Frame delimiting

Unanswered Question
Jun 14th, 2009

At which layer of the OSI model frames are delimited?

I was always thinking it is mainly physical layer, cuz it is concerned with bits and other stuff, though recently encountered some sources, where it was said that frame delimiting is also performed at layer two.

If it is so, can anyone explain the difference in frame delimiting among these two layers.

Thanks in advance!

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scottmac Sun, 06/14/2009 - 13:15

Bits/pulses = Layer one

Frames = Layer two

Packets = Layer three

Segments = Layer four

Good Luck

Scott

Giuseppe Larosa Sun, 06/14/2009 - 20:44

Hello Igor,

usually frames have a way to signal the start of the frame.

The exact fields and values used are technology dependent.

For example HDLC and PPP have a starting byte called flag with value 0x7E.

The case of ethernet is different: missing a continuos clock the sender uses a preamble that is made of 64 bits to allow the receiver to synchronize with the bit stream representing the frame.

7 bytes are set to 0xFF and last is 0xFE.

Ethernet at all speeds require also a silence after the frame with a specific minimum value called inter-frame-gap: roughly IFG (considering also the 8 bytes preamble) is 20 bytes. (20.2)

OSI layer1 doesn't know what frames are but it just sends and receives the bit sequence equivalent to a frame.

Hope to help

Giuseppe

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Posted June 14, 2009 at 1:05 PM
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