WIRELESS

From the July 2005  issue of Communications News

Big deployments, better solutions

Legacy wireless LAN solutions may not provide adequate coverage for today’s network users.

by Joel Vincent

Many organizations are looking at extensive wireless LAN (WLAN) deployments throughout their facilities. Most WLAN vendors, however, offer access points (APs) and controllers based on hardware technology that has not changed significantly since the days when WLAN meant one AP, and users are realizing that legacy AP technology presents major challenges when stretched to deliver pervasive coverage in large facilities.

Initial WLAN applications were limited to e-mail or network access, but companies are now rolling out voice-over-IP (VoIP) and mission-critical corporate applications on their wireless networks. As these applications multiply, so does the need for broader and more robust wireless coverage. Assuming that any organization wanting to deploy a large-scale WLAN wants also to support any current or future applications, the requirements for wireless infrastructure can be grouped into five categories:

Pervasive coverage.
The system should provide complete, continuous coverage of the entire facility, including hallways, stairwells, lobbies and even outdoor patios. Once users begin using applications like VoIP, they will be unhappy to have their calls disconnected anywhere.

High user density.
Many business workers already own at least two wireless-enabled devices (a PDA and a laptop), and the trend toward more devices is increasing. As tethered users see the benefits of wireless mobility, they will add to the device count. The WLAN infrastructure should be able to support a high number of users on any AP.

Appropriate quality of service (QoS).
With multiplying applications and users, the network will need to support different levels of QoS. Voice users should have continuous access, for example, while e-mail users can do with best-effort access. 802.11g users will expect faster connections than 802.11b users on the same network.

Seamless roaming.
Any organization planning to deploy wireless IP voice will need a WLAN infrastructure that allows seamless roaming from one AP coverage area to another. Cellular phone users are not aware when they move from one base station’s coverage area to another, and the WLAN should support similar transparency.

Deployment and management simplicity.
Like any other piece of IT infrastructure, the WLAN system should not be unnecessarily complex or expensive to deploy and maintain.

Continuous coverage.
Placing two APs adjacent to one another is not possible if they are both broadcasting on the same channel, since the resulting co-channel interference confuses clients within range (causing them to flip-flop from one AP to the other), corrupts packets in the air and typically makes reliable connections impossible. Since APs can broadcast on different channels (three channels for 802.11b/g and 11 channels for 802.11a), however, arranging adjacent APs using alternating channels to minimize co-channel interference is possible, as is routing traffic from them through a WLAN controller. Some vendors also allow users to regulate the power output of APs to further minimize the potential for co-channel interference.

Alternating channels and adjusting AP power levels helps facilitate the placement of adjacent APs to provide broader coverage, but there will always be coverage gaps in such an arrangement. In addition, with only three channels to work with (in 802.11b and 802.11g), alternating channels is a limited solution because, eventually, adjacent APs will be on the same channel. While reducing AP power outputs can help reduce co-channel interference in such cases, it can also exacerbate the problem by increasing signal-to-noise ratios, leading to higher packet corruption and lower performance.

User density.
Legacy WLANs all use the same collision-avoidance mechanism to enable multiclient access, and the mechanism makes supporting more than five or six clients per AP difficult. WLAN throughput drops off significantly when more clients than this are present.

Quality of service.
Legacy WLAN APs do not distinguish between the needs of one client and another. If one client is connected via 802.11b and another is connected via 802.11g, the faster 802.11g client’s access and throughput will be slowed by the lower performance of the 802.11b client. In addition, a VoIP user gets the same level of service from the AP as a data user, even though IP voice requires a far more consistent level of packet delivery to ensure voice quality.

Seamless roaming.
Because legacy APs are not aware of each other, wireless clients make roaming decisions and must re-authenticate with the network as they move from one AP coverage area to the next. This requirement causes delays or interruptions in access that can degrade the quality of voice calls or even disconnect them.

Deployment and management simplicity.
Because legacy WLAN systems use APs that were not designed to work together and which do not deliver high user density or per-user QoS, a lot of planning, configuration and management is involved in making them work together. To manage co-channel interference, engineers must conduct site surveys and map out alternating channel AP deployments, for example.

Whenever the space’s RF characteristics change (due to remodeling, office reconfigurations or furniture changes), the WLAN system must be remapped and reconfigured. In addition, voice applications or large user populations typically require overlay 802.11a networks, which makes deploying and managing the overall WLAN infrastructure more difficult and expensive.

Newer AP and controller architectures can do a better job of delivering pervasive WLAN coverage over large areas. In these systems, the APs are designed to work together. Every AP is aware of every other AP, and the WLAN controller unifies the infrastructure to deliver better performance.

Modeled on the cellular telephone architecture and based on a type of WLAN architecture specifically created to solve large-deployment issues, while remaining 100% Wi-Fi certified, this alternative addresses:

Pervasive coverage.
All APs in this system are aware of one another and APs coordinate their broadcasts to deliver complete coverage without co-channel interference.

User density.
The APs manage each client connection. As a result, the system can schedule access to each AP and reduce access delays caused by standard collision-avoidance mechanisms. Each AP can control the length of time each client has access to the channel, thereby enabling reliable access for up to five times as many clients per AP as in legacy systems.

Quality of service.
This architecture delivers specific levels of QoS for each client, ensuring appropriate service levels depending on access speed or application, and can reserve bandwidth over the air for delay-sensitive applications like IP voice. All clients automatically get the appropriate QoS, so this system eliminates the need to segregate groups of users on 802.11a WLANs. Moreover, if the organization has both 802.11g and b clients, the WLAN infrastructure’s client-specific awareness ensures that 802.11g users are not handicapped by the presence of 802.11b users.

Seamless roaming.
Because all APs are aware of all clients and all other APs, the client does not have to re-authenticate with the network each time it roams from one AP coverage area to the next. Instead, all the APs look like one large AP to the client, and it only authenticates once: when it first connects to the network. As a result, there is no handoff delay and IP voice calls or other applications requiring continuous packet flow can be used with the highest level of service.

Deployment and management simplicity.
All APs are able to operate on the same channel, eliminating the need to create deployment maps of alternating channels. Rather, IT engineers can simply deploy APs at reasonable intervals based on coverage ranges. When the space must be reconfigured to accommodate office reconfigurations or when additional coverage or more throughput is desired, the IT department can simply add or move APs to adjust.

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Joel Vincent is director of product marketing at Meru Networks, Sunnyvale, Calif.