction Motivation Rate-Based Channel Assignment algorithm

Spanning Tree Formation Rate-Hop and Local Traffic Count Metrics for RB-CA Channel Assignment Algorithm

Performance evaluation Conclusion

Introduction A WMN consists of a set of stationary wireless mesh routers

forming a multi-hop wireless backbone Some routers act as gateways linked to the Internet via high-

speed wires Each mesh router offers Internet access for wireless mesh

clients

Introduction The network capacity and the performance of WMN can be

increased by using multiple non-overlapping channels As a low-rate link tends to occupy the wireless channel

longer than the high-rate link, the overall performance of network is subject to low-rate links

This problem is often referred to as performance anomaly

Motivation - performance anomaly

• All routers in the figure form a spanning tree, where node A is the parent of nodes B, C, and D • We assume that the total number of channels is three• The offered load is enough on all the links

Motivation - performance anomaly

• Each flow starts at t = 10, 15, and 20 seconds from nodes B, C, and D, respectively

Rate-Based Channel Assignment algorithm Our focus is assigning a channel and selecting a path based

on the data rate of possible links in WMNs In general, each router joins one or multiple gateways by

constructing one or multiple spanning trees. However, we restrict routers to join only one spanning tree

And they are equipped with two interfaces: parent interface and child interface

Rate-Based Channel Assignment algorithm

Spanning Tree Formation Rate-Hop and Local Traffic Count Metrics for

RB-CA Channel Assignment Algorithm

Spanning Tree Formation In WMNs, each router on the tree periodically broadcasts

Mesh Advertisement (MA) messages to its 1-hop neighbors on every channel

Initially, the gateway connected to the Internet via wires only broadcasts MAs. After a while, nodes that joined the tree can send periodic MAs

A MA includes the source address of the MA, internet accessibility, channel number of the child interface, and the primary and secondary metrics.

Spanning Tree Formation (1)

A

X

Y

B

gateway

X sends periodic Mesh Advertisement (MA) to its 1-hop neighbors

Y joins node X by sending a Mesh Join (MJ) message whichincludes its own address

X creates an entry for child node Y and replies with MeshAcknowlegment (MAck)

X forwards the received MJ message to the correspondingparents on the path to the gateway. Then, each parent adds Y’sentry to its routing table

Spanning Tree Formation (2)

A

X

Y

B

gateway

X sends periodic Mesh Advertisement (MA) to its 1-hop neighbors

Y joins node X by sending a Mesh Join (MJ) message whichincludes its own address

X creates an entry for child node Y and replies with MeshAcknowlegment (MAck)

X forwards the received MJ message to the correspondingparents on the path to the gateway. Then, each parent adds Y’sentry to its routing table

Y sends a Mesh Disjoin (MD) message to its previous parent (say node B) after it receives MAck

Accordingly, all parents of node B delete the entry for child node Y from their routing tables

Rate-Hop and Local Traffic Count Metrics for RB-CA We consider a MCMR WMN based on IEEE 802.11b. The

available data rates are 11Mbps, 5.5Mbps, and 2 Mbps. The primary metric is rate-hop (RH), which reflects the data

rate of a path on the tree and a smaller RH is preferred The secondary metric is Local Traffic Count (LTC)

rate-hop (RH)

Since a higher data rate provides better performance, three RHs have the following relationship:

rate-hop (RH)

Note that low-rate links significantly degrade network performance due to the performance anomaly. Thus, the priority :

rate-hop (RH)

But, in terms of RH, smaller RH has higher priority. Thus, we have:

rate-hop (RH)• In practice, for specific values for RH11Mbps, RH5.5Mbps, and RH2Mbps, we set RH11Mbps to a positive constant. Then, RH5.5Mbps and RH2Mbps in eq. (2) have the lower bound.

• Thus, we restrict the range of RH2Mbps as follows:

where k is a tunable parameter to set the upper bound on RH5.5Mbps and RH2Mbps in eq. (2).

• Moreover, a path which consists of more than five 11Mbps- links supports lower data rate than 2 Mbps-link. Thus, k is set to five

rate-hop (RH)

• We can obtain the RH metric of node n expressed as

• Each node maintains the RH in the routing table entry for data forwarding from or to the gateway• update the metric with the recent RH in a MA from its parent.• The updated RH is advertised by nodes on the tree via periodic MAs.

• RH11Mbps, RH5.5Mbps, and RH2Mbps are assigned to 10, 21, and 43, respectively, based on eqs. (1)~(3).

Local Traffic Count (LTC)• In a case that a node discovers multiple parents having equivalent RH metrics

• A node n on the tree locally computes LTC by adopting the following weighted averaging technique, where α [0,1]∈

Every node periodically counts the amount of transmittedand received traffic in bytes through its child interface

Then, it updates the amount as LTCnew and broadcasts the updated LTC metric via periodic MAs.

Local Traffic Count (LTC)• Since traffic loads typically converge on the gateway in WMNs, the traffic variation also rapids near the gateway whereas that of nodes in the last hop is relatively slow.

• Thus, the nodes near the gateway use a large α value to reflect the up-to-date traffic loads as much as possible.

Channel Assignment Algorithm Initially, node X assigns random channels for its parent

interface and child interface Upon receiving the MAs from several nodes on the tree, node

X finds the “best parent node” based on the RH metric in the MA to assign a proper channel for the parent interface

Node X finishes channel assignment for its parent interface, it then selects the least loaded channel for its child interface to reflect the current channel condition.

Channel Assignment Algorithm In a distributed manner, physically r-hop neighboring nodes

on the tree periodically exchange Channel Traffic Count (CTC) messages and maintain CHannel Table (CHT) which contains the load information per channel

Then, each node periodically checks the least loaded channel and changes the current channel of its child interface if necessary

When a node decides to change its child interface’s channel, it informs child nodes of the new channel via CHannel Change (CHC).

Performance evaluation ns-2 network size is 2500 m by 2500 m a single gateway and several (5 to 30) routers All nodes are randomly distributed and each node is equipped

with two interfaces The transmission and interference ranges are 250 m and 550

m The number of available channels is 12 Each node uses 11, 5.5, and 2Mbps based on the 802.11b

corresponding to the ranges of 100, 200, 250 m, respectively

Performance evaluation

Performance evaluation

Conclusion We proposed the Rate-Based Channel Assignment (RB-CA)

algorithm for MCMR WMN. The proposed algorithm alters a low-rate single-hop path to a

high-rate multi-hop path. the proposed scheme shows improved performance in packet

delivery ratio and end-to-end delay

Thank You !