The round trip delay through a multi-hop connection can be calculated in a similar manner to the single hop case. The following figure shows a block diagram of a two-hop system consisting of a Radio Equipment Controller (REC), a network Radio Equipment unit (RE) and a node RE.
The round trip delay between the REC and the node RE is obtained by first measuring T14 (1) at the REC and Toffset at the node RE. These are measured in the same way as described in R21 Calculation, T14 at the master port of the REC and Toffset at the slave port of the node RE.
The round trip delay T14(1) is calculated based on the time difference between the transmission and reception of the synchronization byte at the REC. Section 4.2.9 of the CPRI Specification v7.0 describes how the data that arrives at the network RE from the node RE is sent onto the REC several basic frames (N) after the synchronization byte. To calculate the round trip delay in the multi-hop system this must be accounted for by adding N x TC, where TC is the basic frame length, to T14 (1).
This gives:
Multi-hop round trip delay = T14 (1) + (N x TC) - Toffset