Tag Archives: NB-IoT

NB-IoT Flows for NIDD

EPC, EUTRAN, LTE, Mobile Networks, RFCs & Standards, SDM, , ,

In our last post we covered the basics of NB-IoT Non-IP Data Deliver (NIDD), and if that acronym soup wasn’t enough for you, we’re going to take a deep dive into the flows for attaching, sending, receiving and closing a NIDD session.

The attach for NIDD is very similar to the standard attach for wideband LTE, except the MME establishes a connection on the T6a Diameter interface toward the SCEF, to indicate the sub is online and available.

The NIDD Attach

The SCEF is now able to send/receive NIDD traffic from the subscriber on the T6a interface, but in reality developers don’t / won’t interact with Diameter, so the SCEF exposes the T8 API that developers can interact with to access an abstraction layer to interact with the SCEF, and then through onto the UE.

If you’re wondering what the status of Open Source SCEF implementations are, then you may have already guessed we’re working on one! PyHSS should have support for NB-IoT SCEF features in the future.

NB-IoT provides support for Non-IP Data Delivery (NIDD) over 3GPP Networks, but to handle this, some new network elements are introduced, in a home network scenario that’s the SCEF and the SCF/AS.

On the 3GPP side the SCEF it communicates to the MME via the T6a Interface, which is based upon Diameter.

On the side towards our IoT Service Consumers (in the standards referred to as “SCS/AS” or “Service Capabilities Server Application Servers” (catchy names as always), via the RESTful HTTP based T8 interface.

I’ve written about Non-IP Data support in 5G for transporting Ethernet, but there’s another non-IP use case in 3GPP networks – This time for NB-IoT services.

Procedures

S1 Attach

The start of the S1 Attach procedure is very similar to a regular S1 attach.

The initial S1 PDU Connectivity Request indicates in the ESM Message Container that the PDN Type is Non IP.

S1 PDU Connectivity Request from attach procedure

Other than that, the initial attach procedure looks very similar to the regular S1 attach procedure.

On the S6a interface the Update Location Request from the MME to the HSS indicates that this is an EUTRAN-NB-IoT Radio Access Type.

And the Update Location Answer APN Configuration contains some additional AVPs on the APN to indicate that the APN supports Non-IP-PDN-Type and that the SCEF is used for Data Delivery.

The SCEF-ID (Diameter Host) and SCEF-Realm (Diameter realm) to serve this user is also specified in the APN Configuration in the Update Location Answer.

This is how our MME determines where to send the T6a traffic.

With this, the MME sends a Connection Management Request (CMR) towards the SCEF specified in the SCEF-ID returned by the HSS.

The Connection Management Request / Response

The MME now sends a Diameter T6a Connection Management Request to the SCEF in the Update Location Answer,

In it we have a Session-Id, which continues for the life of our NIDD session, the service-selection which contains our APN (In our case “non-ip”) and the User-Identifier AVP which contains the MSISDN and/or IMSI of the subscriber.

To accept this, the SCEF sends back a Connection-Management-Answer to confirm we’re all good to go:

At this point our SCEF now knows about the subscriber who’s just attached to our network, and correlates it with the APN and the session-ID.

On the S1 side the connection is confirmed and we’re ready to roll.

Mobile Originated Data Request / Response

When the UE wants to send NIDD it’s carried in NAS messaging, so we see an Uplink NAS transport from the UE and inside the NAS payload itself is our HEX data.

Our MME grabs this out and sends it in the form of of a Mobile-Originated-Data-Request (MODR) to the SCEF, along with the same Session-ID that was setup earlier:

At this stage our Non-IP Data is exposed over the T8 RESTful API, which we won’t cover in this post.

SMS over Diameter for Roaming SMS

EPC, GSM, LTE, Mobile Networks, RFCs & Standards, , , ,

I know what you’re thinking, again with the SMS transport talk Nick? Ha! As if we’re done talking about SMS. Recently we did something kinda cool – The world’s first SMS sent over NB-IoT (Satellite).

But to do this, we weren’t using IMS, it’s too heavy (I’ve written about NB-IoT’s NIDD functions and the past).

SGs-AP which is used for CSFB & SMS doesn’t span network borders (you can’t roam with SGs-AP), and with SMSoIP out of the question, that gave us the option of MAP or Diameter, so we picked Diameter.

This introduces the S6c and SGd Diameter interfaces, in the diagrams below Orange is the Home Network (HPMN) and the Green is the Visited Network (VPMN).

The S6c interface is used between the SMSc and the HSS, in order to retrieve the routing information. This like the SRI-for-SM in MAP.

The SGd interface is used between the MME serving the UE and the SMSc, and is used for actual delivery of the MO/MT messages.

I haven’t shown the Diameter Routing Agents in these diagrams, but in reality there would be a DRA on the VPLMN and a DRA on the HPMN, and probably a DRA in the IPX between them too.

The Attach

The attach looks like a regular roaming attach, the MME in the Visited PMN sends an Update Location Request to the HSS, so the HSS knows the MME that is serving the subscriber.

S6a Update Location Request to indicate the MME serving the Subscriber

The Mobile Terminated SMS Flow

Now we introduce the S6c interface and the SGd interfaces.

When the Home SMSc has a message to send to the subscriber (Mobile Terminated SMS) it runs a the Send-Routing-Info-for-SM-Request (SRR) dialog to the HSS.

The Send-Routing-Info-for-SM-Answer (SRA) back from the HSS contains the info on the MME Diameter Host name and Diameter Realm serving the subscriber.

S6t – Send-Routing-Info-for-SM request to get the MME serving the subscriber

With this info, we can now craft a Diameter Request that will get sent to the MME serving the subscriber, containing the SMS PDU to send to the UE.

SGd MT-Forward-Short-Message to deliver Mobile Terminated SMS to the serving MME

We make sure it’s sent to the correct MME by setting the Destination-Host and Destination-Realm in the Diameter request.

Here’s how the request looks from the SMSc towards our DRA:

As you can see the Destination Realm and Destination-Host is set, as is the User-Name set to the IMSI of the UE we want to send the message to.

And down the bottom you can see the SMS-TPDU, the same as it’s been all the way back since GSM days.

The Mobile Originated SMS Flow

The Mobile Originated flow is even simpler, because we don’t need to look up where to route it to.

The MME receives the MO SMS from the UE, and shoves it into a Diameter message with Application ID set to SGd and Destination-Realm set to the HPMN Realm.

When the message reaches the DRA in the HPMN it forwards the request to an SMSc and then the Home SMSc has the message ready to roll.

So that’s it, pretty straightforward to set up!

NB-IoT NIDD Basics

EPC, EUTRAN, LTE, Mobile Networks, RFCs & Standards, Software, , , , , , ,

NB-IoT introduces support for NIDD – Non-IP Data Delivery (NIDD) which is one of the cool features of NB-IoT that’s gaining more widespread adoption.

Let’s take a deep dive into NIDD.

The case against IP for IoT

In the over 40 years since IP was standardized, we’ve shoehorned many things onto IP, but IP was never designed or optimized for low power, low throughput applications.

For the battery life of an IoT device to be measured in years, it has to be very selective about what power hungry operations it does. Transmitting data over the air is one of the most power-intensive operations an IoT device can perform, so we need to do everything we can to limit how much data is sent, and how frequently.

Use Case – NB-IoT Tap

Let’s imagine we’re launching an IoT tap that transmits information about water used, as part of our revolutionary new “Water as a Service” model (WaaS) which removes the capex for residents building their own water treatment plant in their homes, and instead allows dynamic scaling of waterloads as they move to our new opex model.

If I turn on the tap and use 12L of water, when I turn off the tap, our IoT tap encodes the usage onto a single byte and sends the usage information to our rain-cloud service provider.

So we’re not constantly changing the batteries in our taps, we need to send this one byte of data as efficiently as possible, so as to maximize the battery life.

If we were to transport our data on TCP, well we’d need a 3 way handshake and several messages just to transmit the data we want to send.

Let’s see how our one byte of data would look if we transported it on TCP.

That sliver of blue in the diagram is our usage component, the rest is overhead used to get it there. Seems wasteful huh?

Sure, TCP isn’t great for this you say, you should use UDP! But even if we moved away from TCP to UDP, we’ve still got the IPv4 header and the UDP header wasting 28 bytes.

For efficiency’s sake (To keep our batteries lasting as long as possible) we want to send as few messages as possible, and where we do have to send messages, keep them very short, so IP is not a great fit here.

Enter NIDD – Non-IP Data Delivery.

Through NIDD we can just send the single hex byte, only be charged for the single hex byte, and only stay transmitting long enough to send this single byte of hex (Plus the NBIoT overheads / headers).

Compared to UDP transport, NIDD provides us a reduction of 28 bytes of overhead for each message, or a 96% reduction in message size, which translates to real power savings for our IoT device.

In summary – the more sending your device has to do, the more battery it consumes.
So in a scenario where you’re trying to maximize power efficiency to keep your batter powered device running as long as possible, needing to transmit 28 bytes of wasted data to transport 1 byte of usable data, is a real waste.

Delivering the Payload

NIDD traffic is transported as raw hex data end to end, this means for our 1 byte of water usage data, the device would just send the hex value to be transferred and it’d pop out the other end.

To support this we introduce a new network element called the SCEF –Service Capability Exposure Function.

From a developer’s perspective, the SCEF is the gateway to our IoT devices. Through the RESTful API on the SCEF (T8 API), we can send and receive raw hex data to any of our IoT devices.

When one of our Water-as-a-Service Taps sends usage data as a hex byte, it’s the software talking on the T8 API to the SCEF that receives this data.

Data of course needs to be addressed, so we know where it’s coming from / going to, and T8 handles this, as well as message reliability, etc, etc.

This is a telco blog, so we should probably cover the MME connection, the MME talks via Diameter to the SCEF. In our next post we’ll go into these signaling flows in more detail.

If you’re wondering what the status of Open Source SCEF implementations are, then you may have already guessed I’m working on one!

Hopefully by now you’ve got a bit of an idea of how NIDD works in NB-IoT, and in our next posts we’ll dig deeper into the flows and look at some PCAPs together.