rk as an example in NGMN. For the URLLC service, the target is 1-2 ms. The E2E latency is important as a reduction in the latency improves the performance of many applications, and some of them are not feasible unless the latency is low enough. Propagation delay is unavoidable in transport links (although paths can be optimized), while low speed or congested links, excessive buffers, and complex operations in the data plane of transport and network devices can - and should - be completely avoided. Even a single low-performance node or link in the transport network may in the worst case introduce significant unnecessary latency for a large number of users, as every transport link serves an aggregate of end users. 9 Transport RTT 10 ms (MBB) Air If gNB-DU gNB-CU Server Server RTT 1 ms (URLLC) Air If Server Figure 9.6 RTT components. The round trip time (RTT) budget is divided into the air interface, UE processing, baseband processing, transport legs, core function, and connectivity to the server, as shown in Figure 9.6. For the 10 ms e2e target of mobile broadband (MBB), the transport legs on F1 and NG may not contribute more than a few milliseconds (e.g. 1-4 ms RTT) to the latency, depending on the latency consumed by the other components. Transport needs to be assessed as part of the whole system. As in Figure 9.6, for the e2e latency, the low-latency service necessitates an architec- ture where the UPF, gNB, and server are collapsed and present in the same site or anyway close to the radio, and also the transport architecture is different from that of the MBB. From the system operation viewpoint, the F1 and NG interfaces tolerate longer than 1-4 ms RTT. All the delay-critical radio functions (fast retransmissions, radio schedul- ing, Radio Link Control [RLC] protocol) are located in the gNB-DU. That makes the F1 and NG interfaces non-delay-critical. An increase in the delay on these interfaces mainly impacts the e2e delay and performance. Low-latency fronthaul instead is delay critical from the system operation 