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Brief Description of the 3GPP Network Environment

               This section describes the most important concepts of the 3GPP environment for understanding the transition scenarios. The first part of the description gives a brief overview to the GPRS network as such. The second part concentrates on the IP Multimedia Core Network Subsystem (IMS).

1.  GPRS Architecture Basics
             This section gives an overview to the most important concepts of the  3GPP packet architecture. For more detailed description, please see
   [1].   From the point of view of this document, the most relevant 3GPP  architectural elements are the User Equipment (UE), and the Gateway
   GPRS Support Node (GGSN). A simplified picture of the architecture  is shown in Figure 1.   The UE is the mobile phone. It can either be an integrated device comprising a combined GPRS part, and the IP stack, or it might be a  separate GPRS device, and separate equipment with the IP stack, e.g.,    a laptop.
            The GGSN serves as an anchor-point for the GPRS mobility management. It also serves as the default router for the UE.  The Peer node mentioned in the picture refers to a node with which  the UE is communicating.

      --         ----       ************       ---------
     |UE|- ... -|GGSN|--+--* IPv4/v6 NW *--+--|Peer node|
      --         ----       ************       ---------
            Figure 1:  Simplified GPRS Architecture

              There is a dedicated link between the UE and the GGSN called the  Packet Data Protocol (PDP) Context. This link is created through the
   PDP Context activation process. During the activation the UE is  configured with its IP address and other information needed to   maintain IP access, e.g., DNS server address. There are three  different types of PDP Contexts: IPv4, IPv6, and Point-to-Point  Protocol (PPP).
                   A UE can have one or more simultaneous PDP Contexts open to the same or to different GGSNs. The PDP Context can be either of the same or  different types.
 
2  IP Multimedia Core Network Subsystem (IMS)

             IP Multimedia Core Network Subsystem (IMS) is an architecture for  supporting multimedia services via a SIP infrastructure. It is  specified in 3GPP Release 5. This section provides an overview of the 3GPP IMS and is not intended to be comprehensive.
            The IMS comprises a set of SIP proxies, servers, and registrars. In addition, there are Media Gateways (MGWs) that offer connections to
non-IP networks such as the Public Switched Telephony Network (PSTN). A simplified overview of the IMS is depicted in figure 2.

          +-------------+ +-------------------------------------+
          |             | |                            +------+ |
          |             | |                            |S-CSCF|---
    |     |             | |                            +------+ |
  +-|+    |             | |                             /        |
  | |     |   SIP Sig. | |      +------+          +------+       |
  | |----|------+------|--|----|P-CSCF|----------|I-CSCF|        |
  | |     |             | |     +------+          +------+       |
  | |-----------+------------------------------------------------
  +--+    | User traf. | |                                       |
   UE     |             | |                                      |
          | GPRS access | |      IP Multimedia CN Subsystem      |
          +-------------+ +-------------------------------------+
           Figure 2: Overview of the 3GPP IMS architecture

The SIP proxies, servers, and registrars shown in Figure 2 are as follows.

  - P-CSCF (Proxy-Call Session Control Function) is the first contact point within the IMS for the subscriber.
  - I-CSCF (Interrogating-CSCF) is the contact point within an operator's network for all connections destined to a subscriber  of that network operator, or a roaming subscriber currently located within that network operator's service area.
  - S-CSCF (Serving-CSCF) performs the session control services for  the subscriber. It also acts as a SIP Registrar.

            IMS capable UEs utilize the GPRS network as an access network for accessing the IMS. Thus, a UE has to have an activated PDP Context
to the IMS before it can proceed to use the IMS services. The PDP Context activation is explained briefly in section 3.1. The IMS is exclusively IPv6. Thus, the activated PDP Context is of PDP Type IPv6. This means that a 3GPP IP Multimedia terminal uses exclusively IPv6 to access the IMS, and the IMS SIP server and proxy support exclusively IPv6. Hence, all the traffic going to the IMS is IPv6, even if the UE is dual stack capable - this comprises both This, of course, does not prevent the usage of other unrelated services (e.g., corporate access) on IPv4.