Figure 6-1.—The OSI model showing the seven layers. system, all   seven   software   layers may   be implemented. Although there is no requirement for any  hardware  or  software  vendor  to  adhere  to  the principles  set  forth  in  the  OSI  model,  there  is  a worldwide  trend  in  the  computer  industry  toward acceptance and conformance to these standards. Ideally,   if   the   hardware,   network   software, application software, and cabling were all supplied by the  same  manufacturer,  there  would  be  relatively  few problems for users to contend with when designing and  implementing  a  network.  Everything  would  work together  rather  smoothly.  However,  a  computer manufacturer’s  architecture  can  make  it  difficult  to interconnect  hardware  offered  by  other  competing manufacturers  or  vendors.  The  protocols  used  by communications  devices  are  also  highly  complex  and are  often  completely  different  from  one  manufacturer to  another. Then  there  is  the  network  software. Usually, the network software from one LAN vendor will not work with that of a competitor; neither will the application programs. Even the cabling must be selected for a specific local-area network. HARDWARE  LEVEL The hardware level contains the first two layers of the OSI reference model. They are the physical layer and  the  data-link  layer. These  are  concerned primarily with the actual hardware used in a network. Physical Layer The   physical   layer   is   concerned   with   the transmission of the unstructured raw bit stream over a   physical   medium. It   describes   the   electrical, mechanical, and functional interfaces to the carrier. The physical layer carries the signals for all the higher layers  as  follows: Voltages and pulse encoding of bits Media and media interface (cables, connectors, NIC, and so on) Line  discipline  (full-  or  half-duplex) Pin   assignments Data-Link Layer The   data-link   layer   provides   error-free transmission   of   information   over   the   physical medium. This allows the next higher layer to assume virtually  error-free  transmission  over  the  link.  The data-link   layer   is   responsible   for   getting   data packaged  and  onto  the  network  cable.  It  manages  the flow  of  the  data  bit  stream  into  and  out  of  each network node as follows: Creates  and  recognizes  frame  boundaries Checks  received  messages  for  integrity Manages channel access and flow control Ensures  correct  sequence  of  transmitted  data The  data-link  layer  detects  and,  when  possible, corrects errors that occur in the physical layer without using  the  functions  of  the  upper  layers. It  also provides  flow-control  techniques  to  ensure  link-buffer capacity  is  not  exceeded. TRANSPORT  LEVEL The next three layers of the OSI reference model make up the transport level, also known as the  subnet. The  transport  level  defines  the  software  protocols 6-5