Transcript T.38 - SIP Forum

Analysis of T.38 protocol
Survey results, carrier’s remarks and suggestions
Contact Points:
Dr. Jerzy Soldrowsky
(editor of T.38 document in i3F WS “Technical Aspects”)
Telecom Poland
email: [email protected]
Dr. Alessandro Forcina
(chairman of i3F WS “Technical Aspects”)
Telecom Italia Sparkle
email: [email protected]
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Survey results
based on 10 Carriers answers
Purpose:
To investigate most frequent faults during
fax connection.
To determine most suspected:
1.Call phase
2.Section in connection chain
3.Most frequently used profile
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Declared faults per call phase
10
9
Number of answers
8
7
6
5
4
3
2
1
0
Voice call
establishment
Fax
discrimination
phase
Transition to
T.38
Capabilities
negotiation
Facsimile
transmission
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Post message
procedure
Disconnection
Declared faults per connection chain section
Outgoing direction
Carrier A owned VoIP network (3)
TDM network (1)
Third party owned VoIP network (7)
BG
(4)
IP network
Carrier A
IP network
Carrier B
Interconnection link
BG
(7)
BG
Incoming direction
9
8
Number of answers
7
6
5
Fax type:
4
3
G.3 fax
2
B/W
1
0
Terminal - Gateway
Gateway - Gateway
CHF - CHF
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Most frequently used profile
•
•
•
•
•
•
Transport – TCP/UDPTL/RTP - 2/7/2
Transition method – MGC and Autonomous 4/5
Error correction method – FEC/Redundancy/none 4/4/2
Training – local/transferred 3/6
Modulation – V.17/ 14.4 kbit/s
SG3 fax – officially not supported, G.711 VBD, G3
fallback or unstable
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Carrier’s remarks and other
suggestions
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Wrong error codes
• CPE gateways for Retail not correctly handling the SIP ReInvite with T.38 media offered in the SDP. If the media type is not
supported, the UAS (originating gateway) must send a 415
(Unsupported Media) error to the requesting UAC (terminating
gateway). Many gateways sent 488 (Not Acceptable Here) or 606
(Not Acceptable), those errors are also handled by the network
gateway (UAC) and the media reverts to G.711 which allows the
fax to complete.
• H.248 is reported back to the MGC as error code 401 if a media
attribute like T.38 is not supported on the MG. MGCP returns as
error code 518. When these error codes are received by the MGC,
the call is torn down as a Signaling Failure.
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Terminal – Gateway incompatibility
• If Internet-aware Fax devices supports T.38 only and Media
Gateway supports G.711 upspeed only it is not possible to rollback
to G.711 upspeed mode when a T.38 session request is not
accepted by the other endpoint.
• Signalling is setup properly (T.38 ports opened), but fax device
doesn’t generate a required messages for T.38 implementation on
VoIP GW-A using ATAs or for GW-B own fax protocols for
conversion
Fax over IP (FoIP)
T.30
Gr. 3
Fax
T.38
T.30
VoIP
GW-A
PSTN
Gr. 3
Fax
ATA
Gr. 3
Fax
VoIP
PSTN
GW-B
Gr. 3
Fax
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Terminal – Gateway incompatibility
• Non standard fax of different types (encrypting equipment).
• Proprietary Inband protocol for Encryption key distribution to support
fax encryption
• High Quality fax (High resolution) with very long distance calls
(Europe to Japan/Australia)
Those problems have been solved by the NGN vendor by
modifying in a proprietary way the T38 protocol (between their
own media gateways) or by disabling T38 on the fly and moving
to G711.
•
•
•
•
Most failures on VoIP GW-B (VoIP network)
No T.38 implementation on VoIP GW-B
Incompatibilities of ATAs or fax machines (IAF)
Buffer and timer errors
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SDP Offer/Answer
• ITU-T T.38 Annex D 2.1.3.1: it is worth considering the possibility to
further define the SDP parameters, in order to specify whether each
one is negotiable or declarative, and the range of permitted values
and default values.
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Fax discrimination
• Echo canceller (EC): ITU-T T.38 does not specify if the use of Echo
Canceller is recommended when switching from audio to T.38 during
fax discrimination phase. It could happen that echo canceller use
affects fax setup announcement tones, thus impacting fax startup
phase.
• For most of the fax calls that fail, we observe that the common
indication is ‘no signal detected’ This immediately follows T.38 (reInvite) which is supposed to be after fax tone detection CNG (calling
tone) or CED (called tone). CED and CNG however, are optional
tones for T.38, presumably because, although the T.30 fax protocol
requires them, not all fax machines comply. Thus the Preamble
(V21) is always present in the signalling, but not necessarily CED.
When we check the T38 media negotiation (in particular UDPTL),
the signals appear to be in order, the right media ports are opened,
and yet no signal is detected. Staff have reported an observation
that T.38 fax “no signal detected” failures occur more frequently for
fax calls when CED is not present, that for T.30 compliant signals
which have CED. This has not been checked rigorously however.
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Transition problems
• MG incompatibility in autonomous transition method. It needs the
Media Gateway to be able to generate/understand supplier
dependant (not standard) RTP packets (e.g. Cisco NSE) in order to
indicate the will to switch from audio to T.38.
• SIP ReINVITE messages for switching from audio to G.711 (for
‘CED’ tone transmission): ITU-T T.38 does not specify any
indication, to use in the SDP part of the SIP ReINVITE message, as
a reason regarding modem or fax transmission. As a result, the Call
Agent uses global configuration for G.711 audio channel setup (e.g.
use of VAD or audio-driven configuration) that can affect the
subsequent fax tone sending.
• SIP ReINVITE messages for switching to T.38: ITU-T T.38 does not
specify if the “user=phone” string in the “From” and “To” SIP fields is
globally accepted between carriers.
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During facsimile transmission
• Loss of packets
• Delay and jitter
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