IEEE Wireless Communications • February 2005 37 1536-1284/05/$20.00 © 2005 IEEE RADIUS server FW NAT DHCP server (7) (6) (9) (8) (3) ISP The authors focus on the GGSN functions for IP connection including the Access Point Name processing, IP address allocation, tunneling technologies and Quality of Service management. A CCEPTED FROM O PEN CALL INTRODUCTION The Universal Mobile Telecommunications Sys- tem (UMTS) is a third-generation mobile net- work evolved from GSM and General Packet Radio Service (GPRS) [1, 2]. UMTS enables high-speed packet switched data (up to 2 Mb/s) with quality of service (QoS) to access versatile multimedia services anytime and anywhere. Fig- ure 1 shows the UMTS (Release 99) architec- ture [1]. In this figure the dashed lines represent signaling links, and the solid lines represent data and signaling links. The core network consists of two service domains, the circuit-switched (CS) and packet-switched (PS) service domains. In the CS service domain, UMTS connects to the public switched telephone network (PSTN; Fig. 1a) through the mobile switching center (MSC; Fig. 1b). In the PS service domain, UMTS con- nects to the external packet data network (PDN; Fig. 1c) through the serving GPRS support node (SGSN; Fig. 1d) and gateway GPRS support node (GGSN; Fig. 1e). The SGSN in the PS domain plays a similar role as the MSC in the CS domain. The GGSN provides interworking to the external PDN, and is connected with SGSNs via an IP-based GPRS backbone network. The UMTS Terrestrial Radio Access Network (UTRAN) consists of Node Bs (the UMTS term for base stations; Fig. 1f) and radio network con- trollers (RNCs; Fig. 1g) connected by an asyn- chronous transfer mode (ATM) network. A mobile station (MS) or user equipment communi- cates with one or more Node Bs through radio interface Uu based on wideband code-division multiple access (WCDMA) radio technology [3]. General design guidelines for a GPRS core network have been intensively studied [1, 4, ref- erences therein]. However, IP connectivity offered by the GGSN is seldom elaborated on in depth in the literature. This article focuses on GGSN functionality. Several interfaces based on IP are defined for the GGSN. The Gn interface between the SGSN and GGSN uses the GPRS Tunneling Protocol (GTP) to transport user data and control signaling [5]. The GGSN connects to the PDN through the Gi interface. The Ga inter- face between the GGSN and charging gateway (CG; Fig. 1j) uses the GTP protocol to transfer call detail records (CDRs). The GGSN also pro- vides an interface to the network management system (NMS; Fig. 1k) using protocols such as Simple Network Management Protocol (SNMP). Before an MS can access any mobile data ser- vice, the packet data protocol (PDP) context for the service must be activated, which specifies the application-layer PDP and routing information for the communication session. The PDP context is maintained in the MS, SGSN, and GGSN, which will be elaborated on later. This article concentrates on the design of the GGSN functionalities for IP connection, includ- ing IP address allocation, IP packet routing, and transfer. We describe general GGSN functionali- ties and elaborate on access point name (APN) and IP address allocation. We show the inter- working techniques between the GGSN and the external PDN, and address GGSN QoS issues. In these sections we describe how the GGSN functions are implemented based on our experi- ence as a mobile operator (i.e., Chunghwa Tele- com.). For readers who are not familiar with GPRS/UMTS, background information is avail- able in the literature. For example, general GPRS/UMTS network architecture was described in [4]. In [6] the GPRS/UMTS net- work was introduced from the viewpoint of wire- less Internet access. The functionality YUAN-KAI CHEN, NATIONAL CHIAO TUNG UNIVERSITY YI-BING LIN, PROVIDENCE UNIVERSITY ABSTRACT In the Universal Mobile Telecommunications System, the gateway GPRS support node (GGSN) provides IP connection between the mobile telecommunications network and exter- nal packet data networks (e.g., the Internet). Specifically, the GGSN exercises session man- agement to transfer user packets between mobile stations and external data networks. In this arti- cle we focus on the GGSN functions for IP con- nection including access point name processing, IP address allocation, tunneling technologies, and QoS management. Based on our experience as a mobile operator, we give several examples to show how these functions can actually be implemented in a commercial mobile network. IP C ONNECTIVITY FOR G ATEWAY GPRS S UPPORT N ODE