IMS basics thing to Know Before starting

Why IMS?

IMS is an Architectural framework to provide a standardization for mobiles phones to have Voice call services over an IP packet-switched network instead of Switched-circuit-style network(old).

IMS stand on or mainly depend on or upgraded version of 3GPP

What is 3GPP?

3rd Generation Partnership Project is a  (3G) mobile phone system specification based on evolved Global system for Mobile communications(GSM). 3GPP became the industry standards forum that defined UMTS and more recently LTE also.










MGCF (Media Gateway Control Function): performs protocol conversion between ISUP and the IMS call control protocols, communicates with CSCF and controls the connections for media channels in an IMS-MGW

BGCF(Breakout Gateway Control Function):  Controls the transfer of calls to and from the PSTN.



SLF(Subscription Locator Function ): Locates the database containing subscriber data in response to queries from the I-CSCF or AS

AS(Application Server): Provides value-added IP multimedia services.

HSS(Home Subscriber Server): Includes the Home location Register(HLR) and the Authentication center (AuC)

 Advantages of IMS:

• Independent of the access mechanism
• Seamless services provisioning across all(i.e., PSTN,ISDN,PSDN,WAN/LAN/CATV,ETC.) and also for evolving mobile networks.
• Mobility provided by Access technology independent user Identity Module(UIM)
• Access Convergence
• Functionality apportioned between visited network and home network
• home network control of services
• Open interfaces.Sourcing applications from anywhere, common for all user equipment and all application servers.

The main part of IMS architecture is IMS Core Network element: CSCF(call session control function)

Several types of SIP servers known as CSCF are used to process SIP signaling packets in the IMS domain:

1. Proxy CSCF
2. Interrogating CSCF
3. Serving CSCF

The CSCF elements are responsible for SIP session control and implements the logics for the following

functions:

user authentication

call routing

controlling the generation of call detail records (CDRs) for accounting purposes.

Serving Call Session Control Function(S-CSCF):

 It is the central node of the signaling plane.S-CSCF uses DIAMETER Cx and DX interfaces to the HSS to download and upload user profiles -it has no local storages of the user, all it get from HSS.

• It handles SIP registrations.
• It decides to which application server(AS) the SIP message will be forwarded,in order to provide their services.
• It provides routing services ,typically using ENUM lookups.
• There can be many S-CSCFs in the network for load distribution and hgh availability reasons.It's the HSS that assigns the S-CSCF to a user, when it's queried by the I-CSCF.

Interrogating Call Session Control Function (I-CSCF):

It is a SIP function located at the edge of an Administrative domain.

 Proxy Call Session Control Function (P-CSCF):

  P-CSCF is the first point of contact for the IMS terminal