Perdition wrote:
Loop 1 = Local Digital
Loops the local DTE back to itself using as liitle of the local
modem interface as possible. Tests if the DTE can talk to itself
(Most sync DTEs could not, more commonly a BERT tester would be
used as DTE.)
Loop 2 = Remote Digital
Loops the digital interface of the modem (in which the loop is
set) back towards the far end. Tests to see if the far DTE can
talk to itself via both modems and the telco line.
Loop 3 = Local Analog
Loops the local modem analog signal back to itself. Tests if the
local modem can train up to itself. Also would cause local DTE to
see itself, like Loop 1, but using almost all of modem.
Loop 4 = Remote Analog
Loops the telco line back to itself towards the far end (from
where the loop is set). Tests to see if the one modem (far end)
can train up over the double-backed telco line, looking to see
which modem is operating marginally.
Some vendors used slightly different names for these loops, like
Modem Check, DC Busback, etc. Also some looped in both directions
where V54 only called for one direction. All could be set by local
operater pushing a button or switch. Some optionally had Remote
Control capabilities from front panel of local modem. Still others
could be commanded from a central Network Management system.
In older modems these loops were implemented via physical switch
contacts rerouting the signals as appropriate. Later modems used
electronics (solid-state IC chips) to do the rerouting.
(Wow, so my 35 years of working with leased line analog modems has
not gone to total waste after all ;-)
--reed
