Capacity of Wireless Mesh Networks: Comparing Single-Radio, Dual-Radio, and Multi-Radio Networks

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 • 1st 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.