common representation for data which can be used between the application processes. The presentation layer relieves the applications from being concerned with   data   representation,   providing   syntax independence  as  follows: Encodes  data  in  a  standard  way  (integers, floating point, ASCII, and so on) Provides   data   compression   to   reduce   the number of bits that have to be transmitted Provides  data  encryption  for  privacy  and authentication APPLICATION  LEVEL/LAYER The  final  level  is  the  application  level,  which consists  of  the  application  layer.  The  application layer serves as the window for the application process to access the OSI environment. This layer represents the services that directly support users and application tasks.  It  contains  a  variety  of  commonly  needed protocols for the following items: Network  virtual  terminals File   transfers Remote  file  access Electronic   mail Network   management USING THE OSI MODEL A  communications  system  that  does  not  use  a layered architecture can be designed. A specifically designed   communications   system   is   faster,   more efficient, requires less software code, and eliminates redundant functions and activities. Why, then, is the OSI   reference   model   considered   the   standard   in designing  networks  and  writing  software?  It  is considered the standard primarily because the use of a  layered  architecture,  such  as  the  OSI  reference model,  provides  the  network  with  flexibility  and migration. The  greatest  advantage  of  your  using  layer architecture in a network is hardware independence. As   advances   in   technology   continue,   it   is   not necessary  to  scrap  a  network  completely  because  one component  has  been  superseded.  For  example,  if  you have a network and need to upgrade the cable to a type that can handle increased data at a faster rate, the layered architecture of the OSI model will allow you to make this replacement to the physical layer without changing  the  other  layers. LAN  TOPOLOGIES The  physical  arrangement  of  the  components  of  a LAN  is  called  its  configuration  or  topology.  The three major types of configurations, or topologies, of a LAN are the linear bus,  the star, and the  ring. You can   also   create   hybrid   topologies   by   combining features  of  these  configurations.  For  example,  several bus networks can be joined together to form a ring of buses. Each  topology  requires  the  components  of  a  LAN to  be  connected  in  a  different  arrangement.  These components are also referred to as nodes. A node is any  point  on  a  network  where  data  can  be  sent (transmitted)  or  received—a  workstation,  a  server, and so on. LINEAR BUS NETWORK The linear bus topology is like a data highway. That is, all components or nodes are connected to the same cable, and the far ends of this cable never meet, as  shown  in  figure  6-2.  Linear  bus  LANs  are  best suited to applications involving relatively low usage of the bus coupled with the need to pass relatively short messages from one node to another. In many such  networks,  the  workstations  check  whether  a message  is  coming  down  the  cable  before  sending their  messages. Since  all  nodes  share  the  bus,  all messages must pass through the other workstations on the way to their destinations. Each node checks the address attached to the message to see if it matches its own address. Bus topologies allow individual nodes to be out of service or to be moved to new locations without disrupting service to the remaining nodes. 6-7