Cisco 7921G Administration Guide - Page 32

Understanding WLAN Standards and Technologies Chapter 2 Overview of the VoIP Wireless Network Many devices operate in the 2.4 GHz bandwidth including cordless phones and microwave ovens and can interfere with wireless communications. Interference does not destroy the signal, but can reduce the transmission speed from 11 Mbps to 1 Mbps. RF interference can affect voice quality over the wireless network. • 5 GHz-Divided into several sections called Unlicensed National Information Infrastructure (UNII) bands and has four channels each. The channels are spaced at 20 MHz to provide non-overlapping channels and more channels than 802.11b or 802.11g. Wireless Modulation Technologies Wireless communications uses the following modulation technologies for signaling: • Direct-Sequence Spread Spectrum (DSSS)-Prevents interference by spreading the signal over the frequency range or bandwidth. DSSS technology multiplexes chunks of data over several frequencies so that multiple devices can communicate without interference. Each device has a special code that identifies its data packets and all others are ignored. Cisco wireless 802.11b/g products use DSSS technology to support multiple devices on the WLAN. • Orthogonal Frequency Division Multiplexing (OFDM)-Transmits signals by using RF. OFDM is a physical-layer encoding technology that breaks one high-speed data carrier into several lower-speed carriers to transmit in parallel across the RF spectrum. OFDM, when used with 802.11g and 802.11a, can support data rates as high as 54 Mbps. Table 2-1 provides a comparison of data rates, number of channels, and modulation technologies by IEEE standard. Table 2-1 Comparison of Data Rates, Number of Channels and Modulation Technologies by IEEE Standard Item Data Rates 802.11b 1, 2, 5.5, 11 Mbps Non-overlapping Channels Wireless Modulation 3 (Japan uses 4) DSSS 802.11g 6, 9, 12, 18, 24, 36, 48, 54 Mbps 3 (Japan uses 4) 802.11a 6, 9, 12, 18, 24, 36, 48, 54 Mbps Up to 23 DSSS, ODFM ODFM AP, Channel, and Domain Relationships APs transmit and receive RF signals over channels within the 2.4 GHz or 5.1 to 5.8 GHz frequency band. To provide a stable wireless environment and reduce channel interference, you must specify non-overlapping channels for each AP. The recommended channels for 802.11b and 802.11g in North America are 1, 6, and 11. Regulatory domains determine the number of channels that wireless communications can use within the frequency band. Table 2-2 lists regulatory domain, frequency band range, and operating channels for four regulatory domains. The Cisco Unified Wireless IP Phone 7921G uses the fourth domain (product number is CP-7921G-W) for all other regions in the world. Wireless LANs in the rest of the world use 802.11d to identify band ranges and channels. Cisco Unified Wireless IP Phone 7921G Administration Guide for Cisco Unified Communications Manager Release 7.0 2-4 OL-15985-01