etween them being referred to as the Path. Between the first byte of the VC4 and the POH byte of a particular VC12. the two path termination points at B and C, there is no legitimate Each pointer is located in a pre-defined position within the VC4, mechanism for altering any of the information in the POH, hence if hence once the VC4 POH has been located, it is a simple matter of 42/10 The Synchronous Digital Hierarchy (SDH) Bytes allocated Block of 63 Bytes allocated to VC12#38 pointers to VC12 #1 (35 bytes Payload 9 bytes (these are actually distributed throughout the VC4) VC4 = 2349 bytes / 125 u S tail end of previous first few bytes of VC12#38. group of 35 bytes in VC12#38 Remainder of this group of 35 bytes are delivered in the following VC4 Figure 42.10 Fixed location of VC12 pointers within a VC4 counting bytes in order to locate each of the 63 pointers to the conversely, one byte being repeated. (See Figure 42.11.) Either way, VC12s. (See Figure 42.10.) the resulting change of phase of VC12 relative to VC4 is tracked by The trigger for a rephasing of a VC12 relative to its VC4 is when a corresponding change in pointer value. the fill of the VC12 input buffer (into which the incoming VC12 is Because of the importance of pointers for locating low order VCs written prior to loading into the VC4) exceeds a pre-determined (VC12s) within high order VCs (VC4s), the combination of a low threshold. At this point, what appears to be a fairly conventional order VC plus its pointer is referred to as a Tributary Unit (TU). In justification occurs, with either an extra VC12 byte being loaded this case, the combination of a VC12 plus its pointer constitutes a into the VC4 (into the overflow position mentioned earlier) or, TU12. Block of Bytes allocated VC12#38INPUTBUFFER FILL 63 pointers to VC12 # 38 Nearly full Buffer fill continuously reducing Nearly empty o 125 250 375 Time(us) 125 S Buffer fill drops below critical level Phase change of VC12#38. One byte of VC12#38 is written into the VC4 twice 250 u S Phase change 