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Changing the Dynamics of Network Analysis

Changing the Dynamics of Network Analysis. J. Scott Haugdahl CTO, WildPackets, Inc. scott@wildpackets.com www.wildpackets.com. What’s Changing about Network Analysis?. Unlike data protocols, VoIP is sensitive to Delays Congestion Jitter Buffering at the receiver

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Changing the Dynamics of Network Analysis

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  1. Changing the Dynamics of Network Analysis J. Scott Haugdahl CTO, WildPackets, Inc. scott@wildpackets.com www.wildpackets.com

  2. What’s Changing about Network Analysis? • Unlike data protocols, VoIP is sensitive to • 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

  3. 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

  4. 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

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

  6. VoIP Jitter Analysis G.711 every 20 ms is good 2.9 ms recovery – not bad Good thing we have that jitter buffer!

  7. 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

  8. 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

  9. Example RTCP Packet

  10. Packets Get There But In Time? Cause: Competition with data protocols

  11. 802.11 Wireless VoIP Analysis is Essential • Diagnose pre- and post-deployment problems using expert events such as • 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

  12. A VoIP over WLAN Problem Cause: Excessive environmental interference on channel 11.

  13. “Hidden” Wireless Errorsare Costly • Lowering the data rate on a retry may get the data through but… • It’s very inefficient • 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. No 802.11 Ack Frame at 11 Mbps Same Frame at 5.5 Mbps Over 3x bandwidth wasted to send one voice packet

  14. Thank You! J. Scott Haugdahl CTO, WildPackets, Inc. scott@wildpackets.com www.wildpackets.com

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