Figure 6-7.—A ring network using the token passing access  method. Two representative protocols, which control line discipline, are the Binary Synchronous Communications Protocol   (Bisync)   and   the Synchronous Data Link Control (SDLC). Bisync Bisync  is  a  half-duplex  protocol  that  transmits strings  of  characters  at  lower  speeds  over  dial-up circuits. The  information  movement  is  in  one direction  at  a  time,  with  each  data  transfer  being answered by an acknowledgement. SDLC SDLC is a control procedure that sends multiple blocks of data and returns a single acknowledgement for  many  blocks,  thereby  increasing  the  amount  of time  spent  transmitting  data.  The  bits  that  are  put before and after the message at the transmitting end are removed at the receiving end, so only the message is presented to the user. The hardware chosen for the network plays a part in  the  choice  of  network  protocol.  Most  users  and many  of  the  vendors  who  build  the  clone  type  of equipment  would  like  to  see  universal  interfaces, while  others  think  that  the  availability  of  different specifications   will   lead   to   a   proprietary   set   of equipment, even though they favor the overall OSI specifications. LAN  SYSTEMS When  you  decide  to  install  a  LAN  system,  the type  of  topology  used  in  the  initial  wiring  of  the system will have a major effect on the type of system that  can  be  used.  There  are  many  LAN  systems available,  each  with  advantages  and  disadvantages.  In the following paragraphs, we briefly examine some of the available LAN systems. The   Institute   of   Electrical   and   Electronics Engineers (IEEE) has developed a set of standards for local-area networks. These standards encourage the use  of  common  approaches  for  LAN  protocols  and interfaces. The IEEE LAN standards were developed by a committee of engineers and classified as the 802 standards. The 802 standards are broken down even further  to  define  the  protocols  and  topology  used  in  a LAN. Some of the standards we are concerned with are  the  following: IEEE 802.3—Carrier sense multiple access/collision detection (CSMA/CD) IEEE  802.4—Token  Bus IEEE  802.5—Token  ring ETHERNET The EtherNet local-area network was developed by  Xerox,  the  Intel  Corporation,  and  the  Digital Equipment Corporation. It became the model for the development   of   the   IEEE   802.3   standard.   The original standard defined a maximum throughput for EtherNet  of  10  Mbit/s,  but  it  has  been  revised  to support  throughput  of  much  higher  rates. When operating over coaxial cable, EtherNet has a 20-Mb per  second  throughput  speed.  For  high-demand environments,  such  as  engineering  or  graphics, EtherNet is often the choice. It is a bus topology and uses   CSMA/CD   protocol. It  is  available  in  the following  three  versions:  standard  EtherNet,  ThinNet, and twisted-pair EtherNet. Standard EtherNet and ThinNet both use coaxial cable. Standard   EtherNet   is   somewhat   more expensive and more difficult to install than ThinNet, 6-11