Understanding the Impact of IMS on your Session Controller Investment

Introduction

Will IMS (IP Multimedia Subsystem) revolutionise communications networks overnight? Perhaps not, but operators are preparing to deliver a new set of ubiquitous multimedia services. To achieve this, standards and networks will have to evolve over a period of time. This means that equipment purchased today must be IMS ready. Only if this approach is taken can an operator ensure that network investment is optimised.

First, let's take a look what an operator needs today.

Immediate Operator Needs

Today operators are addressing the immediate needs associated with deploying VoIP (Voice over IP) and MoIP (Multimedia over IP) services to business and residential customers. This means dealing with problems such as service reach, Security, QoS and regulatory compliance. The key to the long-term success of the new services will depend on the ability of the underlying infrastructure to adapt to the evolving IMS standards.

Effective session control is at the heart of solving these issues. This has led to the widespread adoption of Session Border Controllers (SBCs) to provide solutions to these problems.

In an access network SBCs increase service reach by allowing access to customers behind NAT devices, thus increasing the potential subscriber base. SBCs can police the calls and actual media being admitted to the access network to prevent overbooking of resources and thus a loss of quality.

SBCs at network interconnect points help to secure the network by performing topology hiding, preventing both network and personal details from being propagated, and limiting the range of DoS attacks.

As VoIP services become subject to regional regulatory requirements, operators must comply by providing Emergency Call Handling and Lawful Interception facilities. Again, SBCs are ideally placed to deliver these functions.

For operators to develop their networks in the future to embrace IMS, it is essential that they have the built in flexibility to evolve in step with the standards.

Session Control in the IMS

At the heart of the IMS are three Call Session Control Functions (CSCFs), each of which has a distinct role to play in the delivery of ubiquitous services. We will look in more detail at the role of the Proxy-CSCF (P-CSCF).

The P-CSCF is the first point of contact within the IMS for a caller. It ensures that the user is registered with the correct network (if the user is roaming) and that messages are correctly routed to the service network once registration has occurred.

The P-CSCF is in the position to detect which services should be hosted by the visitor's network. This is important not only in terms of content services but also in the provision of Emergency Call Handling and Lawful Intercept. It can also provide defence against SIP signalling attack.

TISPAN also defines an Interconnect Border Control Function (IBCF) to formalize signalling interconnect between networks and Border Gateway Functions (BGF) to handle the media.

The IBCF formalizes the signalling interconnect between networks. Its functions include protecting and policing the signalling, topology hiding and conversion between IPv4 and IPv6 (where required). Whilst 3GPP networks are IPv6 only, 3GPP2 and wireline evolutions are likely to be a mixture of IPv4 and IPv6. The IBCF controls a BGF in the media path that protects media exchanged across the operator boundary.

The SBCs sitting in the access network today will be split to meet the signalling requirements of the P-CSCF and media requirements of the access BGF. The Interconnect SBCs will be similarly split to deliver topology hiding and IBCF for the signalling and an Interconnect BGF for the media. Only products which elegantly evolve to provide this functionality can be considered to be protecting investment and be future proof.

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