Page 116 of 378 ITG Engineering Guidelines
553-3001-202 Standard 1.00 April 2000
Adjust ping measurements
One-wa
y
as compared to roundtrip
The statistics are based on round trip measurements, whereas the QoS
metrics in the Transmission Rat ing mo del a re one-way. In orde r to m ake the
comparison compatible, the delay and packet error ping statistics are to be
halved.
Ad
j
ustment caused b
y
ITG processin
g
The measurements are taken from host to host . T h e
Transmission Rating QoS metrics are from end user to end user, and would
include components outside the intra net. The statistic for delay needs
to be further modified by adding 93 ms to accoun t for the pr ocessing a nd jitte r
buffer delay of the ITG nodes.
No adjustment needs to be made for error rates.
If the intranet measurement barely meets the round trip QoS objectives, the
craftsperson needs t o be aware that t her e is a poss ibilit y th at th e one-w ay QoS
is not met in one of the direction of flow. This can be true even if the flow is
on a symmetric route due to the asymmetric behavior of data processing
services.
Late packets
Packets that arrived outside of the window allowed by the jitter buffer are
discarded by the ITG. To determine which samples to ignore, first
calculate the averag e one-way delay based on all the samples. Then add
500 ms to that. This is the maximum delay. All sample s whose o ne-way dela y
exceed this maximum are considered as late packets and are removed from
the sample. Calculate the percen tage of late packets, and add that to the packet
loss statistic.
Network delay and packet loss evaluation example
From ping data, calculate the average one-way delay (halved from ping
output, and adding 93 ms ITG processing delay) and standard deviation for
latency. Do a similar calculation for packet loss without adjustment.
Adding a standard deviation to the m ean of both delay an d loss is for planni ng
purposes. A customer may want to know whether traffic fluctuation in their
Intranet will reduce the user’s QoS.