Multiplexer User's Guide
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4.3.1 Start-up procedure
The only approach to activate the multiplexer protocol is entering the appropriate AT
command AT+CMUX=0. This enables the multiplexer control channel. The next step is to set
up the multiplexer control channel as described in Chapter 4.3.2.
4.3.2 DLC establishment
The multiplexer control channel must be set up as the first channel followed by all other
DLCIs. To do so, a SABM frame (see Chapter 4.2.3) must be sent to the GSM engine.
The module responds either with a UA frame if the DLCI was set up, or with a DM frame if
the DLCI was not set up.
No provision is made for repeating the request if a response is not received.
The state machine requesting the multiplexer control channel DLCI = 0 is the "initiating
station", while the other is called the "responding station".
4.3.3 Information transfer
A response is not essential for every command – for example, an unsolicited result code
does not require a response.
SABM: P = 1
Address Field = DLCI of channel to be established
UA: F = 1, DLCI is being established
DM: F = 1, not ready, DLCI is not established
Address Field = DLCI of requested channel
Figure 7: DLC establishment
P = 0, C/R = 1
P = 0, C/R = 0
Figure 8: Information transfer
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