Mesh Capacity BDMC00040-C02

8/3/2019 Mesh Capacity BDMC00040-C02

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White Paper

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.

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



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

Capacity of Wireless Mesh Networks



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. Understandingthe 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

Copyright © 2006 BelAir Networks BDMC00040-C02 3

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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/Nperformance, 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



1 43 52

Coverage area on channel 6



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 anduse 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 inmore 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



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Per-AP Capacity, Wire in the Middle

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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 802.11

MAC protocol.With only one radio dedicated to backhaul at each node, all of the mesh APs must usethe 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 (Figure 6).

Figure 6: Dual-Radio Wireless Mesh,String of Mesh APs

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. 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.



Coverage on channel 6




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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 MeshLike 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)



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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 802.11a chipsets today. In the near future this will be a good application for 802.16d 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 s ingle 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.



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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.



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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 diff erent approaches when deployed in a linear f ashion around a

wired connection in the middle.

Figure 11 Multi Radio Mesh Per AP Capacity





The capacity of each node in these graphs has been scaled to reflect 802.11 ‘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



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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.



• 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 flexiblechannel 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 customizedto meet the application requirements and budget by using the appropriate mix of the different

wireless mesh approaches.





For more information on single, dual and multi-radio wireless mesh networks, visit www.belairnetworks.com.

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: 613.254-7070 North America Toll Free: 1-877-BelAir1 (1-877-235-2471)

e: [email protected]

Or Visit Our Web Site at

www.belairnetworks.com





Copyright© 2006 BelAir Networks

Specifications may vary by region.

To find out more, contact BelAir Networks:

[email protected]

[email protected]

1-877-BelAir1 (1-877-235-2471)

1-613-254-7070

www.belairnetworks.com BDMC00040-C02

Mesh Capacity BDMC00040-C02

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