Method and apparatus to enable fallback to circuit switched domain from packet switched domain

1. A wireless transmit/receive unit (WTRU) comprising:

a processor comprising:

a Non-Access Stratum (NAS) configured to

determine whether to perform a Circuit Switched (CS) fallback based on an Internet Protocol (IP) Multimedia Subsystem (IMS) registration status, and

send a service request indicating a request to perform a Circuit Switched (CS) fallback on a condition that the WTRU is attached to a CS domain; and

an Access Stratum (AS) configured to

receive the CS service request from the NAS,

send an indication of the CS service request in a Radio Resource Control (RRC) message to an Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (E-UTRAN), wherein the RRC message includes CS service information, and

receive a handover command in response to the CS service request.

2. The WTRU of claim 1 , wherein the AS is further configured to send an indication of the CS fallback to the NAS.

3. The WTRU of claim 1 , wherein the AS comprises:

a legacy AS; and

a Long Term Evolution (LTE) AS configured to receive the service request from the NAS, send the indication of the CS service request in the RRC message to the E-UTRAN, and receive the handover command.

4. The WTRU of claim 3 , wherein the legacy AS is configured to send an indication of the CS fallback to the NAS on a condition that the legacy AS receives a handover parameter from the LTE AS.

5. The WTRU of claim 1 , wherein the NAS comprises:

a Mobility Management (MM) unit configured to notify an Evolved Packet System (EPS) Mobility Management (EMM) of a CS call, wherein the EMM is configured to send a notification of the service request to the AS and send an indication to the MM that the WTRU is operating in a CS domain.

6. The WTRU of claim 1 , wherein the AS is configured to set an establishment cause in the RRC message to a value that indicates an originating CS service.

7. The WTRU of claim 1 , wherein the AS is configured to provide details of the CS service in an establishment cause.

8. The WTRU of claim 1 further comprising:

a Mobile Node (MN) configured to determine whether to fallback to CS based on an Internet Protocol (IP) Multimedia Subsystem (IMS) registration status.

9. A method for circuit switched (CS) fallback performed by a wireless transmit/receive unit (WTRU), the method comprising:

determining whether to perform a CS fallback based on an Internet Protocol (IP) Multimedia Subsystem (IMS) registration status;

receiving a CS service request from a Non-Access Stratum (NAS) of the WTRU, the service request indicating a request to perform a CS fallback on a condition that the WTRU is attached to a CS domain;

sending an indication of the CS service request in a Radio Resource Control (RRC) message to an Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (E-UTRAN), wherein the RRC message includes CS service information; and

receiving a handover command in response to the CS service request.

10. The method of claim 9 further comprising:

sending an indication of the CS fallback to the NAS.

11. The method of claim 9 further comprising:

notifying an Evolved Packet System (EPS) Mobility Management (EMM) of a CS call;

sending a notification of the service request to an Access Stratum (AS) of the WTRU; and

sending an indication to a Mobility Management (MM) unit that the WTRU is operating in a CS domain.