Quantcast Token Passing

Figure 6-6.—A bus network using the CSMA/CD access method. arrangements  used  to  make  sure  each  workstation  has fair  and  equal  access  to  the  network.  The  access method used is governed primarily by the topology and  the  protocol  of  the  network.  The  principal  access methods are contention and token passing. Contention The  contention  method  features  Carrier  Sense Multiple  Access  (CSMA)  and  Carrier  Sense  Multiple Access  with  Collision  Detection  (CSMA/CD).  The CSMA/CD method is shown in figure 6-6. Access for both is on a first-come, first-served basis. The CSMA access scheme is very similar to that of a citizens band (CB) radio. Stations with data to send listen to the channel  and  wait  until  it  is  clear  to  transmit.  With CSMA/CD, when two or more workstations transmit simultaneously, their messages will collide. As soon as   a   workstation   detects   a   collision,   it   ceases transmission, monitors the network until it hears no other  traffic,  and  then  retransmits.  Most  contention networks assign a unique retry algorithm to vary the wait-and-retry  period. This  algorithm  reduces  the likelihood  that  after  a  collision,  two  workstations  will transmit retries simultaneously. Token Passing Token  passing  is  an  orderly  access  method  and  is shown in figure 6-7. Each workstation passes on the opportunity to transmit to its closest neighbor until a station  is  found  with  a  message  to  send. This permission  to  transmit  is  called  a  token.  When  a workstation with data to send is handed a token, part of  the  token  is  changed,  indicating  it  is  carrying  a message, and then data is transmitted with the token. The  token  is  then  passed  around  the  network,  and every  station  checks  whether  the  message  is  intended for it. The receiving station copies the message from the  token,  but  then  passes  the  unchanged  token  along the network. When the transmitting station receives the same token, it knows the message has been passed around the network. The transmitting station erases the  message  and  puts  the  empty  token  back  into circulation   on   the   network. The  amount  of information  that  maybe  transmitted  during  possession of  the  token  is  limited  so  that  all  workstations  can share  the  cable  equally. PROTOCOLS Network  protocols  are  an  important  component because   they   define   how   networks   establish communications between   elements,   exchange information, and terminate communications. Protocols  have  two  major  operational  functions.  They establish  the  circuit  for  transmission  (handshaking) and   for   the   transmission   itself.   Transmission   is conducted  subject  to  the  line  discipline.  The  line discipline is the sequence of operations that actually transmits   and   receives   the   data,   handles   the error-control  procedures,  handles  the  sequencing  of message   blocks,   and   provides   for   validation   for information  received  correctly. 6-10


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