You can read the ROUND_TRIP_DELAY register or calculate the round-trip delay from the delay components (Rx path delay, Tx path delay,cable delay, and Rx-to-Tx switching latency). The round_trip_delay field of the ROUND_TRIP_DELAY register in an REC master records the total round-trip delay from the start of the internal transmit radio frame in the REC to the start of the internal receive radio frame in the REC, that is, from SAP to SAP, in CPRI REC and RE masters.
CPRI v7.0 Specification requirements R-20 and R-21 address the round-trip delay. Requirement R-20 addresses the measurement without including the cable delay, and requirement R-21 is the requirement for the cable delay. Both requirements state that the variation must be no more than ±16.276 ns.
To monitor the round-trip delay, you must check periodically to confirm that the variation in measurements over time is small enough that the requirements are met.
Note: The round trip delay is the sum of the Tx path delays through the REC master and the RE slave, the Rx path delays through the REC master and the RE slave, the cable delay, and the Tx-to-Rx switching delay, sometimes referred to as the loopback delay. The Tx-to-Rx switching delay is labeled T_S_Rx_Tx in the figure. The Tx-to-Rx switching delay depends on the loopback path and the device.