Avaya MCC1 Maintenance Procedures page 364

Additional maintenance procedures
DS1 CPE loopback jack (T1 only)
Figure 110: Network Interface at "Dumb" Block
Figure notes
1 Span Section 1
2 Span Section 2
3 120A2 (or later) Integrated
Channel Service Unit (ICSU)
4 RJ-48 to Network Interface (up
to 1000 Feet) (305 m)
364
5 Loopback Jack
6 "Dumb" Block (Demarcation Point)
7 Interface Termination or Fiber MUX
8 Central Office
Maintenance Procedures
December 2003