3.18.4. Multi-Hop Delay Measurement
To determine the round-trip delay of a full multi-hop system, you must add together the values in the ROUND_TRIP_DELAY registers of the REC and RE masters in the system, plus the delays through the external routers, and subtract the loopback delay from all the hops except the final hop.
Round-trip delay = Σi round_trip_delay (hop i) + Σj (TBdelayUL + TBdelayDL)(j) – Σj T_S_Rx_Tx(j)
where the REC and RE masters in the configuration are labeled i = 0, 1, ...,n and the routing layers in the configuration, and their uplink and downlink delays, are labeled j = 0, 1, ...(n-1).
As the equation shows, you must omit the loopback delay from the single-hop calculation for all but the final pair of CPRI link partners. The loopback delay is only relevant at the turnaround point of the full multi-hop path.
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