PDF of this tutorialAs Figure 8 illustrates, three means are used to provide Internet access using wireless technology: satellite broadcast, terrestrial broadcast, and cellular/PCS.
Figure 8. Wireless Access
Cellular
Internet access can be provided via existing cellular systems using voiceband modems. Because cellular channels are narrowband, access rates are limited to 9.6 kbps for advanced mobile phone service (AMPS) and time division multiple access (TDMA) systems and to 14.4 kbps for code division multiple access (CDMA) systems. Cellular digital packet data (CDPD) is a technique that enables the data rate of AMPS to be extended to 19.2 kbps. CDPD achieves the higher rate by inserting Internet protocol (IP) packets directly into cellular channels that do not contain voice traffic (i.e., channels that are temporarily idle).
Techniques are being investigated to provide Internet access and other data services using personal communications services (PCS). PCS data standards are being investigated by a joint technical committee of ANSI T1 and the Telecommunications Industry Association (TIA), the International Telecommmunication Union–Telecommunications Standardization Sector (ITU–T), and others.
Terrestrial Broadcast
The multichannel multipoint distribution service (MMDS), sometimes called "wireless cable," can provide Internet-access downlinks over a distance of about 50 km from a central-transmitter site. MMDS downlinks combined with telephony uplinks provide a complete Internet-access arrangement. MMDS operates in the 2-GHz frequency band with 33 channels, each capable of supporting downlink data rates that are currently about 10 Mbps. Technology improvements are expected to increase data rates to 27 Mbps in the future.
Local multipoint distribution service (LMDS) is similar to microwave multipoint distribution service (MMDS) in that it will use microwave transmission to provide Internet-access downlinks and wireline telephony to provide uplink access. LMDS will use transmitters operating in the 28-GHz frequency band with each transmitter covering a distance of about 5 km. The relatively close transmitter spacing, coupled with the fact that LMDS will have about four times the bandwidth of MMDS, should enable LMDS to serve a much higher density of Internet users than MMDS.
Satellite Broadcast
Several approaches have been proposed for using satellites to provide Internet-access downlinks. Some proposals are based on using a single fixed-position satellite, whereas others would use clusters of satellites. Proposed data rates vary from low-speed, single-user channels to shared channels with rates greater than one Mbps.
The first widely available system operates in the 12-GHz band and uses a data rate of 400 kbps. Equipment at the end-user location consists of a dish antenna, approximately 52 cm in diameter, a microwave receiver, and a digital decoder card that plugs directly into a PC computer bus. Satellite systems also use telephony circuits for uplink access.
