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•    To provide high speed packet data rates:

-    2 Mbps for fixed environments

-    384 Kbps for pedestrian

-    144 Kbps for vehicular traffic

The exact spectrum band allocated to IMT-2000 by ITU is 1885 MHz-2025 MHz and 2110 MHz-2200 MHz, as shown in figurę 3. It can be noticed that the spectrum allocation is very similar in Europę and Japan, but in the United States great part of the spectrum is already used by second generation systems. As a conseąuence, American network operators will need to gradually replace their existing infrastructure with similar third generation technologies.

In November 1999 the ITU recognized that the following radio interface technology proposals fulfilled IMT-2000’s reąuirements and accepted them as compatible:

•    IMT Direct Spread (IMT-DS; also known as UTRĄ)

•    IMT Multicarrier (IMT-MC)

Of these, the most important radio interface standard is Universal Terrestrial Radio Access (UTRĄ), which is a radio interface based on Wideband-CDMA (WCDMA) technology and operates in two modes: Freąuency Division Duplex (FDD) and Time Division Duplex (TDD). It has received sup-port both from the European Telecommunications Standards Institute (ETSI) and the Association of Radio industries and Businesses (ARIB).

In December 1998 the 3rd Generation Partnership Project (3GPP) Agreement was formalized and signed. This collaboration agreement gave rise to a standardization organization which com-prises several telecommunications standards bodies, or Organizational Partners. The organization is called 3GPP as well and its goal is to produce specifications for UMTS, a third generation system based on two important technologies: the UTRĄ radio interface and the extended GSM/GPRS network. Later, the organization’s scope was extended to include the maintenance and further development of GSM’s technical specifications as well as GPRS’ and EDGE’s.

Due to ITU’s policy of not excluding any serious candidate from IMT-2000 specification, a second radio interface exists. This radio interface, as mentioned above, is called IMT-MC and its use is promoted by the 3GPP2 standardization organization. While 3GPP specifies a new radio interface, 3GPP2 specifies an interface backward compatible with IS-95 systems, which makes transition to 3G systems much easier. This alternative third generation proposal is known as CDMA2000.

2.2 Universal Mobile Telecommunications System’s architecture

A UMTS network is logically divided into two parts, which are referred to with the generic terms Core Network (CN) and a Generic Radio Access Network (GRAN) [2, 8, 10, 11]. The core network reuses several elements already present in GPRS and GSM networks, and consists of two overlapping domains: Circuit-Switched (CS) domain and Packet-Switched (PS) domain. CS domain is madę up of those entities which allocate dedicated resources for user traffic and control signals when the connections are established, and release them when the sessions finish. Generally, voice calls are always handled by the components belonging to CS domain. The entities in the PS domain transport user data in the form of autonomous packets, which are routed independently of each other; this scheme overcomes the limitations of 2G networks to transmit data efficiently. It is through the CN that the user can set up a connection to and from external packet data networks

6

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