WLAN Standards

WLANs use three unlicensed frequency bands: 900 MHz, 2.4 GHz, and 5 GHz. These bands are all in the Industrial, Scientific, and Medical (ISM) frequency range. Higher frequency bands allow greater bandwidth, but have smaller transmission ranges. Within all bands, the data rate decreases as the client moves away from the AP.


802.11b Standard

802.11b is a widely adopted standard that operates in the 2.4 GHz range and uses Direct Sequence Spread Spectrum (DSSS). It has four data rates: 1, 2, 5.5, and 11 Mbps. 802.11b provides from 11–14 channels, depending on country standards, but only three channels have nonoverlapping frequencies: 1, 6, and 11. Cisco recommends a maximum of 25 users per cell; expect an actual peak throughput of about 6.8 Mbps.


802.11a Standard

802.11a operates in the 5 GHz range and uses Orthogonal Frequency- Division Multiplexing (OFDM). It has eight data rates: 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. 802.11a provides from 12–23 nonoverlapping channels, depending on country regulations. Portions of the 5 GHz range are allocated to radar, so 802.11a uses Dynamic Frequency Selection (DFS) to check for radar signals and choose a different channel if it detects them. It also uses Transmit Power Control (TMC) to adjust client power, so that they use only enough to stay in contact with the AP. DFS and TMC are part of the 802.11h specification. Cisco recommends a maximum of 15 users per cell; expect an actual peak throughput of about 32 Mbps.


802.11g Standard

802.11g operates in the same 2.4 GHz range as 802.11b and uses the same three nonoverlapping channels: 1, 6, and 11. It can provide higher data rates; however. 802.11g uses DSSS to provide 1, 2, 5.5, and 11 Mbps throughput, which makes it backward compatible with 802.11b. It uses OFDM to provide 6, 9, 12, 18, 24, 36, 48, and 54 Mbps throughput, as does 802.11a.

802.11b/g access points can register both 802.11b and 802.11g clients. Because 802.11b clients do not understand OFDM messages, when 802.11b clients register, the AP implements an RTS/CTS protection mechanism against collisions. When a client wants to talk, it sends an RTS message. The AP must answer with a CTS message before the client is allowed to transmit. This creates overhead for the AP and causes a drop in overall throughput for all clients. Cisco recommends a maximum of 20 users per cell; expect an actual peak throughput of about 32 Mbps.


Wireless Security

Wireless security methods, listed from weakest to strongest, include:

• Wired Equivalent Privacy (WEP)—It uses static keys, weak authentication, and is not scalable. n 802.1x Extensible Authentication Protocol (EAP)—Uses RADIUS for authentication, dynamic keys, and stronger encryption. Cisco supports it via Lightweight EAP (LEAP) and Protected EAP (PEAP).

• Wi-Fi Protected Access (WPA)—This is a Wi-Fi Alliance standard. Uses Temporal Key Integrity Protocol (TKIP) for encryption, dynamic keys, and 802.1x user authentication. Cisco supports it via Lightweight EAP (LEAP), Protected EAP (PEAP), and Extensible Authentication Protocol-Flexible Authentication via Secure Tunneling (EAP-FAST).

• WPA2—The Wi-Fi Alliance’s implementation of the 802.11i standard, which specifies the use of Advanced Encryption Standard (AES) for data encryption and uses 802.1x authentication methods. Can also use TKIP encryption.

WPA/WPA2 Authentication

When a host wanting WLAN access needs to be authenticated in a network using WPA or WPA2, the following steps occur:

Step 1. An 802.1x/EAP supplicant on the host contacts the AP (or WLAN controller, if it is a lightweight AP) using 802.1x.

Step 2. The AP or WLAN controller uses RADIUS to contact the AAA server, and attempts to authentication the user.

Step 3. If the authentication succeeds, all traffic from the client to the AP is encrypted.