Codecs
A codec is a accessory or affairs able of assuming encoding and adaptation on a digital
data beck or signal. Various types of codecs are acclimated to encode and break or compress
and decompress abstracts that would contrarily use ample amounts of bandwidth on
WAN links. Codecs are abnormally important on lower-speed consecutive links area every bit
of bandwidth is bare and activated to ensure arrangement reliability.
One of the best important factors for a arrangement ambassador to accede while building
voice networks is able accommodation planning. Arrangement administrators charge understand
how abundant bandwidth is acclimated for anniversary VoIP call. To accept bandwidth, the administrator
must apperceive which codec is actuality activated beyond the WAN link. With a thorough
understanding of VoIP bandwidth and codecs, the arrangement ambassador can apply
capacity planning tools.
Coding techniques are connected by the ITU. The ITU G-series codecs are amid the
most accepted standards for VoIP applications.
Following is a account of codecs accurate by Cisco IOS gateways:
■ G.711: The all-embracing accepted for encoding blast audio on a 64 kbps channel.
It is a PCM arrangement operating at an 8 kHz sample rate, with 8 $.25 per sample.
With G.711, the encoded articulation is already in the actual architecture for agenda articulation delivery
in the PSTN or through PBXs. It is broadly acclimated in the telecommunications field
because it improves the signal-to-noise arrangement after accretion the bulk of data.
There are two subsets of the G.711 codec:
■ mu-law: mu-law is acclimated in North American and Japanese buzz networks.
■ a-law: a-law is acclimated in Europe and abroad about the world.
Both mu-law and a-law subsets use digitized accent agitated in 8-bit samples. They
use an 8 kHz sampling bulk with 64 kbps of bandwidth demand.
■ G.726: An ITU-T Adaptive Differential Pulse Cipher Modulation (ADPCM) coding at
40, 32, 24, and 16 kbps. ADPCM-encoded articulation can be alternate amid packet
voice, PSTN, and PBX networks if the PBX networks are configured to support
ADPCM. The four bit ante associated with G.726 are generally referred to by the bit
size of a sample, which are 2-bits, 3-bits, 4-bits, and 5-bits, respectively.
■ G.728: Describes a 16 kbps Low-Delay Cipher Excited Linear Prediction (LDCELP)
variation of CELP articulation compression. CELP articulation coding charge be translated into a
public telephony architecture for commitment to or through the PSTN.
■ G.729: Uses Conjugate Structure Algebraic Cipher Excited Linear Prediction
(CS-ACELP) compression to cipher articulation into 8 kbps streams. G.729a (that is, G.729
Annex A) requires beneath computation, but the lower complication is not after a tradeoff
because accent affection is hardly worsened. Also, G.729b (that is, G.729
Annex B) adds abutment for VAD and CNG, to account G.729 to be added able in its
bandwidth usage. The appearance of G.729a and G.729b can be accumulated into
G.729ab. Accepted G.729 operates at 8 kbps, but there are extensions that provide
6.4 kbps (Annex D) and 11.8 kbps (Annex E) ante for hardly worse and better
speech quality, respectively.
■ G.723: Describes a dual-rate accent coder for multimedia communications. This
compression address can be acclimated for burden accent or audio arresting components
at a actual low bit bulk as allotment of the H.324 ancestors of standards. This codec has
two bit ante associated with it:
■ r63: 6.3 kbps; appliance 24-byte frames and the MPC-MLQ (Multipulse LPC with
Maximum Likelihood Quantization) algorithm
■ r53: 5.3 kbps; appliance 20-byte frames and the ACELP algorithm
The college bit bulk is based on ML-MLQ technology and provides a somewhat higher
quality of sound. The lower bit bulk is based on CELP and provides arrangement designers
with added flexibility.
■ GSM Full Bulk Codec (GSMFR): Introduced in 1987, the GSMFR accent coder has
a anatomy admeasurement of 20 ms and operates at a bit bulk of 13 kbps. GSMFR is a RPE-LTP
(Regular Pulse Excited—Linear Predictive) coder. To address VoiceXML scripts that
can action as the user interface for a simple voice-mail system, the arrangement must
support GSMFR codecs. The arrangement messaging charge be able of recording a
voice bulletin and depositing the bulletin to an alien server for after retrieval.
This codec supports the Cisco basement and appliance accomplice components
required for account providers to arrange unified messaging applications.
■ Internet Low Bit Bulk Codec (iLBC): Designed for attenuated bandage speech, it after-effects in
a burden bit bulk of 13.33 kbps for 30-ms frames and 15.20 kbps for 20-ms frames.
The algorithm is a adaptation of Block-Independent Linear Predictive Coding, with the
choice of abstracts anatomy lengths of 20 and 30 milliseconds. The encoded blocks accept to
be encapsulated in a acceptable agreement for transport, such as RTP. This codec enables
graceful accent affection abasement in the case of absent frames, which occurs in connection
with absent or delayed IP packets.
86 Authorized Self-Study Guide: Cisco Articulation over IP (CVOICE)
Note iLBC is accurate on Cisco AS5350XM and Cisco AS5400XM Universal
Gateways with Articulation Feature Cards (VFCs) and IP-to-IP gateways with no transcoding and
conferencing.
The arrangement ambassador should antithesis the charge for articulation affection adjoin the amount of
bandwidth in the arrangement back allotment codecs. The college the codec bandwidth, the
higher the amount of anniversary alarm beyond the network.