Chapter 1 Overview of IMS

Chapter 1 Overview of IMS

1.1 What is IMS

IMS is the abbreviation of IP Multimedia Subsystem (IP Multimedia Subsystem), which has two main features:

1) It is a complete voice solution based on all IP:

- All IP means that the protocols and interfaces used by IMS follow the TCP/IP protocol stack. This is mainly compared with the CS (Circuit Switched) voice solution in the 2/3G era, because CS uses a large number of non-IP protocols such as MAP, ISUP and other SS7 protocols, and even ATM-based link layer technology physical interface.

- In addition, IMS is a complete voice solution, which can independently complete voice audio and video services, supplementary services, etc., and provide a complete operation solution, such as billing, operation support, contract data distribution, etc. This makes IMS significantly different from some fixed-line VOIP voice solutions (such as enterprise voice solution networks built by companies such as Avaya and Cisco for some enterprises), because the voice solutions of these enterprises may not need to consider too much billing and security. (for internal use only), data distribution, mobility and switching, high concurrency, high availability (such as network element group pool, remote disaster recovery, etc.), Qos (because the LAN generally has no shortage of bandwidth), etc. However, the operator's carrier-grade voice network must be considered. In addition, the network element of the enterprise-level voice solution is relatively single, and one or more sets of VOIP servers (IP PBX) can be deployed in a centralized manner to provide services, and then a hardware IP phone (or an IP phone like Eyebeam) can be placed on the desk of the company's employees. In this way, a soft IP Phone that supports the SIP protocol is sufficient), and usually only follows the RFC specification, and does not need to support the 3GPP specification.

2) IMS has nothing to do with access:

- IMS itself is the realization of the business level, that is, it does not care about the access part underneath. IMS can support fixed network (wired or WIFI) access and mobile network (2/3/4/5G) access. For this reason, the industry is also promoting IMS-based FMC (convergence of mobile and fixed networks). In the mobile communication network, IMS can also be used as the voice service core network of 2G, 3G, 4G, and 5G. Just for the sake of distinction, it is called VoLTE in 4G and VoNR in 5G.

- Because of this, IMS only focuses on the implementation of upper-layer services, and does not provide access itself. However, in order to better serve users, it is necessary to better interact with the access layer to complete some specific functions, such as area code completion and obtaining user location information.

This article will focus on NR access.

IMS was first introduced in 3GPP R5. According to the definition of 3GPP, IMS is a subsystem composed of various core network devices that provide multimedia services such as voice, video, text, and chat, and combine these services to deliver them in the PS: Packet Switch domain. Simply put, IMS is a subsystem that provides mobile multimedia services on the PS domain (which can be EPC or 5GC+4/5G access network).

When 3GPP introduced IMS, other standard organizations also completed their own IMS specifications in parallel. The most famous of these are ETSI TISPAN (Telecommunications and Internet Converged Services and Protocols for Advanced Networking, Telecommunications and Internet Converged Services and Advanced Network Protocols) and IETF. TISPAN, formed by the merger of two ETSIs, is committed to formulating relevant specifications for the transition from circuit-switched networks to packet-switched networks, and strives to formulate an architecture that can serve both networks, which is what we call the Next Generation Network (NGN). In December 2005, TISPAN completed NGN version 1 including IMS components. IETF is the standard giant of the Internet (data communication network), which defines a large number of IP-based protocols and specifications, including the SIP protocol used in IMS. But unfortunately, the IETF has not defined how to use it, especially in the mobile field.

3GPP regards SIP as the control protocol of multimedia communication, and builds a clear framework for IMS based on SIP. Including the functional description of related network elements, the interconnection and addressing between network elements, and related processes, etc., and defines the interoperability of different operators, the interoperability of different access networks, and the interoperability between IMS terminals of different manufacturers. Compared with IETF's SIP service model, 3GPP IMS has the following changes:

- SIP compression/implicit registration/deregistration and re-authentication initiated by the network, enhanced authentication based on USIM/ISIM card, policy control (Rx, Gx, N7 and other interfaces, dedicated load establishment, Qos guarantee), billing (online + offline ), CSCF and AS interface (iFC, ISC interface), mobility and roaming model, regulator requirements (emergency call, location information provision, lawful interception, number portability, subscription data release, etc.)