Codecs and Required Bandwidth

Codecs and Required Bandwidth

The required bandwidth for a single call, in one direction, is 64 kbps. As the G.711 codec samples 20 ms of voice per packet, 50 such packets need to be transmitted per second. Each packet contains 160 voice samples, which gives 8000 samples per second. Each packet is sent in one Ethernet frame. With every packet of size 160 bytes, headers of additional protocol layers are added. These headers include RTP + UDP + IP + Ethernet, with a preamble of sizes, 12 + 8 + 20 + 26, respectively. Therefore, a total of 226 bytes, or 1808 bits, must be transmitted 50 times per second, or 90.4 kbps, in one direction. For both directions, the required bandwidth for a single call is 100 pps or 180.8 kbps, assuming a symmetric flow.

  • Waveform codecs:– Directly encode speech in an efficient way by exploiting temporal and/or spectral characteristics
    – Attempt to reproduce input signal’s waveform by minimizing error between input and coded signals
  • Source codecs / vocoders:– Estimate and efficiently encode a parametric representation of speech

ITU Codec Examples

Waveform

  • Time Domain/ Predictive Coders
    – G.711 PCM (A / μ-law):  64 kbit/s , 56 kbit/s
    – G.726 ADPCM : 32 kbit/s, 24 kbit/s, 16 kbit/s
  • Subband Coders
    – G.722
  • Transform Coders

Source

  • – G.728 LD-CELP : 16 kbit/s
  • – G.729 CS-ACELP : 8 kbit/s
  • – G.723.1 MP-MLQ : 6.4 kbit/s

 

Common ITU-T Codecs and Their Defaults

Codec           Data Rate (kbps)      Datagram Size(ms)     A/D Conversion Delay (ms)    Combined Bandwidth (Bidirectional) (kbps)

G.711u            64.0                        20                               1.0                                            180.80
G.711a             64.0                       20                               1.0                                            180.80
G.729                 8.0                       20                             25.0                                              68.80
G.723.1              6.3                       30                              67.5                                             47.80
G.723.1              5.3                       30                              67.5                                             45.80