ITU - Telecommunications Standardization Sector
Q1103-010
Study Group 16 – Question 11
Location:
Source:
Title:
Contact:
Streatley, Berkshire, UK. 18th – 20th March 2003
CommWorks, a 3Com Company
Proposed Changes to V.92 Related to MOH
Michael Nicholas
Tel: +1 847 262 2673
CommWorks, a 3Com Company Fax: +1 847 262 0479
USA
Email: mailto:[email protected]
Abstract
This document describes six issues related to ITU-T V.92 Modem On Hold procedures and
proposes that the ITU-T to amend V.92 to address some of the issues and create an appendix
to V.92 to address the remaining issues.
Attention: This is not a publication made available to the public, but an internal ITU-T Document intended only for use
by the Member States of the ITU, by ITU-T Sector Members and Associates, and their respective staff and collaborators in
their ITU related work. It shall not be made available to, and used by, any other persons or entities without the prior written
consent of the ITU-T.
Q1103-010
1.
Modem On Hold V.250 "AT Commands"
The Appendix to ITU-T Recommendation V.250 was amended during the development of
Recommendation V.92 to add the additional AT commands that would be needed to support
V.92. It was not amended to describe new uses that were made to existing AT commands.
Modems implementing the V.92 Recommendation shall use the following AT commands as
described:
ATO - This command is used by the modem that initiated Modem On Hold in order to return
to the online state. This command will cause the modem to do the necessary signalling (eg
flashhook) to the telco to switch the line back to the modem that was placed on hold and then
respond appropriately to ANSam with QC1a or CM. If ANSam is not present, it should
disconnect.
ATH - This command is used by the modem that initiated Modem On Hold to initiate a
disconnect. Depending on the states of the modems, this could be used to initiate sending
MHclrd, QC1a with bits set for cleardown, CM with no modulations available, or a CP
sequence with drn=0.
Proposed Solution: Describe the new usage of these AT commands as described above either
in an appendix to V.92 or V.250.
2.
Clarification of 9.10.2.1 Modem on Hold request
In 9.10.2.1 of Recommendation V.92, it says: "If signal QC or signal CM are received, the
modem shall proceed with Phase 1 of the start-up procedure, assuming the role of an answer
modem and disregarding information received in previous phase 1 signals."
Earlier, in the same paragraph, it says: "...and condition its receiver to detect signals from
Phase 1 of the start-up procedure." This does not specify what should occur if a Phase 1
signal other than QC or CM are received.
There is a possibility of the modem in the On Hold state to falsely detect other Network
signals or voice (including music) as low speed modulations such as those described in
Recommendations V.21, V.22 or V.32; therefore, the On Hold modem shall only look for the
V.8 or V.92 Phase 1 signals CM and QC1a. Similarly, the modem that initiated Modem On
Hold shall only send V.8 or V.92 Phase 1 signals CM or QC1a. It cannot use QC2a or V.8
bis signals because the On Hold modem will already be sending ANSam. Neither should it
send other low speed modulation signals such as V.21, V.22, or V.32.
"Assuming the role of the answer modem" applies only to the V.8, V.34, V.90, and V.92
signals. It does not refer to V.42 or other "upper layer" protocols.
Proposed Solution: Amend V.92 to correct section 9.10.2.1 and add two notes.
Add the words "QC and CM" to the clause: "...and condition its receiver to detect signals QC
and CM from Phase 1 of the start-up procedure."
Note: The responding modem should condition its receiver to detect only QC and CM
because of the possibility of falsely detecting network signals, voice, or music as low speed
modulations. The initiating modem therefore may only send QC or CM.
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Note: The words "Assuming the role of the answer modem" applies only to the V.8, V.34,
V.90, and V.92 signals. It does not refer to V.42 or other "upper layer" protocols.
3.
Clarification of 9.10.1 Transmission of MH sequences and all figures related to
MOH
Section 9.10.1 of Recommendation V.92 says: "If Tone RT is transmitted before an MH
sequence, its duration shall be at least 20 ms if the tone was preceded by another MH
sequence, or at least 50 ms otherwise."
This section is confusing because it gives two possible choices for the initiating modem: 1)
do not send Tone RT, 2) send at least 50 ms of Tone RT. Perhaps the intention was if the
initiating modem is initiating modem-on-hold procedures due to the user making an outgoing
call request, then the modems currently would be in data mode. For this case, the initiating
modem needs to send a 70 +/-5 ms wink as described in Amendment 1 followed by RT in
order to cause the responding modem to leave data mode and start detecting modem on hold
sequences.
If, however, the initiating modem is initiating modem-on-hold procedures due to call-waiting,
the network interruptions themselves would cause the responding modem to already be in a
retrain state sending RT. In this case, the initiating modem would not need to send RT.
Further, it is not clear when, if ever, the initiating modem would have previously sent an MH
sequence, nor why it would then send 20 ms of Tone RT. This is not covered in any of the
drawings.
Another issue is that the modem responding to the Modem on Hold request cannot know
beforehand whether it will be receiving a Retrain or an MH sequence. If the initiating
modem sends RT before an MH sequence as described in 9.10.1 and illustrated in the figures,
the responding modem may detect this as retrain. The subsequent MH sequence may be
falsely detected as the phase reversal of a retrain.
The case of the originator (which in V.90 and V.92 is the server, which is typically the
answerer in remote access server applications) is covered in section 9.10.1.1 of V.92. That is
an even worse case because the originator is supposed to send the phase reversal first. The
case of the answerer being the initiator (which is the typical case for V.92 remote access
server applications) is not covered, perhaps because it was not foreseen that the subsequent
MH sequence could be falsely detected as a phase reversal.
One solution is that RT should not be used preceding an MH sequence. This would be a
technical change to the recommendation, making existing modems non-compliant with the
recommendation. This would not be acceptable.
Alternatively, extra care may be taken in the design of the responding modem's receiver. For
example, the responding modem could delay in responding with retrain until it is certain that
it has not detected a sync pattern indicating the start of an MH or INFO sequence. A note
could be added to the recommendation regarding this situation.
Q1103-010
Proposed Solution: Amend V.92 to add one of the following notes to section 9.10.1.1
Note: If the initiating modem sends Retrain Tone before an MH sequence, it may cause the
responding modem to falsely detect the start of the MH sequence as the phase reversal of a
Retrain, causing undesired operation.
or
Note: The responding modem should continue to detect MH sequences during retrain in case
the initiating modem sends RT first, since the following MH sequence could look like the
phase reversal of a retrain.
4.
Clarification of MOH sequences
The following diagrams illustrate the sequence of signals between the modems, their DTEs,
and the telco during a Modem On Hold session. The first is taken from PCM0-064, which
removed the country-specific telco signalling that is included in the later diagrams.
Proposed Solution: Add the following diagrams in an Appendix to V.92, possibly removing
references to country-specific network interactions.
Q1103-010
A transmits
B Receives
A Receives
Data
B
Data
A
B Transmits
Data
A
Data
B
Call Waiting interrupts
Received Data
Call
Waiting
Alert
Signal*
* Note Call w aiting and caller ID sequences
are national regulatory dependant
Caller-ID
Request
*
RT
Caller
ID
RT
RT
 30s
RT
MHREQ
MHREQ
User Intervention
Occurrs
I
MHACK
MHACK
ANSam
ANSam
Depending upon User ResponseDCE may;
-Cleardow n and ONHook
-Ignore Request and continue w ith Data
-Take the Hold for time T1(Flash Hook)
II
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The following diagrams depict possible typical application scenarios of the V.92 Modem-onhold procedures. It is proposed that they be used as the basis for an appendix to V.92.
The diagrams are intended to be independent of which is APCM or DPCM and are referred to
as Modems A and B. The Points marked I and II are continuations to one or more diagrams.
A transmits
B Receives
A Receives
Data
B
Data
A
B Transmits
Data
A
Data
B
Call Waiting interrupts
Data
SAS
CAS
D
RT
Caller
ID
RT
RT
MHREQ
RT
MHREQ
RT
MHACK
MHACK
MHREQ
MHREQ
MHACK
MHACK
User Intervention
Occurrs
ANSam
ANSam
Depending upon User ResponseDCE may;
-Cleardow n and ONHook
-Ignore Request and continue w ith Data
-Take the Hold for time T1(Flash Hook)
I
Figure 1 Modem A interrupted by Call Waiting
II
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A transmits
B Receives
A Receives
Data
A
B Transmits
Data
A
Data
B
RT
Data
B
RT
RT
RT
MHREQ
MHREQ
MHACK
MHACK
MHREQ
MHREQ
MHACK
MHACK
User Intervention
Occurrs
ANSam
ANSam
Depending upon User ResponseDCE may;
-Cleardow n and ONHook
-Ignore Request and continue w ith Data
-Take the Hold for time T1(Flash Hook)
I
II
Figure 2 Modem A Requests a Modem on Hold
I
II
A Receives
B Receives
ANSam
B Transmits
ANSam
V.8 CM
V.8 CM
V.8 JM
V.8 JM
V.8 CJ
V.8 CJ
ON HOOK
ON HOOK
User declines Hold request and clearsdow n modem
connection
Figure 3 Modem A User declines the Modem on Hold and clears down
Q1103-010
I
II
A Receives
B Receives
ANSam
ANSam
QC1 or
V.8 CM
QC1 or
V.8 CM
QCA1 or
V.8 JM
QCA1 or
V.8 JM
Modem Continues to
Connect as described in
XXX
Modem Continues to
Connect as described in
XXX
User declines Hold request and continues w ith Data
Connection.
Figure 4 Modem A User declines the Modem on Hold and continues with data
connection
5.
Documentation of circuits 104 and 106 during MOH
The actions of circuits 104 and 106 are not described in relation to MOH.
The responding modem should be in a state similar to a retrain. Therefore, it shall turn OFF
circuit 106 and clamp circuit 104 to binary one when it detects an MH sequence, if it has not
already done so. (ie because it has detected Tone RT as part of retrain procedures).
Presumably, the initiating modem will already be in an online command mode so that it may
be issued V.250 commands to initiate a modem-on-hold procedure. Thus, circuits 104 and
106 will be indicative of that mode and not of data mode.
Proposed solution: Amend V.92 to add the above descriptions of the actions of circuits 104
and 106 to 9.10.1.2 and 9.10.1.1, respectively for responding and initiating actions.
6.
Apparent Contradiction between section 9.10.1.1 and Amendment 1
In Amendment 1, the initiating modem is told to send 70+-5 ms of silence, then send either
Tone RT or MH. However, section 9.10.1.1 says that MH cannot be sent until Tone RT (or
an MH response sequence) is detected. So what happens if Tone RT has not been detected
before the end of the 70+-5 ms silence period? Further, section 9.10.1 says that the sending
of Tone RT is optional. What should the initiating modem send? One approach is for the
initiating modem to send RT until the responding RT has been detected and at least 50 ms has
Q1103-010
passed. For the no RT case, there is no way to satisfy all the requirements. This is related to
#3 above, and diagrams as in #4 above would help show these types of problems. However,
to force sending RT until RT is detected might be viewed as a technical change that would
make existing modems non-compliant.
Proposed Solution: Amend section 9.10.1.1 to remove the words "and either Tone RT is
received or an MH response sequence is detected."