IMS Technology

IMS enables a packet -based Network to provide multiple services on single  control/service layers via different access networks.

IMS Requirements:

1. IP Multimedia sessions
2. QOS
3. Service control
4. Roaming
5. Internetworking
6. Rapid service creater
7. Multiple access

IMS Protocols:

1. SCF(Session Control Function)

       Cirtuit Switched network :-

                          TUP (Telephony User Part)

                           ISUP(ISDN User Part)

                           BICC(Bearer independent call control)

     Packet Switched Network:-

                                 SIP

                                   H.323

 2) AAA(Authentication, Authorization and Accounting)

                                Diameter

 3) Other protocols

                            Megaco(H248)

                                  RTP/RTCP

IMS was originally standardized by 3gpp.

Key components or Nodes of IMS architecture:

1. HSS and SLF
2. CSCF
3. AS
4. BGCF
5. Media Gateways
6. MRF

1. HSS & SLF :

HSS(Home Subscriber System)========:

• It is a database of all subscriber and server data.
• It is an Evolution of HLR(Home location register) which is in GSM.
• It contain User Profiles used by control layer
• It contain subscription information used by service layer

User profile contains

• . User identity
• .allocated s-cscf name
• . Registration information and roaming profile
• .authentication parameters
• .control and service information

SLF (subscriber location function):

• An SLF is needed to map user address when multiple  HSSs are used.
• Network with Single HSS do not need SLF, On other hand networks with more than one HSS require SLF.
• Both HSS and  the SLF  communicate through the Diameter protocol

2. CSCF(Call session control function):This is a sip server. There are  three types of CSCFs , depending on Functionalities they provide :-

1. PCSCF(proxy)
2. SCSCF(Serving)
3. ICSCF(interrogating)

PCSCF:

1. It is the first point of contact between  IMS terminal (UE) and IMS network.
2.  Its main functionalities are:-

• It establishes number of IP sec security associations ( the ability to detect the content of message has changed  since its creation) towards the IMS terminal.
• It verifies the correctness of sip requests sent by IMS terminal and  forwards sip messages to SCSCF.
• it forwards registration requests  received from UE  to I-CSCF
•  It forwards requests  and answer to the UE.
• It also Includes Compressor  and de-compressor of SIP messages.
• It authenticate  the User  and asserts the identity of the user  to  other nodes in the network.
•   It  also include PDF(policy decision Function).It is integrated with PCSCF or Standalone unit.PDF authorizes media plane and manages Quality of service over media plane.

 3) The PCSCF may be located either in Visited network or Home network.

ICSCF:

I CSCF  is Sip proxy located at the edge of  an administrative domain.

• It's IP Address is published in the DNS of the domain(using NAPTR and SRV type of DNS records)
• It has an interface  to SLF and HSS.
• It Queries the HSS using Diameter cx Interface to retrieve the user location.
• It  also implements interface to Application servers, to route requests that are addressedto services rather regular users.
• It may optionally encrypt  the parts of sip messages that contain sensitive information about the domain, DNS names and capacity. This functionality is called THIG(Topology hiding inter-network gateway.
• It is located in Home Network , In some special cases such as ICSCF(THIG) it may be located in visited network as well.

SCSCF:

S CSCF is the central node of the signalling plane.

• It is a sip server  always located  in home network.
• It Uses Diameter  cx and DX to upload or download user profiles, it has no local storage. All necessary information is stored in HSS.
• It handles SIP registrations, Which allows to bind User location/IP address and SIP address.
• It sits on path of Signalling message and can inspect every message.
• It decides to which application servers the sip message will be forwarded, in order to provide services.
• It  provide Routing Services typically using ENUM lookups.
• It enforces the policy of the network operator.

MRF:(Media Resource Function)    It provides a source of media in the home network .

• It is used for playing Announcements(audio/Video)
• It is used for Multimedia Conferencing( ex: Mixing audio streams)
• It is used for TTS(text-speech Conversion) and Speech recognition.
• It is used for transcoding between different codec
• It is used for obtain statistics and do any sort of media analysis.

It is mainly divided into two types:

MRFC and MRFP

MRFC(Media resource function controller):

• It is a signalling plane node that  acts as a Sip user agent for S-cscf and which controls the MRFP with a H.248 interface.

MRFP:(Media resource function processor):

• It is media plane node that implements all media related  functions, such as playing and mixing media.

MRF is located in Home network

BGCF(Break Out Gateway Control Function) :

• It is a Sip server  used for routing  Calls between the IMS terminal and PSTN phone.
• It routes based on Telephone numbers.
• It break out occurs in same network as the BGCF  then the BGCF select a MGCF that will be responsible  for  internetworking  with the PSTN  and forwards the signalling to MGCF. Other wise it forwards signalling to BGCF  of another operator.
• The MGCF then receives the signalling from BGCF and  manages the internetworking with PSTN network.

The PSTN/CS Gateway:

The internetworking with CS network is realized by several components for signaling, media and control functions.

SGW(Signalling Gateway):

• It is an interface  with signalling plane of CS network.
• It performs Lower layer protocol conversion.
• It transforms ISUP over MTP  into ISUP over SCTP/IP.

MGCF(Media Gateway Control Function):

• It performs call control protocol conversion between Sip and ISUP.
• It interfaces SGW over SCTP.
• It controls MGW with a H.248(Megaco) interface.

MGW: (Media Gateway)

• It is an interface with Media plane of CS network.
• It converts RTP to PCM
• It also performs media transcoding when Codecs doesn't match.

Application Server(AS):

              AS is a sip entity that hosts and executes services .

• It interface with  the S-CSCF and I-CSCF using Sip and HSS using Diameter.
• This allows third party  providers  and easy integration and deployment of their value added services to the IMS infrastructure.

There are three different types of Application servers:-

 SIP AS :- It hosts and executes IP multimedia services based on sip.

 OSA-SCS(open service Access-service capability server):-  It inherits OSA capabilities to access the IMS securely from external network.

IM SSF(IP multimedia Service switching system Function):    It allows a GSM SCF(GSM service control function)to control an IMS session. IMS SSF provides intelligent gateway functionality between sip based IMS network an IN systems that use  protocols such as CAMEL,INAP,AIN and MAP.