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Maintaining voice-over-IP (VoIP)
quality is largely a matter of minimizing network delay and jitter, which is
variability in packet arrival time. Codecs require a steady, consistent
stream of packets to provide adequate playback quality. Packets arriving too
late or out of order will result in jerky, jumbled playback (referred to as
jitter).
While the jitter buffer on a VoIP phone can mask mild delay and jitter
problems, severe jitter can overwhelm the jitter buffer, resulting in
excessive delay.
Another item to monitor is packet loss. Networks tend to either sporadically
drop single packets (these periods are called “gaps†in packet loss) or
large numbers of packets in a “burst.†Packet loss concealment techniques
typically have no trouble handling packet loss during gap periods; sustained
bursts are difficult to manage.
Managing VoIP requires hard numbers beyond subjective user assessments of
quality. Beyond monitoring the network parameters, having an overall quality
score, such as a mean opinion score (MOS) or R-factor score can also be a
useful VoIP network health index. VoIP monitoring tools calculate the MOS
and R-factor scores using a formula known as the E-model. An analyzer
calculates how much the various impairment factors (such as codec, jitter,
delay and packet loss) affect the typical user’s perception of call quality.
VoIP quality may be degraded when any of the following conditions arise: MOS
score falls under 3.5; R-factor score drops below 70; or unidirectional
delay exceeds 150 ms.
When a user cannot get a dial tone or there are excessive delays in ringing
the other party’s phone, VoIP performance is unacceptable. Examining a
display that shows how the call is progressing between the parties and the
call manager or gateway can reveal the problem. A connection dynamics
display (also known as a ladder diagram) highlights which party is not
responding or which party is responding slowly. Having this capability can
dramatically cut down troubleshooting time.
Metrics can provide a good indication of current VoIP performance. An
analysis tool that tracks, stores and analyzes long-term activity will
define what is considered normal for a particular VoIP environment. This
insight will help you intelligently configure alarms on the monitoring tool
to notify you when VoIP performance deviates from the norm–allowing you to
become aware of problems as they arise so you can immediately resolve them.
If you have not deployed VoIP on the network yet, baselining activity is
crucial to determine how the network will handle the new traffic and what
changes need to be made to effectively support VoIP communication.
An analyzer that can provide detailed insight into VoIP communication is
needed to successfully manage VoIP, as the monitoring tools that come
standard with the VoIP equipment may not be adequate. An analyzer that can
detect and resolve audio problems, provide
real-time call detail records, and display VoIP connection dynamics is the
best option.
Charles Thompson is manager of sales engineering for Network
Instruments, Minnetonka, Minn.
For more information:
www.rsleads.com/610cn-260
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