[Asterisk-Users] QoS for improvements

[Jean-Christophe Heger]

That's funny, people having good bandwidth always have a better way to
do it. You should feel lucky, because no one provides 768kbps upstreams
in Switzerland, except if you want to pay 1'000 USD per month for a
leased line.

There is nothing complicated, just mathematics.

Here is the formula:

MTU: Maximum transmit unit = 1492 Bytes (ADSL)
UP: Up stream
t: time spent for a full framed packet (1492 Bytes)

t = 8 * UP / MTU

128k upstream -> 91 ms
256k upstream -> 45 ms
512k upstream -> 23 ms
768k upstream -> 15 ms

Using a codec, such as GSM or G.729, will take around 20 to 30 ms for
encoding and decoding.

While you wait for a full framed packet to go through the ADSL line, a
voip packet will wait need 20 ms for encoding, 91 ms for waiting (at the
most), 30 ms to go to the destination (at the best), and 10 ms to be
decoded. Total: 20+91+30+20 = 161 ms. Even worse, the jitter balance
will play around 100 ms, because of waiting on full framed packed.
That's what I call "breaking the jitter", because not all equipment does
support such jitters.

Depending on the line and the distance (hops), you can easily add 50 ms,
bringing the total around 200 ms. Therefore we consider a conversation
as good, when the overall delay is under 150 ms. Bringing the MTU to 700
ms does bring back the overall delay to this target, and the jitter to
50 ms.

Regarding the results, 768 kbps up stream is working even without QoS (<
100 ms). PFIFO (TOS=lowdelay) is good enough for perfect communications.

So, any other magical solution ?

Jean-Christophe


Michael Graves a écrit :

>Sometimes this all sounds so complicated....but it needn't be. I
>suppose it can vary with the size of your installation.
>
>I use a m0n0wall router on a Covad DSL line. Using m0n0's traffic
>shaping feature I establish inbound and outbound pipes which are
>bandwidth restricted to just less than my mesured average DSL rate.  I
>then break my traffic into three priority ques in each direction;
>highest priority, medium priority, low priority.
>
>I assign all IAX traffic in/out to the highest priority que, and map
>all IAX ports to the * server inside the LAN.
>
>In fact, I just used the Magic Shaper Wizard in m0n0, then added IAX
>specific entries to give it highest priority. The whole process took
>about a half hour. Just as easy as the Linksys BEFSR-81 that I had
>before, but more reliable and more controllable.
>
>Now to be fair, I only have about 6 phones. I use IAX2 to all my ITSPs
>and SIP in-house only. My DSL is 3M down / 768k up.
>
>Michael
>
>On Fri, 06 May 2005 08:37:20 +0200, Jean-Christophe Heger wrote:
>  
>