The latency values given in the Call for Scores are one-way latency values — specifically, the time from the conducting impulse in Dresden to the point at which a musician at a remote location sees or hears it. As noted in the Technical Annex, these values are inclusive of all sources of delay: the video transmission chain, network travel time, and monitoring setup (mouth-to-ear latency).
*Note on figures below: the Call for Scores published indicative zone ranges (Zone A: under 20 ms; Zone B: 20–30 ms; Zone C: 30–40 ms). Without the high-performance GÉANT research network, the network round-trip between Dresden and a Zone C location would typically be around 60–80 ms. GÉANT reduces this to approximately 40 ms round-trip (around 20 ms one-way), enabling the zone values stated in the Call to be achieved in practice. The figures below reflect current estimates based on the confirmed GÉANT infrastructure.*
For a Zone C location such as Athens, this one-way latency — from the conductor’s beat to the moment a musician sees the baton — is approximately 35 ms: around 15 ms for the video chain plus around 20 ms for the network. This falls within the Zone C range stated in the Call.
Your observation about four signal legs is correct when considering the complete feedback loop: the conductor’s beat travels out to the remote musicians (two legs via Frankfurt), and their sound travels back to Dresden (two further legs via Frankfurt). For Zone C, this full round-trip — from the conductor’s beat to the moment the conductor hears the sonic result — is approximately 65 ms.
Composers should therefore work with the one-way latency as the operative figure: this is what each musician receives and must respond to. The full feedback loop (what the conductor hears) involves the additional return path but does not alter the compositional challenge for the remote ensembles.