standardized  data  encoding,  data  compression,  and data encryption and decryption as required. APPLICATION  LEVEL/LAYER—  The final level of the OSI reference model is the application level  and  it  consists  of  the  application  layer.  This layer directly supports the users and application tasks. USING THE OSI MODEL—  When you use a layered architecture (such as the OSI reference model) to  design  a  communications  network,  it  is  possible  to update specific items in the network without having to replace the entire system. LAN TOPOLOGIES— The physical arrangement of the components of a LAN is called its topology.  The  three  basic  topologies  used  in  building a  LAN  are  the  linear  bus,  the  star,  and  the  ring. Hybrid   topologies   can   be   created   by   combining different features of each. LINEAR  BUS—  The   linear   bus   topology connects all the nodes to a common straight cable. All the nodes on the network share the common bus. This topology is very reliable since a failure of one or more  nodes  does  not  affect  the  bus. The disadvantages  of  the  linear  bus  are  the  need  for minimum  distances  between  nodes  to  avoid  signal interference,   and   the   loss   of   data   caused   by the simultaneous transmission by two nodes. STAR  NETWORK—  In  a  star  network,  each node is connected directly to the central computer. All communications between the nodes have to pass through the central computer. Star networks allow the network administrator to give selected nodes higher priority   and   also   allow   centralized   running   of diagnostic programs. RING NETWORK— In a ring network, all nodes are connected to a common cable, and the cable starts and  ends  at  the  network  server. In  this  type  of network, communications are always in one direction, and  the  data  being  transmitted  is  passed  through  each node  in  the  ring. A  major  disadvantage  of  this network is that when a node fails, it can completely halt all communications on the network. 6-15 ACCESS METHODS— Once the topology of a network  is  determined,  the  method  by  which  the nodes  access  the  network  must  be  determined.  In some cases, the access method is determined by the topology  of  the  network.  Some  of  the  access  methods used   in   networks   are   as   follows: carrier   sense multiple access (CSMA), carrier sense multiple access with   collision   detection   (CSMA/CD),   and   token passing. In  the  CSMA  method,  each  node  monitors  the network line for activity. When the node detects that there  is  no  activity  on  the  network,  it  will  send  its data.  A  problem  occurs  when  two  or  more  nodes attempt  to  use  the  network  at  the  same  time.  This situation causes a collision of the data packets and a possible loss of data. In the CSMA/CD method, when a  collision  is  detected,  each  node  ceases  transmission and  retransmits  when  it  senses  that  activity  on  the network  is  completed. In  token  passing,  a  software  token  is  passed  to each  node  in  an  orderly  manner. The  method  is similar to the Roll Call mode of operation of the Link- 11  system  described  in  chapter  2.  When  the  node with the token has no data to transmit, it passes the token  to  the  next  unit.  When  the  node  has  data,  it transmits it when it receives the token, and when it completes  its  transmission,  passes  the  token  to  the next  node. LAN  SYSTEMS—  There  are  several  types  of systems that can be installed in a LAN. The topology used has a major effect on the system the LAN will be capable  of  using. A  few  of  the  LAN  systems available  are  as  follows: EtherNet,   STARLAN, ARCnet,  and  the  IBM  Token  Ring.  EtherNet  is generally a linear bus network using the CSMA/CD protocol  for  network  access.  STARLAN  is  a  star topology   that   also   uses   the   CSMA/CD   access protocol.  ARCnet  is  a  distributed  star  network  that uses a token passing access protocol. The IBM Token Ring network is a star ring network that uses the token passing access protocol. NETWORK OPERATING SYSTEM BASICS—  The  network  operating  system  has  five basic  subsystems  to  control  the  operation  of  the