1. Capacity of Wireless Mesh Networks: Comparing Single- Radio, Dual-Radio, and Multi- Radio Networks By: Alan Applegate

2. Introduction  Common Topologies:  Bus Network  Backbone  All nodes hear all transmissions  Star Network  Central switch/router  Nodes only hear transmissions intended for them  Mesh  All nodes logically connected to each other  Connection to only one node required  Packets forwarded through nodes to reach destination

3. Introduction Cont’d  Types of Mesh Networks:  Wired  Physical connection of every node to every other node  Wireless  Ad-hoc  Permanent infrastructure  Shared  Switched  Single-Radio  Dual-Radio  Multi-Radio

4. History of Mesh Networks  1 st wireless mesh networks were mobile ad-hoc  Wireless stations dynamically participated in a peer- to-peer network (i.e. mobile p2p)  Mesh used because it allowed a node to participate in a network without needing to communicate with every other node  Also very flexible

5. Why We Need Wireless Mesh Networks  Traditional approach of a collection of WiFi access point’s connected to a backhaul network is extremely expensive  Wireless mesh ad-hoc networks are much more flexible  Lower cost  Wider coverage  More reliable

6. Why Wireless Mesh Works  Wireless works best with LoS  Permanent wireless infrastructure mesh systems can utilize forwarding capabilities of the mesh architecture to maneuver around physical obstacles  Less expensive than using high-power signals to blast through obstructions as used with traditional point to multipoint systems  Maintains LoS for best quality signal  Redundancy = Reliability  High levels of frequency re-use between mesh links  Increases overall system capacity  Works well in dense urban areas

7. Terminology  Single-Radio (Shared Mesh Network)  A wireless mesh network that uses a single- radio to communicate to all neighboring nodes  Total B/W between all nodes  Mesh Access Points (MAPs)  Include both mesh interconnection links and client access

8. Terminology  Mesh Points (MPs)  Mesh nodes used exclusively for forwarding  Dual-Radio Shared MAP  Uses separate access and mesh link radios  Only the mesh link radio is shared  B/W shared between mesh links and client access

9. Terminology  Multi-Radio (Switched Mesh Network)  Uses multiple radios to communicate via dedicated mesh links to each neighboring node in the mesh  All B/W of radio channel dedicated to neighboring node’s link  Uses separate access and multiple mesh link radios

10. Terminology  Mesh Cluster  The collection of mesh APs that “home” to a particular wired egress connection  Mesh Portal  The mesh point located at the egress connection

11. Single-Radio Shared Wireless Mesh  Description  Each AP node acts as a regular AP that supports WiFi client access as well as forwarding traffic wirelessly to other mesh points  Same radio used for access and wireless mesh links  Omni-directional antenna

12. Single-Radio Shared Wireless Mesh  Pros:  Lowest cost deployment of a wireless mesh network  Simplest implementation

13. Single-Radio Shared Wireless Mesh  Cons:  Every packet generated must be repeated on the same channel to send it to at least one neighboring AP until reaching the mesh portal  Creates excessive traffic  More APs = More traffic dedicated to forwarding  Very little channel capacity available to support users

14. Single-Radio Shared Wireless Mesh  Cons cont’d:  Capacity reduction as a result of forwarding is between 1/N and (1/2)^N where N is equal to the number of mesh link hops.  i.e. Capacity available to users decreases with each additional AP

15. Single-Radio Shared Wireless Mesh  Cons cont’d:  Use of 1/N or (1/2)^N depends on a number of factors including topology, location of the mesh portal and interference domain  Interference domain = # of nodes whose transmissions will be sensed by and hence block the transmission of other nodes.  1/N is the most optimistic, which is achieved via routing protocols that optimize forwarding and eliminate unnecessary transmissions.

16. Single-Radio Shared Wireless Mesh  All clients and mesh APs must operate on the same channel and use the 802.11 MAC protocol to control contention for the physical medium.  Entire mesh acts like a single AP, all APs and clients must contend for a single channel

17. Dual-Radio Shared Wireless Mesh  Description:  Separate radios for client access and mesh links  Operate @ different frequencies  Typical configuration:  2.4 Ghz client access  5 Ghz mesh link  802.11 MAC

18. Dual-Radio Shared Wireless Mesh  Pros:  Improved capacity and scalability over single-radio  Client access not affected by mesh link forwarding

19. Dual-Radio Shared Wireless Mesh  Cons:  Mesh link contention still limits capacity  Sometimes results in blocking other APs  Results in reduced system capacity as the network grows  Dual-radio systems are a significant improvement over single-radio mesh designs and provide for more potential growth of a mesh cluster

20. Multi-Radio Switched Wireless Mesh  Description:  Separates access and mesh links  Multiple radios  Typically uses directional antennas  Creates a dedicated link between mesh points  aka multiple point to point  Based on 802.11a  Operates @ unlicensed 5 Ghz band

21. Multi-Radio Switched Wireless Mesh  Pros:  Provides increased capacity by overcoming shared mesh limitations inherent to single and dual-radio mesh architectures  No longer a shared network  Individual mesh links have dedicated radio channel  Very rich mesh topologies possible

22. Multi-Radio Switched Wireless Mesh  Pros cont’d:  Low contention  Much higher performance than dual- or single-radio  More capacity and scalability  More nodes = more capacity  Capacity only limited by wired backhaul  Co-existence eliminated  Interference reduced  Backhaul latency low and predictable  Mesh link range increased (directional antennas)

23. Conclusion  The capacity of wireless mesh networks is directly affected by the shared network contention of the mesh links between mesh points used to forward packets  Single-radio best for small mesh clusters at the edge of the network  Dual-radio represents evolution in the growth of a mesh network

24. Conclusion Cont’d  Multi-radio separate wireless access and mesh links  This eliminates in-channel mesh forwarding and shared mesh link contention  Results in high capacity system that can scale to support large networks with broadband service for many users.