6 Connection types

03.463GPPTechnical Realization of Facsimile Group 3 Service - non transparentTS

Table 1/03.46 shows the connection elements attributes applicable to these Teleservices, extracted from GSM 03.10.

Table 1/03.46: Connection elements

Protocol type

Access to TAF

Radio interface

Intermediate

BS‑MSC/IWF

of fig. 6

of the Mobile

connection element

rate

connection

GSM 03.10

Station

RA1 to RA2

element

Model 6: Speech

‑‑‑‑

Speech/GSM

‑‑‑‑

CCITT A‑law

Model 7:

C/D/S UDI

C/D/S UDI

C/D/S UDI

C/D/S UDI

Facsimile

‑ 9.6 kbit/s

‑ 12 kbit/s

‑ 16 kbit/s

‑ 64 kbit/s

Group 3 NT

(‑ 4.8 kbit/s)

(‑ 2.4 kbit/s)

C = circuit switched S = synchronous

D = duplex UDI = unrestricted digital information

Figure 4/03.46 shows the scheme of a typical GSM PLMN connection for these teleservices, considering R and S access, respectively, at network termination.

To support the CCITT Recommendation T.30, requiring different transmission rates, the following strategy shall be implemented:

‑ the channel on the radio interface shall be a full rate channel used for RLP protocol transmission (non‑transparent network support);

‑ no modification procedure (Channel Mode Modify: CMM) shall be performed during the data phase of the call;

‑ the user rate of the MT2 is preferably set to 9 600 bit/s;

‑ the transmission rate between the fax adaptor and the associated facsimile terminal at both ends shall be the same, i.e. there will be only one common end‑to‑end transmission rate at any given time;

‑ the negotiation of the message speed shall be end‑to‑end between the two facsimile terminals; this allows also for a message speed of 7 200 bit/s to be used;

‑ the connection between the fax terminals is divided into three logical sections (fax terminal ‑(1)‑ fax adaptor ‑(2)‑ fax adaptor ‑(3)‑ fax terminal);

‑ flag stuffing is applied by the fax adaptor towards the associated facsimile terminal within the constraints of CCITT Recommendation T.30, to keep the data link active whenever a procedure delay occurs;

‑ the BCS protocol elements and the facsimile coded data are buffered at both ends of the radio interface (in the fax adaptors), if necessary, to guarantee data integrity;

‑ a specific fax adaptor protocol (FA protocol) is provided between both the fax adaptors to cater for the appropriate link control.

R
DTE │ DCE
╔═══╗ ╔═════════╗ V ╔═════< >═════╗ ╔═══╗
║ G ║ ║ ║<—-114—–║ ║ ║ G ║
║ r ║ ║ ║<—-115—–║ ║ ║ r ║
║ o ║ ║ ║ ║ ║ ║ o ║
║ u ║ ║ ║—–108—->║ ║ ║ u ║
║ p ║ ║ ║<—-107—–║ P ║ ║ p ║
║ ║ ║ ║ ║ M ║ ║ ║
║ 3 ║ ║ ║—–105—->║ L S ║ ║ 3 ║
║ ║2-w║ Fax ║<—-106—–║ C ║ 2-w║ ║
║ t ╟─┼─╢ Adaptor ║<—-109—–║ MT2 M / ╟── ─ ─┼─╢ t ║
║ e ║ ║ ║ ║ I ║ ║ e ║
║ r ║ ║ ║<—-125—–║ N W ║ ║ r ║
║ m ║ ║ ║ ║ F ║ ║ m ║
║ i ║ ║ ║—–103—->║ ║ ║ i ║
║ n ║ ║ ║<—-104—–║ ║ ║ n ║
║ a ║ ║ ║ ║ ║ ║ a ║
║ l ║ ║ ║<—-102—->║ ║ ║ l ║
╚═══╝ ╚═════════╝ ╚═════< >═════╝ ╚═══╝

Figure 4a/03.46: Standard Teleservice connection

S

╔═══╗ ╔════════╤══╗ V ╔═════< >═════╗ ╔═══╗
║ G ║ ║ │ ║———->║ ║ ║ G ║
║ r ║ ║ │ ║ X bit ║ ║ ║ r ║
║ o ║ ║ │ ║<———-║ ║ ║ o ║
║ u ║ ║ │ ║ ║ ║ ║ u ║
║ p ║ ║ │ ║———->║ P ║ ║ p ║
║ ║ ║ │ ║ SA bits ║ M ║ ║ ║
║ 3 ║ ║ │ ║<———-║ L S ║ ║ 3 ║
║ ║2-w║ Fax │ ║ ║ C ║ 2-w║ ║
║ t ╟─┼─╢Adaptor │TA║———->║ MT1 M / ╟── ─ ─┼─╢ t ║
║ e ║ ║ │ ║ SB bits ║ I ║ ║ e ║
║ r ║ ║ │ ║<———-║ N W ║ ║ r ║
║ m ║ ║ │ ║ ║ F ║ ║ m ║
║ i ║ ║ │ ║ ║ ║ ║ i ║
║ n ║ ║ │ ║———->║ ║ ║ n ║
║ a ║ ║ │ ║ D bits ║ ║ ║ a ║
║ l ║ ║ │ ║<———-║ ║ ║ l ║
╚═══╝ ╚════════╧══╝ ╚═════< >═════╝ ╚═══╝

Figure 4b/03.46: Optional Teleservice connection

6.1 Protocol model

Figure 5/03.46 depicts the protocol model for this Teleservice, deduced from model 7 of figure 6/GSM 03.10.

It should be noted that depending on the particular implementation the R reference point may not explicitly exist. In this case the LAPB protocol and consequently the LAPB entities operating across this interface may be omitted. The protocol stack at the radio interface, however, is not affected by this consideration, i.e. RLP and L2RBOP always apply.

The main point to be underlined is that all the protocol modules specific for this Teleservice are confined in the fax adaptor functions at both the MT and MSC/IWF ends. This includes the layer 2 entity function for LAPB to be operated towards the standard TAF for synchronous non‑transparent bearer capability.

Mobile station Base station MSC/IWF
<——————————————> <————> <————————————->
Fax adaptor R I/F Radio I/F BSS-MSC I/F Fax adaptor
——————–>| | | |<——————-
T.30┌────────┐ | FA protocol | ┌────────┐T.30
────┤ ├ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┤ ├────
│Fax │ | ┌──────┐ | L2RBOP | ┌──────┐ | │Fax │
│Adaptor │ | │ L2R ├ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┤L2R ├ ─ ─ ─ ─ ─ ─ ┤Adaptor │
│Function│ | LAPB ├──┬───┤ | RLP | ├───┬──┘ | │Function│
│ ├ ─ ─ ─ ─ ─ ─ ─ ─ ┤L2│RLP├ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┤RLP│ | │ │
└────────┘ | └──┴───┘ | | └───┘ | └────────┘
\\ | // \\ | ┌────────┐ | // |
\\ | // ┌────┐ | │\───┬──/│ | ┌───┐ |
\\ | // │RA1’│ | │RA1’│RA1│ | │RA1│ |
\\ | // └────┘ | └────┴───┘ | └───┘ |
\\ | // \ | / \ | / |
┌───────┐|┌───────┐ ┌───┐|┌───┐ ┌───┐|┌───┐ |
│I/F cct├─┤I/F cct│ │FEC├─┤FEC│ │RA2├─┤RA2│ |
└───────┘|└───────┘ └───┘|└───┘ └───┘|└───┘ |

Figure 5/03.46: Protocol model for non‑transparent support

6.2 Principles of the Facsimile Protocol Adaptation

The basic approach of the present document for facsimile group 3 is:

‑ to use the standard non‑transparent network support (including e.g. standard MT) as basically defined in technical specifications GSM 07.01, GSM 07.03, and GSM 09.07;

‑ to use the CCITT Recommendation T.30 procedure at both ends of the connection between the fax adaptor and the associated facsimile terminal and to pass the protocol elements according to CCITT Recommendation T.30 functionally unchanged wherever possible;

‑ to use a specific protocol between both the fax adaptors across the radio interface; and

‑ to intervene within the fax adaptors in order to concatenate the applicable connection sections.

Basically there are four problem areas:

‑ support of facsimile group 3 with a digital connection type;

‑ unpredictable delays on the radio interface due to actual RLP working conditions (ARQ);

‑ the need to change the transmission rate "locally" in the fax adaptors both in the MS and in the MSC/IWF and to adapt it to the constant user rate of the TAF;

‑ the inability to support some CCITT Recommendation T.30 features.

To overcome these problems some particular functions within the fax adaptors are necessary, such as:

‑ buffering of BCS frames and facsimile coded data prior to transfer;

‑ autonomous interventions such as BCS command inhibiting within the fax adaptors;

‑ autonomous interactions between any fax adaptor and the associated facsimile terminal such as BCS command/response repetition; and

‑ provision of a fax adaptor protocol as interchange protocol between the fax adaptors.

Following this strategy, an interchange model is defined concentrating on the facsimile relevant components. According to this model three connection sections can be distinguished:

a) between fax adaptor and associated facsimile terminal;

b) between the both fax adaptors and again;

c) between fax adaptor and associated facsimile terminal.

The fax adaptors fully relying on the standard supporting layers (e.g. TAF) will also have to cater for the correct establishment and control of these layers including traffic channel synchronization and status information exchange in particular with respect to circuit 106 and circuit 109 (according to CCITT Recommendation V.24). Once these circuits have been set to ON (traffic channel synchronization), they must be kept in the ON condition during the entire facsimile phase of a connection (refer subclause 6.3 "Procedure Interrupts").

╔═════════╗ ╔═════════╗ ╔═════════╗ ╔═════════╗
║ ║ ║ ║ ║ ║ ║ ║
║ Fax ╟──╢ FA ╟─ ─ ─ ─ ─ ─ ─ ─╢ FA ╟──╢ Fax ║
║ ║ ║ ║ ║ ║ ║ ║
╚═════════╝ ╚════╤════╝ ╚════╤════╝ ╚═════════╝
┌────┴─────────────────────────┴────┐
│ standard supporting layers │
└───────────────────────────────────┘

CCITT Recommendation T.30
├─ ─── ─── ─── ─── ─── ─── ─── ─── ─── ─ ┤

CCITT Recommendation CCITT Recommendation
T.30 Fax adaptor protocol T.30
├───────────┤├─────────────────────────┤├───────────┤
<—-> <—->
interactions interactions
<— interventions <———> interventions —>

Figure 6/03.46: Communication model

6.2.1 Fax Adaptor Protocol

To cater for the appropriate facsimile transmission some protocol elements and their use (procedure) are defined. These protocol elements are exchanged between both fax adaptors. They are defined as follows and are structured as outlined in annex A:

‑ BCS element:

The BCS element is used to relay CCITT Recommendation T.30 BCS frames. It includes the possibility also to transmit parts of an entire BCS frame (segmentation). This will apply when the BCS frame content ‑ excluding the address and control fields of the BCS frame ‑ to be included exceeds a length of 20 octets (e.g. non standardized frames) which otherwise would cause unacceptable delays when relaying the BCS frame as a whole. For that purpose, the BCS frame element carries information on the integrity of a BCS frame element.

To guarantee the overall integrity of the BCS frames each single element includes a sequence number which is set to zero within each first BCS element carrying either a BCS command or BCS response (i.e. triggered by the preamble). It is incremented by one with each successive element belonging to the same BCS command/response transmitted across the radio interface. The counting is carried out independently for each direction of the element transmission. The number of BCS elements which can be transmitted within one sequence (BCS command/response) is limited to 256.

The fax adaptor receiving those elements checks the correct sequence of the numbers and, by this means, is able to detect loss of BCS elements and to act accordingly.

‑ BCS abort element:

The BCS abort element is used to indicate to the remote fax adaptor that an error occurred during the BCS frame reception and the transmission of the related BCS frame is to be aborted.

‑ BCS transmit request element:

The BCS transmit request element is used in two ways:

a) to request initial transmission of a BCS command or response after the preamble element preceding each BCS element sequence carrying such a command/response has been recognized;

b) to request retransmission of previously transmitted BCS elements in case of an error between both the fax adaptors. This applies e.g. if a sequence error has been recognized. The retransmission starts with the BCS element the number of which is indicated in the transmit request.

‑ Preamble element:

The preamble element is transmitted once for each received preamble as soon as the preamble has been recognized. It is used to switch on the fax modem and to start transmitting the preamble at the remote side of the radio interface 300 ms after the preamble element reception. The duration of the preamble must be the minimum still permitted by CCITT Recommendation T.30.

‑ Normal data element:

The normal data element is used to relay buffered facsimile data which have been received and transcoded when not operating in error correction mode.

‑ Error correction data element:

The error correction data element is used to relay buffered facsimile data frames received when operating in error correcting mode. It contains the entire respective data frame excluding the address and control fields of the frame.

‑ End of data element:

The end of data element is used to indicate the end of the message transmission. Subsequently, a preamble element is to follow.

‑ TCF element:

The TCF element TCF_OK or TCF_NOK is used to inform the opposite fax adaptor function about the result of the autonomously performed TCF phase.

6.2.2 Interactions and interventions within the fax adaptors

Interactions and interventions within the fax adaptors are necessary:

‑ where protocol elements cannot be passed due to the differences between the PSTN and the GSM system;

‑ where the content of protocol elements has to be aligned with the capabilities of the supporting GSM PLMN;

‑ where BCS commands are repeated by the facsimile terminal after a time‑out due to transmission delay across the radio interface;

‑ where BCS commands must be repeated autonomously by the fax adaptor after a time‑out when no response has been received from the associated facsimile terminal;

‑ where the retransmission of BCS commands is requested by the associated facsimile terminal sending a CRP frame due to recognized transmission errors;

‑ where the retransmission of BCS element(s) is requested by any fax adaptor using the BCS transmit request;

‑ during the transmission of facsimile coded data where the document content is transcoded to save transmission capacity at the radio interface (however, this does not apply when using the CCITT Recommendation T.30 error correction mode).

To perform the necessary interactions and interventions the fax adaptors both in the MS and in the MSC/IWF have to monitor the BCS frames and the facsimile coded data exchanged between the two facsimile terminals and to act as detailed below:

‑ reception of CCITT Recommendation T.30 BCS protocol elements from the associated facsimile terminal discarding BCS commands repetitively received due to time‑out in the facsimile terminal;

‑ transmission of CCITT Recommendation T.30 BCS protocol elements to the associated facsimile terminal autonomously repeating BCS commands towards the accepting facsimile terminal if necessary after time‑out or on request by CRP;

‑ control of the half duplex connections between the fax adaptor and the associated facsimile terminal;

‑ storage of BCS commands/responses completely received from the associated facsimile terminal;

‑ relay of the CCITT Recommendation T.30 protocol elements between the two facsimile terminals by using the appropriate FA protocol elements and mapping them onto the particular L2R protocol (L2RBOP) elements and vice versa as indicated below;

‑ changing information elements of the BCS frames indicating capabilities which cannot be supported by the PLMN;

‑ transmission/reception of the CCITT Recommendation T.30 training check frames (TCF) to/from the associated facsimile terminal;

‑ phasing/training with the associated facsimile terminal;

‑ transcoding of the normal facsimile coded data received from the facsimile terminal and writing them into a buffer in the fax adaptor associated with the transmitting facsimile terminal;

‑ reading of the facsimile coded data from the buffer for transmission across the radio interface using the appropriate FA protocol elements;

‑ writing of the facsimile coded data received across the radio interface into a buffer at the receiving end of the connection section between MS and MSC/IWF;

‑ reading and reverse transcoding of the buffered normal facsimile coded data and transmitting them to the associated facsimile terminal;

‑ control of a response transmission timer, to guarantee the reception of a response right in time (refer to subclause 7.2.1.1).

The algorithm for mapping the CCITT Recommendation T.30 information onto the L2R protocol elements consists of three steps:

‑ generation of the appropriate FA protocol element including the CCITT Recommendation T.30 protocol element (BCS frames or facsimile coded data);

‑ generation of a single LAPB I‑frame including the FA protocol element in the information field;

‑ segmentation of a particular LAPB I‑frame into L2RBOP PDUs according to GSM 07.03.

To regenerate the original CCITT Recommendation T.30 protocol element the actions must be reversed at the remote fax adaptor.

The support of guard tones by the fax adaptor in the MSC/IWF is an implementation option.

6.2.3 Training Check

The training check sequence (TCF) as per CCITT Recommendation T.30 is exchanged only locally between the fax adaptor and the associated facsimile terminal. However, the subsequent exchange of CFR or FTT is, in principle, end‑to‑end. The training check sequence sent by the fax adaptor must have the minimum duration permitted (ref. to CCITT Recommendation T.30).

As a consequence of this local procedure, the fax adaptors have to check the received TCF whether the quality requirements are satisfied. Depending on the result of that check, the fax adaptor recognizing a bad line will eventually change the CFR to be a FTT, indicating a negative training result and transmits the TCF_element (TCF_OK or TCF_NOK).

With the knowledge of the TCF check result of the opposite fax adaptor function and the received response frame (CFR or FTT) from the fax apparatus, the fax adaptor, which generated the TCF, is able to deduce the following phase.

The message transfer phase in the receiving fax adaptor function is entered upon reception of TCF element TCF_OK and CFR. The modem training at transmission speed shall start after reception of the first facsimile coded data from the opposite fax adaptor function or autonomously 5.5s after reception of CFR.

In the other cases, the transmitting fax adaptor function shall repeat the last DCS/TCF sequence 3s after reception of the response, if no new DCS frame from the opposite fax adaptor function is available.

Only in case of a mobile to mobile call (refer to subclause 6.2.4/ GSM 03.46) a TCF_NOK leads to transmission of an invalid TCF.

If no TCF element was received, because of a RLP link reset, until a CFR is received from the fax apparatus, the message phase shall be entered autonomously 5.5s after reception of the CFR or the first facsimile coded data in the receiving adaptation function.

6.2.4 Mobile to mobile calls

In this section, an interim solution is described. The final solution depends on the progress in the ITU‑T and is expected by the end of 1995.

To get the information about the nature of the call, a GSM specific country code inside the NSF frame is used to identify an inter MSC call.

The GSM country code (refer to CCITT Recommendation T.35) is coded as follows:

bit 8 7 6 5 4 3 2 1
┌───┬───┬───┬───┬───┬───┬───┬───┐

│ 1 │ 1 │ 0 │ 0 │ 1 │ 1 │ 1 │ 1 │

└───┴───┴───┴───┴───┴───┴───┴───┘

In case no NSF has to be transmitted, the IWF shall insert a special NSF frame defined as follows:

FCF FIF
| |
NSF, GSM country code, stuff byte (0)

After reception of an NSF frame in the IWF with a GSM country code, the IWF shall enter a specific inter MSC procedure with the following interventions:

‑ Insertion of NSC respectively NSS with the above mentioned coding before sending of (CIG)DTC or (TSI)DCS.

‑ Discarding of received NSC and NSS frames which are generated by the opposite IWF.

‑ The transmission of TCF sequence is triggered by the reception of the TCF Discriminator. In case of TCF_NOK, an invalid TCF is transmitted to the opposite IWF.

‑ To save time and to check also the quality of link between the two MSCs, a BCS frame FTT is generated by the IWF and sent to the opposite IWF after reception of an invalid TCF. In this case the response CFR or FTT from the mobile side shall not be transferred to the other MSC. If an IWF receives an FTT from the opposite IWF the preceding DCS frame should not be automatically repeated after expiration of the command/response timer (T=3s, refer to CCITT RecommendationT.30).

‑ The response time supervision (Ts=1.6s, refer to subclause 7.2.1.1) within the IWFs is inactive, that means, a response shall be accepted as a valid frame by the receiving IWF when the BCS carrier is detected within 3s after transmission of the previous command.

‑ The message transfer phase is only entered after reception of the first facsimile coded data from the radio link.

‑ The facsimile coded data shall be transferred between the IWFs with minimum scan line transmission time equal to 0 ms (refer to CCITT Recommendation T.4).

6.2.5 Facsimile Message Transfer

6.2.5.1 Message Transcoding

To save transmission capacity at the radio interface the content of the document shall be transcoded. This applies only, when using the normal facsimile data transfer, i.e. not with the error correction mode.

The facsimile coded data received by the fax adaptor from the facsimile terminal is transcoded and transmitted to the corresponding fax adaptor across the radio interface, where it is transcoded in the reverse direction and transmitted to the receiving facsimile terminal.

The transcoding is based on the minimum line length capability of the T.30 protocol for the normal facsimile data transfer. According to this the transmitting facsimile terminal has to fill up each coded scan line with FILL information to conform to this requirement (ref. CCITT Recommendation T.4).

To take advantage from that, the fax adaptor associated with the transmitting facsimile terminal will force this to use (at least) the standard value of 20 ms by replacing the applicable parameter value of the exchanged DIS/DTC messages. All FILL information of the facsimile coded data received from the facsimile terminal will be deleted prior to forwarding the data across the radio interface. The fax adaptor associated with the receiving facsimile terminal must recognize and store the originally requested minimum line length to be able to regenerate to correct line length.

It should be noted that the CCITT Recommendation T.4 document coding may be 1‑dimensional or 2‑dimensional and, in addition, uncompressed. The fax adaptors have to take care of this when transcoding the document content.

6.2.5.2 Generation of the normal data element

The normal facsimile coded data which have been transcoded and buffered as described in the present document is segmented for transmission across the radio interface into blocks of max. 936 bits (afterwards constituting a sequence of max. 5 L2RBOP PDUs). Each such block is contained in the information field of a normal data element of the FA protocol (see annex A).

The facsimile message transfer is finalized by a trailing end of data element which allows the transmitting fax adaptor to switch off the message speed modem.

It may be necessary to align the content of the normal data element to octet boundary at the end of the facsimile message transmission, i.e. after the RTC. This is done by appending ‘0’s to the RTC. This fill information may be omitted by the remote fax adaptor.

6.2.5.3 Generation of the error correction data element

The content of a FCD frame, if received correctly, is stored by the fax adaptor. Each such block is contained in the information field of a error correction data element of the FA protocol (see annex A).

The facsimile message transfer is finalized by a trailing end of data element which allows the transmitting fax adaptor to switch off the message speed modem.

6.3 Procedure interrupts

Procedure interrupts are only supported in Teleservices 61; in case of Teleservice 62 any attempt to invoke procedure interrupts by MMI on the MT (see subclause 6.4 below) will have no effect.

6.4 Radio channel modification

This applies to Teleservice 61 only, if a change of the radio channel during the call swapping from speech to facsimile or vice versa is required. For this purpose the in‑call modification procedure (ICM) as detailed in GSM 04.08 is carried out.

The change from speech to facsimile is initiated by MMI at the MS as in other data services starting the ICM procedure via MODIFY signalling . As a basic requirement for this transition, circuit 108.2 (according to CCITT Recommendation V.24) towards the MT must be in the ON condition.

Additionally the data call direction (DCD) must be known to both the FAs because of a correct tone handling. The DCD identifies the call direction from the calling to the called station according to CCITT Recommendation 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 in OFF condition indicates "mobile terminated", whereas CT105 ON indicates "mobile originated". The detection of the CT105 condition and subsequent triggering of the MODIFY message has to be done 3 sec after the 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)" IE within the MODIFY message. If the MODIFY message has contained this IE, the same IE shall be included in the MODIFY COMPLETE (ACK) message. On the basis of RCSD and additional information about the initial call setup direction (e.g. transaction identifier flag ‑ ref. 04.07), the FA/IWF shall resolve the actual DCD.

At DCD condition "mobile originated" the FA/IWF has to transmit a CNG tone if neither CED nor a BCS signal has been already received, otherwise nothing. At DCD condition "mobile terminated" the FA/IWF has to transmit a CED. At the moment when CT107 goes to ON condition the FA/MT has to generate CED if CT105 was in ON condition, otherwise nothing (see subclauses 8.2.1.1 and 8.2.2.1).

If during the facsimile call the return to speech is necessary (T.30 procedure interrupt request), this must be initiated by the mobile fax machine as well as by the fixed network fax machine (ref. to the diagrams in figure II.14/15). Upon receipt of the alert operator tone the request will be accepted by manual intervention via MT (phone off‑hook) and is reflected to the FA by CT106 and CT109 going to 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 PSTN side a guard timer is necessary to protect against the possible loss of the response to the PRI‑Q(PIN/PIP) sent by the MS. After the execution of ICM (MODIFY message), CT107 is set to OFF condition finishing the fax data phase. Subsequent re‑selection of the data phase will be done 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 FA R‑I/F (CT106/109 in OFF condition) to the fax apparatus which must remain functionally connected to the FA to maintain the connection.

A subsequent reverse change to facsimile phase is also carried out by manual intervention at the MT causing ICM. This will be immediately reflected by circuit 107 going to OFF condition. The successful completion of ICM is indicated towards the fax adaptor by circuit 107 going to ON (provided circuit 108/2 is still in the ON condition).

During the speech phase of a procedure interrupt, the condition of the facsimile terminal of the MS is not reported across the CCITT Recommendation V.24 interface. It may remain functionally connected to the fax adaptor, but in a suspended state.

The precise operation of the fax adaptor for the support of procedure interrupt is implementation dependent.

6.5 Performance constraints

In order to perform the procedures described in the present document the MS and the IWF environment should be designed to be able to transmit and receive facsimile data continuously without any need to flow control the procedure by themselves. This applies specifically for the RLP, L2R, and the LAPB entities within the MT or the IWF, respectively, as well as for the fax adaptor itself.

Furthermore, the RLP entity should be able to make error recovery by using the SREJ command/response. The parameters (timers, repetition counters, etc.) should be set to appropriate values using the negotiation capability of the RLP.

The appropriate setting of parameter values applies also for the LAPB procedure.

In addition it is strongly recommended to operate at an user access rate of 9 600 bit/s, even if the facsimile terminal(s) are not able to work with 9 600 bit/s message speed.