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10: Ethernet - Ethernet II (version 2, or DIX) frame - Preamble and SFD (Start Frame Delimiter) part 15



Preamble and SFD (Start Frame Delimiter):
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Preamble and SFD are also part of the frame but they are not considered to be fields that accompany the frame up to the receiver (that is why I let them the last things to explain). They function only at the very beginning, or rather before the actual transmission of data. They both act as a means that  notifies the receiver to be ready because data frame in going to be transmitted. For that reason, it is positioned at the beginning of the frame as seen in this figure.

The function of preamble is to put a set of zeroes and ones on the network so that the receiver figures out that a sender is on the threshold of initiating a data transmission. Consider this figure.

The ones and zeroes that appear on the picture are the preamble per se. They are used to synchronize the clocks of devices, for their clocks might be in different positions. For example, suppose that PC-1 in the above figure  wants to send a set of data to PC-4. The first thing that the sender will do, in order to establish a connection, is to send 56 bits 10101010 10101010 10101010 10101010 10101010 10101010 1010101 to make the receiver aware of the data that is going to be sent. When the clocks of the sender and receiver are synchronized, then the role of preamble is finished. To mark the end of the preamble and the start of the actual frame, the sender sends other 8 bits 10101011 different from previous ones. Take a look at this example.
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As you see in the figure, the clocks are synchronized thanks to the preamble ; and the the last bit of the preamble and the first bit of the actual frame is broken by SFD ( Start Frame Delimiter). In a nutshell, the preamble  consists of a group of bits put on the network  to allow devices on the network to easily synchronize their receiver clocks. Af for the SFD, it is used to mark the end of the preamble and indicates the beginning of the frame.
10: Ethernet - Ethernet II (version 2, or DIX) frame - Preamble and SFD (Start Frame Delimiter) part 15 Reviewed by BOUFTIRA on 3:17:00 PM Rating: 5

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