Wireless LAN Design

Chapter Description

This chapter discusses wireless LAN (WLAN) technology and describes how it improves mobility. After introducing WLANs as strategic assets to corporate networks, WLAN standards and components are discussed. The security and management of WLANs are explored, followed by design considerations for WLANs.

Wireless Design Considerations

The following sections discuss some items that should be considered when designing and provisioning a wireless network.

Site Survey

Site surveys, originally introduced to make the most of scarce resources, are sometimes seen as unnecessary in this age of inexpensive WAPs, where wireless saturation seems so economical. Maybe the days of serious physical surveying, where one would look under the ceiling tiles, are long gone, but you should still perform surveying to determine the optimal locations for WAPs to minimize channel interference while maximizing the range.

Whether you are performing an in-depth site survey or a rudimentary one, you should ask the following questions:

  • Which wireless system is best suited for the application?
  • Does a line-of-sight requirement exist between antennas?
  • Where should the WAP be located so that it is as close as possible to clients?
  • What potential sources of interference are in this building? Example sources are cordless phones, microwave ovens, natural interference, or other access points using the same channel.
  • Should any federal, provincial, or local regulations and legislation be considered in this deployment?

WLAN Roaming

WLANs are relatively inexpensive to deploy compared to wired networks, and because, as shown earlier in Figure 5-2, throughput is directly related to the proximity of WAPs, network managers often install WAPs to provide overlapping signals, as shown in Figure 5-6. Using this overlapping design, coverage (radius) area is traded for improved throughput.


Figure 5-6 Overlapping Signals Eliminate Dead Spots

Note that these overlapping signals must be in nonoverlapping channels. This scenario, however, requires WLAN roaming. WLAN roaming plans consider that as a user moves away from a WAP and is therefore losing signal strength, his connection should seamlessly jump to a WAP that provides a stronger signal.

Point-to-Point Bridging

It is not always feasible to run a network cable between two buildings to join their respective LANs into a single Layer 3 broadcast domain. If the two buildings are a reasonable distance apart and preferably in direct line of sight with each other, wireless bridges can be configured, as shown in Figure 5-7. It takes two WAPs to create one logical two-port bridge. In this mode, WAPs are operating in a dedicated point-to-point bridge mode and therefore are no longer operating as wireless access points for clients.


Figure 5-7 Point-to-Point Bridging

Design Considerations for Wireless IP Phones

Because wireless IP phones have different coverage and wireless characteristics than common wireless clients, a system administrator should conduct another site survey.

Another consideration for wireless IP phones is roaming. The roaming described in the "WLAN Roaming" section, earlier in this chapter, is Layer 2 roaming. With Layer 2 roaming, devices keep their IP address and therefore the changing to another switch would not be noticeable by users. Layer 3 roaming would mean that a device would have to change its IP address; this would mean an interruption in the user's connection. If the connection was to a wireless IP phone, the call would be disconnected; this scenario would likely be unacceptable to users. When wireless IP phones are used, the network needs to be equipped with a Cisco Catalyst 6500 Series Wireless LAN Services Module (WLSM). WLSM, an integral component of SWAN, provides aggregation of access point radio management information, thus enabling Layer 2 and Layer 3 roaming and client mobility management.

Layer 2 roaming refers to an IP phone switching WAP within its subnet of origin. Layer 3 roaming refers to an IP phone switching connectivity from a WAP in its subnet to a WAP located in another subnet. Prior to WLSM, Layer 3 roaming was an issue because the phone would end up in a subnet to which its IP address and default gateway wouldn't belong.