Generation 2.5 (2.5G) is a term commonly used to describe the second technology used in a specific application or industry. 2.5G systems provide more services and features than second generation (2G) technology but less than the third generation (3G).

One of the key attributes of 2.5G mobile systems is their ability to transmit data in packet data form. These systems are usually designed to allow the same radio channel to share packet data and voice signals. To ad packet data capability, mobile devices and base stations are modified to include new software that allows them to differentiate and schedule the transmission of data and voice signals (new protocols). Packet data switching equipment is then added to the mobile system allowing it to route data packets within the network or to other data networks (such as to the Internet).

Figure 1.1 shows a 2nd generation digital cellular system that has been upgraded to offer medium speed packet data services. This diagram shows that the existing 2nd generation digital radio channel 

Figure 1.1., Packet Digital Cellular System (2 1/2 Generation)

General Packet Radio Service (GPRS)

General Packet Radio Service (GPRS) is a portion of the GSM specification that allows packet radio service on the GSM system. The GPRS system adds (defines) new packet channels and switching nodes within the GSM system. The GPRS system provides for theoretical data transmission rates up to 172 kbps.

Enhanced Data Rates for Global Evolution (EDGE)

Enhanced Data Rates for Global Evolution (EDGE) is an evolved version of the global system for mobile (GSM) radio channel that uses new phase modulation and packet transmission to provide for advanced high-speed data services. The EDGE system uses 8 levels of Phase Shift Keying (8PSK) to allow one symbol change to represent 3 bits of information. This is 3 times the amount of information that is transferred by a standard 2 level Gaussian Minimum Shift Keying (GMSK) signal used by the first generation of GSM system. This results in a radio channel data transmission rate of 604.8 kbps and a net maximum delivered theoretical data transmission rate of 384 kbps. The advanced packet transmission control system allows for constantly varying data transmission rates in either direction between mobile radios.

CDMA2000™ 1xRTT 

CDMA2000™ is a 3G standard that allows operators to evolve from their existing IS-95 networks to offer 3G services. The original CDMA2000™ proposal contained two distinct evolutionary phases, the first known as 1xRTT used the same 1.25 MHz channels as IS-95

Evolution Data Only (1xEVDO)

The evolution of existing systems for data only (1xEVDO) is an evolved version of the CDMA2000™ 1xRTT system. The 1xEVDO system uses the same 1.25 MHz radio channel bandwidth as the existing IS-95 system that provides for multiple voice channels and medium rate data services. The 1xEVDO version changes the modulation technology to allow for data transmission rates up to 2.5 Mbps. The 1xEVDO system has an upgraded packet data transmission control system that allows for bursty data transmission rather than for more continuous voice data transmission.

Evolution Data and Voice (1xEVDV)

The evolution of existing systems for data and voice (1xEVDV) is an evolved version of the CDMA2000™ 1xRTT system that can be used for data and voice service. The 1xEVDV system provides for both voice and high-speed data transmission services in the same 1.25 MHz radio channel bandwidth as the existing IS-95 system. The 1xEVDV Vision allows for a maximum data transmission rate of approximately 2.7 Mbps.