Institutionen för systemteknik Department of Electrical Engineering Master thesis Design and implementation of a test tool for the GSM traffic channel Theo Öjerteg ISY-LITH-EX-3169-2002 2002-06-04 TEKNISKA HÖGSKOLAN LINKÖPINGS UNIVERSITET Department of Electrical engineering Linköpings tekniska högskola Linköping University Linköpings Universitet S-581 83 Linköping 581 83 Linköping Design and implementation of a test tool for the GSM traffic channel Examensarbete utfört i datatransmission vid Linköpings Tekniska Högskola av Theo Öjerteg Reg nr:LiTH-ISY-EX-3169-2002 Handledare: Michael Lundkvist, Danjel McGougan, Enea Epact Examinator: Ulf Henriksson, ISY Linköping 2002-06-04 Avdelning, Institution Datum Division, Department Date 2002-06-04 Institutionen för Systemteknik 581 83 LINKÖPING Språk Rapporttyp ISBN Language Report category Svenska/Swedish Licentiatavhandling ISRN LITH-ISY-EX-3169-2002 X Engelska/English X Examensarbete C-uppsats Serietitel och serienummer ISSN D-uppsats Title of series, numbering Övrig rapport _____ URL för elektronisk version http://www.ep.liu.se/exjobb/isy/2002/3169/ Titel Design och implementation av ett testverktyg för GSM talkanal. Title Design and implementation of a test tool for the GSM traffic channel. Författare Theo Öjerteg Author Sammanfattning Abstract Todays’ systems for telecommunication are getting more and more complex. Automatic testing is required to guarantee quality of the systems produced. An actual example is the introduction of GPRS traffic in the GSM network nodes. This thesis investigates the need and demands for such an automatic testing of the traffic channels in the GSM system. A solution intended to be a part of the Ericsson TSS is proposed. One problem to be solved is that today’s tools for testing do not support testing of speech channels with the speech transcoder unit installed. As part of the investigation, a speech codec is implemented for execution on current hardware used in the test platform. The selected speech codec is the enhanced full rate codec, generating a bitstream of 12.2 kbit/s, and gives a good trade-off between compression and speech quality. The report covers the design of the test tool and the implementation of speech codec. Particularly performance problems in the imple- mentation of the encoder will be addressed. Nyckelord Keyword GSM, speech codec, traffic generation, test tool, test automation, DSP, telecommunication, TSS, encoder, decoder Contents CHAPTER 1 Preface . 1 CHAPTER 2 Introduction . 3 2.1 Background . 3 2.2 Assignment . 4 2.3 Limitations . 5 2.4 Purpose of this report . 5 2.5 Reading instructions . 5 CHAPTER 3 GSM basics. 7 3.1 Telecommunication overview . 7 3.2 Networking elements. 7 3.3 Interfaces . 9 3.4 Inside the equipment. 10 CHAPTER 4 Speech coding . 13 4.1 General . 13 4.2 Human speech model . 13 4.3 Different codecs. 15 4.4 Enhanced full rate codec . 16 4.5 Speech encoding . 19 4.6 Speech decoding . 22 Design and implementation of a test tool for the GSM traffic channel vii 4.7 Discontinuous Transmission Mode . 23 4.8 Voice Activity Detection . 24 4.9 Substitution and muting . 26 4.10Comfort Noise . 26 4.11Homing . 27 4.12Transmission parameters . 27 CHAPTER 5 Analysis. 29 5.1 TSS overview . 29 5.1.1 Overview . 29 5.1.2 TSS architecture . 29 5.2 Problems. 30 5.3 Use cases . 31 5.3.1 Use case 1. 32 5.3.2 Use case 2. 32 5.3.3 Use case 3. 33 5.3.4 Use case 4. 33 5.3.5 Use case 5. 34 5.3.6 Use case 6. 34 5.3.7 Use case 7. 35 5.3.8 Use case 8. 35 5.4 Requirements . 36 CHAPTER 6 Design . 39 6.1 Proposed solution . 39 6.2 Functional overview . 40 6.2.1 PCM Frame adaption. 40 6.2.2 TRAU/TRAB Frame adaption. 41 6.2.3 Speech codecs . 41 6.2.4 PCM conversion. 42 6.2.5 Traffic generator . 42 6.2.6 Statistics . 43 6.2.7 User interface . 44 6.3 Communication interfaces . 46 6.3.1 A interface . 46 6.3.2 Abis interface . 46 viii Design and implementation of a test tool for the GSM traffic channel 6.3.3 TSS Interface . 47 6.3.4 Signal interface. 47 6.4 Software implementation. 48 6.4.1 External traffic generator . 49 6.4.2 Driver modules . 49 6.4.3 Proxy module . 49 6.4.4 Traffic generator links . 50 6.4.5 Traffic generators . 51 6.4.6 Traffic generator manager . 51 6.4.7 PCI Interface. 52 6.4.8 ESSI Interface . 52 6.4.9 Codecs and frame adapters . 52 CHAPTER 7 Implementation and Test . 53 7.1 Implementation of test tool . 53 7.2 Implementation of speech codec . 54 7.2.1 Multichannel mode . 54 7.2.2 DSP implementation. 54 7.3 Tools . 55 7.4 Problems . 55 7.4.1 Performance of generated code . 55 7.4.2 Size of generated code. 57 7.4.3 Word length . 58 7.4.4 Performance of individual routines . 58 7.5 Testing . 60 7.5.1 Multichannel mode . 60 7.5.2 DSP implementation. 60 7.6 Improvements . 61 CHAPTER 8 Conclusions . 63 8.1 Need of system . 63 8.2 Implementation so far . 63 8.3 Alternative solutions . 64 Design and implementation of a test tool for the GSM traffic channel ix APPENDIX A Generated code examples . 65 APPENDIX B Original C-code . 69 References . 71 Glossary . 75 Index . 79 x Design and implementation of a test tool for the GSM traffic channel CHAPTER 1 Preface Thanks to all people at Enea Epact and Ericsson who have been answering my questions and helping with whatever problem I’ve had. Also a lot of thanks to my wife who has been very support- ive when I really wanted to throw the computer out of the window. Linköping 2002-06-04 Theo Öjerteg Design and implementation of a test tool for the GSM traffic channel 1 2 Design and implementation of a test tool for the GSM traffic channel CHAPTER 2 Introduction 2.1 Background This master thesis work was done as part of the programme for Computer Science and Engineering at Linköping Uni- versity. The employer was Enea Epact, but the real customer was Ericsson Radio Network Center. With the introduction of GPRS in the GSM network, the need for verification of the traffic channels increase. For example, a useful scenario for this new tool would be when there are a lot of GPRS traffic going on through the BSC, then a voice call is being set up and GPRS traffic channels has to be pre-empted. It would also be interesting to use the system for generating background load on the traffic chan- nels, while performing other automatic tests. The thesis consists of two parts. The first part consists of an analysis to find out the needs and to see how it is possible to implement the system. The second part involves implement- ing part of the system. Design and implementation of a test tool for the GSM traffic channel 3 Assignment 2.2 Assignment The customer has a product called Test and Simulation Solutions (TSS), used for testing nodes in the Global Sys- tem for Mobile Communications (GSM) and General Packet Radio Systems (GPRS). The TSS consists of special hard- ware as well as software and is used to simulate traffic in mobile cell phone networks. Within TSS there is a problem when it comes to circuit switched networks like the GSM. The test tool is only able to set up the connections using the signalling channels, the physical channels used for traffic are not verified with this tool. When it comes to packet switched networks, like the GPRS, this problem has been handled. The first assignment was to see how the product TSS could be enhanced with a new tool allowing for verification of the GSM traffic channels. This includes how to insert traffic (i.e. simulated speech) into the physical channel, how to generate it, and how to control the tool in a way that is con- sistent with the rest of the product. To do this, several sub- areas have to be studied. The GSM network itself has to be studied in order to find out scenarios where this new tool can become useful. What standards are there that the new tool must comply to in order to connect to the GSM net- work? And what about the user interface part, what are the common standard the tool must comply to in order to be eas- ily integrated with the TSS? The second assignment for this master thesis was to imple- ment part of the system proposed. In this case the choosen part is the enhanced full rate speech codec, which is the most commonly used codec of cellular phone systems. This part will involve examining the speech compression and decompression algorithms to be able to implement them on the target hardware. 4 Design and implementation of a test tool for the GSM traffic channel Limitations 2.3 Limitations Only the speech encoding and decoding functions will be implemented since this is an area where there is not enough knowledge at the employer or the customer. This is basically one function which can be developed detached from the rest of the test system and easily integrated later. The full test system, with traffic generator, user interface etc., will not be implemented within this master thesis due to limitations in time. Also this area is more well known and the both the employer and the customer has a lot of experi- ence in this area.