# Ethernet

Author

Bryan Newbold <>

## 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
Type Length 16
Data 46 to 1500
Checksum 32

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.

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.

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.

## 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 $0\leq r \leq 2^k$, 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, by Charles Spurgeon. O'Reilly, 2000