USSD In LTE explained with call flows.

LTE comes under the fourth generation of mobile communication. It provides a much higher speed than its previous generation, 3G. LTE is a data-only network. USSD service will be available on LTE too. On LTE, all services will be on the data network. This will be after the full implementation of IMS. IMS is for deploying multimedia services over IP networks.

Voice over LTE (VoLTE) is an example of an IMS application. If the network does not support IMS, the roaming network does fall back to 3G. For instance, if a phone is attached to the LTE network and a mobile subscriber dials a USSD Code. The UE falls back on 3G and does a location update with the HLR over the ss7 network, which attaches to the phone.

Then the roaming network sends the USSD to the home network. Once the USSD session completes, UE again attaches to LTE. This tutorial will cover USSD session details over the LTE network.

With the pure LTE network, UE will use IMS for USSD. The protocols and call flows will be changed. The protocol for USSD will be SIP or session initiation protocol. The USSD over LTE or 4G specification is given in 3GPP Standard.

USSD Protocol Sack in LTE:

USSD Protocol Stack LTE
USSD Protocol Stack LTE

The USSD in LTE uses a sip protocol stack. Sip is an application layer protocol that is specified in  RFC 3261. As IMS uses internet protocol, sip runs over UDP transport protocol. UDP uses internet protocol. Application over USSD uses the services of the SIP layer for sending USSD to the home network.

Application in UE requests the SIP layer to initiate the transaction. For a USSD initiated by the core network, the SIP layer indicates the application for a new request from the network. The SIP layer maintains a session. With IMSI phone will have a standard application for USSD.

USSD Network in LTE :

USSD Network LTE
USSD Network LTE

LTE is the 4G network. The protocols and the network nodes have been changed in LTE compared to the previous generation, which is 3G. The UE interacts with P-CSCF or Proxy call control session function. This is the roaming node in the IMSI network. A kind of VLR in a 3G network. It interacts with S-CSCF or the Serving call control session function. Which in turn connects to the USSI AS. USSI As terminates the USSD session functions and interacts with the applications for USSD in-home network. It can be seen as USSDGW in a 3G network.

Mobile initiated USSD in LTE:

Mobile Initiated Ussd LTE
Mobile Initiated Ussd LTE

Similar to the 3G network, LTE provides handset-initiated USSD. When mobile, a user dials the USSD code. A SIP INVITE message is sent from the roaming network to the USSI As. The USSI As is located in the core network. SIP message has USSD code as a stri g. The USSI AS identifies the code.  All is OK, and the USSD application recognizes the USSD code.

The application replies with the USSD menu, which is the string SIP INFO message, and the response menu string Invite message carries SDP. SDP is setting up for media or voice sessions, but there will be no voice in this case. SDP has port number zero; similarly, in response to INFO, SDP has port number zero. To end the session, any side can send a BYE SIP message to another node.

Network initiated USSD In LTE:

This is where the core network wants to display a USSD  menu to the handset over the LTE network. The flow is similar to the mobile-initiated call flow. But the INVITE starts from USSI AS.