> 4.10.1 | Restrictions on Multiplex mode

> In multiplex mode, data and fax calls can only be set up on logical channel 1. Due to this restriction, AT commands have a different behavior on channels 2+3 compared to channel 1. Several commands are not available, others return different responses. This section summarizes the concerned commands. For general rules and restrictions to be considered in Multiplex mode please refer to [7].

> Table 4.1: | Availability of AT Commands on Virtual Channels

> Command | Behavior on channel 1 | Behavior on channel 2+3
> +++ | not usable, but see note 2) | not usable, but see note 2)
> AT+CBST | as described | not usable
> AT+CRLP | as described | not usable
> AT+CG... (GPRS commands) | as described | see note 3)
> AT+F... (Fax commands) | as described | not usable
> AT&S | as described | not usable
> ATA | as described | no Data Calls
> ATD | as described | no Data Calls
> ATDI | as described | not usable
> ATL | as described | not usable
> ATM | as described | not usable
> ATO | as described | not usable
> ATS6 1) | as described | not usable
> ATS7 1) | as described | not usable
> ATS8 1) | as described | not usable
> ATS10 1) | as described | not usable
> ATS18 1) | as described | not usable

> 1) Siemens GSM engines support the registers S0 - S29. You can change S0,S3,S4,S5,S6,S7,S8,S10 and S18 using the related ATSn commands (see starting from ATS0). The other registers are read-only and for internal use only!
> 2) The applicability of the +++ escape sequence depends on the customer's external application based on the Mulitplexer Protocol. Recommendations for implementing an appropriate modem status command (MSC) are provided in [7], Section "Escape Sequence".
> 3) PDP contexts can be defined on any channel, but are visible and usable only on the channel on which they are defined (thus it is not possible to define a context on channel 2 and activate it on channel 3). GPRS connections can be established on two channels at a time.

> Table 4.2: | Summary of AT commands with Different Behavior in Multiplex Mode

> Command | Description
> AT\Q | It is recommended to use hardware flow control (AT\Q3). XON/XOFF flow control (AT\Q1) is not supported in Multiplex mode. See note regarding AT\Qn settings stored with AT&W if Multiplex mode is active.
> AT&V | Different default configurations on channels 1, 2 and 3.
> AT&W | Different user profiles can be stored on each channel.
> AT+IPR | Before you start Multiplex mode, it is recommended to set the ME to 57600 bps (minimum should be 4800 bps). For GPRS we suggest to use 115200 bps or 230400 bps. In Multiplex mode, the write command AT+IPR=<rate> will not change the bit rate currently used, but the new bit rate will be stored and becomes active, when the module is restarted.
> AT+IPR=0 | Multiplex mode cannot be activated while autobauding is enabled.
> AT+CALA | On each channel an individual <text> message can be stored. but only one time setting applies to all channels. This means an alarm <time> set on one of the channels overwrites the time setting on all remaining channels. Therefore, the total number of alarm events returned by the read command AT+CALA? will always be <n>=0, no matter whether individual text messages are stored. When the alarm is timed out and executed the ME sends the URC only on the channel where the most recent alarm setting was made. The alarm time will be reset to "00/01/01,00:00:00" on all channels.
> AT+CMEE | Presentation mode can be separately configured for each channel.
> AT+CNMA | If Multiplex mode is activated the +CNMI parameter will be set to zero on all channels, if one channel fails to acknowledge an incoming message within the required time.
> AT+CNMI | Phase 2+ parameters can only be used on one channel. The parameter for <mt> and <ds> on the other channels have to be set to zero. If either a SM or a Status Report is not acknowledged, all +CNMI parameter will be set to zero on all channels.
> AT+CFUN | If the ME is in Multiplexer mode, it is not recommended to activate SLEEP mode with AT+CFUN=<fun>. The best approach to properly control SLEEP mode in this case is to issue the PSC messages described in [7], Section "Power saving control (PSC)".
> AT+CPMS | Parameter <mem3> will be the same on all instances, but the settings of <mem1> and <mem2> may vary on each instance.
> AT^SSDA | If one instance is set to <da>=1 and <mt>=1, then all other instances must be configured for <mt>=0.