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ip_multimedia_subsystem

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IMS Use Case

The IP Multimedia Subsystem is a standardized architecture for voice and video communications and messaging in both fixed and mobile networks. The architecture as a whole is described in http://www.3gpp.org/DynaReport/23228.htm. This architecture describes a large number of functional elements. In practical implementations, several IMS functions are often combined into single VNFs.

IMS makes use of three standardized protocols:

  • SIP (Session Initiation Protocol) - used to set up and tear down voice and video calls, and to transport text messages
  • Diameter - used for communication with back-end functions such as user authentication, billing and policy control
  • RTP (Real-Time Protocol) - used to transport voice and video media streams

SIP and Diameter support the control plane for IMS, while RTP supports the user plane.

The key elements of IMS are as follows:

  • IMS core (Call Session Control Function, Breakout Gateway Control Function) - performs user authentication and registration, routes SIP messages between and among all the other IMS elements
  • Access Session Border Controller (Proxy Call Session Control Function, Access Transfer Control Function, Access Transfer Gateway) - provides the secure edge of the IMS network facing its users
  • Interconnect Session Border Controller (Interconnect Border Control Function, Transit Gateway) - provides the secure edge of the IMS network facing other network operators
  • AS (Application Server) - supports all application layer functions including telephony calling features, store-and-forward message delivery etc
  • MRF (Media Resource Function) - provides certain media-related functions such as mixing media for multi-way calls
  • HSS (Home Subscriber Server) - stores details of subscribers, including authentication credentials
  • Billing (Charging Data Function) - collects charging events and creates Call Detail Records
  • MGCF / MGW (Media Gateway Control Function / Media Gateway) - supports interconnect with TDM networks

Phased Testing of vIMS

A complete vIMS implementation requires the deployment of numerous different VNFs, and is therefore complex to achieve in practice. For the purposes of testing vIMS on OPNFV, it makes sense to focus first on those elements that are most performance-sensitive.

Most of the elements of IMS operate exclusively in the control plane. The only exceptions are the SBC functions and the Media Resource Function. The SBC functions are heavily loaded in the user plane since they are required to relay all of the media streams associated with calls established in the IMS domain. The MRF is relatively lightly loaded since it is only invoked in calls that require media processing such as multi-way calls.

Virtualized SBCs are known to apply heavy stress to the networking fabric of the underlying infrastructure. In the user plane, SBCs relay large numbers of RTP media streams, most of which comprise small packets containing audio payload. The capacity of a vSBC in terms of concurrent media sessions is therefore highly dependent on the ability of the virtualization infrastructure to handle the transport and switching of larger numbers of small (~100 byte) packets.

By contrast, the control plane functions in an IMS network are not packet- or bandwidth-intensive. Their performance tends to be compute-limited, not network-limited, and they therefore do not tend to suffer performance degradation introduced by limitations in the network virtualization infrastructure.

It is therefore proposed that the first phase of performance testing in the vIMS domain be focused on vSBC.

ip_multimedia_subsystem.1415903467.txt.gz · Last modified: 2014/11/13 18:31 by Martin Taylor