lexed in with enterprise or other types of data streams. Non- PTP traffic may consist of packets with a very large maximum transmission unit (MTU) size, what are known as jumbo frames. Even when PTP messages are in a high-priority queue, a jumbo frame that has already started to transmit will cause the following PTP message to have to wait. One method to reduce this effect is to have the device support a time-sensitive networking (TSN) technique known as frame preemption (see the section "IEEE TSN" in Chapter 4). Frame preemption allows the transmission of a non-express packet to be interrupted and an ex- press packet to be forwarded before recommencing the transmission of the long frame. The other end of the link does the reassembly of the preempted frame (note that this feature requires hardware support at both ends of the link). Of course, as interface speeds increase, the time taken to send a jumbo frame decreases, and SO the time waiting for transmission of those frames decreases. Despite that, this technique is still considered useful for very tight timing requirements in the front-haul-if interface speeds are low (less than, say, 25-50 Gbps). Remember that this applies only for cases with transit PTP traffic because a properly designed boundary clock will not suffer this problem (the timestamping is not done until the frame is ready to transmit). Similarly, for a T-TC, any waiting time for the transit PTP packet will be re- flected in the value written to the correctionField, effectively nullifying the delay. For details of frame preemption, see IEEE 802.1Q-2018 (which absorbed 802.1Qbu-2016) and 802.1CM-2018, "Time-Sensitive Networking for front-haul," and the 802.1CMde-2020 amendment. The type and design of the network element itself generates PDV. Some devices just suffer from high PDV, because reducing it was not a design goal, or the engineers designed the device with some feature that causes it (such as oversubscription). Remember that the main issue is not the time taken to switch the PTP message bu