the addition of boundary clocks affects PDV. PRTC / T-GM Unaware Unaware Unaware Unaware Slave clock router router router router PRTC / T-GM Application Correction applied correctionField PRTC / T-GM Application Figure 9-21 PDV Accumulation Without and With Boundary Clock and Transparent Clocks The other case is where the PTP phase/time is used in combination with a physical frequency sig- nal, most commonly SyncE. For that case, the frequency is carried using a physical method where PDV is not an issue. Note, however, that the SyncE will have wander, and that impacts the phase/time in the slave clock (see the earlier section "Dynamic Time Error"). In a packet-based transport, the phase/time signal is delivered over the network in packets or frames, and these packets become mixed in (multiplexed) with other traffic in the various network elements they pass through. Packet networks are not inherently synchronous, SO there can be a dif- ference in the rate at which packets are queued for transmission and the rate at which they are trans- mitted. This difference is handled by allowing time gaps between packets, buffering/queuing, or discarding packets. The packets are also routed via intermediate switches and routers that cause delay due to pro- cessing, buffering, and queuing. Then, at a point of convergence, multiple packet streams may con- verge at a single choke point and are then queued until their turn to proceed comes. The resulting contention for a single resource introduces variable delay, and during times of congestion and over- subscription, packets will be dropped. Because it is possible that individual packets may take different paths, a stream of messages from master to slave may display significant PDV when the packets arrive at the destination. In complex topologies, packets may even arrive out of sequence, resulting in devices holding packets until re- ordering is possible. A similar problem arises with packet fragmentation and reassembly. To help improve the service quality that otherwise would be 