> 3.1.1 | Basic Requirements

> • | If your product provides several physical AT command interfaces please note that only the first serial interface ASC0 and the USB interface support multiplexer mode. Other interfaces, such as the second serial interface ASC1 of some products, do not support the multiplexer protocol. At a time, multiplexer mode can be started only on one physical interface. Bear in mind that all other AT command interfaces are disabled when multiplexer mode is active. See also Section 3.1.6 for further detail.

> • | The GSM engine supports Basic mode and UIH framing according to the GSM 07.10 specification. With the Siemens multiplexer protocol version 4, error recovery mode and extended UIH_E framing on the data channels have been introduced. UIH_E is a proprietary Siemens specific frame type, for details see Section 5.5.

> • | Character framing must be configured for 8 data bits, no parity and 1 stop bit (8N1). Please note that if your product includes AT commands for changing the character framing make sure that 8N1 is enabled before you activate the multiplexer.

> • | RTS/CTS hardware flow control (AT\Q3) is recommended for use with multiplexer mode. If used, it needs to be set before multiplexer mode is entered. On the USB interface, RTS/CTS hardware flow control is not relevant.

> • | Several customer software applications may be able to change the selected settings. These settings will be stored in the non-volatile memory and used whenever the module is powered up again. In this case the multiplexer fails to start. To avoid this, it is recommended to re-synchronize all settings before using the multiplexer mode again.

> • | Before closing the multiplexer make sure that there is no ongoing activity on one of the channels. For example, check that voice, CSD or GPRS connections have ended and wait until all pending AT command responses are received. The periodic output of parameters (such as cell information delivered by AT^MONI) must be terminated before closing the multiplexer.