7 Use of terminal adaptation functions

03.463GPPTechnical Realization of Facsimile Group 3 Service - non transparentTS

According to the protocol model of the connection types (figure 5/03.46) there are two classes of TAFs to be considered.

7.1 Standard TAFs for synchronous services

The TAFs are those described in GSM 07.03 for synchronous bearer capabilities in the non‑transparent mode, i.e. presently for LAPB only. The rate adaptation functions shall comply with GSM 04.21. The interchange signalling mapping is in accordance with GSM 07.03.

7.2 Specific TAFs for facsimile service

Integral part of an end‑to‑end connection for this Teleservice is the fax adaptor function, located at both the PLMN ends and in charge of:

‑ establishment and maintenance of a LAPB link between the fax adaptation function and the standard synchronous terminal adaptation function according to GSM 07.03, where applicable;

‑ establishment and maintenance of an L2RBOP link between the fax adaptors in the MS and in the MSC/IWF according to GSM 07.03;

‑ transcoding of the document content to be transmitted across the radio interface as described in clause 6 of the present document;

‑ adaptation of the CCITT Recommendation T.30 protocol procedures to the GSM PLMN environment and generation of the fax adaptor protocol elements as described in clause 6 of the present document.

The main features relevant to the CCITT Recommendation T.30 adaptation functions are detailed in the following.

For better clarification only, in the following a double configuration will be referenced:

‑ transmitter adaptation function, established at the PLMN side where the terminal is located actually performing document transmission;

‑ receiver adaptation function, established at the PLMN side where the terminal is located actually receiving the facsimile document.

The proper configuration is settled on both network sides by detecting DIS/DTC frame just at the beginning of the phase B in the CCITT Recommendation T.30 protocol procedure.

The optional error correction mode, as defined in CCITT Recommendation T.4‑Annex A and CCITT Recommendation T.30‑Annex A may be fully supported, provided some specific features are included in the fax adaptor procedure.

These features are relevant to:

‑ additional BCS frames to be detected;

‑ handling of the message phase.

The overall framework as described in clause 6 of the present document applies, i.e. also the procedures of the error correction mode are, in principle, run end‑to‑end between the two facsimile terminals.

The error correction mode is entered upon detection of the relevant bits in the DIS/DTC frame.

The working principle of the present document is based on the detection, control, deletion, and generation of key messages in the end‑to‑end dialogue between the facsimile terminals.

While in BCS phases, the following frames have to be detected:

‑ DIS/DTC, to monitor all operational parameters of the transmitting terminal;

‑ DCS, to realize the actual operational parameters, e.g. message transmission speed accepted by the sender terminal and the relevant message transfer direction (see table 2/CCITT Recommendation T.30);

‑ CFR and MCF, to trigger the message phase;

‑ CTC/EOR, fixing the retransmission strategy by the facsimile transmitting terminal (error correction mode);

‑ CTR/ERR, acting as confirmation message and so closing a BCS phase before a new message phase;

‑ PPR, as above, but after the fourth consecutive PPR request, the BCS phase continues with either CTC or EOR (error correction mode);

‑ DCN, to initiate the call release procedure.

Furthermore, all BCS command messages have to be monitored to eliminate repeated command messages at the local fax adaptation function and to initiate a repetition of those command messages, if necessary, at the remote fax adaptation function. Additionally, all BCS response messages have to be monitored to be able to clear the former condition.

7.2.1 BCS phase

7.2.1.1 BCS command/response procedures

The CCITT Recommendation T.30 procedure is segmented in sections of associated BCS commands and responses. Any command sent by a facsimile terminal must be answered by an appropriate BCS response (refer to Appendix III, CCITT Recommendation T.30). To guarantee that transmission and reception of responses take place right in time, a response transmission (including preamble) shall never be started by the fax adaptor function associated with the command sending terminal later than Ts (response time supervision =  1.6s) after reception of previous command. According to the command/response relationship, the fax adaptor receiving a BCS command from its associated facsimile terminal is further on designated as commanding fax adaptor. Similarly, the fax adaptor receiving a BCS response is called the responding fax adaptor. The procedure is as follows:

Prior to receiving any BCS command/response the preceding preamble is recognized and the "preamble element" is transmitted to the remote fax adaptor. There it is used to start transmission of the preamble towards the associated facsimile terminal with a delay of 300 ms. The preamble preceding a BCS response must be sent with minimum duration permitted (ref. to CCITT Recommendation T.30). If necessary, a preamble must be "aborted" and restarted after an appropriate period as indicated in CCITT Recommendation T.30.

In addition, the recognition of the "preamble element" is immediately confirmed to the other fax adaptor by sending a "BCS transmit request element" (with sequence number set to zero). As this confines a synchronization point, no further protocol transaction can take place except exchanging another "preamble element" until the "BCS transmit request element" has been recognized by the related fax adaptor.

The commanding fax adaptor when receiving the initial "BCS transmit request element" as confirmation to the "preamble element" (condition 1), starts forwarding the received BCS command using "BCS elements" and applying segmentation, if necessary (ref. to subclause 6.2.1). A BCS command which has been received correctly from the associated facsimile terminal (condition 2), is stored within the commanding fax adaptor. If both conditions 1 and 2 apply, any forwarding of BCS commands repetitively received from the facsimile terminal is further on inhibited. Any "BCS transmit request element" received from the responding fax adaptor is answered accordingly, i.e. by the requested BCS element(s), if any. If an error occurs during the reception of the BCS command from the associated facsimile terminal, the commanding fax adaptor transmits a "BCS abort element" towards the responding fax adaptor and regards the whole BCS command as not received. Any "BCS transmit request element" is ignored at this stage. The inhibit status is cleared when either an appropriate BCS response is received and forwarded to the facsimile terminal or the connection is released.

The responding fax adaptor receiving "BCS elements" checks their correct sequence (refer to subclause 6.2.1) and starts reassembling and transmitting the BCS command once either a complete BCS frame or at least the second element of a segmented BCS frame is received from the commanding fax adaptor and no sequence error has been detected. Any "BCS element" received after successful reception of a complete BCS command is ignored. If a BCS command consists of a sequence of BCS frames preceded by a single preamble each BCS frame is forwarded separately across the radio interface. The sequence has to be reconstituted at the responding fax adaptor when sending to the associated facsimile terminal. If necessary, flags are transmitted between the BCS frames.

If during a transmission any required subsequent element is not available, the responding fax adaptor aborts the BCS command transmission to the facsimile terminal, and the rest of the BCS command is not transmitted. The same applies, if due to residual errors on the radio interface (e.g. RLP link reset) "BCS elements" are missing which can be detected by a wrong sequence number. In these cases, any "BCS element" following the error event (e.g. with a number out of sequence) is ignored. The responding fax adaptor may therefore need to use a "BCS transmit request element" in order to ask for (re)transmission of outstanding elements, if any.

If a "BCS abort element" has been received by the responding fax adaptor and the transmission is aborted due to this fact, the complete BCS command including the preceding "preamble element" will be retransmitted autonomously by the commanding fax adaptor.

Contiguously received parts of a BCS command received from the radio interface are stored in the responding fax adaptor. The stored BCS command when completed, is used for autonomous retransmissions towards the associated facsimile terminal which may start at the earliest possible time according to CCITT Recommendation T.30. No repetition counter is provided.

The responding fax adaptor when receiving the initial "BCS transmit request element" as confirmation to the "preamble element" (condition 1), starts forwarding the received BCS response using "BCS elements" and applying segmentation, if necessary (ref. subclause 6.2.1). A BCS response which has been received correctly from the associated facsimile terminal (condition 2), is stored within the responding fax adaptor. Any "BCS transmit request element" received from the commanding fax adaptor is answered accordingly, i.e. by the requested BCS element(s), if any.

If an error occurs during the reception of the BCS response from the associated facsimile terminal, the responding fax adaptor transmits a "BCS abort element" towards the commanding fax adaptor and regards the whole BCS response as not received. Any "BCS transmit request element" is ignored at this stage. The autonomous command repeat status is cleared when both conditions 1 and 2 apply.

The commanding fax adaptor receiving "BCS elements" checks their correct sequence (refer to subclause 6.2.1) and starts reassembling and transmitting the BCS response once either a complete BCS frame or at least the second element of a segmented BCS frame is received from the responding fax adaptor and no sequence error has been detected. Any "BCS element" received after successful reception of a complete BCS response is ignored. If a BCS response consists of a sequence of BCS frames preceded by a single preamble each BCS frame is forwarded separately across the radio interface. The sequence has to be reconstituted at the commanding fax adaptor when sending to the associated facsimile terminal. If necessary, flags are transmitted between the BCS frames.

If during a transmission any required subsequent element is not available, the commanding fax adaptor aborts the BCS frame transmission to the facsimile terminal, and the rest of the BCS frame is not transmitted. The same applies, if due to residual errors on the radio interface (e.g. RLP link reset) "BCS elements" are missing which can be detected by a wrong sequence number. In these cases, any "BCS element" following the error event (e.g. with a number out of sequence) is ignored. The commanding fax adaptor may therefore need to use a "BCS transmit request element" in order to ask for (re)transmission of outstanding elements, if any.

If a "BCS abort element" has been received by the commanding fax adaptor and the transmission is aborted due to this fact, the complete BCS response including the preceding "preamble element" will be retransmitted autonomously by the responding fax adaptor.

Contiguously received parts of a BCS response received from the radio interface are stored in the commanding fax adaptor. The stored BCS response when completed, is used for autonomous answering towards the associated facsimile terminal which may start at the earliest possible time according to CCITT Recommendation T.30.

After transmission of a response towards the fax apparatus by the fax adaptor function, after which the fax adaptor function waits for message data (CFR, CTR, MCF after MPS, PPS_MPS, PPS_NULL, ERR after EOR_MPS, EOR_NULL) a repeated BCS command shall be locally responded, without transferring a preamble element towards the radio link.

The fax adaptors have to take care of the control of the local modem. The condition is derived from the reception of certain fax adaptor protocol elements. Additionally, there must be a control of the half duplex transmission path towards the associated facsimile terminal. For that purpose, the receiver signal is monitored and the transmitter is only activated, if no receive signal is active.

7.2.1.2 Compatibility checking

Some features cannot be supported in the GSM PLMN environment. The fax adaptor function is in charge of dealing with such compatibility checking which is carried out by monitoring certain BCS frames (DIS/DTC).

‑ Group 1 and group 2 equipments are not supported by the Teleservice as described in the present document.

‑ Error limiting mode cannot be supported.

‑ Only standard 300 bit/s Binary Coded Signalling is supported. To this purpose the fax adaptor will ignore the 2400 bit/s capability within the phase B of the CCITT Recommendation T.30 procedure by looking for DIS frames from CCITT Recommendation V.21 modem only.

‑ Only facsimile message speeds up to and including 9 600 bit/s are supported. For this purpose the fax adaptors are responsible to carry out appropriate actions, e.g. to set the applicable fields in the DIS frame accordingly.

‑ It is not possible to support non‑standard facilities since some of these contain proprietary methods of changing the modem speed, invisible to the IWF and hence impossible to track. If an NSF frame occurs, the country code has to be exchanged in the IWF to the GSM specific country code, which is not supported by a standard fax group 3 apparatus and therefore no private procedure is entered (refer to subclause 6.2.4). However, non‑standard BCS frames may carry information which does not affect the correct operation within the PLMN, but is essential for the user (e.g. passwords). Those elements, if any, will be passed end‑to‑end by the fax adaptors unless an explicit deletion is required by the user (e.g. by specific input at the fax adaptor associated with the mobile station).

7.2.1.3 Message speed checking

Although it is strongly recommended to use the maximum user rate of the MT2 (9 600 bit/s), a particular user may signal a lower user rate. In this case the fax adaptors have to carry out the following additional procedures:

‑ max speed indicated in the call set‑up message is 4 800 bit/s:

‑ if the fax adaptor receives a DIS or DTC indicating CCITT Recommendation V.29 only, the call shall be released;

‑ if CCITT Recommendation V.29 and V.27ter is indicated, this shall be changed to V.27ter;

‑ if other values are indicated, no action has to be taken;

‑ max speed indicated in the call set‑up message is 2 400 bit/s:

‑ if the fax adaptor receives a DIS or DTC indicating CCITT Recommendation V.29 only, the call shall be released;

‑ if CCITT Recommendation V.29 and V.27ter or V.27ter only is indicated, this shall be changed to V.27ter fallback;

‑ if CCITT Recommendation V.27ter fallback is indicated, no action shall be taken.

7.2.1.4 Control of transmission rate

The controlling entity of the fax adaptor recognizes when a change of the transmission rate ‑ and in conjunction with this a change of the modem function ‑ has to commence. The transmission rate is then changed only locally, i.e. between the facsimile terminal and the fax adaptor at both the MSC/IWF and the MS ends.

The actual message speed and the modem function are derived from the content of the related BCS frames (DIS/DTC, DCS, CTC).

7.2.1.5 Clocking

The fax adaptor or the GSM facsimile machine will acquire received data bit timing on circuit 115 (according to CCITT Recommendation V.24). The transmitter element timing circuit 114 shall be synchronized to circuit 115.

The clock rate at the CCITT Recommendation V.24 interface will reflect the user rate of the MT2 as indicated in the set‑up message.

7.2.2 Message phase

During the message phase (phase C of CCITT Recommendation T.30) a single bit pattern has to be detected, the EOL character (see subclause 4.1.2/CCITT Recommendation T.4), a unique code word that can never be found within a valid line of facsimile coded data, and is used, as per CCITT Recommendation T.4:

‑ to identify the start of message phase;

‑ to control the buffer level;

‑ to mark the end of message phase (6 consecutive instances).

7.2.2.1 Normal facsimile data

The message phase (see figure II.6/GSM 03.46 and II.7/GSM 03.46, respectively) at both the PLMN ends is triggered by the transit of a frame (either the CFR or the MCF) sent by the receiving terminal to confirm a previous frame from the transmitting terminal, and marking the end of a BCS phase.

The terminal adaptation function associated with the receiving terminal after receiving facsimile coded data or autonomously 5.5s after detecting the trigger frame (CFR or MCF) will change the modem function to V.27ter or V.29 CCITT Recommendation and initiate the training at the applicable speed.

Following the training segment, 0s bits will be stuffed towards the facsimile terminal (FILL sequence, see subclause 4.1.2 of CCITT Recommendation T.4), disregarding all information received from the radio interface, until a EOL character is detected, that will mark the beginning of the real phase C (see figure 1/CCITT Recommendation T.4).

If due to a preceding error the message phase cannot be entered, this training must be aborted when a new BCS element is received by the transmitting fax adaptor.

7.2.2.2 Error correction facsimile data

As these facsimile coded data between the fax adaptor and the facsimile terminal are structured in HDLC frames, the handling of this procedure segment will exploit such formatting. The content of such an HDLC frame is further on called a block.

Each such block is included in the information field of a error correction data element of the FA protocol which is processed for transmission across the radio interface as outlined in clause 6.

The message phase (see figure II.8/GSM 03.46) at both the PLMN ends is triggered by the transit of a confirmation frame (CFR, MCF, PPR, CTR or ERR) sent by the receiving terminal and marking the end of the BCS phase.

If four consecutive PPR are counted within the same "partial page", the BCS phase continues.

The transmitter adaptation function will enter the message phase as per CCITT Recommendation T.30 standard procedure.

The terminal adaptation function associated with the receiving terminal after receiving facsimile coded data or autonomously 5.5s after detecting the trigger frame will change the modem function to V.27ter or V.29 CCITT Recommendation and initiate the training at the applicable speed.

Following the training segment, HDLC flags will be stuffed towards the facsimile terminal until a FCD frame is detected, that will mark the beginning of the real phase C.

If due to a preceding error the message phase cannot be entered, this training must be aborted when a new BCS element is received by the transmitting fax adaptor.

7.2.2.3 Buffering of facsimile coded data

The following subclauses only apply, when using the normal facsimile data transfer, i.e. not with the error correction mode.

7.2.2.3.1 Transmitter adaptation function

In the transmitter adaptation function the facsimile coded data being received from the facsimile terminal are transcoded stripping of FILL information and written into the buffer.

If there is enough information available, this data is read out from the buffer, and a FA protocol element is generated which is processed as described in clause 6 to be transferred to the receiver adaptation function using one of the standard TAFs referred to in subclause 7.1. For that purpose the data is segmented in blocks (see subclause 6.2.5.2).

Due to the ARQ techniques of the RLP the throughput across the radio interface may be less than the message speed between the transmitting facsimile terminal and the transmitter adaptation function, i.e. the content of the buffer may increase. When a certain threshold is reached from which the fax adaptor can derive that the actual page cannot be transmitted successfully, the connection may be prematurely released.

If the throughput at the radio interface is greater than the message speed between the transmitting facsimile terminal and the transmitter adaptation function (e.g. when the end‑to‑end speed is lower than 9 600 bit/s), the buffer may be empty most of the time.

7.2.2.3.2 Receiver adaptation function

In the receiver adaptation function FILL information is transmitted to the facsimile terminal at the beginning of each page, if necessary, to bridge the gap between the training sequence and the real facsimile coded data. In case of normal fax data the FILL 0’s can be expanded up to 5s only and therefore after these up to two white scan lines should be inserted, if necessary.

The facsimile coded data received across the radio interface are re‑generated from the LAPB, L2R and FA protocol elements, reversely transcoded according to the knowledge of the fax adaptor, and written into the buffer. The reverse transcoding consists of insertion of FILL information before the facsimile coded data is forwarded to the facsimile terminal to comply with the recognized minimum line length as defined in CCITT Recommendation T.4.

At the beginning of each page the facsimile coded data to be sent to the facsimile terminal is not read out from the buffer until at least 2 instances of EOL or an RTC have been received or the following buffer size limit, depending on the end to end data transfer rate, has been exceeded:

2 kByte for 2,400 bit/s;

4 kByte for 4,800 bit/s;

6 kByte for 7,200 bit/s;

8 kByte for 9,600 bit/s.

Once this procedure has been started, i.e. during the page transmission, the facsimile coded data is transmitted, however, the following EOL is delayed by inserting additional FILL information, if necessary, until the pre‑set threshold (2 EOLs or the buffer size limit) is reached again. If the actual coding line is going to exceed 5 s, the threshold is temporarily reduced, i.e. the following EOL is sent. However, the buffering algorithm shall try to reach the pre‑set threshold again as fast as possible (by inserting FILL also before following EOLs). If no EOL is available to be transmitted to the facsimile terminal for a period greater than 5 s, the connection will be released by an ordinary receiving facsimile terminal (ref. CCITT Recommendation T.4).

7.2.3 Disconnect procedure

The transmitter adaptation function, upon detection of the DCN frame (see CCITT Recommendation T.30) sent by the local terminal to indicate the end of the facsimile transmission, initiates the disconnect procedure.

7.2.4 Timeouts

The overall fax adaptation function is in principle bound to the timing constraints associated with the end‑to‑end CCITT Recommendation T.30 procedure. This means that, no matter of the reference configuration used at the mobile station, either the "standard" one (figure 2a/GSM 03.46) or the "GSM facsimile machine" (figure 2d/GSM 03.46), the progress of the call will be mainly subject to the CCITT Recommendation T.30 typical timing protections, settled externally.

However, due to the specific conditions caused by the GSM PLMN system, there is the need for a special support with respect to BCS command repetitions as explained above. For that purpose, the fax adaptors will provide means for local time‑out. The timer will be started and stopped as described in the applicable clauses of the CCITT Recommendation T.30.