[SIPForum-techwg] FW: Clarification regards modem passthrough with reference to IP PBX / Service provider Interoperability

[Chris Gatch]

1. If upon the detection of 2100 Hz tone if the system switches to g.711
codec after initiating a reINVITE, what is to be done upon the reception
of Phase reversal on the terminating side?

 

The originating side needs to receive the phase reversal such that echo
cancellers can be disabled. This means that the terminating gateway
needs to pass this information along. 

 

If NTE (RFC2833) had been negotiated in the original session, the phase
reversal can be sent as an NTE event to the originating side. It seems
this would be independent of the reINVITE (for G.711 negotiation)
meaning the event could be sent prior to the session being renegotiated
for G.711. This also implies that the originating gateway must be
capable of not only translating the NTE event back to the 2100 Hz tone
on the TDM side but also acting on the event and disabling any active
echo cancellation for the session. 

 

If NTE had not been negotiated on the original session, then the 2100 Hz
tone with phase reversal needs to be transmitted inband (in the voice
codec). Assuming the original session is not using G.711, the tone will
not be reliable until the reINVITE renegotiates with the G.711 codec. It
is possible / likely that the originating side will not detect the 2100
Hz tone until G.711 is in use. It is reasonable that the G.711
negotiation would occur well in advance of the ceasing of the 2100 Hz
tone (with or without phase reversal) by the terminating station.

 

2. How do we indicate to the remote gateway as regards the phase
reversal? 

 

Answered in 1. Via NTE events if NTE is negotiated. Inband via G.711
once session is renegotiated for G.711.

 

3. Do we disable ECAN upon receiving the phase reversal signal or how is
it?

 

Yes.

 

4. In the SIP re-INVITE, how do we specify whether it is a fax call or a
modem call?

 

The concept of the "inband method" is that gateways are unaware the
session is being used for fax transport. I know of no way to indicate
fax for this method. Alternatively, if using T.38, then the negotiation
of T.38 would indicate fax usage. Is there a reason that SIP would need
to indicate fax or modem call if all it is doing is G.711?

 

From: Binod Roay (broay) [mailto:broay at cisco.com] 
Sent: Tuesday, April 10, 2007 11:53 AM
To: Chris Sibley; Chris Gatch
Subject: Clarification regards modem passthrough with reference to IP
PBX / Service provider Interoperability

 

Hi Chris,

 

I am Binod Roay working with Cisco Systems. We are trying to implement
the modem passthrough to be compliant with the SIP forum draft in this
regard.

I had some doubts as regards the document
sf-draft-twg-IP-PBX_SP_Interop-sibley-siipconnect  in the sip forum.

The doubt is as regards section 15.5 of that document. Please direct me
to the appropriate person in case you aren't the ones who created this
section.

 

 

15.5 Fax and Modem Calls
When performing in-band transport of fax or modem calls, any device that
supports fax and/or modem transmissions
MUST upon recognition of a 2100 Hz tone (+/- 15 Hz) tone:
  1. Switch the active codec in use on the call to G.711 (if a codec
other than G.711 was previously in use).
  2. Disable the high pass filter.
  3. Disable voice activity detection (VAD) and comfort noise generation
(CNG).
  4. Switch from any adaptive/dynamic jitter buffer in use to a
fixed-length jitter buffer. (A RECOMMENDED
depth of 200-ms is suggested when switching to a fixed-length jitter
buffer.)


Renegotiation of the session media attributes MUST be performed using
the SIP reINVITE request as described in RFC
3261 [8] or the SIP UPDATE request as described in RFC 3311 [12].
Superior performance of fax transmissions over packet networks can be
achieved by utilizing the ITU-T T.38 [22] fax
relay specification (as opposed to in-band transport). In-band fax
transmissions are especially problematic over packet
networks, especially for calls that traverse the public Internet or
other network that doesn't offer adequate QOS.
Accordingly, it is RECOMMENDED that Enterprise devices utilize T.38 fax
relay when possible.
Trunking Gateways MUST support the ITU-T T.38 [22] specification and
Enterprise devices SHOULD support the
specification. It is important to note that steps 1-4 outlined above for
in-band transport of fax/modem calls do not apply, to fax calls only,
for implementations utilizing T.38 fax relay.

 

Questions

 

 

1. If upon the detection of 2100 Hz tone if the system switches to g.711
codec after initiating a reINVITE, what is to be done upon the reception
of Phase reversal on the terminating side?

2. How do we indicate to the remote gateway as regards the phase
reversal 

3. Do we disable ECAN upon receiving the phase reversal signal or how is
it ?

4. In the SIP re-INVITE, how do we specifiy whether it is a fax call or
a modem call ?


Low-speed modems (V.22bis and below) and faxes work with echo cancellers
in place, and so for
these connections the echo canceller should be active. High-speed modems
disable the PSTN echo
cancellers by adding phase reversals to their answer tones. The PSTN
network detects these phase
shifts, and are aware that the echo cancellers must be disabled. In a
voice over packet situation, the
same information needs to be used to disable the echo canceller.

 

 

Could you please provide your inputs ?

 

Thanks and regards,

Binod

 



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