Analysis of T.38 protocol

1. 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: jerzy.soldrowski@telekomunikacja.pl Dr. Alessandro Forcina (chairman of i3F WS “Technical Aspects”) Telecom Italia Sparkle email: alessandro.forcina@telecomitalia.it

2. Survey resultsbased on 10 Carriers answers Purpose: To investigate most frequent faults during fax connection. To determine most suspected: • Call phase • Section in connection chain • Most frequently used profile

3. Declared faults per call phase

4. Outgoing direction Carrier A owned VoIP network(3) BG (4) IP network Carrier A Interconnection link TDM network(1) BG (7) Third party owned VoIP network(7) BG Incoming direction IP network Carrier B Declared faults per connection chain section • Fax type: • G.3fax • B/W

5. Most frequently used profile • Transport – TCP/UDPTL/RTP - 2/7/2 • Transition method – MGC and Autonomous 4/5 • Error correction method – FEC/Redundancy/none4/4/2 • Training – local/transferred 3/6 • Modulation – V.17/ 14.4 kbit/s • SG3 fax – officially not supported, G.711 VBD, G3 fallback or unstable

6. Carrier’s remarks and other suggestions

7. Wrong error codes • CPE gateways for Retail not correctly handling the SIP Re-Invite 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.

8. Fax over IP ( FoIP ) T.30 T.38 T.30 Gr. 3 VoIP Gr. 3 PSTN GW-A Fax Fax VoIP PSTN GW-B Gr. 3 Gr. 3 ATA Fax Fax 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

9. 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

10. SDP Offer/Answer • ITU-T T.38 AnnexD 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.

11. 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 (re-Invite) 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.

12. 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.

13. During facsimile transmission • Loss of packets • Delay and jitter