Ethernet II Vs IEEE 802.3 Frame format

[pushpam.kumar]

I was reading through some of the documentation available on the support forum where i found some interesting differences. It was posted some where that  Ethernet II frames have 8 bytes of Preamble while IEEE 802.3 Frame has 7 bytes of Preamble and then 1 byte of SFD.

here is some of my doubts :

1. Is preamble a part of Layer two header i.e. do we count 8 bytes of preamble in the minimum/maximum frame size of Ethernet ?

2. What  is the exact function of Preamble and  SFD ?

3. Why do we need this SFD , and how is it different in Ethernet II where we don't have SFD but only 8 bytes of preamble.

[Richard Burts]

No the preamble is not part of the layer 2 header and does not count in the minimum/maximum frame size. The preamble was instituted to help synchronize clocking for devices on the Ethernet and to correct for any possible "drift" in clocking between devices.

HTH

Rick

[pushpam.kumar]

Hi Richard,

got it that Preamble is not part of L2 Header.

what I want to understand is both of the Ethernet format uses 8 bytes of data for this sync purpose.  Then how is the 8 bytes of Ethernet II different from 8 bytes and IEEE 802.3 which has one byte of SDF.

Thanks,

Pushpam Kumar

[rasmus.elmholt]

Hi

as far as i know it is the same. The names are just different. The data/signal is the same in ethernetv2 and 802.3. Otherwise it would not be compatible.

[Richard Burts]

The difference between the preamble of Ethernet II and 802.3 with Start Frame Delimiter is in semantics (what you call things). From the perspective of the bits on the wire they are exactly the same. Ethernet II says that the preamble is 8 octets consisting of a stream of binary 1010 and ending with binary 1011. 802.3 changed the semantics, saying that preamble was shorter and adding the SFD, saying that preamble is 7 octets of repeating binary 1010 and then SFD is an octet of 10101011. So from the perspective of bits on the wire they are the same while from the perspective of how we name the components they are different. 

HTH

Rick

[pushpam.kumar]

Thanks a lot Richard for the explanation...

good knowledge to have that the bit stream is same irrespective of the nomenclature !

Thanks again,

Pushpam Kumar

[Richard Burts]

Pushpam

You are welcome. These forums are excellent places to learn about Cisco networking. I hope to see you continue to be active in the forums.

HTH

Rick