METRODATA NetTESTER FAMILY USER MANUAL
76-02-107 Rev.A
Page 125 of 133
By now excluding the processing time, the actual network latency can be calculated
as
T_latency = T_rtt
– T_soft
And assuming a symmetric network with equal latency in both directions, the one
way delay may be approximated as
T_oneway = T_latency / 2
TWAMP is normally configured to send a test PDU once per second to measure the
round-trip latency.
Additionally, packet delay variation or jitter is measured independently in both
directions as follows:
T_outward = ABS( T_rx1
– T_Tx1 )
T_inward = ABS( T_rx2
– T_tx2 )
The variation is calculated by comparing the delay of the current packet with that of
the previous packet, such that
Variation_out = ABS( T_Outward( n )
– T_outward ( n-1 ) )
And
Variation_in = ABS( T_inward( n )
– T_inward ( n-1 ) )
Now, because the variation will typically be small, the drift of the clocks is
insignificant allowing for the jitter/variation to be measured in both directions
separately.
PACKET LOSS MEASUREMENT
Each TWAMP PDU that is generated is given a sequence number. A PDU is
declared lost if there is a gap in the sequence numbers. In this way, packet loss is
estimated based on the loss of the test PDUs, and due to the low, one packet per
second, PDU rate this is considered synthetic packet loss measurement.
