Also known as US Digital Cellular (USDC), US TDMA and NA-TDMA (North American TDMA), and specified in IS-54, which evolved into IS-136. D-AMPS is a North American 2G digital cellular radio standard that essentially is a digital version of the earlier analog AMPS specification. D-AMPS operates in the same 800 MHz band as AMPS and the two can coexist in the same network. D-AMPS uses the same 30 kHz bands as AMPS, and supports up to 416 frequency channels per carrier. D-AMPS employs time division duplex (TDD) to subdivide each frequency channel into six time slots, each of which operates at 8 kbps. Each call initially uses two time slots in each direction, for a total of 16 kbps.The standard recommends speech compression using vector-sum excited linear predictive coding (VSELP) at an average rate of 7.95 kbps that can burst up to 48 kbps. IS-136 is known as a dual-mode standard because both D-AMPS and AMPS can coexist on the same network, with both using the same 21 control channels for call setup, call handoff, and call teardown.Thereby, IS-136 offers carriers the advantage of a graceful transition from analog to digital. IS-136 also includes a nonintrusive digital control channel (DCCH), which adds features such as short message service (SMS) and caller ID. D-AMPS supports symmetric data communications at up to 9.6 kbps per channel through paired downlink and uplink time slots, and as many as three channels can be aggregated for speeds up to 28.8 kbps. D-AMPS is considered to be at the end of its technological life cycle and is being replaced by GSM/GPRS and CDMA2000. The RF modulation technique is /4 differential quaternary phase shift keying (/4 DQPSK). See also 2G, AMPS, analog, caller ID, carrier, CDMA2000, cellular radio, channel, DCCH, digital, downlink, GPRS, GSM, handoff, IS-54, IS-136, modulation, /4 DQPSK, RF, SMS, symmetric, TDD, time slot, uplink, and VSELP.