Types of CSCF | IMS-IPV6 MULTIMEDIA SUBSYSTEM

The first point of contact for the IMS terminal is a Proxy-CSCF (P-CSCF), which is a SIP proxy assigned to an IMS terminal during the registration, which does not change during the registration duration. It also involves authenticating the user and establishing IPsec security integration with the IMS terminal. The other nodes trust the P-CSCF, and do not have to re-authenticate the user.

The Policy Decision Function (PDF) is also involved, which is used for policy control, bandwidth management, and authorizes media plane resources, such as QoS over the media plane.

Proxy-Call Session Control Function (P-CSCF)

P-CSCF serves as a Back-to-Back User Agent (SIP B2BUA) and is the initial SIP signaling contact point for subscribers. The P-CSCF is responsible to forward SIP registration messages from the the User Element (UE) and subscriber's endpoint, to the Interrogating-CSCF (I-CSCF) and as a consequence; call set-up requests and responses to the Serving-CSCF (S-CSCF). The P-CSCF maintains the mapping between physical UE IP address and logical subscriber SIP URI address and a security association, for both confidentiality and authentication. It supports the E911 emergency call and local routing within the visited network, session timers, accounting, and admission control. The DIAMETER protocol (Rx interface) is used by the session admission control to query the external Policy and Charging (PC) Rule Function (PCRF) element for resource reservation and bandwidth-based admission control. The P-CSCF interacts with AGW (defined in subsection 3.4.5) for the boundary control at the media layers and signaling, including Port Translations (NAPT) lawful intercept and pinhole firewall, Network Address and etc.

Serving-CSCF (S-CSCF)

A S-CSCF serves as a central node in the signaling plane. It also serves as a SIP server, which performs session control. S-CSCF uses Diameter D_x and C_x interfaces in connection to the HSS to upload and download user profiles, without having a local storage for the user. All necessary data is loaded from the HSS.

The SIP registrations are handled at this signaling plane. This permits to bind the SIP address, the user location, and to decide the destination application servers for which the SIP message will be forwarded to. This is done in order to provide the routing services using Electronic Numbering lookups and enforcing the network operator's policy.

To offer load distribution and high availability, multiple S-CSCFs can coexist in the same network, however the S-CSCF is assigned to the user through the HSS when I-CSCF is queried.

Interrogating-CSCF (I-CSCF)

An I-CSCF is another SIP proxy that provides service locator functionality. The followings are within its major functions:

1. Registration: Registration is assigning a S-CSCF to a user via SIP registration.
2. Session Flows (SFs): A routing message may include a session flow, which is part of a SIP request received at the S-CSCF from another network. It can also be a part of routing intra-domain SIP requests between users on different S-CSCFs.

IMS-IPV6 MULTIMEDIA SUBSYSTEM

IMS, also known as IP Multimedia Subsystem is an IP-based multimedia and telephony core network technology, which was introduced by 3GPP and 3GPP2 standards and is based on IETF Internet protocols. IMS is based on a set of specifications describing the Next Generation Networking (NGN) architecture involving the implementation of IP based telephony and multimedia services. It contains the specification of a framework and a complete architecture enabling the integration of video, voice, data and mobile network technology on top of an IP-based infrastructure (Amirth, n.d.).

IMS is access technology independent since it supports IP-to-IP session over wired IP, wireless (i.e., 802.11,802.15, CDMA, etc), and packet data along with GSM/EDGE/UMTS and other packet data applications.

 IMS Architecture

In IMS, the networking infrastructure is subdivided into individual functions with standardized interfaces (reference point) between each of them. Every reference point defines both the operating functions and the protocol over the. Figures 1 and 2 show the IMS architecture overview.

The IMS architecture is split into three main layers containing a number of individual entities.

 

Figure 1: IMS architecture

Figure 2 presents a graphical overview of the IMS core entities. The combination of legacy mobile signaling networks, other IP multimedia networks, and the PSTN entities form the external interface functions. The combination of BGCF, CFCF, MGCF, and MRCF form the Sequencing and Control Functions. The combination of HSS and SLF form the Storage and Reference Functions. UE covers the User Interface Functions and the combination of MGW and MRFP form the Media Processing Functions. The interfaces between BGCF and MGCF and CDCF are of Ml type. The interface between MGCF and CSCF is Mg and the interface between WGW and MGCF is of M1 type.

 

Figure 2: IMS core network 

The traffic types are of two major categories; Data/Bearer and signaling/control types. The PSTN link between PSTN and MGW entities contain CSD (TDM) traffic and the Data/bearer, such as Mb interface, contains IPv6 traffic. The rest of the interfaces contain signaling and control traffic, including: MEGACO (H.248), ISUP, SIP, QoS (COPS), MAP/TCAP, and other types of traffic.

Transport and Endpoint Layer

This layer is involved in setting up sessions and providing bearer services and initiating and terminating SIP signaling procedures. This layer is also responsible for providing the media gateways to convert the VoIP data to the PSTN TDM format.

Session Control Layer

This layer includes Call Session Control Function (CSCF), which provides the routing for the SIP signaling messages and the endpoints for the registration. The routing functionality enables the SIP signaling to be routed to the correct application servers. Through communicating with the transport and endpoint layer, the CSCF is able to guarantee QoS.