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---
format: rst
categories: cs
toc: yes
...
========
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
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