SCEF in NB-IoT  – S6t-S6a-S6b Interfaces explained.

For NonIP data delivery in IoT or the Internet of Things, there is a SCEF network node. The full form is the Service Capability Exposure Function. IoT is the network of connected devices over the Internet. SCEF exposes API to the IoT device to access. Accessing a device may be getting parameters from sensors or instructing the device from a server. T8 is the interface between an application server over the network and an IoT device.

Why is SCEF required? 

          SCEF is a gateway between the third-party network applications and the devices over a low-power NB-IoT network. With NB-IoT, a device can work for many years (e.g., 10 Years) without changing a battery. SCEF provides interfaces for supporting NonIp data delivery and exposes an external API interface for accessing a device from a third party.

T8 interface in SCEF :

Till now, all device communication was via the telecom operator’s core network. A device attaches to the HSS/HLR  of a network operator and does data transfer via PGW/GGSN. IoT devices need to send and receive data from a third-party application server using a low-power data transfer mechanism in NB-IoT.  T8 is an HTTP-based interface that provides access to the device. For example, an air conditioner company provides IoT-enabled devices in air conditioners to adjust air conditioner temperature remotely. For that, a SIM needs to install on the mobile device for instruction over mobile data. Based on sensors, the device can also send the current room temperature to an application server. So it’s two-way communication.

With the SCEF, the company can send the instructions to the air conditioner and receive the room temperature information.

SCEF network interfaces:

SCEF has various network interfaces over the diameter protocol along with the T8. The following is the list of interfaces.

T6aThis is the interface between MME and SCEF for attaching a mobile device with the SCEF and transferring NonIp data to/from the mobile device between MME and SCEF.
T6bT6b is between SGSN and SCEF, T6b interface is similar to the T6a.
S6tDiameter Protocol-based interface between HSS and SCEF. SCEF uses the interface for accessing SIM card information in the roaming network from HSS.
T8Describe in an earlier section.

SCEF network procedure for SIM registration:

The NB-IoT Sim cards are regular SIM cards. To enable a SIM to use SCEF,  provisioning of a NonIp Apn requires in HSS against the SIM card IMSI. When a mobile phone with LTE SIM (with standard APN) switches on, the sim card authenticates with the HSS. After authentication, SIM does the location update over S6a/S6d interface for the initial attach procedure. During Initial Attach, SIM also doses GTP signaling with the PGW.

With NoIp Apn, MME/SGSN does the diameter signaling with the SCEF for a SIM instead of GTP signaling. If all works well, the sim will be ready to send/receive NonIp data using SCEF. NonIp APN has the URL of the SCEF. It is similar to APN and has a URL of PGW.