Transcript Over 3x bandwidth wasted to send one voice packet No

Changing the Dynamics of
Network Analysis
J. Scott Haugdahl
CTO, WildPackets, Inc.
[email protected]
www.wildpackets.com
What’s Changing about Network
Analysis?
• Unlike data protocols, VoIP is sensitive to
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Delays
Congestion
Jitter
Buffering at the receiver
• Convergence can affect performance
• Analysis is highly dependent on where the data is
gathered
• 802.11 compounds VoIP analysis challenges
www.voipdeveloper.com
August 8-10, 2006
Santa Clara, California
Hyatt Regency Santa Clara
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
www.voipdeveloper.com
August 8-10, 2006
Santa Clara, California
Hyatt Regency Santa Clara
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
www.voipdeveloper.com
August 8-10, 2006
Santa Clara, California
Hyatt Regency Santa Clara
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
 A G.711 packet sent
 Packets are buffered and
delayed at the Receiver
every 20 ms
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. . .. .. . . ........
 Packet jitter and drops
 The “jitter” buffer releases
a G.711 packet every 20 ms
Packets delayed more than the buffer delay
(100 ms as an example) are dropped
www.voipdeveloper.com
August 8-10, 2006
Santa Clara, California
Hyatt Regency Santa Clara
VoIP Jitter Analysis
G.711 every 20 ms is good
2.9 ms recovery – not bad
Good thing we have that jitter buffer!
www.voipdeveloper.com
August 8-10, 2006
Santa Clara, California
Hyatt Regency Santa Clara
End-to-end Voice Quality Analysis
The call goes through the network and…
HQ user IP
Remote user IP
… note the decrease in quality at the other end
www.voipdeveloper.com
August 8-10, 2006
Santa Clara, California
Hyatt Regency Santa Clara
RTP Control Protocol (RTCP)
• Defines RTP Sender and Receiver Report (combined)
– Found in the same RFC as RTP, RFC 3550, which obsoletes RFC 1889
– Contains total packet and byte counts, packet loss, and jitter information
– Optional, but all VoIP end-nodes should implement it!
• Enhanced by RTCP Extended Report (XR)
– RFC 3611
– Can be sent by non-recipients such as PSTN gateways
– Defines multiple report blocks with detailed information such as
• Loss RLE Report – Similar to RTCP RR but noting specific RTP packets that
were lost
• Duplicate Packet RLE Report
• Packet Receipt Times Report
• Detailed jitter, loss rate, discard rate, computed MOS scores, echo, noise, and
other information as end-node capable
www.voipdeveloper.com
August 8-10, 2006
Santa Clara, California
Hyatt Regency Santa Clara
Example RTCP Packet
www.voipdeveloper.com
August 8-10, 2006
Santa Clara, California
Hyatt Regency Santa Clara
Packets Get There But In Time?
Cause: Competition with data protocols
www.voipdeveloper.com
August 8-10, 2006
Santa Clara, California
Hyatt Regency Santa Clara
802.11 Wireless VoIP Analysis is Essential
• Diagnose pre- and post-deployment problems using
expert events such as
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Excessive wireless retransmissions
Recovery and data rate changes during RTP sessions
Excessive jitter
VoIP protocol signaling errors
Late packet arrival analysis at end-points
• Use an analyzer either side of an access point to
perform call and quality analysis for converged
networks
– Full seven layer analysis including encrypted packets on the
wireless using phone WEP keys; 802.11 media analysis is
always available regardless of encryption
www.voipdeveloper.com
August 8-10, 2006
Santa Clara, California
Hyatt Regency Santa Clara
A VoIP over WLAN Problem
Cause: Excessive environmental
interference on channel 11.
www.voipdeveloper.com
August 8-10, 2006
Santa Clara, California
Hyatt Regency Santa Clara
“Hidden” Wireless Errors
are Costly
• Lowering the data rate on a retry may get the data through but…
– It’s very inefficient
No 802.11 Ack
Frame at 11 Mbps
Same Frame at 5.5 Mbps
Over 3x bandwidth wasted to send one voice packet
• Retries at same speed and then lowered are even worse
• Sender can bounce up and down
• We need detailed operational WLAN analysis to see this and determine
the impact and to help optimize our physical environment, AP and client
settings, etc.
www.voipdeveloper.com
August 8-10, 2006
Santa Clara, California
Hyatt Regency Santa Clara
J. Scott Haugdahl
CTO, WildPackets, Inc.
[email protected]
www.wildpackets.com
www.voipdeveloper.com
August 8-10, 2006
Santa Clara, California
Hyatt Regency Santa Clara