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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:
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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.
