ScreenOS Cookbook

Chapter 3. Wireless

3.0. Introduction

Wireless local area networks (WLANs) provide obvious benefits over wired networks, in that they link network devices without wires. WLAN uses spread-spectrum or Orthogonal Frequency-Division Multiplexing (OFDM) modulation technology (based on radio waves to enable communication between devices in a limited area, also known as the basic service set).

IEEE 802.11 is the standard defined for the WLAN. It uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) as the access method. Multiple standards are defined under the 802.11 umbrella standards; 802.11a was the first standard defined for WLAN, and it uses OFDM at a 5 GHz frequency, providing 54 Mbps throughput (in theory). The most commonly used standards, however, are the 802.11b and 802.11g, and they operate in Direct Sequence Spread Spectrum(DSSS) and OFDM in a 2.4 GHz frequency, providing 11 Mbps and 54 Mbps. 802.11g is backward-compatible with 802.11b; however, 802.11b is not backward-compatible with 802.11a. Therefore, you would see a lot of wireless Access Point (AP) devices that support 802.11b/g on the same radio and 802.11a on a separate radio. The NS-5GT supports only the 802.11b/g standard; the SSG5 and SSG20 support the 802.11a/b/g standards using dual-band radio.

The 802.11 Standards

The 802.11b/g is prone to interferences from common household devices, such as microwaves and cordless phones. The channels available for 802.11b/g are 1–13, with the most commonly used channels ...