Wireless communication system for performing combined service between terminals having different communication environments

1. A method by a Combined Circuit-Switched and Internet protocol Multimedia Subsystem (CSI) Application Server (AS) for supporting a CSI session between an Internet protocol Multimedia Subsystem (IMS) User Equipment (UE) and a CSI UE, the method comprising:

generating, by the CSI AS, a first message including information about IMS registration status of the IMS UE and information about radio environment capability of the IMS UE,

wherein the radio environment capability indicates whether circuit-switched and packet-switched services are simultaneously supported;

sending, by the CSI AS, the first message to the CSI UE in a Circuit-Switched (CS) domain; and

sending, by the CSI AS, a response message to the IMS UE.

2. The method of claim 1 , wherein the CSI AS exchanges capability information with the CSI UE on behalf of the IMS UE.

3. The method of claim 1 , wherein the IMS UE supports communications over only a Packet-Switched (PS) domain, and

wherein the CSI UE supports communications over both the CS domain and the PS domain.

4. The method of claim 1 , wherein capability information of the IMS UE is included in a header of the first message.

5. The method of claim 1 , the method further comprising:

receiving, by the CSI AS from the CSI UE, a second message including capability information of the CSI UE in response to the first message.

6. The method of claim 1 , wherein the first message is a Session Initiation Protocol (SIP) based INVITE message.

7. The method of claim 5 , wherein the capability information of the CSI UE includes at least one of a Multiple Radio Access Bearer/Dual Transfer Mode (MultiRAB/DTM), predetermined IMS service information, or capability version information of the CSI UE.

8. The method of claim 1 , further comprising:

receiving, by the CSI AS, a request message from the IMS UE,

wherein the request message is for an initial setup of a session including both a voice component on the CS domain and a non-voice component on a Packet-Switched (PS) domain.

9. The method of claim 8 , wherein the CSI AS performs a session splitting by sending the voice component to the CS domain and sending the non-voice component to the PS domain.

10. The method of claim 1 , wherein the first message is sent via a Media Gateway Control Function (MGCF).

11. The method of claim 10 , wherein the first message is converted into an ISUP (ISDN (Integrated Services for Digital Network) User Part) message by the MGCF and the converted ISUP message is forwarded to the CSI UE.

12. The method of claim 1 , wherein the first message further includes a capability version.

13. A method by a Combined Circuit-Switched and Internet protocol Multimedia Subsystem (CSI) User Equipment (UE) for communicating over a CSI session between an Internet protocol Multimedia Subsystem (IMS) UE and the CSI UE, the method comprising:

receiving, by the CSI UE from a CSI Application Server (AS), a first message in a Circuit-Switched (CS) domain, wherein the first message including information about IMS registration status of the IMS UE and information about radio environment capability of the IMS UE is generated by the CSI AS when the CSI AS receives a request message from the IMS UE,

wherein capability information of the IMS UE is added to the first message by the CSI AS, and

wherein the radio environment capability indicates whether circuit-switched and packet-switched services are simultaneously supported; and

sending, by the CSI UE to the CSI AS, a second message including capability information of the CSI UE in response to the first message.

14. The method of claim 13 , wherein the capability information further includes a capability version.

15. A Combined Circuit-Switched and Internet protocol Multimedia Subsystem (CSI) Application Server (AS) for supporting a CSI session between an Internet protocol Multimedia Subsystem (IMS) User Equipment (UE) and a CSI UE, the CSI AS comprising:

a transceiver; and

a processor configured to:

generate a first message including information about IMS registration status of the IMS UE and information about radio environment capability of the IMS UE,

wherein the radio environment capability indicates whether circuit-switched and packet-switched services are simultaneously supported,

control the transceiver to send the first message to the CSI UE in a Circuit-Switched (CS) domain, and

control the transceiver to send a response message to the IMS UE.

16. The CSI AS of claim 15 , wherein the first message further includes a capability version.

17. A Combined Circuit-Switched and Internet protocol Multimedia Subsystem (CSI) User Equipment (UE) for communicating over a CSI session between an Internet protocol Multimedia Subsystem (IMS) UE and the CSI UE, the CSI UE comprising:

a transceiver; and

a processor configured to:

control the transceiver to receive, from a CSI Application Server (AS), a first message in a Circuit-Switched (CS) domain,

wherein the first message including information about IMS registration status of the IMS UE and information about radio environment capability of the IMS UE is generated by the CSI AS when the CSI AS receives a request message from the IMS UE,

wherein capability information of the IMS UE is added to the first message by the CSI AS, and

wherein the radio environment capability indicates whether circuit-switched and packet-switched services are simultaneously supported, and

control the transceiver to send, to the CSI AS, a second message including capability information of the CSI UE in response to the first message.

18. The CSI UE of claim 17 , wherein the capability information further includes a capability version.