O

OCN (Operating Company Number), tiered rates and, 100–102

OC-3 circuit

identifying problems with, 260

overview, 56

OC-12 circuit, identifying problems with, 260

1MB lines, changing long-distance carriers with, 121–127

online billing, 103. See also invoices

Open System Interconnection Reference Model (OSI model), lower layers of, 304–305

Operating Company Number (OCN), tiered rates and, 100–102

optical multiplexers (optical MUX), OC-3 circuits and, 56

order numbers

on DLR, 156

on FOC document, 151

on technical cut sheet, 162

outages

expecting credits after, 332

handling large, 230

PBX systems and, 31

protecting toll-free systems from, 77

outlying areas, making calls to, 49–50

out-of-band signaling

FGD protocol and, 145–146

ISDN protocol and, 140–143

overview, 140

SS7 protocol and, 143–145

outpulse signal protocol, dedicated circuits and, 137–139

overflow routing, to switched lines, 182–183

overlapping calls, checking for, 105–106