AMERITEC AM-48 - Figure 6-4. 4-Wire End-To-End Testing

AM-48 Test Set (18-0015) Connection & Configuration
0015-S06
6
-11
End-to-end testing is the most reliable for characterizing the near-to-
far and far-to-near impairments, but it has the disadvantage of
requiring two (2) test sets and an operator at each test site.
CPU
DTE
RCV
RCVXMIT
XMIT
<SND>
<RCV> <SND>
<RCV>
AM-48
AM-48
MODEM
MODEM
Telephone
Network
Figure 6-4. 4-Wire End-to-End Testing
Loopback Testing . The distant end of a 4-wire transmission line can
be looped back (1) manually, or (2) with a commandable loopback
device such as a Western Electric Model 829. The AM-48 at the near
end sends a signal which travels to the distant end where it makes a
U-turn and travels back to the AM-48, which measures the received
signal.
The advantage of this configuration is that it requires only one (1)
operator and test set at a central site. Although this method is more
convenient and less costly than end-to-end testing, it is also less
reliable. The disadvantage is that it does not characterize
transmission impairments in each direction. Instead of detecting
impairments, a loopback measurement could actually cover them up.
An apparently satisfactory loopback measurement may contain
impairments in each direction which cancel each other. Loopback
testing may be useful, providing its limitations are recognized.
Note: A loopback test can not be done on 2-wire because signals
can not be sent and measured simultaneously on the same
pair of lines.
To do a loopback test with a Western Electric Model 829:
1. Connect the AM-48 at the near end of the 4-wire line under test.