Ethernet

Necessary knowledge: [HW] Controller module, [AVR] USART, [LIB] Serial Interface, [LIB] Alphanumeric LCD

Ethernet is summarizing networking technologies for LANs. It was introduced first in 1980 and later standardized as IEEE 802.3 in 1985.

When talking about Ethernet standards, those are tightly connected with the OSI physical layer. Ethernet provides service up to and including the data link layer. As transfer medium, Ethernet started with coaxial cables, with the 10BASE5 standard, later replaced by twisted pair and after that by fiber optic links.

The possible transmission rate with Ethernet started with 10 megabits per second, up to currently 100 gigabits per second. Most computer networks are currently using 100 Mbit/s infrastructures, tending to 1GB/s.

A system utilizing Ethernet divides a networking (data) stream into small pieces, called frames. Each frame needs to contain the sender (source) and receiver (destination) addresses, the MAC adresses and data for error checking. In the middle section, an Ethernet frame is carrying data including headers from other protocols, like the Internet Protocol (IP protocol). The error checking is realized by a 32-bit cyclic redundancy check that is able to detect errors in the data.

A 802.3 Ethernet frame consists of a 8byte long preamble, with bit synchronization information, followed by a 1 byte sequence (“10101011”) as a Start Frame Delimiter. The next two times six bytes are carrying the destination and source address (each usually 48 its long).

Next 2 bytes are specifying the length of the data segment, followed by the so called Data Unit (bytes 46 to 1500). The 802.3 has a maximum of 1500 MTU (the maximum transmission unit), Ethernet with LLC and SNAP and PPPoE only 1492. For other types of Ethernet, the MTU (maximum bytes) can be also from 1500 to 9000. Next 4 bytes are for the error detection of corrupted data. This check is an 802.3 version of CRC. This 32 bit code has an algorithm applied to it which will give the same result as the receiver of the link, provided that the frame was transmitted successfully.