nued, and finally the neighboring switch reassembles the outgoing frame and forwards it. Reflecting on Table 6.6.1, it is obvious that at high rates frame pre-emption is not really needed to reduce delay, and its real purpose is to avoid complete starvation of normal traffic when there is an abundance of express traffic. Some TSN mechanisms assume that all (or at least most) network forwarding elements have access to high-accuracy (sub-microsecond) timing information, obtained, for example, by use of the PTP (IEEE 1588v2). Once the entire network is thus synchronized, a new set of mechanisms becomes available that can provide guaranteed upper bounds on E2E latencies. The base mechanism of TSN is the time-sensitive queue defined in IEEE 802.1 Qbv, which mimics time-division multiple access schemes by opening and closing at precise timeslots. Timeslot schedules may be dynamically computed by a centralized management system that configures the network nodes using the Stream Reservation Protocol (SRP). In this way express traffic classes are serviced without interruption, effectively eliminating queuing delay, and rendering residence time deterministic. This enables guaranteeing upper bounds on E2E latencies. A readily understood, but non-optimal, method of exploiting time-sensitive queues with- out intricate signaling is called cyclic queuing (previously called peristaltic queuing) (IEEE 802.1Qch). In this scheme all switches simultaneously forward all packets of the same traf- fic class in the same timeslot. Before outputting, the priority marking is incremented, SO that the packets exit the next switch in the following time slot. The E2E latency is hence the number of switches traversed times the timeslot duration. 6.6.4.3 Achieving the Required Reliability Mobile communications were originally considered relatively unreliable, due in large part to poor coverage and the customer's understanding of the limitations of the air interface. However, due to its ubiquity, people and businesses have become more and more de