> Hi,
>
> Does someone know how PPP is used on asynchronous lines?
> (NOT in hdlc framing)
> I means how boundaries of frames are detected? as it seems
> to me that via an UART, the reception is in character mode...
>
>
>
> regards,
> Pierre Vandwalle
>
I do NOT have the details about this in front of me just now, but there
is a start/stop mode of HDLC. As far as I remember, it is using one start
bit, 8 data bits and one stop bit in every transmitted character.
I think that the start stop mode restricts the link service datagrams
to be a multiple of eight bits (or some amount of full octets if you want).
The first data bit in every character is the least significant bit in a octet.
The start/stop variant of HDLC is NOT inserting single bits in the data
stream to avoid interferance with the framing pattern. Instead byte stuffing
is used. Maybe the occurence of the framing pattern in the link service
datagrams will result in frames with up to approx. 20 timeslots (two
character) per octet of data, but maybe I am wrong at this point.
I was not able to find any description of some algorithm for
resynchronization of the character transmitter and charater receiver
if some line noise makes the receiver out of sync.
If I still correctly remember som fragments of the start/stop HDLC protocol,
then you have a couple of options when you are not sending frames:
1. You can extend, in time, the state then the transmitter sends the
stop bit.
2. You can send flags (probably 0011111101 sent periodically).
In the first case it will be simple to insert a pause of enough length
if you think that the receiver in the other end may be out of sync, but
that was not as I know mentioned in the ISO standard for HDLC.
In the second case, you will probably be able to receive 0x7e or 0xf9
characters depending of how the receiver is interpreting the periodical
pattern. In that case you may want to know if the HDLC protocol will
guarantee that a conventional start/stop receiver hardware very soon
will find the correct set of 1-to-0 transitions to interpret as stop bit
to start bit transitions. Maybe the HDLC protocol is very simple when you
use it for PPP, because probably only UI (Unumbered Information) frames
are used, but I have not been analyzing the problem more in detail.
As long as the receiver is correctly in sync with start and stop bits,
then the frames is easily separable using flag sequences (proably single
0011111101 paterns).
Maybe I can find some HDLC standard documents if I get some time.
For the moment, I can't remember the ISO numbers for the documents.
/Harley
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