Mobile phone system of generation 2.5 are regarded as an intermediate step between
the second generation system , such as GSM, D/E-AMPS,
ADC and JDC, and the tird generation technologies such as IMT-2000.
in particitular, 2.5G provides higher bandwidths to support new services, including
mobile multimedia. The most interesting standards are
CDMA-related: IS-95B and IS-95C, offering 64 and 144 kbit/s, respectively
IS-95A was a 14.4 kbit/s circuit-switched service. IS-95B achieves 64kbit/s
by eliminating the header overhead and a transition to packet-switching.
High-Speed Circuit-Switched Data (HSCSD)
HSCSD replaces the GSM coding scheme. It add less redundancy than used for
speech transmission. Meanwhile, four instead of one timeslot can be used.
This allows the transmission of 57.6 kbit/s. HSCSD may be implemented in the
base station just by a software upgrade.
General Packet Radio Service (GPRS)
GPRS also is an extension of GSM that uses multiple time slots within a TDMA
frame. GPRS further allows the transmission of Internet TCP/IP messages to
multiple users within a single GSM slot.
Enhanced Data Rates for Global Evolution (EDGE)
EDGE allows the transmission of up to 384kbit/s by involving 8PSK modulation
instead of GMSK.
Justin Chang of AT&T Research Labs explains the goals of EDGE.
EDGE is an evolution on GPRS and GSM.
