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Chapter 8: Network
8.1 General DiscussionA one-sentence description of the 3GPP concept might be: "A CDMA packet-based air interface combined with a GSM + GPRS core network." ITU's 3G concept, known as IMT-2000, includes several other accepted technologies for 3G systems. However, the referenced WCDMA + GSM combination will be the most widely used. The simple reason for this is the fact that among the second-generation mobile cellular networks, GSM is by far the most widely used technology. All of the significant 3G proposals for IMT-2000 are those that successfully protect the investments in their 2G legacy networks in a 3G world. The 3GPP work strives to protect the GSM investments and bring the markets into 3G. The present-day operators will not want to invest in new networks if they can iarecyclely their existing GSM networks. Most big network manufacturers and operators are actively supporting this approach. So for these operators this 3G solution is a good deal, because they can continue to use their upgraded GSM networks. In many cases the radio access network can be updated to conform to the 3G requirements. The mobile phone users however are not so lucky, as they will need new phones that are capable of accessing WCDMA base stations. Most probably, these phones will have to be dual-system GSM + WCDMA phones at first, since the UTRAN radio access network coverage will be quite limited at service launch. The UTRAN may only provide coverage in urban hot-spots in the beginning, as the old GSM networks will be used to provide wide-area service.
The IMT-2000 network is divided into two logical concepts, the core network (CN) and the generic radio access network (GRAN). The noble idea behind this arrangement is that the GRAN will be capable of connecting, perhaps simultaneously, to several different core networks, such as GSM, B-ISDN + IN, or a packet data network. The GRAN could be implemented, for example, as a GSM BSS, DECT, LAN, CATV, or Hiperlan2, network. 3GPP has also specified a new dedicated UMTS radio access network (RAN) called the UTRAN (UMTS Terrestrial RAN). An important requirement for the GRAN implementations is that they conform to the Iu interface specifications. Note however that the 3GPP Release 99 specifications only contain provisions for the GSM-MAP (including GPRS) and the ANSI-41 core networks.
In GSM terms, the GRAN contains the base station subsystem; that is, the BTS and the BSC. In the 3GPP specifications, the generic GRAN concept is translated into a concrete UTRAN network in which the base transceiver station (BTS) has the curious name Node B. The new name for the base station controller (BSC) is the radio network controller (RNC).
Between the GRAN and the core network we find the Iu interface, and between the GRAN and the UE we see the Uu interface (radio interface). See Figure 8.1.
8.2 Evolution from GSMIt must be noted that a GSM phase 2+ network provides a smooth transition path to UMTS, especially if the operator also operates a GPRS network. GSM networks have been updated little by little to include more and more features. Table 8.1 (from ) shows how well the future GSM 2.5G network will comply with the UMTS requirements.
As we can see in Table 8.1, a GSM network with all the add-ons very closely mimics a UMTS network. The only difference is the more flexible and capable UMTS air interface, which can handle different bearer types at the same time. Real-time services are confined to dedicated connections whereas non-real-time low bandwidth services can quite easily use shared communication channels, which can more easily be changed dynamically.
UMTS can also achieve higher bit rates, but it must be noted that the differences between UMTS and 2.5 GSM are not so large from the user point of view. A GSM with all the 2.5G upgrades could achieve close to 200 Kbps user data rates. In theory (and in the marketing talk) GSM could approach 384 Kbps rates. If a UTRAN network wants to exceed this speed, it has to use very low spreading factors, and allocate the resources of a base station mostly to one user. When a GSM base station offers close to 200 Kbps speeds to one user, it only uses one of its frequency carriers; there are probably several other carriers still available for other users. But a typical WCDMA base station has only one downlink frequency carrier, and if one user is provided with a downlink connection of over 2 Mbps, then other users are left with nothing. In practice the situation is not so bad, since high-speed traffic is typically bursty and not continuous, and thus several users can have high data rates momentarily. There are also techniques to enhance the UTRAN's downlink capacity further, like sectorization, smart antennas, and addi- tional scrambling codes.
We can see that a GSM + GPRS combination provides a very good foundation for the UMTS core network building process. The biggest operator investment will clearly be building out the radio access network. We are excluding the operator license fees here, as they are not technology related to the network's implementation. Some of the latest GSM base stations are, however, said to be upgradeable to UTRAN standards.
Note that the easy accommodation of GSM for the UMTS requirements may also be a problem for new UMTS operators, since the existing GSM (non-UMTS) operators can provide almost the same services without the extra UMTS investments. The competition will be hard for the new UTMS operators in the early phases, and it is especially hard for the new UMTS operators, which do not have an existing 2G network. It is very expensive for them to build out wide area coverage 3G networks, while they don't have any income from existing networks. Furthermore, in many countries the operating licenses are very expensive. The combined cost burden from the licensing fees, interest and network construction can push a new green-field operator into a very unfavorable position. Telecommunication authorities may have to do some creative thinking to find ways to help these companies. One way to ease the situation would be to force the current 2G network operators to lease their networks to new 3G operators, so that they can provide wider coverage for their customers from the beginning. There are already successful examples of this concept in the GSM world. New GSM-1800 operators have been able to provide wider coverage by leas- ing GSM-900 capacity from existing operators in some countries. Note that these old operators are competitors for the new networks, so the telecommunication authority must be very careful in their decisions so that free competition is not obstructed more than absolutely necessary.
8.3 UMTS Network StructureFigure 8.2 depicts the UMTS architecture at the very highest level. This chapter concentrates on both the core network (CN) and the UTRAN. Section 8.4 discusses the CN, and section 8.5 handles the UTRAN. The interfaces between the UE and the UTRAN (Uu interface) and between the UTRAN and the CN (Iu) are open multivendor interfaces. Note that most of the first seven chapters of this book are dedicated to the Uu interface, with its WCDMA technology....
Table of Contents
|1.1||History of Mobile Cellular Systems||1|
|1.2||Overview of 3G||8|
|1.3||Proposals for 3G Standard||10|
|1.6||3G Evolution Paths||23|
|2||Principles of CDMA||25|
|2.1||Radio-Channel Access Schemes||25|
|3||WCDMA Air Interface: Physical Layer||49|
|3.3||Spreading and Scrambling Codes||81|
|3.6||Data Through Layer 1||97|
|4||Modulation Techniques and Spread Spectrum||101|
|5.4||Autocorrelation and Cross-Correlation||118|
|6.6||Channel Coding in UTRAN||129|
|7||Wideband CDMA Air Interface: Protocol Stack||131|
|7.6||RRC Protocol States||183|
|7.7||Location Management in UTRAN||187|
|7.8||Core Network Protocols in the Air Interface||190|
|7.10||Packet Data Convergence Protocol||196|
|7.13||Dual-System Protocol Stack in UE||201|
|8.2||Evolution from GSM||204|
|8.3||UMTS Network Structure||206|
|8.5||UMTS Terrestrial Radio Access Network||213|
|8.6||GSM Radio Access Network||216|
|8.9||UMTS Network Evolution--Release 5||243|
|9.1||Importance of Network Planning||251|
|9.2||Differences Between TDMA and CDMA||251|
|9.3||Network Planning Terminology||255|
|9.4||Network Planning Process||256|
|9.5||Network Planning in WCDMA||262|
|10.4||Service Providers Versus Operators||298|
|11.1||RRC Connection Procedures||303|
|11.2||Radio Bearer Procedures||306|
|11.5||Random Access Procedure||340|
|12||New Concepts in the UMTS Network||343|
|12.2||High-Speed Downlink Packet Access||355|
|12.3||Multimedia Broadcast/Multicast Service||358|
|12.4||Multimedia Messaging Service||361|
|12.7||Gateway Location Register||374|
|12.9||Adaptive Multirate Codec||381|
|12.10||Support of Localized Service Area||384|
|14.1||Justification for 3G||407|
|14.2||Path into the Market||409|
|14.3||Applications As Competition Tools||410|
|14.6||Traffic Characteristics of 3G Applications||422|
|14.8||Examples of 3G Applications||427|
|Appendix A||Cellular User Statistics||487|
|Appendix B||3GPP Specifications||491|
|Appendix C||Useful Web Addresses||509|
|Appendix D||Nokia Communicator||513|
|Appendix E||Standardization Organizations and Industry Groups||515|
|About the Author||523|