Interworking with External Networks

8.4 Interworking with External Networks

A packet domain network that is a PLMN supporting GPRS is interconnected at the Gi interface to an external data packet network (IP networks). The Gi interface is located between the GGSN and the external data packet network. The GGSN is seen as an IP router by the external IP network. Figure 8.16 shows an IP network interworking.

Figure 8.16: IP network interworking. ( From -[1].)

The packet domain network supports interworking with networks based on the IP. It may also support interworking with networks based on the point-to-point protocol (PPP).

8.4.1 Interworking with PDNs Based on IP

The packet domain network provides either a direct transparent access to the Internet or a nontransparent access to the intranet/ Internet service provider (ISP). In a direct transparent access, the MS IP address is allocated by the GPRS PLMN operator within its addressing space. No user authentication or authorization process is performed for direct transparent access during the PDP context activation procedure. In a nontransparent access to an intranet/ ISP, the MS IP address is allocated within the intranet/ISP addressing space. User authentication from a server (e.g. Radius, DHCP) belonging to the intranet/ISP is required during the PDP context activation procedure.

Figure 8.17 shows an interworking with PDN based on IP.

Figure 8.17: Interworking with PDN based on IP.

An L2 or PPP link exists between the data terminal equipment (DTE) and mobile terminal (MT) to encapsulate the IP packets only if the DTE and MT are two separate pieces of equipment making up the MS. The PPP layer implemented in the MS acts as a PPP server and the layer in the DTE acts as a PPP client.

Note that the PPP protocol consists of the following elements:

• A multiprotocol datagram encapsulation method;

• A LCP for establishing, configuring, and testing the data link;

• A family of network control protocols (NCPs) for configuring and establishing the different network-level protocols; the IPCP is implemented for IP configuration.

During a PDP context activation procedure, a PPP link is established between the DTE and MT in several phases:

• LCP configuration for the PPP connection (e.g., maximum size of the PDUs exchanged, activation or nonactivation of the authentication phase, activation or nonactivation of link-quality monitoring, compression of protocol, address and command fields);

• PAP or CHAP authentication phase (optional);

• IPCP negotiation (e.g., negotiation of IP parameters: Van Jacobson header compression, primary address of the DNS server, secondary address of the DNS server).

Figure 8.18 illustrates a PDP context activation for nontransparent access to ISP/intranet. The authentication request (PAP, CHAP) and IP configuration request are tunneled in protocol configuration options (PCOs) between the MT and the GGSN during the PDP context activation procedure. The GGSN will deduce from the APN the ISP address via a DNS server and the servers used for IP address allocation and for user authentication-as in the scenario below, where DHCP is used for host configuration and address allocation and RADIUS is used for authentication with the CHAP protocol.

Figure 8.18: PDP context activation for non transparent access to ISP/intranet.

8.4.2 Interworking with PDNs Based on PPP

A packet domain network may interwork with networks based on the PPP at Gi reference. Either PPP connections are terminated at GGSN or PPP frames are tunneled via the layer-two tunneling protocol (L2TP) at Gi reference.

Figure 8.19 shows an interworking with PDNs based on PPP access.

Figure 8.19: Interworking with PDNs based on PPP access.

Figure 8.20 illustrates a PDP context activation for access to a PDN based on PPP. The MS IP address is allocated within the intranet/ISP addressing space. In the scenario below, we have assumed that the PDP address was given at the subscription. User authentication (PAP, CHAP) and retrieval of configuration parameters for connection to IP networks may be performed with Radius or DHCP server during PPP link establishment after the PDP context activation procedure.

Figure 8.20: PDP context activation for access to packet data network based on PPP.