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Page 1 of 3 Session Control in the IP Multimedia Subsystem The IP Multimedia Subsystem (IMS) defined by 3GPP provides an enabling architecture that is access independent. This is central in the move towards convergence. Now each access type is being 'enabled' to work with a multimedia IMS core, be it DSL, WLAN, GPRS or emerging technologies, such as WiMAX.
Session Border Controllers (SBCs) that can be physically separated into signalling and media elements are an essential part of building converged networks based on an IP Multimedia Subsystem (IMS) core. Between access and core networks, SBCs provide signalling security, Lawful Interception, Emergency Call Handling and, where required, NAPT, firewalling and traffic policing at the media layer. Additionally, they can direct users to the correct home network. SBCs have the advantage that, unlike most elements in this newly defined architecture, the functionality is deployable today.
In recent years a number of forces have lead operators towards IP convergence. Services can be accessed from a range of devices and users were rightly questioning why they could not have uniform access to services regardless of the type of access network being used. Seeing declining revenues, wireline providers started to consider ways to access wireless revenues. Wireless operators, in turn, have seen handset sales peak and must now consider ways to expand their markets; the most obvious area being converged business services.
This white paper provides an overview of the different strands of standardization that are providing the base for converged IP networks to become a reality and describes some of the key functional elements. It then shows how Newport's 1460 SignallingProxy and MediaProxy can be used to provide practical, secure and reliable access to an IMS core for wireless and wireline services.
Background: A Convergence of Standards
As a result of IP Multimedia Subsystem becoming the convergence architecture of choice, a number of standards bodies are involved in defining converged architectures in both fixed and wireless networks. This section reviews the main players in this arena. 3GPP 3GPP (Third Generation Partnership Project) is an amalgam of existing standards bodies, brought together to drive the specification and standardization of 3rd generation mobile. Standards bodies from Europe, USA, Japan, China and South Korea are involved. It was at Release 5 that 3GPP introduced the IP Multimedia Subsystem.
IMS is basically an overlay to the packet-switched domain using Session Initiation Protocol (SIP) to provide multimedia services over IP. It re-uses much of the IETF definition of SIP, adding customizations for the requirements of mobility. It is important to note that the 3GPP IMS does not provide service equivalence to the circuit-switched domain. Hence, the migration towards an IMS only network, replacing a circuit-switched core, can be seen only as a long-term vision currently.
An important principle in the 3GPP Release 6 standards is that the IMS core is access technology independent. Meaning that any specific requirements for mobile access should be dealt with by the access network, for example, compression requirements to optimize bandwidth usage. Practically, the IMS's access independence is still not a reality for fixed network access - this is where TISPAN comes in. ETSI TISPAN The role of TISPAN (Telecoms & Internet converged Services & Protocols for Advanced Networks) in ETSI is to standardize converged multimedia networks using IMS as its core architecture. This means adding the ability for fixed network access to interface to IMS and also requesting 3GPP to enhance the IMS specification where it has been found to be wireless specific. With the objective of moving existing PSTN functionality onto an IP core, IMS is now being focused on providing PSTN emulation; effectively PSTN equivalence services. PSTN simulation services provide a definition of what must be provided as a minimum, for example, malicious call trace, but also allow for multimedia to provide additional enhancements to the service. 3GPP2 The 3GPP2 group is using the IMS as a base for their Multimedia Domain (MMD) architecture. This will allow CDMA2000-based access networks to provide third generation mobile services. The 3GPP2 core definition follows the IMS definition closely but there are differences to allow for the differences in radio technology. Interestingly, 3GPP2 has also allowed the use of IPv4 and the separation of GGSN and P-CSCF (see later).
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