ayload that contains the timing information. To allow the packet to pass over the communica- tions path, the payload is wrapped inside a header and footer (denoted respectively as H and F in Figure 3-6). Because the delay of every packet on these types of networks is nondeterministic, time- critical packets sent out periodically will vary in their transit time-what is known as packet delay variation (PDV). PDV is a significant problem in deployment of packet-based timing distribution because of its effect on the accuracy of the recovered clock. "Chapter 9. PTP Deployment Consider- ations", covers strategies to manage PDV in more detail. Payload Payload Payload Significant instants Figure 3-6 Packet Timing Signal One more factor to determine is at what precise time the packet "departs" or "arrives." This is de- termined by agreeing that one precise location of the message is the significant instant. The timestamp should represent the time at which this significant instant passes the timing reference point of the clock. By convention, the location is agreed to be the end of the start-of-frame delim- iter, but it can be defined differently in any given packet technology, provided the definition is con- sistently applied. For further background reading on timing based on packet networks, significant instants, and more, see Chapter 6, "Description of packet timing concepts," of ITU-T G.8260. 3.1.1 Jitter and Wander When the timing signal (technically, the significant instant of that signal) shows short-term, high- rate variations from the ideal position, it is called jitter. Alternatively, slow, long-term time vari- ations are known as wander. So, the question becomes what is considered to be "short term." According to the ITU-T (in G.810), the division between jitter and wander for physical time distri- bution is at 10 Hz, SO variations that occur at a frequency at or above 10 Hz are referred to as jitter, and those below 10 Hz are called wander. Packet-based methods such as PTP might use a different cutoff (0.1 Hz),