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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

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


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.



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



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.



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.



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



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.



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



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.



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.



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



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


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