 path termination points at B and C, there is no legitimate mechanism for altering any of the information in the POH; hence if the receiving path termination detects any discrepancy between the POH and the content of the VC-12 payload (i.e., the C12), this indicates that the VC-12 payload has somehow become corrupted during its journey across the SDH network. Although path-level monitoring is sufficient for a PTO to ascertain what error rate is being inflicted by the SDH network on its customers' 2-Mbit/s circuit, it provides no infor- mation whatsoever on the source of the errors-that is, which network element has gone faulty. This task is dealt with by the addition of still further overhead information, which will be described shortly. Before this, it is necessary to examine the way in which several VC-12s are multiplexed into a higher-rate signal. Multiplexing Virtual Containers The general principle of this multiplexing operation is fairly straightforward. Several VC-12s, which should, hopefully, be synchronous with one another, are loaded into a larger synchronous container, which subsequently has its own POH added, thus creating a larger (i.e., higher-bitrate) VC (see Figure 10.4). In ETSI countries, this operation often, but not always, results in the creation of a VC-4, which has a payload that is large enough to accom- modate up to 63 VC-12s. Unfortunately, as discussed in The Basis of SONET and SDH section, complications arise in this operation because the network element that is performing this task might not itself have created all the VC-12s (see Figure 10.8). This leads to the possibility that not all the VC-12s are completely synchronous with one another or, more important, with the VC-4 into which they are being loaded. The solution to this problem comes in two parts. First, the internal structure of the VC-4 has been purposely designed to allow each VC-12 to run slightly faster, or slower, than the VC-4 rate. This is done by designating certain bytes in the C4 as overflow bytes (one per Chapter 1