Wireless Mess Network #2

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.The capacity analysis is somewhere between 1/N times the channel capacity and (1/2)^N times the channel capacity where N is the number of wireless hops in the longest path between a client and the wired infrastructure.

Figure 2: Single-radio Wireless Mesh Capacity

Figure 2 shows AP capacity estimates for a single-radio Wi-Fi mesh network using these equations. User capacity available at each AP declines as you add more APs to the network and increase the number of wireless hops.The starting capacity is 5 Mbps because the network is a single channel of 802.11b, which has a raw data rate of 11 Mbps and useful throughput measured at the TCP/IP layer of about 5 Mbps. This throughput is shared between the access traffic and the backhaul traffic in a single radio mesh. Throughout this paper, the vertical axis can be scaled to reflect the radio capacity. Some good rules of thumb are; 5Mbps for 802.11b only mode, 11Mbps for 802.11b/g mixed mode and 22Mbps for 802.11g only mode.The latter is not typically deployed in public environments due to backwards compatibility with the large pool of 802.11b devices.

It doesn’t really matter which of the equations is closer to real world behavior. 1/N is more optimistic, but neither scales to support large networks. Capacity available in each cell declines rapidly as more APs are added.

There are mesh protocols that optimize the forwarding behavior and eliminate unnecessary transmissions. But the best these optimizations can do is to bring the network closer to 1/N performance, which is inadequate for most permanent infrastructure applications today. Single-radio mesh systems will not deliver broadband performance to the user population throughout a very large coverage area.

This analysis may seem harsh, but it is actually oversimplified. It assumes perfect mesh forwarding, no interference and perfect coordination of the Wi-Fi channel access.That will never happen, so real world throughput and capacity will usually be even lower.

Figure 3: Single-radio Mesh Architecture, String of Mesh APs
To illustrate this point, consider a linear string of mesh APs arranged so that each one can hear only one adjacent neighbor on either side (Figure 3).This is not a likely real world deployment, but it simplifies the analysis and we will use this example to compare each of the wireless infrastructure mesh approaches. Throughout this paper we will also assume that client access load is evenly distributed across the mesh APs. In this string of APs with the wired connection on the end, N the number of hops from figure 4, is same as the number of mesh APs.

The total channel capacity is 5 Mbps.You can see that 1/N performance is basically not achievable. N=5, so each AP should have 1 Mbps of capacity. All of the traffic from the entire mesh network will have to flow through AP5 to get to the wired network. If each mesh AP accepts a load of exactly 1 Mbps of traffic from its clients, then AP5 will have to forward 4 Mbps of traffic from APs 1, 2, 3 and 4; and has exactly 1 Mbps of capacity left for its local clients. For this to work, there would have to be perfect contention, interference and collision management. The mesh APs would have to coordinate their transmissions with each other and perfectly control the transmissions of all their respective clients. That is not how Wi-Fi works.

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 configuration in Figure 3 has minimal connectivity required to complete the mesh and minimum interaction between adjacent APs for a 5-node mesh AP network.APs 2, 3, and 4 can hear two other APs; and AP1 and AP5 hear one other AP each. Each time AP3 transmits,AP2 and AP4 must defer and hold off their transmissions since they are using the 802.11 MAC protocol, which is essentially “listen before talk”.Whenever that hold-off doesn’t happen, collisions and retransmissions occur resulting in more congestion and lower capacity.

A capacity analysis of these systems should include both the effects of the mesh forwarding and the effects of the shared network backhaul, which can be significant. Consider the string of Mesh APs in Figure 3. If we move the wired backhaul from AP5 to AP3, what happens to the capacity?

N, the number of forwarding hops, is reduced from 5 to 3, so we might expect the capacity to be higher than the N=5 capacity shown in Figure 4. However, due to the shared network behavior and the fact that AP3 can hear more mesh AP neighbors than AP5, the capacity is actually lower as shown in Figure 4. (Note:The x axis in Figure 4 is the number of Mesh APs, not the number of wireless hops in the longest path through the mesh.)

Figure 4: Single-radio Mesh String,Wired Connection in the Middle

The 1/N equation we used earlier predicts that per-AP capacity will be 1.67 Mbps when N=3. However, when we factor in the effects of contention and interference when the wired connection is in the middle of a string of 5 APs (Figure 3 with the wired connection at AP3), the estimated capacity is .58 Mbps. This matches the (1/2)N prediction of .56 Mbps when N = 5.

The string of mesh APs that we have described so far is not a typical mesh configuration.The cluster of mesh APs shown in Figure 5 is a more common example of a small mesh network.

Figure 5: Single-radio Mesh Cluster

In this case, contention and interference would reduce the capacity available for client access beyond what we have described in the string of APs examples previously discussed. Large coverage mesh APs in these systems have high power radios and high gain antennas.The mesh APs can hear each other at a much greater range than they can hear the clients they support, because most Wi-Fi client devices are low power with low gain antennas.

In this cluster, AP3 can hear all the other APs except for AP5.All traffic for the entire mesh network flows through AP3 so it will frequently hold off the other APs, limiting their ability to handle traffic from their local clients.A more complicated formula is required to characterize the impact of neighboring mesh APs in a shared backhaul network as well as the mesh forwarding.

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.

to be continued...
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