Brief Introduction to the SIP protocol in IMS

I believe everyone is in touch with the SIP protocol. In the previous article, we also discussed the content of the SIP protocol. If you are not clear about it, you can review it a little. We know that the SIP protocol has a huge development potential in the network communication field. Here we will explain the content of the IMS and SIP protocols.

1. Brief Introduction to IMS and SIP protocols

The IP Multimedia core subsystem (IMS) is a subsystem proposed by the third-generation mobile communication partner project (3GPP) to support IP multimedia services. It is characterized by the adoption of Session Initiation Protocol (SIP ), communication has nothing to do with the access mode. It provides multiple media services, including separation of control functions and bearer capabilities, separation of call and session, separation of applications and services, separation of services and networks, and integration of mobile and Internet services. IMS conforms to the development trend of communication network convergence.

SIP is designed based on the two most successful services of the Internet, Web and e-mail. drawing on Internet standards and protocol design ideas, adhering to the principles of simplicity, openness, scalability, and reusability, it provides a method to combine simple applications into complex services to form multimedia communication networks and provide multimedia services. SIP establishes and controls various types of point-to-point media sessions in a convenient way. similar to the Internet protocol, it adopts a modular structure, request/response mode, and text-based mode. Therefore, it is very simple and flexible to use, and is easy to upgrade and expand. SIP consists of basic SIP protocols and a series of SIP extensions for mobile businesses. the basic protocol of SIP is defined by the request instruction document (RFC) 3261 of the Internet Engineering Task Group (IETF), and the SIP Extension is composed of a series of RFC documents, including RFC3455, RFC3311, RFC3262, and RFC 3325.

2. SIP protocol in IMS

Due to the flexibility of SIP, 3GPP adopts SIP as the session control protocol in R5 to design IMS.3GPP. Instead of defining a new SIP protocol, it only uses the SIP defined by IETF in some way. therefore, there are also specific requirements for SIP in public mobile networks, such as low bandwidth, roaming, security requirements, service quality (QoS), and billing control.

In the SIP model, to establish a session, the user proxy client initiates a request to the user proxy service. requests are routed through the proxy server in the network. in addition, to provide the location information of the user agent, you need to map the SIP address to an IP address. this model is selected for the 3GPP IMS architecture.

In IMS, the user agent is the network entity of the call session control function (CSCF). The user agent in IMS is the proxy server and the registration server in the User device (UE). There are three types of CSCF:

1) Service CSCF (S-CSCF), act as a registration server and activate application business control based on user data;

(2) The proxy CSCF (P-CSCF) is the first contact point of UE in IMS network; the SIP signaling message is transmitted between P-CSCF and UE;

3) Querying CSCF (I-CSCF) is the first contact point for an external network, especially for an external IMS network.