digital  data  pulses  into  a  form  acceptable  to  the various  types  of  communications  channels. The equipment  most  often  transmits  digital  data  over  a distance  by  varying  a  continuous  analog  signal  in amplitude, frequency, or phase. Communications channels that can pass data in two  directions  (transmit  and  receive)  are  known  as duplex   channels. Single-direction   channels   are simplex channels.  Duplex channels may operate in one  of  the  following  two  modes:  half-duplex  or fill-duplex.  Half-duplex  channels  transmit  data  in one direction, pause, and then receive data coming from  the  opposite  direction.  Full-duplex  channels,  on the   other   hand,   can   transmit   and   receive   data simultaneously. TYPES  OF  COMMUNICATIONS  CHANNELS (TRANSMISSION  MEDIA) In  the  fleet  and  at  shore  activities,  you  will encounter  several  forms  of  communications  channels. The most common channels are landlines and radio communications. Landlines Landlines are physical lines or cables that connect the digital equipment. Originally, landlines referred to telephone lines and were limited to carrying analog audio  frequencies  (voice  frequencies).  For  digital information  to  be  carried  over  these  lines,  the limits. The   signal   can   be   varied   in   amplitude (amplitude   modulation),   frequency   (frequency modulation), or phase (phase modulation) to convey the  data.  We  will  discuss  each  type  of  modulation later in this chapter. A digital signal has a limited set of values (1 or 0, true  or  false,  etc.). A  limited  number  of  discrete pulses  can  be  transmitted  in  a  fixed  period.  The unique sequence of the bits represents the data. Digital  equipments  (computers  and  peripherals) within  a  system  normally  communicate  with  each other  in  pure  digital  pulses  (serial  and  parallel). Transmitting   digital   information   over   a   distance requires  the  use  of  special  equipment  to  convert characteristics of one or more tones or carriers in the audio-frequency  range  had  to  be  modified  in amplitude, frequency, or phase. Today,  telephone  lines  are  commonly  used  in many network applications. Bulletin boards, such as BUPERS ACCESS, use existing telephone lines; but many  landline-based  systems  use  dedicated  lines. Dedicated lines are common in local area networks (LANs).  In  a  LAN  system,  several  computers  are joined  together  to  share  information  with  all  the  users on the system. System connections are made using coaxial,   dual-coaxial,   fiber-optic,   or   twisted-pair cable. The type of cable depends on several factors, such  as  the  number  of  users  on  the  LAN  and  the maximum distance between workstations. The device used to convert the digital data into a form  usable  by  the  communications  channel  and  back to digital data is known as a modem. Modem   is   an   acronym   for   MOdulator Demodulator.  The  modulator  function  converts  the data   of   the   transmitting   system   into   discrete modifications of the tone or carrier signals. The de- modulator converts the data-carrying tone or carrier signal into digital data for the receiving system. Radio Radio  waves  have  been  used  for  teletype  and voice   communications   for   many   decades. The advantages of radio-based systems are that they are more  mobile  and  can  communicate  over  barriers  such as large bodies of water. Tactical information links, like  those  we  will  cover  in  chapter  2,  are  almost exclusively   radio-based. Radio  communications  are  based  on  frequency ranges or  radio-frequency  bands.   The   frequency range  of  the  carrier   frequency   determines  the operational characteristics of the system. Table 1-1 illustrates  the  international  frequency  bands  and  their uses.  The  tactical  digital  information  systems  used  by the  Navy  generally  use  portions  of  the  hf and  uhf bands. 1-2