CSMA/CA  (carrier  sense  multiple  access  with  collision avoidance) system, the media-access method uses RTS (ready to send) and CTS (clear to send) signals before sending a frame onto the network. A node transmits only after the node has requested access to the line and access has been granted. Other nodes will be aware of the RTS/CTS transmission and will not try to transmit at the same time. RTS.— A hardware signal sent from a potential transmitter  to  a  destination  to  indicate  that  the transmitter  wishes  to  begin  a  transmission.  If  the receiver is ready, it sends a CTS signal in return. CTS.— A hardware signal sent from a receiver to a transmitter to indicate that the transmitter can begin sending. ACTS signal is generally sent in response to an RTS signal from the transmitter. NETWORK  SYSTEM  CONNECTIONS The testing and evaluation of network connections is accomplished with the same test equipment that is used to test network components. This equipment includes   voltmeters,   ammeters,   volt-ohm- milliammeters,  and  line  scanners.  All  of  this  test equipment checks the voltage, resistance, and current that  passes  through  the  cable  and  the  connectors between the network nodes. Any increase or decrease in voltage or current or an increase in the resistance will cause communications problems for the users. Whether the cable is pre-made or you make it, you should always test the cable before it installed into the network. This will alleviate the possibility of installing a bad cable or connector to the system. Any time that you  can  detect  a  bad  connector  will  be  to  your advantage, since each connector has a limited number of connections before it has to be replaced. COMMUNICATION LINE PROBLEMS Communication  line  problems  fall  into  three general  categories:  excessive  noise,  cabling,  and backbone  connections.  With  proper  testing  and precautions,  these  problems  can  be  taken  care  of  before they  happen. EXCESSIVE NOISE Noise is the term for random electrical signals that become part of a transmission, and that serve to make the signal (information) component of the transmission more difficult to identify. Noise can take various forms, including  the  following: l l l Impulse noise: voltage increases that last for just a   short   period,   usually   for   only   a   few milliseconds. White noise: random background noise. Crosstalk:   interference   on   one   wire   from another. There are limits set on the allowable levels for each of these types of noise. A noise filter can be used to remove random noise from a signal. In a transmission, signal-to-noise ratio (SNR) is the ratio  between  the  signal  and  noise  levels  at  a  given point, usually at the receiving end of the transmission. The SNR value is generally expressed in dB. The SNR can be used to determine how long a cable segment can be before the signal loss is unacceptably high. The SNR also helps to determine whether a particular type of cable will work for the intended use. Cable testers can help determine whether a particular type of cable is appropriate in a specific environment. In  general,  digital  signals  have  a  much  higher  SNR than  analog  signals.  Because  analog  signals  in  a broadband network must be confined to a portion of the total  bandwidth,  filtering  and  other  signal-cleaning measures are necessary This confinement makes the signal more delicate and subject to distortion. Several types of filtering maybe used to help clean a broadband transmission. The filters are distinguished by the filtering technique they use as well as by where in the transmission process they are applied. For   example,   filters   applied   early   in   the transmission,  prior  to  modulation,  are  known  as baseband or premodulation filters. Those applied after the   modulation   are   known   as   passband   or postmodulation filters. CABLING Cables are good media for signals, but they are not perfect. The signal at the end of the cable should be as loud and clear as at the beginning, but this will not be true. Any transmission consists of signal and noise conponents.  Even  a  digital  signal  degrades  when transmitted over a wire. This is because the binary information must be converted to electrical form for transmission, and because the shape of the electrical signal changes over distance. 3-5