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PPP Framing
There are three HDLC frame variations used by PPP to transmit ULP data. The type
of transmission frame employed is determined by kind of transport connection
being used for transmission. These variations are defined in RFC 1662.
For asynchronous transport using modems, AHDLC framing is used. AHDLC framing
is implemented in software as part of the PPP interface driver. It makes use of a
defined frame delimiter character for marking the beginning and end of AHDLC
frames. It also employs an escape character to indicate to the receiver how to
interpret the next octet value. In order for the escape characters to be interpreted
properly with certain octet values, the link transport must support 8-bit values,
otherwise the data will be misinterpreted. These character combinations are never
used to represent user data. AHDLC also utilizes some type of in-band or
out-of-band flow control mechanism, but that is left up to the actual hardware and
software implementation. AHDLC's processing of the datastream on a per octet
basis does add some additional overhead to the data handling process.
For synchronous transport over dedicated DS-X leased lines or ISDN, bitsynchronous
HDLC (BHDLC) framing is used. BHDLC is ideal for transport
implementations where framing is accomplished by the hardware interface. Where
AHDLC uses special escape characters for structuring the user data, BHDLC instead
uses bit stuffing techniques to present the user data in interpretable form. Also like
AHDLC, an end-of-frame marker is employed to indicate the end and beginning of a
new frame (01111110). To ensure that the end-of-frame marker and user data are
not mistaken, bit stuffing is used after any sequence of five consecutive ones. If
after five consecutive ones the next bit is a zero, the zero is a stuffing bit and should
be ignored. If a one is the next bit, the octet is complete. By using bit stuffing, the
octet boundaries are no longer valid, so the entire bitstream must be interpreted to
transport the data correctly. Hence the name, bit-synchronous. BHDLC's
bit-synchronous transmission mechanism is also more efficient than AHDLC
because it allows the datastream to be processed on a per packet basis instead of a
per octet basis.
The last variation is octet-synchronous HDLC. This variation is rarely used except for
SONET and SDH applications. Octet-synchronous uses the same start/end delimiter
and escape character that AHDLC uses, the difference being in the handling of the
escape characters. Octet-synchronous HDLC was originally developed for ISDN
transport applications. It was never implemented, though, as BHDLC is more
efficient for ISDN transport applications.
Regardless of the framing variation, the PPP frame format is essentially the same for
each transmission variation, as illustrated in Figure 5.22.