D-AMPS competed against GSM and systems based on code-division multiple access (CDMA).
D-AMPS uses existing AMPS channels and allows for smooth transition between digital and analog systems in the same area.
[2] IS-136 added a number of features to the original IS-54 specification, including text messaging, circuit switched data (CSD), and an improved compression protocol.
[3] The evolution of mobile communication began in three different geographic regions: North America, Europe and Japan.
In Japan, the 1G standards were: Nippon Telegraph and Telephone (NTT) and the high capacity version of it (Hicap).
The early systems used throughout Europe were not compatible to each other, meaning the later idea of a common 'European Union' viewpoint/technological standard was absent at this time.
AMPS also had a poor security system which allowed people to steal a phone's serial code to use for making illegal calls.
This method separates calls by time, placing parts of individual conversations on the same frequency, one after the next.
Capacity was increased over the preceding analog design by dividing each 30 kHz channel pair into three time slots and digitally compressing the voice data, yielding three times the call capacity in a single cell.
To maintain compatibility with the existing AMPS cellular telephone system, the primary forward and reverse control channels in IS-54 cellular systems use the same signaling techniques and modulation scheme (binary FSK) as AMPS.
The digital verification color code (DVCC) is the equivalent of the supervisory audio tone used in the AMPS system.
Each base station has its own preassigned color code, so any incoming interfering signals from distant cells can be ignored.
This technique allows a bit rate of 48.6 kbit/s with 30 kHz channel spacing, to give a bandwidth efficiency of 1.62 bit/s/Hz.
The major disadvantage with this type of linear modulation method is the power inefficiency, which translates into a heavier hand-held portable and, even more inconvenient, a shorter time between battery recharges.
Once a call comes in the mobile switches to a different pair of frequencies; a voice radio channel which the system carrier has made analog or digital.
In case of high noise, FACCH embedded within the digital traffic channel overrides the voice payload, degrading speech quality to convey control information.
The slow associated control channel or SACCH does not perform handoffs but conveys things like signal strength information to the base station.
The IS-54 speech coder uses the technique called vector sum excited linear prediction (VSELP) coding.
Both the transmitter and the receiver support both analog FM and digital time-division multiple access (TDMA) schemes.
The antenna focuses and converts RF energy for reception and transmission into free space.
The control panel serves as an input/output mechanism for the end user; it supports a keypad, a display, a microphone, and a speaker.
A pragmatic effort was launched to improve IS-54 that eventually added an extra channel to the IS-54 hybrid design.
IS-136 systems needed to support millions of AMPS phones, most of which were designed and manufactured before IS-54 and IS-136 were considered.
IS-136 added a number of features to the original IS-54 specification, including text messaging, circuit switched data (CSD), and an improved compression protocol.
AT&T Mobility, the largest US carrier to support D-AMPS (which it refers to as "TDMA"), had turned down its existing network in order to release the spectrum to its GSM and UMTS platforms in 19 wireless markets, which started on May 30, 2007, with other areas that followed in June and July.