Primer: Mesh Configurations and Applications

Outdoor mesh gets the most attention and arguably offers the most compelling value proposition, but indoor and hybrid indoor/outdoor meshes also are useful, especially in hard-to-wire environments. Today's wireless mesh systems are proprietary, but that's not necessarily a reason to postpone deploying one. After all, most large organizations try to standardize on their critical infrastructure elements to minimize support costs, so multivendor interoperability isn't critical as long as each system supports open standards like Ethernet and Wi-Fi.

Unfortunately, mesh capabilities aren't integrated with modern enterprise Wi-Fi systems, so if you want to extend your existing WLAN with mesh backhaul connections, you must integrate it with a mesh vendor's technology.

The BasicsMost WLAN APs are deployed in a modified star topology, as part of a broader wired Ethernet system (see "Conventional WLAN With Wired Distribution System" left). Although clients can roam from one access point to the next, traffic to and from clients at the edge of the wireless network follows a static path from client to AP to Ethernet switch and into the backbone network.

In a mesh architecture, access traffic at the edge of the network is dynamically routed across a wireless mesh that interconnects the APs and possibly other Ethernet devices. "Municipal Network Using Mesh Architecture" (right), shows a small municipal wireless network that uses wireless links between APs rather than Ethernet cabling to move traffic from wireless clients to the backbone. Client traffic at the edge that's destined for systems on the wired network can take alternate paths through the mesh. Mesh nodes optimize those paths.

This wireless mesh architecture is similar to the wired mesh used on the Internet, where routers make forwarding decisions using dynamic routing protocols. In both cases, the specific path that packets take through intermediate nodes is transparent to clients. Many factors, including traffic levels, link capacity, routing-protocol efficiency and overhead, can impact overall performance. Networks with small diameters (small hop counts) generally will have better throughput and latency characteristics than those with large diameters, which experience a performance hit for every intermediate hop. To overcome this, you'll probably want fast and dedicated mesh backhaul connections.

Design OptionsAlthough some mesh systems limit themselves to providing wireless backbone services, most provide a combination of backbone/infrastructure and client-access services. Thus, a Wi-Fi client can connect to a node that's simultaneously acting as an infrastructure device for the mesh backbone. In these systems, the mesh node must handle standard Wi-Fi access (usually 802.11b/g but sometimes 802.11a as well), ingress traffic from other mesh nodes, egress traffic to other mesh nodes and, in some cases, an Ethernet connection to the wired network.

The simplest wireless mesh design uses a single-radio for access, ingress and egress. Its distinguishing characteristics are low cost and simplicity. In a single-radio design, both client access and communication between mesh nodes take place over one radio, which dynamically switches its function from AP-mode to mesh-node (see "Solo Radio" left).

This system, which typically uses 2.4-GHz 802.11b/g radios, is the least expensive system to deploy, but offers limited performance and capacity. That's because the single radio in each mesh node must time-slice between client access, ingress and egress. To overcome these limitations, you must design your network to minimize hop counts. So about one-third to one-half of all mesh nodes also should have connections into the wired network, directly over Ethernet or over a dedicated point-to-point, or point-to-multipoint, fixed wireless backhaul system. Large, single-radio mesh networks are typically deployed in conjunction with 5-GHz multipoint wireless backhaul systems from Alvarion or Motorola. That can mean higher costs as well as network-management complications.

More sophisticated wireless mesh networks use a multiradio design, separating the access, ingress and egress functions. In a two-radio design (see "The Power of Two" right), client access traffic takes place on one radio channel (usually 2.4-GHz 802.11b/g) while mesh ingress/egress traffic uses a different channel (usually 5-GHz 802.11a).

By separating access and mesh functions, the dual-radio design offers some performance and design benefits. You get an increase in the overall system capacity and more flexible allocation of your available RF channels. The trade-off, however, is that dual-radio systems tend to cost more than single-radio systems, because hardware for dual-radio setups is more expensive and they generally rely on 5-GHz radios for mesh communications. These signals are heavily attenuated by buildings and foliage, so you'll need more mesh nodes for this kind of a system than for a single-radio, 2.4-GHz design. And deployment may require more complex radio-link engineering to ensure line of sight between mesh nodes.

The last and arguably most sophisticated mesh network design uses three or more radios per node. These additional radios can be used for one of two purposes. First, you can create a multisector and/or multichannel access system using directional antennas to extend range and provide more client access capacity. Second, you can optimize mesh traffic by separating ingress and egress traffic onto different radios/channels. This multiradio approach offers excellent performance, but is more costly and complex to install.

Keep in mind that adding lots of radios to a mesh node doesn't always guarantee superior performance. That's because other factors, including radio efficiency, routing protocols and mesh diameter, also contribute to performance. As a general rule, however, more radios translates to better performance and capacity--albeit at a higher cost.

Since no wireless mesh standards yet exist, interoperability among different vendors' systems is limited. It's theoretically possible to construct large mesh networks using gear from multiple vendors, but most organizations opt for a single vendor. As the popularity of wireless mesh increases, the need for interoperability will increase as well. This is particularly true for systems designed for the consumer market, where ease of installation is a key requirement.The IEEE 802.11s task group began consideration of proposals for a mesh extension to the 802.11 standard in July. The leading proposal, which received an 83 percent approval by voters, is known as SEEMesh (Simple, Efficient and Extensible Mesh). SEEMesh is backed by a large group of high-profile vendors including Intel, Motorola, Nokia, NTT DoCoMo and Texas Instruments. Details about this proposal were not available at press time, but it's oriented toward making standardized mesh capabilities available to the broadest possible consumer market.

Close behind (with approval by 76 percent of voters) is a proposal submitted by the Wi-Mesh Alliance, which includes Accton, Nexthop, Nortel and Philips. The Wi-Mesh proposal addresses a broader range of needs, including both single- and multiradio design elements, quality of service and security enhancements. Details of this proposal are available at www.wi-mesh.org.

Players in the wireless mesh market include large and established network vendors like Nortel and Motorola as well as smaller companies focused exclusively on mesh. With the exception of Motorola, none of the leading broadband fixed wireless companies have jumped into the wireless mesh market, perhaps because they're focused on WiMax. Nor are any enterprise WLAN infrastructure providers offering mesh, though Cisco is expected to include mesh support on the enterprise WLAN product from its Airespace acquisition.

Most mesh suppliers are focusing on specific vertical markets, like public safety, health care and education, while also pitching mesh as a horizontal solution for enterprises (see "Mesh WLAN Vendors" page 70).

The most interesting market is municipal wireless broadband. This is a battleground because of both the technical challenges of scalability, security and quality of service, and the public policy debate over the role of local governments in delivering wireless broadband service that could compete with DSL and cable. But the hot debate hasn't stopped municipalities from diving in. Early network deployments, including Chaska, Minn.'s network, which has been heavily promoted by Tropos, have achieved some success. But serious questions remain on their scalability and business models because many municipalities have faced unexpectedly high deployment costs. Politics aside, municipal wireless mesh is likely to gain momentum for public safety, if not for public access. Cities worldwide will serve as live laboratories for the technology.Get Ready

Before you decide whether to deploy wireless mesh today or wait until the technology matures and standards emerge, assess both your long-term wireless strategy and short-term needs. Wireless mesh may be a sound tactical choice for temporary installations or where available wired connections to your Ethernet network are limited. In these cases, it's relatively easy to deploy mesh and perform basic traffic engineering.

In more ambitious outdoor installations that span large distances, however, exercise more care to ensure that the system will scale to meet growth needs at a reasonable cost. While one or more pilot installations should be considered an essential element of any large-scale rollout, scalability is the most critical system attribute. You'll need to inject enough traffic into the system to confirm that your mesh is up to the task, or talk to your counterparts with similar implementations to find out how they handled it.

Dave Molta is Network Computing's senior technology editor. Write to him at [email protected]

In the market for mesh? Here's a rundown of some of the leading providers of wireless mesh systems.• BelAir Networks. BelAir was an early advocate of multiradio, carrier-class system design, but the company is branching out to the lower end of the market. Its high-end BelAir 200 offering, the keystone of its "cellular LAN" architecture, integrates up to four radios, high-gain directional antennas and standby batteries into a carrier-class enclosure designed to be mounted outdoors. Aside from providing outdoor access, the system is frequently used to beam Wi-Fi signals into buildings from the outside, akin to cellular phone services. BelAir's recently announced single-radio system, scheduled for availability this month, provides a lower-cost offering for installations where performance requirements are more modest.

• Firetide. While most mesh providers deliver both mesh infrastructure and Wi-Fi access, Firetide focuses on mesh infrastructure with an Ethernet interface that can be attached to a Wi-Fi AP or to any other Ethernet-capable device. Firetide has begun to develop mesh-based video surveillance, for example--a natural for a rapidly growing public-safety/homeland security market--using video gear originally designed for Ethernet networks. The company offers both indoor and outdoor products, aggressively priced from $695 to $1,995. Although the value is high, you won't get the capacity of competing multiradio system designs.

• Motorola. Motorola has focused heavily on public safety and government operations, where it has significant penetration with its radio systems. Its proprietary mesh offering is Mesh Enabled Architecture (MEA). Motomesh, Motorola's upcoming multiradio system, supports MEA radios and Wi-Fi concurrently and unlike MEA, emphasizes location-tracking capabilities. Motorola also has a multipoint fixed-wireless product line, called Canopy, that is sometimes used to provide backhaul services on Tropos' mesh networks.

• Nortel Networks. Nortel offers a dual-radio mesh system, called Wireless Mesh Network Solution, that uses 802.11g for access and 802.11a for mesh backhaul. Deployments include some universities and NASA, but the City of Taipei's network is the most interesting as a test of mesh scalability: The system is expected to include 10,000 mesh APs by the end of the year. Nortel is also a lead player in the Wi-Mesh Alliance.

• Strix Systems. Strix Systems made its entry into the market with its Access/One Indoor Wireless System (IWS), a spiffy modular multiradio design that lets you stack up to three radio modules into a single mesh node. More recently, the company has branched into the somewhat hotter outdoor market. Its Outdoor Wireless System (OWS) combines up to six radios into a single NEMA enclosure, which provides considerable flexibility. The outdoor system also interoperates with its indoor system, facilitating indoor/outdoor installations. Unlike Belair's outdoor system, Strix's offering does not include integrated antennas. This enhances system flexibility but complicates installation (and detracts from aesthetics).• Tropos. Tropos is a leader in the market by virtue of its large installed base. Its single-radio 802.11g system design is aggressively priced and easy to deploy. To overcome the limitations of the single-radio design, Tropos uses its predictive wireless routing protocol (PWRP) to optimize routing decisions and minimize routing overhead. Still, the company's recommendation that the number of intermediate mesh hops shouldn't exceed two is a clear indication that its mesh is capacity-constrained. This means that a much larger proportion of mesh routers needs to be connected to the wired network. Where this is impractical, Tropos often employs Motorola's Canopy 5 GHz multipoint backhaul system.