Voice Gateways: Voice CODECs and DSP Resources

Many different codecs available, three you should know for sure:

G.711 – 64Kbps – MOS 4.3 – Uncompressed – PPS (Packets Per Second) 33/50 – Payload Bytes 240/160 – Sample Size ms 30/20
G.729 – 8Kbps – MOS 3.92 – Compressed – PPS (Packets Per Second) 33/50 – Payload Bytes 30-20 – Sample Size ms 30/20
iLBC – 15.2Kbps – MOS 4.14 – Open Source – PPS (Packets Per Second) 33/50 – Payload Bytes 50/38 – Sample Size ms 30/20

Three quality evaluation methods:

Mean Opinion Score – MOS – People
Perceptual Evaluation of Speech Quality – PESQ – Automated
Perceptual Evaluation of Audio Quality – PEAQ – Automated

G.729 builds a codebook on the fly of samples. Example of the word ‘cow’.
Must be LATIN based. Music On Hold can cause problems with G.729 due to not being LATIN and therefore not understood.

Transcoding on the VG from 1 CODEC to another.


G.711  Example

50 PPS
160 Bytes

Add L3 40 bytes…
200 Bytes

Add L2 Overhead..
Ethernet = 18
218 Bytes

218 * 50 = 10900 Bytes per second

* 8 for Bytes to Bits
= 87200 Bits per second Total 87.2Kbps

*G.711 via Ethernet is not as efficient BW wise as PBX

Managing DSP Resources

  • DSPs offload media processing from voice gateways

Media Termination Point (MTP – Holding a call that is on hold)
Conferencing (3 people on a single Cisco phone with built in DSP resources)

  • DSPs are limited in the number of calls they can handle (High/Med/Low)
  • DSPs can be directed using DSPFarms
  • Google for ‘Cisco DSP Calculator’

Basic DSP setup

voice-card 1
dsp services dspfarm

dspfarm profile 1 conference
codec g711alaw

associate dsp profile with SCCP/CUCM etc… beyond CVOICE scope and seen in CIPT exams