Wireless LANs - LAN Wiring - page 314

you can imagine a wave on a lake passing through an area of swamp grass, you can
get an idea of what happens when radio waves are attenuated by passing through a
plasterboard wall, or even a cement structure such as a wall or a building floor.
Radio transmission typically occurs at a particular base frequency that is called
the carrier frequency , or just the operating frequency . Operating frequencies are
divided into bands by the national laws, and by international treaty. Each band
serves a particular purpose for things such as broadcast radio and TV, public-serv-
ice radio communications, and private radio services. Transmission on some bands
requires an operating license that may be for a single operating frequency or may
cover the entire band. Licensed bands include broadcast radio and TV, public serv-
ice and business band two-way radios, and most microwave links. On other bands,
unlicensed operation is allowed, as long as transmit power is limited and approved
transmitters are used. Examples include citizens band, family radio service (FRS),
and of course wireless LANs such as 802.11 WiFi.
Typical frequency bands in the United States (see Fig. 13.1) are 0.540 to 1.600
MHz for AM radio, 88.1 to 107.9 MHz for FM radio, 850 to 950 MHz for cellu-
lar phones, and “900 MHz” for two-way radio and paging. Most of the police and
fire departments operate in three bands, 150 to 175 MHz, 450 to 475 MHz, and

Audio Frequencies 300 Hz

3000 Hz

Voice and
Music

20,000 Hz Percussion-Cymbals

Radio Frequencies 530 kHz

AM Broadcast Radio

HF-Range 1630 kHz

1800 kHz

Short-wave Radio

30 MHZ

VHF-Range 54-88 MHz

88-108 MHz

TV Channels 2-6
FM Broadcast Radio

UHF-Range 450 MHz

800 MHz
850 MHz
900 MHz
1 GHz

UHF TV Broadcast
Two-way Radio
Cellular Radio
Pagers and Two-way Radio
Long-range Radar Microwave

ISM Band 2.4 GHz Wireless LANs and Phones

UNII Band 5.3-5.8 GHz Wireless LANs, etc.

FIGURE 13.1
Common radio-frequency bands.