write  this  file  to  sector  xyz  on  the  disk.  The  file  server converts   the   high-level   calls   from   the   users   to   low- level   disk   commands,   thus   providing   effective   disk management.   The   file   server   maintains   the   list   of privileges   and   authorizations   for   each   user.   This protects   the   data   files   from   unauthorized   access   and protects  the  data.  An  example  of  this  is  that  one  user may  be  authorized  total  access  to  a  data-base  file  to update  the  file,  while  another  user  may  be  authorized read-only  access  to  the  information.  Still  a  third  user may  be  denied  access  to  the  file  altogether. A  network  file  server  is  a  special-purpose  unit  that can  reside  in  either  a  dedicated  computer,  or  one  of the   workstations   (a   PC)   that   has   a   hard   disk containing   the   software   of   the   network.   When   the network  server  is  used  solely  for  serving  the  network and is  NOT  used  as  a  workstation,  it  is  referred  to  as a dedicated   server.   If  the  server  can  also  be  used  as a   workstation,   it   is   referred   to   as   a   nondedicated server. Some  networks  do   NOT  have   a   single   dedicated file  server.  Instead,  they  use  a   distributed  approach in  which  any  of   the   nondedicated   servers   may   make available  files  that  reside  on  their  hard  disks.  Under these   circumstances,   any   workstation   on   the   network can   use   or   copy   these   files.   Moving   files   back   and forth   on   such   a   network   establishes   a   temporary relationship,   you   might   say,   between   the   two   PCs involved.  One  PC  acts  as  the  server,  and  the  other  PC acts  as  the  receiving  workstation. Print  Server The  print  server  is  a  software  routine  that  allows all   the   workstations   on   the   LAN   to   use   a   single printer.  When  the  laser  printer  was  introduced  to  the market,  the  extremely  high-quality  print  and  multiple fonts   made   it   desirous   for   all   correspondence. Unfortunately,   the   cost   of   a   laser   printer   often exceeded  that  of  an  individual  workstation  and  made it  impractical  for  each  workstation  to  have  a  dedicated printer.   The   print   server   solved   that   problem   by accepting  requests  for  print  jobs  from  the  network users   and   directing   them   to   the   printer.   The   print server  makes  sure  one  job  is  completed  before  a  new job  is  started.  Print  server  routines  are  included  in almost   all   network   operating   systems   on   the   market today. WORKSTATIONS Workstations  is  another  name  for  the  PCs  used  on a  network.  The  PCs  can  be  of  the  same  brand,  such  as Zenith,   or   they   can   be   a   combination   of   different brands,   such   as   IBM,   Zenith,   Compaq,   along   with other   PC   compatible   computers   (clones).   Each   PC can  be  configured  differently.  Some  might  have  their own  hard  disk  drives;  others  might  have  expanded memory.  Still  others  might   NOT  even   have   diskette drives  or  printer  ports  of  their  own.  Instead,  these  less expensive   workstations   use   the   storage   and   printing resources  available  through  the  network.  Even  though a  PC  may  be  part  of  a  LAN  system,  you  can  use  it independently  as  a  stand-alone  PC  at  any  time  or  you can  use  it  as  part  of  the  LAN. THE   OPEN   SYSTEM   INTERCONNECTION (OSI)   REFERENCE   MODEL Over   the   past   few   years,   a   number   of   network standards   or   protocols   (rules   to   live   by)   have   been developed  by  the  International  Standards  Organization (ISO)   to   provide   some   level   of   uniformity   among computer  manufacturers  and  network  vendors.  ISO  is one   of   several   governing   organizations   in   this   field that  has  developed  such  protocols.  ISO  has  defined  a five  level,  seven-layer  architecture.  These  seven  layers of  standards,  shown  in  figure  8-1  define  a  generalized architecture   called   the   Reference  Model  of  Open Systems   Interconnection.   It  is  also  known  as  the OSI   reference   model   or  OSI  model.   The   primary purpose   of   the   OSI   model   is   to   provide   a   basis   for coordinating   the   development   of   standards   that   relate to   the   flexible   interconnection   of   incompatible systems   using   data   communications   facilities. The  OSI  model  does  NOT  define   anyone   vendor’s particular  network  software  as  such,  nor  does  it  define detailed   standards   for   any   given   software.   It   simply defines  the  broad  categories  of  functions  that  each  of the  seven  layers  should  perform.  The  OSI  model  can include   different   sets   of   standards   at   each   layer   that are   appropriate   for   given   situations.   For   example,   in a   very   simple   data   communications   system,   one   that 8-4