Codec packet sizes

[Andrew Rutherford <>]

I got asked about this line:

> >         Cisco-AVpair = "codec-bytes=240"

Why? Decreased bandwidth and increased jitter buffer.

How?

!
voice class codec 1
 codec preference 1 g711alaw bytes 240
 codec preference 2 g729r8 bytes 40
 codec preference 4 g723r63 bytes 96
 codec preference 5 g726r16 bytes 80
 codec preference 8 gsmfr bytes 132
 codec preference 14 g711ulaw bytes 240
!
dial-peer voice 200 voip
 voice-class codec 1
!

(Yes, we're not in North America so u-law is down the list.)

JITTER
------

Most systems calculate the size of their jitter buffer at least 
partially on the payload size they are receiving. If you have lots of 
bandwidth but a little niggling jitter because of large bursts of 
data traffic, increasing the payload size can often make the problem 
go away.

BANDWIDTH
---------
We're trying to decrement the over head from IP packets when going 
over the Internet where we don't have (or want) RTP header 
compression. Eg, G729 is an 8k codec with 10 byte frame size. Default 
is two frames per packet, so that's 20 byte IP + 8 bytes UDP header + 
12 byte RTP header + 20 byte payload = 60 bytes, or a 3x uplift from 
8k to 24k.

Now try the same calculation with a 40 byte payload, and you get 16k. 
60 byte payload, and you get 13.3k. (Now run two calls over a dialup 
modem link. :-)

NOTES
-----

Unless you're sure, don't go past four frames (generally 1/25 of a 
second), and no more than three for G.711. Cisco default to 2 frames.

Cisco 7905/7912 phones and ATA's have a similar tuning variable, 
NumTxFrames, that's global across codecs.

This will marginally increase latency, but in most cases over shared 
data networks without QoS it results in better overall voice quality.

G.711 is restricted to 240 bytes as some Cisco platforms, eg, 2600, 
will reject G.711 packets over that size and it won't be obvious 
unless you turn RTP debugging on, even though the AS5xxx routers will 
quite happily go higher.

G.729 can generally go to 60, although we have found one 802.11 phone 
that can't go past 40.

The router will guide you on appropriate sizes in multiples of frame sizes, 
eg:

gw(config-class)#codec preference 4 g723r63 bytes ?
  Each codec sample produces 24 bytes of voice payload.
  Valid sizes are:
    24, 48, 72, 96, 120, 144.
  Any other value within the range will be rounded down
  to nearest valid size.
  <24-149>  Choose a voice payload size from the list above

The list above doesn't have G.729b only because it confuses codec 
negotiation in certain Cisco phones. See my message on 10/10/2003 for 
details.

--
Andrew Rutherford      
sip:
      244 Pirie Street
Iagu Networks          tel:+61-8-8425-2255       Adelaide SA 5000
http://www.iagu.net/   
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   Australia