2.2 Mobile Computing

Even though wireless data transmission rates are improving, and the trend is expected to continue, it is still instructive to look at the various options currently available for wireless data transmission and the data throughput provided by each.

There are currently five options commonly available for wireless data transmission. These five options are described below [1, 10, 13, 17].

Spread-spectrum packet radio

Wireless local area networks implemented using spread-spectrum packet radio are available. They have a raw data rate of up to 5.7 Mbps, but have a limited range, with a typical radius outside the LAN of 200 to 1000 feet. Bridges using microwave radios are available, but with data rates of only about 1.6 Mbps.

The Ricochet Wireless Network Service [17] also uses spread-spectrum packet radio, but uses a different network topology to provide greater area coverage. However, the raw data rate for this network is 100 Kbps, with throughput in the 9.8-28.8 Kbps range.

Private packet radio

This technology is currently used by two companies (Ardis and RAM Mobile Data) to implement wide area coverage of most of the United States. Transmission rates are in the range of 4.8-19.2 Kbps.

Circuit-switched cellular networks

This is the option offered by most cellular telephone companies. Transmission rates are in the 2.0-20.0 Kbps range.

Cellular digital packet data

This alternative is an attempt to lower the cost of cellular data transmission by sending the data during the idle time between cellular voice calls. Throughput is around 9.8 Kbps with a 1 to 5 second response time.

Satellite

All current system use geosynchronous satellites. This option is rather expensive and currently only supplies a two-way digital transmission rate of 2400 bps per connection.

Other systems using networks of low-earth-orbit satellites have been proposed by various companies (including Motorola and Qualcomm), but these have yet to materialize.