ETSI ETS 300 931 ed.3 (1998-01)

SLOVENSKI STANDARD
01-december-2003
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VNXSLQHWUDQVSDUHQW*60UD]OLþLFD
Digital cellular telecommunications system (Phase 2+) (GSM); Technical realization of
facsimile group 3 transparent (GSM 03.45 version 5.2.1)
Ta slovenski standard je istoveten z: ETS 300 931 Edition 3
ICS:
33.070.50 Globalni sistem za mobilno Global System for Mobile
telekomunikacijo (GSM) Communication (GSM)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN ETS 300 931
TELECOMMUNICATION January 1998
STANDARD Third Edition
Source: SMG Reference: RE/SMG-040345QR1
ICS: 33.020
Key words: Digital cellular telecommunications system, Global System for Mobile communications (GSM)
R
GLOBAL SYSTEM FOR
MOBILE COMMUNICATIONS
Digital cellular telecommunications system (Phase 2+);
Technical realization of facsimile group 3 transparent
(GSM 03.45 version 5.2.1)
ETSI
European Telecommunications Standards Institute
ETSI Secretariat
Postal address: F-06921 Sophia Antipolis CEDEX - FRANCE
Office address: 650 Route des Lucioles - Sophia Antipolis - Valbonne - FRANCE
X.400: c=fr, a=atlas, p=etsi, s=secretariat - Internet: secretariat@etsi.fr
Tel.: +33 4 92 94 42 00 - Fax: +33 4 93 65 47 16
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 1998. All rights reserved.

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ETS 300 931 (GSM 03.45 version 5.2.1): January 1998
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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ETS 300 931 (GSM 03.45 version 5.2.1): January 1998
Contents
Foreword .5
0 Scope .7
0.1 Normative references .7
0.2 Definitions and abbreviations.8
1 Service definition .9
2 Network architecture .9
3 Reference configuration at the mobile station.10
3.1 Fax Adaptor functionality .10
3.2 GSM Facsimile Machine functionality.11
4 Connection types.12
4.1 Information transfer protocol model.13
4.2 Interactions with T.30.14
4.2.1 Link control strategy .14
4.2.1.1 Message detection.15
4.2.2 Speed conversion for BCS phases .15
4.2.3 Compatibility checking.16
4.2.3.1 Group 1 and Group 2 support.16
4.2.3.2 2 400 bit/s handshaking.16
4.2.3.3 Non-standard facilities .16
4.2.3.4 12 000 and 7200 bit/s Facsimile Document Transfer.16
4.2.3.5 Procedure interrupts .16
4.2.4 Speed checking.16
4.3 Radio Channel Modification Procedures.17
4.3.1 In Call Modification (ICM) .17
4.3.2 Channel Mode Modify (CMM).18
5 Use of terminal adaptation functions.19
5.1 Standard TAFs for synchronous services.19
5.2 Specific TAFs for facsimile service.19
5.2.1 Working principle.19
5.2.2 Basic protocol structure.20
5.2.2.1 Frame formats .20
5.2.3 Protocol description.21
5.2.3.1 IDLE state.21
5.2.3.2 BCS-REC state.21
5.2.3.3 BCS-TRA state .22
5.2.3.4 MSG-REC state.22
5.2.3.5 MSG-TRA state .22
5.2.4 DCS and TCF processing .23
5.2.5 DCN (disconnect) frame.23
5.2.6 Clocking.24
5.2.7 Timeouts.24
5.3 Specific TAFs for facsimile service (T.30/A).24
5.3.1 Frame detection .24
5.3.2 Message phase.24
5.3.3 Additional CMM request .25
6 Signalling aspects .25
6.1 Handling of Tonal Signals.25
6.2 Call establishment.25
6.2.1 Mobile Terminated Call - Speech then Fax.25
6.2.2 Mobile Terminated Call - Auto answer .26

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ETS 300 931 (GSM 03.45 version 5.2.1): January 1998
6.2.3 Mobile Originated Call - Speech then Fax . 26
6.2.4 Mobile Originated Call - Auto calling. 27
6.2.5 Mobile Originated Call - Manual calling . 27
6.3 Call release . 27
7 Interworking to fixed networks . 28
7.1 Interworking to PSTN . 28
7.2 Interworking to ISDN .29
Annex I (Informative): Protocol entities from CCITT T.30 and T.4. 30
Annex II (Informative): Procedure examples. 31
History. 44

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ETS 300 931 (GSM 03.45 version 5.2.1): January 1998
Foreword
This European Telecommunication Standard (ETS) has been produced by the Special Mobile Group
(SMG) of the European Telecommunications Standards Institute (ETSI) .
This ETS defines the technical realization of facsimile group 3 using transparent network support within
the digital cellular telecommunications system.
The specification from which this ETS has been derived was originally based on CEPT documentation,
hence the presentation of this ETS may not be entirely in accordance with the ETSI/PNE Rules.
Transposition dates
Date of adoption of this ETS: 2 January 1998
Date of latest announcement of this ETS (doa): 30 April 1998
Date of latest publication of new National Standard
or endorsement of this ETS (dop/e): 31 October 1998
Date of withdrawal of any conflicting National Standard (dow): 31 October 1998

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ETS 300 931 (GSM 03.45 version 5.2.1): January 1998
0 Scope
This European Telecommunication Standard (ETS) deals with the procedures allowing the technical
realization of the Group 3 facsimile Service within the GSM PLMN using transparent Network support,
according to the definition of Teleservice 61 and 62 specified in the GSM 02.03 [2].
0.1 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] GSM 01.04 (ETR 350): "Digital cellular telecommunication system (Phase 2+):
"Abbreviations and acronyms".
[2] GSM 02.03 (ETS 300 905): "Digital cellular telecommunication system
(Phase 2+); Teleservices supported by a GSM Public Land Mobile Network
(PLMN)".
[3] GSM 02.34: “Digital cellular telecommunications system (Phase2+): High Speed
Circuit Switched Data (HSCSD) - Stage1”.
[4] GSM 03.10: "Digital cellular telecommunication system (Phase 2+); GSM Public
Land Mobile Network (PLMN) connection types".
[5] GSM 03.34: "Digital cellular telecommunications system (Phase2+): High Speed
Circuit Switched Data (HSCSD) - Stage2.
[6] GSM 04.02: "Digital cellular telecommunication system (Phase 2+); GSM Public
Land Mobile Network (PLMN) access reference configuration".
[7] GSM 04.08 (ETS 300 940): "Digital cellular telecommunication system
(Phase 2+); Mobile radio interface layer 3 specification".
[8] GSM 04.21 (ETS 300 945): "Digital cellular telecommunication system; Rate
adaption on the Mobile Station - Base Station System (MS - BSS) interface".
[9] GSM 07.01 (ETS 300 913): "Digital cellular telecommunication system
(Phase 2+); General on Terminal Adaptation Functions (TAF) for Mobile
Stations (MS)".
[10] GSM 07.03 (ETS 300 915): "Digital cellular telecommunication system
(Phase 2+); Terminal Adaptation Functions (TAF) for services using
synchronous bearer capabilities".
[11] GSM 09.07 (ETS 300 976): "Digital cellular telecommunication system
(Phase 2+); General requirements on interworking between the Public Land
Mobile Network (PLMN) and the Integrated Services Digital Network (ISDN) or
Public Switched Telephone Network (PSTN)".
[12] CCITT Recommendation T.35: "Procedure for the allocation of CCITT
members' codes".
[13] CCITT Recommendation F.160 Fascicle II.5: "General operational provision for
the international public facsimile".
[14] CCITT Recommendation T.4 Fascicle VII.3: "Standardization of group 3
facsimile apparatus for document transmission".

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ETS 300 931 (GSM 03.45 version 5.2.1): January 1998
[15] CCITT Recommendation T.30 Fascicle VII.3: "Procedures for document
facsimile transmission in the general switched telephone network".
[16] CCITT Recommendation V.21 Fascicle VIII.l: "300 bits per second duplex
modem standardized for use in the general switched telephone network".
[17] CCITT Recommendation V.24 Fascicle VIII.l: "List of definitions for interchange
circuits between data terminal equipment (DTE) and data circuit-terminating
equipment (DCE)".
[18] CCITT Recommendation V.25bis Fascicle VIII.l: "Automatic calling and/or
answering equipment on the general switched telephone network (GSTN) using
the 100-series interchange circuits".
[19] CCITT Recommendation V.27ter Fascicle VIII.l: "4 800/2 400 bits per second
modem standardized for use in the general switched telephone network".
[20] CCITT Recommendation V.29 Fascicle VIII.l: "9 600 bits per second modem
standardized for use on point-to-point 4-wire leased telephone-type circuits".
[21] CCITT Recommendation V.33 Fascicle VIII.l: "14 400 bits per second modem
standardized for use on point-to-point 4-wire leased telephone-type circuits".
[22] CCITT Recommendation X.300 Fascicle VIII.6: "General principles for
interworking between public networks, and between public networks and other
networks for the provision of data transmission services".
[23] CCITT Recommendation V.17 “A 2-wire modem for facsimile applications with
rates up to 14 400 bit/s”.
0.2 Definitions and abbreviations
In addition to those below, abbreviations used in this ETS are listed in GSM 01.04 [1].
BCS Binary Coded Signalling phase of Facsimile transmission as per CCITT T.30
CT105 Interchange Circuit 105 as per CCITT V.24
CT106 Interchange Circuit 106 as per CCITT V.24
CT107 Interchange Circuit 107 as per CCITT V.24
CT108.2 Interchange Circuit 108/2 as per CCITT V.24
CT109 Interchange Circuit 109 as per CCITT V.24
CT114 Interchange Circuit 114 as per CCITT V.24
CT115 Interchange Circuit 115 as per CCITT V.24
FA/MT The Fax Adaptor specifically located at MT side
FA/IWF The Fax Adaptor specifically located at IWF side
MSG Message phase of Facsimile transmission as per CCITT T.30
All protocol entities from CCITT Facsimile Recommendations (T.4 and T.30) apply; in this ETS they are
referenced to in the same way as in the above CCITT Recommendations (see also annex I to this ETS).

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ETS 300 931 (GSM 03.45 version 5.2.1): January 1998
1 Service definition
The fixed network Group 3 Facsimile service, as basically defined in CCITT Recommendation F.160, is an
international telematic service for ISO A4 document transmission between two facsimile stations.
The service specification is comprised of two parts: the control protocol described in CCITT
Recommendation T.30, and the document transmission coding described in CCITT Recommendation T.4.
The GSM facsimile Teleservice is intended to allow facsimile connections between group 3 apparatus
using:
- a GSM PLMN as a stand alone facility, for mobile to mobile communication;
- a GSM PLMN to gain access to fixed networks PSTN and ISDN, for mobile to/from land
communication.
For this Teleservice, the document coding is as CCITT Recommendation T.4 with no modifications. The
protocol used is CCITT Recommendation T.30 modified within the PLMN as detailed in this ETS.
The interworking between different networks is based on CCITT Recommendation X.300.
The particular features of this Teleservice are:
- it uses point-point communication;
- the information transfer capability is dual "Speech"/"Group 3 Facsimile" for Teleservice 61 and
"Group 3 Facsimile" only for Teleservice 62;
- both mobile originated and terminated calls are supported;
- the information transfer mode is circuit, duplex, synchronous and symmetric;
- different end-to-end transfer rates are used within the same call to take advantage of the better
radio path error rate;
- use of a standard synchronous terminal adaptation function (as per GSM 07.03) within the MS.
2 Network architecture
The network architecture applicable to this Teleservice is shown in figure 1/03.45, below.
PLMN
v v
PSTN FAX
FAX BSS MSC/IWF
MT
ISDN
Figure 1/03.45: Network architecture
This shows the case of mobile to fixed network interworking. For mobile to mobile calls, there would
effectively be a loop back within the PLMN, using two IWFs.

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ETS 300 931 (GSM 03.45 version 5.2.1): January 1998
3 Reference configuration at the mobile station
The mobile reference configurations described in this clause are defined as per GSM 04.02.
Mobile Station
R U
2w
m
FAX
a)
FAX /
MT2
Adapt.
S
2w
FAX
b)
FAX /
MT1
TA
Adapt.
2w
FAX
/
c) FAX TA MT1
Adapt.
R
d) FAX
FAX
MT2
Adapt.
FAX
e)
FAX TA
MT1
Adapt.
f)
MT0
Figure 2/03.45: Reference configurations
The Teleservice definition in GSM 02.03 regards the group 3 facsimile terminal as a 2-wire analogue
terminated equipment. In order to connect this to the MT2 a separate "Fax Adaptor" device is necessary.
This configuration, shown in figure 2a/03.45, has to be considered as the standard configuration, so that
all the existing Group 3 facsimile apparatus can be connected to the PLMN.
An alternative realization would be to combine a standard group 3 facsimile machine and the Fax Adaptor
into a specially developed "GSM facsimile machine", directly providing a digital output. Although such a
device must appear to the MT2 as identical to the Fax Adaptor (i.e. with an identical interface and
protocol), it would allow for a significantly smaller and simpler facsimile machine. This configuration is
shown in figure 2d/03.45 and is regarded as a desirable alternative.
In addition of course, it is always possible to realize an MT0, as per figure 2f/03.45, where both the
facsimile and mobile termination functions are considered to be part of one integrated unit.
The remaining configurations concern the use of an S interface and are considered as optional
configurations. Their use is for further study.
The particular terminal adaptation functions used are those detailed in GSM 07.03 and the interface to the
MT2 used is synchronous V.24 with an option for support of V.25bis procedures for autocalling and
autoanswering.
3.1 Fax Adaptor functionality
The Fax Adaptor block, figure 3/03.45, is intended to specifically complement the Group 3 facsimile
apparatus in order to be able to communicate over a GSM PLMN.

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ETS 300 931 (GSM 03.45 version 5.2.1): January 1998
COMPOSITE
MODEM
R
2W
PROTOCOL
MONITORING
&
CONTROL
TONE
HANDLING
Figure 3/03.45: Fax Adaptor scheme
Whether it has to be a function internal to the GSM PLMN, or an external accessory associated with the
Group 3 apparatus, is beyond the scope of this ETS, and in any case, does not affect at all the working of
the procedure as here described.
It can be functionally partitioned in two sections:
- an analogue section, dealing with:
- the modulation and demodulation processes according to CCITT Recommendation V.21,
V.27ter, V.29 and V.17 as explained in T.4 and T.30;
- handling of the signalling on the 2-wire path to the facsimile machine, including autocalling
and autoanswer functions where necessary (see clause 6).
- a digital section, dealing with:
- monitoring and where necessary, manipulation of the T.30 protocol as detailed in the rest of
this ETS;
- overall control of the adaptor;
- connection over the synchronous V.24 interface to the MT as described in GSM 07.03;
- where necessary, autocalling and autoanswering functions according to V.25bis.
In the following this specification will reference the Fax Adaptor functionality, considering the most general
case where it operates as a full-featured (see figure 3/03.45) separate block (see figure 2a/03.45), as this
reference configuration is implicitly or explicitly exhaustive of all service related technical aspects.
3.2 GSM Facsimile Machine functionality
The special GSM facsimile machine shown in the MS configuration of figure 2d/03.45 has a similar
functionality to the digital part of the Fax Adaptor, but without any of the analogue portions.
It appears over the V.24 interface as identical to the Fax Adaptor, i.e. the MT2 needs to have no
knowledge of the particular configuration used.
When necessary this reference configuration, will be explicitly referenced to in the following; otherwise all
technical aspects relevant to the configuration implicitly apply.

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ETS 300 931 (GSM 03.45 version 5.2.1): January 1998
4 Connection types
Table 1/03.45 shows the connection elements attributes applicable to this Teleservice (note), adapted
from GSM 03.10.
NOTE: Teleservice 61 includes both speech and data connection types, but Teleservice 62
only the data connection type.
Table 1/03.45: Elements of connection types
Protocol type Access to TAF Radio interface Intermediate BS-MSC/IWF
of Figure 6 of TS of the Mobile connection rate connection
GSM 03.10 Station element RA1 to RA2 element
Model 6: Speech - Speech/GSM - CCITT
A-law
Model 5: C/D/S UDI C/D/S UDI
C/D/S UDI C/D/S UDI
Facsimile - 14.4 kbit/s - 14.5 kbit/s - 16 kbit/s - 64 kbit/s
Group 3 - 9.6 kbit/s - 12.0 kbit/s - 16 kbit/s - 64 kbit/s
- 4.8 kbit/s - 6.0 kbit/s -  8 kbit/s - 64 kbit/s
- 2.4 kbit/s - 3.6 kbit/s -  8 kbit/s - 64 kbit/s
(note)
Model 5b C/D/S UDI C/D/S UDI C/D/S UDI C/D/S UDI
Facsimile - 14.4 kbit/s 14,5 or 2X12 kbit/s 16 or 2X16 kbit/s - 64 kbit/s
Group 3 -  9.6 kbit/s 12 or 2X6 kbit/s 16 or 2X8 kbits - 64 kbit/s
-  4.8 kbit/s 6 kbit/s 8 kbit/s - 64 kbit/s
-  2.4 kbit/s 6 kbits 8 kbit/s - 64 kbit/s
NOTE: The highest Access Rate actually supported in this teleservice will be consistent with the
highest Access Rate of the Transparent Bearer Service provided by the Network
Operator.
C = Circuit switched S = Synchronous
D = Full-duplex UDI = Unrestricted Digital Information
Table 1a/03.45
Fax modem rates No. of Radio i/f rate Intermediate Rate Padding Scheme
substreams
14.4 kbit/s 1 14,5 kbit/s 16 kbit/s
2 12 kbit/s 16 kbit/s
12.0 kbit/s 1 14,5 kbit/s 16 kbit/s FA(5D + S)
2 12 kbit/s 16 kbit/s FA(5D + S)
9.6 kbit/s 1 12 kbit/s 16 kbit/s
2 6 kbit/s 8 kbit/s
7.2 kbit/s 1 12 kbit/s 16 kbit/s FA(3D + S)
2 6 kbit/s 8 kbit/s FA(3D + S)
4.8 kbit/s 1 6 kbit/s 8 kbit/s
2.4 kbit/s 1 6 kbit/s 8 kbit/s
FA = Padding is performed in the FA
(nD + mS) means that m SYNC frames will be added every n’th DATA frame
The MS must support the combinations in table 1a/03.45, restricted to what has been negotiated between
the MS and the network.
All transitions from one combination in table 1a/03.45 to another combination in the same table, must be
supported by the MS, as long as the fax modem rate is kept constant or the transition of fax modem rate
is performed in steps of 2,4 kbit/s.
The figure 4/03.45 shows the scheme of a typical GSM PLMN connection for this Teleservice, considering
respectively R and S access at Network Termination.

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ETS 300 931 (GSM 03.45 version 5.2.1): January 1998
To grant full support to the CCITT Recommendation T.30, requiring different transmission speeds, the
following strategy shall be implemented:
- PLMN provides for four Access Rates only, that is 14 400 bits/s, 9600 bit/s, 4800 bit/s, 2400 bit/s;
- radio channel modification procedures are used for switching between Speech phase and facsimile
phase, as well as to select the suitable Access Rate (14 400,9600/4800/2400 bit/s) within the
facsimile phase, as resulting from the preliminary end-to-end negotiation between the terminals;
- to transport Binary Coded Signalling (BCS) requiring a synchronous 300 bit/s bit-rate, speed
conversion will be used at both the PLMN ends.
DTE DCE
MT2
MSC/IWF
3.1 kHz audio
P
2w
FAX FAX
2w
L
Adaptor
Adaptor
R I/F M
(V.24)
N
MT1
MSC/IWF
3.1 kHz audio
FAX
P
2w
2w
L
Adaptor
M
S I/F
N
Figure 4b/03.45: Optional Teleservice connection (F.F.S)
4.1 Information transfer protocol model
The Figure 5/03.45 depicts the conceptual protocol model for this Teleservice, deduced from model 5 and
5b of Figure 6/03.10.
The main point to be underlined is that all the protocol modules specific for this Teleservice are confined in
the Fax Adaptation functions at both the MT and IWF ends. Note that model 5b of figure 6/03.10 details
the placing of the splitting and combiner function for multislot configurations.
GR 3 Apparatus
GR 3 Apparatus
FAX
Adaptor
TA
G R 3 Apparatus
GR 3 Apparatus
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ETS 300 931 (GSM 03.45 version 5.2.1): January 1998
Mobile Station Base Station MSC/IWF
Radio I/F
R I/F
BSS-MSC I/F
Fax
Adapt.
Fax
Fax
T.30
T.30
Adaptation
Adaptation
function
function
RA1
RA1'
RA1'
RA1
FEC RA2
FEC RA2
I/F Cct I/F Cct
Figure 5/03.45: Information transfer protocol model
4.2 Interactions with T.30
The philosophy of this specification is to allow the T.30 protocol to pass transparently wherever possible,
through the Fax Adaptors at both ends of the PLMN channel. Manipulations are only made to the protocol
where necessary to overcome problems resulting from the differences between the PSTN and the GSM
system. Basically, these problems fall into four categories:
- supporting facsimile on a digital connection type;
- bit errors during transfer of BCS frames;
- the need to change speed to reduce the impact of bit errors during transfer of Fax encoded
Messages;
- inability to support some features of T.30.
4.2.1 Link control strategy
Though the T.30 procedure is plain half-duplex, between the FAs at both the GSM PLMN ends a
full-duplex mode connection will be established. Information transfer is structured in fixed length blocks,
carrying either data received from the local modem, or idle synchronization patterns.
Establishment, maintenance and release of the data circuit is performed autonomously by each FA, by
properly handling the physical interface to its local modem.
Due to the rigid timeout constrains in T.30 protocol procedure, which will be actually concatenated to the
procedure defined here for the GSM connection part, a strictly forwardonly approach is adopted for data
link control; that is no retransmission means are explicitly introduced to recover corrupted information
blocks.
Although in the majority of cases the T.30 protocol can take care of lost frames and easily recover, there
are some particularly critical points where two consecutive segments in the procedure, without any
interposed acknowledgement, require different transmission speeds (BCS speed/Message speed).
In these cases, to grant that a correct change-over from one modem type to another takes place at both
the PLMN ends, some additional protection is necessary in the GSM environment. This protection,
actually at the beginning and at the end of the Fax page transmission, is given by means of a confirmation
mechanism, whereby the originating FA waits while transmitting a fixed pattern until an explicit
acknowledgement is received from the terminating FA. In these cases an interruption of the normal T.30
information flow may eventually result.

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ETS 300 931 (GSM 03.45 version 5.2.1): January 1998
Aiming at minimizing any possible delay in the end-to-end transmission between the two facsimile
apparatuses, the FAs will pass on information without error control of the message integrity (note). To this
regard, however, it is worth underlining that an essential requirement in this procedure is the detection of
some key messages exchanged between the terminals, carrying basic control information. Hence, in
parallel to the primary information passing process, a continuous monitoring of the information flowing
across the FA is necessary, to detect these messages, fully checked in their logical integrity.
NOTE: All V.21 modems in the FAs shall use the faster response time provided by CCITT
Rec. V.21 (see table 2/V.21) for OFF to ON transition of CT109 and CT106.
In addition, as detailed below, some fields in some frames may need to be changed.
4.2.1.1 Message detection
While in BCS, the following frames have to be detected:
- DIS/DTC, to monitor all operational parameters proposed to the sender terminal by the receiver
terminal;
- DCS, to realize (see table 2/T.30) the actual message transmission speed accepted by the sender
terminal and the relevant message transfer direction;
- NSF, to inhibit Non-Standard Facilities, if explicitly required by the user Fax Adaptor configuration;
- DCN, to initiate the Call release procedure.
During the Message phase (C phase of T.30) a single bit pattern has to be detected, the EOL character, a
unique codeword that can never be found within a valid line of Fax coded Data (see CCITT
Recommendation T.4, subclause 4.1.2), and therefore used to control the buffer level in each FA, as
detailed in the following subclause 5.2.6.
4.2.2 Speed conversion for BCS phases
Given the signalling load caused by the Channel Mode Modify procedure, rather than changing the radio
channel speed to 300 bit/s to carry the BCS frames, a speed conversion mechanism is exploited at both
the PLMN ends, allowing to maintain the Channel rate of the Message phase (14 400/9600/4800/2400
bit/s) during BCS phases.
A speed conversion factor can be defined as:
Fax Message speed (14 400 or 9600 or 4800 or 2400 bit/s)
---------------------------------------------------------------------
Standard BCS speed (300 bit/s)
which will assume the value 8 or 16 or 32 or 48, depending on the actual Message speed negotiated
between the terminals.
On the basis of the above speed conversion factor, in the originating FA up-conversion to Message speed
will be performed according to the repetition algorithm specified in subclause 5.2.3.2.
In the terminating FA, down-conversion to the BCS speed will require a decimation algorithm (see
subclause 5.2.3.3); this algorithm is assumed implementation dependent, and hence its actual definition is
beyond the scope of this ETS.
As the actual Access Rate over the GSM-TCH may change throughout the Call, speed conversion factor
needs to be updated in both FAs.
The FA/IWF will update this value upon successful completion of a CMM procedure (see
subclause 4.3.2).
The FA/MT which does not have direct access to the GSM signalling channel (except for MT0
configuration), shall estimate the actual Access Rate established over the GSM-TCH from the effective
Access Rate of the MT data interface (or any other means in case of MT0); this check shall be performed

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ETS 300 931 (GSM 03.45 version 5.2.1): January 1998
whenever a BCS frame is sent towards the Radio path after which a change for GSM-TCH Access Rate
may be expected (e.g., DIS and DCS frames for Normal Fax mode, PPR and CTC frames for Error
Correction Mode).
4.2.3 Compatibility checking
Some features of T.30 cannot be supported in the GSM PLMN environment. Fax Adaptation function is in
charge of dealing with such compatibility checking.
4.2.3.1 Group 1 and Group 2 support
Group 1 and Group 2 equipments are not supported by the Teleservice as described in this ETS.
To this purpose, any tonal signalling specific to the above kind of Fax apparatus will be ignored by the Fax
adaptation function at MS and IWF.
4.2.3.2 2 400 bit/s handshaking
Only standard 300 bit/s Binary Coded Signalling is supported. To this purpose the FA will ignore this
capability within the B segment of T.30 procedure, looking for DIS frame, by default, from V.21 modem
only.
4.2.3.3 Non-standard facilities
Non-standard facilities are supported in so far they are not in conflict with the standard procedure
described in this GSM ETS. All the relevant BCS frames are passed end-to-end, unless an explicit
deletion is required by the user (e.g. by a specific input at the Fax Adaptor associated with the MS). In this
case, upon detection (see subclause 4.2.1.1) of the NSF frame identifier (the Facsimile Control Field octet
according to T.30 terminology), the whole HDLC frame shall be stripped and replaced by a GSM specific
NSF frame, containing a Country Code and a unique identification of the FA manufacturer, encoded as
per CCITT T.35 and according to national regulations; the Information field of this NSF frame shall have a
maximum length of 6 octets, and shall not require any specific functionality in the PLMN.
4.2.3.4 12 000 and 7200 bit/s Facsimile Document Transfer
End-to-end transfer of T.4 information at 12 000 and 7 200 bit/s can be supported, even if this Bearer
Service is not provided by a GSM PLMN.
To this purpose a 14 400 bit/s or 9 600 bit/s Access Rate shall be used in the PLMN, by exploiting only
part of the available bandwidth. The originating FA will add padding information to data received from the
MSG modem, before transmission over the GSM-TCH; the terminating FA will remove the same
information to restore the net 12 000 or 7 200 bit/s continuous data stream to be transmitted over its local
MSG modem.
The relevant procedures are fully described in sections 5.2.3.4 and 5.2.3.5.
4.2.3.5 Procedure interrupts
Procedure interrupts are only possible in Teleservice 61; in case of Teleservice 62 any attempt to invoke
procedure interrupts by MMI on the MT (see subclause 4.3.1 below) will be ineffective.
4.2.4 Speed checking
Upon receiving DIS/DTC frame, the FA/IWF shall perform a basic speed checking procedure, aiming at
verifying compatibility between the bit-rate possibilities offered by the called Fax apparatus (bit fields 11,
12, 13, 14 in the IF) and the actual Access Rates allowed on the GSM PLMN (2 400/4 800/9 600/14400
bit/s); in addition, some of these bit rates may result inhibited during the call, if a speed lower than 9 600
bit/s has been chosen by the user in the call set up message (for example if a better error rate is
required).
The following cases may occur:

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- some of the bit-rates indicated in DIS/DTC are not allowed on GSM-TCH (e.g. the max speed
indicated in setup is 9 600 bit/s, while DIS/DTC requests V.33); a possible strategy to cater for this
case is altering the indication in DIS/DTC to a compatible value (e.g. V.29) by forcing appropriately
the relevant bits (note); other strategies are possible and allowed, as long as the same goal of
supporting the progress of the call is pursued, by constraining the Fax apparatuses to use as
message speeds the access rates actually available in the PLMN.
NOTE: Is beyond the scope of this specification to specify the implementation strategy
adopted to recalculate the frame CRC after the manipulation; additional delays, if any,
shall not have a significant impact on T.30 operability.
- the full set of bit-rates indicated in DIS/DTC are not compatible with the allowed Access Rates on
GSM-TCH (e.g. the max speed indicated in setup is 2 400 bit/s while DIS/DTC requests V.29 only);
in this case the call shall be failed;
- the full set of bit-rates indicated in DIS/DTC are compatible with the allowed Access Rates on
GSM-TCH; in this case no action shall be taken, independently of the access rate actually
established.
It should be noted that the "14400 bit/s" as max speed indication in the SETUP message, does not
preclude the establishment of a Call towards a Fax GR 3 equipment supporting only 9600, 4 800 and
2 400 bit/s as Message speed. As a matter of fact, due to the multiplexing mechanism for BCS phases the
initial Access Rate (14 400 bit/s) will affect only the connection part between the called MS and the
associated IWF, whilst the appropriate Message speed will be actually negotiated during the initial BCS
phase, and settled upon reading the appropriate field in DCS frame (see subclause 4.2.1.1). Therefore, if
a request for MSG transmission is received, without a valid DCS frame being detected in advance (hence
the message speed is undefined), the FA/IWF shall release the call immediately.
4.3 Radio Channel Modification Procedures
There are two reasons why the characteristics of the radio channel may need to be changed during a call.
Firstly, for the swap from speech to data and vice versa. In this case the In Call Modification procedure as
detailed in GSM 04.08 is carried out.
Secondly, if a T.30 speed drop back is requested, a channel mode modify procedure as detailed in
GSM 04.08 is initiated to match the radio channel rate to the facsimile transmission speed to optimize the
error rate.
4.3.1 In Call Modification (ICM)
Applies only for Teleservice 61.
The speech to facsimile change is initiated by MMI at the MS as in other data services starting the ICM
procedure via MODIFY signalling. CT108.2 ON condition towards MT is a basic requirement to allow
speech/data transition take place.
Additionally, the data call direction (DCD) must be known to both the FA's because of a correct tone
handling. The DCD identifies the call direction from the calling to the called station according to CCITT
T.30 phase "A" tonal signals.
The DCD is derived from the evaluation of the behaviour of the mobile Fax machine. No later than 3 sec
after connecting the Fax apparatus to the line the FA/MT is able to determine the DCD. If a CNG tone or
nothing is detected by the FA/MT the mobile Fax station is the calling station, if a CED tone or a BCS
signal is detected by the FA/MT the mobile Fax station is the called station. The FA/MT indicates this
towards the MT by means of CT105: CT105 OFF condition indicates "mobile terminated", CT105 ON
condition indicates "mobile originated". The detection of CT105 condition and subsequent triggering of the
MODIFY message has to be done 3 sec after reception of the ON condition of CT108.2.
A reverse DCD compared with the initial call setup direction is indicated to the FA/IWF by means of the
"Reverse Call Setup Direction" (RCSD) information element within the MODIFY message. If the MODIFY
message had contained this information element, the same information element shall be included in the
MODIFY COMPLETE message (ACK). On the basis of RCSD and additional information about the initial

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call setup direction (e.g. Transaction Identifier flag as per GSM 04.07), the FA/IWF shall resolve the actual
DCD.
At DCD condition "mobile originated" the FA/IWF has to transmit CNG if neither CED nor a BCS signal
has been already received, otherwise nothing. At the DCD condition "mobile terminated" the FA/IWF has
to transmit CED. At the moment when the CT107 goes to ON condition the FA/MT has to generate CED if
CT105 was in ON condition, otherwise nothing (see subclauses 6.2.1 and 6.2.3).
Selection of the speech phase from the data phase (T.30 Procedure Interrupt request) may be initiated by
the mobile Fax machine as well as by the fixed network fax machine (refer to the diagrams in
figure II.-10/03.45 and II.-11/03.45). Upon receipt of the "alert operator" tone request will be accepted by
manual intervention via MT (phone off-hook), and is reflected to the Fax Adaptor (GSM Facsimile
machine) by CT106 and CT109 going in OFF condition.
Upon monitoring the transit of the necessary sequence of BCS signals specific for PRI the CT108.2 goes
to OFF condition causing ICM from the Fax data phase to the speech phase. In case of Procedure
Interrupt request from the fixed network side (figure II-10/03.45), a guard timer in the MS is necessary to
protect against the possible loss of the response to the PRI-Q(PIN/PIP) sent from the MS. After the
execution of ICM CT107 is set to OFF condition finishing the Fax data phase.
Subsequent re-selection of the data phase will be by manual intervention via the MS causing CT108.2
going to ON condition initiating ICM.
During the speech phase of a procedure interrupt, the "phone off-hook" condition of the MT is reported via
the Fax Adaptor "R" interface (CT106/109 in OFF condition) to the fax apparatus which must remain
functionally connected to the Fax Adaptor to maintain the connection.
The precise operation of the Fax Adaptor for the support of procedure interrupt is implementation
dependent.
4.3.2 Channel Mode Modify (CMM)
The channel mode modification procedure is only initiated by the FA/IWF. Its purpose is to adjust the radio
channel bit rate to match the message speed negotiated end-to-end between the facsimile machines.
The FA/IWF enters the CMM routine upon detecting the DCS frame sent either from the MS or from the
PSTN.
In the first case, FA/IWF being actually the receiving side in the facsimile document transmission, the
CMM procedure is executed as soon as the end of frame is detected.
That is, FA/IWF monitors the DCS frame and, if the requested rate differs from the existing radio channel
rate, then in sequence:
- issues a CMM request to the new rate towards the GSM signalling;
- waits for the acknowledgement of CMM completed (note);
NOTE: Information sent/received to/from the GSM-TCH while waiting for CMM completed is
considered irrelevant, and left implementation dependent in the context of this
Recommendation.
- returns back to the usual information passing process.
In the second case, FA/IWF being actually the transmitting side in the facsimile document transmission,
the CMM procedure is executed after a suitable delay assumed as the time for transferring the DCS frame
across the Radio channel.
That is, FA/IWF monitors the DCS frame and, if the requested rate differs from the existing radio channel
rate, then in sequence:
- waits for a fixed time of 150 m/sec.;
- issues a CMM request to the new rate towards the GSM signalling;

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Looking at the overall Facsimile service architecture in GSM (figure 5/03.45), half-duplex transmission is
initiated in either side of the connection by the actual request on the physical interface (CT109 ON on the
local modem), is then continued on the PLMN traffic channel (TCH), and finally terminated on the
appropriate modem at the remote side.
On the basis of the above assumptions, considering the Fax Adaptor a finite state machine, the whole
Adaptation process can be described by a set of 5 states:
- IDLE state, when the FA is just connected to the GSM-TCH, sending synchronizing patterns over
the radio path;
- BCS-REC state, when the FA is receiving data from the V.21 modem (BCS phases);
- BCS-TRA state, when the FA is transmitting data over the V.21 modem (BCS phases);
- MSG-REC state, when the FA is rece
...