Recommendation 0.11

                    MAINTENANCE ACCESS LINES


1.  GENERAL

































1.5.2  Digital loopback test line

       The digital loopback test line provides a dialable 4-wire  test  line
capability intended both for use  in  measuring  the  error  performance  of
international digital circuits and as a quick method of verifying the continuity 
of wholly digital, non-PCM encoded and mixed analogue/digital  circuits.  It
consists of circuitry that accepts and loops back on a digital basis the signal 
from a circuit. The test signal may be any arbitrary digital test pattern or 
analogue test signal.

       Once the tester has accessed the test line at a remote location,  the
tester may transmit the  desired  analogue  test  signals  or  digital  test
patterns. The tester may examine the returning signal for the received power (or 
continuity) of the analogue  test  signals  or  the  error  performance  (or
continuity) of the digital test patterns.

       The proposed dialling plan for this test line enables a  particular  line
to be selected when the distant switching centre is equipped for  this  type  of
dialling access. If the normal test line number (access code)  is  busy,  it  is
expected that the call should route to a busy indication.


















































3.7    Digital loopback test

       The digital loopback test line provides  a  dialable,  4-wire  test  line
capability; it consists of circuitry that accepts and loops back received octets from 
a digital circuit. The octets when looped back, are retransmitted  so  that  the
positions of the bits within the octet are preserved; that is, the most significant 
bit of the retransmitted octet corresponds to the most significant  bit  of  the
received octet, and so forth.

       The loopback  may  be  integrated  into  the  switching  network  of  the
automatic digital switching machine, or may be provided in a stand-alone mode, having an 
external 4-wire 64 kbit/s  appearance  on  the  switching  machine,  similar  to
existing test lines.