The addressing in IMS is called SIP URI (Uniform Resource Identifier) and looks like an email address. In order to use IMS services, the UE must perform IMS registration. First the UE must obtain an IP connectivity bearer and discover an IMS entry point i.e. P-CSCF. After the P-CSCF discovery, the UE sends a SIP REGISTER request to this Proxy. The P-CSCF processes the REGISTER request and uses the provided home domain name to resolve IP address of the I-CSCF then contact the HSS to fetch the required
capabilities for S-CSCF selection. After S-CSCF selection the I-CSCF forwards the REGISTER request to the S-CSCF. The S-CSCF checks the response and if it is correct downloads a user profile from the HSS and accepts the registration with a 200 OK response as shown in the figure 2.4. Once the UE is successfully authorized, the UE is able to initiate and receive sessions. The UE must keep its registration active by periodically refreshing its registration.

The registration procedure in step wise is explained as follows:

• First, the dedicated signalling Packet Data Protocol (PDP) context is established between User Equipment (UE) and the Gateway GPRS Support Node (GGSN).
• The UE discovers the address of the Proxy Call Session Control Function (PCSCF).
• The UE sends a REGISTER message to home network to perform SIP registration.
• The Interrogating-CSCF (I-CSCF) selects the Serving-CSCF (S-CSCF).
• The S-CSCF downloads the authentication data of the user from the Home Subscriber Server (HSS).
• The UE and the P-CSCF agree on a security mechanism.
• The UE and the network (S-CSCF) authenticate each other.
• IP security (IP-sec) associations between the UE and the P-CSCF are established.
• SIP compression starts between the UE and the P-CSCF.

• The UE learns the route to the S-CSCF.
• The S-CSCF learns the route to the UE.
• The S-CSCF downloads the user profile from the HSS.
• The S-CSCF registers the public user identity of the user.
• The UE becomes aware of public user identity and user current registration state.
• The P-CSCF becomes aware of public user identity and user current registration state.