The Internet Encyclopedia (Volume 3)

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844 WIRELESSINTERNET

IP

Network

RAN

PSTN/

ISDN

GSN

PS-domain

MSC

CS-domain

Core Network

HLR

M

S

Figure 3: A 2.5G cellular network.

equipped with the technology as are some new digi-

tal cameras. Furthermore, products such as headsets,

PCMCIA cards for laptops and PDAs are appearing on the

market.

3G CELLULAR SYSTEMS

Second generation cellular networks are possessed with

very limited data capabilities. Apart from special verti-

cal services such as the SMS, these capabilities are re-

stricted to circuit-switched data up to a data rate of

14.4 Kbps. 2G cellular networks have been extended later

with enhanced data capabilities. The GSM standard, for

instance, has been extended with a packet-switched mode,

GPRS, where timeslots can be assigned toMSs on an on-

demand basis. Also, an MS can combine several timeslots

per frame, in order to achieve higher data rates (theo-

retically up to 171.2 Kbps). This GPRS is much better

suited for the support of end-to-end IP-based services. For

the migration to GPRS, only few changes are required

in the GSM base stations. The core network consisting

of circuit-switched MSCs, on the other hand, needs to

be enhanced by a packet-switched network. In the core

network the switches (MSCs) are augmented by special-

ized routers, the so-called GSNs. From this so-called 2.5

generation on, the core network of a cellular system con-

sists of a circuit-switched part with MSCs, and a packet-

switched part with GSNs (see Figure 3). The major change

going from 2.5G to 3G cellular networks is a complete new

RAN.

For 2G and 2.5G cellular systems, a number of in-

compatible standards and systems are used throughout

the world. For 3G, the International Telecommunications

Union (ITU) has set up IMT 2000 (International Mobile

Telecommunications 2000) as a framework for worldwide

wireless access by linking the diverse system of terrestrial

and/or satellite based networks (ITU-T Recommendation

Q.1701, 1999). The vision for IMT 2000 is to support ad-

vanced applications by providing higher data rates, from

384 Kbps global coverage to 2 Mbps indoor or low-range

outdoor coverage. Further, the systems should be highly

flexible, providing support for both applications that tra-

ditionally use circuit-switched networks and applications

that traditionally use packet-switched networks. A wide

range of date rates is to be supported, with a high granu-

larity.

IMT 2000 comprises a number of cellular systems

(De Vriendt, Lain ́e, Lerouge, & Xu, 2002). First, the

CDMA2000 system is an evolution from the American

CDMA system, IS-95. Further, a number of IMT 2000 sys-

tems are defined by (re)using the core network from GSM.

Three different radio technologies have been defined for

use with this core network. EDGE (enhanced data rates

for GSM evolution) is an evolution from the GSM (and

GPRS) technology, using new modulation techniques, to

provide data rates up to 384 Kbps. The two other ra-

dio technologies have been developed in the context of

UMTS (universal mobile telecommunication system), and

are based on CDMA principles. The main difference be-

tween the two is the duplexing technique used. Duplexing

the two directions of communication can be done in either

frequency or time, resulting in an FDD and a TDD UMTS

variant, respectively. The first one, which is currently most

widely implemented, uses a technique called wideband

CDMA (WCDMA), using 5-MHz carriers to provide data

rates up to 2 Mbps. WCDMA and CDMA2000 both rely

on CDMA technology, but differ from each other in vari-

ous design and implementation aspects, such as clock and