time-division  multiplexer  system.  The  data  from  each user can be in the form of bits, bytes, or blocks. The data signals from all users are compiled into frames for   transmission   on   a single,  high-speed communications channel. Transmit   and   receive   frames   are   used   for half-duplex  communications.  Transmit  frames  are sent  and  a  receive  time  slot  is  enabled  for  return information. In   this   manner,   a   single   carrier frequency and modem may be used to transmit and receive information at a fairly high rate of speed. Since time slots are preset and assigned, if a user has  no  data  to  transmit,  the  time  slot  is  wasted. Advantages of a TDM system include the following: its ability to handle devices with varying speeds, its effectiveness  when  used  with  devices  that  transmit data   almost   continuously,   and   its   simple implementation. Frequency-Division  Multiplexing Frequency-division multiplexing (FDM) divides a band of frequencies into several distinct channels or tones. Each tone carries a portion of the data being transmitted. FDM devices can be complex because a separate  modulator/demodulator  circuit  is  required  for each  tone  used. The   composite   tones   are   then modulated  to  a  single  carrier  frequency  for  radio transmission. FDM allows for the parallel transmission of data over a single communications channel. For example, the  Link-11  communications  system  uses  15  audio tones to transmit 30 bits of parallel data. Each tone transmits   two   bits   of   differential   quadrature phase-shift keyed data. SUMMARY—FUNDAMENTALS  OF  DATA COMMUNICATIONS This chapter introduced you to the building blocks of  a  data  communications  system.  The  following information  summarizes  the  important  points  you should  have  learned. COMMUNICATIONS   SYSTEMS—   Digital data  devices  that  exchange  data  over  distances  are known   as   communications   systems. A  basic communications  system  consists  of  the  following three  components: a  transmitter,  a  receiver,  and  a communications  channel.  The  transmitter  converts digital data into a form (digital or analog) useable by the  communications  channel.  The  receiver  accepts data from the communications channel and converts the data back to its pure digital form. Communications  systems  that  can  transmit  and receive  data  are  known  as  duplex  systems,  while communications systems that are limited to transmit only  or  receive  only  are  simplex  systems.  Duplex systems  that  transmit  data,  pause,  and  then  receive data  are  half-duplex  systems.  Full-duplex  systems can transmit and receive data simultaneously. COMMUNICATION  CHANNELS—  Several types of communications channels are in use today. The   most   common   are   landlines   and   radio communications. Landlines are physical cables that connect  computers;  they  are  common  in  local  area networks. Radio   communications   use   the radio-frequency bands to exchange information. The most common bands used in the Navy are the HF and UHF  bands. DECIBEL  MEASUREMENT  SYSTEM—  The decibel measurement system is used to measure the gain or loss of amplifiers, antennas, communications lines,  and  other  types  of  communications  equipment. A gain of +3 decibels (dB) indicates that the output power of the circuit, compared to the input power, has doubled.  Each  +3  dB  gain  indicates  a  doubling  of power. For example, a signal that has a gain of 6 dB is twice as strong as a signal that has a gain of 3 dB. A S Y N C H R O N O U S T R A N S - MISSION—  Asynchronous  transmission  refers  to data  sent  without  the  use  of  timing  pulses.  Data signals are sent a byte at a time, with start, stop, and parity bits added to each byte. S Y N C H R O N O U S T R A N S - MISSION— Synchronous transmission refers to the sending of long, uninterrupted streams of data with a predefined start and stop sequence. 1-10