In the IMS there are six categories

CSCF ===> Session management and routing Server HSS   ===> Databases IW     ===> Interworking Element SE     ===> Support Entities CE     ===> Charging Entities AS     ===> Application Server

1.Proxy –call session control function

the proxy call session control function is the first contact point for users within IMS, all SIP signaling traffic from or to UE go via the P-CSCF, it validates the request. forward to selected destinations and process and forward the response 

an operator network can be contain one or many P-CSCFs, furthermore P-CSCF provide IPsec or ESP (Encapsulation Security Payload) for SIP signaling and interacts with PDF (Policy Decision Function) for media policing purpose, finally it contributed to the charging process by sending accounting-related info to the CCF ( Charging Collection Functions)

2.Serving –Call Session Control Function

the S-CSCF have a functions ;

• Handling of registration request and users de-registration when needed
• Mutual authentication between user and Network
• Download of user information and service related data from HSS
• Routing of mobile termination traffic to the P-CSCF and mobile originated traffic to the I-CSCF, Breakout Gateway Control Function (BGCF) or the Application Server (AS)
• Media Policing, a process that check the content of the user payload to find out whether it contains media type or codecs which are not allowed for a user
• Accounting-related information to the CCF for offline charging purpose and to the online charging system  (OCS) for online charging purpose

the serving-cscf is the brain of IMS an operator can include multiple S-CSCF with different functionalities

3.Interrogation –Call Session Control Function

the functions of I-CSCF is :

• contact the HSS to obtain the name of the S-CSCF that serving the user, and S-CSCF assignment
• forwarding of SIP requests or response to the S-CSCF
• provisioning of accounting-related information to the CCF
• provisioning of a hiding functionality an optional integrated topology hiding internetwork Gateway (THIG) can be used to hide the configuration, capacity and topology of the network from outside an operator’s network

for scalability and redundancy reason an operator network may contain multiple I-CSCF

4.Home Subscriber Server

the Home Subscriber Server (HSS) in the main data storage for all IMS subscriber and service –related data. the data stored in the HSS include user identities, registration information, access parameters and service triggering information

user identities consist of two types: private and public user identities .. the private user identities is a user ID that assigned  by the home network operator and is used for purposes like registration and authorization. it can be compared to the international mobile subscriber identity (IMSI) in GSM network 

the public user identity is the ID that other users can use for requesting communication with the end user. it server a similar purpose as the TMSI, Temporary Mobile Subscriber Identity,in GSM

A special mechanism called the Subscription Locator Function (SLF) is implemented within I-CSCF, S-CSCF and Application Server , when separately addressable HSSs have been installed within the network, it is used a resolution mechanism to find the proper address of the HSS that holds the subscriber data for a given user identity  

IMS access parameters are used to set up sessions and include parameters like authentication, roaming authorization and allocated S-CSCF names, Serving-treggering information enables SIP service execution the also provides user specification requirement for S-CSCF capabilities ,this information is used by the I-CSCF to select the most suitable S-CSCF for user

there can be more than one HSS in home network depending on the number of mobile subscribers, the equipment capacity and the organization of the network. communication between different HSS function is not standardized

5.Policy Decision Function (PDF)

The Decision Function (PDF) makes policy decisions base on session and media-related information obtain from the P-CSCF in Release 5 it is an integral part of the P-CSCF, it acts as a policy decisions point for service based local policy control

Service based local policy (SBLP) provides parameters for :

• session identification ex: IP address, port numbers,bandwidths,etc
• session authorization to PDF and GGSN depending on the requested bearer ex: Packet data Protocol or PDP context
• session maintenance (PDP context modification or re-establishment)
• session charging by passing an IMS-charging identifier to the GGSN and a GPRS-charging identifier to the P-CSCF

    to be continue …..(^^),\/