IMS Architecture

- IMS architecture giving overview of structure and details of the elements.

The IMS architecture is relatively complicated and this can mean it is expensive to implement and also requires attention to understand.

The IMS architecture can be split down to make it more accessible to understand.

The IMS architecture consists of many different entities which may be collocated or distributed within the network.

IMS architecture basics

The architecture of an IMS system can be split into a number of main elements or areas:

  • User equipment:   As the name implies, the user equipment or UE is part of the IMS architecture resides with the user - it is the endpoint.
  • Access network:   This is the portion of the IMS architecture through which the overall network is accessed.
  • Core network:   This is a major element within the IMS architecture and provides all the core functionality.
  • Application layer:   The application layer contains the web portal and the application servers, which provide the end user with service and enhanced service controls. T

IMS architecture functional view

Although a complete architecture diagram is quite complicated, a general overview provides a more informative view.

A simplified view of the system architecture for an IMS network
Simplified view of the functional IMS architecture

The overall IMS architecture contains a number of main elements:

  • IMS CSCF - Call Session Control Server:   The IMS CSCF is the section of the architecture that provides the registration of the endpoints. It also provides routing for the SIP signalling messages. It also links to the interworking and transport layer to provide QoS. The IMS CSCF can be further split into further entities:

    • Server CSCF:  : This element in the overall IMS CSCF is a session control entity for endpoint devices and it maintains session state.
    • Proxy CSCF:  : This part of the IMS CSCF is the entry point to IMS for devices. The P-CSCF is the first point of contact for the UE and it forwards SIP messages to the user's home S-CSCF. It provides device control interworking security. Within the P-CSCF, the PCF or Policy Control Function provides QoS management.
    • Interrogating CSCF:  : This entity within the IMS CSCF is a session control entity for endpoint devices that maintains session state.
  • Home Subscriber Server, HSS:   This is an important element within the IMS architecture which provides the subscriber data base for the home network.
  • Breakout gateway control function, BGCF:   This entity within the IMS architecture selects the network in which a PSTN breakout is to occur. If this is to occur in the same network as the BGCF, then the BGCF selects a media gateway control function, MGCF
  • Media gateway control function, MGCF:   This entity interworks the SIP signalling. It manages the distribution of sessions across multiple media gateways.
  • Media server function control, MSCF:   This manages the use of resources on media servers.
  • SIP applications server, SIP-AS:   The SIP-AS is a service execution platform on which one or more services are deployed.

IMS core network

As the name implies the IMS core network is at the centre of the network, and accommodates some of the main features within the network as a whole. Typically the entities within the core network address the functions with its basic operation.

Some of the main entities within the core network include:

  • P-CSCF
  • I-CSCF
  • S-CSCF
  • HSS

Much of the core network may be co-located, or alternatively it may be distributed around the network.

Although the path between entities located within the same premises may be shorter and occur more quickly, there are reasons for multiple deployment of the same entities around the network.

  • Network capacity is a key reason for multiple deployment. With load increasing it can be advantageous to have several instances of the same entity to cope with peak demand. Multiple HSSs may be needed to hold details of all the network subscribers, or multiple S-CSCFs may be needed to handle the peak number of SIP sessions that may occur.
  • Geographic distribution can provide real benefits if the network is distributed over a wide area. Although the system would operate if centred on a small area, geographic distribution can reduce the traffic as fewer nodes require to be accessed and latency can also be reduced.
  • Distributing the network over a wide area can add redundancy for instances when one centre may experience a power outage, etc. It is possible, for example for one S-CSCF to take over the user's registration dynamically from another, or HSS data may need to be accessed from different occurrences of the HSS. This approach adds significant resilience to the network and considerably increases the reliability.

IMS access network

The IMS access network is made up of those elements that are associated with communication from the core network to the outside world - external networks and users.

The IMS network can be accessed through various forms of IP Carrier Access Networks, IP-CAN.

The IP-CAN provides the IP connectivity as well as mobility. The IMS terminal sends control plane signalling and media transfer through the IP-CAN to the IMS core network.

By Ian Poole

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