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Understanding Wireless Mesh

Rather than blasting through a building with high power, a wireless mesh system will forward packets through intermediate nodes that are within line of sight and go around the obstruction with robust wireless links operating at much lower power.


May 1, 2008  


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This article focuses on wireless mesh infrastructure systems used for creating large Wi- Fi access networks and examines three different approaches currently available for implementing them. It examines the strengths and weaknesses of each approach with a particular focus on the capacity that is available to users.

Can wireless mesh infrastructure systems deliver enough capacity to support broadband services for a large number of users?

Mesh is a type of network architecture. Originally, Ethernet was a shared bus topology in which every node tapped into a common cable that carried all transmissions from all nodes. In bus networks, any node on the network hears all transmissions from every other node in the network.

Most local area networks (LANs) today use a star topology in which every network node is connected to a switch (switches can be interconnected to form larger networks).

Mesh networks are different in that full physical layer connectivity is not required. As long as a node is connected to at least one other node in a mesh network, it will have full connectivity to the entire network because each mesh node forwards packets to other nodes in the network as required. Mesh protocols automatically determine the best route through the network and can dynamically reconfigure the network if a link becomes unusable.

There are many different types of mesh networks. Mesh networks can be wired or wireless. For wireless networks there are ad-hoc mobile mesh networks and permanent infrastructure mesh networks. There are single radio mesh networks, dual-radio mesh networks and multi-radio mesh networks. All of these approaches have their strengths and weaknesses. They can be targeted at different applications and used to address different stages in the evolution and growth of the network.

The first wireless mesh networks were mobile ad hoc networks with wireless stations moving around and participating in a peer to peer network.

Mesh is an attractive approach for wireless networking since wireless nodes may be mobile and it is common for a wireless node to participate in a network without being able to hear all of the other nodes in the network.

Mobile peer to peer networks benefit from the sparse connectivity requirements of the mesh architecture; and the combination of wireless and mesh can provide a reliable network with a great deal of flexibility.

The popularity of Wi-Fi has generated a lot of interest in developing wireless networks that support Wi-Fi access across very large areas. Large coverage access points (AP) are available for these scenarios, but the cost of deploying these wide area Wi-Fi systems is dominated by the cost of the network required to interconnect the APs and connect them to the Internet — the backhaul network.

Even with fewer APs, it is very expensive to provide T1, DSL or Ethernet backhaul for each access point. For these deployments, wireless backhaul is an attractive alternative and a good application for mesh networking.

Wireless connections can be used between most of the APs and just a few wired connections back to the Internet are required to support the entire network.

Wireless links work better when there is clear line of sight between the communicating stations.

Permanent wireless infrastructure mesh systems deployed over large areas can use the forwarding capabilities of the mesh architecture to go around physical obstacles such as buildings.

Rather than blasting through a building with high power, a wireless mesh system will forward packets through intermediate nodes that are within line of sight and go around the obstruction with robust wireless links operating at much lower power.

This approach works very well in dense urban areas with many obstructions.

There are many different types of mesh systems and they often get lumped together. Since early wireless mesh systems were focused on mobile ad-hoc networks, many people assume that wireless mesh systems are low bandwidth or temporary systems that can not scale up to deliver the capacity and quality of service required for enterprise, service provider and public safety networks. That is not the case. Engineered, planned and deployed effectively, wireless mesh networks can scale very well, while still offering a cost-effective evolution strategy that preserves the network investment.

Understanding the strengths and weaknesses of single, dual, and multi-radio mesh options is the first step.

Single-radio Wireless Mesh: In a single-radio mesh, each mesh node acts as an AP that supports local Wi-Fi client access and forwards traffic wirelessly to other mesh nodes. The same radio is used for access and wireless backhaul.

This option represents the lowest cost entry point in the deployment of a wireless mesh network infrastructure. However, because each mesh AP uses an omni-directional antenna to allow it to communicate with any of its neighbor APs, almost every packet generated by local clients must be repeated on the same channel to send it to at least one neighboring mesh AP.

The packet is then forwarded to another node in the mesh and ultimately to a node that is connected to a wired network.

This packet forwarding generates a lot of traffic. As more mesh APs are added, a higher percentage of the wireless traffic in any cell is dedicated to forwarding. Very little of the channel capacity is available to support users.

There is debate in the industry about the impact of mesh forwarding and actual throughput that is possible in this scenario at the TCP/IP layer of about 5 Mbps.

In a single-radio Wi-Fi mesh network, all clients and mesh APs must operate on the same channel and use the 802.11 Media Access Control protocol. As a result, the entire mesh ends up acting like a single, giant access point-all of the mesh APs and all of the clients must contend for a single channel. This shared network contention and interference reduces capacity further and introduces unpredictable delays in the system as forwarded packets from mesh APs and new packets from clients contend for the same channel.

The capacity in a single-radio mesh is limited by both access and backhaul issues. Optimizing the mesh forwarding protocol will not solve the problem.

The basic capacity is too low and adding more mesh nodes makes it worse-no matter how perfect the mesh protocol.

Single-radio solutions offer the lowest cost entry point in the deployment of mesh networks. In an infrastructure network, single radio mesh systems are best used for small mesh clusters of a few nodes.

Larger systems may be created by providing wired backhaul to one of the nodes in each cluster or using wireless backhaul links to aggregate multiple clusters. Single radio mesh solutions can also be the right approach for mobile, ad hoc peer-to-peer wireless networks where the emphasis is on basic connectivity or used for large sensor network and meter reading networks where the data rate is very low.

Dual-Radio Wireless Mesh: The capacity and scaling ability of wireless mesh infrastructure networks can be improved by using APs that have separate radios for client access and wireless backhaul.

In a dual-radio mesh, the APs have two radios operating on different frequencies. One radio is used for client access and the other radio provides wireless backhaul. The radios operate in different frequency bands, so they can run in parallel with no interference. A typical configuration is 2.4 GHz Wi-Fi for local access and some flavour of 5 GHz wireless for backhaul. Since the mesh interconnection is performed by a separate radio operating on a different channel, local wireless access is not affected by mesh forwarding and can run at full speed.

However, in a dual radio mesh the wireless backhaul is a shared network so it is subject to the same network contention issues that hamper the single radio mesh. The contention on the backhaul network limits capacity and creates
additional latency making the dual radio approach inappropriate for voice traffic.

The backhaul mesh in dual-radio mesh architectures is usually a shared network running the 802.11 MAC protocol. With only one radio dedicated to backhaul at each node, all of the mesh APs must use the same channel for connectivity to the backhaul mesh.

Parallel operation on the backhaul network is not possible, as most of the APs hear multiple other mesh APs. So they must contend for the channel and at the same time generate interference for each other. The result is reduced system capacity as the network grows.

As noted earlier, the useful capacity of each Wi-Fi access coverage cell is 5 Mbps. In dual-radio mesh systems, the access radios of adjacent cells can use different channels. There are three non-overlapping channels in the 2.4 GHz band, so they will be able to operate independently in most cases.

The most likely shared backhaul network protocol is 802.11a, which has a raw date rate of 54 Mbps and useful throughput of approximately 20 Mbps in this type of network.

Capacity is limited because of the behavior of the shared network used for the backhaul. Contention and interference vary depending on the placement of the APs. All of the APs must operate on the same channel for the wireless backhaul and they must be able to hear at least one other AP in order to be part of the mesh.

Typically, each AP will be able to hear at least two or three other APs. Those with more adjacent neighbors will have more contention and generate more interference than isolated mesh APs at the edge of the network.

Multi-Radio Wireless Mesh: Like a dual-radio wireless mesh, a multi-radio wireless mesh separates access and backhaul.

It goes a step further, however, to provide increased capacity by addressing the shared backhaul network issues that limit the dual-radio mesh architecture.

In a multi-radio wireless mesh, multiple radios in each mesh node are dedicated to backhaul.

The backhaul mesh is no longer a shared network, since it is built from multiple point-topoint wireless links and each of the backhaul links operates on different independent channels.

This type of multi-radio design is called a multiple point-to-point mesh. It is possible to create very rich mesh topologies with this multi-radio approach and just a few backhaul radios at each node.

When used for backhaul in this fashion, the performance of a multi-radio mesh is similar to switched, wired connections.

Point-to-point links

The mesh radios operate independently on different channels so latency is very low. There are only two nodes per mesh link, so contention is very low.

In fact, it is possible to run a customized point-to-point protocol that optimizes throughput in this simple two-node contention-free environment. These dedicated point-topoint links are usually in the unlicensed 5.8 GHz band and based on 802.11a chipsets today.

In the near future this will be a good application for 802.16d Wi-Max. These pre-Wi-Max wireless links have a potential throughput of approximately 25 Mbps.

Performance in a multi-radio mesh is much better than the dual-radio or single-radio mesh approaches.

The mesh delivers more capacity and continues to scale as the size of the network is increased-as more nodes are added to the system, overall system capacity grows.

In the real world, large wireless networks require an integrated combination of the three mesh approaches described. It is possible to deploy a very low cost, low capacity network based mostly on single-radio mesh with some multi-radio mesh nodes acting as aggregators for single radio mesh clusters.

Over time, the network can be upgraded with more capacity or better QoS by replacing single-radio mesh nodes at the edge with multi-radio nodes.

Network design should be customized to meet the application requirements and budget by using the appropriate mix of the different wireless mesh approaches.

David Park is vice president of product marketing with Kanata, Ont.-based Belair Networks Inc. Prior to joining the company, he held senior engineering and management positions with Nortel Networks, Bell Northern Research STC Technology, and Marconi Communications.

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In a dual-radio mesh, the APs have two radios operating on different frequencies.