X 10-11 per month), yet there may be some small variance in time among the clocks. Assuredly they will not be phase-aligned. Likewise, SONET must take into account loss of master clock or a segment of its timing delivery system. In this case, switches fall back on lower-stability internal clocks. This situation must also be handled by SONET. Therefore synchronous transport must be able to operate effectively under these conditions where network nodes are operating at slightly different rates. 5 Slips are discussed in Chapter 6. ADVANCED BROADBAND DIGITAL TRANSPORT FORMATS To accommodate these clock offsets, the SPE can be moved (justified) in the positive or negative direction one octet at a time with respect to the transport frame. This is achieved by recalculating or updating the payload pointer at each SONET network node. In addition to clock offsets, updating the payload pointer also accommodates any other timing phase adjustments required between the input SONET signals and the timing reference at the SONET node. This is what is meant by dynamic alignment where the STS SPE is allowed to float within the STS envelope capacity. The payload pointer is contained in the H1 and H2 octets in the line overhead (LOH) and designates the location of the octet where the STS SPE begins. These two octets are viewed in Figure 19.10. Bits 1 through 4 of the pointer word carry the new data flag, and bits 7 through 16 carry the pointer value. Bits 5 and 6 are undefined. Let's discuss bits 7 through 16, the actual pointer value. It is a binary number with a range of 0 to 782. It indicates the offset of the pointer word and the first octet of the STS SPE (i.e., the J1 octet). The transport overhead octets are not counted in the offset. For example, a pointer value of 0 indicates that the STS SPE starts in the octet location that immediately follows the H3 octet, whereas an offset of 87 indicates that it starts immediately after the K2 octet location. Payload pointer processing introduces a signal impairment known as payload adj