Characteristics of Physical Layer Optimization Problems
Exceeding cable design limits, poor quality cabling and connections

Attenuation
A common issue of exceeding the design limits of a media type is the attenuation of the bit-stream transmitted along the media. Attenuation may occur to such an extent that the receiving device cannot always successfully distinguish the component bits of the stream from each other. This ends in a garbled transmission and results in a request from the receiving device for retransmission of the missed traffic by the sender.

A far more common source of attenuation is a poor connection resulting from a loose cable or dirty or oxidized contacts.

Return Loss
Return loss is a measure of all reflections that are caused by the impedance mismatches at all locations along the link. It indicates how well the characteristic impedance of the cable matches its rated impedance over a range of frequencies. The characteristic impedance of links tends to vary from higher values at low frequencies to lower values at the higher frequencies. Return loss is expressed in decibels.

The termination resistance at both ends of the link must be equal to the characteristic impedance of the link to avoid reflections. A good match between characteristic impedance and termination resistance in the end equipment provides for a good transfer of power to and from the link and minimizes reflections.

Return loss is of particular concern in the implementation of Gigabit Ethernet.