Predeployment Testing of Wireless Mesh Networks

A new and resilient Internet infrastructure presents a set of unique testing challenges.

Wireless mesh networking, combining performance, simplicity, and economics, can be used when wired backhaul networks are impractical in applications such as municipal wireless networks, public safety, healthcare, and education. In fact, a recent ABI Research report forecasts a $1.2 billion market for citywide Wi-Fi networks by 2010, and more than 50 municipalities including Chicago, Houston, Philadelphia, and San Francisco already have installed wireless metropolitan networks with several others currently in the deployment process.

Carriers and service providers understand the costs involved in the wide adoption of early-stage technologies and that their customers will not tolerate unreliable new services. If an unpredictable network is deployed, the service provider bears the challenge and cost of customer support and troubleshooting.

If Wi-Fi is to effectively compete with technologies such as CDMA-2000 EV-DO, WCDMA, and UMTS, the technology must evolve from its low-cost consumer roots to a carrier-grade infrastructure with improved performance and robustness. To guarantee the success of wireless mesh technology and establish its credibility for metro-area network applications, carriers and service providers require reliable predeployment testing.

Step-by-step predeployment testing in a controlled laboratory environment is the most efficient way of introducing complicated networking technology such as wireless mesh networking. Thorough testing reveals technological flaws and weaknesses, helping vendors fix problems before product shipment.

Such testing must be automated or it lacks the scalability to test all possible load, motion, background interference, and device configuration scenarios. The absence of automation may allow bugs and network vulnerabilities to remain unchecked.

Wireless mesh networks present unique testing challenges because this wide-area infrastructure supports mobile users and high-throughput data, voice, and video applications. Wireless mesh nodes use sophisticated routing algorithms to direct traffic to its destination instead of connecting through traditional cabled backhaul. For that reason, mesh nodes cannot be tested in the same way as single-access points. They must be tested together as a self-configuring, self-healing system.

Mesh Networking Performance Variables
Mesh nodes discover each other and determine the optimum frequency scheme for communicating with neighboring nodes and local clients. Because they promptly respond to environmental changes and fault conditions by rerouting traffic or reconfiguring the frequency channel scheme, mesh nodes must constantly be aware of network conditions.

The most critical factors impacting user satisfaction in a communications network are throughput and quality of service (QoS). In a Wi-Fi mesh system, throughput degrades as the number of hops increases. QoS for voice and video is highly dependent on throughput, packet loss, delay, and jitter, which also increase with each successive hop. When testing mesh networks for performance variables, measurements should be performed over different hop counts and plotted vs. hops.

A variety of device and network settings impacts mesh performance and should be measured in a controlled laboratory environment. Some of these variables, such as range, equipment and vendor interoperability, and interference, are applicable to all Wi-Fi network testing. However, a number of variables affect mesh performance:

Node Client Capacity
Node client capacity shows the capability of individual mesh nodes to scale to a large number of clients. Because mesh deployments are primarily well suited for and targeted at municipal or public access hotspots, conference centers, libraries, and hotels, the number of users may be high at times, fluctuate considerably, and have different demands.

Multihop Throughput Performance
Multihop throughput performance measures the throughput for varied topologies and node configurations. Network size is critical in mesh networking. As the deployments succeed and the benefits are justified, customers will want to expand their networks to include bigger and bigger deployments over wider and more difficult coverage areas.

As deployment size increases, so will the number of nodes traversed from a user to the backhaul wired link. Each additional hop can potentially increase the network delay. In addition, the amount of background traffic and the number of network users increase assuming each node also is a user�s node, not only a relay.

Call Capacity
Call capacity tests the capability of the mesh network to deliver real-time services like VoIP. Voice call capacity and quality are measurements of voice traffic across multiple hops and with varying background traffic.

The promise of fixed-mobile convergence/Wi-Fi to cellular roaming such as better coverage, a decrease in dropped calls, and high-quality voice has many service providers ready to jump in the market. Mesh networks must meet this new level of service in addition to allowing regular data access. QoS-specific functions are a key part of supporting real-time communications traffic.

Node Roaming
Node roaming tests verify the robustness of the network design, autoconfiguration, and failure recovery through smooth node-to-node transition. Mesh networks can reroute traffic from a user through different routes; however, each vendor uses its own approach to make this happen seamlessly and efficiently. Evaluating the speed and disruption caused in making this transition will factor into the user�s experience as the system changes.

In addition, Wi-Fi access for radio, TV, phone, and other services is an appealing concept. But if the service is spotty, the user base will revolt. Discovery and self-healing of user connection and node-to-node connections are critical to operation.

Security Tests
Security tests include security performance and multi-basic service set identifier (BSSID) isolation. Security performance measures the impact on performance for different security protocols. Private networks such as the library network, municipal employees� network, or leased virtual corporate networks use security and operate in parallel with other secure and open public networks. This test is focused at the load that security adds to the network and whether it affects other network functions.

Mesh Networking Test Methodology
Performance of wireless mesh devices and systems must be verified under controlled conditions that allow the causes of poor performance to be easily isolated and managed. Multi-radio mesh test setups can be configured for controlled laboratory testing by interconnecting various mesh topologies using a conducted test setup.

By using multiple radios, mesh networks can segregate local client traffic and backhaul traffic to multiple simultaneous channels. Mesh networks that use multi-radio nodes with different channels that communicate with neighboring nodes and local clients tend to have better throughput than early single-radio implementations, which shared one channel for client and backhaul traffic. As the number of radios in mesh nodes significantly impacts performance, measurements must be made with different numbers of radios activated.

An example of how a simple mesh topology can be configured is shown in Figure 1. Mesh nodes are interconnected using programmable or fixed attenuators to emulate a variety of path losses causing the mesh to self-configure and select backhaul and client channels. Client traffic can be emulated using specialized test equipment. Throughput and QoS measurements can be performed at different points in the mesh and cover multiple hops.