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Spirent KB Article
Doc ID: FAQ20006
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Spirent TestCenter: How to get accurate Latency and Jitter measurement with STCv?
In order to obtain the most accurate latency and jitter measurements, the following steps are recommended. Details are provided in the subsections below.
Utilize a stratum one source
Enable NTP Burst
Allow NTP to sync over a longer period
Use recommended deployments
Follow hypervisor configuration recommendations
Enable NTP Burst
STCv Command to enable NTP Burst –
ntp x.x.x.x burst
Provides 100 +/- Microsecond range with accuracy, when using a private NTP server synced to a stratum one clock.
Recommended Deployments for Latency MeasurementsRecommended Deployments for Latency Measurements
In order to eliminate clock differences, deploy a single VM with multiple test ports. If that is not feasible, the next best option is to deploy VMs on the same hypervisor.
Traffic between two STCv ports on the same STCv VM.
Traffic between two STCv VMs on the same Hypervisor.
Traffic between two STCv VMs across two sites.
Two NTP Servers running independently (Not Recommended).
To increase the accuracy of latency measurements, you should:
Use dedicated CPU cores for STCv VM
Configure the hypervisor to isolate the STCv cpu cores with CPU pinning
Not having pinned vCPUs can lead to issues with ‘CPU Steal Time’
Do not allow hypervisor interrupt processing on the STCv cores
Given the nature of NTP, it is possible to get negative latency numbers even after following the recommendations. A negative latency is seen if the following ae true –
The clocks on one of the VMs is ahead of the clock on another VM. This clock difference is inherent in the nature of NTP.
The network latency is smaller than the clock difference.
For example, the picture below shows an example of negative latency
Latency in one direction:
Latency in the other direction:
- 540,741.32 us
In such cases, taking an average of the two latencies cancels out the clock difference and gives an accurate measure of average network latency:
Taking the Average
Why does taking the average cancel out the clock difference? Let’s look at the example below. Let’s say that the actual network latency between the two VMs is
, and the clock difference between the two VMs is
. In other words, when the clock on
, the clock on
will show time
Let’s say that
is traveling from
. It departs
. Note that
clock would show a time of
at that moment. The packet would arrive at
after a period
and would get an arrival timestamp of
, which is traveling from
, will have departure and arrival timestamps of
Latency, which is determined by subtracting departure timestamp from the arrival timestamp, would be:
VM1 to VM2 latency (Packet A)
VM2 to VM1 Latency (Packet B)
clock is ahead of
, the latency measured in
VM1 -> VM2
direction will be higher and the latency measured in
VM2 -> VM1
direction will be lower. If the clock difference
is larger than the network latency
, then we would see negative numbers for latency.
We can, however, determine the true latency
by averaging the two unidirectional latency numbers.
Product : STC Virtual,Spirent TestCenter