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.
David Bombal,
CCIE #11023, CCSI, CCDP, CCIP, CCNP, CCSP, CCVP, CCNA, CCDA
David has over 10 years experience in the IT industry and has been training in the Cisco market for over 8 years. He has worked with Cisco IP Telephony since the Call Manager 3.0 days in 2001.
He is the owner of http://www.CCNAVoice.com which has many resources for the CCNA Voice candidate. Get access to free articles, eBooks, training and more.
http://www.CCNAVoice.com also provides test questions and simulations to help you prepare for your studies, premium access to more detailed information and mentoring.
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