[Linux-aus] SIP

Glen Turner gdt at gdt.id.au
Sat Jul 6 11:00:05 EST 2013

> As I said, the NBN gets around this by provided a dedicated channel for
> VOIP free when you get a data connection.

This is how the NTD presents the service. Underneath it is implemented using ethernet QoS, there is no dedicated channel on the fibre.

> Opus at 64Kbs should provide much better quality than a POTS phone.

Transcoding will reduce that quality, and you will encounter transcoding on most calls due to the low use of Opus. If you want quality to all endpoints then run G.711 A-law. In practice POTS and VoIP quality is typically limited by the quality of the analogue engineering of the audio -- focussing on that can improvise call quality substantially more than by fiddling with codecs.

> ...aggressive codec like G729

G.729 is a fine codec, licensing aside. "Aggressive" implies that it is sacrifices loss in favour of bandwidth. Whereas it actually operates better than all ITU codecs apart from G.711. That is, the lower bandwidth is the result of more complex algorithms (ie: more silicon area and increased power consumption). The poor quality of G.729 on international VoIP providers is very much the result of deliberate under-provisioning to reduce costs.


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