2.5G

Introduction

CDMA2000's 1xRTT is the first technology for the evolution of cdmaOne 2G networks to 2.5G networks.

GPRS represents the first packet-based technology for evolution from 2G GSM networks to 2.5G networks.

Another GSM 2.5G packet technology, Enhanced Data rates for GSM Evolution (EDGE).
Another 2.5G technology that is circuit based, High Speed Circuit Switched Data (HSCSD).

Impetus for 2.5G

The major impetus for 2.5G is the "always-on" capability. Being packet based, 2.5G technologies allow for the use of infrastructure and facilities only when a transaction is required, rather than maintaining facilities in a session-like manner. This provides tremendous infrastructure efficiency and service delivery improvements.

Using GPRS as a bearer for WAP, for instance, will allow for the use of WAP on a per-transaction rather than a per-minute-of-use basis. More importantly perhaps is the ability for GPRS to allow for autonomous service realization through the always-on capability. For example, a GPRS customer could receive content or services without actually manually invoking a service or transaction. This has significant implications for mobile commerce and location based services.

Data Speed

GPRS data speeds are expected to reach theoretical data speeds of up to 171.2 Kbps. However, this is based on optimal conditions in terms of available cell/sector capacity in terms of available time slots, maximum coding scheme (CS-4) as well as moible phone availability to support the maximum number of time slots - eight. More practical data rates are currently in the order of 40-60 Kbps.

CDMA2000 1xRTT data speeds are averaging about 70-80 Kbps.

EDGE will boost data theoretical data rates to 384 Kbps if/when deployed. EDGE accomplishes these higher rates through introduction of a new modulation scheme known as Eight Phase Shift Keying (8PSK). 8PSK provides for up to 3 bits per symbol (rather than GPRS's 1 bit per symbol), facilitating an up to 3 X's improvement over GPRS.

HSCSD will provide speeds of up to 64 Kbps. However, HSCSD perpetuates the inefficient use of spectrum and transmission that is relegated by any circuit switched mechanism.

Prior to the introduction of these technologies, Cellular Digital Packet Data (CDPD), offered only up to 19.2 kbps on AMPS networks. Other current means of mobile data such as NTT DoCoMo's PDC network offer only 9.6 kpbs, such as used for the highly successful iMode.

3G technologies such as CDMA2000 (1xEV-DO and 3x) and W-CDMA will theoreticaly provide up to 2 Mbps in a fixed location. There will, however, be some significant limitations to this theoretical capacity.

2.5G Architecture and Issues

GPRS represents an overlay network deployment to GSM, thus new network elements are placed into the network while existing network elements, such as the HLR, simply require a software upgrade.

CDMA2000 also represents an overlay network, with 1xRTT first requiring new channel cards, and later versions of CDMA2000 requiring new Core Network (CN) infrastructure such as the AAA server and Packet Data Server Node (PDSN) as part of a Mobile IP network infrastructure.

Additional Resources:Books about: 2.5G and 3G Wireless Communications