[LAD] AoIP question

Len Ovens len at ovenwerks.net
Mon Oct 6 23:10:59 UTC 2014

On Mon, 6 Oct 2014, Reuben Martin wrote:

> On Sunday, October 05, 2014 08:47:25 AM Len Ovens wrote:
>> but what
>> does an "enterprise" switch have over any other switch aside from
>> speed? I am sure I am being small minded in my thoughts here. For example,
>> I am expecting very little non-audio bandwidth and I am guessing that the
>> average home switch does not prioritize any style of packet over another.
> "Enterprise" switchs have hardware support for the PTP protocol. The timing of
> PTP without hardware support in the switches is not 100% deterministic.

Meaning jitter may create over/under runs I am guessing. In AES67 switches 
with PTP are recommended but not "required".

>> I guess I am asking what parts of a switcher are important for audio?
> IGMP Snooping built into the switch is very convenient. Otherwise the
> multicast audio transport will flood your ports.

IGMP is one of those things mentioned in the AES67 spec. that I have to 
read still. For some reason they have not bothered to spell out what IGMP 
stands for as they have with just about everything else.

For a one IF studio, or even two or three, I get the idea a second and 
even third NIC may be cheaper. That is, use the NIC already there for 
internet and add another just for audio with a different network. A third 
one may be useful to allow splitting audio from control, but then we are 
back to some kind of switch and the computer NIC may end up being faster 
than a cheaper switch.

I went looking at managed, unmanaged and smart switches in the office and 
up categories. QoS shows up first from the very bottom end of things even 
IGMP shows up pretty well. But there are not a lot of even managed 
switches that have an on board PTP clock/server. Very uncommon. Standard 
QoS does prioritize clock over audio and normal traffic, but I do not know 
if that is enough... I would guess it depends on traffic. Most of them 
specify QoS in terms of number of ques. I would guess this means that the 
switch identifies data streams that should be priortized. The low end 
commonly supports 4 ques, The highest I have seen is 8. How do these ques 
work? One que for each packet type I would guess. But I am not sure.

Anyway this is all theoretical in Linux Audio untill there is at least a 
way for dealing with an AoIP IF.

Len Ovens

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