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Realization of a Mobile Data Application

in TETRAE. Lammerts1 C.H. Slump1 K.A. Verweij2

1Laboratory Signals & Systems, Faculty of Electrical Engineering,University of Twente,

PO Box 217, 7500 AE, Enschede, the Netherlands

Phone: +31-53-4892780 Fax: +31-53-48910602 ITO, PO Box 238, 3970 AE Driebergen

[email protected] [email protected] [email protected]

Abstract — At this moment, in many Europeancountries the analogue PMR (Private Mobile Ra-dio) communications structure is being replaced by

a new digital system based on the TETRA (TEr-restrial Trunked RAdio) standard. This new stan-dard provides higher quality, more flexibility andimproved security of communications.

An interesting aspect of TETRA is that it pro-vides data communications facilities. This makes itpossible to do automatic tracking of the location of police cars, access medical information from an am-bulance or do administrative work at the locationof an incident, and many other things.

The Dutch police has developed an applicationto access their central information systems calledMobipol. This application currently runs on GSM

and RAM Mobitex (a commercial packet-switchednetwork) wireless communications infrastructures.In the future, TETRA will be used instead.

Mobipol uses the TCP/IP protocol to commu-nicate. Unfortunately, TCP/IP functionality hasbeen introduced in the TETRA standard only re-cently

and is not supported in the TETRA Test Config-uration.

This paper describes a protocol design and im-plementation to support Mobipol and other appli-cations using TCP/IP on TETRA. The system is

intended for demonstration purposes and is specif-ically targeted at the TETRA Test Configurationof the Dutch police located in Driebergen. The de-velopment was performed in cooperation with ITO,the IT organization of the Dutch police.

Keywords— TETRA, Mobipol, TCP/IP

I. Introduction to TETRA

What is TETRA?

TETRA[1][2] is a new digital transmission stan-dard for Private Mobile Radio (PMR), developed bythe The European Telecommunication Standards In-stitute (ETSI).

There are a number of reasons why current PMRsystems need to be replaced:

• Capacity is too low.

• No data facilities are available.

• Security is very bad (For example, with an analogsystem, it is easy to listen in on regular police com-munications).

TETRA solves these problems. It is a vendor-independent system providing voice communications,several types of data communications, direct mobile-to-mobile operation, encryption of all traffic and fastcall-setup (300 ms). Most European Public Safety or-ganizations have adopted TETRA as their new PMRstandard.

TETRA is a cellular system, which means that thegeographical area in which the system operates is di-vided into a number of cells. Each cell has its ownbase station.

Security is an important aspect of TETRA. Au-thentication and confidentiality are key requirementsfor public safety organizations, especially for the po-lice. Authentication is performed through the use of an individual key-based authentication algorithm atlog-on. To ensure confidentiality, two types of encryp-tion are used:

Air-interface encryption, which encrypts the radiopath between the terminal and the base station.

End-to-end encryption for critical applications whereencryption is required for the transmission through-

out the sytem to the other terminal.

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248 E. Lammerts, C.H. Slump, K.A. Verweij

TETRA Addressing

In the TETRA system several addresses are used foridentifying an application, mobile station or entity inthe TETRA protocol stack. The main identities areTETRA Subscriber Identities (TSI). The TSI consistsof a 10-bit Mobile Country Code (MCC), a 14-bit Mo-

bile Network Code (MNC) and a 24-bit Short Sub-scriber Identity (SSI). Within a TETRA subdomainSSIs are used to reduce the amount of signalling in-formation.

Each entity in the TETRA system has its own ITSI(Individual TETRA Subscriber Identity). Groups of ITSIs are addressed with Group TETRA Subscriber Identities (GTSI). A mobile can have multiple GTSIs.

TETRA Layering

TETRA is based on the Open Systems Interconnec-tion Reference Model (OSI-RM). TETRA only defineslayers 1-3.

Physical Layer

The physical layer defines the radio characteristicsof TETRA. TETRA can operate at frequencies of 150–900 MHz. European Public Safety users will usethe 380–400 MHz frequency band provided by NATO.The modulation used in TETRA is π

4DQPSK (Dif-

ferential Quadrature Phase Shift Keying). The trans-mission speed is 36 kbits/s per carrier. There aremultiple carriers spaced 25 kHz apart.

Data Link Layer

The Data Link Layer provides the service of trans-mitting frames of information to remote systems. TheTETRA frame structure is shown in figure 1. TETRAuses TDMA (Time Division Multiple Access) technol-ogy to enable four channels on the same carrier. OneTDMA frame consists of four time slots. Each time

slot consists of 510 bits. The TDMA frame is partof a multiframe, consisting of 18 TDMA frames. The18th frame in a multiframe is called the Control Frame(CF) and is used for control signalling. 60 multiframestogether form one hyperframe.

Channel allocation can be performed in a number of ways: trunked, non-trunked and DMO (Direct ModeOperation). Trunking is a way to increase channelcapacity: A channel is only allocated when data hasto be sent. DMO is used when two mobile stationsto communicate directly to each other, without assis-

tance from a base station.

508 509 5101 2 3 4 5

1 time slot = 510 modulating bits duration (= 14.167ms)

1 TDMA frame = 4 time slots (= 56.67ms)

1 2 3 4

1 2 3 4 17control

frame

1 multiframe = 18 TDMA frames (= 1.02s)

18

1 hyperframe = 60 multiframes (= 61.2s)

1 2 3 4 5 60

Fig. 1. TETRA frame structure

Network Layer

The Network Layer provides transparent datatransfer services to applications. Applications neednot be aware of the underlying protocol mechanisms.

The Network Layer also provides Mobility Manage-ment (MM) and Supplementary Services (SS).

The Mobility Management keeps track of the loca-tion of users. When a call arrives for a user, the basestation has to know in which cell the user is located.The MM also takes care of (de-)registering the userswith the system.

Supplementary Services can modify or supplementthe basic services. These include call priority control,late entry, talking party identification, and dynamicgroup number assignment. It also provides services

such as call diversion, call waiting, and call completionon busy.

II. TETRA Data Facilities

TETRA provides the following data services: CM-data (Circuit Mode Data), SDS (Status and ShortData Service), CONP (Connection Oriented NetworkProtocol), SCLNP (Specific ConnectionLess NetworkProtocol) and PDO (Packet Data Optimized. Theseservices are described in the next sections.

Circuit Mode Data

The Circuit Mode Data service provides an end-to-end circuit. The bandwidth required for this con-nection is reserved and therefore guaranteed. If onetime slot is allocated, the available bandwidth is 7.2kbits/s. Optionally, low or high forward error protec-tion can be performed. This reduces the bandwidthavailable to 4.8 kbits/s and 2.4 kbits/s, respectively.It is possible to allocate more than one of the four slotsto a connection, thereby increasing the bandwidth of

the circuit.

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Realization of a Mobile Data Application in TETRA 249

Status and Short Data Service

With SDS, one can send and receive pre-defined aswell as user-defined status messages. These statusmessages are represented by a 16-bit number. There-fore they use very little bandwidth.

Additionally, data messages of up to 256 bytes can

be sent. These messages can be sent to individualstations or to groups of stations.

Connection Oriented Network Protocol

TETRA CONP provides a translation of the prim-itives and parameters of the OSI CONS (ISO 8348 /X.213) and the packets of the X.25 Packet Layer Pro-tocol (ISO 8208). This mapping is done through ISO8878 / X.223.

Services provided are Virtual Call Setup, QoS ne-

gotiation and Data transfer.

Specific ConnectionLess Network Protocol

SCLNP offer a connectionless data transfer ser-vice. Therefore, no connection setup is needed. Themaximum length of each data packet is 2048 bytes.TETRA provides a confirmation of the uplink trans-fer.

Additional services are prioritization of messages,multicast messages, area selection of messages, timestamping and sub-addressing.

Packet Data Optimized

The Packet Data Optimized (PDO) specificationonly supports packet data services. It does not in-teroperate with the other TETRA protocols but it isbased on the same physical radio platform. PDO andthe other TETRA protocols must be assigned differ-ent carrier frequencies.

III. Nokia Status and Short Data ServiceInterface

The Nokia Status and Short Data Service Inter-face (SDSI)[4] is an interface provided by Nokia tointerface with Nokia TETRA equipment. It is imple-mented as a COM (Component Ob ject Model) libraryand requires Microsoft Windows 95/98/NT.

The PC running the SDSI must be connected to ei-ther a Nokia TETRA mobile system or a Nokia DSC(Dispatch System Controller) with a RS232/RS422 se-rial link. With the SDSI, Windows applications cansend and receive SDS messages. The maximum size

of a message is 127 bytes.

IV. TCP/IP

The TCP/IP protocol hardly needs an introduc-tion. It is the most widely used communications pro-tocol nowadays. Every major company involved indata communications sees TCP/IP as the protocol of choice for its products. Its best-known application is

of course the Internet, but it is also used more andmore in private communications networks of compa-nies, government agencies and other institutions.

One of the biggest strengths of TCP/IP is that itwas designed to be used on any kind of network infras-tructure, connecting any kind of computer. The pro-tocols are robust, can work with unreliable commu-nications links and facilitate the building of networksthat keep working even if part of the infrastructurebreaks down.

TCP/IP protocols

The lowest protocol layer in TCP/IP is IP (InternetProtocol). This layer corresponds to the layer 3 (net-work layer) of the OSI layering model. IP providesa connectionless, unreliable data transfer service tothe upper layers. It is the responsibility of the layersabove IP to make sure data is transferred reliably (if required).

The most commonly used layer 4 TCP/IP pro-tocols are TCP and UDP. TCP provides a reliable

connection-oriented service. It contains mechanismsto deal with the unreliability of the service providedby IP. Most applications and services use TCP be-cause of its reliability and the fact that it achievesvery good performance by tuning itself to the prop-erties of the underlying network. UDP provides anunreliable connectionless service. It used most oftenfor very short messages like name lookup (where theTCP overhead would be high), or when very specificquality-of-service requirements must be met (for in-stance, when a low delay is more important than re-

liability).

TCP/IP Programming interfaces

On a computer, the TCP/IP protocol is usually im-plemented as part of the Operating System. Applica-tions access this functionality through an API (Ap-plication Programming Interface). The BSD Socketinterface, introduced with the 4.2BSD release of theUNIX operating system, is the most widely used APIto use TCP/IP. A derivative of this API is WindowsSocket Interface (Winsock), designed for Microsoft

Windows.

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Request: all systems 150–250 bytesReply: no hits or error 150–200 bytesReply: short hitlist 200–250 bytesReply: medium hitlist 300–400 bytesReply: long hitlist > 400 bytesReply: full answer 250–450 bytes

TABLE ISize of Mobipol Queries / Replies

V. Mobipol

Mobipol is a computer application that can be usedby police officers to access central information sys-tems like RDW (number-plates), OPS (wanted peo-ple), CVI and NSIS (National Schengen Information

System). It is also possible to access regional systemsand exchange messages. Mobipol was developed byITO, the IT-organization of the Dutch police.

The Mobipol application communicates with a Mo-bipol server, which in turn is connected to variousinformation systems. Therefore, communications isvery simple from the mobile point of view: It onlyconnects to one system and uses one protocol. Cur-rently, GSM and RAM Mobitex communications in-frastructures are used for the connection between theapplication and the server. These are commercial sys-

tems which do not provide the level of confidentialityand authentication required for this kind of applica-tion. Therefore, the Mobipol application has built-inencryption and authentication functions. Before Mo-bipol can be used, one must login with a personalusername and password. In the future TETRA willprovide the communications infrastructure of choicefor Mobipol.

Mobipol is a 32-bit Windows application that runson Windows 95/NT. It uses the Winsock API to com-municate with the Mobipol server using TCP/IP. De-

pending on what data communications network isused, the PC is equipped with a special WinsockTCP/IP-stack for mobile use.

The data format used by Mobipol to transmit in-formation has been optimized for mobile use. TableI lists the typical size of Mobipol queries and replies.Because Mobipol is optimized for mobile use, the re-quired communications bandwidth is quite small.

VI. Design

The hardware on which the demonstration system

must be realized is shown in figure 2. A mobile PC is

connected to a mobile TETRA system, and a gatewayPC connects the TETRA base station with a localLAN. The local LAN provides a connection to theMobipol server.

Because the Nokia SDSI (which provides only SDSfunctionality) is to be used, only the SDS protocol is

available. Another limitation is that it is not possi-ble to make changes to the Mobipol application. Fig-ure 3 shows a block diagram of the software archi-tecture. The TCP/IP connectivity is realized using“TCP port-forwarding”. The Mobile PC has a lo-cal TCP/IP subsystem using a loopback driver. TheMobipol application is configured to connect to theMobipol server “localhost”. The SDSI–TCP/IP Gate-way on the Mobile PC pretends it is a Mobipol server.When the Mobipol application starts communicating,this gateway encapsulates the data into SDS messages

and sends them through the TETRA system to theGateway PC. The Nokia SDSI library is used to ac-complish this. When the Gateway PC receives thedata, it initiates a TCP connection to the real Mo-bipol server using the LAN/WAN.

The advantages of this approach are:

• Efficient use of bandwidth. No IP headers have tobe sent over TETRA, lowering the overhead.

• No IP-numbers need to be assigned to the MobilePCs.

• The Mobipol application uses a regular TCP/IP

stack which reduces the chance of incompatibilities.• Mobile and Gateway software is almost the same

which lowers the amount of necessary coding.

• The software runs entirely as a user application.Therefore programming and debugging is mademuch easier, and the software does not need anyadjustments to run on Windows 95, Windows NTor future versions of Windows.

Disadvantages of this approach are:

• The Mobile PC’s connectivity is limited to systemsspecifically configured on the SDSI–TCP/IP Gate-

way software. This does not affect the Mobipol,since it is designed to use only one server.

Alternative Designs

Two other designs to support Mobipol on TETRAwere considered: writing of a Windows NT networkdevice driver and writing a new Winsock implemen-tation specifically targeted at the Nokia TETRA sys-tem.

With a Windows NT network device driver is itpossible to provide full IP connectivity over TETRA.

The Point-To-Point Protocol (PPP) which is a built-

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Gateway PC

DSC

DXT

TETRA Base Station

TETRA Mobile

RS232

RS232

Mobile PC

MOBIPOL server Databases

LAN / WAN

Fig. 2. Architecture of Mobipol over TETRA

MOBIPOL client

Mobile PC Gateway PC MOBIPOL server

MOBIPOL serverSDSI <-> TCP/IP Gateway (server side)SDSI <-> TCP/IP Gateway (client side)

TETRA Mobile

Nokia SDSINokia SDSI Nokia SDSINokia SDSI Network DriverNetwork

Loopback Driver

TCP/IP(Winsock)

TCP/IP(Winsock)

TETRA Network LAN / WAN

Fig. 3. Software Architecture

in part of the Microsoft TCP/IP-stack can be used toprovide auto-configuration and header compression.The task of the device driver is to exchange PPP-

frames between the TCP/IP-stack and the NokiaSDSI. Though this solution provides full TCP/IP con-nectivity between the Mobile PC and the LAN/WAN,this solution was rejected because of practical consid-erations: writing of Windows NT device drivers is acomplicated and tedious process.

The other solution that was considered is to writea new Winsock DLL (Dynamic Link Library). The

Mobipol application would use this DLL instead of the standard Winsock DLL. This has the advantagethat it is not necessary to have a TCP/IP stack on theMobile PC; the new DLL simply has no provide thecorrect interface (Winsock API) to the Mobipol appli-cation. This approach allows efficient use of TETRAbandwidth. A drawback of this approach is that writ-ing of a Winsock DLL can be complicated (especiallybecause multiple versions of the Winsock API existsand can be used simultaneously). Furthermore, com-patibility problems could arise if either the Mobipol

application or Windows itself were to be upgraded.

VII. Implementation and Test Results

The main task of the SDSI–TCP/IP Gateway is toprovide a reliable stream service on top of an unre-liable datagram service (SDS). The protocol hat wasdesigned to achieve this will be described briefly in thefollowing sections. A more detailed description can befound in [3].

CRC32

Data

8 bits

Type Ack Sequence Number / Control Number

Connection ID

Fig. 4. SDS Message Format

Protocol

Figure 4 shows format of the SDS messages. The

header has been kept as small as possible. Source and

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destination addresses are not needed in the messages,because we can use the TETRA addresses for thatpurpose. There is no need for a length field either,since TETRA provides that information as well.

The Type bit indicates whether the message is adata (0) or control (1) message. The Ack bit indicates

that the message is an acknowledgment to anothermessage. The meaning of the third field, Sequence

Number / Control Number depends on whetherthe message is a data or control message. In data mes-sages this field provides a sequence number that allowsthe system to reassemble the data stream correctly. Incontrol messages this field indicates the type of controlmessages. Defined types are Begin Connection, aAccept Connection and Reset Connection.

A connection is uniquely identified by the MobileTETRA Address, the Gateway TETRA Address andthe field Connection ID. This allow up to 256 si-multaneous connections per Mobile PC. The integrityof received SDS messages is verified using a CRC-32checksum in the field Crc32.

After the header of the message comes a variablenumber of data bytes. The total size of an SDS mes-sages is limited to 127 bytes by the TETRA system.

When the client-side gateway receives a connec-tion request from the Mobipol client, it sends a Be-

gin Connection control SDS message. This mes-

sage contains the TCP port number on which theconnection request was received. The server-sidegateway uses this to make a TCP/IP connection tothe target host. When this TCP/IP connection isestablished, the server-side gateway sends an Ac-

cept Connection control message to the client-sidegateway. When the connection fails somehow, a Re-

set Connection control message is sent instead.

After succesful establishment of a connection, thegateways exchange SDS messages. Acknowledgment

messages are sent to indicate succesful transfer. Whena gateway does not receive and acknowledgmentwithin a certain timeout-period, the message is re-sent. The timeout-period is adjusted dynamicallyusing measurements of the round-trip time of previ-ous messages. The algorithm used is the same as inTCP[5].

Flow control was omitted from the protocol on pur-pose. The Mobipol client and server systems can pro-cess data orders of magnitude faster than the TETRAcan transmit the data. Therefore there is no need for

it.

Realization

The SDSI–TCP/IP gateway software was writtenusing Microsoft Visual C++ and uses a message-driven approach. Every time an SDS message or TCPdata is received, the gateway is notified by eitherthe Nokia SDSI through COM (Component Object

Model) or the Microsoft TCP/IP stack. The gatewayuses a state machine to implement the protocol cor-rectly. Special measures were taken not to upset theTETRA system. It turned out that sending a largenumber of SDS message in a short time could causethe loss of many messages or even malfunction of theTETRA Mobile station.

Test Results

Testing of the gateway software was performed

in Driebergen at the TETRA Test Configuration of ITO. The software was installed on a Mobile PC(a Pentium-class notebook) connected to a NokiaTETRA Mobile. It was also installed on a Gate-way PC (also a Pentium machine) connected to theDSC/DXT/Base Station. This Gateway PC also hasan Ethernet connection to the ITO LAN. Throughthis LAN communications with the Mobipol serverare possible.

The system was tested with Mobipol, Internet Ex-plorer and a Microsoft Newsreader. The last two ap-

plications are not intended to be used with this soft-ware. However, it is not possible to stress-test it withMobipol because the amount of data traffic generatedis quite low (a few hundred bytes per request, whichhave to be typed in manually). The throughput of the system was measured by downloading a large fileusing Internet Explorer. Several runs showed speedsvarying between 20 and 100 bytes per second.

With Mobipol the situation is different: The Mo-bipol server usually needs 5 to 30 seconds to gener-ate responses to queries. The reason for this is that

the Mobipol server has to connect to several back-enddatabases. These databases can be slow and/or thecommunications links to them can be slow. There-fore, throughput of the communications link is not soimportant here. The extra delay in getting responsesas perceived by the user is more important. By us-ing the Mobipol client application and observing thedebugging output generated by the software an es-timate could be made on the extra delay caused bythe TETRA system. This delay varied between 1 and5 seconds. This means that Mobipol requests over

TETRA typically take 20% more time than Mobipol

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requests over the LAN. For demonstration purposesthis is acceptable.

During testing a number of problems with theTETRA system were observed. Most of these can becontributed to the fact the system is under develop-ment and not entirely bug-free. A severe problem is

that the extensive use of SDS messages places a heavyload on the TETRA control channel. This was evennoticable when only one Mobile PC was used at atime. Therefore it is not likely that the use of severalMobile PCs at a time will be acceptable. In principlea TETRA Mobile station can send SDS messages dur-ing a speech call by means of the 18th frame. This,however, is not implemented yet in the TETRA TestConfiguration in Driebergen.

VIII. Conclusions

In this paper the design and implementation of ademonstration system for the usage of Mobipol in aTETRA Test System was presented. This implemen-tation works as intended. Although the limited band-width provided by the TETRA system proved to besufficient, it is clear that for large-scale usage of Mo-bipol SDS is not a suitable protocol. For this we ex-pect that the new TETRA packet data protocol [6] isbetter suited.

References

[1] ETSI, TETRA Voice + Data – Designers’ Guide, DraftETR 0.1.0, European Telecommunication Standards Insti-tute (1996).

[2] ETSI, TETRA Voice + Data – Designers’ Guide - Part 2 ,ETR 300-2 0.2.0, European Telecommunication StandardsInstitute (1997).

[3] Eric Lammerts, Realization of a Mobile Application inTETRA, Tech.Rep 011N99, Signals & Systems NT, Electri-cal Engineering Dept., University of Twente, The Nether-lands (1999)

[4] Nokia Telecommunications, Nokia Tetra System: Statusand Shortdata Service Interface (SDSI), Interface Descrip-tion, (1998)

[5] J. Postel, Transmission Control Protocol, STD7,ftp://ftp.isi.edu/in-notes/std/std7.txt (1981)

[6] ETSI, Radio Equipment and Systems (RES); TETRA Voice+ Data; TETRA packet data , EPT WG3(98) 005, EuropeanTelecommunication Standards Institute (1998).

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