Multiplexer User's Guide
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4.3 State diagrams
The multiplexer protocol is based on two state machines (see Figure 4). One state machine
initiates the setup of the logical channels, the other one responds to the requests.
The GSM engine can only respond to requests. A higher level for controlling the state
machines is not implemented.
The procedure for setting up the two state machines – the one for the customer µC and the
one for the GSM engine – is shown in Figure 5 and Figure 6.
Executing the AT command AT+CMUX=0 starts the switchover from AT command mode to
the multiplexer protocol and parameterizes the multiplexer control channel DLCI = 0. Both
state machines are entering the DISCONNECTED state and immediately have the option of
setting up the multiplexer control channel DLCI = 0 and other logical channels.
The logical channels are then set up (DLC establishment). If the DLC has been established
successfully the state machine for that particular channel changes to CONNECTED. If the
request is unsuccessful the logical channel cannot be established and the state machine
remains in DISCONNECTED on this particular channel.
Information can be transferred over all channels in CONNECTED. Control commands can be
transferred in the multiplexer control channel DLCI = 0; the other channels transfer data.
The parameters for all logical channels DLCI = 1...4 in DISCONNECTED can be set for the
requested logical channels by parameter negotiation.
Disconnecting individual channels (DLC release) causes the state machine for those
channels to revert to DISCONNECTED. Release of the multiplexer control channel DLCI = 0
corresponds to a CLOSE DOWN. The CLOSE DOWN command switches back.
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