Q. Determine briefly subscriber loop system.  Give some technical specification for subscriber lines.

Ans:

Subscriber Loop System:Each subscriber in a telephone network is connected generally to the nearest switching office by a dedicated pair of wires. Subscriber loop refers to this pair of wires. It is un-widely to run physically independent pairs from each subscriber premises tothe exchange. It is far easier to lay cables comprising a number of pairs of wires for different geographical locations and run individual pairs as needed by subscriber premises.

Commonly four levels of cabling are used as displayed in figure. At the subscriber end, drop wires are taken to a distribution point. Drop wires are the individual pairs which run into the subscriber premises. At distribution point, drop wires are connected to wire pairs in distribution cables. Many distribution cables from nearby geographical locations are terminated on a feeder point where they are linked to branch feeder cables which, in turn, are linked to main feeder cable. Main feeder cables carry a larger number of wire pairs, usually 100-2000, than distribution cables which carry normally10-500 pairs. Feeder cables are terminated on a main distribution frame (MDF) at the exchange. Subscriber cable pairs emanating from exchange are also terminated on the MDF.

Exchange  pairs  and  subscriber pairs are interconnected at the MDF by jumpers. MDF thus provides a flexible interconnection mechanism which is very useful in reallocating cable pairs and subscriber numbers. Technical specifications for subscriber lines include:

1.  Diameter of wire

2.  D.C. resistance per Km

3.  Attenuation per K