Appendix B: MUX
December 31, 2007 AT Commands Reference Manual B-5
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.
Advance Mode Frame Unpacking
The receiver (referring to the receiver of a frame) examines the frame between the two flag octets.
Upon receipt of a control escape octet, and prior to FCS calculation, it does the following:
• Discards the control escape octet.
• Restores the octet that immediately follows by complementing its 6th bit (XOR with 0x20).
MUX State Procedures
Valid channel indexes in MUX state are 0 to 4, where 0 is the index of the control channel and 1-4
are indexes of the information channels (IDLC).
The following table describes the basic procedures (Open/Close/Err) performed when the G24 is
in MUX state.
Table B-3: MUX State Procedures
Procedure Description
Unrecognized
Frames/Invalid Frames
The G24 ignores unrecognized and invalid frames.
SABMi Reception –
Channel Establishment
Upon receiving a SABMi frame (i = channel index), the G24 checks
whether i is within the valid range and is not already established. It them
tries to establish the new channel.
If the G24 succeeds, it sends a UA frame. If it is not successful, it sends
a DM frame.
DISCi Reception –
Disconnect Channel
Upon receiving a DISCi frame (i = channel index), the G24 checks
whether i is within the valid range and represents an open channel. If so,
the G24 attempts to close the channel.
If the G24 is successful, it sends a UA frame. If it is not successful, it
sends a DM frame.
Closing a channel hangs up any active CDSs. However, active voice
calls are not hung up, even after the IDLC is closed.
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