Reliability—Designed  to  be  operational  more than  99%  of  the  time,  including  the  periods  of  poor propagation. Countermeasures—Because  of  antenna  directiv- ity, the system is difficult to jam. Additionally, the sys- tem  should  not  be  susceptible  to  nuclear  disturbances  of the ionosphere. Application—Because  of  the  bandwidth  capa- bility   and   minimum   site   requirements,   los   is   well adapted to moderate distance point-to-point multichan- nel  communications  (with  repeaters),  transmission  of closed  circuit  TV,  transmission  of  radar  information from   outlying   sites,   communications   relay   between locations in congested areas, and “antenna farms.” TROPOSPHERIC  SCATTER  SYSTEM At  microwave  frequencies,  the  atmosphere  has  a scattering  effect  on  electromagnetic  fields  that  allows for   over-the-horizon   communications.   This   type   of communications   is   called   tropospheric   scatter,   or troposcatter for short. Troposcatter takes place mostly at low altitudes, but some effect takes place at altitudes of  up  to  10  miles.  Under  the  right  conditions, troposcatter can take place over hundreds of miles. A tropospheric scatter microwave system consists of one or more point-to-point hops (or sections). Each hop  is  designed  so  it  can  be  integrated  into  the worldwide  communications  network  of  the  Defense Communications   System   (DCS).   Troposcatter   links have  the  following  characteristics: Propagation—Free space as affected by the tro- posphere. Communications   capacity/bandwidth—Up   to 600 4-kHz voice channels; wideband, can accept TV. Range—Up to 800 km (500 statute miles). RF Power—High; up to 75 kilowatts depending upon bandwidth, quality, and range. Coverage—Point-to-point   only. Antennas—Both   transmitting   and   receiving antennas  are  horn-driven  paraboloids  providing  high gain and narrow beam widths. Reliability—Designed  to  be  operational  more than 99% of the time, including periods of poor propa- gation. Countermeasures—Extremely   difficult   to   jam. Should  not  be  susceptible  to  nuclear  disturbances  of  the ionosphere. Application—Meets  the  communications  re- quirements  between  HF  sites  within  its  minimum skywave one-hop distance of about 400 miles and line- of-site of about 30 miles. It is especially useful where conditions prevent the use of line-of-sight communica- tions or if adverse propagation conditions interfere with other transmission methods. MULTIPLEXING As  we  mentioned  earlier,  the  rf  spectrum  has become   very   congested.   The   maximum   number   of transmissions  taking  place  in  the  rf  spectrum  is  being increased   through   the   use   of  m u l t i p l e x i n g . Multiplexing  refers  to  the  simultaneous  transmission of  two  or  more  messages  over  the  same  medium  or channel   at   the   same   time.   Multiplexing   may   be achieved in various ways, but the most common meth- ods   are   time-division   multiplexing   (tdm)   and   fre- quency-division  multiplexing  (fdm).  Although  several types  of  multiplexing  equipment  are  available  in  the fleet today, the AN/UCC-1D is the most common. TIME-DIVISION  MULTIPLEXING Time-Division Multiplexing (Tdm) is a method of combining  analog  signals  for  serial  transfer.  The  signals are  sampled  at  intervals  and  interwoven  for  transmis- sion. The speed of this multiplexed signal is faster than the  original  individual  channel  speed  by  a  multiple equal to the number of combined signals. For example, if 5 signals are multiplexed, the data speed of each sig- nal must be multiplied by 5 to keep the signals in syn- chronization.  Tdm  also  results  in  an  increase  in  the signal bandwidth because of the increased data speed. Time-division multiplexing also can be used with digital  signals,  but  this  method  is  usually  called synchronous multiplexing. FREQUENCY-DIVISION   MULTIPLEXING Unlike  tdm,  which  samples  a  portion  of  the  sine wave,    frequency-division    multiplexing    (fdm) transmits  and  receives  for  the  full  360  degrees  of  the sine   wave.   A   channel   is   subdivided   into   smaller segments   of   equal   size,   called   subchannels.   Each subchannel carries a separate signal. Fdm used by the Navy  can  generally  be  divided  into  two  categories, voice   and   tty   communications.   You   can   find   more information on multiplexing in NEETS, volume 17. 2-13