The native AMPS environment is harsh for data transmission. RF modulation and coding techniques, such as vocoders, ADPCM2.34 trunking compression and FM pre-emphasis-which are optimized for voice transmission-distort data and disallow the use of standard (landline) modems on cellular channels. Cell handoffs, channel reassignments and power level change commands are all transmitted in-band by the system, further distorting the data channel. Finally, static, signal fading and interference make cellular a noisy channel for data applications. Standard landline modems typically react by either losing data or hanging up.
Despite these challenges, the ubiquity of AMPS analog cellular coverage and demand for wireless data services have motivated development of technology supporting cellular-based data services. Today, circuit-switched cellular data is the most widely used mobile data service.
Special modem technology has been developed by AT& T Paradyne (Enhanced Throughput Cellular or ETC), Microcom (MNP10), Motorola (EC2) and others to optimize the capabilities of AMPS channels for data transmission. These enhanced cellular data protocols allow approximately 9 kbps under normal conditions, with 14.4 kbps becoming increasingly available. Data compression also increases the effective throughput enjoyed by cellular data users.
Modem pools supporting these new protocols are now being deployed by cellular service providers. These modem pools provide a gateway function bridging the specialized cellular modem protocols and standard landline modem protocols. A special code is added to the dialed digit string at the mobile, which alerts the cellular switch to connect the call to the modem pool rather than simply placing a call. The gateways allow continued interoperability of AMPS with conventional modems.
The modem pool concept has been extended with backbone packet services offered by AT& T, MCI, Sprint and others. Typical of these offerings is MCI's Xstream Air Network, depicted in Figure 2.4, an X.25-based backbone supporting cellular modem-based data applications. Access to Xstream is via third party cellular service providers. Special 800-number access is available from anywhere to the MCI cellular modem pool, which supports both MNP10 and ETC protocols.
Another data solution for the cellular airlink is a single-sided protocol such as Air True by Air Communications, Inc. This protocol only requires support at the transmitting side (presumably the mobile) of the airlink to effectively counter the debilitating effects of the cellular environment. It recognizes network event (call control) messages for cell handoff, power control and channel changes for what they are rather than interpreting them as random noise. After the interrupting network event message has completed, the mobile transmitter resumes where it left off, increasing the effective bandwidth.
Channel characteristics are not the only challenge for AMPS-based data applications. Current cellular systems are voice-oriented and thus have usage accounting mechanisms, which are based on time of usage rather than actual data transmission. The time-of-usage billing schemes typically begin with a minimum usage of one minute. A significant departure from previous per-minute billing practices is embodied in the UPS package tracking system, supported since 1992 by a large number of cooperating cellular service providers.
In 1995, Bell South Wireless, Inc., announced a wireless telemetry solution named Cellemetry, which would be licensable by at most one cellular service provider per market. Cellemetry operates over the AMPS control channels and thus is limited by the amount of data which can be transported by cellular registration and paging messages (32 bits) and the amount of additional traffic which can be borne on the control channel without impacting the primary purposes of these control channels-cellular registration and paging.