It is important as a CCNA Voice candidate to understand the various codecs that are supported by Cisco IP phones and the bandwidth that they each use.

Various codecs are supported on Cisco IP Phones including:

• G711
• G729
• G729a
• iLBC

G711

G711 provides toll quality voice - in other words, the best voice quality. There are two versions of G711 -

• G711ulaw - used in the USA, Canada & Japan (also called g711 mulaw)
• G711alaw - used in the rest of the world

A really clever guy called Nyquist worked that if you sample voice at two times its highest frequency, you can reproduce voice exactly on the other end. Nyquist decided that human voice conversation on telephones is in the range 0 to 4000 Hz.

Thus 2 x 4000 Hz = 8000 samples a second. When you call from an analog landline at home, a machine in the telco cloud is sampling your voice at 8000 times a second. Each of those samples is represented by an 8 bit code word.

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

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

However, to get a better signal to noise ratio, logarithmic sampling is used. This means that more samples are taken closer to zero and fewer samples are taken as the sound wave moves away from zero.

G729

However, by using clever voice sampling, the voice bandwidth can be reduced from the 64kbps required for G711. An example of this is G729. G729 is the codec that is common in Cisco environments. This uses 8 kbps.

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

G.729 offers high quality, robust speech performance at the price of complexity. We will not cover the details of the algorithm here - please refer to other documents for detailed explanations.

G729a

A variant of G.729 is G.729a which uses the same amount of bandwidth, but takes less resources to produce. It however, has a slightly lower quality than G729. It also uses 8kbps of bandwidth.

iLBC

The Internet Low Bandwidth Codec (iLBC) is a newer codec supported on the new Cisco IP Phones. It has several advandages over the previous codecs.

• Was designed for packetized communications
• Is royalty free
• Has better quality than G729
• Enables graceful speech quality degradation in the case of lost frames
• 13.3 kbps of bandwidth used

It however is only supported on new Cisco handsets, so if older handsets are in use, it will not be available to those.

Others

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

Summary

Various codecs are supported on Cisco IP phones. Each has various advantages and disadvantages. iLBC has several advantages over other codecs, but is only supported on newer Cisco IP phones.

Comments

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elieser  - g729   |2008-12-24 23:27:01 PLEASE, EXPLAINE THE CODEC!!! Reply 2   0

Nawas Ahamed  - Coded   |2009-06-23 20:54:14 Hi, codec is short form of coder and decoder. codec defines how a signal is sampled to represent the original signal at the transmission end and how the original signal is reproduced at the receiving end Reply 0   0

kundangem   |2009-06-09 13:52:48 Hi David , Till now my concept in codec was not clear. But after reading your article i understood the concept Its will be better if you expalain more in codec. Thanks a lot... Reply 0   0

ankit  - ausomeeeeeeeeeeeeeeeeeee   |2009-10-24 21:38:07 that was really gud n ausome.keep posting new things. Reply 0   0

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