< Technology

> Intro

> Polynomial

> Calculation

• 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.

ESC	Enc.	Data	Remarks
		7e	Start of HDLC frame
		ff	Address field: address all
7d	23	03	Escaped encapsuled control byte
		c0	Start of LCP (Link Control Packet)
		21
7d	21	01	Type 1 = Configuration request
7d	21	01	Packet ID = 1 (1st packet sent)
7d	20	00	Length of packet = 0017 hex = 23 including Type. Note, high byte preceeds low byte
7d	37	17
7d	22	02	Modem Control Characters (ACCM) follow Length = 6 including ACCM
7d	26	06
7d	20	00
7d	2a	0a
7d	20	00
7d	20	00
7d	25	05	Magic Number (refer to RFC 1662) Length = 6 including Magic Number
7d	26	06
7d	20	00
		2a
		2b
		78
7d	27	07	Address & Control Field Compression = ON Length = 2 including ACF
7d	22	02
7d	28	08	Protocol Field Compression = ON (for IP) Length = 2 including PFC
7d	22	02
7d	2d	0d	Data (?)
7d	23	03	Data (?)
7d	26	06	Data (?)
		a5	Frame Check Sequence low byte high byte
		f8
		7e	End of HDLC frame
ESC	Enc.	Data	Remarks
Enc. = Encapsuled data byte (+ 20 hex)

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.