Understanding IP Multimedia Subsystem (IMS)

IP Multimedia Subsystem Introduction

IMS is an intelligent services platform which supports the delivery of multimedia applications and content across both wireless and wireline networks. IMS also provides for multiple, simultaneous sessions across one or more devices enabling greater user control over access to information.

IMS is the core network technology that promises to transform communications, blending services for users, and even blending their lifestyles. However, this transformation of networks and services is taking longer than many people familiar with the technology anticipated.

The need for IMS has been driven by many factors including convergence in many areas including:

  • Convergence of Technologies: IP is the common network protocol
  • Convergence of Networks: Wireless and wireline networks sharing nodes
  • Convergence of Access: Handsets that access more than one network
  • Convergence of Services: Features that follow the user across networks
  • Convergence of Content: Access the same content across multiple networks
  • Convergence of Revenue: User’s demand is divided across fixed and mobile
  • Convergence of Control: Customer control over service provider features

At its core, IMS is relied upon for certain core services such as Voice over LTE (VoLTE).

However, the hope for IMS is that it would also become a Value-added Service (VAS) application enabler.

IMS Architecture, Framework, and Databases

Both wireless and wireline networks throughout the telecom industry have traditionally been highly vertically integrated. Each network contained all the required network nodes and support systems for billing and services separate from other networks. This level of network separation was often repeated geographically within each network as well, with separate networks built in adjacent geographic markets.

In each case, each vertically integrated network contained its own access network elements (base stations, cell sites), switches (TDM circuit switching), gateways and customer authentication and control nodes such as the Home Location Register (HLR). The various support systems used for provisioning, billing and vertical features (voice mail) were also dedicated separately to wireline and wireless networks.

IMS Architecture, Framework, and Databases

Both wireless and wireline networks throughout the telecom industry have traditionally been highly vertically integrated. Each network contained all the required network nodes and support systems for billing and services separate from other networks. This level of network separation was often repeated geographically within each network as well, with separate networks built in adjacent geographic markets.

In each case, each vertically integrated network contained its own access network elements (base stations, cell sites), switches (TDM circuit switching), gateways and customer authentication and control nodes such as the Home Location Register (HLR). The various support systems used for provisioning, billing and vertical features (voice mail) were also dedicated separately to wireline and wireless networks.

IMS Planes

The IMS architecture is organized into three functional planes:

  • Services Plane: contains the content and administrative platforms including media servers with media content, billing systems and application servers.
  • Control Plane: This Plane contains all the call control and authentication functions for call setup, hand-offs, and billing recording.
  • Transport Plane: The Network (or Transport) Plane contains all the traditional network switching and transport nodes found in today’s wireline and wireless networks and is the point of interconnection for next generation networks and devices.

One of the central tenants of IMS is that it is device independent. IMS relies heavily upon Session Initiation Protocol (SIP) for signaling and control. As long as an end-point and/or device is SIP-capable, it will theoretically work in an IMS environment.

SIP works in conjunction with the Session Description Protocol (SDP) to initiate multimedia sessions. SDP describes multimedia session functions such as session initiation and session announcement. SIP is used to establish sessions according to the following general steps.

IMS Network Elements

The IMS core network consists of the following key components:

  • Home Subscriber Server (HSS): The HSS is analogous to the Home Location Register (HLR) as employed by CDMA and GSM operators. The HSS employs the Subscriber Location Function (SLF) to map addresses and users. It is essentially a user database. The user can be identified through a combination of International Mobile Subscriber Identity (IMSI), Temporary Mobile Subscriber Identity (TMSI), International Mobile Equipment Identity (IMEI) and Mobile Subscriber ISDN Number (MSISDN); in a similar fashion as that of present day mobile networks. Additionally, the HSS also employs the SIP based Uniform Resource Identifiers (URI) such as IP Multimedia Private Identity (IMPI) and IP Multimedia Public Identity (IMPU).
  • Call Session Control Function (CSCF): The CSCFs are SIP application servers. Different types of CSCFs are Proxy CSCF (P-CSCF), Serving CSCF (S-CSCF) and Interrogating CSCF (I-CSCF). P-CSCF forms the first point of contact with the IMS user. It handles user registration, user authentication, signaling message inspection, policy control, quality assurance and similar functions. P-CSCF can be present in the home as well as visitor networks. S-CSCF manages the session control functions. Present in the home location, its chief function is to invoke the user identity parameters from the HSS. The HSS assigns an S-CSCF when queried by I-CSCF. Specifically, the S-CSCF handles binding of user location and SIP address, numbering look-ups, message routing and similar functions. I-CSCF interfaces IMS networks to each other. It acts as a forwarding point.
  • Media and Application Servers: The media servers are the workhorses of the network and handle functions such as media mixing, manipulation and management. The media server is implemented through a SIP User Agent and is known as Media Resource Function Controller (MRFC) and Media Resource Function Processor (MRFP) respectively. In addition to the CSCF functionality, the SIP application servers also handle the functioning of individual applications and services. It is this flexibility of the application server arrangement that makes IMS agile and scalable for services deployment. The application servers can be in the home network or in third party application provider domains.
  • Gateway Functions: Gateway functions manage interfacing IMS networks with non-IMS networks. These could be break-out gateways or PSTN gateways.