Signal  quality  degrades  for  several  reasons, including attenuation, crosstalk, and impedance. Attenuation Attenuation is the decrease in signal strength, measured in decibels (dB) per 100 feet. Such loss happens  as  the  signal  travels  over  the  wire.  Attenuation occurs more quickly at higher frequencies and when the cable’s  resistance  is  higher. In  networking  environments,  repeaters  are responsible for cleaning and boosting a signal before passing it on. Many devices are repeaters without explicitly saying so. For example, each node in a token- ring network acts as a repeater. Since attenuation is sensitive to frequency, some situations require the use of equalizers to boost different-frequency signals the appropriate   amount. Crosstalk Crosstalk is interference in the form of a signal from a neighboring cable or circuit; for example, signals on different pairs of twisted wires in a twisted pair cable may interfere with each other. A commonly used measure  of  this  interference  in  twisted-pair  cable  is near-end  crosstalk  (NEXT),  which  is  represented  in  dB. The higher the dB value, the less crosstalk and the better is the cable. Additional shielding between the carrier wire and the outside world is the most common way to decrease the effects of crosstalk. Impedance Impedance,  which  is  a  measure  of  electrical resistance,  is  not  directly  a  factor  in  a  cable’s performance.  However,  impedance  can  become  a factor if it has different levels at different locations in a network. In order to minimize the disruptive effects of different  impedances  in  a  network,  special  devices, called baluns, are used to equalize impedance at the connection. Impedance  does  reflect  performance  indirectly. The higher the impedance, the higher is the resistance; the higher the resistance, the greater is the attenuation at higher frequencies. Line Conditioning Line conditioning tries to eliminate the effects of certain types of distortions on the signal. It becomes 3-6 more necessary as transmission speeds increase. Two types of line conditioning are available: C conditioning tries to minimize the effects of distortion  related  to  signal  amplitude  and distortion  due  to  envelope  delay. D conditioning tries to minimize the effects of harmonic distortion in addition to the amplitude and envelope delay distortions handled by type C conditioning. A  line  driver  is  a  component  that  includes  a transmitter and a receiver; it is used to extend the transmission range between devices that are connected directly to each other. In some cases a line driver can be used in place of a modem, for short distances of 10 miles or less. To test a particular section of cable, you can use a line-testing tool. A line monitor is a low-end line- testing tool that tells you if the line is intact. A high-end line-testing tool can do very precise measurements using time domain reflectometry (TDR). A TDR is a device used to test the integrity of a section of cable before the cable is even unwound. This diagnostic method  uses  a  signal  of  a  known  amplitude  and duration,  which  is  sent  along  a  stretch  of  cable. Depending on the amount of time the signal takes to return   and   on   the   cable’s   nominal   velocity   of propagation, the TDR can determine the distance the signal traveled and whether there are any shorts or opens  in  the  cable. BACKBONE  CONNECTIONS In  addition  to  the  inherent  problems  of  the  cabling, backbone  connections  add  the  problems  that  come  with the use of connectors. They have only a limited number of times that they can be connected before they have to be replaced. These connectors are used in several places along the backbone, each presenting one more place for trouble to start. Some of the places that connectors are used are: At the server At the repeater, concentrator, and the gateway Along the backbone for  each drop or tap At the splice and coupler (used with  fiberoptic) Each of the connections uses a different type of connector, each with its own limitations. For example: