From 2be1fedb1ece4ef5ebb4427b2b3dcba6eac94277 Mon Sep 17 00:00:00 2001 From: User Date: Mon, 25 Jan 2010 17:28:27 +0000 Subject: renames --- Ethernet.page | 154 ---------------------------------------------------------- 1 file changed, 154 deletions(-) delete mode 100644 Ethernet.page (limited to 'Ethernet.page') diff --git a/Ethernet.page b/Ethernet.page deleted file mode 100644 index d455f21..0000000 --- a/Ethernet.page +++ /dev/null @@ -1,154 +0,0 @@ ---- -format: rst -categories: cs -toc: no -... - -======== -Ethernet -======== -:Author: Bryan Newbold - -.. contents:: - -History -======= -Bob Metcalfe first described the ethernet system in a memo while working at the -Xerox Palo Alto Research Center in 1973 ([oreilly]_). It was based on the -earlier *Aloha* system developed by Norman Abramson at the University of -Hawaii. *Aloha* was a radio network for inter-island communications. - -Structure -========= - -Data Frames -~~~~~~~~~~~ -The frame format is the same for ethernet systems of any speed. - -==================== =========== -Section Size (bits) -==================== =========== -Preamble 64 -Destination Address 48 -Source Address 48 -Type Length 16 -Data 46 to 1500 -Checksum 32 -==================== =========== - -.. topic:: Preamble - - The preamble originally served as a "warm up" period for hardware to - stabilize and prepare for the rest of the frame. - - The last 8 bits are the preamble are a particular "start frame delimiter" - pattern to help hardware notice the beginning of signal transmission. - -.. topic:: Source and Destination addresses - - Manufacturers are given a 24 bit id (first half), and are expected to - ensure the uniqueness of the second half of each device's address. - - If the first bit of the destination is high (1), the destination is - multicast; if low (0), then unicast. Under IEEE, if the second bit is high, - global administration rules should be followed, and if low, local - administration rules should be followed. - - The IEEE-SA (Standards Association) delegates addresses. - -.. topic:: Checksum - - The checksum (or Frame Check Sequence) is a Cyclic Redundancy Checksum. - -Inter-Frame Gap -~~~~~~~~~~~~~~~~~ -The Inter-Frame Gap (IFG) is a pause in between signal frames; it is 96 -bit-times long. - -.. _collision: - -Collisions -============= -Collision management is the heart of Ethernet; a collision occurs when two -devices try to transmit at the same time and their signals overlap and the -transmission is unreadable. This problem is made worse the further apart -stations are because it takes longer for signal fronts to propagate, so there -is a longer overlap period when the first device has started transmitting but -the second hasn't noticed and may start transmitting on its own. The network -size (in distance, not nodes) is determined by the maximum collision period -(also called slot time). This period is 512 bit-times, so usually the faster a -network is, the smaller it should be (in distance); at gigabit speeds the -slot is expanded, see Speeds_. - -Under Ethernet, if any device detects a collision, it sends out a 32 bit -"collision enforcement jam signal" to ensure that all devices notice the -collision. Then each device that wishes to transmit waits a random amount of -bit-times r before transmitting again. If there is another collision, a new -r is randomly chosen with a larger maximum; up to 15 retries are allowed before -the devices will give up and drop the frame. The constraints for the random -backoff time r are `$0\leq r \leq 2^k$`:latex:, where r is in bit times and -k is the number of retry attempts up to 10 (for 11th to 15th retries, use -k=10). - -Speeds -========= -At 10mb, bit time is 100ns (nano-seconds); at 100mb, 10ns; at 1gb, 1ns. - -At gigabit speeds, the slot time would limit network size to 20 meters, so -it gets padded with a null "carrier extension" signal to 4096 bits (512 bytes). -On high speed half-duplex gigabit links (which are rare!), frame bursting is -a technique to allow several small data frames to be transmitted at the same -time: one frame is sent regularly to capture the channel, then up to 65536 -bit times of short non-carrier extended (only IFG) frames are sent, then -a single last regular frame is sent. - -Phrases -========= - -Auto-negotiation - Optional protocol for direct links (eg, from switch to switch) to - determine network speed. Can be reinitiated by dropping and restarting - the link. - -802.1 - IEEE working group for higher level networking. - -802.2 - IEEE working group for "logical link control" (LLC), which is related - to the type/length frame field. - -802.3 - For _`CSMA/CD` LANs - -802.5 - For Token Ring LANs - -CSMA/CD Protocol - Stands for Carrier Sense Multiple Access/Collision Detect. The two main - implementations are DIX (DEC-Intel-Xerox) and IEEE. (Can't remember how - this works...) - -VLAN - Optional 4-byte field in between "source" and "type" fields. The first - two bytes are a valid type field; this increases the max length to - 1522 bits. Only used by switches. - -Channel Capture - This phenomena is a result of _collision management. After a few - sequential collisions, the device that ultimately transmits first gets - its backoff reset to zero, while all other devices retain a high backoff; - this means the first device is much much more likely to retransmit again - faster than any of the others, and if it has a lot of frames to send it - will dominate the carrier until it is done transmitting. - -Jabber - When a broken station/device keeps a carrier open and prevents other - transmissions. - -PAUSE - A PAUSE frame is a MAC control (higher level, type 0x8808) frame which - causes switches to stop for up to 65536 slot times to allow a switch or - station to clear its buffers. - -.. [oreilly] `Ethernet: The Definitive Guide`:title:, by Charles Spurgeon. - O'Reilly, 2000 -- cgit v1.2.3