EXFO FTB-800 Series for FTB-1 - Manual (Page 464)

Test Functions
454 FTB-700G/800 Series
RTD
RTD
Note: Not available in Through mode.
Round Trip Delay (RTD) measurements are needed to quantify the time it
takes for a signal to cross the network and come back. Usually, transport
delay is due to two factors: long configured paths and transit times through
the network elements along the path. Therefore, RTD measurements are
significant in systems that require two-way interactive communication,
such as voice telephony, or data systems where the round-trip time directly
affects the throughput rate.
From the Test menu, tap Functions, and the RTD tab.
Note: To do Round Trip Delay test, the remote NE should be configured to provide
a loopback. However a local DSn test can be configured to use loopback
codes allowing RTD testing.
Note: Be aware that RTD requires error free operation conditions to provide
reliable results. Therefore, RTD results could be affected by error injection
or error introduced by the network.
Mode
Allows the selection of the round trip delay test mode. Choices are Single
(default) and Continuous. For CPRI/OBSAI test application, RTD operates
in Single mode.

Single allows testing the round trip delay once.

Continuous allows testing the round trip delay continuously in a
repetitive manner (one RTD measurement every 2 seconds).