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ETHERNET TECHNOLOGY

Part 1: Frames, Collisions, and 10 Mbps LANs

by James Antonakos

Start ý Ethernet Frame Format ý The Interframe Gap ý Collision Or No Collision ý Detecting Errors ý Random Waiting Period ý 10-Mbps Ethernet ý 10BaseFý Errors In Ethernet LANS ý More To Come ý Sources and PDF

ERRORS IN ETHERNET LANS

When working with Ethernet systems, it is useful to be familiar with some common problems encountered in actual networks.

The maximum transmission unit (MTU) is the maximum frame size allowed on the network. For the Ethernet, the MTU may range from 68 to 1500, with many ISPs setting their lines to an MTU of 576. If the MTU is too small, large frames will be fragmented into two or more smaller frames, contributing to excess use of the bandwidth and increased collisions. Windows users may adjust their MTU by modifying the Registry.

An out-of-control Ethernet transmitter may generate a frame that is longer than 1526 bytes. Other conditions on the network, such as bad terminations or a failing power supply in a hub or switch, may cause distortions that resemble a jabbering device. Repeaters are designed to prohibit retransmission of any frames for a short period of time when jabbering is detected, thus preventing the network from being saturated with meaningless signals.

A runt is any transmitted frame whose length is less than the minimum frame size, even a short frame with a valid FCS.

When all the bits of a frame have been received, it is possible that the last bit received is not the last bit of the final byte in the frame. In other words, the number of bits in the frame is not a multiple of eight. This is called an alignment error. This will most likely cause the FCS to be invalid and the frame will be discarded. Intermittent connections will contribute to this type of error, as will collisions, which terminate with an unknown number of bits transmitted followed by the jam sequence.

Cabling errors typically occur in either the coax or UTP. Problems that can occur in either of these areas are shown in Table 5.

Removing the 50-ohm terminating resistor from the end of a 10Base2 or 10Base5 cable will cause distorted signals to reflect off the end instead of being absorbed by the terminating resistor (because of high-frequency properties of transmission lines). The signal reflections cause repeated collisions, effectively shutting down the entire cable segment.

And finally, the Ethernet exhibits poor performance when its utilization is 60% or more. Excess utilization (including plenty of lost bandwidth because of collisions) is typically the result of too many stations operating in the same collision domain. Replacing hubs with switches or routers will partition the network into multiple collision domains, each containing a smaller number of stations. The improvement in performance will be noticeable.

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