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VoIP PacketsIn the Air and Over the Wire
J. Scott HaugdahlCTO
www.wildpackets.com
VoIP Packet Analysis
• Invaluable for granular VoIP analysiso 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 analysiso Can involve multiple protocols and IP
addresseso Filtering can be tricky, capture at end-user
• VoIP voice stream analysiso RTP streams are two-way and independento 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 streamso Analysis close to listener is besto Listening to independent (i.e. one-way) streams is
besto Ability to vary the jitter buffer during playback
assists in determining the optimal jitter buffer size
Filtering Down to a StreamFrom this…
To this
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, mustbe converted using codec info
. . .. .. . . ..............
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
.......
End-to-end Voice Quality Analysis
HQ user IP Remote user IP
… note the decrease in quality at the other end
Replay the VoIP call at different jitter buffer settings
The call goes through the network and…
Special Considerations forVoIP over Wireless LANs
• Traditional protocols can be bursty but VoIP requires predictability
o VoIP works best with a nice steady delivery of packets
o Limited bandwidth + far more error prone than wired = unpredictability
• Security of utmost importance as WLAN “has no boundaries”
o Unlike LAN, WLAN signaling is totally exposedo Secure authentication and encryption is a musto Encryption can impact latency which impacts VoIP
• Roaming users may experience handoff delays
o Can lead to annoying clicks, delays, or dropped calls
Special Wireless Considerations (continued)
• There will be many frames received with CRC errors
o Retries are via the 802.11 management protocolo Retries may or may not be at the same data rateo Network utilization, packets per second statistics,
etc. can be deceiving• DOS attacks worse than on wired LANS
o Much easier to disrupt both VoIP signaling and data
o Even with encryption and firewalls, DOS attacks are still easy by “occupying the air” at 2.4 GHz (b/g) or 5 Ghz (a)
• Protocols are more complicated, it’s not just the voice
o Signaling, gatekeepers, call managers, and advanced features require the use of wired services
“Hidden” Wireless Errorsare Costly
• Lowering the data rate on a retry may get the data through but…
o 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 consumed to send one frame
No 802.11 Ack
WLAN Fault Analysis is Essential
• Diagnose pre- and post-deployment problems using expert events such as
o Excessive wireless retransmissionso Recovery and data rate changes during RTP
sessionso Excessive jittero VoIP protocol signaling errorso 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
o Full seven layer analysis including encrypted packets on the wireless using phone WEP keys; 802.11 analysis always available regardless
Simultaneous LAN/WLAN Analysis
VoIP over WLAN Steam Analysis
Good thing we have that jitter buffer!
G.711 every 20 ms is good 2.9 ms recovery – not bad
A VoIP over WLAN Problem
Cause: Excessive environmentalinterference on channel 11.
Another VoIP over WLAN Problem
Cause: Competition with data protocols
Maintaining a Quality VoIP Experience
• The dynamic nature of networks and usage patterns requires vigilant 24x7 monitoring
o Remote probes/engines/sensors are critical to wireless fault analysis
• Placement is not in the data center!• Probes at deployed access points may not detect rogue access points• Consoles allow multi-location WLAN analysis
o A quality expert system can help identify weaknesses in performance, DOS attacks, optimize security and performance, etc.
o Portable analyzers are good for pinpointing problem spots• Pay close attention to factors that affect VoIP
o Latency, dropped packets, jitter, signaling problems, excessive wireless retransmissions, late packet arrival, etc.
o Utilize a tool that shows relationships between SSIDS, access points, stations, and channels to manage, troubleshoot, and optimize for VoIP
• For wireless, be realistic about the number of users per access point
• Implement QoS end-to-end but do not give VoIP absolute priority
