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THE DRAWINGS CONTAINED IN THIS RECOMMENDATION HAVE BEEN DONE IN AUTOCAD.
Recommendation U.201
INTERWORKING BETWEEN THE TELETEX SERVICE
AND THE TELEX SERVICE
(Melbourne, 1988)
1 Introduction
1.1 This Recommendation defines the procedures to be followed for interworking
between the Teletex service and the telex service.
1.2 This Recommendation is one of the a series which define interworking
between the telex service and the Teletex service.
The other Recommendations are:
a) Recommendation F.200: Teletex service
b) Recommendation T.390: Requirements for interworking with the telex
service
c) Recommendation F.201: Interworking between the Teletex service and
the telex service - General principles
2 Basic interworking service
2.1 Types of interworking
2.1.1 Interworking between the Teletex service and the telex service consists of
two directions:
a) the telex to Teletex direction;
b) the Teletex to telex direction.
2.2 Methods of interworking
2.2.1 considering:
(a) that the Teletex service can be provided upon various networks (see
Recommendation F.200, S 2);
(b) that an Administration1) can provide the Teletex service on more than
one network (e.g. PSTN and PSPDN . . .);
(c) that the technical constraints of the existing networks (e.g. numbering
plans, . . .),
the two following methods of interworking between the telex service and
the Teletex service can be provided:
i) interworking with one-stage selection;
ii) interworking with two-stage selection.
2.2.2 The service requirements of each method are described in Recommendation
F.201, SS 3 and 4 respectively.
3 Telex access to a conversion facility (CF)
3.1 One-stage selection
Recommendation F.201, S 3 describes the service principles for this method
of interworking.
3.1.1 Access from a manual telex terminal
Interworking in the telex to Teletex direction using one-stage selection,
in case of manual terminals, is described in Figure 1/U.201 and associated notes.
Figure 1/U.201 - T0900100-89
Notes relative to Figure 1/U.201:
Note 1 - The total selection information of the Teletex subscriber should not be longer
than 12 digits.
Note 2 - See Recommendations T.90 and U.70.
Note 3 - The Teletex answerback sent to the telex subscriber is a rearranged Teletex
terminal identification, as follows:
» ¼ = Part 2 of eq [»] « ╪
TTX-id \b(\a\co2\hs2(
= » Part 4
,of, TTX-id))
1 1 1 up to 12
1) or a recognized private operating agency.
Fascicle VII.2 - U.201 PAGE1
up to 15 up to 4
20 characters
where:
* "TTX-id" is for Teletex terminal identification according to Figure 2/F.200 (Red
Book).
* Q is the telex network identification code as provided in Recommendation F.69 (Red
Book).
* [ ] indicate an optional part or signal.
i) [»] before "« ╪" does not exist if Part 4 of TTX-id is included.
ii) Part 2 of TTX-id has the same definition as given in S 7.5.4 of Recommendation
F.200.
iii) Part 4 of TTX-id, whenever included, has the same definition as given in S 7.5.6
of Recommendation F.200 but the minimum of letters could be reduced to one
(instead of three);
iv) The length of the Teletex answerback is set to 20 characters, the eventual unused
positions are filled with "»" according to Recommendation F.60.
v) As far as possible, the mnemoric part is included in the Teletex answerback.
Examples of TTX-A/B:
1 2 3 4 5 6 7 8 9 10 11 12 1314
15 16 17 18 19 20
¡ ¼ ║
PAGE16 Fascicle VII.2 - U.201
6 1 5 1 9 1 7 = » F T
Z » » » « O
¡ ¼ ║ 1 2 3 4 5 6 7 8 9 1011
12 =
Fascicle VII.2 - U.201 PAGE1
» « N L
¡ ¼ ║ 1 2 3 4 5 6 7 8 = » M
N E M O « F
PAGE16 Fascicle VII.2 - U.201
Note 4 - The CF shall accept the possible occurrence of an extra WRU at this stage. The
answerback returned by the CF is the Teletex answerback obtained during the validation
call. (See Note 3.)
Note 5 - The CF shall wait at least 15 seconds for the message input. See Annex for
abnormal conditions during text input.
Note 6 - i) This procedure is recommended but not mandatory. (See Recommendation F.60.)
ii) The answerback returned by the CF is the Teletex answerback obtained during the
validation call. (See Note 3.)
Note 7 - i) The end of input (EOI) signal is the plus symbol (ITA No. 2 combination No.
26 Figure case) repeated four times (+ + + +). This EOI signal need not be
transferred to the Teletex side.
ii) If the caller clears without the EOI signal, the network will endeavour to
deliver the message as received. However, customers must be clearly warned that
the network cannot guarantee delivery in these circumstances.
Note 8 - i) The CF shall wait 800 ms for WRU after EOI. If no WRU is received within
this period, the terminal is considered as a manual one. A MOM signal is then
sent by the CF at each call attempt. If a WRU is sent by the telex terminal
within this period the terminal is considered as a telex automatic emitting
device.
ii) When the EOI signal is received, the conversion facility shall immediately
establish the delivery call to the Teletex terminal. In the case of
unsuccessful calls on the PSPDN and CSPDN, the conversion facility shall make
several attempts with five-second intervals measured from the end of one
attempt to the beginning of the next. At each attempt, the MOM signal followed
by service signals defined in Recommendation U.70, if appropriate, shall be
sent to the waiting telex terminal. As a whole, the telex line is not to be
held for a period exceeding 30 seconds. If all these attempts are unsuccessful,
the conversion facility shall send the service message ITL->TTX->╪/(ITL - I'll
transmit later) to the waiting telex terminal and clear down. It shall then be
the responsibility of the Administration operating the conversion facility to
effect delivery of the message by whatever means are available.
Note 9 - This answerback returned by the CF is the Teletex answerback obtained during the
delivery call to the Teletex terminal and having the same format and content as described
in Note 3. This answerback is an "on-line" delivery acknowledgement for the telex user.
3.1.2 Access from a telex automatic emitting device (TAED)
Interworking on the telex to Teletex direction, using one-stage selection,
in case of TAEDs is described in Figure 2/U.201 and associated notes.
Figure 2/U.201 - T0900110-89
Notes relative to Figure 2/U.201:
Remark - Taking into account that most of the notes related to the two figures are
similar, the notes related to Figure 2/U.201 only emphasize the differences from notes to
Figure 1/U.201.
Notes 1 to 5 - Identical to notes of Figure 1/U.201.
Note 6 - Same as Note 7, Figure 1/U.201.
Note 7 - i) The CF shall wait 800 ms for WRU after EOI. The terminal is considered a
telex automatic emitting device (TAED) if a WRU is sent within this period.
The answerback returned by the CF is the Teletex answerback obtained during the
validation call (see Note 3) to confirm that the connection still exists. It is
not a proof of delivery.
No progress signal (e.g. MOM, ITL«TTX«╪) will be returned by the CF to the
telex automatic emitting device after the EOI signal.
ii) When the EOI signal is received, the CF should establish immediately a delivery
call to the Teletex terminal.
Note 8 - The CF shall make several attempts with five-second intervals measured from the
end of each attempt to the beginning of the next. It is the responsibility of each
Administration operating the conversion facility to effect the delivery of the message by
whatever means are available.
3.2 Two-stage selection
3.2.1 Service principles
Recommendation F.201, S 4, describes the service principles for this
method of interworking.
3.2.2 Access from a manual terminal
3.2.2.1 Interworking in the telex to Teletex direction, using two-stage
selection, in case of manual terminals, is described in Figure 3/U.201 and
associated notes.
Figure 3/U.201 - CCITT 69202
Notes relative to Figure 3/U.201:
Note 1 - An example of telex procedures is:
FIGURE - CCITT 74090
If a WRU is received by the CF, it shall return its answerback. Coding of the CF
answerback is the following:
¡ ¼ = CF NATIONAL NUMBER « » T«TTX«╪ up to 7 digits
Fascicle VII.2 - U.201 PAGE1
Note 2 - Address input: see S 3.2.2.2 of this Recommendation.
Note 3 - WRU signal should be sent immediately after the address input.
Note 4 - If the returned telex answerback cannot be processed according to Recommendation
U.74 and if no telex address was input, and no skipping indication was given [see S
3.2.2.2.1 b) of this Recommendation], then the CF shall request the telex address with a
specific prompt signal:
¼ ║ » ADD
Remark - The telex address input in response to this ADD must be terminated with a "+",
Combination No. 26 of ITA2 in figure case.
If a telex address is not received within 15 seconds after the prompt signal ADD has been
sent, the CF should proceed by sending back a GA signal. In this case, an NDN signal
cannot be sent to this telex user.
Note 5 - Validation of the Teletex address by means of a validation call or data base
retrieval of the Teletex address (see Recommendation F.201).
Note 6 - Validation result (see S 3.2.2.4 of this Recommendation).
Note 7 - See Note 5, Figure 1/U.201.
Note 8 - EOI is coded as four ITA2 Combination No. 26 "++++" in figure case. This EOI
signal needs not to be transferred to the Teletex side.
Note 9 - Content of IMA is defined in S 3.2.2.5.
Note 10 - The telex user may maintain connection at this step of the procedure in order to
obtain an on-line delivery acknowledgement (see S 3.2.2.7 of this Recommendation).
Note 11 - The CF may offer the follow-on message facility at this step (see S 3.2.2.8 of
this Recommendation).
Note 12 - After the call reattempt procedures have been completed and the message cannot
be delivered to the Teletex terminal, a non-delivery notification (NDN) should be sent to
the originating telex customer (see S 3.2.2.6 of this Recommendation).
PAGE16 Fascicle VII.2 - U.201
3.2.2.2 Address input
3.2.2.2.1 Single address input format
a) Case 1 - If the telex subscriber has a processable answerback according
to Recommendation U.74, minimum address input is:
FIGURE - CCITT 74101
Teletex address is the call-numeric string necessary to select the Teletex
terminal, i.e.
- if there is only one network supporting the Teletex service, the
Teletex address is the national Teletex number;
- if there is more than one network supporting the Teletex service, the
Teletex address includes the DNIC/TCC number according to
Recommendation X.121. DNIC or TCC may be separated from the national
Teletex number by a hyphen (-), Combination No. 1 of ITA2.
b) Case 2 - If the telex subscriber has a non-processable answerback
according to Recommendation U.74, he should input the following string:
FIGURE - CCITT 74111
Telex address is the Recommendation F.69 code followed by the national telex
number. Any spaces shall be ignored by the CF.
c) Case 3 - A telex subscriber may omit specifying the calling address,
either inadvertently or intentionally, by transmitting:
FIGURE - CCITT 82050
In this case no "ADD" signal should be sent by the CF.
d) Remark - In all cases the signal "+" is not to be used within the
address input for other purposes than to indicate "end of address".
3.2.2.2.2 Multiple address input facility
- This facility enables the calling telex subscriber to deposit messages
addressed to multiple Teletex recipients.
- If this facility is provided by the CF, the calling telex subscriber
should separate each Teletex address (including or not the mnemonic part) by:
¼ ║
- The input of the last address, the calling telex address (if required)
and the EOA signal, shall follow the rules established for the single address in
S 3.2.2.2.1 of this Recommendation.
- The calling telex subscriber should know whether the called CF offers
the multi-address facility. However, if the facility is invoked by the calling
telex subscriber but not offered by the CF, this will be treated as an abnormal
condition and the suggested reaction of the CF is in the annex to this
Recommendation.
3.2.2.3 Request for positive delivery notification (PDN)
- If this facility is provided by the CF the method of requesting a PDN
shall be to follow the Teletex address (or each Teletex addresses if
multi-address is also offered) for which a PDN is required, by the sequence:
, ACK
- Example: (in this example both the PDN and the multi-address facilities
are offered)
. . .
3029 - 500 9145 = XYZ, ACK
3029 - 500 9090
+
. . .
- The possibility of requesting a PDN for all Teletex addresses by means
of a single indicator, is for further study.
- The calling telex subscriber should be aware whether the CF offers this
facility or not.
However, if a calling telex subscriber requests a PDN from a CF which does
not offer this facility, this shall be treated as an abnormal condition and is
described in the annex to this Recommendation.
3.2.2.4 Validation result
3.2.2.4.1 Positive validation result
- The format of the positive validation result is:
Fascicle VII.2 - U.201 PAGE1
¼ » VAL « ¡ Telex terminal identification
║
- The Teletex terminal identification shall be in accordance with Note 6
to Figure 7/U.201.
- The format of the progress signal following the validation result is:
¼ ║ » GA
- If the CF offers the multi-address input facility, validation of
addresses shall continue until a valid address is found.
- If the timing constraints have been met (i.e. a valid address has been
found within 5 seconds of receipt of the EOA signal) it is the Teletex terminal
identification of this valid address which is returned with the validation
result, in the format described above.
- Where time to find a valid address exceeds 5 seconds after receipt of
the EOA signal, the CF shall behave according to S 4.1.4 of Recommendation F.201
and Table 1/F.201.
3.2.2.4.2 Negative validation answer
- If the validation leads to a negative result, the CF should send at
least the telex service signal "NP" or, if available, other appropriate service
signals according to Recommendation U.70. The CF should then clear the call.
- Where the CF offers multi-address input, all addresses shall be
validated. As this may be difficult in real-time, the CF should return the
addresses found to be invalid in a single non-delivery notification.
4.2.2.4.3 Action of the CF following validation
The action of the CF following validation shall be as prescribed in S
4.1.4 of Recommendation F.201, and Table 1/F.201.
3.2.2.5 Input message acknowledgement
The input message acknowledgement (IMA) is to be returned by the CF to the
calling telex subscriber after the EOI.
This information is used as the message reference in case of notifications
(NDN or PDN).
The format of the input message acknowledgement is:
¼ IMA « ¡ yy « mm « dd « hh:mm ¼ ║ xxx xxx ¼ ║
║
»
where the xxx xxx¼ ║ is an additional reference number and is optional.
3.2.2.6 Notifications format
3.2.2.6.1 Non-delivery notification (NDN) format
- The NDN message has the format and content described in Figure 4/U.201:
C¡ ¼ ║CF NATIONAL NUMBER « » T « TTX « ╪ [»]
PAGE16 Fascicle VII.2 - U.201
o¼ ║ ¼ ║
n¼ ║ NDN [« TELETEX]
t¼ ║ ¡ YY « MM « DD « HH:MM ¼ ║ Date and time of the CF
e¼ ║ » TELETEX « ADDRESS ¡:XX - - - XX Address received by the CF
n¼ ║ » IMA « ¡ YY « MM « DD « HH:MM ¼ ║ [xxxxxx (The same information as given
t¼ ║ » CAUSE ¡ :xxx ¼ ║ Telex service signal of the
Note - Text given in [ ] is optional.
¡ Figure shift
¼ Carriage return
║ Line feed
» Letter shift
« Space
FIGURE 4/U.201
Fascicle VII.2 - U.201 PAGE1
When the multi-address facility is offered, one separate NDN is returned
to the originator, for each non-delivered address (recipient).
3.2.2.6.2 Positive delivery notification (PDN) format
- The PDN message has the format and content described in Figure 5/U.201.
C¡ ¼ ║CF NATIONAL NUMBER « » T « TTX « ╪ [»]
o¼ ║ ¼ ║
n¼ ║ PDN [« TELETEX]
t ¼ ║ ¡ YY « MM « DD « HH:MM ¼ ║ Current date and time of the CF
e ¼ ║ » TELETEX ADDRESS ¡:XX - - - XX Teletex address received by the
n¼ ║ » YOUR « MESSAGE « REF ¡:
t¼ ║ » IMA « ¡ YY « MM « DD « HH:MM ¼ ║ [xxxxxx Information provided by the CF
PAGE16 Fascicle VII.2 - U.201
Note - Text given in [ ] is optional.
¡ Figure shift
¼ Carriage return
║ Line feed
» Letter shift
« Space
FIGURE 5/U.201
Fascicle VII.2 - U.201 PAGE1
3.2.2.7 Text delivery
3.2.2.7.1 Text delivery with ODA
If the CF provides the On-line delivery acknowledgement (ODA) facility, it
sends a MOM signal immediately after the IMA.
The CF attempts to establish the delivery call within a maximum pariod of
30 seconds, with several attempts (at least one in the case of PSTN). Attempts
should be made at 5-second intervals measured from the end of one attempt to the
beginning of the next.
A MOM signal is returned after each attempt followed eventually by network
service signals.
If the message delivery succeeds the CF returns to the telex user the
called party's Teletex answerback, as described in Note 6 of Figure 7/U.201, and
clears the call.
If the Teletex call establishment fails within 30 seconds, the CF sends
service signal "ITL" and clears the call, the procedure is then as in S 3.2.2.7.2
below.
3.2.2.7.2 Text delivery without ODA
If the ODA facility is not provided, the CF sends a service signal "ITL"
immediately after the IMA, and clears the call.
After sending an ITL signal, in all cases, the CF should attempt to
deliver the message within four hours. The CF should make at least 16 series of
four calls, with 15 minutes between each series. (These figures may be revised in
some cases, e.g. in the case of a PSTN.)
If the delivery fails despite the performance of the cycle of delivery
attempts, the CF should send a non-delivery notification (NDN) with the format
described in S 3.2.2.6.1.
3.2.2.8 Follow-on message facility
- Where this facility is offered, rather than clearing the call (as
stated in SS 3.2.7.1 and 3.2.7.2) the CF should generate a prompt in the format
below:
¼ ║ TTX « NBR:
- The CF shall wait up to 15 seconds for the start of a new input to
appear, after this delay the CF shall clear the call.
- The CF shall treat the follow-on message as if it had been received in
a completely separate session.
3.2.3 Access from a telex automatic emitting device (TAED)
3.2.3.1 Interworking on the telex to Teletex direction, using two-stage
selection, in case of TAEDs is described in Figure 6/U.201 and appending notes.
Figure 6/U.201 - CCITT 69
Notes relative to Figure 6/U.201:
Note 1 - An example of a call procedure by means of telex automatic emitting device is
shown below:
FIGURE - CCITT 74120
a) Network generated WRU.
b) CF answerback may be preceded by date/time information and register numbers,
etc. which may be followed by a recorded message.
c) WRU generated by telex automatic emitting devices.
d) Clear CF answerback for validation purposes.
e) Telex automatic emitting devices transmit their answerback at this stage in
accordance with Recommendation F.60, S A.1.
Notes 2, 3 and 4 - See S 3.2.3.2.
Note 5 - Same as for Figure 3/U.201
Note 6 - Negative validation result, if it occurs, is returned according to S 3.2.3.4.2 of
this Recommendation.
Notes 7 and 8 - Same as for Figure 3/U.201.
Note 9 - The clearing procedure is in accordance with Recommendation S.20.
Note 10 - See S 3.2.3.4 of this Recommendation.
- If the validation result is positive, but abnormal conditions have occurred during
text input (see annex to this Recommendation), the CF should send a service code instead
of its answerback.
- If the validation result is negative and arrives after text input, the CF
answerback will be replaced by a service signal as defined in Recommendation U.70.
Note 11 - If the subscriber is still on-line, at this stage, the CF should clear the call.
If the follow-on message facility is offered the CF should wait 15 seconds before
clearing, for a possible new message input. No prompt should be sent to the user by the
CF.
Note 12 - Same as for Figure 3/U.201.
PAGE16 Fascicle VII.2 - U.201
3.2.3.2 Address input
3.2.3.2.1 Single address input format
a) Case 1 - Whether or not the telex subscriber has a processable
answerback according to Recommendation U.74, he should input his telex
address as described below:
FIGURE - CCITT T0900120-89
- The start of address "CI" informs the CF that prompts that
validation information and WRU signal must not be returned to the
telex automatic emitting devices.
- Teletex address and telex address have the same definition as in S
3.2.2.2.1 to this Recommendation.
b) Case 2 - A telex subscriber may omit his address input either
inadvertently or intentionally by transmitting:
FIGURE - CCITT T0900130-89
c) Remark
In all cases:
- no "ADD" signal should be sent by the CF;
- the signal "+" is not to be used within the address line for other
purposes than to indicate "end of address".
3.2.3.2.2 Multi-address input format
(See S 3.2.2.2.2, Recommendation U.201.)
3.2.3.3 Positive delivery notification (PDN) facility request
(See S 3.2.2.3.)
3.2.3.4 Validation result
3.2.3.4.1 Positive validation answer
- The result of the address validation, if positive, should be sent to
the user together with the IMA, after successful completion of the message input.
- The positive validation result, in the format defined in S 3.2.2.4.1 of
Recommendation U.201, and the IMA in the format defined in S 3.2.2.5, should
follow without pause, the transmission of the CF's answerback in response to the
first "WRU" requested by the user after the end of message input "EOI" (see step
referred by Note 10 in Figure 6/U.201).
- If no answerback is requested by the telex user at this stage (Note 10,
Figure 6/U.201) positive validation answer and IMA cannot be returned.
- In all cases, if a second WRU is issued by the telex user, at this
stage (Note 10, Figure 6/U.201), the CF shall return only its answerback.
- This is in order to allow a positive match, if the telex TAED performs
a comparison between the answerbacks returned at the start and the end of the
call.
- Where the multi-address input facility is offered the CF should
continue validating addresses until a valid one is found, and return the result
of the first positive one in the way described above.
3.2.3.4.2 Negative validation answer
If the validation result is negative, the CF should transmit sequences of
characters "T" according to Recommendation S.4 to interrupt the telex terminal
transmission. If the terminal continues to transmit for more than 20 seconds, the
CF shall clear the call. This will be followed, after a pause of one second, by
the appropriate service signal, according to Recommendation U.70, and clear the
call.
Where the multi-address input facility is provided and all addresses are
found to be invalid, this should be treated as a negative validation result and
the CF shall act in accordance with S 4.1.4 of Recommendation F.201 and Table
1/F.201.
3.2.3.5 Notification formats
Positive and negative notification (PDN and NDNs) formats are as described
in SS 3.2.2.6.2 and 3.2.2.6.1 respectively.
4 Delivery procedure to Teletex from a CF
4.1 Service principles
Recommendation F.201, SS 3.2 and 4.2 describe the service principles for
this direction.
4.2 Text delivery
After clearing, the CF should attempt to deliver the message within four
hours. The CF should make at least 16 series of four calls, with 15 minutes
Fascicle VII.2 - U.201 PAGE1
between each series. (These figures may be revised in some cases, e.g. in the
case of a PSTN.)
If the delivery fails despite the performance of the cycle of delivery
attempts, the CF should send a non-delivery notification (NDN). This information
is sent to the Teletex user with the complete reference of the related message in
order to allow the Teletex user to take further action. No further delivery
action shall be taken by the CF.
Text delivery to the telex terminal is described by Figure 7/U.201, for
both one- and two-stage selection CFs.
The main difference between these two CFs is the content of the answerback
of the CF.
Figure 7/U.201 - CCITT 69163
Notes relative to Figure 7/U.201:
Note 1 - See Recommendation T.90.
Note 2 - a) Case of one-stage selection CF
If a WRU is received by the CF, it shall return the rearranged Teletex terminal
identification, as described in Note 3 to Figure 1/U.201.
b) Case of two-stage selection CF
If a WRU is received by the CF, it shall return its answerback. Coding of the CF
answerback is the following:
¡ ¼ ║ CF NATIONAL NUMBER « » T « TTX « ╪
» up to 7 digits «
Note 3 - This step is only necessary if the answerback was not available during normal
telex procedures.
Note 4 - a) Case of one-stage selection CF
PAGE16 Fascicle VII.2 - U.201
Service message indicating that the call is being made by a telex automatic emitting
device and showing clearly the origin of the call. The coding of the service message to be
returned is the following:
» ¼ ║ CI ¼ ║ TTX « ╪
b) Case of two-stage selection CF
Service message indicating that the call is being made by an automatic installation and
showing clearly the origin of the call.
The coding of the service message to be returned is the CF telex answerback [as
described in Note 2 b] preceded by:
¼ ║ CI
Note 5 - Reference of the message is the Teletex date and time of the message input, in
the following format:
¼ ║ » REF « ¡ yy « mm « dd « hh:mm ¼ ║ xxx xxx ¼ ║
Remark - xxx xxx ¼ ║ is an additional reference and is optional.
Note 6 - a) Case of one-stage selection CF (see Note 3 to Figure 1/U.201).
b) Case of two-stage selection CF
This rearranged Teletex terminal identification (or "Teletex answerback") contains the
Teletex subscriber directory information:
- DNIC or TCC and national number according to Recommendation X.121 if there is more
than one network for Teletex service (in this case, DNIC or TCC are separated from the
national number by a hyphen (-), Combination No. 1 of ITA2);
- National number if only one network.
If space is available, the Teletex answerback will contain the mnemonic part of the
Teletex identification.
Note 7 - The CF shall transmit to the telex terminal the stored message in the format in
which it was originated.
Note 8 - After text transmission is completed, the (telex) answerback should be taken and
compared with that received at the start of message delivery. In the event of a mismatch
of answerbacks, the (telex) answerback shall be taken again, and if there is a match with
that received at the start of delivery, the delivery shall be deemed successful. If there
is a second mismatch, the call shall be cleared and one further attempt to deliver the
message may be made after an interval of at least three minutes. In this case, the text
shall be preceded by "POSSIBLE DUPLICATE MESSAGE".
Note 9 - After text transmission is complete, the CF shall send to the telex terminal the
Teletex answerback, as described in Note 6 to this figure.
Note 10 - The acknowledgement call is mandatory if the telex call is unsuccessful, but
optional if the telex call succeeds.
Note 11 - Administrations should advise their customers of the meaning and possible
consequences of using special telex character sequences (see Recommendation S.4) in the
submitted text.
ANNEX A
(to Recommendation U.201)
Reactions to abnormal conditions during the telex input
A.1 Telex connection clearing without the end of input signal
After a clear without the end of input (EOI) signal, the conversion
facility should forward the message to the Teletex subscriber.
A.2 Telex subscriber pausing during input of address information
If there is a delay greater than 15 seconds at the start of the address
input or between characters within the address input, the CF shall clear the
connection.
A.3 Telex subscriber stopping transmission without sending the end of input
signal
After at least a 30 seconds time-out, the conversion facility should send
a prompt "GA" to the telex subscriber in order to request more information input
(e.g. a text or the end of the input signal). If after a further 30 seconds
time-out there is no more information, then the conversion facility should send
the input message acknowledgement signal, followed by a service message BK. After
this the conversion facility should clear the call.
A.4 Telex subscriber sending a WRU to the conversion facility during text
input
i) In case of one-stage selection procedure, the CF should return the
rearranged Teletex answerback (see Note 3 of Figure 1/U.201).
ii) In case of two-stage selection procedure, in any step of the procedure,
the conversion facility should return its answerback after receiving a
WRU. In addition:
- if WRU is followed by text, message input is continued after the
conversion facility answerback. Also the WRU is deleted from the
message text;
Fascicle VII.2 - U.201 PAGE1
- if WRU is followed by a clear from the telex network, the
conversion facility proceeds as in S A.1 above;
- if WRU is followed by an idle condition, the conversion facility
proceeds as in S 3 above.
A.5 Telex subscriber sending a text after the end of input signal
Any characters received after the end of input signal will be ignored. The
conversion facility should use the "TTT . . ." characters to attempt to stop the
telex transmission and if successful, then send an input message acknowledgement
signal followed by clearing. After clearing, the message should be normally
forwarded to the Teletex terminal.
This does not apply in a case of a two-stage selection CF offering the
follow-on message facility.
A.6 Telex subscriber clearing after the end of input signal and before the
input message acknowledgement signal
The message shall be normally forwarded to the Teletex terminal.
A.7 Telex subscriber sending national variants of ITA2 characters (figure
shift characters of F, G and H)
These combinations could either be converted into a Teletex code which is
a non-telex character (e.g. "*"), or into the national use of these combinations.
The choice is a national matter.
A.8 The conversion facility detecting signal distorsion during text input
Reactions to the detection of distorsion are a national matter.
A.9 Telex subscriber sending a bell signal
The conversion facility has to ignore the bell signal in text input.
A.10 CFs storage capacity overflow during telex message input
- In order to avoid memory overflow occurring during message input, a
guaranteed message length of 12 000 characters is defined.
- The CF should return an "NC" service signal if guaranteed storage space
is not available.
- Messages exceeding the guaranteed length will continue to be accepted
if storage is available.
- If the number of characters received by the conversion facility during
a message input exceeds the available storage to that input, the
conversion facility should discard the excess characters and no attempt
should be made by the conversion facility to overwrite previously
stored characters. When this occurs, the conversion facility should
immediately attempt to prevent the telex subscriber from sending
further characters by transmitting a sequence of "TTT. . ." characters
for a maximum of 20 seconds.
If the calling terminal stops transmission within this period, the
conversion facility should return the message length exceeded
indication, "LDE", return IMA in case of the two-stage selection
procedure and then behave as normal, as if the text input phase had
finished.
If the terminal continues to transmit characters after this period, the
conversion facility should forcefully clear the connection.
- The conversion facility should attempt to deliver the message text,
accepted and stored, preceded by a special text prefix to indicate to
the called Teletex subscriber that the message may be incomplete.
A.11 Request of optional facilities, when not provided by the CF
If a user requests an optional facility not offered by a CF (e.g.
multi-address or positive delivery notification), the CF shall use sequences of
character "T" (according to Recommendation S.4) to attempt to stop the telex
transmission. After the successful stop, the CF should then send an "NA" service
signal.
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