How to design a wireless LAN: key issues: RF coverage sufficient capacity and accounting for RF signal-loss factors - Wireless

Communications News, Sept, 2003 by Michelle Mclean

Designing enterprise wireless LANs (WLANs) is a new craft, even for many experienced network architects. When designing a wired network, most IT managers are familiar with the steps to ensure sufficient capacity for the users and applications. With IEEE 802.11 wireless LANs, a new factor comes into play: the tradeoff between radio-frequency (RF) coverage and bandwidth capacity per user. Planning for both capacity and coverage is one of the key design issues for WLANs. In short, if you plan for sufficient capacity, adequate coverage will follow.

The biggest change in mindset when designing WLANs is that they provide shared, not switched, connections. Many WLAN designers mistakenly focus only on providing adequate coverage, rather than sufficient bandwidth, for their users. While coverage may be the design point if you are deploying a single access point (AP)-based network for the conference room or a workgroup, the application demands of an enterprise make bandwidth the real design criteria. Quite simply, you will not deliver enough bandwidth if you plan for coverage alone.

Key issues to consider when deploying a WLAN include determining RF coverage from a single AP, ensuring sufficient capacity to support the user population and accounting for RF signal-loss factors.

The distance at which a particular throughput can be achieved will vary with 802.11 a or 802.11 b WLANs. 802.11b has a data rate of 11 Mbps inside a radius of 100 feet when indoors. Similarly, 802.11a should deliver 36 Mbps within a 75-foot radius.

Many APs will automatically decrease their data rates as the RF signal degrades because a lower-frequency signal is more likely to get through when there is interference. One user associated at 1 Mbps will slow the entire group, since the AP will take longer to communicate with that user, taking bandwidth from all other connected users.

To avoid degrading the performance for all users associated to an AP, consider setting minimum association rates that force users to associate with a new AP once their throughput falls below the minimum rate. By designing smaller cells with higher throughput, you can create an enterprise-quality experience.

The number of users and their applications are major drivers of bandwidth requirements. The network architect must account for the number of users within the AP's cell diameter. In a large office or where user density is high, you should design smaller cells to achieve a higher data rate, since walls and other objects will not naturally create the cells by attenuating or blocking the RF signal. With smaller cells, you will need to re-use frequencies more often and thus ensure that the channels do not overlap.

Determining how much bandwidth each user will need is critical, as your calculations will define the user experience, as well as the number of APs required. A good rule of thumb for an 802.11a network is to allow for 2 Mbps downstream and upstream per user, which delivers about the same user experience as being on a wired LAN. For an 802.11b network, a rule of thumb is to allow for 500 Kbps each way, which delivers a user experience similar to a DSL connection.

The 802.11 specification includes 802.11a, 802.11b and 802.11g. 802.11b features a raw data rate of 11 Mbps and provides three non-overlapping channels. 802.11a offers a peak throughput of 54 Mbps and provides eight to 12 non-overlapping channels. 802.11g offers a peak throughput of 54 Mbps, provides the same three non-overlapping channels available in 802.11b, and is backward-compatible with 802.11b clients.

Because of several factors, actual throughput on a wireless system is much lower than the technology's specified data rate. For instance, with even a one-way transmission on a 54-Mbps system, the best possible throughput is approximately 30 Mbps. For an 802.11b network, the best possible throughput is 4 Mbps to 6 Mbps.

For an enterprise environment with high-throughput needs, 802.11a is likely the better choice, as it has higher throughput and more non overlapping channels. Including an overlay of either 802.11b or 802.11g will provide support for older devices that were purchased prior to the availability of 802.11a.

A major difference between designing for wired and wireless LANs is the impact of objects on RF signals. Walls, doors, windows and other fixed objects in the building will absorb RF signals, causing signal loss. The building construction also has an impact: Concrete absorbs more signal than wood. Look for tools that automate these calculations.

CALCULATING AP LOCATIONS

The next step is to define the size of the area where high-speed coverage is desired by calculating its width and length. An office building can be divided into multiple areas for planning. For example, you may plan for the engineering organization separately, as engineering applications are more bandwidth-intensive than the office applications used by sales and marketing.

Plan for a hotspot area, such as a conference room, separately from the rest of the enterprise, as it will have different access and quality-of-service requirements. Calculate the expected total bandwidth needed to serve the area by multiplying the average bandwidth needed per user by the number of users in that coverage area.