VoIP Packets

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VoIP Packets In the Air and Over the Wire J.
Scott Haugdahl CTO www.wildpackets.com
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VoIP Packet Analysis

• Invaluable for granular VoIP analysis
• Packet variance analysis (jitter), check for
  dropped packets at selected points in the path,
  late packet arrivals, out of sequence packets,
  examine RTCP reports, derive MOS scores, etc.
• VoIP signaling analysis
• Can involve multiple protocols and IP addresses
• Filtering can be tricky, capture at end-user
• VoIP voice stream analysis
• RTP streams are two-way and independent
• Filter at end-points by IP, then selectively
  analyze each direction

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Quality of Experience (QoE)

• Check for consistent packet delivery and verify
  QoS policies such as 802.11e as well as
  prioritization of packets sourced from layer 3
  devices
• For wireless, analyze impact of hand-off between
  access points
• Compare derived MOS scores for overall voice
  quality
• Playback captured VoIP RTP voice streams
• Analysis close to listener is best
• Listening to independent (i.e. one-way) streams
  is best
• Ability to vary the jitter buffer during playback
  assists in determining the optimal jitter buffer
  size

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Filtering Down to a Stream
From this
To this
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Real-time Transport Control (RTCP)
Contains valuable information sent from receiver
to sender. Not all devices support it, but should!
Lost packet stats
Latest jitter NOT in milliseconds, must be
converted using codec info
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Jitter
.......

• Jitter is the variance in packet delivery
  intervals to the listener
• Jitter buffer adds additional delay to voice
  reaching the ear piece in case other packets need
  to catch up

Ž
Packets are buffered and delayed at the Receiver
Œ
A G.711 packet sent every 20 ms
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