Short note: Ethernet

Ethernet is the most widely used LAN technology, standardised as IEEE 802.3. It was developed at Xerox PARC (Robert Metcalfe, 1973) and later specified by DEC, Intel and Xerox (DIX, 10 Mbps). All stations share a common channel and access it using CSMA/CD (Carrier Sense Multiple Access with Collision Detection). It uses Manchester coding at 10 Mbps.

Physical types

In 10BaseX, "10" = 10 Mbps, "Base" = baseband, the last part = cable type or length (×100 m).

Bus (10Base5/10Base2):
 T|==+=====+=====+=====+==|T   T = terminator
     H     H     H     H
Star (10Base-T):  H--[Hub/Switch]--H
                        |
                        H

Frame format

+--------+---+----+----+-----+---------+-----+
|Preamble|SFD| DA | SA | Len | Data+Pad| FCS |
|   7    | 1 | 6  | 6  |  2  | 46-1500 |  4  |
+--------+---+----+----+-----+---------+-----+

• Preamble (10101010…) for clock sync; SFD (10101011) marks start.
• 48-bit MAC destination and source addresses.
• Length/type field; data 46–1500 bytes (padded to 46).
• FCS: 32-bit CRC. Frame size 64–1518 bytes.

CSMA/CD operation

1. A station senses the channel; if busy it waits until idle (1-persistent).
2. It transmits and listens at the same time.
3. If a collision is detected, it stops, sends a 32-bit jam signal, and waits a random binary exponential backoff time: after the $n$-th collision it picks $K$ from $0$ to $2^{\min(n,10)} - 1$ and waits $K \times 51.2\ \mu\text{s}$ (slot time = 512 bit times at 10 Mbps).
4. After 16 failed attempts the frame is dropped.

The 64-byte minimum frame makes sure a sender is still sending when a collision from the far end (up to 2500 m with 4 repeaters) reaches it, i.e. frame time ≥ round-trip time.

Advantages and limitations

• Simple, cheap, easy to install and expand; no central control.
• Good performance at light to moderate load.
• At heavy load collisions waste capacity; access delay is not deterministic (unlike token ring); no priority.

Modern Ethernet uses switches and full-duplex links, so each link is collision-free and CSMA/CD is no longer needed; speeds have grown from 10 Mbps to 100 Gbps and beyond.