Manual of VDSL2 Mini MEG101AE-R4
Page #11
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Some words about xDSL cabling
There are no two cables which are exactly identical. Physics matters for every cable. Of
course everybody has concerns about the length of the cable, but that is just an indication.
Once you talk to your customer about the cables, you will get an estimation of the cable
length. Watch out for interconnects and ask your customer about them. Every
interconnect box influences the main physical factors of the cable:
Impedance and Capacitance
This one is not science, but it gives you some hints and indications. With a good
multimeter you should be able to measure the impedance of your cable. The impedance
of a telephone wire should be between 50 and 150 Ohm per kilometer (thin cables have
high impedance-thick cable have lower impedance)
Disconnect all equipment on Side A and Side B. Then do a short on Side A of the two
wires you want to use. Next measure the impedance with a digital multimeter on Side B
from wire 1 to wire 2. That value divide by two to get a rough idea of the impedance of your
cable. You should also measure the isolation impedance of the cable. Therefore
disconnect the short of Side A again and measure from wire 1 to wire 2. The value you get
should be several MOhms.
This means, once your customer tells you the route would be 500m long, but your result
measuring the impedance is 200 Ohms or more, something is not as it should be. 200
Ohms (this is not science!) equal to a thick cable of 4km or a thin cable of 1km.
If the physics of your cable are within the functional limit of your xDSL device, you get a
link, if not, then not. Remember, there is no “small guy” inside any xDSL modem that will
run along your cable with a folder rule and say “go” if the wire is x meters long. All is about
physics.
