ETSI ETS 300 471 ed.1 (1995-12)

Radio Equipment and Systems (RES); Land mobile service; Access protocol, occupation rules and corresponding technical characteristics of radio equipment for the transmission of data on shared channels

DE/RES-02-14

Radijska oprema in sistemi (RES) - Storitev kopenskih mobilnih komunikacij - Dostopovni protokol, pravila zasedenosti in ustrezne tehnične značilnosti radijske opreme za prenos podatkov po kanalih s souporabo

General Information

• Status: Published
• Publication Date: 14-Dec-1995
• Technical Committee: RES 2 - Land Mobile
• Current Stage: 12 - Completion
• Due Date: 31-Dec-1995
• Completion Date: 15-Dec-1995
• Ref Project: SIST ETS 300 471:1998 - Radio Equipment and Systems (RES); Land mobile service; Access protocol, occupation rules and corresponding technical characteristics of radio equipment for the transmission of data on shared channels

Standards Content (Sample)

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Radio Equipment and Systems (RES); Land mobile service; Access protocol, occupation rules and corresponding technical characteristics of radio equipment for the transmission of data on shared channels33.060.20Sprejemna in oddajna opremaReceiving and transmitting equipmentICS:Ta slovenski standard je istoveten z:ETS 300 471 Edition 13SIST ETS 300 471:1998en01-DYJXVW-19983SIST ETS 300 471:1998SLOVENSKI
STANDARD



SIST ETS 300 471:1998



EUROPEANETS 300 471TELECOMMUNICATIONDecember 1995STANDARDSource: ETSI TC-RESReference: DE/RES-02-14ICS:33.060.20Key words:access, data, mobile, protocol, radio, testingRadio Equipment and Systems (RES);Land mobile service;Access protocol, occupation rules and corresponding technicalcharacteristics of radio equipment for thetransmission of data on shared channelsETSIEuropean Telecommunications Standards InstituteETSI SecretariatPostal address: F-06921 Sophia Antipolis CEDEX - FRANCEOffice address: 650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCEX.400: c=fr, a=atlas, p=etsi, s=secretariat - Internet: [e-mail].: [phone] - Fax: [phone]Copyright Notification: No part may be reproduced except as authorized by written permission. The copyright and theforegoing restriction extend to reproduction in all media.© European Telecommunications Standards Institute 1995. All rights reserved.SIST ETS 300 471:1998



Page 2ETS 300 471: December 1995Whilst every care has been taken in the preparation and publication of this document, errors in content,typographical or otherwise, may occur. If you have comments concerning its accuracy, please write to"ETSI Editing and Committee Support Dept." at the address shown on the title page.SIST ETS 300 471:1998



Page 3ETS 300 471: December 1995ContentsForeword.51Scope.72Normative references.73Definitions.84General.84.1Sharing speech/data.84.2Sharing data/data.84.3Conformity to this ETS.84.4Interpretation of the measurement results.95Technical characteristics of the equipment.96Access protocol.96.1General.96.2Principles.96.3Procedure.96.4Carrier sensing.106.5Observation time.106.6Initiation of the transmitter.116.7Duration of the RF channel occupation (time interval).116.7.1Transmissions of data packets exceeding 300 ms.116.7.2Transmissions of data packets not exceeding 300 ms (speech/datachannels).117Examples of sharing situations.128Methods of measurement for receiver delays.178.1Carrier sense delay.178.1.1Definition.178.1.2Method of measurement.178.1.3Limit.188.2Receiver opening delay.188.2.1Definition.188.2.2Method of measurement.188.2.3Limit.199Measurement uncertainty.19Annex A (normative):
General arrangements for measurements involving the use of radiatedfields.20A.1Test fixture.20A.1.1Description.20A.1.2Calibration.20A.1.3Mode of use.21A.2Acoustic coupler.21A.2.1General.21A.2.2Description.21A.2.3Calibration.22A.3Bibliography.22SIST ETS 300 471:1998



Page 4ETS 300 471: December 1995Annex B (informative):Comments concerning the random part of the observation time.23Annex C (informative):Comments concerning the traffic loading of shared channels.24History.25SIST ETS 300 471:1998



Page 5ETS 300 471: December 1995ForewordThe access protocol described in this ETS is based on the access protocol published as annex F ofI-ETS 300 113 [3].Transposition datesDate of adoption of this ETS:17 November 1995Date of latest announcement of this ETS (doa):31 March 1996Date of latest publication of new National Standardor endorsement of this ETS (dop/e):30 September 1996Date of withdrawal of any conflicting National Standard (dow):30 September 1996This ETS can be implemented for equipment meeting the requirements of ETSI standardse.g. ETS 300 113 [1] and ETS 300 390 [2].SIST ETS 300 471:1998



Page 6ETS 300 471: December 1995Blank pageSIST ETS 300 471:1998



Page 7ETS 300 471: December 19951ScopeThis ETS applies to equipment designed to operate within the professional mobile radio service and to theassociated frequency planning.This ETS applies to equipment for the transmission of data on shared channels; it also contains twomethods of measurement used for the assessment of receiver timing parameters.This ETS specifies an access protocol and occupation rules for data communications on radio channelsshared by different users. It may be used for data communications over channels originally intended forspeech use. This ETS gives freedom for the use of any bit rate, any constant envelope modulation or anytype of protocol which fulfills the normative parameters of this ETS to access a shared radio channel.The access protocol specified in this ETS also permits the sharing of speech and data communication.This access protocol applies to single frequency simplex operation (and two frequency repeateroperations with the repeater in duplex mode and the mobile units in simplex mode). This access protocolis applicable for:-multiple data only users, independent of each other, which do not share a common central controlfacility, but may share a common single or two frequency radio channel;-multiple mixed analogue speech and data users, independent from each other, which do not sharea common central control facility, but may share a common single, or two frequency, radiochannel and where speech is to have priority over data transmissions.This access protocol is not applicable for data users with common central control facilities or for trunkedsystems operating on dedicated non shared channels.In the case of analogue transmissions, the corresponding access protocol is known as the"radio-discipline" of the users.Within the limits set out in this ETS, each group of users may use its own communication protocol.2Normative referencesThis ETS incorporates by dated and undated reference, provisions from other publications. Thesenormative references are cited at the appropriate places in the text and the publications are listedhereafter. For dated references, subsequent amendments to or revisions of any of these publicationsapply to this ETS only when incorporated in it by amendment or revision. For undated references the latestedition of the publication referred
to applies.[1]prETS 300 113 (1995): "Radio Equipment and Systems (RES); Land mobileservice; Technical characteristics and test conditions for radio equipmentintended for the transmission of data (and speech) and having an antennaconnector".[2]prETS 300 390 (1995): "Radio Equipment and Systems (RES); Land mobileservice; Technical characteristics and test conditions for radio equipmentintended for the transmission of data and speech and using an integralantenna".[3]I-ETS 300 113 (1992): "Radio Equipment and Systems (RES); Land mobileservice Technical characteristics and test conditions for non-speech andcombined analogue speech/non-speech equipment with an internal or externalantenna connector, intended for the transmission of data".SIST ETS 300 471:1998



Page 8ETS 300 471: December 19953DefinitionsFor the purpose of this ETS, the following definitions apply:bit: Binary digit.block: The smallest quantity of information sent over the radio channel. A constant number of useful bitsare always sent together with the corresponding redundancy bits.packet: One block or a contiguous stream of blocks sent by one (logical) transmitter to one particularreceiver or one particular group of receivers.burst (physical): Transmission of a small number of consecutive packets within a period not exceeding300 ms in accordance with this access protocol.transmission (physical): One or several packets transmitted between power on and power off of aparticular transmitter.window: A set of inter-related transmissions resulting from the action of the "initiating transmitter", andlimited in time by the access protocol and corresponding occupation rules to a duration of tt + Dtt.session: A set of inter-related exchanges of packets occupying one or several windows or parts thereof (ifapplicable). It corresponds to a complete interactive procedure for interchanging data between users,comprising initiation, data transmission and termination procedures. The session can be short(e.g. 2 packets) or long (e.g. one full page of text).message: User data to be transferred in one or more packets in a session.initiating transmitter: The initiating transmitter is the station that has completed the "observation time"(see subclause 6.5) and therefore starts a transmission. This initiates a window and triggers the timer tt.reply: A transmission by a station as an answer to the "initiating transmitter". This reply can be anacknowledge ("ACK") or a negative acknowledge ("NACK") or a longer packet of useful information.4General4.1Sharing speech/dataThis access protocol gives speech priority over data on mixed speech/data channels. In order to limitannoyance to speech users, the duration of data transmissions shall be limited in accordance withsubclause 6.7.4.2Sharing data/dataOn frequencies assigned only to data communication users (without common central control facilities) theaccess protocol provides access to independent users with equal priority.4.3Conformity to this ETSA signed declaration shall be provided as a confirmation that the equipment meets the requirements ofthis access protocol. This may be submitted by the manufacturer with the application form for tests.In the case where the controlling software for the equipment has not been engineered by themanufacturer of the radio equipment, the party responsible for engineering the controlling software shallprovide a signed declaration that the equipment meets the requirements of this access protocol.Measurements of receiver carrier sense delay and opening delay shall be performed in an accredited testlaboratory, possibly together with the tests corresponding to ETS 300 113 [1].SIST ETS 300 471:1998



Page 9ETS 300 471: December 19954.4Interpretation of the measurement resultsThe interpretation of the results recorded in a test report for the measurements described in this ETS shallbe as follows:a)the measured value related to the corresponding limit shall be used to decide whether anequipment meets the minimum requirements of this ETS;b)the actual measurement uncertainty of the test laboratory carrying out the measurement, for eachparticular measurement, shall be included in the test report;c)the values of the actual measurement uncertainty shall be, for each measurement, equal to or lowerthan the figures given in clause 9 (absolute measurement uncertainties).5Technical characteristics of the equipmentThe timing requirements that the equipment shall fulfil in order to operate correctly and efficiently can befound in ETS 300 113 [1], subclauses 5.1.7 and 5.1.8, ETS 300 390 [2], subclauses 5.1.5 and 5.1.6 andsubclauses 8.1.3 and 8.2.3 of this ETS.The corresponding methods of measurement can be found in ETS 300 113 [1], subclauses 8.8 and 8.9,ETS 300 390 [2], subclauses 8.5 and 8.6 and in subclauses 8.1 and 8.2, of this ETS.6Access protocol6.1GeneralThe access protocol shall be used for each occupation of the Radio Frequency (RF) channel for sharingdata/data and for sharing speech/data with automatic channel access.6.2PrinciplesThe equipment determines whether or not the channel is, and has been, idle for a certain period (theobservation time) by means of carrier sensing.The observation time consists of a fixed part and a randomly selected part. When the channel stillappears to be idle at the end of the observation time, the transmitter is initiated and powered up within aspecified time.The duration of the emission is limited (see subclause 6.7).6.3ProcedureThe equipment shall determine whether or not the channel is, and has been, idle for a certain period (theobservation time to), by means of carrier sensing (see subclause 6.4). The observation time to consists ofa fixed part tf and a randomly selected part tr.If the channel is occupied during part of the observation time, the process shall be repeated (withoutchanging m (see subclause 6.5).If the channel appears to be idle throughout the observation time, the transmitter shall be initiated andpowered up within the specified time (attack time; see ETS 300 113 [1], subclause 8.8 or ETS 300 390 [2],subclause 8.5). The channel can then be seized for the duration of one time interval. The maximum lengthtt of this interval depends on the frequency sharing category (data/speech, data/data).If a re transmission is required (due, for example, to a "collision", i.e. a simultaneous channel access byseveral users), the observation process shall be repeated and the channel shall be detected as idle priorto a repetition of a transmission.SIST ETS 300 471:1998



Page 10ETS 300 471: December 1995Within one time interval, the following radio traffic may take place:a)from a base station to one or several mobiles;b)from a mobile to a base station;c)between mobiles.To ensure that no other user can access the channel during a time interval, the reversion time tc, the timebetween transmission of a message and reception of the corresponding acknowledgement and/or reply,shall not exceed 50 ms. The reversion time tc is the time between the switch off of one transmitter and theswitch on of the other.
The points of switch off and switch on are considered to be the points at which thetransmitter power is 50 % of the rated carrier power.6.4Carrier sensingCarrier sensing is the detection of whether the RF level in the receive channel exceeds a given threshold.The carrier sense shall be able to detect RF signals with different types of modulation (e.g. F3E, G3E,F1D, F2D, G1D).The channel shall be regarded as in use during the observation time (see subclause 6.5) if the level of theRF signal on the channel exceeds a certain level.
For equipment meeting ETS 300 113 [1] or I-ETS 300113 [3], this threshold level shall be as defined in table 1.Table 1: Threshold levelsBandThreshold level indBmmV emf30 - 137 MHz12> 137 - 300 MHz6> 300 MHz0The carrier sense delay is specified in subclause 8.1.3.6.5Observation timeThe observation time shall start within 10 ms after each time that the RF channel has become idle. It shallalso start at power on.The observation time to is the sum of the fixed part tf and the random part tr = n ´ tito = tf + n ´ ti.The fixed part, tf, of the observation time shall be:-on pure data channels:60 ms ± 1 ms;-on combined speech/data channels:2 000 ms ± 1 ms;except as specified in subclause 6.7.2.The increment time ti shall be 50
ms ± 0,1
ms.The number n is a random integer number from 1 to m (1 £ n £ m); this means that 1 to m is the eventfield of the random number n. The random number n shall be determined by use of a random generatorwith a uniform distribution (see also annex B).SIST ETS 300 471:1998



Page 11ETS 300 471: December 1995To achieve short delays during low traffic, the observation time should be short, i.e. m should be small.Therefore in this access protocol, m shall be set to 4 for the first trial.A short random part of the observation time however, increases the probability of several userssimultaneously accessing the channel for a time interval.Therefore, when a re transmission i
...

SLOVENSKI STANDARD
SIST ETS 300 471:1998
01-oktober-1998
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Radio Equipment and Systems (RES); Land mobile service; Access protocol, occupation
rules and corresponding technical characteristics of radio equipment for the transmission
of data on shared channels
Ta slovenski standard je istoveten z: ETS 300 471 Edition 1
ICS:
33.060.20 Sprejemna in oddajna Receiving and transmitting
oprema equipment
SIST ETS 300 471:1998 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST ETS 300 471:1998

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SIST ETS 300 471:1998
EUROPEAN ETS 300 471
TELECOMMUNICATION December 1995
STANDARD
Source: ETSI TC-RES Reference: DE/RES-02-14
ICS: 33.060.20
access, data, mobile, protocol, radio, testing
Key words:
Radio Equipment and Systems (RES);
Land mobile service;
Access protocol, occupation rules and corresponding technical
characteristics of radio equipment for the
transmission of data on shared channels
ETSI
European Telecommunications Standards Institute
ETSI Secretariat
F-06921 Sophia Antipolis CEDEX - FRANCE
Postal address:
650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCE
Office address:
c=fr, a=atlas, p=etsi, s=secretariat - [e-mail]
X.400: Internet:
Tel.: [phone] - Fax: [phone]
Copyright Notification: No part may be reproduced except as authorized by written permission. The copyright and the
foregoing restriction extend to reproduction in all media.
© European Telecommunications Standards Institute 1995. All rights reserved.
New presentation - see History box

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SIST ETS 300 471:1998
Page 2
ETS 300 471: December 1995
Whilst every care has been taken in the preparation and publication of this document, errors in content,
typographical or otherwise, may occur. If you have comments concerning its accuracy, please write to
"ETSI Editing and Committee Support Dept." at the address shown on the title page.

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SIST ETS 300 471:1998
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ETS 300 471: December 1995
Contents
Foreword .5
1 Scope .7
2 Normative references.7
3 Definitions.8
4 General.8
4.1 Sharing speech/data.8
4.2 Sharing data/data.8
4.3 Conformity to this ETS.8
4.4 Interpretation of the measurement results.9
5 Technical characteristics of the equipment.9
6 Access protocol.9
6.1 General .9
6.2 Principles .9
6.3 Procedure .9
6.4 Carrier sensing.10
6.5 Observation time.10
6.6 Initiation of the transmitter .11
6.7 Duration of the RF channel occupation (time interval).11
6.7.1 Transmissions of data packets exceeding 300 ms .11
6.7.2 Transmissions of data packets not exceeding 300 ms (speech/data
channels).11
7 Examples of sharing situations .12
8 Methods of measurement for receiver delays.17
8.1 Carrier sense delay.17
8.1.1 Definition .17
8.1.2 Method of measurement .17
8.1.3 Limit.18
8.2 Receiver opening delay.18
8.2.1 Definition .18
8.2.2 Method of measurement .18
8.2.3 Limit.19
9 Measurement uncertainty.19
Annex A (normative): General arrangements for measurements involving the use of radiated
fields .20
A.1 Test fixture .20
A.1.1 Description.20
A.1.2 Calibration.20
A.1.3 Mode of use .21
A.2 Acoustic coupler.21
A.2.1 General .21
A.2.2 Description.21
A.2.3 Calibration.22
A.3 Bibliography.22

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ETS 300 471: December 1995
Annex B (informative): Comments concerning the random part of the observation time. 23
Annex C (informative): Comments concerning the traffic loading of shared channels. 24
History. 25

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ETS 300 471: December 1995
Foreword
The access protocol described in this ETS is based on the access protocol published as annex F of
I-ETS 300 113 [3].
Transposition dates
Date of adoption of this ETS: 17 November 1995
Date of latest announcement of this ETS (doa): 31 March 1996
Date of latest publication of new National Standard
or endorsement of this ETS (dop/e): 30 September 1996
Date of withdrawal of any conflicting National Standard (dow): 30 September 1996
This ETS can be implemented for equipment meeting the requirements of ETSI standards
e.g. ETS 300 113 [1] and ETS 300 390 [2].

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ETS 300 471: December 1995
Blank page

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ETS 300 471: December 1995
1 Scope
This ETS applies to equipment designed to operate within the professional mobile radio service and to the
associated frequency planning.
This ETS applies to equipment for the transmission of data on shared channels; it also contains two
methods of measurement used for the assessment of receiver timing parameters.
This ETS specifies an access protocol and occupation rules for data communications on radio channels
shared by different users. It may be used for data communications over channels originally intended for
speech use. This ETS gives freedom for the use of any bit rate, any constant envelope modulation or any
type of protocol which fulfills the normative parameters of this ETS to access a shared radio channel.
The access protocol specified in this ETS also permits the sharing of speech and data communication.
This access protocol applies to single frequency simplex operation (and two frequency repeater
operations with the repeater in duplex mode and the mobile units in simplex mode). This access protocol
is applicable for:
- multiple data only users, independent of each other, which do not share a common central control
facility, but may share a common single or two frequency radio channel;
- multiple mixed analogue speech and data users, independent from each other, which do not share
a common central control facility, but may share a common single, or two frequency, radio
channel and where speech is to have priority over data transmissions.
This access protocol is not applicable for data users with common central control facilities or for trunked
systems operating on dedicated non shared channels.
In the case of analogue transmissions, the corresponding access protocol is known as the
"radio-discipline" of the users.
Within the limits set out in this ETS, each group of users may use its own communication protocol.
2 Normative references
This ETS incorporates by dated and undated reference, provisions from other publications. These
normative references are cited at the appropriate places in the text and the publications are listed
hereafter. For dated references, subsequent amendments to or revisions of any of these publications
apply to this ETS only when incorporated in it by amendment or revision. For undated references the latest
edition of the publication referred to applies.
[1] prETS 300 113 (1995): "Radio Equipment and Systems (RES); Land mobile
service; Technical characteristics and test conditions for radio equipment
intended for the transmission of data (and speech) and having an antenna
connector".
[2] prETS 300 390 (1995): "Radio Equipment and Systems (RES); Land mobile
service; Technical characteristics and test conditions for radio equipment
intended for the transmission of data and speech and using an integral
antenna".
[3] I-ETS 300 113 (1992): "Radio Equipment and Systems (RES); Land mobile
service Technical characteristics and test conditions for non-speech and
combined analogue speech/non-speech equipment with an internal or external
antenna connector, intended for the transmission of data".

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ETS 300 471: December 1995
3 Definitions
For the purpose of this ETS, the following definitions apply:
bit: Binary digit.
block: The smallest quantity of information sent over the radio channel. A constant number of useful bits
are always sent together with the corresponding redundancy bits.
packet: One block or a contiguous stream of blocks sent by one (logical) transmitter to one particular
receiver or one particular group of receivers.
burst (physical): Transmission of a small number of consecutive packets within a period not exceeding
300 ms in accordance with this access protocol.
transmission (physical): One or several packets transmitted between power on and power off of a
particular transmitter.
window: A set of inter-related transmissions resulting from the action of the "initiating transmitter", and
limited in time by the access protocol and corresponding occupation rules to a duration of t + Δt .
t t
session: A set of inter-related exchanges of packets occupying one or several windows or parts thereof (if
applicable). It corresponds to a complete interactive procedure for interchanging data between users,
comprising initiation, data transmission and termination procedures. The session can be short
(e.g. 2 packets) or long (e.g. one full page of text).
message: User data to be transferred in one or more packets in a session.
initiating transmitter: The initiating transmitter is the station that has completed the "observation time"
(see subclause 6.5) and therefore starts a transmission. This initiates a window and triggers the timer t .
t
reply: A transmission by a station as an answer to the "initiating transmitter". This reply can be an
acknowledge ("ACK") or a negative acknowledge ("NACK") or a longer packet of useful information.
4 General
4.1 Sharing speech/data
This access protocol gives speech priority over data on mixed speech/data channels. In order to limit
annoyance to speech users, the duration of data transmissions shall be limited in accordance with
subclause 6.7.
4.2 Sharing data/data
On frequencies assigned only to data communication users (without common central control facilities) the
access protocol provides access to independent users with equal priority.
4.3 Conformity to this ETS
A signed declaration shall be provided as a confirmation that the equipment meets the requirements of
this access protocol. This may be submitted by the manufacturer with the application form for tests.
In the case where the controlling software for the equipment has not been engineered by the
manufacturer of the radio equipment, the party responsible for engineering the controlling software shall
provide a signed declaration that the equipment meets the requirements of this access protocol.
Measurements of receiver carrier sense delay and opening delay shall be performed in an accredited test
laboratory, possibly together with the tests corresponding to ETS 300 113 [1].

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ETS 300 471: December 1995
4.4 Interpretation of the measurement results
The interpretation of the results recorded in a test report for the measurements described in this ETS shall
be as follows:
a) the measured value related to the corresponding limit shall be used to decide whether an
equipment meets the minimum requirements of this ETS;
b) the actual measurement uncertainty of the test laboratory carrying out the measurement, for each
particular measurement, shall be included in the test report;
c) the values of the actual measurement uncertainty shall be, for each measurement, equal to or lower
than the figures given in clause 9 (absolute measurement uncertainties).
5 Technical characteristics of the equipment
The timing requirements that the equipment shall fulfil in order to operate correctly and efficiently can be
found in ETS 300 113 [1], subclauses 5.1.7 and 5.1.8, ETS 300 390 [2], subclauses 5.1.5 and 5.1.6 and
subclauses 8.1.3 and 8.2.3 of this ETS.
The corresponding methods of measurement can be found in ETS 300 113 [1], subclauses 8.8 and 8.9,
ETS 300 390 [2], subclauses 8.5 and 8.6 and in subclauses 8.1 and 8.2, of this ETS.
6 Access protocol
6.1 General
The access protocol shall be used for each occupation of the Radio Frequency (RF) channel for sharing
data/data and for sharing speech/data with automatic channel access.
6.2 Principles
The equipment determines whether or not the channel is, and has been, idle for a certain period (the
observation time) by means of carrier sensing.
The observation time consists of a fixed part and a randomly selected part. When the channel still
appears to be idle at the end of the observation time, the transmitter is initiated and powered up within a
specified time.
The duration of the emission is limited (see subclause 6.7).
6.3 Procedure
The equipment shall determine whether or not the channel is, and has been, idle for a certain period (the
observation time t ), by means of carrier sensing (see subclause 6.4). The observation time t consists of
o o
a fixed part t and a randomly selected part t .
f r
If the channel is occupied during part of the observation time, the process shall be repeated (without
changing m (see subclause 6.5).
If the channel appears to be idle throughout the observation time, the transmitter shall be initiated and
powered up within the specified time (attack time; see ETS 300 113 [1], subclause 8.8 or ETS 300 390 [2],
subclause 8.5). The channel can then be seized for the duration of one time interval. The maximum length
t of this interval depends on the frequency sharing category (data/speech, data/data).
t
If a re transmission is required (due, for example, to a "collision", i.e. a simultaneous channel access by
several users), the observation process shall be repeated and the channel shall be detected as idle prior
to a repetition of a transmission.

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SIST ETS 300 471:1998
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ETS 300 471: December 1995
Within one time interval, the following radio traffic may take place:
a) from a base station to one or several mobiles;
b) from a mobile to a base station;
c) between mobiles.
To ensure that no other user can access the channel during a time interval, the reversion time t , the time
c
between transmission of a message and reception of the corresponding acknowledgement and/or reply,
shall not exceed 50 ms. The reversion time t is the time between the switch off of one transmitter and the
c
switch on of the other. The points of switch off and switch on are considered to be the points at which the
transmitter power is 50 % of the rated carrier power.
6.4 Carrier sensing
Carrier sensing is the detection of whether the RF level in the receive channel exceeds a given threshold.
The carrier sense shall be able to detect RF signals with different types of modulation (e.g. F3E, G3E,
F1D, F2D, G1D).
The channel shall be regarded as in use during the observation time (see subclause 6.5) if the level of the
RF signal on the channel exceeds a certain level. For equipment meeting ETS 300 113 [1] or I-ETS 300
113 [3], this threshold level shall be as defined in table 1.
Table 1: Threshold levels
Band Threshold level in
dBμμV emf
30 - 137 MHz 12
> 137 - 300 MHz 6
> 300 MHz 0
The carrier sense delay is specified in subclause 8.1.3.
6.5 Observation time
The observation time shall start within 10 ms after each time that the RF channel has become idle. It shall
also start at power on.
The observation time t is the sum of the fixed part t and the random part t = n × t
o f r i
t = t + n × t .
o f i
The fixed part, t , of the observation time shall be:
f
- on pure data channels: 60 ms ± 1 ms;
- on combined speech/data channels: 2 000 ms ± 1 ms;
except as specified in subclause 6.7.2.
The increment time t shall be 50 ms ± 0,1 ms.
i
The number n is a random integer number from 1 to m (1 ≤ n ≤ m); this means that 1 to m is the event
field of the random number n. The random number n shall be determined by use of a random generator
with a uniform distribution (see also annex B).

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ETS 300 471: December 1995
To achieve short delays during low traffic, the observation time should be short, i.e. m should be small.
Therefore in this access protocol, m shall be set to 4 for the first trial.
A short random part of the observation time however, increases the probability of several users
simultaneously accessing the channel for a time interval.
Therefore, when a re transmission is required (e.g. in the case of no acknowledge, see figure 2), the
observation process shall be repeated and the channel shall be detected as idle during a new t befor
...