Technical information

Table Of Contents
Software Procedures Related to RS232 HW Lines
B-4 AT Commands Reference Manual January 31, 2007
MUX Modes
The following table describes the differences between each mode in a non-ERM environment.
Note: The length is still required in the information field of the UIH frame.
Advance Mode Transparency Mechanism
There is a set of protected characters that cannot appear between the start and end flags. These
characters are protected by the transparency mechanism within the following fields: address,
control, information and FCS. This mechanism is detailed in the packing/unpacking subsections
that follow.
Advance Mode Frame Packing
The transmitter (referring to the sender of a frame) examines the frame between the opening and
closing flag sequences (including the address, control and FCS fields) and, following completion
of the FCS calculation, does the following:
When encountering protected characters, complements the 6th bit of the octet (XOR with
0x20).
Inserts a control escape octet immediately preceding the octet resulting from the above, prior
to transmission.
Table B-1: MUX Mode Differences
Feature Basic Mode Advanced Mode
Start flag 0xF9 0x7E
Close flag 0xF9 0x7E
Length field Yes No
Data transparency No Yes
Frame UIH (or UI) UIH (or UI)
Processing cost Low High
Recovery of
synchronization
Slow Quick
Table B-2: Protected Characters
Protected Characters Value Encoded Value
Flag sequence (SOF,
EOF)
0x7e 0x7d, 0x5e
Control escape 0x7d 0x7d, 0x5d
XON 0x11 0x7d, 0x31
XOFF 0x13 0x7d, 0x33