> 5.3 | Overview of Siemens Multiplexer Protocol Versions

> This chapter summarizes the differences of the existing multiplexer protocol versions.
> Siemens multiplexer protocol version 1:

> • | No version check.

> • | No break signal is sent.

> Siemens multiplexer protocol version 2:

> • | First version including the version check.

> • | Additional features: Transparent signals DTR and RTS, escape sequence +++ transportable via MSC.

> Siemens multiplexer protocol version 3:

> • | Includes all features of version 2.

> • | Transparent signals DSR, CTS, RING and DCD.

> • | Send MSC request from module to host after version check on every channel to signal the initial state.

> Siemens multiplexer protocol version 4:

> • | Ιncludes all major features of version 3.

> • | Implements chapter 6 of the GSM 07.10 standard “Error recovery mode option”.

> • | MSC frame will still transmit V.24 status lines as in V3, but the flow control bit (FC) is always ignored.

> • | See Section 5.5 for Siemens specific implementation details.

> Modem status command (MSC):

> 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

> Figure 13: MSC as used in multiplexer protocol version.
> V.24 signals in multiplexer protocol version 3:

> 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

> Figure 14: V.24 signals as used in multiplexer protocol version 3
> V.24 signals in multiplexer protocol version 4:

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

> Figure 15: V.24 signals as used in multiplexer protocol version 4
> Table 6 summarizes version specific differences in handling the modem status command MSC and the V.24 signals.

> Table 6: | Version differences for MSC

> Version number | RTC | RTR | RTC | RTR | RING | DCD | FC
> | Host application ⇒ Module | Module ⇒ Host application |
> 1 | 1 | 1 | Not used | Used
> 2 | DTR | RTS | Not used | Used
> 3 | DTR | RTS | DSR | CTS | RING | DCD | Used
> 4 | DTR | RTS | DSR | CTS | RING | DCD | Set, but ignored