Network - Ethernet

Map Of Internet 1973


Ethernet is a LAN technology part of the Data link layer (OSI level 2) that creates ethernet packet known as frame that are carried out physically by ethernet cables.

Ethernet Cables


TCP/IP sends a TCP packet known as segment:


Introduced in 1973, Ethernet is the best way to wire your computer to a network. But it was very expensive, costing thousands of dollars for an interface board. Intel's 82586 chip (1982) simplified the interface, dropping network prices to “just” 1000.

At the time, Ethernet chips usually just translated data packets into a stream of network bits, computing checksums. Intel's chip went much further: it included a coprocessor to move data between the network and memory independent of the main processor (i.e. DMA).

The secret to Ethernet is simplicity. Instead of controlling when a computer can talk on the network (like Token Ring), a computer can send data when the network appears idle. If two computers try at the same time, a “collision” occurs and they retry.

To recover from a collision, each computer waits a random time and tries again. If there's another collision, they double the wait time and keep trying. (Kind of like two people talking at the same time, pausing, and trying again.) This is “ exponential backoff”.

On the 82586 chip, exponential backoff is implemented with three circuits: a 10-bit counter to generate a pseudorandom number, a 10-bit mask register to select an (exponentially scaled) part of the counter, and a delay counter to loop that many times before trying again.

Each bit is implemented with a “dynamic latch”. When the clock is high, the pass transistor turns on, connecting the two transistors in a stable loop, holding the bit. When the clock is low, the transistor capacitance holds the value (like DRAM)A or a new value can be input.

With illustration :

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