[Foip] Call Flow Observations

[Sebastien BOIRE-LAVIGNE]

Hi John,

 

Good points, let me know if you feel the following makes more sense.

 

In 4.1, the UA1 that is detecting the CNG doesn't support fax, so for
the First "detection" (by UA1), we assume that it is a minimal detection
that can be implemented in a voice device (CNG). For the second
"detection", I agree that the UA2 may relay a signal coming from the
PSTN (emit CED/ANSam/V.21 preamble), detect CED and generate it on its
own PSTN (emit CED/ANSam/V.21 preamble). As such we should replace the
CNG Detected of the UA2 by "CED/ANSam/V.21 preamble detected/generated".
This would be more like the other scenarios.

 

In 4.2 & 4.3, The GW receiving the call from the PSTN and will actively
listen for CNG. If it detects one "it preempt" the receiving side from
emitting/detecting the CED/ANSam/V.21 preamble, since it "knows" that it
will never do so. (This a case where the UA1 is a voice only termination
device, not a gateway). The GW then triggers the T.38 Invite (which is
usually not its role).

 

Maybe we can create a new call flow! Like if we need another one ;)

 

Since the CNG is repeated couple of time, maybe we could combine the
last part of 4.1 (first inviting UA2 in voice, then switch to T.38) but
use the first part of 4.2 & 4.3 and have the GW detect the CNG and
initiate the invite to the second UA. I have never seen this scenario in
the field, but it may replace 4.2 & 4.3 in a more "standard" fashion:

 

What do you think, should we add this as scenario 4.4?

 

 

                   GW                      UA1 (voice only)

                                  |                       |

                                  |     INVITE (voice)    |

                                  |---------------------->|

                                  |                       |

                                  |     100 Trying        |

                                  |<----------------------|

                                  |                       |

                                  |     200 OK (voice)    |

                                  |<----------------------|

                                  |                       |

                                  |        ACK            |

                                  |---------------------->|

                                  |                       |

                                  |                       |

                                  |     RTP Media         |

                             CNG  |<=====================>|

                          ------->|                       |

                          emitted |                       |

                                  | CNG                   |

                                  |<---------             | 

                                  | detected              |

         UA2 (fax enabled)        |                       |

         |    INVITE (voice)      |                       |

         |<-----------------------|                       |

         |                        |                       |

         |    100 Trying          |                       |

         |----------------------->|                       |

         |                        |                       |

         |    200 OK (voice)      |                       |

         |----------------------->|                       |

         |                        |         BYE           |

         |       ACK              |---------------------->|

         |<-----------------------|      (tear down)

         |                        |

         |                        |

         |      RTP Media         | 

         |<======================>|  CNG

         |                        |<--------

CED/ANSam/      |                        | emitted

V.21 preamble   |                        | 

-------->|                        |

Gene. /detected |                        |

         |    INVITE (T38)        |

         |----------------------->|

         |                        |

         |    200 OK (T38)        |

         |<-----------------------|

         |                        |

         |    ACK                 |

         |----------------------->|

         |                        | 

         |    T.38 Fax Flow       | 

         |<======================>|




 

 

 

SBL

 

From: foip-bounces at sipforum.org [mailto:foip-bounces at sipforum.org] On
Behalf Of Lunsford, John
Sent: Wednesday, March 10, 2010 01:12
To: foip at sipforum.org
Subject: [Foip] Call Flow Observations

 

I have a question about the call flows in the 02/19/10 version of the
call flow document. In all the flow diagrams except those in section 4,
a fax call is recognized as such by the detection of both the CNG and
CED or ANSam call tones and possibly by recognizing V.21 7E flags. Is
there some reason why this is not important in the section 4 maps? They
show recognition of the CNG tone only.

 

John Lunsford

QualityLogic, Inc.

 

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