Chapter 3. TETRA overview: a cellular system different from GSM

Transcription

1 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

2 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

3 TETRA s main parameters are given in table 3.1. Parameter Carrier Spacing Modulation Carrier Data Rate Voice Coder Rate Access Method User Data Rate Maximum Data Rate (4 channels) Max Low Protected Data Rate Max Heavily Protected Data Rate Value 25 khz pi/4-dqpsk 36 kb/s ACELP (7.2 kb/s after coding is added) TDMA with 4 time slots/carrier 7.2 kb/s per time slot 7.2 x 4 = 28.8 kb/s 4.8 x 4 = 19.2 kb/s 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 available 1, 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) 1 This is the gross data rate, which is defined as the total number of physically transferred bits per second over a communication link, including actual data, protocol overhead and redundant data. 3

4 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 services 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 modifications 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 2 The initial P can stand for either Private or Professional, and there s no fixed compromise on this. 4

5 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 environments 3. It s a set of standards defined by the European Telecommunications Standards Institute (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 3 As a matter of fact, the hardware used in the present project, TETRAFlex R, is a 100% IP-based system. 5

6 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 Infrastructure. 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

7 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

8 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 different 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

9 effort to deal with the satellite aspects and possibilities of the technology. Direct Mode Operation (DMO). It ensures terminal-to-terminal communication ( walkietalkie ) independently of the TETRA network. It also provides communication between 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) 9

10 (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 language 4. (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 connection (e.g. ISDN), as opposed to the air interface specification (I1). The Network Management 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. 4 Keep 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

11 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 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 channels 5 ) are used for traffic purposes and represent the Traffic Physical 5 Note how it exists an abuse of language when it comes to talking about channels. It is important to know 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

12 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

13 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. 13

14 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 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

15 long as it s located within the mobile-to-mobile range area 6. 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 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. 6 This range will vary depending upon the type of sub-i6 interface employed. This issue will be addressed later in this section. 15

16 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 I6 Direct mode: radio interface gateway from trunked mode, as in Fig

17 Figure 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 Figure Direct mode coverage using an independent repeater. If the mobile-to-mobile communication range is not enough to allow direct communication 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. 17

18 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 technologies 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. 18

19 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 ensured). 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. 19