tus of Alice's UA entity (or Bob's) by sending a NOTIFY request. 40) The TAS entity responds with 200 OK. 3.3. Deregistration The deregistration process to the IMS network is described in Figure 3.3. 1), 2) The deregistration phase starts when the Alice's UA entity (or Bob's) transmits a REGISTER message which parameter expires of the header Contact has a zero value. 3) The I-CSCF entity transmits to the HSS entity the DIAMETER LIR message to retrieve the IP address of S-CSCF. P-CSCF I-CSCF S-CSCF SIP REGISTER SIP REGISTER DIAMETER LIR DIAMETER LIA SIP REGISTER DIAMETER SAR DIAMETER SAA SIP NOTIFY SIP NOTIFY SIP 200 OK SIP 200 OK SIP NOTIFY SIP 200 OK SIP NOTIFY SIP 200 OK SIP 200 OK SIP 200 OK SIP 200 OK Figure 3.3. Mobile deregistration to IMS network Basic Procedures 4) The HSS entity provides the IP address of the S-CSCF entity in the DIAMETER LIA message. 5) The I-CSCF entity sends to the S-CSCF entity the REGISTER request. 6) The S-CSCF entity transmits to the HSS entity the DIAMETER SAR message informing about deregistration of the mobile. The HSS entity still retains the identity of the S-CSCF entity to allow the use of services when the mobile is not registered, such as call forwarding to voicemail. 7) The HSS entity transmits the DIAMETER SAA message to acknowledge the previous request. 8), 9) The S-CSCF entity informs Alice's UA entity (or Bob's) of its deregistration in a NOTIFY message. 10), 11) The 200 OK message is a response to the NOTIFY request. 12) The S-CSCF entity informs the TAS entity about the deregistration of Alice's UA entity in a NOTIFY message. 13) The 200 OK message is a response to the NOTIFY request. 14) The S-CSCF entity informs the P-CSCF entity about the deregistration of Alice's UA entity in a NOTIFY message. 15) The 200 OK message is a response to the NOTIFY request. 16), 17), 18) The 200 OK message is a response to the REGISTER request. 3.4. Detachment The detachment procedure to the EPS network is described in Figure 3.4. 1) Upon receipt of 200 OK response to the REGISTER re