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New Member

PPP Frame Format quiry

I am designing a piece of network hardware as a project and I have found details regarding the PPP frame format that state that the beginning and end of an entire PPP frame is marked by the flag "01111110". i have two questions:

a) Does this mean that the same byte sequence of "01111110" will not appear anywhere else in the packet, including in the data field ????

b) On Cisco's web site i have found details that say the second flag comes after the data field with a two byte FCS that comes after it, but I thought that the Final flag was the last byte of the PPP Frame.

Thanks in Advance for any help

  • Other Network Infrastructure Subjects
New Member

Re: PPP Frame Format quiry

The PPP frame is based on HDLC. The RFC is probably your best bet for information:

In short, though, the flag sequence must not show up in the data field. Bit stuffing is used to insure this. As for the flag, only one is required between two frames. So the "final" flag (after the FCS) is as much the start of the next frame as it is the end of the last one.

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