number of cells per packet but is typically in the single millisecond range due to the higher data rate of a complete multiplex as compared to a single VoIP flow. Playout delay considerations do not differ materially between TDMoIP and VoIP, however, so both work best on paths with controlled packet delay variation (strong overprovisioning or "QoS"). Timing recovery Native TDM networks rely on hierarchical distribution of timing. Somewhere in the network there is at least one extremely accurate primary reference clock with a long-term accuracy of 1 x 10^-11. This node, which offers Stratum 1 accuracy, provides the reference clock to secondary nodes with Stratum 2 accuracy. The secondary nodes then provide a time reference to Stratum 3 nodes. This hierarchy of time synchronization is essential for the proper functioning of the network as a whole. Packets in the PSN reach their destination with delay that has a random component, known as packet delay variation (PDV). When emulating TDM transport on such a network, this randomness may be overcome by placing the TDM packets into a jitter buffer from which data can be read out at a constant rate for delivery to TDM end-user equipment. The problem is that the TDM source time reference is no longer available, and the precise rate at which the data are to be "clocked out" of the jitter buffer is unknown. In certain cases timing may be derived from the TDM equipment at both ends of the PW. Since each of these clocks is highly accurate, they necessarily agree to high order. The problem arises when at most one side of the TDMoIP tunnel has a highly accurate time standard. For ATM networks, which define a physical layer that carries timing, the synchronous residual time stamp (SRTS) method may be used; IP/MPLS networks, however, do not define the physical layer and thus cannot specify the accuracy of its clock. Hence, in many cases the only alternative is to attempt to recover the clock based exclusively on the TDMoIP traffic, a technology known as "adaptive clock recovery". This is possible since the source TDM device is producing bits at a constant rate determined by its clock, although this rate is hidden by the PDV. The task of clock recovery is thus an "averaging" process that negates the effect of the random PDV and captures the average rate of transmission of the original bit stream.