l l l Verify  the  domain  name. Look for the maps subdirectory; it should be the same as the domain name. If it is not there, you will  need  to  run  the  system  initialization command. If the above fails, ensure that all the files to be mapped are present on the server. If any have been deleted, they will have to be restored from the latest system saves. One of the best ways to avoid server malfunctions is to conduct maintenance on the server. It is important to set up a schedule for your server and strictly adhere to it. To check the hardware, you should do at least the following  things: l l l l l Clean the server carefully but thoroughly. Check  cabling  and  connections  for  tightness  and signs  of  bending  or  stress.  Do  not  disconnect connectors   unless   necessary,   since   many connectors are rated for a limited number of matings. If  possible,  check  the  cabling  with  a  line analyzer. Run thorough diagnostics on the storage medium and on other system components to identify the components that are likely to fail and to deal with these before they actually do fail. Check the quality of your power line by using a line  tester. The hardest part of server maintenance is finding the  time  to  conduct  the  maintenance,  since  the  network will have to be offline to conduct. In many cases, server maintenance will need to be during off peak hours, late night or early morning, when there is little or no use. DATA  COLLISIONS A  data  collision  is  the  simultaneous  presence  of signals from two nodes on the network. A collision can occur when two nodes each think the network is idle and both start transmitting at the same time. Both packets involved in a collision are broken into fragments and must be retransmitted. Collision Detection To detect for a collision, nodes check the dc voltage on the line. A voltage level of two or more times higher 3-4 than expected indicates a collision, since this means there are multiple signals traveling along the backbone at  the  same  time. In a CSMA/CD (carrier sense multiple access with collision detection) systems, all workstations or nodes attached to the network monitor the transmission medium at all times. When a node needs to send data, it waits until the line is quiet and then transmits. If two or more nodes happen to transmit data at the same instant, a collision occurs. Each node detects the collision and then   waits   for   a   variable   amount   of   time   (as programmed in the NIC’s microprocessor) before testing  the  bus  again  and  retransmitting.  Since  each node waits for a different amount of time, say 10/1000 and 20/1000 of a second, it is very unlikely that the collision will occur a second time. The CSMA/CD detection method is further illustrated in figure 3-1. Collision   Avoidance To avoid collisions, nodes can send special signals that indicate a line is being used for a transmission. In a Figure  3-1.—A  bus  network  using  the  CSMA/CD  access method.