LTN Definition

Some devices send a few bytes a day and must run for years on one battery. LTN is the network that ETSI describes for exactly those devices. This page defines LTN and puts numbers on its low throughput. Then it lists the main characteristics from ETSI GS LTN 003 and the applications that LTN targets.

What is it ?

LTN stands for Low Throughput Network. You can easily guess what it is from the name itself. ETSI set up an Industry Specification Group for it. The group wrote three group specifications: GS LTN 001 for use cases, GS LTN 002 for the functional architecture, and GS LTN 003 for protocols and interfaces.

Yes, it is special type of network that is specialy optimized for low throughput data traffic.

Then you may ask. How low it should be to be called 'Low Throughput' ? If you have been addicted too much about LTE, LTE Advanced. You may have difficult even imagining low throughput case. Probably the lowest throughput you may think of would be Cat 0 throughput which is about 1 Mbps.

Is this low enough ?

No. It is way, way, way high throughput for LTN.

Then, what kind of throughput you are thinking about in LTN ?

It is about 10~100 bits per sec typically and even in max throughput case it would be just about 50 k bits per second.

Are you sure ? Do you have such a many application with this little/tiny throughputs that we have to think of brand new network architecture ?

The answer is Yes. I will post a separate page for LTN application later.

Let's put numbers on it. GS LTN 003 defines two radio implementations for the interface between the end point and the access point [1]. Both use the same radio spectrum. The first is UNB, Ultra Narrow Band. Its uplink uses BPSK at 100 baud in a 100 Hz channel, or 600 baud in a 600 Hz channel in the USA. Each UNB uplink frame carries a payload of 0 to 12 bytes. The second is OSSS, Orthogonal Sequence Spread Spectrum. It spreads the signal over 8 kHz to 500 kHz, depending on the spreading factor. Its data rate ranges from 30 bps to 50 000 bps. So the 10 to 100 bits per second above describes UNB, and the 50 k bits per second describes the fastest OSSS setting.

The LTN Network Architecture, UNB PHY and UNB MAC pages go into each of these parts in detail.

LTN is a class of network, not one product : ETSI describes it through an architecture and a set of interfaces, and two different radio implementations fit under it.
UNB trades data rate for range : a 100 Hz channel collects very little noise, so the receiver can decode very weak signals. GS LTN 003 recommends a receiver sensitivity better than -135 dBm.
Most of the traffic goes up : UNB is mainly an uplink system. A device that wants downlink data opens a reception window with a fixed delay and duration after each uplink.

What are the major characteristics of LTN ?

Low throughput is only the starting point. It buys two other properties, long range and long battery life, and those two are what make LTN useful. The three characteristics below are quoted from the introduction of GS LTN 003.

I think ETSI GS LTN 003 is well describing the major characteristics of LTN in terms of Protocol and Radio Access. It states as follows :

Long Range Communication : LTN enables long-range data transmission (distances up to 40 km in open field) and/or communication with buried underground equipment.
Low Engergy Consumption : LTN operates with minimal power consumption in the device modems that allows operation on several years even with the standard batteries.
Low Throughput : LTN implements low throughput along with advanced signal processing that provides effective protection against interferences.

Each characteristic has a number behind it in the same specification. For long range, the recommended receiver sensitivity is better than -135 dBm, for both UNB and OSSS. For low energy consumption, the device simply transmits very little. In Europe, the UNB uplink band from 868.0 MHz to 868.6 MHz allows at most 25 mW and a mean transmission time of 1 %. For protection against interference, UNB keeps each message inside a very narrow channel, and OSSS spreads it with orthogonal sequences. GS LTN 003 states that UNB allows large numbers of end points in a cell without spectrum interference. It also states that OSSS gives good spectrum efficiency when the nodes are dense.

The three characteristics are one trade : the very low data rate is what buys the long range and the long battery life.
Regulation shapes the design : in Europe, the 1 % limit on mean transmission time and the 25 mW power limit apply. They decide how often and how strongly a UNB device can transmit.

Main Target Application

The characteristics above point to one kind of application. Many devices each send a few bytes now and then, often from places that are hard to reach or to power. GS LTN 003 names this target explicitly, and it also gives LTN a second role next to cellular networks.

With all the characterestics mentioned above, LTN can be a good solution to the applications as follows :

M2M : LTN is particularly well suited for low throughput machine to machine communication where data volumn is limited and latency is not a strong requirement.
Complement to cellular : LTN could also cooperate with cellular networks to address use cases where redundancy, complementary or alternative connectivity is needed.

The second item matters because LTN does not have to replace anything. A device can keep a cellular link for large or urgent data. It can then use LTN as a backup, or as a cheap channel for small regular reports. GS LTN 001 covers the use cases themselves, and GS LTN 003 leaves the business view to that document.

LTN suits small, regular, delay tolerant messages : latency is not a strong requirement, so the network can trade it for range and battery life.
LTN can sit next to cellular : ETSI describes it as a partner for redundancy or alternative connectivity, not only as a competitor.

Reference

[1] ETSI GS LTN 003 V1.1.1 (2014-09) : Low Throughput Networks (LTN); Protocols and Interfaces