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 (OSI)  to  provide  some  level  of  uniformity  among computer  manufacturers  and  network  vendors.  OSI  is one of several governing organizations in this field that  has  developed  such  protocols.  These  seven  layers of standards, shown in figure 6-1, 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   any   one 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 uses a simple point-to-point link, the software at the higher level layers (say 5, 6, and 7) might   be   very   simple   or   possibly   nonexistent. However,  in  a  very  complex  data  communications 6-4