specifically  to  act  as  a  server.  In  either  case,  the computer normally has a hard disk and the software needed to run the network system. A network  server is able to control network traffic as well as the sharing of  other  resources,  such  as  application  programs,  disk space,  data  files,  and  printers.  There  are  several different types of servers, and each has a particular function.  In  newer  systems,  some  separate  server functions are combined into a central file server. The servers we will look at are the disk server, the file server, and the print server. Disk Server The  disk  server  was  the  first  of  the  network operating systems. In the early days of PC networks, very few computers were equipped with a hard disk. When the hard disk became affordable, manufacturers were asked to develop a system to allow several users to share a single hard drive. The earliest disk servers were   multiplexer   that   polled   each   connected computer for requests to write a file on the hard drive or  to  retrieve  a  file  from  the  disk.  The  multiplexer then responded accordingly. A major problem with this process was that it did not allow for any type of security, data organization, or disk management. As LAN technology evolved, the development of the disk server software in the early 1980s addressed some of these issues. The disk server is a software routine  that  was  installed  on  each  computer  in  the network. The disk server software allowed each PC to access the shared hard drive as if it were a local drive.  In  other  words,  the  computer  thought  the  drive was installed in the computer, but in reality, the drive was remotely located on the network. The   disk   server   also   provided   for   some information sharing. One purpose of a network is to allow multiple users access to the same information. One  problem  encountered  with  early  disk  servers occurred when two or more users updated the same file  at  the  same  time.  When  the  file  was  saved  by both users, the updates of one of the users was lost. A method of preventing this information loss is file locking. File locking means that when one user accesses a file, all the other users are prevented from accessing that file until the first user is finished with it.  As  you  can  see,  this  method  severely  limits  the number of users able to access the information. Another method used to prevent data loss is record locking.  In  a  data-base  environment,  many  users could access the same data file, but when a record was being  modified  by  one  user,  the  other  users  were locked out of the record being modified. A data file can be updated by several users without threatening the integrity of the data by using this technique. Although the disk server was used in most LANs developed before 1985, a major problem still existed in   maintaining   data   integrity.   The   two   methods covered in the previous paragraphs provided for data management, but not for reliable disk management. A disk drive stores information on the next available block on the disk. When the disk server was used, it was not uncommon for two users to try to write data to  the  same  block  at  the  same  time.  When  this happened, the second user would overwrite the data just written by the first user, causing a loss of data. The  development  of  the  file  server  in  1983  solved  all of the problems encountered with the disk server. File Server Currently,  all  local-area  networks  require  some type of file server. In most cases, the file server is a dedicated  PC  or  minicomputer.  The  file  server performs   the   processing   of   the   network   control software  and  the  central  processing  and  storage  point of  the  application  software  and  data  files  of  the network. The file server has a hard disk with a very large  storage  capacity. The file server manages the hard disk and ensures that multiple requests for the same file do not conflict with  each  other.  In  the  disk  server  environment,  each PC workstation manages its I/O with the disk through low-level sector calls. In the file server environment, each  workstation  communicates  with  the  central  disk through the use of high-level calls to the file server. A high-level call can be a request to open a particular file or to store a file, while a low-level call maybe to write this file to sector  xyz on the disk. The file server converts the high-level calls from the users to low- 6-3