This is an old revision of the document!
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 and Diameter support the control plane for IMS, while RTP supports the user plane.
The key elements of IMS are as follows:
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.