What Is an Ethernet Preamble?

Ethernet is the most widely used set of standards for the physical properties of networks. It also defines how data gets onto the cable of the network and how it travels down the wire of the cable. The preamble is a series of zeros and ones that precede arriving data.

Data travels around networks in segments. This prevents one connection hogging the line for any length of time and blocking network access all other computers. Network software places this segment of data in a packet and puts headers on the front to describe the data and to indicate its source and destination. This network software than passes its packets to an Ethernet program, which converts the data packet into bits in as structure called a frame. These bits are then sent down the wire of a cable as an electrical pulse.

Frames do not travel down the wire one immediately after another. A slight silence is between them, which is called an Inter Frame Gap. The gap represents the time it takes to transmit 12 bytes of data and so it is a different length of time for different grades of Ethernet networks. Fast Ethernet requires a gap of 9.6 microseconds and Gigabit Ethernet requires a minimum gap of 0.096 microseconds. The next event before the frame arrives is the preamble. The preamble is described by two categories. It is said to be an 8 byte sequence with the last two bits being a different pattern, or it is counted as 7 identical bytes followed by a Start of Frame byte.

The first seven bytes of the preamble are all the same: 10101010. The last byte, or the Start of Frame byte is slightly different: 10101011. The 8 bytes of the preamble and the Start of Frame create a pattern of 64 bits. They are not officially counted as part of the Ethernet frame. The frame begins immediately after the Start of Frame, without a gap.

The preamble functions like the outriders in a presidential motorcade. They tell everyone ahead to wake up and pay attention: something important is coming. Apart from being a "get ready" notification, the preamble also serves as a clock synchronization device. Bits are represented as an electrical voltage -- high or low. The receiver needs to keep pace with the standard interval, because it is only with a regulated clock that it can tell where one bit ends and the next starts. A 10101010 signal is easy to detect, but a 11111111 signal might be interpreted as 1111111 if the receiver's clock is set too slow. The preamble demonstrates the pace of arriving data and repeats the 1010 pattern long enough to enable the receiver to set its clock.