s, the path overhead can be aligned in any column other than the first three columns when the STS-1 frame is depicted as a 9 X 90 byte two-dimensional array. In examining the overhead byte designators shown in Figure 6.7, note that rows 1 through 3 represent section overhead, resulting in 9 section overhead bytes. The A1 and A2 bytes are used to denote the beginning of an STS-1 frame. Thus, these 2 bytes are framing characters. A1 has the bit composition 11110110 or hex (hexadecimal) F6, while A2 has the bit Figure 6.7 STS-1 overhead Section bytes. Overhead Overhead Transport Overhead Overhead Chapter Six composition 00101000 or hex 28. Because the A1 and A2 bytes appear horizontally in the two-dimensional matrix, they are also referred to col- lectively as the horizontal sync pulse. The third byte in the first row in the section overhead is the JO byte. The function of this byte is to verify that patching operations are per- formed correctly. Thus, the JO byte represents a section-level trace byte. The second row in the section overhead area commences with the B1 byte. The B1 byte is a bit interleaved parity code (BIP-8) byte. In an STS-1 frame even parity is used to check for transmission errors over a regen- erator section, and its value is calculated over all bits of the previous frame. Thus, the B1 byte provides an error performance monitoring capability for a SONET section. The second byte in the second row is the E1 byte. This byte repre- sents a section-level order wire for voice communications between regen- erators, huts, and remote terminal locations. The use of this byte pro- vides a 64 kbit/s pulse-code modulation (PCM) voice channel between sections. The third byte in the second row is the F1 byte. This byte represents a section user data channel, providing a 64-kbit/s path that can be read from or written to at each location where section terminating equip- ment is located. The third row in the section overhead area contains three bytes labeled D1, D2, and D3. These bytes form a section data commun