> 4.3.9 | Modem Status Command (MSC)

> The Modem Status Command is used for software flow control.

> Comman d | Length | DLCI | V.24 signals | Break Signals (optional)
> 1 octet | 1 octet | 1 octet | 1 octet | 1 octet

> Command:

> Bit 1 | Bit 2 | Bit 3 | Bit 4 | Bit 5 | Bit 6 | Bit 7 | Bit 8
> 1 | C/R | 0 | 0 | 0 | 1 | 1 | 1

> C/R bit: Indicates whether the sequence is a command or a response.
> Length: Length = 2 , EA-Bit = 1
> DLCI:

> Bit 1 | Bit 2 | Bit 3 | Bit 4 | Bit 5 | Bit 6 | Bit 7 | Bit 8
> 1 | 1 | DLCI

> V.24 signals:

> Bit 1 | Bit 2 | Bit 3 | Bit 4 | Bit 5 | Bit 6 | Bit 7 | Bit 8
> 1 | FC | RTC | RTR | reserved 0 | reserved 0 | RING | DCD

> FC bit: Flow control, included in all multiplexer versions
> FC = 1: no frames are accepted
> The following bits for V.24 status lines are included in the Siemens multiplexer protocol from version 3 onwards. To benefit from these functions activate at least version 3. Otherwise version 1 will be used. See Section 5.2 and Section 5.3 for details on version control handshake and version differences.
> Direction host application → module (for request only) as of multiplexer protocol V3:
> RTC: mapped to DTR
> RTR: mapped to RTS
> Bit 5, 6, 7, 8 are not valid.
> Direction module → host application (for request only) as of multiplexer protocol V3:
> RTC: mapped to DSR
> RTR: mapped to CTS
> RING: mapped to RING
> DCD: mapped to DCD
> Bit 5, 6 are not valid
> The mappings are valid only for an MSC request as of multiplexer protocol version 3. Information on older multiplexer protocol versions can be gathered from Section 5.3.
> The response to any MSC must be always the same data already sent. Please keep in mind that it is impossible to remap any response bits.
> Break signal (optional):

> Bit 1 | Bit 2 | Bit 3 | Bit 4 | Bit 5 | Bit 6 | Bit 7 | Bit 8
> 1 | Not supported

> Usually the break signal octet carries information about a break condition detected from the host application in the data stream for the DLC.
> This command supports no parameters. Instead we use this optional parameter to transport the escape sequence detection from the host to the module. If the customer application detects an escape sequence (usually +++), it sends this optional octet with bit 1 set to 1. The module calls its original escape sequence.
> Note:
> A problem may occur if a connection breaks down, for example because of poor network coverage, when a data transmission is in progress and the internal transmission buffer of the module is full. In such case, the "NO CARRIER" result code will be appended to the data stream and put into the buffer, but the modem status and control signals are instantly transmitted over the control channel DLCI 0 (as defined in GSM 07.10). As a result, the modem status and control signals signals are indicated faster than the data stream.