sung@mcnc.org (Wayne Sung) (12/01/88)
In looking for simple devices to keep track of a running network, I have found a couple. 1) an audio vu meter hung across an Ethernet provides a surprisingly good reading of utilization. On the meter I tried, using a calibrated packet generator, 97% (limit of the packet generator) is just about at the right edge of the meter. The red band begins at just about 50%. So the percent readings on the scale can be halved to get actual load. The way this works is that although the meter does not actually have 10 MHz frequency response, it acts on the envelop of the packet and integrates it. Thus both lots of small packets or large packets give roughly equal readings. I have not been pleased with the calculated usage figures found in our bridging devices as they do not take arrival rate into account, also the calculation time of one second is too long. The meter has a 'calculation' time due to mechanical characteristics which gives a better idea of how instantaneously loaded the net is. 2) I also took apart a cheap Ethernet board (find the cheapest one you can). This one had a thinwire transceiver built in and costs only slightly more than a transceiver alone. Every Ethernet adapter has a serializer (it may be only one chip or it may be several) that provides a signal called (you might have guessed) carrier detect. This corresponds to the envelop of the packet. With this signal pulled out, you can put a frequency counter on it and measure actual packets per second. On a working system this is not as useable as the meter above but for checking the performance of a given device it is good. We send a stream of packets into say a bridging device and see how many come back out. On the output side since there will only be the tested device sending the frequency counter gives a stable reading. There is often also a collision detect pin which can be used with a frequency counter in a running system but I do not use this arrangement.