is  used  to  manage  the  shared  resources  of  the  network. The  hardware  may  be  a  PC  or  a  computer  designed specifically  to  act  as  a  server.  A   network  server   is able  to  control  network  traffic  as  well  as  the  sharing of  other  resources,  such  as  application  programs,  disk space,   data   files,   and   printers.   In   either   case,   the computer  normally  has  a  hard  disk  and  the  software needed  to  run  the  network  system.  There  are  several different   types   of   servers,   and   each   has   a   particular function.   In   newer   systems,   some   separate   server functions  are  combined  into  a  central  file  server.  The servers   we   will   look   at   are   the   disk   server,   the   file server,  and  the  print  server. Disk  Server The   disk   server   was   the   first   of   the   network operating  systems.  In  the  early  days  of  PC  networks, very  few  computers  were  equipped  with  a  hard  disk. When  the  hard  disk  became  affordable,  manufacturers were  asked  to  develop  a  system  to  allow  several  users to  share  a  single  hard  drive.  The  earliest  disk  servers were   multiplexers   that   polled   each   connected computer  for  requests  to  write  a  file  on  the  hard  drive or   to   retrieve   a   file   from   the   disk.   The   multiplexer then  responded  accordingly.  A  major  problem  with this  process  was  that  it  did  not  allow  for  any  type  of security,   data   organization,   or   disk   management. As  LAN  technology  evolved,  the  development  of the  disk  server  software  in  the  early  1980s  addressed some   of   these   issues.   The   disk   server   is   a   software routine  that  was  installed  on  each  computer  in  the network.   The   disk   server   software   allowed   each   PC to  access  the  shared  hard  drive  as  if  it  were  a  local drive.  In  other  words,  the  computer  thought  the  drive was  installed  in  the  computer,  but  in  reality,  the  drive was  remotely  located  on  the  network. The   disk   server   also   provided   for   some information  sharing.  One  purpose  of  a  network  is  to allow  multiple  users  access  to  the  same  information. One   problem   encountered   with   early   disk   servers occurred   when   two   or   more   users   updated   the   same file   at   the   same   time.   When   the   file   was   saved   by both  users,  the  updates  of  one  of  the  users  was  lost. A   method   of   preventing   this   information   loss   is file   locking.   File   locking   means   that   when   one   user accesses  a  file,  all  the  other  users  are  prevented  from accessing  that  file  until  the  first  user  is  finished  with it.   As   you   can   see,   this   method   severely   limits   the number  of  users  able  to  access  the  information. Another  method  used  to  prevent  data  loss  is  record locking.   In   a   data-base   environment,   many   users could  access  the  same  data  file,  but  when  a  record  was being   modified   by   one   user,   the   other   users   were locked  out  of  the  record  being  modified.  A  data  file can   be   updated   by   several   users   without   threatening the  integrity  of  the  data  by  using  this  technique. Although  the  disk  server  was  used  in  most  LANs developed  before  1985,  a  major  problem  still  existed in   maintaining   data   integrity.   The   two   methods covered  in  the  previous  paragraphs  provided  for  data management,   but   not   for   reliable   disk   management. A  disk  drive  stores  information  on  the  next  available block  on  the  disk.  When  the  disk  server  was  used,  it was  not  uncommon  for  two  users  to  try  to  write  data to   the   same   block   at   the   same   time.   When   this happened,   the   second   user   would   overwrite   the   data just  written  by  the  first  user,  causing  a  loss  of  data. The  development  of  the  file  server  in  1983  solved  all of  the  problems  encountered  with  the  disk  server. File   Server Currently,   all   local-area   networks   require   some type  of  file  server.  In  most  cases,  the  file  server  is  a dedicated   PC   or   minicomputer.   The   file   server performs   the   processing   of   the   network   control software  and  the  central  processing  and  storage  point of  the  application  software  and  data  files  of  the network.  The  file  server  has  a  hard  disk  with  a  very large   storage   capacity. The  file  server  manages  the  hard  disk  and  ensures that  multiple  requests  for  the  same  file  do  not  conflict with  each  other.  In  the  disk  server  environment,  each PC  workstation  manages  its  I/O  with  the  disk  through low-level  sector  calls.  In  the  file  server  environment, each  workstation  communicates  with  the  central  disk through  the  use  of  high-level  calls  to  the  file  server. A  high-level  call  can  be  a  request  to  open  a  particular file  or  to  store  a  file,  while  a  low-level  call  maybe  to 8-3