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T.38 issue

T.38 issue. London 22-24 March. 2010. T.38 implementation status. Am.1: SG3 Fax, Auto Switchover. June 1998. March 2002. April 2004. Sept. 2005. April 2007. SG3 fax implementation. SG3 fax support. SG3 fax method. SG3->G3 fallback.

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T.38 issue

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  1. T.38 issue London 22-24 March. 2010

  2. T.38 implementation status Am.1: SG3 Fax, Auto Switchover June 1998 March 2002 April 2004 Sept. 2005 April 2007

  3. SG3 fax implementation SG3 fax support SG3 fax method SG3->G3 fallback If SG3 terminal will try to connect to G3 terminal -> fallback to T.38 If SG3 terminal will try to connect to SG3 terminal -> G.711 will be chosen Recommendation: 1. Implement fallback procedure? 2. Announce that SG3 is not supported? 3. Implement last T.38 edition?

  4. SG3 fax fallback Fax G3/V.34 SG3 GW Fax G3/V.34 CNG/CI CNG/CI ANSam ANSam CM 2,6 – 4 s DIS/V.21 1. SG3 -> G3 fallback procedure 2. G3 gateway call flow (SipForum) 3. Let SG3 terminals negotiate V.34?

  5. V.152 (VBD) implementation • Disable VAD, comfort noise and CRTP • provided DSD capability is supported as described in • Amendment 1 to V.152 Annex B). • Set the jitter buffer to a fixed value. • Disable echo suppressors (ITU-T G.164) • Enable echo cancellers in case of V.21 • and disable in case of V.34 (ITU-T G.168) V.152 implementation Recommendation: Assure automatic gateway speed up to VBD on fax/modem tones. Target solution: Amendment 1 to V.152 Annex C = V.21 sensitive echo cancellers

  6. DTMF Digits, Telephony Tones and Telephony Signals Implementation status Use to transport fax tones. RFC 2833 mostly supported but rarely used: Recommendation: Always declare RFC 2833 payload for VoIP call setup. Even if bandwidth is poor, tones/signals should be transported and recognized by far end.

  7. Fax tones/events

  8. ECM implementation status Gateways ECM enabled ECM disable implementation ECM can cause multiple retransmissions when QoS is poor. ECM disable for less then 50% of carriers. Recommendation: Carriers declare low packet loss so -> ECM to be negotiated by terminals.

  9. ECM disable by gateway Disable ECM by gateway setting bit 27 of DIS to 0 (Table 2/T.30). Fax G3 G3 GW Fax G3/ECM CNG CNG CED CED Set bit 27=0 DIS/V.21/ No ECM DIS/V.21

  10. Autonomous switchover v=0 c=IN IP4 124.124.124.222 m=audio 2222 RTP/AVP 0 (… additional attributes may be included) m=image 4444 udptl t38 a=T38FaxVersion:1 a=T38FaxRateManagement:transferredTCF a=T38FaxUdpEC:t38UDPFEC a=T38FaxMaxBufferSize:2000 a=T38MaxDatagram:512 a=T38MaxBitRate:14400 Autonomous switchover implementation Autonomous switchover can eliminate problems with large Re-Invite delay.It is supported by less than 50% of carriers. Recommendation: Autonomous switchover should be used if possible. Implement latest T.38 edition. Important note: RTP port numbers must be negotiated in initial SDP O/A. If not, (port=0) next O/A are required what will cause additional delay.

  11. Cisco protocol & Encryption Cisco protocol use Problems with fax encryption. Recommendation: Problem of interoperability between gateways using Cisco protocol and standard gateways as well as encryption problem appear rarely and can be now neglected. Encryption for UDPTL is planned to be standardized by IETF which should be target solution.

  12. V.150.1 support Recommendation: Modem relay cannot be used.

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