ITU - Telecommunications Standardization Sector
PCM'01-023R1
Study Group 16 - Question 11
_____________________________________
Rapporteur Meeting, Nice, France, 2-4 April 2001
SOURCE:
3Com
CONTACT:
Michael Nicholas
Phone: +1 (847) 342-6772
Fax:
+1 (847) 342-6785
Email: Michael_Nicholas@3Com.com
TITLE:
Automatic Detection of Analog Modems and Facsimile Machines
_____________________
Abstract
This contribution proposes how xOIP gateways may distinguish between Fax and Modem calls
as well as V.18.
Intellectual Property Statement
The individuals preparing this contribution know of patents, the use of which may be essential to a standard
resulting in whole or in part from this contribution.
ITU- Telecommunications Standardization Sector
Study Group 16 - Question 11
Nice, France, 2-4 April, 2001
1 Introduction
Contribution PCM01-021 outlines a procedure for how a call may be switched between VoIP,
FoIP, and MoIP. It recommends that xOIP gateways start in VoIP mode and when modem/fax
signals being sent in both directions indicate that it is a modem or fax call, that the gateways
switch from VoIP to MoIP or FoIP. This contribution explicitly defines the Fax and Modem
signals to use and when and how to switch. It proposes using RTP packets based partly on
RFC2833, but expanding on RFC2833, which incomplete. This contribution is roughly related
to H.248 Annex F, which defines how Fax and V.18 signals may be detected. However, H.248
Annex F procedures are inadequate and it is anticipated that the procedures defined here will
supersede those defined in H.248 Annex F.
This contribution assumes that MoIP Type 0 and possible Types 4 and 5 are supported. The
other Types (1, 2, 3) as described in the MoIP Issues list may not be supported by these
procedures.
2 Signals Sent by Answering Modems
Note that a call could be made using 'reverse frequency dialling'. These procedures are
symmetric and will still work as long as the gateways are looking for both 'originating' and
'answering' modem signals.
2.1 ANS, ANS/, ANSam, ANSam/, ANSpn, ANSpcm
Most modem and fax connections involve sending some form of V.25 or V.8 (or V.92) 2100 Hz
answer tone. RFC 2833 defines Tone and Event packets which may be sent when these are
detected, but defines no procedures. In order to meet timing requirements for modem and fax
connections, detecting an entire 1s in order to observe two phase reversals as well as 15 Hz am
modulation will not work. It is recommended that event packets not be used and that tone
packets be used in the following manner, with some modifications to RFC2833:
2.1.1 2100 Hz (ANS or CED) detected
When 2100 Hz is seen, the gateway shall clamp off its VoIP path to the other gateway and start
sending Tone packets indicating 2100 Hz. As was described in document PCM01-021, the
gateway which detected 2100 Hz will need to indicate to the other gateway which scenarios it
supports. It will do this by 'piggybacking' another RTP packet with bits set to indicate support
for FoIP, MoIP Type 0, MoIP Issue 1 (which may be Types 4 or 5 as described in the MoIP
Issues List), or V.18 (or other call functions defined in V.8).
2.1.2 ANSam detected
When 15 Hz modulation is seen in addition to 2100 Hz, change from sending 2100 Hz Tone
packets, to sending 2100 Hz with 15 Hz modulation Tone packets. Again, the gateway should
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Automatic Detection of Analog Modems and Facsimile Machines
PCM'01-023R1
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ITU- Telecommunications Standardization Sector
Study Group 16 - Question 11
Nice, France, 2-4 April, 2001
'piggyback' additional RTP packets indicating support for FoIP, MoIP Type 0, MoIP Issue 1,
V.18, etc.
2.1.3 ANS/ or ANSam/ detected
When a 180 degree phase reversal is seen, switch to sending 2100 Hz Tone (optionally plus 15
Hz modulation) plus with 180 degree reversal bit set. This would be a modification to RFC
2833. This bit would remain set in each additional Tone packet until another 180 degree phase
reversal is seen at which point it would be toggled back to cleared, and so on. Again, the gateway
should 'piggyback' another RTP packet indicating support for FoIP, MoIP Type 0, MoIP Issue 1,
V.18, etc.
2.1.4 ANSpn
The proprietary ANSpn should be detected as ANS (or possibly ANSam if it is additionally
modified by 15 Hz amplitude modulation) and is not supported by this contribution.
2.1.5 ANSpcm
It is not necessary to detect ANSpcm, since ANSpcm will only be sent after the call has switched
to MoIP mode.
2.1.6 Additional Information
The gateway which is re-generating the 2100 Hz tone needs to take care not to cause extraneous
phase reversals in its tone generator routine. The level and duration fields in the RFC2833 RTP
packets may be used by the manufacturers as desired. For example, the gateway generating the
2100 Hz signal may decide to generate 2100 Hz at a lower level to meet national restrictions.
When 2100 Hz is no longer detected, the gateway should send Tone packets indicating 2100Hz
is no longer seen.
When these Tone packets are sent, both gateways will know that the call may be a fax or modem
call for the one direction. They cannot yet switch to MoIP or FoIP until they know the other
direction is also modem or fax.
2.1.7 Example
If a client modem is calling to a server modem which sends ANSam/ through an xOIP link, it
will first see a small amount, maybe 50ms, of 2100 Hz which is passed through the xOIP link as
VoIP G.711 or G.729 or other codec. Then it will receive the 2100 Hz as generated by the
nearest gateway. After maybe 200ms, that will change to ANSam. Finally, after 450ms it will
see the first phase reversal. The initial 50ms or so of 2100 Hz may cause a spurious phase
reversal. It is not anticipated that this should cause any problems for network echo cancellers.
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Automatic Detection of Analog Modems and Facsimile Machines
PCM'01-023R1
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ITU- Telecommunications Standardization Sector
Study Group 16 - Question 11
Nice, France, 2-4 April, 2001
2.2 USB1
When USB1 is seen, the gateway shall clamp off its VoIP path to the other gateway, if it has not
already done so, and start sending Tone packets indicating 2250 Hz. As was described in
document PCM01-021, the gateway which detected 2250 Hz will need to indicate to the other
gateway which scenarios it supports. It will do this by 'piggybacking' another RTP packet with
bits set to indicate support for FoIP, MoIP Type 0, MoIP Issue 1 (which may be Types 4 or 5 as
described in the MoIP Issues List), or V.18 (or other call functions defined in V.8). It needs to
send this for the case that 2100 Hz is not sent first.
2.3 V.32bis AC
When V.32bis AC is seen, the gateway shall clamp off its VoIP path to the other gateway, if it
has not already done so, and start sending Tone packets indicating 3000+600 Hz (this may need
to be defined for RFC2833). As was described in document PCM01-021, the gateway which
detected V.32bis AC will need to indicate to the other gateway which scenarios it supports. It
will do this by 'piggybacking' another RTP packet with bits set to indicate support for FoIP,
MoIP Type 0, MoIP Issue 1 (which may be Types 4 or 5 as described in the MoIP Issues List), or
V.18 (or other call functions defined in V.8). It needs to send this for the case that V.32bis AC is
sent first.
2.4 V.21 1650 Hz Mark Frequency
When V.21 1650 Hz Mark Frequency is seen, the gateway shall clamp off its VoIP path to the
other gateway, if it has not already done so, and start sending Tone packets indicating 1650 Hz.
As was described in document PCM01-021, the gateway which detected V.21 1650 Hz Mark
Frequency will need to indicate to the other gateway which scenarios it supports. It will do this
by 'piggybacking' another RTP packet with bits set to indicate support for FoIP, MoIP Type 0,
MoIP Issue 1 (which may be Types 4 or 5 as described in the MoIP Issues List), or V.18 (or
other call functions defined in V.8). It needs to send this for the case that V.21 1650 Hz is sent
first.
There are some open issues with this still. Should MoIP support V.21, since it is not usually
used with an error correcting protocol? Should MoIP support V.21 or treat it as V.18
Automode? This is left for further discussion.
2.5 V.23 1300 Hz Mark Frequency
When V.23 1300 Hz Mark Frequency is seen, the gateway shall clamp off its VoIP path to the
other gateway, if it has not already done so, and start sending Tone packets indicating 1300 Hz.
As was described in document PCM01-021, the gateway which detected V.23 1300 Hz Mark
Frequency will need to indicate to the other gateway which scenarios it supports. It will do this
by 'piggybacking' another RTP packet with bits set to indicate support for FoIP, MoIP Type 0,
MoIP Issue 1 (which may be Types 4 or 5 as described in the MoIP Issues List), or V.18 (or
other call functions defined in V.8). It needs to send this for the case that V.23 1300 Hz is sent
first.
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Automatic Detection of Analog Modems and Facsimile Machines
PCM'01-023R1
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ITU- Telecommunications Standardization Sector
Study Group 16 - Question 11
Nice, France, 2-4 April, 2001
There are some open issues with this still. Should MoIP support V.23, since it is not usually
used with an error correcting protocol? Should MoIP support V.23 or treat it as V.18
Automode? 1300 Hz is also used as the data mode calling tone. With reverse frequency
dialling, this may be mistaken as V.23 Answer Mode Mark frequency. This is left for further
discussion.
2.6 V.8 bis CRe (1375 Hz + 2002 Hz, followed by 400 Hz)
V.8 bis signals present a unique problem and need special handling since it might be the case that
the client and server modems support V.8 bis, but only one or neither of the MoIP gateways
support V.8 bis. Therefore, the gateways which do not support V.8 bis must still detect the V.8
bis signals to prevent sending them unknowingly as VoIP G.711 data. When V.8 bis CRe is
detected, the gateway shall send Tone Packets to the other gateway indicating the V.8 bis tones
were detected.
As was described in document PCM01-021, the gateway which detected V.8 bis CRe will need
to indicate to the other gateway which scenarios it supports. It will do this by 'piggybacking'
another RTP packet with bits set to indicate support for FoIP, MoIP Type 0, MoIP Issue 1
(which may be Types 4 or 5 as described in the MoIP Issues List), or V.18 (or other call
functions defined in V.8).
The other gateway shall then send CRe if it supports V.8 bis. If it does not support V.8bis, it
may either send nothing (waiting for the server modem to timeout, fallback to V.8 and send 2100
Hz answer tone) or it may generate ANSam.
3 Signals Sent by Originating Modems
3.1 1100 Hz CNG Tone
The 1100Hz Fax Calling Tone should be detected and signalled in 1100 Hz Tone packets. It
may also be sent in VoIP G.711 or G.729 or other codec, since this signal alone is not enough to
indicate that the call is Fax and it may not be sent by the originating Fax machine in all cases.
The gateway shall not piggyback an RTP packet indicating xOIP capabilities.
3.2 1500 Hz Proprietary Calling Tone
The proprietary 1500Hz Fax Calling Tone may be detected and signalled in 1500 Hz Tone
packets. It may also be sent in VoIP G.711 or G.729 or other codec, since this signal alone is not
enough to indicate that the call is Data and it may not be sent by the originating Fax machine in
all cases. The gateway shall not piggyback an RTP packet indicating xOIP capabilities.
3.3 V.21 Flags
When V.21 hdlc flags are seen and the gateway knows that the other gateway is capable of
MoIP, it shall switch to FoIP mode and send one or more RTP packets to the other gateway to
inform it to switch to FoIP mode as well.
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Automatic Detection of Analog Modems and Facsimile Machines
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ITU- Telecommunications Standardization Sector
Study Group 16 - Question 11
Nice, France, 2-4 April, 2001
3.4 V.8 CI
V.8 CI should be sent in VoIP mode. Just as in 1100 Hz CNG tone, this is not mandatory and is
not an indication that both ends of the connection are modems.
3.5 V.8 CM (and V.92 QCA1a or QCA1d)
When the start of V.8 CM is seen and the gateway knows that the other gateway is capable of the
call function indicated in V.8, it shall switch to that mode (MoIP, V.18, etc) and send one or
more RTP packets to the other gateway to inform it to switch to that mode as well. It is not
necessary to detect an entire CM sequence, only the Preamble and Call Function are necessary.
As long as it does not look like HDLC flags (indicative of Fax), the gateway may switch to
MoIP. In fact, it is desirable that the gateway does switch modes before the end of the second
CM or the server modem (in the case of client calling server via MoIP) will have received the
V.8CM from the client modem. This is not what it needs. The server modem needs the V.8CM
from the gateway closest to it, which may support different modulations such as V.91.
When the gateways have switched to MoIP mode, they each negotiate modulations locally.
3.6 V.8 bis CRd (1529 Hz + 2225 Hz, followed by 1900 Hz)
V.8 bis signals present a unique problem and need special handling since it might be the case that
the client and server modems support V.8 bis, but only one or neither of the MoIP gateways
support V.8 bis.
If V.8 bis CRd is detected, and the gateway supports V.8 bis and knows that the other gateway is
capable of MoIP, it shall switch to MoIP mode and send one or more RTP packets to the other
gateway to inform it to switch to MoIP mode as well.
When the gateways have switched to MoIP mode, they each negotiate modulations locally.
3.7 V.92 QCA2a or QCA2d
V.92 V.8 bis-like signals present a unique problem and need special handling since it might be
the case that the client and server modems support V.8 bis, but only one or neither of the MoIP
gateways support V.8 bis.
If V.92 QCA2a or QCA2d is detected and the gateway knows the other gateway supports MoIP,
the gateway shall clamp off the data being sent via VoIP to the other gateway, switch to MoIP
mode , and send one or more RTP packets telling the other gateway to switch to MoIP mode as
well.
When the gateways have switched to MoIP mode, they each negotiate modulations locally.
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Automatic Detection of Analog Modems and Facsimile Machines
PCM'01-023R1
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ITU- Telecommunications Standardization Sector
Study Group 16 - Question 11
Nice, France, 2-4 April, 2001
3.8 V.32bis 1800 Hz AA
When V.32bis 1800 Hz AA is seen and the gateway knows that the other gateway is capable of
MoIP, it shall switch to MoIP mode and send one or more RTP packets to the other gateway to
inform it to switch to MoIP mode as well.
When the gateways have switched to MoIP mode, they each negotiate modulations locally.
3.9 V.22bis S1
When V.22bis S1 is seen and the gateway knows that the other gateway is capable of MoIP, it
shall switch to MoIP mode and send one or more RTP packets to the other gateway to inform it
to switch to MoIP mode as well.
When the gateways have switched to MoIP mode, they each negotiate modulations locally.
3.10 V.22 SB1
When V.22 SB1 is seen and the gateway knows that the other gateway is capable of MoIP, it
shall switch to MoIP mode and send one or more RTP packets to the other gateway to inform it
to switch to MoIP mode as well.
When the gateways have switched to MoIP mode, they each negotiate modulations locally.
3.11 103-Type 1270 Hz
When 103-Type 1270 Hz is seen and the gateway knows that the other gateway is capable of
MoIP, it shall switch to MoIP mode and send one or more RTP packets to the other gateway to
inform it to switch to MoIP mode as well.
When the gateways have switched to MoIP mode, they each negotiate modulations locally.
There are some open issues with this still. Should MoIP support 103-Type, since it is not usually
used with an error correcting protocol? Should MoIP support 103-Type or treat it as V.18
Automode? This is left for further discussion.
3.12 V.21 980 Hz Mark Tone
When V.21 980 Hz is seen and the gateway knows that the other gateway is capable of MoIP, it
shall switch to MoIP mode and send one or more RTP packets to the other gateway to inform it
to switch to MoIP mode as well.
When the gateways have switched to MoIP mode, they each negotiate modulations locally.
There are some open issues with this still. Should MoIP support V.21, since it is not usually
used with an error correcting protocol? Should MoIP support V.21 or treat it as V.18
Automode? This is left for further discussion.
3Com
Automatic Detection of Analog Modems and Facsimile Machines
PCM'01-023R1
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ITU- Telecommunications Standardization Sector
Study Group 16 - Question 11
Nice, France, 2-4 April, 2001
3.13 V.23 390 Hz Mark Tone
When V.23 390 Hz is seen and the gateway knows that the other gateway is capable of MoIP, it
shall switch to MoIP mode and send one or more RTP packets to the other gateway to inform it
to switch to MoIP mode as well.
When the gateways have switched to MoIP mode, they each negotiate modulations locally.
There are some open issues with this still. Should MoIP support V.23, since it is not usually
used with an error correcting protocol? Should MoIP support V.23 or treat it as V.18
Automode? This is left for further discussion.
3Com
Automatic Detection of Analog Modems and Facsimile Machines
PCM'01-023R1
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