Interfaces used for MO-MT

The European Telecommunications Standards Institute (ETSI) has defined various interfaces and protocols for mobile telecommunications networks. While ETSI primarily focuses on the standardization of telecommunications protocols and interfaces in general, it does not specify interfaces for specific stages of a Mobile Originated (MO) to Mobile Terminated (MT) call within a 5G network. Instead, the 3rd Generation Partnership Project (3GPP), which works in close cooperation with ETSI, defines the specific protocols and interfaces for mobile communication networks, including 5G.

In the context of 5G, the key 3GPP interfaces and protocols that are used for different stages of an MO to MT call are as follows:

1. Stage 1: Call Initiation (MO):
   • In this stage, the UE initiates the call setup by sending a request to the 5G Core Network (5GC) using the N1 interface. The specific protocol used for this stage can be the NGAP (Next-Generation Application Protocol), which is part of the 3GPP’s specifications.
2. Stage 2: 5GC Call Setup:
   • During the call setup phase, various 3GPP interfaces and protocols come into play. Some of the key interfaces involved include:
     • N2 interface: This interface connects the UE to the gNB (Next-Generation NodeB) in the Radio Access Network (RAN).
     • N3 interface: It connects the gNB to the Access and Mobility Management Function (AMF) within the 5GC.
     • N4 interface: This interface connects the gNB to the Session Management Function (SMF) within the 5GC.
   • These interfaces are part of the 3GPP’s specifications and play a crucial role in setting up and managing the communication session.
3. Stage 3: Voice Communication:
   • Once the call is successfully set up, voice communication occurs over the established radio link. The specific protocols used for voice communication can include codecs such as AMR (Adaptive Multi-Rate) or EVS (Enhanced Voice Services), as well as the 5G NR (New Radio) air interface protocol for data transmission.
4. Stage 4: Call Termination and Release:
   • Call termination involves signaling between the UE, gNB, AMF, and SMF using various 3GPP-defined interfaces and protocols.
   • The interfaces N1, N2, N3, and N4, which were used in call setup, may also be involved in the termination and release of the call, depending on the network architecture and signaling procedures.
5. Billing and Charging:
   • While not specific to ETSI or 3GPP interfaces, billing and charging information may be collected and transmitted using various protocols and interfaces within the network, often involving interfaces to the Charging Function in the 5GC.

It’s important to note that the specific interfaces and protocols used in a 5G network may vary depending on the network architecture, deployment scenario, and network operator’s configuration. However, the 3GPP standards are the primary reference for the design and implementation of interfaces and protocols in 5G networks.