SCEF in NB-IoT  – S6t-S6a-S6b

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 SCEF require? 

          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 expose 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. For IoT devices, there is a 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 providing IoT-enabled devices in air conditioner,  to adjust air conditioner temperature remotely. For that, a SIM needs to install in the mobile device for instruction over mobile data.  Based on sensors, the device can send the current room temperature to an application server too.  So its 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 are the list of interfaces.

Interface Description
T6a This is the interface between MME and SCEF for attaching a mobile device with the SCEF and For
Transfering NoIp data to/from the mobile device between MME and SCEF.
T6b T6b is between SGSN and SCEF, T6b interface is similar to the T6a.
S6t Diameter Protocol based interface between HSS and SCEF. SCEF uses the interface for accessing SIM card information in the roaming network from HSS.
T8 Describer 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 does the authentication 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 signalling with the PGW and gets attached to the network.

With NoIp Apn, MME/SGSN does the diameter signalling with the SCEF for a SIM, in place of GTP signalling. 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 have a URL of PGW.

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