BY AXXO

BOM DIA readers (Portuguese). So you people thought we would go next to 3G from 2G? Think again. There is 2.5G.

2.5G is a stepping stone between 2G and 3G cellular wireless technologies. The term “second-and-a-half generation” is used to describe 2G-systems that have implemented a packet switched domain in addition to the circuit switched domain. It does not necessarily provide faster services because bundling of timeslots is used for High Speed Circuit Switched Data services (HSCSD) as well.

2.5G provides some of the benefits of 3G (e.g. it is packet-switched) and can use some of the existing 2G infrastructure in GSM and CDMA networks. GPRS is a 2.5G technology used by GSM operators. Some protocols, such as EDGE for GSM and CDMA2000 1x-RTT for CDMA, can qualify as “3G” services (because they have a data rate of above 144 kbit/s), but are considered by most to be 2.5G services (or 2.75G which sounds even more sophisticated) because they are several times slower than “true” 3G services.

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). Yet another 2.5G technology that is circuit based is High Speed Circuit Switched Data (HSCSD).

IMPETUS

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 realisation 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 were 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 mobile 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 average about 70-80 Kbps.

EDGE boosts 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 three bits per symbol (rather than GPRS’ one bit per symbol), facilitating an up to 3 X’s improvement over GPRS. HSCSD provides 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 theoretically provide up to 2 Mbps in a fixed location. There will, however, be some significant limitations to this theoretical capacity.

ARCHITECTURE & 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 (Home Location Register), 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.
So, next week we’ll have 3G? I dunno… wait and watch.
Till then, Adeus.