ecision. In reality, this is not the case. The time counter that we use to maintain time, and which is captured to generate the timestamp, has a finite resolution-typically in the order of a few nanoseconds. Additionally, hardware limitations generally mean that there is some variation packet-to-packet as to what is defined as the "exact moment" of sending or receiving a message. Both of these factors effectively add noise to the timestamps which can impact the accuracy with which synchronization can be achieved. The above calculations made the assumption that the network is symmetrical, at least in terms of the delays experienced by the timing packets. Specifically, it assumed that the one-way delay for master to slave (which is what we need) is exactly half the round-trip delay of master-slave-master (which is what we can measure). In a perfect network, this may be a valid assumption, but in real networks, there will in general be asymmetries. These can be caused by the network infrastructure itself, such as different fiber lengths in each direction when separate transmit and receive fibers are used, or different optical pathlengths if two wavelengths are used on the same fiber, or can also be caused within the nodes themselves by factors such as where in the interface circuitry the timestamping. A much bigger problem is what is called Packet Delay Variation (PDV)-or jitter as it is commonly known to network engineers. This refers to the fact that each message sent between the master and slave likely would not take the same time to complete the journey, even though the physical path is unchanged. This results from delays in any switches or routers between the master and slave caused by other traffic on the network. Ethernet and the overlying protocols all support the concept of some form of quality of service to allow messages that are considered time-sensitive, such as the synchronization ones, to be prioritized over other packets, but none of these mechanisms can get around the issue of head of line blocking,