Tetr Avs Gsmasc Ipaperv2


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Tetr Avs Gsmasc Ipaperv2

  1. 1. TETRA or GSM-ASCI network for Public Safety Let the users decide TETRA MoU Association Association House South Park Road Macclesfield SK11 6SH United Kingdom www.tetramou.com May 2004
  2. 2. TETRA or GSM-ASCI Page 2 of 13 1. Introduction This white paper has been produced by the TETRA MoU Association in response to a need from the Public Safety and Security radio user community who want to understand whether the proposed enhancements to GSM cellular technology could provide viable radio communications system for PSS operations. The proposed enhancements are known as the ASCI Features (Advanced Speech Call Items) and were originally included in the GSM standards for railway radio communications purposes, then known as GSM-R. Some proponents of GSM-ASCI technology also use the German term “GSM-BOS” to describe the technology. The ASCI feature set consists basically of three features: • Voice Group Call Service • Voice Broadcast Service • Enhanced Multi-Level Precedence and Pre-emption service This paper studies the capabilities of GSM-ASCI technology in comparison with stated and known Public safety user requirements and in comparison with TETRA, which is a purpose-built ETSI standard and technology for mission critical radio operations such as PSS. This paper is addressing aspects like radio coverage, capacity, availability of service, response times, specialised functionality and cost. At the end of the document is a list of published studies and documents to allow readers to make their own conclusions. ---- Although every reasonable effort has been made to ensure the accuracy of the factual information presented in this document, the use of this information is at the liability of the reader. TETRA MoU Association Ltd May 2004
  3. 3. TETRA or GSM-ASCI Page 3 of 13 2. Mission critical user needs The main needs of the mission critical PMR users, such as the ones representing the Public Safety and Security (PSS) forces can be briefly categorised as follows: • specialised functionality in group communications and dispatching, with instant connection and including purpose-built security, dynamic management of talkgroups, emergency calls, prioritisation of communications etc • response times, typical requirements for voice call set-up time are in the range 0.3 to 1 second, with 0.5 s often cited as the requirement for wide area operation • seamless radio coverage throughout the whole served area, including guaranteed availability of coverage under exceptional conditions – including the means to maintain communication during network outage • incident capacity; the need for radio capacity increases during major incidents and accidents and that capacity must be guaranteed to the rescue and law enforcement forces • uncompromised voice quality allowing the listener to recognise who is speaking, even under excessive background noise These requirements – when talking about the Public Safety and Security operations and agencies – should not be taken as “needs” that can either be met fully, partly or not met at all, but rather as mandatory baseline requirements. We should remember that occupational safety of the officers as well as health and life of other citizens of the society may depend on how each of these requirements is fulfilled by the communications tools. Specifically in Europe for the member states of the Schengen Treaty the technical and tactical requirements for cross-border communication of police forces and customs are documented by the Schengen Telecom group. • Efficient cross-border cooperation itself is one of the fundamental requirements of the European PSS users. TETRA MoU Association Ltd May 2004
  4. 4. TETRA or GSM-ASCI Page 4 of 13 3. Radio coverage Two issues are vital in estimating the adequacy of cellular service to the mission critical PMR users: 1. Complete coverage under normal operating conditions 2. Mechanisms to guarantee coverage for rescue forces under exceptional conditions In addition, there is a need to provide dedicated frequencies for Air to Ground and Ground to Air communication for the rescue forces. Dedicated frequencies are needed to prevent the airborne transmitter from causing interference to ground radios even at distances of hundreds of kilometres. Many Public Safety forces also state Air- Ground-Air communication needs as “mandatory”. 3.1 Radio coverage during normal operation It is generally accepted that the current coverage of GSM networks is adequate or can be upgraded to be adequate even in rural areas without major difficulty. The possible issue is rather the adequacy of the radio channel capacity for all the talkgroup members in group communication. Also the battery back-up requirements of PSS are far more extensive than those of commercial mobile services. The number of cells needed to cover an area with GSM is much higher than that with TETRA. The consequences of this fact on continuity of radio communication is discussed later in this document. The channel capacity and spectrum requirements are likewise discussed later in this document. 3.2 Coverage for exceptional conditions and network outage PMR technologies have special arrangements to provide basic radio coverage even under network failure situations, such as Base Station Fallback and Direct Mode Operation (DMO) supported by TETRA systems and radio terminals. Base station fallback operation is not supported by GSM. The question of providing communication tools during network outage remains open. GSM-R in its original railway variant defines DMO to be done by analogue FM channels and those channels for railway usage are identified at the 870 MHz frequency band by a European level spectrum decision. These channels are reserved for railway use only. Other proprietary solutions have been proposed to replace DMO in Public Safety use of GSM-ASCI, such as to use PMR technologies like TETRA DMO. If TETRA DMO would be chosen as the DMO solution, the next question would be: How are the DMO Repeater and TETRA MoU Association Ltd May 2004
  5. 5. TETRA or GSM-ASCI Page 5 of 13 DMO Gateway implemented in GSM-ASCI? It is obvious that any solution would need for example transcoding that typically reduces voice quality. 3.3 Airborne coverage and channels TETRA can easily support dedicated frequencies for Air-Ground-Air communications. And can do this with the same radios. Devoting GSM frequencies to this purpose so that the same frequency is not reused within say 200 km distance likely will be challenging, taking into account for example new need for cross-border coordination of the GSM frequencies. There have been proposals to deploy airborne coverage by use of dedicated GSM cells but these seem to lead either to a large amount of cells or reduced capacity per channel if extended cell range is used. In both cases the availability and cost of GSM spectrum for this purpose will likely become an issue. The likely outcome would be that the Air-Ground-Air communication would need separate PMR radios in any case. TETRA MoU Association Ltd May 2004
  6. 6. TETRA or GSM-ASCI Page 6 of 13 4. Response times and continuity of communication Public Safety radio communication has connection set-up time requirements measured in fractions of a second for well-known reasons related to human safety and occupational safety issues, and the PMR systems like TETRA can support this requirement. The set-up time of a group call in GSM-ASCI is longer – in the order of seconds - and does not meet this requirement. The set-up time could be mitigated by keeping the radio terminal permanently in the group call traffic channel. This method however has some important consequences in capacity calculations that are discussed later in this document. Authentication of talk group parties also seems to have to be sacrificed to achieve faster set-up times. The cell reselection process in GSM-ASCI when radios are in a group call does not happen seamlessly, but instead, the mobile terminal first loses connection to the earlier serving cell and then registers to the new cell, which causes a break in communication. Remembering that the number of cells in a GSM-ASCI network would be many times more than that in a TETRA network this can lead to even frequently lost speech item transmissions to radio terminals. PMR communication is characterised by many short speech items transmitted between the talkgroup members, with typical duration of the speech item transmission being only 5 to 10 seconds. Thus any break of 10 seconds can totally destroy a vitally important message. Further, to ensure that all talkgroup members always hear the communication, a mechanism usually called Late Entry is needed to rejoin lost talkgroup members in the call, for example after each cell reselection in a GSM-ASCI network. In TETRA the Late Entry service is implemented efficiently. In GSM-ASCI the interval between Late Entry messages may be even more than 10 seconds, which means that again a total speech item transmission could be lost. TETRA MoU Association Ltd May 2004
  7. 7. TETRA or GSM-ASCI Page 7 of 13 5. Public Safety radio functionality 5.1 Voice quality The Public Safety users require voice coding that allows the listener to identify the person who is talking and the voice to be received clearly even during excessive background noise around the talking party. The vocoder used in TETRA was specifically designed to reject background noise and experience from the field confirms this. 5.2 Security In law enforcement operations it is of fundamental importance to both ensure privacy of communication and to prevent any malicious attacks against the communications system. TETRA – being a purpose built technology – meets these security needs and specifically meets the requirements of the Schengen Telecom Group with its secure mutual authentication of both radio terminal and base station, its encryption algorithms (one of them being specifically devoted to the European police forces) and end-to-end encryption capability of both one-to-one and group calls. The group call mechanism of GSM-ASCI does not authenticate all participants of the group call. Furthermore, GSM-ASCI can sometimes omit authentication to obtain faster call set-up, leaving a back door open for potential denial of service attacks. GSM-ASCI does not support mutual authentication that would eliminate the possibility of denial of service attacks by fake base station transmitters. Whether the encryption algorithms of GSM are acceptable to the police forces of the Schengen countries is something that the governments of the Schengen countries should perhaps study and make their conclusions. The group call mechanism used in GSM-ASCI connects the dispatchers to the group by use of a conference bridge. Remembering that conference bridges work by doing arithmetic summing of digital voice samples, it looks impossible to use end-to-end encryption in group calls. In TETRA this kind of problem naturally does not exist. Also the possibility for erasing tactical and security information in the terminal during terminal disable – one of the Schengen requirements – seems not to be met by GSM-ASCI. Public cellular systems are based on the paradigm where the operator controls its users, their numbers and subscriptions. Information privacy between the end-user organisations and towards operator staff cannot be guaranteed unless this paradigm is changed. TETRA systems can typically make a difference between technical and TETRA MoU Association Ltd May 2004
  8. 8. TETRA or GSM-ASCI Page 8 of 13 operational management with subscriber management distributed under control of the user organisations. 5.3 Communications features Some features and supplementary services should be considered when evaluating the possible use of GSM-ASCI: • Direct Radio-to-radio communication (Direct Mode Operation, DMO) is missing from GSM and has to be done by proprietary solutions. i.e. by PMR • GSM-ASCI lacks the possibility to send data or status messages to group addresses, but instead the messages have to be distributed individually to each recipient • GSM-ASCI terminals cannot receive signalling when in group call (due to set-up time requirements the terminal may have to be in the group call permanently) and could thus miss e.g. emergency related signalling • The talkgroups in GSM-ASCI are statically defined. This may present a major challenge in some operational scenarios that are based on dynamic talkgroup allocations and would require supplementary service like the TETRA DGNA (Dynamic Group Number Assignment). This becomes most evident during major incidents and disasters or any other occasions requiring quick modifications to normal operational procedures in the field. • As already mentioned the Late Entry message to rejoin a terminal to a GSM-ASCI talkgroup may come so late that it literally is Late Entry – missing a speech item transmission totally • To allow efficient incident communication structures the talkgroups and their dispatchers need to be configurable in a flexible way. The limitation of GSM-ASCI to allow only a maximum 5 dispatchers per talkgroup could pose a difficulty. As seen, there are several issues in which the Public Safety radio users need to consider quite carefully, whether their operational scenarios can actually be supported by GSM-ASCI in a satisfactory way or not. 5.4 Cross-border communication The Schengen requirements apply specifically to cross-border cooperation of police forces and customs. The requirements assume international roaming and common cross-border talkgroups plus present detailed functional requirements. There is no indication on how international talkgroups meeting the Schengen requirements could be built with GSM-ASCI. Most European countries rely on TETRA. TETRA MoU Association Ltd May 2004
  9. 9. TETRA or GSM-ASCI Page 9 of 13 6. Traffic capacity Public Safety operations are group centric and thus require substantial amount of resources for multiple simultaneous group calls. Digital trunking systems like TETRA have solved the resource need effectively by applying quasi-transmission trunking and reserving traffic channels only at sites where group members are present at each moment. Because GSM systems operate at 900 MHz or 1800 MHz frequencies they need many more cells to reach similar coverage than TETRA systems operating at 400 MHz. Due to high existing subscriber density the GSM systems in practice deploy even more cells – the GSM networks apply capacity limited cell planning, microcells etc. The “shifting area” group call in TETRA reserves traffic channel only at sites having registered group members, whereas the GSM-ASCI group call area is fixed and a traffic channel is reserved at every site within the group area for each group. In a GSM-ASCI network the number of reserved traffic channels per talkgroup can thus be e.g. ten times bigger than in a TETRA network. Another important capacity issue is related to the call set-up times required for group calls. Normally the set-up time provided by GSM-R is in the order of seconds, which does not meet the requirements quoted by Public Safety agencies. To overcome this deficiency it has been proposed that group calls should be kept permanently open – i.e. terminals kept in the group call traffic channel – for talkgroups requiring fast call set-up. A study published in 2004 at Helsinki University of Technology makes some interesting conclusions on the required number of radio channels in a GSM-ASCI network that could meet the set-up time requirements of Public safety operations: • As there is no shifting area group call available, the talkgroups designed to cover a large area consume a huge amount of resources, when they reserve group traffic channels at each of the many sites • The sub-second call set-up time requirement can be met only by keeping the radio in the group call traffic channel – “open channel” type of operation. This means that traffic channels must be permanently allocated to all talkgroups needing fast call set-up in every cell. This indicates that the benefit of the trunking effect is totally lost. The study also presents some cost calculations based on these capacity conclusions, ending up with quite impressive figures that indicate that with quite moderate talkgroup area sizes and percentages of talkgroups really requiring fast call set-up, the cost of TETRA MoU Association Ltd May 2004
  10. 10. TETRA or GSM-ASCI Page 10 of 13 the GSM-ASCI infrastructure actually would exceed that of the TETRA infrastructure due to the enormous amount of radio channels. required In addition to investment cost this continuous reservation of channels would naturally reflect on the operational expenditure and user tariffs. The capacity figures presented in the study actually raise another question related to the amount and availability of radio spectrum for this operation in the 900 MHz GSM frequency bands, because the required additional channel capacity likely cannot be assumed to be ready built and left so far unused in the current GSM networks. The channel capacity required for rescue operations during a major incident would be quite substantial taking into account the fixed talkgroups areas and lack of dynamic regrouping in GSM-ASCI. Assuming that the priority mechanisms provided in the network can effectively take the resources away from ordinary users and give those to the rescue forces, we come to the question whether the ordinary subscribers of the network can actually be left without the possibility of mobile communication at the moment when they likely feel to need that desperately. We have to remember that GSM subscribers while in their home country cannot select another network but are tied to the subscription that they happen to have. The GSM service provider would obviously need to consider whether to block normal subscribers from the service during a crisis or to invest in such an amount of spare capacity and spectrum (if such extra spectrum is available) that there is effectively double capacity available waiting for the next disaster. TETRA MoU Association Ltd May 2004
  11. 11. TETRA or GSM-ASCI Page 11 of 13 7. Cost issues 7.1 Network infrastructure cost The common perception is that a GSM network provides lower capital expenditure than digital PMR. Also sharing of network infrastructure with commercial users should give cost benefit. Certainly one can assume that GSM base stations are cheaper than digital PMR base stations simply because of the volume difference. Surprisingly the publicly available data from GSM-R railway projects does not support the perception that GSM when implemented with ASCI features would be more economical than digital PMR. The study from Helsinki University of Technology in 2004 is suggesting that the need for network capacity and also cost would actually be clearly higher in the case of GSM-ASCI requiring fast call set-up time for group calls and group area being reasonably large. It appears in the study, that if more than 30 % of talkgroups require fast call set-up and the group should cover more than 200 to 500 square kilometres, the GSM-ASCI network cost becomes higher and increases very rapidly if the percentage figure and/or area is further increased. It is fair to assume that the existing GSM networks would have some extra capacity available for GSM-ASCI use without new investment, but if the need for new capacity is several TRX modules per base station site, it is very likely that additional network investment is really needed and someone has to cover that cost one way or another. Also the battery-back-up arrangements need to be totally rebuilt for PSS. The possible consequences on national spectrum policy including spectrum pricing and competitive licensing terms in the case of extra spectrum for PSS usage are currently not fully understood. 7.2 Mobile terminals cost Another common perception is that GSM phones are much cheaper than digital PMR radios. However, making a Public Safety radio to meet the specific needs does not seem to depend much on the selected air interface signalling technology when volumes are comparable. This means that for example a police radio with TETRA signalling or police radio with GSM signalling for a market of 500 000 units would be produced at approximately same cost. The GSM-ASCI police radio market would be very limited due to the fact that most European governments already have made their choice in favour of TETRA. Furthermore, to support DMO communication the GSM-ASCI police radio would actually need another radio built in – with consequences on cost, size & battery life. Hence the cost of GSM-ASCI police radios would by common sense be higher than the cost of TETRA police radios, and the number of available models more limited. Naturally, all cost estimations remain only theoretical exercises as long as nobody has actually produced such products. TETRA MoU Association Ltd May 2004
  12. 12. TETRA or GSM-ASCI Page 12 of 13 8. Conclusions From the issues discussed in previous chapters we can make the following conclusions concerning GSM-ASCI and Public Safety usage: • The call set-up time requirements of Public Safety would require the GSM-ASCI terminals to stay permanently on the group call traffic channel. This may lead to drastically bigger capacity needs and cost. Slow cell handovers may compromise officer safety. • The group call in GSM-ASCI is wasting capacity by keeping radio channels reserved at sites having no talkgroup members • GSM-ASCI cannot maintain service in the event of network outage in the same way as TETRA base station fall-back operation mode provides • GSM does not have inherent Direct Mode, and thus add-on solution like PMR channels and radios is needed for the users in any case. • Providing Air-Ground-Air communication with helicopters and airplanes is practically impossible with GSM • GSM even if complemented with the ASCI features cannot meet the level of security that TETRA provides. The authentication mechanism of GSM leaves possibility for malicious fake calls. Only TETRA provides encryption algorithms that meet the Schengen requirements. End-to-end encryption is not supported in GSM- ASCI, especially not for group calls. • Talkgroups of GSM-ASCI cannot be modified dynamically • GSM even if complemented with the ASCI features cannot support SDS or status messaging to group addresses. Neither is any other type of signalling to the terminal supported when the terminal is in group call. • The number of dispatch stations per talkgroup in GSM-ASCI is limited to max 5, which may present a challenge during incidents To summarise, it appears that GSM complemented with the ASCI feature set in Public safety operations would: • have functional deficiencies compared to the stated requirements of the Public Safety user community – and the Schengen Treaty • lack standard fall-back communication mechanisms • lack cross-border communication support • perhaps lead to a massive need for new network capacity • introduce a new technology risk for the radio terminal purchasers TETRA MoU Association Ltd May 2004
  13. 13. TETRA or GSM-ASCI Page 13 of 13 9. Literature The issue of possible use of public mobile networks and GSM derivatives for Public Safety radio communication has been addressed in various studies during recent years. The conclusions made in the studies vary radically from in favour to against. The readers are encouraged to make their own judgment on the basis of the published reports that include for example the following. The Schengen document is the official statement of requirements for the member states of the Schengen Treaty. Konvergering av trådlöse nett. Forstudie for Naerings- og haldelsdepartementet. Nexia/Preview, June 2002. The report is available in Norwegian language at: http://www.odin.dep.no/archive/nhdvedlegg/01/03/Konve041.pdf The Use of Commercial Cellular Mobile Networks as a Solution for Public Safety Users in Norway. Norwegian Ministry of Justice, August 2002. The report is available at: http://odin.dep.no/archive/nhdbilder/01/06/GSMfo052.pdf Oavhengig vurdering av konklusjoner I rapporten “Konvergering av trådlöse nett”. Närings og haldelsdepartementet & Gartner Group, September 2002. Available in Norwegian language at: http://www.odin.dep.no/archive/nhdbilder/01/06/NHDTe071.pdf Funktionell och finansiell analys av alternativa tekniker för gemensamt radiokommunikationsnät för skydd och säkerhet. Stelacon AB, September 2002. Available in Swedish language at address: www.sou.gov.se/rakel/PDF/Stelacon analys alternativa tekniker 20020923.pdf Reti professionali mobili. Quali tecnologie? inRete, Periodico informativo del Centro Tecnico, Presidenza del Consiglio dei Ministri, numero 5, 2003. Available in Italian language at: www.ct.rupa.it/CENTRO-TEC/Periodico- /INRETE_5.pdf English translations of many of the above documents can be found at the website of the TETRA MoU Association at www.tetramou.com (--> TETRA Facts --> Market). The usage of mainstream technologies for public safety and security networks. Simon Riesen, Helsinki University of Technology, 2003. Available both in English and German languages at www.tetramou.com (--> TETRA Facts --> Market). Digital Radio Communications network for Security Organisations (Tactical and Operational Requirements). Schengen working party on Telecommunication, SCH/I-Telecom (95)18, June 1995 TETRA MoU Association Ltd May 2004