Cabletron Systems EMM-E6 - Manual (Page 39)

Monitoring Hub Performance 2-23
Using the EMM-E6 Hub View
“legal” collisions, as opposed to the OOW collisions described below) are a
natural by-product of a busy network; if you are experiencing high numbers of
collisions, it may be time to redirect network traffic by using bridges or routers.
Extremely high collision rates can also indicate a data loop (redundant
connections) or a hardware problem (some station transmitting without listening
first).
Total Errors
The total number of errors of all types recorded by this network (channel),
module, or port since the statistics window was opened or the Reset button was
pressed.
Alignment Errors
The total number of misaligned packets recorded since the statistics window was
opened or the Reset button was pressed. Misaligned packets are those which
contain any unit of bits which is less than a byte — in other words, any group of
bits fewer than 8. Misaligned packets can result from a packet formation problem,
or from some cabling problem that is corrupting or losing data; they can also
result from packets passing through more than two cascaded multi-port
transceivers (a network design which does not meet accepted Ethernet spec).
CRC Errors
CRC, or Cyclic Redundancy Check, errors occur when packets are somehow
damaged in transit. When each packet is transmitted, the transmitting device
computes a frame check sequence (FCS) value based on the contents of the packet,
and appends that value to the packet. The receiving station performs the same
computation; if the FCS values differ, the packet is assumed to have been
corrupted and is counted as a CRC error. CRC errors can result from a hardware
problem causing an inaccurate computation of the FCS value, or from some other
transmission problem that has garbled the original data. The CRC error counter
shows the total number of CRC errors that were recorded since the statistics
window was opened or the Reset button was pressed.
OOW Collisions
The total number of out-of-window collisions recorded since the statistics
window was opened or the Reset button was pressed. OOW collisions occur
when a station receives a collision signal while still transmitting, but more than
51.2
µ
sec (the maximum Ethernet propagation delay) after the transmission
began. There are two conditions which can cause this type of error: either the
network’s physical length exceeds IEEE 802.3 specifications, or a node on the net
is transmitting without first listening for carrier sense (and beginning its illegal
transmission more than 51.2
µ
s after the first station began transmitting). Note
that in both cases, the occurrence of the errors can be intermittent: in the case of
excessive network length, OOW collisions will only occur when the farthest
stations transmit at the same time; in the case of the node which is transmitting
without listening, the malfunctioning node may only fail to listen occasionally,
and not all of its failures to listen will result in OOW collisions — some may
simply result in collisions (if the 51.2
µ
s window has not yet closed), and some
will get through fine (if no one else happens to be transmitting).