nsmitted from left to right. 3. The succeeding rows are transmitted in sequential order, from top to bottom, as depicted in Figure 3-24. Order of transmission Section overhead (SOH) Figure 3-24 A1: Frame alignment SONET frame. A2: Frame alignment B1: Section BIP-8 Frame N D1-D3: Section data communication channel E1: Orderwire F1: Section user channel JO: Section trace Z0: Section growth Line overhead (LOH) B2: Line BIP-8 D4-D12: Line data communication channel E2: Orderwire Frame N+1 H1 and H2: Pointer value 125 microseconds H3: Pointer action byte K1 and K2: APS channel MO: STS-1 line REI SOH & LOH SPE boundary M1: STS-N line REI S1: Synchronization messaging 250 microseconds Z1: Growth Z2: Growth STS path overhead (POH) B3: Path BIP-8 C2: STS path signal label F2: Path user channel G1: Path status H4: Multiframe indicator J1: STS path trace N1: Tandem connection maintenance/path data channel Z3 and Z4: Growth VT path overhead J2: VT path trace V5: VT path Z6 and Z7: VT path growth VT line overhead M1: Line quality S1: Synchronization message Chapter 3 3.3.2 SONET Overhead Detailed definitions for the following SONET overhead can be found in ANSI T1.105. The following paragraphs provide an abbreviated listing of the available overhead bytes. Not all bytes will be utilized on customer-to- network applications. The SONET overhead is active across the NI, as shown in Figure 3-25. The section overhead is shown in Figure 3-26, the STS path overhead is shown in Figure 3-27, the line overhead is shown in Figure 3-28, and the VT overhead is shown in Figure 3-29. 3.3.3 Synchronization In normal synchronous operation, the timing of the SONET frame at all interfaces points on the network heading in the direction of the customer installation needs to be traceable to a Primary Reference Source (PRS) as specified in ANSI T1.101. Also, in normal synchronous operation, the timing of the SONET frame at all NIs points heading in the direction away from the customer installation (CI)¹ needs to be traceable to the PRS. During norm