|author||bnewbold <firstname.lastname@example.org>||2012-02-27 23:33:45 -0500|
|committer||bnewbold <email@example.com>||2012-02-27 23:33:45 -0500|
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+:Author: Bryan Newbold <firstname.lastname@example.org>
+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.
+The frame format is the same for ethernet systems of any speed.
+Section Size (bits)
+Destination Address 48
+Source Address 48
+Type Length 16
+Data 46 to 1500
+.. 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.
+The Inter-Frame Gap (IFG) is a pause in between signal frames; it is 96
+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
+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.
+ Optional protocol for direct links (eg, from switch to switch) to
+ determine network speed. Can be reinitiated by dropping and restarting
+ the link.
+ IEEE working group for higher level networking.
+ IEEE working group for "logical link control" (LLC), which is related
+ to the type/length frame field.
+ For _`CSMA/CD` LANs
+ For Token Ring LANs
+ 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...)
+ 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.
+ 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.
+ When a broken station/device keeps a carrier open and prevents other
+ 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