4 Connection types

3GPP43.045Release 17Technical realization of facsimile group 3 transparentTS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

Table 1/43.045 shows the connection elements attributes applicable to this Teleservice (note), adapted from 3GPP TS 43.010.

NOTE: Teleservice 61 includes both speech and data connection types, but Teleservice 62 only the data connection type.

Table 1/43.045: Elements of connection types

Protocol type

of Figure 6 of TS

3GPP TS 43.010

Access to TAF

of the Mobile

Station

Radio interface

connection element

Intermediate

rate

RA1 to RA2

BS-MSC/IWF

connection

element

Model 6: Speech

–

Speech/GSM

–

ITU-T

A-law

Model 5:

Facsimile

Group 3

C/D/S UDI

– 14.4 kbit/s

– 9.6 kbit/s

– 4.8 kbit/s

– 2.4 kbit/s

(note)

C/D/S UDI

– 14.5 kbit/s

– 12.0 kbit/s

– 6.0 kbit/s

– 3.6 kbit/s

C/D/S UDI

– 16 kbit/s

– 8 kbit/s

C/D/S UDI

– 64 kbit/s

Model 5b

Facsimile

Group 3

C/D/S UDI

– 14.4 kbit/s

– 9.6 kbit/s

– 4.8 kbit/s

– 2.4 kbit/s

C/D/S UDI

14,5 or 2X12 kbit/s

12 or 2X6 kbit/s

6 kbit/s

6 kbits

C/D/S UDI

16 or 2X16 kbit/s

16 or 2X8 kbits

8 kbit/s

C/D/S UDI

– 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/43.045

Fax modem rates	No. of substreams	Radio i/f rate	Intermediate Rate	Padding Scheme
14.4 kbit/s	1 2	14,5 kbit/s 12 kbit/s	16 kbit/s 16 kbit/s
12.0 kbit/s	1 2	14,5 kbit/s 12 kbit/s	16 kbit/s 16 kbit/s	FA(5D + S) FA(5D + S)
9.6 kbit/s	1 2	12 kbit/s 6 kbit/s	16 kbit/s 8 kbit/s
7.2 kbit/s	1 2	12 kbit/s 6 kbit/s	16 kbit/s 8 kbit/s	FA(3D + 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/43.045, restricted to what has been negotiated between the MS and the network.

All transitions from one combination in table 1a/43.045 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 4/43.045 shows the scheme of a typical PLMN connection for this Teleservice, considering respectively R and S access at Network Termination.

To grant full support to the ITU-T 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.

Figure 4b/43.045: Optional Teleservice connection (F.F.S)

4.1 Information transfer protocol model

The Figure 5/43.045 depicts the conceptual protocol model for this Teleservice, deduced from model 5 and 5b of Figure 6/43.010.

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 6/43.010 details the placing of the splitting and combiner function for multislot configurations.

Figure 5/43.045: 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 PLMN 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 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 PLMN 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 PLMN 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.

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 ITU-T 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 ITU-T 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 the present document.

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 PLMN 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 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 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 the present document.

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 the present document. 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 PLMN specific NSF frame, containing a Country Code and a unique identification of the FA manufacturer, encoded as per ITU-T 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 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 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:

– 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 3GPP TS 24.008 is carried out.

Secondly, if a T.30 speed drop back is requested, a channel mode modify procedure as detailed in 3GPP TS 24.008 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 ITU-T 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 call setup direction (e.g. Transaction Identifier flag as per 3GPP TS 24.007), 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.‑10/43.045 and II.‑11/43.045). 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 (PLMN 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‑10/43.045), 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 PLMN 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 PLMN signalling;

– waits for the acknowledgement of CMM completed;

– returns back to the usual information passing process.

If the CMM procedure fails, the IWF will initiate the release of the call. However, in calls restricted to one channel and radio interface rate not higher than 12 kbit/s, this does not necessarily apply.

As it will be seen below (see subclause 5.2.4), in both cases the execution of the CMM procedure has to be seen like an exception within the usual information passing process, even overlapping it to maintain the overall end-to-end fixed delay.