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----
-format: rst
-categories: cs
-toc: no
-...
-
-========
-Ethernet
-========
-:Author: Bryan Newbold <bnewbold@mit.edu>
-
-.. 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