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This article was published in the August 2000 issue of
IEEE Personal Communications.

ABSTRACT

 

Today's wireless core network is based on a circuit-switched SS7 architecture similar to that found in wireline telecommunications networks. With the advent of IP technologies and the tremendous growth in data traffic, the wireless industry is evolving its core networks toward IP technology. Wireless telecommunications started as an offshoot of wireline telephony, and the absence of global standards resulted in regional standardization. Two major mobile telecommunications standards have dominated the global wireless market, namely, TDMA/CDMA developed by TIA in North America and GSM developed by ETSI in Europe. As we move toward third-generation wireless, there is a need to develop standards which are more global and collaborative. Recently, the global wireless industry has created two new global partnership projects ,3GPP and 3GPP2, to address the issue of the limited data capabilities of 2G systems, motivating the PPs to start work on 3G wideband radio technologies that can provide higher data rates. This work resulted in 3G wireless radio technologies that will provide data rates of 144 kb/s for vehicular, 384 kb/s for pedestrian, and 2 Mb/s for indoor environments, and meet the ITU IMT-2000 requirements. Now that the radio technology standards to support higher data rates have been developed, the PPs are focusing on development of standards for all IP networks. In this short article, we discuss the genesis of 3GPP and 3GPP2 IP work, outlining the important architectural differences of the two groups. Currently, 3GPP and 3GPP2 offer divergent proposals that need to be harmonized if convergence toward an IP-based mobile telecommunications networks is to become a reality.

 

 

The 3GPP and 3GPP2 Movements Toward an All-IP Mobile Network

 

Girish Patel, Nortel Networks
Steven Dennett, Motorola

 

Today's wireless core network is based on a circuit-switched Signaling System No. 7 (SS7) architecture similar to that found in wireline telecommunications networks. With the advent of IP technologies and the tremendous growth in data traffic, the wireless industry is evolving its core networks toward IP technology.

Wireless telecommunications started as an offshoot of wireline telephony, and the absence of global standards resulted in regional standardization. Two major mobile telecommunications standards have dominated the global wireless market: time-/code-division multiple access (TDMA/CDMA) developed by the Telecommunications Industry Association (TIA) in North America, and the Global System for Mobile Communications (GSM) developed by the European Telecommunications Standards Institute (ETSI) in Europe. As we move toward third-generation (3G) wireless, there is a need to develop standards which are more global and collaborative. Recently, the global wireless industry has created two new partnership projects to address this issue:

The limited data capabilities of 2G systems motivated the PPs to start work on 3G wideband radio technologies that can provide higher data rates. This work resulted in 3G wireless radio technologies that will provide data rates of 144 kb/s for vehicular, 384 kb/s for pedestrian, and 2 Mb/s for indoor environments, and meet the International Telecommunication Union (ITU) International Mobile Telecommunications (IMT)-2000 requirements [3]. Now that the radio technology standards to support higher data rates have been developed, the PPs are focusing on development of standards for all IP networks.

In this short article we discuss the genesis of 3GPP and 3GPP2 IP work, outlining the important architectural differences between the two groups. Currently, 3GPP and 3GPP2 offer divergent proposals that need to be harmonized if convergence toward an IP-based mobile telecommunications network is to become a reality. Note that this provides a snapshot at present, since the work in both fora is progressing rapidly.

3GPP Network Architecture

In evolving to an IP core network, 3GPP has decided to base it on GPRS [4]. While the GPRS-based approach provides packet data access in 3GPP, some operators felt that the work toward an all-IP network was not moving rapidly. Hence, in early 1999 a group of wireless operators invited a group of leading vendors to work in a focus group called 3G.IP to address the requirements for an all-IP wireless network architecture. The intent was to formulate fast track technical solutions that would be introduced into 3GPP.

Shortly after, 3GPP formed an ad-hoc group to perform a feasibility study for an all-IP network based on 3G.IP inputs. The group developed requirements for network architecture based on an all-IP network. The IP-based architecture would have to support both stream and best effort services. The implication of this is that the mobile terminal would include IP-based clients. The group also agreed to base access mobility on GPRS. The feasibility study has resulted in a draft architecture for an all-IP network, a simplified version of which is shown in Fig. 1 [5].

An essential principle of the framework was to provide separation of service control from connection control. 3GPP essentially started with GPRS as the core packet network, and overlaid it with call control and gateway functions required for supporting voice over IP (VoIP) and other multimedia services. The functions are provided via Internet Engineering Task Force (IETF)-developed protocols to maintain compatibility with the industry direction in all-IP networks.

To support VoIP, call control function analogous to call control in a circuit-switched environment are provided by the call state control function (CSCF). The mobile terminal communicates with the CSCF via SIP or H.323 protocols that support VoIP. The CSCF performs call control functions, service switching functions, address translation functions, and vocoder negotiation functions. For communication to the public switched telephone network (PSTN) and legacy networks, PSTN gateways are provided. To support roaming to 2G wireless networks, roaming gateway functions are also provided.

The GPRS serving node, SGSN, uses existing GSM registration and authentication schemes to verify the identity of the data user. This makes the SGSN access-technology-dependent. The GPRS home location register (HLR) is enhanced for services that use IP protocols. The data terminal makes itself known to the packet network by doing a GPRS attach. The IP address is anchored in the GPRS gateway node, GGSN, during the entire data session. This limits the mobility of the data terminal to within GPRS-based networks. To provide mobility with other networks a foreign agent (FA) as per the Mobile IP architecture [6] can be incorporated in the GGSN.

3GPP has accepted IP technology for its new work items in 2000. Until now, the 2000 work has consisted primarily of framework discussions, so detailed solutions are not finalized.

3GPP2 Network Architecture

3GPP2 has created a new packet data architecture building on the CDMA 2G and 3G air interface data services. 3GPP2 has taken advantage of 3G high data rates and existing work in IETF on Mobile IP [6] to enhance the network architecture to provide IP capabilities. For additional details of the Mobile IP application in 3GPP2 architecture see [7]. One advantage of using globally accepted IETF protocols is ease in interworking and roaming with other IP networks. The other major advantage is that it can provide private network access (virtual private networking) via a Mobile IP tunnel with IP security [7].

3GPP2 has undertaken the work to enhance the IP architecture for multimedia services (including voice). The idea is to capitalize on the synergies of Internet technologies and use a single network for all services. Figure 2 shows an interpretation of the all-IP network (actual requirements are currently being developed).

The essential attributes of an all-IP network are end-to-end IP connectivity, distributed control and services, and gateways to legacy networks. In the 3GPP2 architecture, IP connectivity reaches all the way to the base station transceiver (BTS). Both the base station controller (BSC) and BTS are contained in the IP-based radio access network node in Fig. 2. This means that the BSC will be a router-based IP node containing some critical radio control functions (e.g., power control, soft handoff frame selection). The remaining control functions, such as call/session control, mobility management, and gateway functions, are moved out to the managed IP network. This allows distributed and modular control architecture. The functions of the packet data serving node (PDSN) in the 2G architecture [7] are distributed as shown in Fig. 2. Since much of the communication will be between wireless and legacy terminals, gateway functions are provided for roaming to 2G wireless networks and interworking with the PSTN.

In the 3GPP2 architecture, the mobile terminal uses Mobile-IP-based protocols to identify itself. The PDSN contains foreign agent (FA) functionality as per the Mobile IP architecture [7]. When the mobile terminal attaches to the FA, the FA establishes a Mobile IP tunnel to the home agent (HA) and sends a registration message to the HA. The HA accesses the authorization, authentication, and accounting (AAA)server to authenticate the mobile terminal. The IP address of the mobile terminal is now anchored in the HA for the duration of the data session. The data device connected to the mobile terminal can be handed over to any other access device that supports Mobile IP. Thus, this approach can provide mobility across different access networks (wireless, wireline, etc.). However, since it essentially uses address translation to provide mobility, it cannot do fast handoff due to the latency of address updates from distant agents. There has been considerable research to address the latency issue via schemes such as Cellular IP [8], Hawaii [9], and TeleMIP [10]. Essentially all of them propose some form of hierarchy with local/gateway routers, which can reduce latency by reducing updates from the remote HA. These schemes could be used to optimize Mobile IP application in 3GPP2.

Convergence of the Two Approaches

In provision of mobility for IP sessions, the 3GPP and 3GPP2 architectures are different because of the underlying base networks and evolution strategies. In 3GPP, GPRS-based mobility was already defined, so the IP network enhancements were considered on top of GPRS. On the other hand, 3GPP2 needed to develop a mobility mechanism for packet data since one did not exist previously. 3GPP2 has decided to use Mobile IP as the basis for packet data mobility. To illustrate the similarities and differences of the two approaches, mobility needs to be separated into three levels: air interface mobility, link-level mobility, and network-level mobility. Air interface mobility supports cell-to-cell handoff within a radio access network. Link-level mobility maintains a Point-to-Point Protocol (PPP) context across multiple radio access networks. Network-level mobility provides mobility across networks.

In both approaches, air interface mobility is handled in the radio access network. Air interface mobility is specific to the radio technology, so harmonization of the two will depend on the harmonization efforts underway for global CDMA. In 3GPP, link-level mobility is handled by GPRS Tunneling Protocol (GTP) [4]. GTP is used to provide mobility to other 3GPP-defined networks. The 3GPP architecture also provides an option in which an FA may be located in the GGSN. This allows roaming from GPRS-based networks to other IP access networks. In 3GPP2, link-level mobility is provided by defining a tunneling protocol as an extension of Mobile IP. The Mobile IP architecture allows the mobile to have a point of presence and roam across any IP network. Registration and authentication in the 3GPP architecture for access and data networks are integrated, and use the schemes used for wireless. In the 3GPP2 architecture, the registration and authentication for access and data networks are performed separately. For a data network, authentication and registration as defined in Mobile IP [6] is used. Hence, the data architecture is access-independent.

A common set of IP mobility protocols are needed to provide network-level mobility between different access networks, including wireless. IETF is developing a suite of protocols (Mobile IP) to achieve such mobility. Mobile-IP-based protocols could provide a good approach for network-level mobility. A new forum, the Mobile Wireless Internet Forum (MWIF) [11], begun recently by major global CDMA carriers, intends to drive a single open mobile wireless Internet architecture that enables seamless integration of mobile telephony and Internet services and is independent of access technology. It intends to influence both 3GPP and 3GPP2. Hopefully, cross-forum discussions between 3G.IP, MWIF, 3GPP, and 3GP2 will result in achieving the objectives set by MWIF. This will be to the benefit of the global wireless industry.

 

References
 [1] 3GPP home page

[2[ 3GPP2 home page

[3] ITU-R Draft Rec. M, "Detailed Specifications of the Radio Interfaces of IMT-2000," Doc 8/126.

[4] G. Brasche and B. Walke, "Concepts, Services, and Protocols of the New GSM Phase 2+ General Packet Radio Service," IEEE Commun. Mag., vol. 35, no. 8, Aug. 1997.

[5] 3GPP TR 23.922, "Architecture for an All IP Network," Dec. 1999.

[6] C. Perkins, "Mobile IP", IEEE Pers. Commun., this issue.

[7] P. J. McCann and T. Hiller, "An Infrastructure for Cellular CDMA Networks Using Mobile IP," IEEE Pers. Commun., this issue.

[8] A. Valkó et al., "Design and Performance of Cellular IP Access Networks," IEEE Pers. Commun., this issue.

[9] R. Ramjee et al., "IP-Based Access Network Infrastructure for Next Generation Wireless Data Networks," IEEE Pers. Commun., this issue.

[10] S. Das et al., "TeleMIP: Telecommunication Enhanced Mobile IP Architecture for Fast Intra-Domain Mobility," IEEE Pers. Commun., this issue.

[11] MWIF home page

Biographies
 Girish Patel is director of wireless standards in Nortel Network's Wireless Solutions Business Unit. His group is actively involved in the setting of standards for wireless in committees such as TIA, T1, and ITU. He has been in the telecommunications industry for more than 20 years, the last 7 in wireless. He has extensively contributed to the setting of standards in a number of other areas, such as ISDN SS7 and data, and has published in a number of conferences and publications. He also holds two patents on SS7 and ISDN. He has a B.Tech. (Hons) degree in electronics and communication engineering from the Indian Institute of Technology and an M.S. degree in electrical engineering from Loughborough University, United Kingdom.

Steve Dennett is director of U.S. standards for Motorola's Personal Communications Sector, and chair of the 3GPP2 Steering Committee. He has 20 years of experience in cellular communications, where he actively participated as a member of technical staff, senior staff engineer in the area of cellular network architecture and standards. In his current role he manages US based standards activities. He is an active participant in the TIA/TR45 standards forum. In this capacity, he chairs Working Group IV of TR45.5. This working group was responsible for the ITU-R, IMT-2000, cdma2000 RTT candidate proposal which was submitted to ITU-R in June of 1998.