15.2 3GPP Network Evolution


Chapter 9 describes the Release 1999 standard network. The main standardization effort was in the new radio access network and the WCDMA air interface. Release 4 and 5 standards focus on the core network evolution.

15.2.1 3GPP RAN Evolution

Second-generation GSM networks that were introduced in the early 1990s have experienced phenomenal growth over the last decade and have become the predominant wireless technology in the world today, with a market share of 70% or more. While the acceptance and numbers of subscribers in wireless cellular networks has been growing rapidly , the other phenomenon of the 1990s has been the exponential growth of the Internet and the number of Internet users. We have in essence a wireless network that primarily serves voice and messaging (SMS), and the Internet, which is a data-centric network that offers information, entertainment, e-commerce, and a slew of other services. With the natural evolution toward convergence of voice and data networks and the demand for Internet access and data services in wireless cellular networks, radio technologies defined for second-generation digital networks are evolving to accommodate packet data and services based on packet networks.

The initial GSM specifications provided only basic transmission capabilities for supporting data services. Data rates were in the 9.6-Kbps range. Release 96 specified high-speed circuit switched data services (HSCSD). The theoretical limit of HSCSD with 14.4-Kbps channel coding is 115.2 Kbps. However, practical limits allow data rates up to 64 Kbps.

After five years of intense standardization efforts, the result is an evolution path for GSM that is seen to be smooth, competitive, and cost-efficient. The first steps of this evolution were specified in Release 97 standards when GPRS was introduced. GPRS is able to deliver packet data services efficiently over existing GSM networks. The theoretical maximum throughput in GPRS networks is 160 Kbps per mobile station using all eight channels without error correction.

Voice capacity in wireless networks has always been an issue. In order to maximize the ROI (return on investment), operators have been asking for higher voice capacity over the existing spectrum. Adaptive multirate codec (AMR), which is included in Release 98, increased the spectral efficiency and quality of speech services significantly. The AMR codec contains a set of fixed-rate speech and channel codecs in addition to fast inband signaling and link adaptation. It operates in the full-rate and half-rate GSM channel modes.

Enhanced data rates for GDM evolution (EDGE) was introduced in Release 99. EDGE introduced more efficient modulation, coding, and retransmission schemes. The net effect of this was a significant boost to the performance of data services. EDGE as specified will enhance the throughput per time slot of both HSCSD and GPRS. Enhancement to HSCSD is called ECSD (enhanced circuit switched data), and enhancement to GPRS is called EGPRS (enhanced general packet radio service). ECSD in reality is not implemented since EGPRS is a superior mechanism for packet data access. The enhancement is equivalent to tripling the data rates for HSCSD and GPRS. This is accomplished by using 8-PSK modulation in addition to the existing GMSK. EGPRS is built on top of GPRS. One major change in EGPRS over GPRS is the link quality control, which in EGPRS also supports incremental redundancy. EGPRS also includes QoS capabilities that allow support for real-time services as well.

WCDMA was standardized as UTRAN (UMTS terrestrial radio access network) by 3GPP as part of Release 99. Characteristics of WCDMA have been covered in Chapter 9. With WCDMA, bit rates of 384 Kbps and a maximum of 2 Mbps were introduced. With significant gains in capacity, multiservice capability can now be introduced in WCDMA-based networks. Subscribers can have voice and data sessions ongoing at the same time.

GSM/EDGE radio access network (GERAN) has now been standardized as another radio access network for UMTS in Release 5 by 3GPP. With this UMTS now supports two access technologies, namely, UTRAN and GERAN. GERAN supports the same traffic classes as UTRAN and interfaces to the core network via the Iu interface. Hence from an UMTS vision perspective, GERAN provides the same services via the core network as UTRAN.

Another evolution step forward in Release 5 of 3GPP is the standardization of HSDPA (high-speed downlink packet access) for WCDMA. With HSDPA it is now possible to have data rates of a maximum of 10 Mbps on the downlink. The HSDPA concept offers over 100% higher peak user bit rates than Release '99. HS-DSCH bit rates are comparable to DSL modem bit rates. The mean user bit rates in large macrocell environments can exceed 1 Mbps and in small micro cells 5 Mbps. The HSDPA concept is able to support efficiency not only for non-real -time UMTS QoS classes but also for real-time UMTS QoS classes with guaranteed bit rates.

Release 6 of 3GPP, which is expected in 2003, is currently studying the introduction of WLAN technologies based on 802.11 as another access type, in addition to UTRAN and GERAN. Release 6 will primarily focus on services and the IP multimedia subsystem.

Evolution of radio technologies beyond Release 6 is not a working item yet. However, it is expected that as the capacity requirements for voice and data keep increasing, radio technology will keep evolving to meet these requirements. As new spectrum is opened up, new radio technologies will be developed as well in addition to adapting the current technologies for the new spectrum. As UMTS also evolves to the point where radio networks such as UTRAN, GERAN, and WLAN coexist, mobility across these networks in a seamless manner will also become important. It is expected that multimode terminals with capability for any access type will become available. Operators will deploy UMTS networks in a way that makes use of the capabilities of the access networks. Hence WCDMA-based UTRAN base stations will be deployed where capacity for voice and data is high, and GSM/GPRS base stations will be used in less demanding environments. For existing GSM spectrum, EDGE offers a smooth evolution path to support packet data access and new services based on these. So it is very likely that operators who have GSM networks will upgrade via the EDGE route. For new spectrum, WCDMA offers higher capacity, better spectral efficiency, higher data rates, and QoS support. Existing and new operators who have spectrum for WCDMA will deploy such networks on a gradual basis as demand for data services rises.

The term 4G or fourth generation at this time is a marketing term. No specifications as to what exactly 4G means have been defined. The term is generally used without necessarily referring to any spectrum or radio technologies. At the moment it simply means radio networks with data rates that are higher than what current 3G can offer.

15.2.2 3GPP Release 4 Standard

The 3GPP Release 4 standard introduces the possibility to split the mobile services switching center (MSC) to separate control and switching parts . The MSC server implements the call control and mobility management functions of the MSC. The MSC server controls the user plane part of MSC, called media gateway function (MGW). The MGW terminates the user plane transport channels from both the circuit switched (CS) and packet switched (PS) domains. A channel can be an AAL2/ATM virtual circuit or an RTP/UDP/IP media stream. The MGW can also convert between different types of media so that one party of the call may be in the CS domain (AAL2) while the other uses PS domain VoIP (RTP) call facilities. The MSC server controls the switching in the MGW over a MEGACO (ITU H.248) interface. The roots for the MSC server development lie in the voice over IP (VoIP) networking, where similar development has taken place earlier.

The general architecture of UMTS Release 4 is the same with the Release 1999 standard. Access stratum services and functions are the same.

15.2.3 3GPP Release 5 Standard

The 3GPP Release 5 standard introduces the IP multimedia subsystem (IMS) into the UMTS architecture. In Release 5, the access and core networks are like in the Release 4 standard.

The IMS is the service machinery to support IP services on top of the packet switched network domain (Figure 15-1). The IMS services are based on the Session Initiation Protocol (SIP) defined in IETF, and for which 3GPP has defined specific extensions. SIP is used by the user in the terminal to communicate with the network, and for signaling between the IMS network elements. The IMS is mainly involved in service control, whereas the PS core network provides the transport for the user plane payload. The IMS supports VoIP services, presence services, instant messaging, and other IP services.

Figure 15-1. IP multimedia subsystem configuration.

graphics/15fig01.gif

Release 5 introduces minor modifications to the PS core network, mainly for the support of QoS for IP services and the control by the IMS of QoS provisioning.

The IMS group service and system specification (3GPP TS 23.228) describes the mechanisms to support multimedia services in a mobile environment. The services themselves are not described, but a general framework where the service can be plugged in is given. Internet application development is seen as the fast way to create the services. When the service is attached to IMS, it adds strong authentication and roaming- related functions, which are typically not supported by the service itself.

Call session control function (CSCF) and proxy CSCF (P-CSCF) are the heart of the SIP session management inside the network. The P-CSCF is the mobile subscriber's first contact point for the SIP services inside the visited network. The P-CSCF forwards the SIP signaling to the CSCF in the home network. CSCF verifies the SIP identity with the authentication token from the home subscriber server (HSS), which also generates session-specific cryptographic keys. Service control always takes place in the home network; therefore, the end user has the ability to access the same services and experience the same quality whether the subscriber is roaming in a far-away foreign network or connecting directly to the home network. The IP multimedia media gateway function (IM-MGW) terminates circuit and packet switched media flows and provides media conversion and payload processing in largely the same way as the CS-MGW does in the Release 4 standard. Other elements in the IP multimedia subsystem manage the switching resources.

15.2.4 3GPP Release 6 Standard

Release 6 of 3GPP has many work items for standardization that span the radio interface, radio network, packet core and the IMS. Release 6 is expected to be completed in the second half of 2003. Some of the enhancements for the radio interface in UTRAN are are :

  • Multiple input/multiple output (MIMO) antennas

  • Terminal power saving features

  • Improvements in inter-system and inter-frequency measurments

The transport network in the RAN, which is currently defined as ATM, is also being revised and the possibility of using IP in the RAN is being studied. Some of the other changes in Release 6 will be the support for multicast broadcast and multicast services (MBMS). With MBMS, the operators could deliver services such as advertising to users. MBMS also brings IP multicast to wireless networks. This creates yet another source of revenue for the operators.

Within the core network, the IP multimedia subsystem is being enhanced for the messaging and presence services. Group management within the scope of IMS is also being addressed in this release. Emergency calls in the packet switched network are also being addressed. A feasibility study on wireless LAN and UMTS interworking is in place. Actual inclusion of such a solution is expected in Release 7. Release 6 is also addressing an important issue which is network sharing. Since the cost of deployment of 3G is becoming a limiting factor, network sharing offers a solution by which multiple operators could share a common radio network. Other enhancements will be in the area of QoS for packet services especially services such as streaming audio and video. The open service architecture (OSA) specifications are also being worked on. Enhancements to the Generic User profile, ability to discover terminal capabilities and push services are being added to OSA.

The emphasis is shifting from adding more features to the network architecture to services. There is a far greater need to ensure that services can be developed and deployed in an efficient manner. Also the ability for third parties to provide services to cellular subscribers is becoming more apparent and is being taken into consideration in the evolution of the 3GPP architecture.



IP in Wireless Networks
IP in Wireless Networks
ISBN: 0130666483
EAN: 2147483647
Year: 2003
Pages: 164

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