Presented by Tae-Seok Kim Joint Channel Assignment and Routing for Throughput Optimization in Multiradio Wireless Mesh Networks Mansoor Alicherry Randeep Bhatia Li (Erran) Li Bell Laboratories, Lucent Technologies Presented by Tae-Seok Kim

Wireless LAN/MAN Evolution Internet/WAN Wireless routers with gateway functionality interface with wired network Gateway Internet/WAN Wired backbone Wireless Mesh backbone Wireless routers form a self organized & self configured wireless mesh backbone Multi-hop wireless routing Only last hop is wireless Wireless Mesh Network (WMN) Traditional wireless networks Access points connected to a wired backbone

WMN performance issues Internet Active Inactive Interference limits capacity of wireless mesh backbone Performance degrades as network size increases Throughput degrades as the number of wireless hop increases. Performance improvement options: Multi-channel, Multi-radio, Routing

WMN single radio nodes and multiple channels Internet Active 1 2 When used with single radio nodes may result in network partitioning. One partition per channel Multiple channels can be used to lower interference More simultaneous transmissions are possible

WMN multiple radio nodes and multiple channels Internet 3 Active 1 4 channels 2 radios per node 2 4 If enough node-radios and enough channels then interference may be completely eliminated In practice small number of avail. channels and few radios per node Interference still an issue Channel assignment is crucial

Channel assignment and routing go hand in hand Internet Routing Paths 4 channels 2 radios per node Internet 3 1 2 4 3 4 1 2 Routing Paths Channel assignment and routing paths must be jointly computed. Channel assignment and routing are inter-dependent. channel assignment: link bandwidths and interfere extent traffic routing: the traffic flows for each link Channel assignments need to be done in a way such that the communication requirements for the links can be met.

WMN design problem: Joint channel assignment and routing to maximize throughput Given: A WMN G= (V,E) K orthogonal channels Number of radios I(u) per node u. Traffic load l(u) for each node u directed for the internet. Find: A channel assignment for node radios (from the set 1..K) A set of packet routes over the WMN backbone and their associated flow (traffic) A fair throughput measure . Such that: An interference free communication schedule can route l(u) traffic for each node u A periodic (with period T) time slotted schedule is assumed. is maximized

Wireless model Interference Range Defines interfering links in E Communication Link Defines communication links (e.g. ) Communication Range Link Interferes with link Does not interfere with link Interfere link set for ( , ) is not a communication link Link e has the maximum rate of c(e) May depend on distance

Steps for WMN design problem Routing: Solving LP Channel Assignment Flow Scaling Interference Free Link Scheduling

Scheduling (given the routing and channel assignment) Internet Internet 2 channels 1 2 Necessary condition: (e.g. c(2) = 8) c(q)- approximation algo. for scheduling Sufficient condition: Scheduling problem is NP-hard even for 1 channel

Scheduling necessary condition continued To show: In a given slot of an interference free link schedule either link e is active or at most c(q) links from can be simultaneously active Do not interfere

Scheduling necessary condition continued How many nodes all apart can be located in a circle of radius How many non-overlapping circles of radius can be packed in a circle of radius The answer is c(q) (e.g. c(2) = 8)

Routing Problem: LP formulation (May not result in feasible channel assignment) Internet 4 channels 2 radios per node l()=2 l()=0 l(): node traffic load Each link of capacity c(e) Let q=2, c(q) = 8 Find maximum st. all incoming node flow can be routed to the internet node: Flow conservation constraints Scheduling necessary condition Node radio constraints Capacity constraints

Channel Assignment Algorithm Phase 1 4 channels 2 radios per node [a] [b] [c] [d] Internet [a] [b] [d] [c] Internet Each f(e,i) = 1/4 1 1 1 1 Basic Flow Transformation Step: For an link e move flow f(e,i) for channel i to flow f(e,j) for channel j. Normalize (by splitting) nodes to have approx. same number of radios I (I=2 here) Using only first I channels, transform flows such a way that the network has a large number of connected components with small intra-component interference Interference of a connected component A: At end of the phase channel assignment is feasible however some constraints may be violated

Channel Assignment Algorithm Phase 2 Internet [a] [b] [d] [c] 1 4 channels 2 radios per node [a] [b] [c] [d] [a] [b] [c] [d] Group into at most K (=4) groups the connected components in all the channels (first I (=2) channels): A group can only contain connected components from the same channel Greedily merged such that the merging causes the least increase in max group interference Assign links in the i-th group to channel i Scale flow (and ) to satisfy scheduling and node constraints At end of the phase channel assignment is still feasible, all constraints satisfied by flow scaling

The overall algorithm Theorem: The overall algorithm (scheduling, routing and channel assignment) is a Kc(q)/I approximation algorithm for the joint routing and channel assignment problem to maximize throughput. K is number of available channels I is minimum number of radios per node c(q) depends on the interference model parameter q ( ). For instance c(2)=8.

Simulation Setup 802.11a radios 60 nodes networks Link rates distance dependent (Max. 54 Mbps) Max. num of available channels = 12 channels 60 nodes networks Grid networks: 8X8 grid, Grid size , Random node distribution on grid points Random connected networks: Nodes placed randomly in 500mX500m area (9 random connected topologies ) 20 randomly chosen nodes have load of 20Mbps each Vary number of gateway nodes from 2 to 12 Vary number of radios per node from 1 to 4 (equal per node) Vary number of available channels from 1 to 12

Varying the number of available channels As the number of available channels increase the per node throughput generally increases

Varying the number of radios per node & number of gateway nodes As the number of radios per node increase the per node throughput generally increases (the number of channels is 12) Highest increase in going from 1 to 2 radios per node As the number of gateways increase the per node throughput generally increases

Algorithm performance compared with the worst case approximation bound Upper Bound: LP optimal value(may not be feasible) λ* # of gateways: 8, # of radios: 3, # of channels: 12 Worst Case Bound: Approximation ratio performance bound (is feasible) λ*l(v)/W, where W=Kc(q)/I Proposed Algorithm At least 5.3 and 8.3 times better than Worst Case Bound At most 4.0 and 2.4 times worse than the Upper Bound

Conclusion Multiple radios and multiple channels can be used to alleviate interference in WMNs Channel assignment and routing is key Designed an efficient algorithm for joint channel assignment and routing for WMN The algorithm has a constant approximation bound Is able to make use of multiple radios/channels for increased per node throughput Future work Dynamic Channel Assignment Routing based on distributed routing protocols (e.g. OSPF)