# Ethernet
Ethernet is an inexpensive OSI Layer 2 technology for connecting hosts at fast speeds.
Defined by 802.3.

It is common in LANs where fast speeds and cost-effective cabling is desired.


# Ethernet frame
TODO eth frame ascii diagram

- Preamble is 7bytes and warns listening devices to get ready for an incoming frame.
- SFD (Start Frame Delimiter) marks the actual start of the frame and useful data.
- 10BASE-T (10Mbps) Ethernet uses Manchester encoding.
- Frames can vary between 64 - 1500 bytes.
        - Frames larger than 1500 bytes are called *jumbo frames* or *baby giant frames*.
        - Frames smaller than 64 bytes have added padding.

## Ethertype
* IPv4 = 0x0800
* ARP = 0x0806


# MAC address
Each NIC is identified (almost uniquely, in practice doesn't matter) by a 48-bit hex address called the MAC address (Media Access Control).

Switches use a CAM table to keep track of port/MAC pairings. This is how a switch switches.
Hubs were OSI layer 1 only devices that amplified signals from a port out all the other ports.


# History
RJ-45s are derived from RJ-11s which are derived from RJ-10s. This explains why the pinout for the wiring is the way it is on ethernet. The RJ-10 was the a 4 pin - 4 conductor connector for telephones. Pins 2 & 3 were Line 1 Tip and Ring, Pins 1 & 4 were Line 2 Tip and Ring. The RJ-11 superseded the RJ-10, but it kept the Line 1 and Line 2 wires in the same place in the centre of the connector with an extra pair on the 1 and 6 pins. RJ-45 now had 8 pins. But it kept telephone lines 1 & 2 in the same place in the centre of the connector but added new twisted pairs on pins 1 & 2 and 7 & 8. The original idea was for the telephone to use the telephone portion of the connector and the network to use the 1-2 & 7-8 pairs. However, by the 2000's, the dropping cost of ethernet cable and the prevalence of digital PBX and VOIP meant that networking ended up taking over the entire connector.