Showing results for 
Search instead for 
Did you mean: 

I want to know how preamble works


In the Outbound and Inbound pdu's there is a field call preamble I want to know how it works and the methodology behind.    


Giuseppe Larosa
Hall of Fame Master Hall of Fame Master
Hall of Fame Master

Hello Jonathan,

when the ethernet frame has to be transmitted at OSI layer1 as a bit stream there is the need to provide to potential receivers a way to synchronize with the bit stream. Each bit is transmitted as a signal over a time interval.

In ethernet there is no permanent clock signal that provides the timing information on when to sample the signal, and when no frames are sent there is silence.

The preamble provides to potential receivers a short training interval before the effective frame starts.

The preamble is made of a known sequence of bytes with known value. First 7 bytes are made of 10101010 and last octect is 10101011 and is called start of frame delimiter

In this way potential receivers can tune and synchronize before information bits of the real frame start.


Hope to help


I don't quite understand yet but I think it is used to sequence and esemble packets together that might not arrive at the same time but with the preable and the start of frame the network can identify which packet below to which?

Thank you for your time...

Hi there,

to put it in easy words, preamble is like a fixed known binary pattern that devices on the ethernet network use to show the start of a packet. it shows the start of a packet. as Giuseppe said, ethernet is Not a synchronized network (synchronized means, devices on the line expect a packet at every fraction of time, by default) so there might be times that there is nothing on the ethernet line, the method for devices to understand there is a packets comming is to detect the KNOWN preamble bit stream.

keep in mind that in OSI Layer 1, packets are sent as flat 0 and 1 streams: 101000101010101011100011010101.

if you get thousands of this 1 / 0 in a line, how would u want to know where is the begining, where is the end of oone packet? well, preamble and CRC.

Hope it Helps,


Hope it Helps!



as The la Rosa and you have mention this method is used to identify parts of a packet or frame that has been sent not in a synchonized manner, But is does the number has a sequence so the network or layer 1 can identify which frame belongs to which, For example in this long extended number will be the same in each portion ofthe whole package or it is a sequence to order the frame? how does the computer knows which one is first and which one is second or it just put one number to pick up the packets pertaining to that frame?...

Thank you guy for the explanation.    

since I don't know if you are completely familiar with Sync and Async network connections, Let me clarify somethings first; in Syncronous connections as soon as the both ends of the connection negotiate and start the link, the nodes define start/end of a packet in a timely fashion, i.e. they agree that every 1 second the signals transmit a complete packet over the link. So, in order to maintain this Time Sync between the nodes, they should continuesly send "data" to eachother over the link, even if there is not any actual data coming from the upper layers or the user.


On the other hand, in Async networks i.e. Ethernet, instead of that timings and Sync-ing the connection between the nodes, a rule has been put in place so that machines can distinguish the start and end of each packet in the signals that are transmitted over the link. The goal is achieved by defining a known bit sequence (by the standard) in each packet's header and trailer, called Preamble and CRC respectively. By this standard everytime a network device receives the said bit sequences, it knows that a packet is begining or ending over the link.


I think what you are asking is related to packet fragmentation which occures in the Layer 2, it has a sequence number that is related to the acting layer 2 protocol. Preamble gets added after this process.


Please rate if this answer is helpful,


Hope it Helps!


I think what I meant was the sequence number... Like the last octect in a package is put as the beggining of the sequence number of the next package so it can be identify and reasamble is it?

Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: