ccie voice written Q&A

Q.51
You are the network administrator at 1234.com. Your newly appointed trainee wants to know which fields in the output from show call active voice command indicates that packet loss is occuring. What will be your reply?





How to use the show call active voice command to identify jitter and typical voice quality symptoms
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Solution # K10995848
Title How to use the show call active voice command to identify jitter and typical voice quality symptoms
Resolution Jitter is the difference between when a voice packet is expected to arrive and when it is actually received, due to a delay in the networking path.
Received packets can be held in a de-jitter buffer that controls the variations in packet arrival before sending it to the codec. The amount of time a packet is held in the de-jitter buffer is determined by its size; this, in turn, determines the delay. De-jitter buffer size can be configured by issuing the playout-delay command.

Small gaps in the stream are hidden by using concealment techniques. For example, instead of having gaps of silence when packets arrive outside the de-jitter buffer playout delay, they are filled with predicted intervals determined from recent past frames.

A general idea of jitter in the network can be determined by repeatedly issuing the show call active voice command while a call is in progress. Ideally, these parameters should stay relatively steady. If they do, that is an indication of smooth packet flow. However, if jitter is present, there are sharp, short term spikes such as those shown in these two sample outputs:




GapFillWithSilence=950 ms
GapFillWithPrediction=1980 ms
GapFillWithInterpolation=0 ms
GapFillWithRedundancy=0 ms
HiWaterPlayoutDelay=350 ms
LoWaterPlayoutDelay=25 ms
ReceiveDelay=29 ms
LostPackets=0
EarlyPackets=0
LatePackets=83
.
.
GapFillWithSilence=1040 ms
GapFillWithPrediction=2350 ms
GapFillWithInterpolation=0 ms
GapFillWithRedundancy=0 ms
HiWaterPlayoutDelay=40 ms
LoWaterPlayoutDelay=28 ms
ReceiveDelay=35 ms
LostPackets=0
EarlyPackets=0
LatePackets=99
The incrementing number of late packets in these sample outputs reveal a degree of jitter. The silence insertion indicated by an increase in the GapFillWithSilence value manifests itself as choppy voice. The predictive insertion, indicated by an increase in the GapFillWithPrediction value, tends to manifest itself as synthetic voice.

To alter the amount of voice signal that is buffered to avoid jitter buffer under-runs or over-runs, issue the playout-delay command. Care should be taken in configuring the playout-delay command since it will have an impact on the overall delay budget of the path of the call. Larger values result in larger return trip delays, but reduce the likelihood of under-runs at receiving de-jitter buffers. Depending on the severity, this could result in a choppy or robotic voice.

Note: Configuring the playout-delay command is not necessarily the appropriate solution in all cases. Severe jitter should be located and eliminated from the network where possible.

By altering the gateway configuration, messages are sent to the Digital Signal Processor (DSP) to modify the handling of the voice stream. Before Cisco IOS® Software version 12.1 (5) T, the playout-delay command was configured on the voice port. From Cisco IOS Software version 12.1 (5) T and later, the command is typically configured under the dial peer, but can still be placed under the voice port.

The two modes of configuration for playout delay are adaptive and fixed:

Adaptive allows the jitter buffer to grow and shrink for the duration of the call within a configured range by issuing the playout-delay {nominal value | maximum value | minimum {default | low | high}} command.
Fixed is set at the beginning of a call by issuing the playout-delay mode {adaptive | fixed [no-timestamps]} command.
For more information on VoIP, refer to Playout Delay Enhancements.

For more sample voice quality symptoms, refer to Recognizing and Categorizing Symptoms of Voice Quality Problems.


Problem Type Ask a how to ... question, Voice quality problems (Audio quality: Choppy voice, hissing, echo, etc.)
Voice Quality Garbled voice, Choppy voice (Listen to short or long choppy voice samples.), Robotic voice (Listen to short or long robotic voice samples.), Synthetic voice (Listen to short or long synthetic voice samples.)
How to ... How to ... IOS show commands, How to ... voice quality
Failure Type Jitter
IOS Show Commands show call active voice (brief)
Voice Gateways IOS gateways


links:
http://supportwiki.cisco.com/ViewWiki/index.php/How_to_use_the_show_call_active_voice_command_to_identify_jitter_and_typical_voice_quality_symptoms
http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a008019ab88.shtml