Avaya BCM 2.5 IP Telephony Guía de configuración Pagina 24

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24 Chapter 1 Introduction
P0937663 02.0
Jitter Buffer
Voice frames are transmitted at a fixed rate, because the time interval between frames is constant.
If the frames arrive at the other end at the same rate, voice quality is perceived as good. In many
cases, however, some frames can arrive slightly faster or slower than the other frames. This is
called jitter, and degrades the perceived voice quality. To minimize this problem, configure the IP
telephone with a jitter buffer for arriving frames. This is how the jitter buffer works:
Assume a jitter buffer setting of five frames.
The IP telephone firmware places first five arriving frames in the jitter buffer.
When frame six arrives, the IP telephone firmware places it in the buffer and sends frame one
to the handset speaker.
When frame seven arrives, the IP telephone buffers it and send frame two to the handset
speaker.
The net effect of using a jitter buffer is that the arriving packets are delayed slightly in order to
ensure a constant rate of arriving frames at the handset speaker. The disadvantage of using a jitter
buffer is that the speech arrives delayed by the number of frames in the buffer. For one-sided
conversations, this is not an issue. For conversations where one party tries to interrupt the other
speaking party, it is annoying because by the time the voice of the interrupter reaches the
interruptee, the interruptee has spoken (2*jitter size) frames past the intended point of interruption.
In cases where very large jitter sizes are used, some users revert to saying "OVER" when they wish
the other party to speak.
Possible jitter buffer settings, and corresponding voice packet latency (delay) for the Business
Communications Manager system IP telephones are:
None
Small (.06 seconds)
Medium (.12 seconds)
Large (.18 seconds)
QoS routing
When it sends a voice frame onto the network, the IP telephone firmware places some header
information on the frame.
The header contains the network address of the sending and receiving IP telephones, and a TOS
(Type Of Service) byte, which contains a routing priority.
The IP telephone firmware establishes the TOS byte to the highest possible priority so that as the
voice frame travels through the network, the routers it encounters give it higher routing priority
than competing data frames resulting from file transfers, WEB downloads, e-mails, etc. This
process of prioritizing data frames is Quality of Service (QoS) routing.
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