Specifications
Ethernet Loop Testing Page
E-9
assistant, the remote station is probably down. If no communication
with the multicast assistant group is possible, then the last resort
is a LoopDirect to the general broadcast address.
If
this fails then
either no one else is turned on or the local station
is
broken.
If
it
succeeds, it is again most likely that the remote station
is
down.
If some loopback assistant station can communicate with the remote
node but the local station cannot, the local station can then test for
the direction in which communication does not work. The LoopAssisted
function, using the assistant node that responded previously as the
assistant, can be used to detect either transmit or receive problems.
By repeating the above test with different remote stations, it can be
determined whether the local station or
the
remote station
is
at
fault, thus isolating the problem to
a
particular transmitter or
receiver.
When a station finds itself unable to communicate,
it
can report this
in whatever high level way is available. An operator or control
center can then respond by attempting to isolate and repair the
problem.
E.3.2 Remote Control Test Example
When a control center receives a report that a station cannot
communicate properly, it
cqn use the Loop Testing functions to
investigate the problem. It can first diagnose its own ability to
communicate with the station.
If
this communication appears to work,
the control center can similarly check its ability to communicate with
the remote station that the reporting station could not reach.
If
the control center can communicate with both stations, it can then
use LoopAssisted, full assistance, with one of the nodes as assistant
and the other as remote to see
if
they can communicate. Similarly it
can use transmit and receive assistance to determine which direction
is
a problem.
Similar checks using other stations can
be
used
to
isolate
the
problem
to a particular transmitter or receiver.
E.4
Operation
I
This section describes the operation of the Loop Test Server and
Requester.
Loop Test operation does not depend on particular stations being able
to receive multicast messages. Those stations willing to volunteer as
loop testing assistants respond to multicast address
CF-00-00-00-00-00, and are known as the loopback assistance multicast
group.