How LTE works and why airtime doesn’t “disappear” on 3G
MTN recently told MyBroadband it bills customers for data used by devices to connect to its LTE network.
Airtime was deducted from two MTN SIMs installed in LTE-capable smartphones used in our tests on disappearing data.
“LTE is an always-connected technology, and as a result of LTE standards, any compatible device connecting to the LTE network receives an IP address,” said MTN.
“These charges only occur when an LTE device, containing an LTE SIM card, is in an LTE coverage area. This does not occur during any 3G or Edge data sessions.”
How your phone connects
Radisys explains in a whitepaper on LTE signalling procedures that to get service from the network, a device has to select the network and camp on a cell.
For this to happen, the device must synchronise with the network. From there, it requires certain information:
- Public Land Mobile Network ID, which is comprised of the mobile country code and mobile network code.
- Tracking Area ID
- Cell ID
- Radio and Core Network capabilities for its network selection.
The network broadcasts this information in collections known as Master Information Blocks, and System Information Blocks.
Once a device is synchronised and has a cell, it goes goes through several procedures – including attaching to the Non-Access Stratum (NAS) system.
LTE vs 3G
Unlike earlier mobile data technologies in the GSM family, LTE runs exclusively on Internet Protocol (IP) – the same network system that powers the Internet.
Radisys said that after a device has selected the radio network it is connecting to, it tries to attach to the Non-Access Stratum.
NAS is the highest level in the control plane of the Evolved Packet System (EPS) and manages communications session set-up, and maintains sessions while devices move around the network.
The functions of the NAS are grouped into two categories:
- EPS Mobility Management (EMM)
- EPS Session Management (ESM)
ESM is where an IP address is allocated to the device and data sessions are managed so that you can access the Internet.
However, the 3GPP standards document for NAS also describes how the ESM handles procedures for transporting “control plane” user data between the network and the device.
“Control plane” data is generally used to refer to information used by the network to perform various control functions, such as setting up network sessions and keeping them alive.
Its complement is “user plane” data, a term typically used to refer to data we consume when accessing the Internet.
Telkom, Vodacom, and Cell C said they view control plane data sent during session management differently to “regular” mobile data usage.
MTN to change its billing
In older GSM technologies, such as High Speed Packet Access (HSPA) and Enhanced Data rates for GSM Evolution (EDGE) – referred to as 3G and 2G – similar signalling happens between the phone and the network.
However, this is done over a protocol known as Signalling System 7. The MyBroadband article Hidden activity when you use your smartphone provides a detailed look at how this signalling works and why it is necessary.
On LTE, all of the signalling described above runs over Internet Protocol – some of which MTN billed as data traffic.
While MTN’s competitors confirmed that LTE is an IP technology, none of them bill for signalling traffic between devices and the network.
Vodacom and Cell C, as well as Telkom, told MyBroadband that they have never billed for this traffic.
MTN said it was not aware other mobile networks were zero-rating their LTE session traffic, and said it would implement a solution in the near future.
“As a matter of priority, we are going to adjust our tariffs and align our data charging in LTE with our competitors. This will be implemented within the next 30 days,” said MTN.
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