Cisco CCNA Voice Certification Test Tutorial – Codecs Explained – G711, G729, G729a, ILBC

It is significant as a CCNA Voice applicant to realize the a variety of codecs that are supported by Cisco IP phones and the bandwidth that they each use.

A variety of codecs are supported on Cisco IP Telephones which include:

-G711

-G729

-G729a

-ILBC

G711

G711 provides toll excellent voice – in other terms, the most effective voice quality. There are two variations of G711 –

-G711ulaw – utilised in the United states, Canada & Japan (also identified as g711 mulaw)
-G711alaw – used in the relaxation of the earth

A definitely clever man known as Nyquist worked that if you sample voice at two times its highest frequency, you can reproduce voice exactly on the other close. Nyquist determined that human voice dialogue on telephones is in the assortment to 4000 Hz.

Consequently 2 x 4000 Hz = 8000 samples a 2nd. When you simply call from an analog landline at house, a machine in the telco cloud is sampling your voice at 8000 situations a second. Just about every of individuals samples is represented by an 8 little bit code phrase.

Consequently 8000 samples a second X 8 bits per sample = 64000 bps or 64 kbps.

Both G711ulaw and G711alaw use 64 kbps of bandwidth without having headers.

Nevertheless, to get a far better signal to sounds ratio, logarithmic sampling is used. This implies that much more samples are taken nearer to zero and fewer samples are taken as the audio wave moves away from zero.

G729

Nevertheless, by making use of clever voice sampling, the voice bandwidth can be minimized from the 64kbps required for G711. An case in point of this is G729. G729 is the codec that is prevalent in Cisco environments. This utilizes 8 kbps.

The G.729 speech coder is an 8 kbps Conjugate-Composition Algebraic-Code-Enthusiastic Linear Prediction (CS-ACELP) speech compression algorithm approved by ITU-T.

G.729 delivers significant excellent, strong speech effectiveness at the price tag of complexity. We will not include the aspects of the algorithm below – remember to refer to other files for detailed explanations.

G729a

A variant of G.729 is G.729a which makes use of the exact volume of bandwidth, but takes much less means to deliver. It nevertheless, has a a little decreased top quality than G729. It also employs 8kbps of bandwidth.

iLBC

The Web Low Bandwidth Codec (iLBC) is a more recent codec supported on the new Cisco IP Phones. It has a number of advandages about the earlier codecs.

– Was intended for packetized communications

– Is royalty totally free

– Has superior top quality than G729

– Permits swish speech top quality degradation in the scenario of misplaced frames

– 13.3 kbps of bandwidth used

It on the other hand is only supported on new Cisco handsets, so if older handsets are in use, it will not be readily available to these.

Other people

There are other may perhaps other codecs obtainable, but these are not necessarily supported on the Cisco IP Handsets.

Summary

Various codecs are supported on Cisco IP telephones. Just about every has various advantages and drawbacks. iLBC has quite a few positive aspects in excess of other codecs, but is only supported on more recent Cisco IP telephones.