Codecs are used to convert an analog voice signal to a digitally encoded version. Codecs vary in the sound quality, the bandwidth required, the computational requirements, etc.
Each service, program, phone, gateway, etc typically supports several different codecs, and when talking to each other, negotiate which codec they will use.
As an example, a Cisco ATA-186 supports these codecs: G.723.1, G.711a, G.711u, G.729a (please note that ATA-186 has x2 FXS ports, there is only one port can run on (G.729) with two simultaneously active ports, second one will be on G.711a/u)
As an example, a Cisco 7960 supports (Firmware P0S3-06-0-00): G.711a, G.711u, G.729a
Some codecs require payment of royalties for their use in a product or program. See Codec Patents for more information.
- AMR Codec
- BroadVoice Codec 16Kbps narrowband, and 32Kbps wideband
- DoD CELP – 4.8 Kbps
- GIPS Family – 13.3 Kbps and up
- GSM – 13 Kbps (full rate), 20ms frame size
- iLBC – 15Kbps,20ms frame size: 13.3 Kbps, 30ms frame size
- ITU G.711 – 64 Kbps, sample-based Comes in two flavors: A-law and mu-law
- ITU G.722 – 48/56/64 Kbps ADPCM 7Khz audio bandwidth
- ITU G.722.1 – 24/32 Kbps 7Khz audio bandwidth (based on Polycom’s SIREN codec)
- ITU G.722.1C – 32 Kbps, a Polycom extension, 14Khz audio bandwidth
- ITU G.722.2 – 6.6Kbps to 23.85Kbps. Also known as AMR-WB. CELP 7Khz audio bandwidth
- ITU G.723.1 – 5.3/6.3 Kbps, 30ms frame size
- ITU G.726 – 16/24/32/40 Kbps
- ITU G.728 – 16 Kbps
- ITU G.729 – 8 Kbps, 10ms frame size
- LPC10 – 2.5 Kbps
- Speex – 2.15 to 44.2 Kbps
Some tables comparing different codecs:
Audio Examples of Different Codecs
News
- 2005-05-09 – Free online codec conversion tool available at asteriskguru.com.
Some potentially useful info:
Keep in mind the link-layer framing that is used as well. ATM (what most of the internet backbones use) has 53 byte fixed cells. There are 5 bytes of headers. Only one packet can exist in a group of cells (a group is 1 or more cells until the whole packet is sent). This means that if you try to send a 80 byte IP packet over an ATM link it will be chopped into 2 ATM cells with 16 bytes of padding. This is only 83% efficient. By adjusting your sample size you may find that your throughput can be increased because you transfer more useful data and less padding. Make your sample size too small and you have a lot of IP overhead, make it too large and it can cause problems with the call quality (think of a 30ms jitter buffer and 30ms sample sizes, in effect you have no jitter buffer because packets cant be reordered, jitter cant be controlled, etc). Its a fine balance, but something to consider. Even if you don’t use ATM the internet backbones often do, so this is something that may make slightly faster transfers and better network efficiency. DSL, E1, T1, SMDS, OC1, OC3, OC12, etc generally all use ATM, so it’s quite common.
See also
- A complete list of ITU G series recommendations
- Asterisk codecs
- Bandwidth consumption
- Codec Software
- Open Source Codecs
April 7, 2023
Phone System 