etherfaq - Newsgroups comp.dcom.lans.ethernet From...

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Newsgroups: comp.dcom.lans.ethernet From: [email protected] (BARR DOUG) Subject: Ethernet FAQ Organization: University of Colorado, Boulder Date: Tue, 5 Jan 1993 20:51:40 GMT This has not been posted for a while, so I am taking the liberty of posting it: Q: What is a runt? A: A packet that is below the minimum size for a given protocol. With Ethernet, a runt is a frame shorter than the minimum legal length of 64 bytes (at Data Link). Q: What causes a runt? A: Runt packets can be caused accidentally or intentionally. If accidental, they are most likely the result of a faulty device on the network, or software gone awry. If intentional, they may be designed to be runts for a specific reason. SNMP (Simple Network Management Protocol) is often sent as runt packets so that many devices will simply ignore it. Q: What is a jabber? A: A blanket term for a device that is behaving improperly in terms of electrical signalling on a network. In Ethernet this is Very Bad, because Ethernet uses electrical signal levels to determine whether the network is available for transmission. A jabbering device can cause the entire network to halt because all other devices think it is busy. Q: What causes a jabber? A: Typically a bad network interface card in a machine on the network. In bizarre circumstances outside interference might cause it. These are very hard problems to trace with layman tools. Q: What is a collision? A: A condition where two devices detect that the network is idle and end up trying to send packets at exactly the same time. (within 1 round-trip delay) Since only one device can transmit at a time, both devices must back off and attempt to retransmit again. The retransmission algorithm requires each device to wait a random amount of time, so the two are very likely to retry at different times, and thus the second one will sense that the network is busy and wait until the packet is finished. If the two devices retry at the same time (or almost the same time) they will collide again, etc. Q: What causes a collision? A: See above. Ethernet is a CSMA/CD (Carrier Sense Multiple Access/ Collision Detect) system. It is possible to not sense carrier from a previous device and attempt to transmit anyway, or to have two devices attempt to transmit at the same time; in either case a collision results. Ethernet is particularly susceptible to performance loss from such problems when people ignore the "rules" for wiring Ethernet. Q: What is a jam? A: When a workstation receives a collision, and it is transmitting, it puts out a jam so all other stations will see the collision also. When a repeater detects a collision on one port, it puts out a jam on all other ports, causing a collision to occur on those lines that are transmitting, and causing any non-transmitting stations to wait to transmit.
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Q: What is a broadcast storm?
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