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Alexander Shcheblikin
Asked: 2017-04-27 14:36:19 +0800 CST

How does buffering work in a chain of Asterisk servers?

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I have two Asterisk servers in a chain like this:

SIP client (G.729) <-SIP-> Asterisk 1 (converting G.729 to G.711) <-IAX2-> Asterisk 2 -> terminated to analog line.

When no buffering was configured on either of Asterisks, the sound was terrible when received on the analog line (it was perfectly OK in the other direction - I assume the SIP client has buffering enabled for reception.)

As the two Asterisk servers are on LAN, I thought buffering will make most sense on Asterisk 1 facing the remote SIP client.

However, enabling buffering on Asterisk 1 made no difference to sound quality. Enabling buffering on Asterisk 2 somehow did the trick.

Watching the stats on Asterisk 2 confirms this:

CLI> iax2 show netstats
                           -------- LOCAL ---------------------  -------- REMOTE --------------------
Channel               RTT  Jit  Del  Lost   %  Drop  OOO  Kpkts  Jit  Del  Lost   %  Drop  OOO  Kpkts FirstMsg    LastMsg
IAX2/ast1-5936       1   60  140    79   0    43 16712     64    0   40     0   0     0    0      0 Rx:NEW      Rx:ACK

As you can see about 25% of received packet are Out-of-order (OOO), which I suppose is fixed by the configured jitterbuffer.

So my question is:

  1. How could the wrong order of packets survive codec translation on Asterisk 1 and make it through to Asterisk 2? and
  2. How jitterbuffer can fix errors introduced a hop away (I am sure the culprit is the SIP client - CsipSimple - as other SIP clients sound much better without buffering than this one) and why jitterbuffer on the closer server couldn't do that?
asterisk

1 Answers

  1. The info you posted doesn't sound right. The first asterisk (outside facing) server receiving packets will likely suffer from high packet latency, out of order packets, etc. That's is where you would normally add the jitter buffer.

    If the second server is showing traffic suffering from high latency or out of order packets, and these two hosts are on the same LAN segment, something is wrong on your LAN. Sniff your LAN to see what's happening there (use wireshark). If you are seeing a lot of retransmits you may have bad connections/wiring/NIC/etc.

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