🚀 go-pugleaf

The network interface itself is a small (~1.5cm*~3cm) pc board with 3
components on it.  (I haven't seen one for about 6 months, so I hope I
remember everything correctly.)  The components are wired in series.
They consist of a capacitor (.1uF < value < 1uF), a resistor, and a
device packaged in a WE case of the sort used for the 100A varistor
found across a U1 receiver in a phone handset.  This device was black
and had a red band painted around the middle (sic), so it must be
symmetrical.  I would guess it is a varistor rated somewhere in the
tens of volts.  I would guess that an AC voltage higher than the
varistor voltage is applied to the subscriber line and the impedance
is measured.  This test signal sounds a lot like a ringing voltage,
but it might be of a higher frequency to prevent ringing the
subscriber's ringer.

The first network interface I saw was just the plain, unenclosed pc board
connected across the protector block (lightning arrester), but I believe it
is more popular to use the type where the pc board is installed inside a
WE type 625 modular jack with the "Network Interface"+legalese label on it.

Since the network interface is installed on the telco side of the "point
of demarcation," I would suspect that its purpose is to allow the telco to
test the continuity of the subscriber's cable pair upon receiving a trouble
report from the subscriber so that they would not have to send a repairperson
to the customer premises.  Whether they would actually do this and lose
the exorbitant fee of a premises visit is another question.  I don't think
that a network interface or a lack thereof is that critical at this time.
Anyone who has heard the words "network interface"  knows how to test
his line at the protector and if you report trouble, the telco will probably
send a repairperson anyhow.

-Ross Snyder