FAX handling in VoIP

1
FAX handling in VoIP
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What ?

• Fax is used heavily over the regular phone(PSTN)
  system
• Expectation is that it works over a voip call

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So what is the problem

• Most if not all codecs do not transport FAX
  faithfully
• Codecs are optimized for speech
• Tend to distort anything in the spectrum that
  does not look like speech
• G.711 is the notable exception because it doesnt
  really compress
• Note The PSTN network uses G711 as well

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Problems contd..

• A fax call starts as a voice call
• Then there is a training or tone exchange mode
  to switch over the call to fax mode

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Solutions

• Store and forward fax
• Fax is received first and then sent over the IP
  network using SMTP/email
• Fax is sent as TIFF body
• Real time fax
• Passthru
• Relay

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Real time fax passthru/relay

• Passthru
• On detecting fax, switch the call over to G.711.
  Fax signaling happens using tones end to end.
• Relay
• On detecting fax, switch the call over to a
  special fax relay mode. This includes definition
  of special fax signaling events over IP and a
  special encapsulation for the fax data.

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Fax passthru signaling(G.711 switchover)

• There needs to be end-to-end transmission of fax
  tones. So switchover needs to happen quickly
• Also switching the codec over needs signaling and
  synchronization
• Over the signaling channel
• Uses special signaling messages
• Cons
• Could be more delay in the signaling stream
• Needs sync between the signaling and media
  components
• Over the RTP stream
• Uses the RTP events

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Problems with just using G.711(passthru)

• Works a lot of the time, but
• Very sensitive to jitter
• Very sensitive to loss

9
T.38 (Real time fax relay)

• ITU standard for Fax relay over IP
• Can be used with H.323 or SIP or MGCP
• Not a call setup protocol
• Specifies
• transmission of fax control messages over IP
• Fax codec
• Can use TCP or UDP for actual data
  transmission FEC type redundancy usually employed
  with UDP

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Reading exercise

• Section in text on T.38/fax relay