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Unformatted text preview: check, to detect errors
in the frame.
Since the byte value 0x7e is the flag character, PPP needs to escape this byte when it appears in
the information field. On a synchronous link this is done by the hardware using a technique called
bit stuffing [Tanenbaum 1989]. On asynchronous links the special byte 0x7d is used as an escape
character. Whenever this escape character appears in a PPP frame, the next character in the frame
has had its sixth bit complemented, as follows:
1. The byte 0x7e is transmitted as the 2-byte sequence 0x7d, 0x5e. This is the escape of
the flag byte.
2. The byte 0x7d is transmitted as the 2-byte sequence 0x7d, 0x5d. This is the escape of
the escape byte.
3. By default, a byte with a value less than 0x20 (i.e., an ASCII control character) is also
escaped. For example, the byte 0x01 is transmitted as the 2-byte sequence 0x7d, 0x21.
(In this case the complement of the sixth bit turns the bit on, whereas in the two previous
examples the complement turned the bit off.)
The reason for doing this is to prevent these bytes from appearing as ASCII control
characters to the serial driver on either host, or to the modems, which sometimes interpret
these control characters specially. It is also possible to use the link control protocol to
specify which, if any, of these 32 values must be escaped. By default, all 32 are escaped.
Since PPP, like SLIP, is often used across slow serial links, reducing the number of bytes per
frame reduces the latency for interactive applications. Using the link control protocol, most file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm (6 of 11) [12/09/2001 14.46.33] file:///D|/Documents%20and%20Settings/bigini/Documenti/homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm implementations negotiate to omit the constant address and control fields and to reduce the size of
the protocol field from 2 bytes to 1 byte. If we then compare the framing overhead in a PPP frame,
versus the 2-byte...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12