Cabletron Systems EMM-E6 manual Total Errors, Alignment Errors, CRC Errors, OOW Collisions, 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∝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 ∝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 ∝s window has not yet closed), and some will get through fine (if no one else happens to be transmitting).

Monitoring Hub Performance 2-23