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Diffstat (limited to 'Ethernet')
| -rw-r--r-- | Ethernet | 107 |
1 files changed, 103 insertions, 4 deletions
@@ -1,14 +1,15 @@ ======== Ethernet ======== +:Author: Bryan Newbold <bnewbold@mit.edu> -.. warning:: Under progress! +.. 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 +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 @@ -34,16 +35,114 @@ Checksum 32 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 |