ymbol mini-slot, UE capability #2 10.4 Latency Best-case latency Release 8 Low band FDD 3.5 GHz TDD Low band FDD with mini-slot Figure 10.13 Best-case two-way latency with different 5G deployment options. Ping measurements with 3.5 GHz 5G Figure 10.14 Latency measurement with 3.5 GHz TDD. 10.4.2 Low Latency Architecture When the radio latency becomes lower in 5G, it becomes more important to minimize the total end-to-end latency beyond the radio network. The signal propagation speed in optical fiber is 2 X 108 m per second. The two-way propagation time in 1000 km direct fiber is 10 ms, in 100 km 1 ms, and in 10 km 0.1 ms; see Figure 10.15. If we want to achieve 1 ms end-to-end RTT including radio and transport, the content must be close to the radio, which implies a maximum of a few tens of kilometers or preferably even a shorter 10 5G Performance Round trip time in fiber 1000 km 100 km 10 km Figure 10.15 Theoretical two-way propagation delay in direct optical fiber. Measured in Helsinki Measured in Oulu Measured in Oulu Gateway in Helsinki Gateway in Helsinki Gateway in Helsinki Server in Helsinki Server in Helsinki Server in Oulu Ping 11 ms Ping 20 ms Ping 29 ms 600 km Latency 11 ms Latency 20 ms Latency 29 ms Helsinki Figure 10.16 Impact of distance on practical latency. distance. Therefore, local content and a local gateway will be relevant for low latency use cases. The impact of distance on the latency can be demonstrated with RTT measurements in an LTE network. The measurement case is illustrated in Figure 10.16. The local oper- ator has a core network gateway located in Helsinki. Ping measurements over the LTE network show a latency of 11 ms in Helsinki. The same measurement in Oulu shows a 20 ms latency, that is, an additional latency of 9 ms. The distance from Helsinki to Oulu is 600 km, which in theory corresponds to an additional latency of 6 ms, but the practi- cal latency is always more than the theoretical minimum. The third measurement was done in Oulu while selecting a local server in Oulu. The m