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> Intro • Examples |
— FCS (CRC): Examples — Example HDLC Frame for PPP Here is a HDLC frame I captured. I had two computers, each connected to a modem, the modems were connected to my small in-house telephone exchange. Computer 1 running Windows called computer 2 with the RAS service (PPP) just as if it would call up an ISP (Internet Service provider). The modem of computer 2 answered the call, on the computer run a small packet capture program I wrote. The initiating computer sent 10 PPP packets until it gave up with the message, that it would get no answer. Of course, I had not programmed computer 2 to answer the PPP request. I could analyze the captured PPP HDLC frame, though.
Consider all data bytes (bold) in the white fields to determine the FCS for sending a frame. When analyzing the received frame, take all data bytes in the white and red fields. This must result in f0b8 hex for an uncorrupted frame. Examples for Calculation Understanding and calculating the checksums is a bit tricky and not always really obvious. However, if you follow «The calculation loop step by step», you can calculate a small frame by hand, using a hex-calculator. I have done so for the «Example HDLC Frame for PPP» and for an example from a correspondent, who did not quite got it right. If you are not yet enlightened, perhaps you try to follow the examples yourself. |
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© 2004 - 2018 by Horo Wernli. |