Specifying Compensation for a PTP Delay Asymmetry

In this section

This section describes how to compensate for a PTP path delay asymmetry.

PTP delay asymmetry

PTP path delay asymmetry is defined as half the difference between the propagation times in two opposite directions. There can be a number of causes for such an asymmetry:

• Mismatch in the link speeds between master and slave
• Mismatched fiber lengths if separate fibers are used for the two directions resulting in a 2.5-ns asymmetry per meter of length difference since the delay in optical fibers is 5 ns/m and the asymmetry is half the difference between propagation times
• Different routes for uplink and downlink
• Poor equipment design

The asymmetry can be calculated or measured, and the value can be used for compensation.

Example

This figure shows an example where the cable in one direction is 4 m and the cable in the other direction is 104 m. In theory this results in a delay asymmetry of 250 ns (100 m × 2.5 ns/m).

However, the delay asymmetry can also be measured and monitored by the SmartNIC using PPS and the monitoring tool (see DN-0449 and DN-0793).

PTP delay asymmetry compensation

ntservice.ini code line

The syntax for the ntservice.ini code line compensating for a PTP delay asymmetry is:

'PTPDelayAsymmetry' '=' ( '–999000000' | '–998999999' |  '–998999998' | ... | '0' | '1' |  '2' | ... | '999000000' )

Example:

PTPDelayAsymmetry=10000

Default value

This table shows the default value.