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 802.1 la), 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.