[Asterisk-Users] Asterisk SIP Packet Time (20ms)

Sean Cheesman scheesman at gdsworks.com
Tue Dec 23 08:54:38 MST 2003

The problem occurs when the software is expecting the packet in a certain
timeframe so that it can reassemble it in a timely manner.  It's not a big
deal with a web page or something along that lines.  But when a voice
application cannot get reassembled in a timely manner, you'll surely notice

-----Original Message-----
From: Joel Maslak
To: asterisk-users at lists.digium.com
Sent: 12/23/2003 10:41 AM
Subject: Re: [Asterisk-Users] Asterisk SIP Packet Time (20ms)

On Tue, 23 Dec 2003, Rich Adamson wrote:

> If a collision or dropped packet occurs (in a voip udp environment)
> is no way to retransmit the missing/damaged packet. Missing one packet
> a big deal, but if you have collisions and/or dropped packets, there
is a
> very high probability that lots of packets will be dropped. If too
> are dropped, you'll hear the result in the undecoded voice as choppy
> voice.

Actually, collisions occur at Layer 2, not Layer 3, and the layer 2
hardware automatically resends packets involved in a collision - layer 3
is never aware of it happening (although it may cause additional delay).
Eventually the ethernet card will give up if too many collisions occur
during retries, but this is very rare in practice unless the network is
*VERY* loaded.

> Assuming alaw/ulaw codecs in use (about 80k bps), a half duplex 10 meg
> ethernet would handle roughly 20-25 rtp sessions before bumping into
> problem (your milage may vary). The majority of the folks on this list
> seem to be running home/soho systems and would likely never run into
> issue. But the heavier users will.

For a duplex mismatch, my experience is that if one end on a 100 Mb/sec
link is half and the other is full, bandwidth is limited to about 8
max.  This is based on some tests I've accidentally conducted.  If you
to send 9 Mb/sec over that link, yes, some packets will get dropped as
they simply won't fit.  (But I do agree that for a half-half link, you
get about 20 Mb/sec)

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