Load Balance Based Access Point Association in Wireless Mesh Networks

1. Load Balance Based Access Point Association in Wireless Mesh Networks Ma Tianze Computer Network and Protocol Testing Laboratory, Dept. of Computer Science & Technology, Tsinghua Univ.

2. Outline • Problem challenge • Related works • AP association algorithm • Indirect users balancing extension • Experiment

3. Problem challenge • WMN Characteristics • Wireless Mesh Network is a multi-hop wireless network • The data sent by a user will be transferred among the multi-hop path of the backbone to the mesh-router which has a back hole to the internet • A new AP association mechanism is needed because conventional association method using signal strength as the only metric may be not quite suitable for WMN

4. WMN Characteristics

5. Problem challenge • Factors Considered in AP Association • Signal strength: users would like to select an AP with good signal quality • Quantity of AP’s users: If an AP serves too many users, the channel access delay of each user will be too large because of channel access competition. • Multi-hop characteristic of WMN: Delay and congestion of the data path between AP and back holes

6. Related works • Dynamic Cross-Layer Association in 802.11-based Mesh Networks • George Athanasiou, Thanasis Korakis, Ozgur Ercetin, Leandros Tassiulas • IEEE INFOCOM 2007 • Defines an Airtime cost and make it the AP selection metric instead of 802.11 RSSRI

7. Related works • Joint Optimal Access Point Selection and Channel Assignment in Wireless Networks • Koutsopoulos.I, Tassiulas.L • Networking, IEEE/ACM Transactions • Proposed a SCLB algorithm to assign users who stays in the overlap area of APs

8. AP association algorithm • Some Definitions • A set of APs • A set of users • association coefficient • The coefficient increases when the AP is appropriate for the user, which embodies the preference of users in AP association

9. AP association algorithm • Network Model • Each AP has an coverage • The coverage of two or more APs is divided into several overlap areas • The algorithm has several passes • During a pass, all the overlap areas are traveled in turn

10. AP association algorithm • Algorithm Steps • 1. All the overlap areas are traveled in turn • 2. When it goes to a overlap area • For each AP who forms this area, work out how many users should be assigned to it • Using a best-bipartite-matching algorithm to satisfy above assignment and make the summation of association coefficient maximum • 3. After all the areas have been traveled, if the association of users an APs is the same with last pass, the algorithm halts. Else go to step 1

11. AP association algorithm • work out how many users should be assigned to each AP • Purpose: After the assignment, the number of users of each AP can be balanced to the best of our abilities

12. AP association algorithm • Using a best-bipartite-matching algorithm to satisfy above assignment • We have already worked out how many users should be assigned to each AP in a overlap area • The question is which user should be assigned to which AP • Purpose: make the summation of association coefficient maximum

13. AP association algorithm • best-bipartite-matching • Nodes in left side are users • Nodes in right side are APs split by number of users to be assigned to it in the area • Weight of edges is association coefficient • If a best-bipartite-matching is found, the summation of association coefficient maximum

14. Indirect users balancing extension • Indirect Users • An AP in WMN not only has direct users connected to it, but also has indirect users which would take it as a relay node of data transfer • Indirect users can also be seen as load of AP • If indirect users load of APs are not balanced, congestion may occurred on the data path

15. Indirect users balancing extension • As an extension to the above algorithm • we define a load degree which summarizes the direct and indirect users to scale the load of APs • The balancing of direct users will be replaced by the balancing of load degree.

16. Experiment • Tools • C++ codes to implement the algorithm • Performance Metrics • The overall throughput • End-to-end delay • Fairness of users • Compared with • Pure IEEE 802.11

17. Thank you!