be  the  following:  the  NCS  having  an  incorrect  PU address entered in the DTS, low transmitter power out from  the  NCS,  an  excessively  noisy  frequency,  or weak PU receivers. Figure  3-17.—Several  PUs  not  responding  to  NCS  call-ups. The LMS-11 also has several off-line modes that allow you to save data onto a disk and analyze the data in detail. The off-line modes include a frame-by- frame display to analyze each frame of a transmission. This allows you to analyze the data of a particular PU and shows the status of each bit position. Remember that when you are doing a frame-by-frame analysis, the data has not been decrypted. More information on all modes of the LMS-11 can be found the  System   Operation   and   Maintenance Instructions, Organization   Level,   Link   Monitor System AN/TSQ-162(V)1, EE-190-AB-OMI- 010/TSQ-162(V)1. SUMMARY—LINK-11  FAULT  ISOLATION This chapter introduced you to some of the tools available  to  ensure  the  Link-11  system  is  operating  at maximum  efficiency. The  following  information summarizes  some  of  the  important  points  you  should have  learned. LINK-11  MYTHS  AND  FACTS—  Through time,  several  myths  about  troubleshooting  Link-11 have evolved. We explored some of the myths and tried to determine the facts. Some of the myths are as follows: Changing  NCS  solves  net  problems.  This  is only  true  if  the  NCS  is  causing  the  net problems. Changing  frequency  solves  net  problems. Again,  this  is  true  only  if  the  frequency  is noisy   or   is   being   jammed   by   another frequency. More  power  improves  Link-11  performance. This is a myth. In fact, too much power can actually  degrade  the  Link-11  net. Dummy   PUs   improve   link   quality.   Again, this  is  a  myth.  This  myth  evolved  from  the time of the CP-642 family of computers and the  AN/USQ-36  DTS.  Back  then,  a  dummy PU would help improve link quality, perhaps by providing a time delay for the computer to process  all  received  data. Radio  Silence  reduces  net  cycle  time.  This could be true if the unit that goes radio silent is transmitting less than 38 frames of data. As a rule, if a unit goes radio silent, its PU should be deleted from the polling sequence until the unit is ready to rejoin the net. LINK-11 POFAS— The two POFAs used in the Link-11 system are the single-station POFA and the multi-station POFA. SINGLE-STATION  POFA—  The   single-station POFA is an end-around test that will test most of the DTS, the computer input and output circuits, and the audio path if the radio is not removed from the test path.  A  single-station  POFA  does  not  check  the receive timing circuits. When a single station POFA is  completed,  a  printout  is  produced  that  lists  the errors  detected  during  the  test.  To  be  considered successful, a single station POFA should always run with  zero  errors. MULTI-STATION   POFA—   A   multi-station POFA is a test between two or more units. The multi- station   POFA   closely   represents   normal   link operations. A   multi-station   POFA   requires coordination among all units participating. All units should   be   within   25   miles   of   each   other   when attempting  a  multi-station  POFA.  Since  the  multi- station POFA actually transmits data over the air, it is subject  to  many  types  of  interference,  and  several attempts may be required before acceptable results are obtained. 3-19