6.5 Performance Monitoring | LANs to WANs: The Complete Management Guide

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The monitoring tools of bridges display a number of statistics about their performance, which network administrators can use to fine-tune the network or head off problems. Such tools can be used to gather baseline statistics when the network is considered to be operating at peak performance. These baseline statistics are saved for comparison against new measurements taken periodically. If there is significant negative deviation, steps can be taken to determine the cause and an appropriate remedy.

Throughput This display shows details about the data packets passing through a particular bridge. The statistics are categorized in terms of radio transmit and receive, and bridge transmit and receive, providing the following information:

Recent rate: Displays the event rates, per second, averaged over the last 10 seconds.
Total: Displays the number of events that have occurred since the statistics were last cleared.
Average rate: Displays the average event rates, per second, since the statistics were last cleared.
Highest rate: Displays the highest rate recorded since the statistics were last cleared.
Packets: Displays the number of packets transmitted or received.
Bytes: Displays the total number of data bytes in all the packets transmitted or received.
Filtered: Displays the number of packets that were discarded as a result of an address filter being set up.
Errors: Displays the number of errors that may have occurred.

Radio errors This display shows the extent of radio receiver and transmitter errors that have occurred at the bridge and includes the following information:

Buffer full frames lost: Shows the number of frames lost due to a lack of buffer space in the unit.
Duplicate frames: Shows the number of frames received more than once. This is usually due to a frame acknowledgment being lost.
CRC errors: Shows the number of frames received with an invalid CRC, which is usually caused by interference from nearby radio traffic. CRC errors can also occur from random noise when the receiver is idle.
Retries: Shows a cumulative count of the number of times a frame had to be retransmitted due to an acknowledgment of it not being received.
Maximum retries/frame: Shows the maximum number of times any one frame had to be retransmitted. An excessive number of retries may indicate a poor-quality radio link.
Queue full discards: Shows the number of times a packet was not transmitted due to too many retries occurring to the same destination. This occurs if packets destined to the same destination are taking up more than their share of transmit buffers.

Ethernet error statistics This display provides details on the receiver and transmitter errors that have occurred at the bridge and includes the following information:

Buffer full frames lost: Shows the number of frames lost due to a lack of receiver buffer space in the unit.
CRC errors: Shows the number of frames received with an invalid CRC.
Collisions: Shows the number of times a collision occurred while the frame was being received. This would indicate a hardware problem with an Ethernet node somewhere on the infrastructure.
Frame alignment errors: Shows the number of frames received whose size in bits was not a multiple of eight. Occasionally, extra bits of data are inadvertently attached to a transmitted packet, causing a frame alignment error.
Over-length frames: Shows the number of frames received that are longer than the configured maximum packet size.
Short frames: Shows the number of frames received that are shorter than the allowed minimum packet size of 64 bytes.
Overruns: Shows the number of times the hardware receives first in first out (FIFO) overflow. This should be a rare occurrence.
Misses: Shows the number of Ethernet packets that were lost due to lack of buffer space on the unit.
Excessive collisions: Shows the number of times transmissions failed due to excessive collisions. This usually indicates the frame had to be continuously retried due to heavy traffic on the Ethernet infrastructure.
Deferrals: Shows the number of times frames had to wait before transmitting due to heavy traffic on the cable.
Excessive deferrals: Shows the number of times the frame failed to transmit due to excessive deferrals. This usually indicates the frame had to be continuously retried due to heavy traffic on the Ethernet infrastructure.
No carrier sense present: Shows the number of times the carrier was not present when a transmission was started. This usually indicates a problem with a cable on the Ethernet infrastructure.
Carrier sense lost: Shows the number of times the carrier was lost during a transmission. This usually indicates a problem with a cable on the Ethernet infrastructure.
Out-of-window collisions: Shows the number of times a collision occurred after the 64th byte of a frame was transmitted. This usually indicates a problem with a cable on the Ethernet infrastructure.
Underruns: Shows the number of times the hardware transmit FIFO buffer became empty during transmission. This should be a rare occurrence.
Bad length: Shows the number of times an attempt was made to transmit a packet larger than the specified maximum allowed.

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