SUBSCRIBER LOOP DESIGN
the wire along a track or road. At intervals somebody is assigned to talk on the transmitter,
and we test the speech level, say, every 5 km. At about 30 km the level of the voice heard
on the test receiver is so low that intelligible conversation is impossible. What drops the
level of the voice as the wire is extended is the loss, which is a function of length.
Let us digress a moment and discuss American Wire Gauge or AWG. It is a standardized
method of measuring wire diameter. Just like the gauge on shotguns, as the AWG number
increases, the wire diameter decreases. The following equivalents will give us a basic idea
of AWG versus diameter.
Signaling limits of a subscriber loop are based on dc resistance. When we go "off-hook"
with a telephone, a certain minimum amount of current must flow in the loop to actuate
the local serving switch. The generally accepted minimum loop current value in North
America is 20 mA. If subscriber loop current is below this value, we have exceeded the
signaling limits. Applying Ohm's law, the loop resistance should not exceed 2400
for the battery feed bridge and we are left with 2000
for the loop itself.
We must account for the resistance of the subset wiring. Budget 300
for this. Thus the
resistance of the wire itself in the loop must not exceed 1700
Once we exceed the signaling limit (the loop resistance, wire only, exceeds 1700
when the telephone goes off-hook, no dial tone is returned. This just means that there is
insufficient loop current to actuate the switch, telling the switch we wish to make a call.
When there is sufficient current, the switch, in turn, returns the dial tone. When there is
insufficient loop current, we hear nothing. If we cannot effect signaling, the telephone
just will not operate. So between the two limiting factors, loss and resistance, resistance
is certainly the most important of the criteria.
Designing a Subscriber Loop
Figure 5.4 is a simplified model of a subscriber loop. Distance
D in the figure is the length
of the loop. As we mentioned above,
D must be limited in length owing to (1) attenuation
of the voice signal on the loop and (2) dc resistance of the loop for signaling.
The maximum loop loss is taken from the national transmission plan.
America, it is 8 dB measured at 1000 Hz. We will use the maximum resistance value
calculated above, namely 1700
Subscriber loop model.
National transmission plan for North America, see Bellcore, BOC Notes on the LEC Networks, latest edition