changing the dynamics of network analysis j. scott haugdahl cto, wildpackets, inc....

Changing the Dynamics of Network Analysis

J. Scott HaugdahlCTO, WildPackets, [email protected]

www.wildpackets.com

www.voipdeveloper.comAugust 8-10, 2006

Santa Clara, CaliforniaHyatt Regency Santa Clara

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



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



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



. . .. .. . . ..............

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

Packets are buffered anddelayed at the Receiver

The “jitter” buffer releases a G.711 packet every 20 ms

A G.711 packet sent every 20 ms

Packets delayed more than the buffer delay (100 ms as an example) are dropped

Packet jitter and drops

Œ

Ž



VoIP Jitter Analysis

Good thing we have that jitter buffer!

G.711 every 20 ms is good 2.9 ms recovery – not bad



End-to-end Voice Quality Analysis

HQ user IP Remote user IP

… note the decrease in quality at the other end

The call goes through the network and…



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



Example RTCP Packet



Cause: Competition with data protocols

Packets Get There But In Time?



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



A VoIP over WLAN Problem

Cause: Excessive environmentalinterference on channel 11.



“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.

Frame at 11 Mbps Same Frame at 5.5 Mbps

Over 3x bandwidth wasted to send one voice packet

No 802.11 Ack



J. Scott HaugdahlCTO, WildPackets, [email protected]

www.wildpackets.com

changing the dynamics of network analysis j. scott haugdahl cto, wildpackets, inc....

Documents

packet loss

total packet

droppedpacket jitter

jitter informationoptional

consistent packet delivery

packet delivery intervals

optimal jitter buffer

buffer delay