It is vital as a CCNA Voice candidate to comprehend the several codecs that are supported by Cisco IP phones and the bandwidth that they each and every use.
Many codecs are supported on Cisco IP Phones including:
-G711
-G729
-G729a
-ILBC
G711
G711 presents toll excellent voice – in other text, the greatest voice high-quality. There are two versions of G711 –
-G711ulaw – utilized in the Usa, Canada & Japan (also known as g711 mulaw)
-G711alaw – used in the rest of the globe
A seriously clever dude named Nyquist labored that if you sample voice at two instances its best frequency, you can reproduce voice accurately on the other finish. Nyquist determined that human voice dialogue on telephones is in the vary to 4000 Hz.
So 2 x 4000 Hz = 8000 samples a second. When you connect with from an analog landline at household, a device in the telco cloud is sampling your voice at 8000 instances a next. Just about every of those people samples is represented by an 8 bit code phrase.
Therefore 8000 samples a 2nd X 8 bits per sample = 64000 bps or 64 kbps.
Equally G711ulaw and G711alaw use 64 kbps of bandwidth without having headers.
Having said that, to get a superior signal to noise ratio, logarithmic sampling is employed. This usually means that much more samples are taken closer to zero and much less samples are taken as the sound wave moves away from zero.
G729
On the other hand, by making use of clever voice sampling, the voice bandwidth can be minimized from the 64kbps expected for G711. An example of this is G729. G729 is the codec that is prevalent in Cisco environments. This takes advantage of 8 kbps.
The G.729 speech coder is an 8 kbps Conjugate-Construction Algebraic-Code-Psyched Linear Prediction (CS-ACELP) speech compression algorithm accredited by ITU-T.
G.729 gives significant top quality, robust speech functionality at the price tag of complexity. We will not deal with the specifics of the algorithm listed here – please refer to other files for in-depth explanations.
G729a
A variant of G.729 is G.729a which uses the similar total of bandwidth, but normally takes significantly less resources to develop. It nonetheless, has a a little decreased quality than G729. It also uses 8kbps of bandwidth.
iLBC
The Net Lower Bandwidth Codec (iLBC) is a newer codec supported on the new Cisco IP Phones. It has several advandages over the earlier codecs.
– Was made for packetized communications
– Is royalty free
– Has far better quality than G729
– Enables sleek speech excellent degradation in the circumstance of lost frames
– 13.3 kbps of bandwidth applied
It nonetheless is only supported on new Cisco handsets, so if more mature handsets are in use, it will not be offered to those.
Other people
There are other may possibly other codecs accessible, but these are not automatically supported on the Cisco IP Handsets.
Summary
Numerous codecs are supported on Cisco IP telephones. Each has various strengths and down sides. iLBC has many positive aspects about other codecs, but is only supported on newer Cisco IP phones.