Understanding Single Radio, Dual Radio and Multi-Radio Wireless Mesh Networks

Transcription

1 White Paper Capacity of Wireless Mesh Networks Understanding Single Radio, Dual Radio and Multi-Radio Wireless Mesh Networks Overview This paper 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 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.

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. Figure 1: Meshing Around Obstructions 2 Copyright 2006 BelAir Networks BDMC00040-C02

3 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 Per-AP Access Capacity (Mbps) /N (1/2)^N Number of hops Copyright 2006 BelAir Networks BDMC00040-C02 3

4 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 b, 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 b only mode, 11Mbps for b/g mixed mode and 22Mbps for g only mode.the latter is not typically deployed in public environments due to backwards compatibility with the large pool of b 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 Coverage area on channel 6 4 Copyright 2006 BelAir Networks BDMC00040-C02

5 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 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 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.) Copyright 2006 BelAir Networks BDMC00040-C02 5

6 6. 00 Per-AP Capacity, Wire in the Middle Per-AP Access Capacity (Mbps) Number of Mesh APs 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 Copyright 2006 BelAir Networks BDMC00040-C02

7 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. 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 flavor 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 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. Copyright 2006 BelAir Networks BDMC00040-C02 7

8 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 (Figure 6) Coverage on channel 6 Coverage on channel 1 Coverage on channel 11 Coverage on channel 56 Figure 6: Dual-Radio Wireless Mesh, String of Mesh APs The most likely shared backhaul network protocol is a, 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. It is difficult to predict the system capacity without making assumptions about AP placement. Figure 7 compares the capacity for the minimal overlap string of mesh APs shown in Figure 6.The backhaul network offers 20 Mbps of capacity, so per-ap capacity is good for a few nodes.after three or four nodes, the per-ap capacity drops off because of the shared network effects. In a more typical mesh cluster with more overlap between mesh APs, useful access capacity could be worse than shown here. 8 Copyright 2006 BelAir Networks BDMC00040-C02

9 6. 00 Per-AP Capacity Wired at End Per-AP Access Capacity (Mbps) Number of Mesh APs Figure 7: Per-AP Capacity in a Dual-Radio Wireless Mesh System,Wire at End. Dual-radio systems are a big improvement over single-radio mesh designs and represent a logical evolution in the growth of a mesh network. However, dual-radio systems alone don t scale to metro dimensions and the high and unpredictable latency on the shared backhaul network makes them a poor candidate for voice over Wi-Fi. 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-to-point 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. (Figure 8) Copyright 2006 BelAir Networks BDMC00040-C02 9

10 Coverage on channel 6 Channel 1 Channel 11 Channel 6 Channel 1 Coverage on channel Coverage on channel 11 Channel Channel 153 Channel 61 1 Channel 53 Channel 6 Figure 8: Multi-Radio Wireless Mesh, String of APs When used for backhaul in this fashion, the performance of a multi-radio mesh is similar to switched, wired connections.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 to point links are usually in the unlicensed 5.8 GHz band and based on a chipsets today. In the near future this will be a good application for d WiMAX.These pre-wimax 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. Figure 9 shows the capacity per-ap for the multi-radio configuration shown in Figure 8.We assume a channel capacity of 23 Mbps for each of the point-to-point wireless backhaul links. In this string of APs, without the direct link between AP1 and AP5, total system capacity is limited to a single channel of the backhaul link because there is only one backhaul link connecting the node to the wired network.this delivers maximum per-ap capacity for up to five mesh APs and then declines with each additional AP. 10 Copyright 2006 BelAir Networks BDMC00040-C02

11 String of Mesh APs Loop of Mesh APs Per-AP Capacity (Mbps) Number of Mesh APs Figure 9: Per-AP Capacity in a Multi-Radio Wireless Mesh String of APs,Wired Connection at the End or Loop. Adding the wireless link between AP1 and AP5 doubles system capacity and delivers maximum per-ap capacity through 10 APs, as shown. So the bottleneck in a multi-radio architecture is not in the wireless mesh. System capacity in this architecture is limited by the wired backhaul. System capacity will increase and per-ap capacity will remain stable as more mesh APs are added to the network as long as there is enough wired backhaul support. Capacity increases beyond those shown in Figure 9 are possible if there are multiple wired network egress points supplying the mesh. A more typical multi-radio mesh configuration is shown in Figure 10. In this design there are multiple paths through the network, and a mesh protocol would eliminate the forwarding loops and minimize the number of hops to the wired backhaul. Larger networks would typically have additional wired egress points to increase capacity and offer more redundancy in the system. Copyright 2006 BelAir Networks BDMC00040-C02 11

12 C 148 Channel 11 C 53 C 161 C = Channel C 61 C 153 Channel 6 Channel 1 Channel 11 Figure 10: Multi-Radio Wireless Mesh,Typical Cluster The estimated capacity of multi-radio mesh is compared to single-radio and dual-radio designs in Figure 11.This chart shows the capacity of the different approaches when deployed in a linear fashion around a wired connection in the middle Figure 11 Multi Radio Mesh Per AP Capacity Multi radio Dual radio, realistic Single radio, realistic Per-AP Access Capacity (Mbps) Number of APs 12 Copyright 2006 BelAir Networks BDMC00040-C02

13 The capacity of each node in these graphs has been scaled to reflect b/g compatibility mode operating at an over the air rate of 54Mbps.The delivered peak tcp/ip rate in this mode is 22-25Mbps. The normal operating scenario of b/g compatibility mode delivers approximately 11Mbps tcp/ip capacity as extra packets are transmitted to enable b client radios to determine the presence of g packets on air. Figure 12 plots the performance of the different mesh modes in terms of the overall system capacity, for a cluster of nodes connected to a single egress point. It is interesting to see that the single and dual radio mesh systems asymptote to a system capacity close to the capacity of the medium (the air) whereas the capacity of the multi-radio system rises until limited by the capacity of the node at the egress point. In practice this means that in single and dual radio systems, adding more nodes to a cluster does not increase the overall system capacity Figure 12 Multi Radio Mesh System Capacity Multi radio Dual radio, realistic Single radio, realistic System Capacity (Mbps) Number of Mesh APs There are many other advantages of the multi-radio mesh approach. Co-existence Most large wireless meshes today are designed to support Wi-Fi clients in the 2.4 GHz band.there are many other Wi-Fi devices out there. It is important for large infrastructure to fit into the RF environment. A single-radio mesh must use the same channel throughout the system. (Similarly, the backhaul mesh in a dual-radio system uses the same 5 GHz channel for the whole system.) It is unlikely that this channel will be the best at each location in a large network. A multi-radio mesh is much more flexible. Each access radio can be assigned a different channel, so the co-existence problem is isolated to the coverage area of a single mesh AP not the whole system. Multi-radio meshes fit into their environment and share the unlicensed spectrum better. Copyright 2006 BelAir Networks BDMC00040-C02 13

14 Interference Multi-radio meshes are very flexible in terms of channel assignment on the access or backhaul radios.they can adapt to interference because each access radio can be set to the channel that is least used in a given area.the backhaul network consists of point-to-point links.they use directional antennas that have high gain, but they project their signals in a narrow pattern in a specific direction.this minimizes the impact of the multi-radio backhaul mesh on other systems in the area. Multi-radio meshes have very little self interference because of flexible channel assignment and multiple radios operating on different channels at the same time. Both dual-radio and single-radio meshes cause self-interference, since all the nodes in the mesh must share a common channel for backhaul. In addition, interference from external networks in one location will disrupt service across and entire dual- or single-radio mesh network. Latency The dedicated point-to-point links in the multi-radio mesh keep backhaul latency low and predictable. Single-radio mesh and dual-radio mesh approaches have a shared backhaul network using a contention based protocol with unpredictable latency. Multi-radio mesh is suitable for voice applications, the others are not. Conclusion Capacity in a wireless mesh infrastructure is affected by the mesh forwarding performance, shared network contention and self interference of the mesh APs. It is important to consider all of these issues when analyzing these systems. Single radio wireless mesh, representing the lowest cost entry point in the deployment of a mesh network, is low capacity and will not effectively scale to implement a complete large network. Single radio mesh is best used in small mesh clusters at the edge of a network. The dual-radio mesh architecture represents the logical evolution in the growth of a mesh network. Dual-radio systems alone don t scale to metro dimensions. Multi-radio mesh systems separate wireless access and backhaul, and use dedicated point-to-point links to form the wireless backhaul mesh.this eliminates both in-channel mesh forwarding and shared backhaul network contention overhead.the result is a high capacity system that can scale to support large networks with broadband service for many users. 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. 14 Copyright 2006 BelAir Networks BDMC00040-C02

15 For more information on single, dual and multi-radio wireless mesh networks, visit About BelAir Networks BelAir Networks is the first company to offer scalable, wide-area Wi-Fi solutions with the highest quality for data, voice and video. BelAir s wireless networking solutions are built on the only multi-service architecture for wide-area wireless broadband deployments of Wi-Fi,WiMAX, and 3G Cellular networks. Built specifically for outdoor deployments, BelAir Networks patent pending solution delivers the lowest cost per user and deploys in days, blending into the physical infrastructure of downtown business districts, hotels and resorts, and college campuses. Founded in 2001, BelAir Networks is a privately held company headquartered in Kanata, Ontario. For More Information Contact: BelAir Networks t: North America Toll Free: BelAir1 ( ) e: info@belairnetworks.com Or Visit Our Web Site at Copyright 2006 BelAir Networks BDMC00040-C02 15

16 Copyright 2006 BelAir Networks Specifications may vary by region. To find out more, contact BelAir Networks: BelAir1 ( ) BDMC00040-C02