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EtherMAC (Ethernet Media Access Control)

Manchester Code

EtherMAC is a part of Ethernet interface. It is not easy to introduce full function of Ethernet because the interface has very complicated structure. This is just a brief explanation regarding Ethernet.
Ethernet can be mainly divided into four hierarchical layers which are physical layer, data link layer, network layer and the upper layers. EtherMAC handles the lowest two layers, physical and data link ones.
An Ethernet cable consists of four signal wires. Two twisted wires are used for output data and another two twisted wires, for input data. This cable has good noise immunity even without electric shield. There are no clock lines so Ethernet is an asynchronous interface. In order to comprehend the concept of Ethernet, 10Base-T is introduced here even though 100Base-TX is the most popular version today because 10Base-T is fundamental and much simpler than 100Base-TX.
10Base-T is an interface which has 10Mbps transfer rate and Base band format with using a twist pair cable. Manchester Code format is applied to the signal transfer. In Manchester Code data “1” and “0” are defined with rise transition and fall one, respectively.

Ethernet does not have any clock lines. A receiver and a transmitter have to synchronize each other, the same as CAN and USB interfaces. The transmitter sends 56 bit continuous pulses preceding data only to synchronize the receiver with the transmitter.
Ethernet has no such hierarchy structure as Host-and-Device relation which is essential in USB. When one unit of the interface would like to output data, the line must be vacant. The unit has to wait if another unit occupies the line. Even though the unit has ensured the line is available and started to transmit data, a collision will occur if one of the other units starts to send data coincidentally. Once the collision occurs, all units stop sending their data and wait for the vacancy of the line. And one unit will start to transmit its data. But another collision occurs because other units would do the same. In order to prevent such permanent collisions, Ethernet has a countermeasure against the collision. When a collision happens, the wait times for the transmitters are determined by random numbers. This is effective for the units which made the collision. But a new unit might make a collision when one of the previous units starts to transmit the data again. One unit can try to send the same data 16 times at a maximum. This is one of the best arbitration methods in the system without Master-and-Slave relation to share communication responsibility.

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