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Home > Success Center > VoIP & Network Quality Manager (VNQM) > VNQM 4.4 Administrator Guide > Quality of Service metrics > Latency

Latency

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With respect to VoIP, latency is a measure of the difference in time between when one caller speaks and when the other caller hears what the first has said. Excessive network latency can cause noticeable gaps and synchronization loss in transmitted conversations, particularly when VoIP is used with other types of data, as in a video conference. If these gaps become large enough, callers may find that they inadvertently interrupt each other while conversing.

IP SLA operations measure latency by sequentially applying four different time stamps to a single test packet in the following way:

  1. Time stamp T1 is applied to a test packet as it leaves the source router.
  2. Time stamp T2 is applied as the test packet arrives at the target router.
  3. Time stamp T3 is applied as the test packet leaves the target router to return to the source.
  4. Time stamp T4 is applied when the test packet returns to the source.

IP SLA operations provide four separate measures of latency by computing differences among the four time stamps in the following way.

latency measure

calculation

Round trip time

T4-T1

Source-to-target latency

T2-T1

Target processing latency

T3-T2

Target-to-source latency

T4-T3

Latency is computed for both source-to-target and target-to-source directions to account for asynchronous network behavior, providing a more detailed picture of the overall network latency.

SolarWinds VoIP and Network Quality Manager calculates latency in the following way:

if (rttMonLatestJitterOperNumOfRTT <> 0)

delay = (convert rttMonLatestJitterOperRTTSum to milli/microseconds depending on rttMonEchoAdminPrecision) / rttMonLatestJitterOperNumOfRTT

else

delay = 0

if (micro/milli second convert value of rttMonLatestJitterOperOWSumSD and rttMonLatestJitterOperNumOfOW has value and rttMonLatestJitterOperNumOfOW <> 0)

delaySD = micro/milli second convert value of rttMonLatestJitterOperOWSumSD / rttMonLatestJitterOperNumOfOW

else

if (micro/milli second convert value of rttMonLatestJitterOperOWSumDS and rttMonLatestJitterOperNumOfOW has value and rttMonLatestJitterOperNumOfOW <> 0)

delayDS = micro/milli second convert value of rttMonLatestJitterOperOWSumDS / rttMonLatestJitterOperNumOfOW

else

delaySD & delayDS = null

rttMonLatestJitterOperNumOfRTT 1.3.6.1.4.1.9.9.42.1.5.2.1.1
rttMonLatestJitterOperRTTSum 1.3.6.1.4.1.9.9.42.1.5.2.1.2
rttMonEchoAdminPrecision 1.3.6.1.4.1.9.9.42.1.2.2.1.37
rttMonLatestJitterOperOWSumSD 1.3.6.1.4.1.9.9.42.1.5.2.1.33
rttMonLatestJitterOperNumOfOW 1.3.6.1.4.1.9.9.42.1.5.2.1.41
rttMonLatestJitterOperOWSumDS 1.3.6.1.4.1.9.9.42.1.5.2.1.37

 
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