Path OverHead information

Date: 2015-10-07; view: 362.

Having mapped the 2Mbil/s bit stream into the C12, the next operation is to generate and attach the Path Overhead byte (POH), which enables this C12 to be identified, monitored for errors and routed through the SDH network. The addition of this POH byte to the C12 creates a Virtual Container 12 (VC12). As mentioned in Section 42.3, the idea is that the POH stays attached to its C12 all the way from the point where it was generated, to the point at which the 2Mbit/s payload exits the SDH network. These two points are the 'Path Termination' points for this VCI2, with the continuous stream of VCs between them being referred to as the Path. Between the two 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 VC12 payload (i.e. the C12), this indicates that the VC12 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 his customers' 2Mbil/s circuit, it provides no information whatsoever on the source of the errors i.e. which network element has gone faulty. This task is dealt with by the addition of still further overhead information, which will he described shortly. Before this, it is necessary to examine the way in which several VC12s are multiplexed into a higher rate signal.