ust- ment jitter. This impairment appears on a received tributary signal after recovery from a SPE that has been subjected to payload pointer changes. The operation of the network equipment processing the tributary signal immediately downstream is influenced by this excessive jitter. By careful design of the timing distribution for the synchronous network, payload pointer adjustments can be minimized, thus reducing the level of tributary jitter that can be accumulated through synchronous transport. 19.2.2.6 The Three Overhead Levels of SONET. The three embedded overhead levels of SONET are Path (POH) Line (LOH) Section (SOH) These overhead levels, represented as spans, are illustrated in Figure 19.11. One important function is to support network operation and maintenance (OAM). The POH consists of 9 octets and occupies the first column of the SPE, as pointed out previously. It is created and included in the SPE as part of the SPE assembly process. The POH provides the facilities to support and maintain the transport of the SPE between path terminations, where the SPE is assembled and disassembled. Among the POH specific functions are: An 8-bit-wide (octet B3) BIP (bit-interleaved parity) check calculated over all bits of the previous SPE. The computed value is placed in the POH of the following frame. Alarm and performance information (octet G1). A path signal label (octet C2); gives details of SPE structure. It is 8 bits wide, which can identify up to 256 structures (28). One octet (J1) repeated through 64 frames can develop an alphanumeric message associated with the path. This allows verification of continuity of connection to the source of the path signal at any receiving terminal along the path by monitoring the message string. Normal values 011000 10-bit pointer value 00000000 SPE octet NNN - IIDIDIDIDID - Negative stuff Positive stuff opportunity octet opportunity octet Legend: Bit assignments: I = increments D = decrement N = new data flag -= unassigned To Actuate: Set new data flag: invert 4 N-bits Negat