Full Name: Advanced Mobile Phone Service (AMPS) Circuit-Switched Cellular

• History
• Technology
• Strengths
• Weaknesses
• Related Technologies
• Deployment
• Markets and Applications

History

Circuit-switched cellular allows for data transmission over existing analog cellular systems (AMPS in North America) originally designed for voice. This became available shortly after cellular service was introduced back in 1984. AMPS was designed by Bell Labs back in 1969.

Technology

AMPS uses the 800 MHz frequency band and was originally designed for voice transmission. Technologies have recently been developed to allow for data over these same voice channels. This cellular data transfer requires a "session" between each end similar to a voice connection. During this session, cellular data transmissions send data over an airlink (i.e. through a nearby cell, the regional switching office, and Public Switched Telephone Network (PSTN)) to a remote computer.

Since cellular phones utilize radio signals, common radio performance impairments such as interference, noise, fading, and overall channel degradation can affect data transmission. Therefore, special "cellular-enhanced" protocols have been introduced to control modulation, data compression, and error correction helping provide faster and more reliable transmissions when both modem ends use the same protocol. The most common of these protocols are ETC (Enhanced Throughput Cellular from AT&T Paradyne Inc.) and MNP-10 (Microcom Network Protocol from Microcom Inc.). Since it is difficult to guarantee both ends use the same protocol, it's common for cellular carriers to implement "modem pools", a collection of cellular and landline modems connected to each other and located at the Cellular Switching Office (CSO) used specifically for cellular data transmissions. This helps guarantee that the enhanced protocols are available when connecting with the cellular data transmission and helps provide conversion from cellular to standard landline protocols.

Billing for AMPS data service is most often based on airtime, normally in one-minute increments. Similar to an AMPS cellular phone, additional charges for long distance/roaming are also applied.

Strengths

• Coverage

Circuit-switched cellular (CSC) networks have been over a decade in the making providing virtually ubiquitous coverage. This means CSC data services can be widely available relying on the existing AMPS cellular infrastructure. No additional equipment is needed for voice, fax, or data. Therefore, wherever a cell phone can go for cellular voice communications, wireless data can go too.

• Stability of Network

AMPS cellular has achieved a high level of maturity and robustness over its lifetime. It is a proven technology that is widely adopted, now for data as well. New modem technologies (e.g. protocols) have also been developed specifically for transmitting data over the CSC network resulting in improvements in reliability and performance.

• Cost

As an alternative to packet-switched services such as CDPD, circuit-switched cellular data communications can be more economical for larger data transfers. Rather than paying for the total amount of data actually sent and received over the cellular connection, users are charged for the time it takes to transfer the data and close the call.

• Payment Structure

Since CSC data connections are almost identical to the typical cellular voice connections, billing procedures are more familiar for the end user. Rather than calculating the total number of kilobytes sent and received like packet data networks normally require, CSC data transfer is billed by the connection time similar to a cellular phone. Since these rates have also become more economical with increased subscriber bases, a variety of rate plans are now available.

• Equipment Selection

Over the years, numerous AMPS cellular devices have become available for both voice and data providing a complete selection for the end user.

Weaknesses

• Cost

Since a connection or "circuit" is established when transferring data over an AMPS cellular network, charges are based on connection time similar to a cellular phone call. This means if the wireless application transfers short bursts of data, the cost of the connection time can outweigh the time of the actual data transmission. In addition to the payment format, additional charges for long distance or roaming can be applied unlike packet-switched networks.

• Delay

Since CSC involves a connected cellular circuit, dial-up delays are introduced. These delays are not only realized when the data connection is to be made but also when the monthly bill based on connection time is received.

• Transmission Rate and Errors

Transmission speeds for circuit-switched connections generally range from 2.4 to 14.4 kbps. Throughput is affected by interference, noise, fading, and overall channel degradation, common RF-related affects, and varies from one location to another. Dropped calls are also not uncommon. However, several modem technologies have been introduced to address these data-related issues.

• Security

Cellular does not provide any data encryption, meaning voice transmissions can be intercepted by an appropriately modified radio scanner. This isn't as easy as it may seem, though, since similar cells and frequencies must be used. This can be especially difficult if the mobile end is moving about. Data, on the other hand, is much harder to track since it is sent through co-operating modems using specialized cellular protocols. These modems may also incorporate a compression standard such as MNP-7 or V.42bis. This makes it very difficult to intercept data transmissions over cellular.

• Complexity

Compared to a packet-switched data technology like CDPD, CSC connections can be more difficult to originally set up and use.

Related Technologies

• Personal Communication Services (PCS) - cellular-like system in the 1900 MHz range within North America ( PCS Information)

Deployment Today, analog cellular services are available throughout the world. In North America, cellular networks (i.e. AMPS) blanket the continent and, within the U.S., most carriers offer circuit-switched data communications services over their cellular networks. Circuit-switched data services are also now being introduced over digital PCS systems such as GSM and CDMA.

Markets and Applications

Circuit-switched cellular lends itself to fax, large data file (i.e. greater than 30 kilobytes), and interactive application transmissions since it makes more sense to be charged by connection time rather than the amount of data sent or received. On the other hand, cellular is a good back-up option for those applications that cross geographical areas which do not have support for other wireless data service options such as CDPD or RAM.