Continuing the TelePresence notes series based on Cisco TelePresence Fundamentals by Cisco Press.
Managing Latency, Jitter, and Loss
A few important facts about latency:
- Latency: The human experience is round-trip in nature. This is referred to as conversational latency, or experience-level latency; 250 ms to 350 ms is the threshold at which the human mind begins to perceive latency and be annoyed by it.
- Latency Target: To maintain acceptable experience-level latency, Cisco recommends that customers engineer their networks with a target of no more than 150 ms of network-level latency, in each direction, between any two TelePresence systems.
- Latency Thresholds: When network-level latency exceeds 250 ms averaged over any 10-second period, the Cisco TelePresence system receiving those packets generates an alarm, and an onscreen message displays to the user.
If the latency from one TelePresence system in Hong Kong to the CTMS in London is 125 ms, and the latency from the CTMS in London to the other TelePresence system in San Francisco is 125 ms, the end-to-end latency from the Ethernet network interface of the Hong Kong system to the Ethernet network interface of the San Francisco system is 250 ms, plus approximately 10 ms added by the CTMS, for a total of 260 ms. The TelePresence System in San Francisco will not realize this and will think that the latency for that meeting is only 125 ms.
A few important facts about jitter:
- Jitter Target: To maintain acceptable experience-level latency, Cisco recommends that customers engineer their networks with a target of no more than 10 ms of packet-level jitter and no more than 50 ms of video frame jitter in each direction between any two TelePresence systems.
- Jitter Thresholds: Cisco TelePresence uses a quasi-adaptive jitter buffer.
- At the beginning of every new meeting, the jitter buffer starts out at 85 ms in depth.
- After monitoring the arrival time of the video frames for the first few seconds of the meeting, if the incoming jitter exceeds 85 ms average, the jitter buffer is dynamically adjusted to 125 ms.
- After that, if the jitter exceeds 125 ms averaged over any 10-second period, the Cisco TelePresence system receiving those video frames generates an alarm and dynamically adjusts the jitter buffer to 165 ms. The alarm is written to the syslog log file of that TelePresence system, and an SNMP trap message is generated.
- No onscreen message is displayed to the user.Any packets exceeding the 165 ms jitter buffer depth are discarded by the receiving TelePresence system and logged as “late packets” in the call statistics.
A few important facts about loss:
- Loss Target: To maintain acceptable experience-level video quality, Cisco recommends that customers engineer their networks with a target of no more than .05 percent packet loss in each direction between any two TelePresence systems. This is an incredibly small amount, and given the complexity of today’s global networks, 0.05 percent loss is not always possible to accomplish.
- Loss Thresholds: The Cisco TelePresence system has a multi-tiered loss thresholds:
- When packet loss (or late packets) exceeds 1 percent averaged over any 10-second period, the Cisco TelePresence system receiving those packets generates an alarm, and an onscreen message appears.
- When packet loss (or late packets) exceeds 10 percent averaged over any 10-second period, the Cisco TelePresence system receiving those packets generates a second alarm, and a second on-screen message appears (unless the hold timer is already in affect).
- If loss (or late packets) exceeds 10 percent averaged over any 60-second period, in addition to the actions described, the system downgrades the quality of its outgoing video.
- Finally, if loss equals 100 percent for greater than 30 seconds, the codec hangs up the call. If the packets begin flowing again anytime up to the 30-second timer, the codec immediately recovers.
