Chapter 3. TETRA overview: a

cellular system different from GSM

Chapter 1 provided a brief overview of TETRA along with some of its advantages

and architectural differences over other mobile technologies like GSM. Although a very

rough idea of TETRA’s pros might be extracted out of there, an in-depth analysis must be

carried out in order to justify the need of such technology.

Thus, in this chapter we are going to deal with technical details of the standard,

explaining at length many of the most complex characteristics of the technology, including

logical channels and its internal operation for the two operational modes of TETRA - the

trunked and the direct mode. Along the way GSM will be covered too, but just briefly

since it is a more well-known technology. Due to the extension of the topic many of the

mentioned details have been included in an Appendix A in order to facilitate the read of

the chapter.

The majority of the information given here has been obtained from [7].

3.1 Introduction

A good starting point to get a clear idea of TETRA is to locate the standard in the

OSI model. The well-known model, given in Figure 3.1, is an attempt to break down a big

task into smaller ones, so that a complex issue becomes easier to tackle. This reasoning, of

course, leads to layers.

1

Figure 3.1. The OSI model.

The model works in such a way that a layer provides services to the layer above itself

and receives services from the layer right underneath. For example, the second layer - the

data link layer, see Figure 3.1- ensures the third layer (the network layer) an error-free link.

To do so, the data link layer adds redundant data to the incoming packets from the network

layer. This redundant data is known as the data link header. After that, the layer calls

the first layer (the physical layer) to send and receive packets until the right information

has achieved its destination. In the peer entity, each layer simply deals with the header

corresponding to its twin layer.

This is the theoretical explanation, and it sounds clear and simple. However, putting

it into practice results in a number of uncertainties. For example, the OSI model does not

specify how systems are implemented, but rather how they communicate with each other.

This particular point becomes interesting in TETRA, for it means that the system is flexible

and there’s room for changes and modifications when it comes to fitting a specific application

into one of the layers.

In TETRA, the standard specified by the ETSI is essentially confined to layers 1-3

of this model, but with some nuances that will be explained over the next few paragraphs.

2

TETRA’s main parameters are given in table 3.1.

Parameter Value

Carrier Spacing 25 kHz

Modulation pi/4-DQPSK

Carrier Data Rate 36 kb/s

Voice Coder Rate ACELP (7.2 kb/s after coding is added)

Access Method TDMA with 4 time slots/carrier

User Data Rate 7.2 kb/s per time slot

Maximum Data Rate (4 channels) 7.2 x 4 = 28.8 kb/s

Max Low Protected Data Rate 4.8 x 4 = 19.2 kb/s

Max Heavily Protected Data Rate 2.4 x 4 = 9.6 kb/s

Table 3.1. Main parameters for a TETRA radio link.

Following the GSM comparison, several similarities with GSM can already be noticed

at this point. For instance, the use of Time Division Multiple Access (TDMA) for sharing a

single frequency channel or the use of a phase modulation scheme. On the other hand, there

are also some important differences, the most remarkable being the date rate. Whereas in

GSM an overall data rate of 22.8 kb/s per channel is available1, in TETRA this value is

equal to 7.2 kb/s. This is approximately a third of the data rate available in GSM. This

point will be discussed and explained later on.

TETRA supports a number of services which can be classified as bearer services and

teleservices. Bearer services are classically defined as those services that provide information

transfer between user network interfaces (e.g. the two data link layers involved in an

individual call) involving only OSI layers from 1 to 3. Bearer services are all the types of

calls supported by TETRA, that is, individual, group, group acknowledged and broadcast

call, for each of the following modes:

• Circuit Mode (Trunked Mode): Voice plus Data (V+D)

1This is the gross data rate, which is defined as the total number of physically transferred bits per secondover a communication link, including actual data, protocol overhead and redundant data.

3

• Packet Connection Oriented Mode

• Packet Connectionless Mode

Therefore, bearer services exclude possible functions associated to end terminals such as

encryption attributes. This types of tasks belong to the teleservices, defined as those ser-

vices that provide the complete capability for communication between users including the

terminal functions. In other words, a teleservices will comprise services from layer 1 to layer

7 of the OSI stack. The teleservices supported by TETRA are voice (either encrypted or

unencrypted) for each of the following calls:

• Individual call

• Group call

• Acknowledged group call

• Broadcast call

Other than this, TETRA has some supplementary services which can be seen as modifica-

tions of the previous but can’t be classified in either type. For example, allocation of access

priority (more commonly known as priority calls) or call forwarding. These aspects of the

standard will be discussed later on.

3.2 Definition and characteristics

It is not an easy task to give a short, clear definition of what is TETRA. As it was

advanced in the first chapter, TETRA is a type of Professional Mobile Radio (PMR)2 that

provides a private group of users (e.g. a company) with a number of services. A PMR is

usually set up by a group of user or a company and includes the whole infrastructure as

well as the mobile stations. Although it’s normally deployed by a company that requires a

private parallel infrastructure to public mobile radio systems like GSM, it can be deployed

for different reasons. For example, it can be often a cheaper solution in the long-term than

2The initial ’P’ can stand for either Private or Professional, and there’s no fixed compromise on this.

4

the usage of GSM. TETRA is also a trunked radio system, which as it was also mentioned

in the introduction means that it’s a computer-controlled system.

Even if this definitions are accurate and true, they do not quite explain what TETRA

provides or what it can be used for. TETRA is more than a mere PMR technology or a

trunked system, and for this reason, the following list has been drawn up in order to put

together everything TETRA is.

• TETRA a 100% digital system, which by the time it was defined, the 90’s, meant an

important step forward to definitely left analogue technologies behind. In addition,

this feature makes the system particularly suitable for IP environments3.

• It’s a set of standards defined by the European Telecommunications Standards Insti-

tute (ETSI). Its specifications are mandatory in Europe and have also been adopted

worldwide in many other places. ETSI is also in charge of updating the standard.

• TETRA was designed to be interoperable with other technologies and easily scalable

in order to provide a flexible solution. The traditional example is a network with a

certain coverage at first which later can be easily expanded/reduced according to its

needs.

• TETRA is the main PMR technology along with TETRAPOL, another PMR system

with similar characteristics. If you put both technologies together, they practically

dominate the entire PMR market.

There are some other important details regarding TETRA. For example, since it’s a private

system, a new infrastructure needs to be deployed to use the technology. This means an

important investment of money and it’s important to bear it in mind.

TETRA OPERATIONAL MODES

Two different operational modes are provided:

• The Circuit Mode, also know as the Trunked Mode, for transmitting both voice and

data (Voice+Data, or V+D). Simply put, in this mode each terminal in the area of

3As a matter of fact, the hardware used in the present project, TETRAFlex R©, is a 100% IP-basedsystem.

5

coverage is allocated a Traffic CHannel (TCH) for the duration of the call, even if the

source is not active. A traffic channel is one of a number of logical channel specified in

the TETRA standard. They are used for managing and maintaining purposes while

the duration of a call. The concept of logical channel is crucial to the operation of the

TETRA system and will be fully addressed later in Section 3.5 and in Appendix A.

• The Direct Mode Operation (DMO), also known as the walkie-talkie mode. Roughly

speaking, it allows users to communicate among each other without actually using the

trunked network. In other words, communication mobile-to-mobile is possible without

intervention of the TETRA network. The implications of this mode will be discussed

in Section 3.6 and Appendix B..

As a matter of fact, there’s a third operational mode defined by TETRA. It is called the

Packet Data Optimised (PDO). It’s the least known due to its lack of commercial use. This

mode is intended to be used only for sending data over a packet switched network, totally

neglecting voice services. Due to voice needs in day-to-day communications, to date no

manufacturer has developed any PDO applications and it’s only used for research purposes.

Interestingly, the work from this standardisation activity has been carried forward

in the Project MESA space - a Partnership Programme between ETSI and TIA.

3.3 TETRA Network Architecture vs GSM’s

Perhaps the best way to obtain a quick, intuitive idea of a new system is to show a

sketch of its architecture. A possible TETRA network architecture is illustrated in Figure

3.2. Why possible? Unlike GSM network architecture given in Figure 3.3, TETRA is

exclusively defined in terms of its interfaces, i.e. the standards defined to communicate

with “the outer world”.

ETSI refers to TETRA networks (in pink) as the Switching and Management Infras-

tructure. This includes the base stations, controllers and all necessary equipment to set up

a TETRA network and allow mobile stations to communicate with/through them.

6

Figure 3.2. TETRA network architecture.

In other words, in TETRA what’s inside the network can be designed and built

up just the way manufacturers want. The only requirement is to meet the standardised

interfaces. This ’do it as you want as long as it works’ philosophy stands out for its flexibility,

for manufacturers are completely free to design their products the way they want.

Figure 3.3. GSM network architecture.

Taking a closer look at Figure 3.2, it can be noticed that, as in GSM, there are

base stations (the base transceiver station, BTS, in GSM; the piece of equipment that

7

connects mobile terminals with the network equipment), a base controller (the base station

controller, BSC, in GSM; also regarded as the intelligence behind the BTSs, the piece of

equipment that actually manages the requirements of the terminals present in the cell. The

BSC reports to the switching center), a gateway to control the access to another networks

such as public telephony, Internet, LAN/WAN networks, etc. The switching centers are not

specified, and it is the equipment at the controller along with the central remote TETRA

core network the ones that will take care of their tasks.

The important and key idea in this section is to keep always in mind that the internal

structure in a TETRA network is never standardised; it’s the set of input/output interfaces

what it is indeed standardised, and that’s exactly what it is going to be studied in the next

section.

3.4 TETRA interfaces

When talking about TETRA interfaces we often refer to the four standards illustrated

in Figure 3.4. The aims of the set are to ensure interoperability, interworking and network

management:

• Air Interface. It standardises the connection between the base station and the radio

terminals. In other words, it ensures interoperability of terminal equipment from

different manufacturers, so that radio terminals can be developed independently with

the certainty that they will be compatible. The air interface is the most complex of

the standards and the hardest to implement.

• Peripheral Equipment Interface (PEI, also known as TEI from Terminal in some

books). This interface standardises the connection between external devices and radio

terminals. The aim of this standard is to facilitate the independent development of

mobile data applications.

• Inter-System Interface (ISI) allows the interconnection of TETRA networks from dif-

ferent manufacturers, that is, it standardises the connection between two base stations.

Speaking of ISI, in the present project it’ll be the main focus of study, making a special

8

effort to deal with the satellite aspects and possibilities of the technology.

• Direct Mode Operation (DMO). It ensures terminal-to-terminal communication (’walkie-

talkie’) independently of the TETRA network. It also provides communication be-

tween terminals also beyond network coverage, something that come in handy in many

professional situations.

Figure 3.4. TETRA interfaces.

These are, in a way, the ’commercial’ standards, or the most common ones. Formally,

ETSI defines TETRA in terms of six different interfaces, two of then being ’in the shade’

due to their minor importance when it comes to noticeable purposes, but vital in certain

situations. They have been listed below along with their ETSI nomenclature.

• I1: Trunked Mode Air Interface

• I2: Line Station Interference

• I3: Inter-System Interface (ISI)

• I4: Terminal Equipment (TE) Interface for a Mobile Station (MS). Also known as

Terminal Equipment Interface (TEI)

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(I4‘: Terminal Equipment (TE) interface for a Line Station (LS) )

• I5: Network Management Interface

• I6: Direct Mode Air Interface: mobile to mobile radio interface (’walkie-talkie’). It’s

commonly known as DMO, even if this entails an abuse of language4.

(I6‘: Direct mode: radio interface gateway from trunked mode)

(I6‘: Direct mode: radio interface via repeater)

The Line Station Interface is designed for special terminals connected over a wireline con-

nection (e.g. ISDN), as opposed to the air interface specification (I1). The Network Man-

agement Interface sees to providing local and remote network management functionality to

TETRA networks. In the recent years this interface has become very important due to the

complexity achieved by modern networks. The information sent over this protocol has to do

with inter-working processes such as accounting, performance, configuration or planning.

3.5 TETRA circuit mode (V+D)

The circuit mode -also called the trunked mode, or the Voice plus Data mode (V+D)

- is undoubtedly the main and most important mode of the ones provided by TETRA.

Basically, it brings all GSM possibilities with a lot of improvements and add-on features such

as interactive calls among several users or broadcast calls. On the drawbacks, the offered

data rates are slower than those in GSM in exchange for spectrum efficiency, compatibility

with analogue PMR and higher level of security. The following table summarises the services

and data rates offered in this mode. The different data rates reflect the fact that up to four

traffic channels can be assigned to the same communication in order to increase the data

rate.

4Keep in mind that the Direct Mode Operation or DMO is exactly that, a mode supported by TETRA,and not an interface as it is often referred to. I hope the context makes this point clear for each case.

10

TETRA Teleservices

Individual call

Group call

Broadcast call

Acknowledged call

TETRA Bearer Services

Circuit mode protected data 4.8/9.6/14.4/19.2 kb/s

Circuit mode heavily protected data 2.4/4.8/7.2/9.6 kb/s

Connection oriented data packet

Connection data packet

Table 3.2. TETRA bearer services and teleservices.

Previously, it was mentioned that in this mode each mobile terminal is allocated

a traffic channel (TCH) for the duration of a call, irrespective of whether that source is

active or not. A traffic channel is a type of channel called logical channel (logical channels

will be explained in the next paragraphs), often bi-directional, that carries user information.

Different types of traffic channels are defined in TETRA depending on the use. For instance,

there are traffic channels for speech, for data applications and for different data message

speeds.

The ’physical’ transmission mechanism used by logical channels is given by a physical

channel that exists within the physical layer. How exactly? Simply by using a specific carrier

frequency/slot. To avoid any confusion, let’s say now that this does not mean that only one

logical channel can be transmitted through a physical channel, on the contrary; a particular

physical channel can and is used for transmitting several logical channels on a shared basis

by using a multiplexion mechanism: TDMA. Let’s take a closer look at this:

As shown in Fig. 3.5 (next page), each TETRA frame is made up of four time slots,

with a duration of 56.67 ms. This frame is repeated over time up to 18 times to make up

a TETRA multiframe. Each of the columns shown, made up of 18 time slots each, is a

physical channel. To make a long story short, this is how the multiframe goes: in normal

operation, time slot 1 of every frame (both uplink and downlink) is allocated for control

purposes. This is known as the Control Physical channel (CP) (first column). The other

3 time-slots (or channels5) are used for traffic purposes and represent the Traffic Physical

5Note how it exists an abuse of language when it comes to talking about channels. It is important toknow when we refer to logical channels and physical channels, since they are not the same thing. Simply put,a physical channel can carry up to four logical channels (by means of TDMA), and these logical channels

11

channels (TP) (second, third and fourth columns).

Figure 3.5. TETRA frame structure.

Therefore, there are four physical channels available per carrier, three for sending

data (TP) and one for controlling purposes (CP). The control physical channel is made

up of two logical channels with identical names, one for the uplink and another one for

can be classified into either control or traffic channels. I hope the context makes this clear when mentioned.

12

the downlink in charge of maintenance. The name of this logical channel is Main Control

CHannel (MCCH) and it’ll be often referred to over the next sections since it turns out to

be a key logical channel in TETRA.

Taking a closer look at the figure, note how the second column, made up of slots 2,

that composes a traffic channel, data is transmitted in the first 17 frames. But, in the 18th

slot, traffic is not transmitted anymore. Instead, it is used for signalling purposes. This

occurs in every single traffic channel in the V+D mode. That means that, in reality, only

17 frames out of 18 are used for data, resulting in a 17:18 data ratio for that traffic channel.

Likewise, the multiframe is repeated 60 times in order to produce a hyperframe, with

a duration of 61.2 s. The task of this frame is related to encryption and synchronisation

procedures. Figure 3.6 shows all possible TETRA frames, including the two groups of 255

bits that compose a timeslot as a result of the pi/4, DQPSK modulation (a symbol is made

up of 2 bits).

Figure 3.6. The different TETRA frame structures.

For a further explanation of the V+D mode, including an in-depth review of logical

channels, see Appendix A.

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3.6 TETRA Direct Mode Operation

Once a thorough breakdown of the trunked mode has been presented it is time to talk

about the so-called ’walkie-talkie’ feature of TETRA, the Direct Mode Operation (DMO).

The services provided by this mode are drawn up here:

• Individual/group circuit mode calls in simplex mode.

• Call set-up with and without presence check.

• Clear and encrypted circuit mode operation.

• Pre-emption capability.

• User defined short message transmission and reception.

• Pre-defined short message transmission and reception.

The most important things to note are: (a) unlike the trunked mode, the direct mode does

not support duplex transmission, and (b) only voice and short messages (as GSM texts) are

available; data traffic is not supported.

3.6.1 Introduction

The trunked mode, seen at length both in the previous subsection and Appendix

A, involves the utilisation of a number of the interfaces defined in Figure 3.4, all of them

regulated by the ETSI. As it has been detailed, this mode shares a lot of similarities with

GSM in its performance and some of its characteristics. In turn, the direct mode is totally

unknown to GSM. It brings the possibility to directly communicate users among each other

without actually using the TETRA infrastructure itself. This feature offers new advantages,

such as communication between terminals beyond the trunked coverage.

There is a reason why the trunked mode was explained in first place. The TETRA

specifications are written in a way that a mobile terminal operating in the trunked mode

can be contacted by a DMO mobile (that is, a mobile station using the direct mode) as

14

long as it’s located within the mobile-to-mobile range area6. While the vice versa is true,

the first option is much more common.

Figure 3.7. Example of the trunked mode and direct mode interaction.

Fig. 3.7 shows a simplified scenario with three different location areas. This figure

proves the ’cozy’ relationship between the two modes. How does the entire system handle

this duality? The solution is directly implemented in the mobile station and requires the MS

to continuously monitor operation of both modes. This mechanism is called ’dual watch’

mode and is illustrated in Fig. 3.8.

In other words, mobile stations are always watching to detect possible terminals in

their DMO coverage area and at the same time exchange signalling data with the trunked

network.

How does this translate into the interfaces defined in Section 3.4? DMO is essentially

based on the I6 interface defined in the set of TETRA standards. Figure 3.9 illustrates the

classical walkie-talkie operation. However, depending on the particular case, this mode can

also make use of the I1 interface as we shall see soon.

6This range will vary depending upon the type of sub-I6 interface employed. This issue will be addressedlater in this section.

15

Figure 3.8. Direct mode dual watch mode operation.

Figure 3.9. I6 direct mode, mobile-to-mobile operation (’walkie-talkie’).

A closer look at I6 shows that the interface actually breaks down to three different

interfaces: I6, I6’ and I6”.

• I6 Direct mode: mobile to mobile radio interface, as in Fig. 3.9.

• I6’ Direct mode: radio interface gateway from trunked mode, as in Fig. 3.10.

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Figure 3.10. Direct mode coverage enhancement with a gateway station.

In this case there’s a mobile gateway station that provides coverage extension and

communicates with the trunked network via the I1 interface. Note that the station must

necessarily be in the coverage area of the network, and thus this case is bound to occur in

the limits of the trunked coverage area. The operation of the station is located at OSI layer

three and goes beyond the scope of the project.

• I6” Direct mode: radio interface via a repeater, as in Fig. 3.11.

Figure 3.11. Direct mode coverage using an independent repeater.

If the mobile-to-mobile communication range is not enough to allow direct communi-

cation between two MSs, then a repeater can be used. In this case the intermediate station

is completely independent of the trunked network and it can be deployed anywhere. For a

detailed explanation on this mode see Appendix B.

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3.7 TETRA Release 1&2

Everything that has been discussed so far is based on TETRA as we know it today.

This is TETRA Release 1, a deeply rooted standard that has been working for over 15 years

now. However, just like any other technology, it needs to evolve to adapt to new times. In

the words of the TETRA Association:

“TETRA Release 1 (Voice + Data) already provides a very comprehensive portfolio

of services and facilities but as time progresses there is a need to evolve and enhance all tech-

nologies to better satisfy user requirements, future proof investments and ensure longevity.

Like GSM moving to GPRS, EDGE and UMTS/3G, TETRA also needs to evolve to satisfy

increasing user demand for new services and facilities as well as gleaning the benefits of

new technology.”

TETRA release 2 provides the following advantages.

• Trunked Mode Operation (TMO) Range Extension. TETRA’s theoretical extension

in Release 1 is 58 km where in Release 2 goes all the way up to 83 km. This is achieved

by modifying uplink and downlink bursts and guard times, amongst others.

• Adaptive Multiple Rate (AMR) Voice Codec. Just like in GSM, the AMR voice codec

is used to provide the highest quality voice with the lowest data rate.

• Mixed Excitation Liner Predictive, enhanced (MELPe) Voice Codec.

• TETRA Enhanced Data Service (TEDS).

Although TETRA Release 2 is not involved in the present project, it is important

to be aware of its existence.

3.8 Key points of the chapter

• Inputs&Outputs standardised. TETRA’s internal structure is not standardised; only

the interfaces that let devices communicate with the outside are standardised.

• Individual calls exist as in GSM, and in addition there are group, acknowledged and

broadcast calls.

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• Two different modes: V+D and DMO.

• Direct Mode (’walkie-talkie’ mode). Mobile terminals can communicate even out or

the coverage area.

• Logical channels play a key role in TETRA’s operation.

• Very fast call setup, 300 ms.

• Safer encryption than public mobile services like GSM (end-to-end encryption en-

sured).

• Greater coverage than GSM (theoretically up to 58 km in TETRA release 1 and 83

km in release 2), which can provide a significant cost saving in terms of the necessary

radio infrastructure.

• Voice has always greater coverage than data. Both values depend on the particular

environment.

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