low-cost  systems  are  STARLAN,  ARCnet,  10Net, and  LANtastic. Zero-Slot Network Operating Systems Zero-slot   network   operating   systems   are appropriate only for networks with very few users and light  usage. They  are  an  inexpensive  and  simple alternative  to  the  NOSs  that  require  expensive network interface cards. Rather than requiring each workstation computer to have a NIC, the computer’s RS-232 serial communications port and twisted-pair cables are used. Because of this, they are very slow and offer limited file transfer abilities. They may not provide  disk  sharing. An  example  of  a  zero-slot system is LANLink. SUMMARY—LOCAL-AREA   NETWORKS This   chapter   introduced   you   to   local-area networks.  The  following  information  summarizes  the important points you should have learned. LOCAL-AREA  NETWORKS—  LANs  are  a combination of hardware and software which allows personal  computers  to  share  information.  The  total number  of  computers  and  the  total  distance  the network can cover are determined by several factors, including  the  type  of  cable  used  and  the  network operating system software. CABLES— Several types of cables can be used to create  a  local-area  network.  They  are  twisted-pair, shielded  twisted-pair,  coaxial,  and  fiber-optic.  The type  of  cable  used  determines  maximum  data  transfer rates and can be a factor when the number of nodes in the network is determined. NETWORK  INTERFACE  CARD—  The network interface card attaches the PC to the network. Most   network   interface   cards   have   built-in microprocessors that control network communications. This frees the PC’s main processor of time-consuming I/O operations. NETWORK SERVERS— The modern network server controls all operations of the network. These operations include controlling network communications,  storing  and  retrieving  files  from shared memory resources, and controlling common printers.  In  older  systems,  each  of  these  functions required a separate server. WORKSTATIONS—   Workstations  are  the personal computers connected to the network. Even if a PC is part of a network, it can still be used in a stand-alone   configuration. OPEN   SYSTEM   INTERCONNECTION OSI  REFERENCE  MODEL—  The open systems interconnection  reference  model  defines  the  protocols network hardware and software manufacturers use to create a network operating system. There are seven layers in the OSI model. These layers are contained in the five levels. HARDWARE  LEVEL—  The  hardware  level contains  the  first  two  layers  of  the  OSI  reference model. These are the physical layer and the data link layer.   The   physical   layer   defines   the   electrical, mechanical,   and   functional   interfaces   for   the transmission of data through the cable. The data link layer  is  responsible  for  error  detection  and  correction of  the  transmitted  data. TRANSPORT LEVEL— The next three layers of the OSI reference model are contained in the transport level, also referred to as the subnet. The three layers of  the  transport  level  are  the  network  layer,  the transport layer, and the session layer. The network layer monitors network activity and controls which path the data is to be transmitted over. The software, controlling the network interface card, stores  the  data  to  be  transmitted,  builds  the  data packets, and routes the data to the correct destination. The  transport  layer  ensures  the  integrity  of  the data  packets. The  session  layer  provides  for  the interface  between  two  or  more  applications  to communicate across the network. PRESENTATION LEVEL/LAYER—  The presentation  level  contains  the  presentation  layer.  The presentation layer formats the data presented to the application level.    The  presentation  layer  provides 6-14