The WGSN Approach

This section describes the architecture and the features of the WLAN-based GPRS Support Node (WGSN). WGSN interworks UMTS with WLAN to support Scenario 3 features described in Section 1.2.

2.1.1 WGSN Network Architecture

Figure 2-1 illustrates the inter-connection between a UMTS network and a WLAN network through WGSN. The UMTS network (Figure 2-1 (1)) provides 3G PS services. The WLAN network (Figure 2-1 (2)) provides access to Internet. The customers are allowed to roam between the two networks as long as the MS is equipped with both a 3G module and a WLAN card.

Figure 2-1. WGSN Architecture

(dashed lines: signaling; solid lines: data and signaling)

The UMTS network includes two sub-networks. The UMTS Terrestrial Radio Access

Network (UTRAN; Figure 2-1 (3)) consists of Radio Network Controllers (RNCs) and Node Bs (i.e., base stations). The radio interface between a Node B and an MS is based on WCDMA radio technology [28]. The UMTS core network (i.e., GPRS network; Figure 2-1 (4)) consists of Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN), which provide mobility management and session management services to mobile users. An SGSN connects to the UTRAN by Asynchronous Transfer Mode (ATM) links, and communicates with the GGSN through an IP-based backbone network. The GGSN connects to the external Packet Data Network (PDN) by an IP-based interface Gi. Both SGSN and GGSN communicate with the Home Subscriber Server (HSS) through the Gr and Gc interfaces, respectively. These two interfaces are based on the Mobile Application Part (MAP) [21]. The HSS is the master database containing all user-related subscription and location information.

The WLAN radio network includes 802.11-based Access Points (APs) that provide radio access for the MSs. The WGSN acts as a gateway between the PDN and the WLAN node, which obtains the IP address for an MS from a Dynamic Host Configuration Protocol (DHCP)[18] server and routes the packets between the MS and the external PDN. The WGSN node communicates with the HSS to support GPRS/UMTS mobility management following 3GPP Technical Specification 23.060 [5]. Therefore, the WLAN authentication and network access procedures are exact the same as that for GPRS/UMTS.

The WGSN node integrates both SGSN and GGSN functionalities. Like an SGSN, the WGSN communicates with the HSS through the Gr interface. On the other hand, like a GGSN, the WGSN communicates with the external PDN via the Gi interface. Therefore, for other GPRS/UMTS networks, the WGSN node and the corresponding WLAN network are considered as a separate GPRS network. The WGSN node can be plugged in any 3G core network without modifying the existing 3G nodes. To integrate the billing system for both UMTS and WLAN, WGSN communicates with the Charging Gateway using the same UMTS protocols (the GPRS Tunneling Protocol (GTP’) protocol implemented in the Ga interface [5] or by FTP).

To access the WGSN services, the MS must be either a 3G-WLAN dual mode handset or a

laptop/Personal Data Assistant (PDA) that equips with both WLAN Network Interface Card (NIC) and a 3G module.

2.1.2 WGSN Features

Based on the seven interworking aspects listed in 3GPP Technical Report 22.934 [3], we describe the features implemented in WGSN [23].

Service aspects: WGSN provides general Internet access and VoIP services based on SIP protocol [46]. Since a Network Address Translator (NAT) is built in the WGSN node, the VoIP voice packets delivered by the Real Time Protocol (RTP) [49] connection cannot pass through the WGSN node. This issue is resolved by implementing a SIP Application Level Gateway (ALG) [13] in the WGSN node, which interprets SIP messages and modifies the source IP address contained in these SIP messages.

In UMTS, an MS must activate the PDP [5] context for VoIP service before a caller from the external PDN can initiate a phone call to this MS. Also, for both UMTS and WLAN, a SIP User Agent (UA) must be activated in an MS before it can receive any incoming VoIP call. Therefore, a SIP-based Push Center (SPC) is implemented in the WGSN node to provide MS terminated SIP services. The SPC is implemented on a SMS-based IP service platform called iSMS [45], where none of the UMTS/GPRS components is modified. SPC also provides push mechanism through WLAN for a WGSN user who does not bring up the SIP UA. Therefore, the SIP terminated services (e.g., incoming VoIP calls) can be supported in WGSN.

Access control aspects: WGSN utilizes the standard UMTS access control for users to access WLAN services. Our mechanism reuses the existing UMTS Subscriber Identity Module (SIM) card and the subscriber data records in the HSS. Therefore, the WGSN customers do not need a separate WLAN access procedure, and the maintenance for customer information is simplified. User profiles for both UMTS and WLAN are combined in the same database (i.e., the HSS).

Security aspects: WGSN utilizes the existing UMTS authentication mechanism [34]. That

is, the WLAN authentication is performed through the interaction between an MS (using 3G SIM card) and the 3G Authentication Center. Therefore, WGSN is as secured as existing 3G networks. We do not attempt to address the WLAN encryption issue [35]. It is well known that WLAN based on IEEE 802.11b is not secured. For a determined attack, Wired Equivalent Privacy (WEP) is not safe, which only makes a WLAN network more difficult for an attacker to intrude. The IEEE 802.11 Task Group I is investigating the current 802.11 Media Access Control Address (MAC) security. WGSN will follow the resulting solution.

Roaming aspects: WGSN provides roaming between UMTS and WLAN. We utilize the standard UMTS mobility management mechanism without introducing any new roaming procedures.

Terminal aspects: A terminal for accessing WGSN is installed with a Universal IC Card (UICC) reader (a smart card reader implemented as a standard device on the Microsoft Windows platform). The UICC reader interacts with the UMTS SIM card (i.e., the UICC containing the SIM application) to obtain authentication information for WGSN attach procedure.

Naming and addressing aspects: The WGSN user identification is based on Network Access Identification (NAI) format [12] following the 3GPP recommendation. Specifically, the International Mobile Subscriber Identity (IMSI) is used as WGSN user identification.

Charging and billing aspects: The WGSN acts as a router, which can monitor and control all traffic for the MSs. The WGSN node provides both offline charging and online charging (for pre-paid services) based on the Call Detail Records (CDRs) delivered to the charging gateway.

Besides the seven aspects listed above, WGSN also provides automatic WLAN network configuration recovery. A WGSN MS can be a notebook, which is used at home or office with different network configurations. The network configuration information includes IP address, subnet mask, default gateway, WLAN Service Set Identifier (SSID), etc. When the MS enters the WGSN service area, its network configuration is automatically reset to the

WGSN WLAN configuration if the MS is successfully authenticated. The original network configuration is automatically recovered when the MS detaches from the WGSN. This WGSN functionality is especially useful for those users who are unfamiliar with network configuration setup.