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CHAPTER 2: Introduction
The Remote MiniBridge uses a variant of HDLC
framing for WAN communication, in accordance with
RFC 1662. In asynchronous mode, the RMB breaks up
the frames into 8-data-bit octets which are preceded by a
start bit and followed by a stop bit. To make this process
transparent (to achieve “frame transparency”), it
normally uses “octet stuffing”—it replaces any flag
sequence (0x7E), control-escape octet (0x7D), or other
single octet in its Async Control-Character Map (ACCM)
with two octets: the control-escape octet followed by the
original octet XORed with 0x20. The ACCM consists of
the first 32 characters in the ASCII character set (those
with octet values up to 0x20). You can disable ACCM
octet stuffing by tying Pin 22 of the RMB’s DB25
connector to ground (refer to
Table 2-1
on the
previous page).
If you want to, you can also enable enhanced
compression, which increases data throughput by
stripping the padding bits from the 64-byte frames.
(When compression is disabled, frames are transmitted
as is.) See
Table 3-1
on page 22.
