IP Multimedia Subsystem
Various voice over IP technologies are available on smartphones; IMS provides a standard protocol across vendors.
IMS was originally designed by the wireless standards body 3rd Generation Partnership Project (3GPP), as a part of the vision for evolving mobile networks beyond GSM.
This vision was later updated by 3GPP, 3GPP2 and ETSI TISPAN by requiring support of networks other than GPRS, such as Wireless LAN, CDMA2000 and fixed lines.
Since it is becoming increasingly easier to access content and contacts using mechanisms outside the control of traditional wireless/fixed operators, the interest of IMS is being challenged.
Each node can also be present multiple times in a single network, for dimensioning, load balancing or organizational issues.
It is similar to the GSM home location register (HLR) and Authentication centre (AuC).
A subscriber location function (SLF) is needed to map user addresses when multiple HSSs are used.
It has the form of a Network Access Identifier(NAI) i.e. user.name@domain, and is used, for example, for Registration, Authorization, Administration, and Accounting purposes.
The IMPU can also be shared with another phone, so that both can be reached with the same identity (for example, a single phone-number for an entire family).
If located in the home network, it can query the HSS with the Diameter Sh or Si interfaces (for a SIP-AS).
The AS-ILCM (Application Server - Incoming Leg Control Model) and AS-OLCM (Application Server - Outgoing Leg Control Model) store transaction state, and may optionally store session state depending on the specific service being executed.
For media, CS networks use Pulse-code modulation (PCM), while IMS uses Real-time Transport Protocol (RTP).
IMS-based PES (PSTN Emulation System) provides IP networks services to analog devices.
IMS-based PES allows non-IMS devices to appear to IMS as normal SIP users.
One of the most important features of IMS, that of allowing for a SIP application to be dynamically and differentially (based on the user's profile) triggered, is implemented as a filter-and-redirect signalling mechanism in the S-CSCF.
This mechanism relies on the authentication performed during the network attachment procedures, which binds between the user's profile and its IP address.
CableLabs in PacketCable 2.0, which adopted also the IMS architecture but has no USIM/ISIM capabilities in their terminals, published deltas to the 3GPP specifications where the Digest-MD5 is a valid authentication option.
To compensate for the lack of IPsec capabilities, TLS has been added as an option for securing the Gm interface.
Although all 3 variants of Digest-MD5 authentication have the same functionality and are the same from the IMS terminal's perspective, the implementations on the Cx interface between the S-CSCF and the HSS are different.
