Take the 2-minute tour ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

In the IEEE 802.3 frame structure there is a 7 byte preamble section for synchronization. If it is really for synchronization does that mean Ethernet is synchronous and not asynchronous?

Ethernet structure for reference: IEEE 802.3

share|improve this question

3 Answers 3

up vote 3 down vote accepted

Your question seems to assume that each frame is transmitted one after the other, synchronously like one frame, then a gap then another, and thus why a need for a preamble. As if a preamble tells it and just tells it that there's a frame coming. And if a preamble, then why can't it be asynchronous. It is a good question. The preamble isn't for that. Or in fact to put it another way. It is actually asynchronous in the sense that you mean. The answer, is that the preamble isn't for synchronization of one frame followed by another e.t.c. synchronously. There is indeed no such synchronization for that. Frames do not arrive synchronously. It is purely for each individual frame so the electronics, a clock I suppose, reading it, knows when each bit is going to come -for that frame- by the time the processing of that frame reaches the start of the SFD(start of frame delimiter). Once the frame has been completely read, it has no idea when the next frame will arrive. In fact it never has any idea when the next frame will arrive.

Below are some quotes from some documents which serve as sources by which I can answer your question.


802.3-2008_section1.pdf
3.2.1 Preamble field
The Preamble field is a 7-octet field that is used to allow the PLS circuitry to reach its steady-state synchronization with the received packet’s timing (see 4.2.5).

4.2.5 Preamble generation
In a LAN implementation, most of the Physical Layer components are allowed to provide valid output some number of bit times after being presented valid input signals. Thus it is necessary for a preamble to be sent before the start of data, to allow the PLS circuitry to reach its steady state. Upon request by TransmitLink- Mgmt to transmit the first bit of a new frame, PhysicalSignalEncap shall first transmit the preamble, a bit sequence used for physical medium stabilization and synchronization, followed by the Start Frame Delimiter. If, while transmitting the preamble or Start Frame Delimiter, the collision detect variable becomes true, any remaining preamble and Start Frame Delimiter bits shall be sent. The preamble pattern is: 10101010 10101010 10101010 10101010 10101010 10101010 10101010 The bits are transmitted in order, from left to right. The nature of the pattern is such that, for Manchester encoding, it appears as a periodic waveform on the medium that enables bit synchronization. It should be noted that the preamble ends with a “0.”

http://en.wikipedia.org/wiki/Start_Frame_Delimiter
The Start Frame Delimiter (SFD) is the 8-bit (1-byte) value marking the end of the preamble of an Ethernet frame. The SFD is immediately followed by the destination MAC address. It has the value 10101011. The preamble of an Ethernet frame consists of a 56-bit (7-byte) pattern of alternating 1 and 0 bits, which allows devices on the network to easily detect a new incoming frame. The SFD is designed to break this pattern, and signal the start of the actual frame.

share|improve this answer
    
@baran: to put it simply, think of the header as Ethernet's version of "3, 2, 1, go!". –  geekosaur Mar 23 '11 at 3:40
    
@geekosaur That is horribly lame. You're lucky comments can't be downvoted! –  barlop Mar 29 '11 at 23:03

Ethernet is Asynchronous.

Asynchronous communication means the transmitter and receiver do not share an external clock signal (such as would be transmitted over a "clock" pin or "clk+/clk-" pair on a cable). Ethernet cables have no clock pins or pairs. Ethernet does not use a separate bit-clock signal shared between the transmitter and receiver, so it is asynchronous.

Because Asynchronous communications busses don't share a separate clock signal, the transmitter has to encode each transmission in a way that allows the receiver to know when one bit ends and the next bit begins. Ethernet's solution for that is to start every transmission with a long series of alternating 0 and 1 bits -- the preamble -- which allows the receiver to temporarily synchronize its bit-clock with the transmitter's clock for the duration of that transmission. As soon as one frame ends and the next begins, the temporary synchronization must begin again.

share|improve this answer

The preamble is needed for electrical reasons so that the MAC can easily detect when the packets starts.

I do not understand your question about Ethernet being "synchron/asynchron".

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.