> 3.2.1 | Flow Control

> Logical flow control
> The internal logical flow control (FC bit in MSC message, see Section 4.3.9) is responsible for the flow control from the host application to the module. For example, if a data call is initiated and the host application transmits data to the module on this logical channel, the module will stop the data transmission from time to time. This happens because the module operates with a bandwidth of 9600 bps on air, but the host application uses a larger width. In this case the module sends an MSC message with the FC bit set. After all data stored in the internal buffer have been sent, the module will send a second MSC message with the FC bit reset. So, logical flow control works like RTS/CTS hardware flow control, but with the FC bit set on every channel. The RTS/CTS lines are not used for flow control because the traffic on the logical channels may cause a temporary loss of bandwidth on another channel. This behavior has no impact on the handshake V.24 lines.
> With the proprietary Siemens multiplexer protocol version 4, the handling of the FC bit has been changed. The FC bit is still transmitted within the MSC message, but ignored. For details see Section 5.5.
> RTS/CTS on the physical interface
> On the physical interface, hardware flow control (AT\Q3) is recommended for use with the multiplexer. For power saving it is indispensable. The setting AT\Q3 needs to be made before switching on the multiplexer. An exception is the USB interface where RTS/CTS flow control is of no use.
> The host application decodes and encodes the data. To prevent loss of data, the application must be able process the information about internal flow control (MSC) regulated by the module. Flow control information is transmitted within the data flow and contains messages whether or not the channel is allowed to send. See Section 4.3.9 for MSC.
> As of Siemens multiplexer protocol version 2, the host application shall permanently activate RTS on all used logical channels. This is because no control data could be sent if RTS was deactivated. If flow control is needed, it is recommended either to set flow control on the logical channels by using the FC bit as explained above for multiplexer version 2 or 3 or, if available, to use the Siemens multiplexer version 4 as explained in Section 5.5.
> RTS/CTS on the logical channels
> The customer application needs to regulate the data flow according to the logical flow control. The implementation of the WinMux is a good example. It maps the 3 decoded channels to 3 serial interfaces as well as the logical flow control information (FC bit in MSC message) directly on the RTS/CTS-control lines.
> In this case CTS superposes the STOP information (data sending disabled) sent by the module to control the data transmission from the customer application to the module. If RTS is reset, a STOP is transmitted to the module to control the data transmission from the module to the customer application. Figure 2 illustrates the data flow.
> Figure 2: Logical flow control and RTS/CTS signaling behind the decoder