Limitations of the Local Loop

The challenge of making a physical layer specification for the subscriber s telephone line is in the transmission characteristics of the twisted pair. Subscriber lines often have a length of several kilometers and are composed of different pieces of wire with different gauges. Some lines have one or more bridged taps; these are pairs that are connected in parallel to the subscriber line (often put in place to allow the same line from the central office to be connected to any one out of a certain set of subscribers). Finally, subscriber lines run in bundles containing tens to hundreds of individual pairs, all of which will cause some electromagnetic signal leakage ( crosstalk) from one pair to another.

Transfer function. The transfer function H(f) of a channel describes the relation of the signal coming out of the channel to the signal going into the channel as a function of frequency. For a copper pair, the main trend is that attenuation increases with frequency and with distance. This means that the useful bandwidth of a copper pair (determined by the highest frequency at which a useful signal can be made out between noise and crosstalk for a given transmitted power spectral density) decreases with increasing loop length. Gauge changes and bridged taps cause reflections that show up as (series of) dips in the transfer function, further reducing the information capacity of the channel.

Crosstalk. ...