Because of the way linear bus cabling is laid out, this  type  of  cabling  is  simple.  The  bus  topology  is very reliable, because if any node on the bus network fails,   the   bus   itself   is   NOT   affected,   and   the remaining  nodes  can  continue  to  operate  without interruption. Many of the low cost LANs use a bus topology and twisted-pair wire cabling. Figure  6-2.—A  bus  network  topology. A  disadvantage  of  the  bus  topology  is  that generally  there  must  be  a  minimum  distance  between workstations  to  avoid  signal  interference.  Another disadvantage  is  that  the  nodes  must  compete  with each  other  for  the  use  of  the  bus.  Simultaneous transmissions   by   more   than   one   node   are   NOT permitted. This problem, however, can be solved by using  one  of  several  types  of  systems  designed  to control  access  to  the  bus. They   are   collision detection,  collision  avoidance,  and  token  passing, which we will cover shortly. Also, there is no easy way for the network administrator to run diagnostics on the entire network. Finally, the bus network can be easily compromised by an unauthorized network user, since  all  messages  are  sent  along  a  common  data highway.  For  this  reason,  it  is  difficult  to  maintain network   security. STAR  NETWORK In a star network,  each component is connected directly to the central computer or network server, as shown in figure 6-3. Only one cable is required from the central computer to each PC’s network interface card to tie that workstation to the LAN. The star is one  of  the  earliest  types  of  network  topologies.  It 6-8 uses  the  same  approach  to  sending  and  receiving messages  as  our  phone  system.  Just  as  a  telephone call from one person to another is handled by a central switching station, all messages must go through the central computer or network server that controls the flow of data. New workstations can be easily added to  the  network  without  interrupting  other  nodes.  This is one of the advantages of the star topology. Another  advantage  of  star  topology  is  that  the network  administrator  can  give  selected  nodes  a higher   priority   status   than   others.   The   central computer looks for signals from these higher priority workstations  before  recognizing  other  nodes.  The  star topology   also   permits   centralized   diagnostics (troubleshooting)  of  all  functions.  It  can  do  this because all messages must first go through the central computer. This can prove invaluable in making sure that network security has not been breached. The main disadvantage of the star topology is its reliance   on   the   central   computer   for   performing almost  all  the  functions  of  the  network.  When  the central computer fails, all nodes also stop functioning, resulting in failure of the entire network. Figure 6-3.—A star network topology.


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