station would have to be transmitting to have both pairs active, and that would constitute a local collision. On UTP networks this is the most common sort of collision observed. There is no possibility remaining for a normal or legal collision after the first 64 octets of data has been transmitted by the sending stations. Collisions occurring after the first 64 octets are called “late collisions". The most significant difference between late collisions and collisions occurring before the first 64 octets is that the Ethernet NIC will retransmit a normally collided frame automatically, but will not automatically retransmit a frame that was collided late. As far as the NIC is concerned everything went out fine, and the upper layers of the protocol stack must determine that the frame was lost. Other than retransmission, a station detecting a late collision handles it in exactly the same way as a normal collision. Interactive Media Activity Drag and Drop: Collision Types After completing this activity, the student will learn the three different types of collisions. Web Links Ethernet Error Descriptions http://www.networkcomputing.com/ netdesign/ t15errors.html
Content 6.2 Ethernet Operation 6.2.7 Ethernet errors Knowledge of typical errors is invaluable for understanding both the operation and troubleshooting of Ethernet networks. The following are the sources of Ethernet error: While local and remote collisions are considered to be a normal part of Ethernet operation, late collisions are considered to be an error. The presence of errors on a network always suggests that further investigation is warranted. The severity of the problem indicates the troubleshooting urgency related to the detected errors. A handful of errors detected over many minutes or over hours would be a low priority. Thousands detected over a few minutes suggest that urgent attention is warranted. Jabber is defined in several places in the 802.3 standard as being a transmission of at least 20,000 to 50,000 bit times in duration. However, most diagnostic tools report jabber whenever a detected transmission exceeds the maximum legal frame size, which is considerably smaller than 20,000 to 50,000 bit times. Most references to jabber are more properly called long frames. A long frame is one that is longer than the maximum legal size, and takes into consideration whether or not the frame was tagged. It does not consider whether or not the frame had a valid FCS checksum. This error usually means that jabber was detected on the network. A short frame is a frame smaller than the minimum legal size of 64 octets, with a good frame check sequence. Some protocol analyzers and network monitors call these frames “runts". In general the presence of short frames is not a guarantee that the network is failing. The term runt is generally an imprecise slang term that means something less than a legal frame size. It may refer to short frames with a valid FCS checksum although it usually refers to collision fragments. Interactive Media Activity Matching: Ethernet Errors Matching After completing this activity, the student will be able to understand the terms and definitions of Ethernet errors. Web Links Ethernet Error Descriptions http://www.networkcomputing.com/ netdesign/ t15errors.html
Content 6.2 Ethernet Operation 6.2.8 FCS and beyond A received frame that has a bad Frame Check Sequence, also referred to as a checksum or CRC error, differs from the original transmission by at least one bit. In an FCS error frame the header information is probably correct, but the checksum calculated by the receiving station does not match the checksum appended to the end of the frame by the sending station. The frame is then discarded. High numbers of FCS errors from a single station usually indicates a faulty NIC and/or faulty or corrupted software drivers, or a bad cable connecting that station to the network. If FCS errors are associated with many stations, they are generally traceable to bad cabling, a faulty version of the NIC driver, a faulty hub port, or induced noise in the cable system. A message that does not end on an octet boundary is known as an alignment error. Instead of the correct number of binary bits forming complete octet groupings, there are additional bits left over (less than eight). Such a frame is truncated to the nearest octet boundary, and if the FCS checksum fails, then an alignment error is reported. This is often caused by bad software drivers, or a collision, and is frequently accompanied by a failure of the FCS checksum. A frame with a valid value in the Length field but did not match the actual number of octets counted in the data field of the received frame is known as a range error. This error also appears when the length field value is less than the minimum legal unpadded size of the data field. A similar error, Out of Range, is reported when the value in the Length field indicates a data size that is too large to be legal. Fluke Networks has coined the term ghost to mean energy (noise) detected on the cable that appears to be a frame, but is lacking a valid SFD. To qualify as a ghost, the frame must be at least 72 octets long, including the preamble. Otherwise, it is classified as a remote collision. Because of the peculiar nature of ghosts, it is important to note that test results are largely dependent upon where on the segment the measurement is made. Ground loops and other wiring problems are usually the cause of ghosting. Most network monitoring tools do not recognize the existence of ghosts for the same reason that they do not recognize preamble collisions. The tools rely entirely on what the chipset tells them. Software-only protocol analyzers, many hardware-based protocol analyzers, hand held diagnostic tools, as well as most remote monitoring (RMON) probes do not report these events. Interactive Media Activity Checkbox: Name That Ethernet Error After completing this activity, the student will be able to understand the terms and definitions of Ethernet errors. Web Links A Brief Explanation of Ethernet Errors Tracked by Observer http://www.networkinstruments.com/ html/osup1021_-_ethernet_ errors_tra.html
Content 6.2 Ethernet Operation 6.2.9 Ethernet auto-negotiation As Ethernet grew from 10 to 100 and 1000 Mbps, one requirement was to make each technology interoperable, even to the point that 10, 100, and 1000 interfaces could be directly connected. A process called Auto-Negotiation of speeds at half or full duplex was developed. Specifically, at the time that Fast Ethernet was introduced, the standard included a method of automatically configuring a given interface to match the speed and capabilities of the link partner. This process defines how two link partners may automatically negotiate a configuration offering the best common performance level. It has the additional advantage of only involving the lowest part of the physical layer. 10BASE-T required each station to transmit a link pulse about every 16 milliseconds, whenever the station was not engaged in transmitting a message. Auto-Negotiation adopted this signal and renamed it a Normal Link Pulse (NLP). When a series of NLPs are sent in a group for the purpose of Auto-Negotiation, the group is called a Fast Link Pulse (FLP)