When negotiating a connection with a remote system the highest common line speed between the two systems is used. In addition, speed buffering, flow control and fixed mode operation are all catered for by the modem. These are described separately below in order to avoid confusion. However, they are closely linked in some respects and the descriptions given should be read carefully.

All K56Flex modems support continuous operation at DTE speeds of up to 115200 bps. For high speed operation, external modems need connecting to a serial port which uses a type 16550 UART. For reliable operation at this speed, your communications software must support the 16550 UART (consult the documentation supplied with your software if you are not sure).

If your system does not have, or cannot support, this type of UART, a maximum DTE speed of 19200 bps is recommended.

To prevent an overflow of information being sent down the telephone line, the modem buffers the data and slows it down to an acceptable rate for transmission across the link. Conversely, data received at the speed of 56000 bps is buffered by the modem and sent to the local DTE at the higher rate.

Speed buffering allows the DTE speed to remain fixed at a constant speed irrespective of the type of connection. An example of speed buffering is highlighted in a DTE or software package that does not allow the use of split speed rates such as V.23. With buffering enabled, a V.23 remote system may be accessed because of the ability of the modem to buffer the data to the correct rate.

A major advantage of speed buffering relates to the modem's in-built error correction and data compression protocols. When using error correction and data compression, data is sent from the DTE to the modem at a higher rate than between the modem and the remote system. This lets you take full advantage of the higher data throughput allowed with these protocols.

Note: The modem continues to communicate with the DTE (computer and terminal) in the same way (speed, parity etc.) as it did for the most recent AT command issued.

Speed buffering is enabled in most modes, except after the \N1 command (external modems only) is issued. When speed buffering, it is essential that some form of flow control takes place to prevent data loss.

If speed buffering is disabled, it is essential that the DTE and line speeds are the same to avoid an overflow or underflow of data and thus corruption. This mode of communication is known as direct mode.

RTS/CTS is the default and preferred method of flow control. It operates by controlling the RTS (ready to send) and CTS (clear to send) lines on the RS232 interface. To function properly, these control lines must be connected between the modem and the DTE by an appropriate cable.

XON/XOFF is another method of flow control. It operates by inserting special characters (XON and XOFF) into the data stream. If the receiving equipment wishes to stop the transmission of data, it sends an XOFF character signal to the transmitting equipment. This tells the transmitter to cease sending data. Transmission only resumes after an XON character signal is sent to the transmitter.

In addition to flow control between the modem and the DTE, XON/XOFF flow control can be used across the communications link to affect flow control between the local and remote modems. This is known as modem-to-modem flow control. It is useful on non-error corrected links to provide flow control between the modems. It allows the DTE-to-modem connection to use RTS/CTS flow control while the modems themselves use XON/XOFF flow control.

32000 to 56000 bps (K56Flex)

2400 to 33600 bps (V.34 ITU-T standard)

4800, 7200, 9600, 12000, 14400 bps (V.32bis ITU-T standard)

4800, 9600 bps (V.32 ITU-T standard)

2400 bps (V.22bis ITU-T standard)

1200 bps (V.22 ITU-T standard or Bell 212A)

300 bps (V.21 ITU-T standard or Bell 103)

1200/75 bps split-speed (V.23 ITU-T standard)

Some of these speeds may be selected individually via the F command (or, optionally, the +MS= command for K56Flex modems), but it is more commonplace to enable automode to allow the modem and remote system to determine the speed they use to talk to each other.

Automatic line speed selection operates by trying each of the modulation standards in turn. When the modems find the first common standard which they both support, the negotiation progresses to determine the fastest speed within that standard that can be used. If no common standard between the two systems is available or the time-out period specified in register S7 is exceeded, the modem does not connect.

In answer mode the sequence of events is as follows: On detection of an incoming call the modem cycles through each valid answer carrier tone. If no connection at one speed is achieved, a slower speed in the sequence is tried until a connection is made or the time-out period specified in register S7 is exceeded.

After the modem connects it continually monitors the line quality. If the quality of the line is found to be below that necessary for reliable operation the modem initiates a fall back to a lower line speed. Equally, if the quality of the line improves, the modem attempts to fall forward to a higher line speed. This ensures that the data throughput is maintained at a maximum appropriate for the current line conditions.

Whilst dialling a remote system in fixed mode, the modem analyses any incoming carrier looking only for the carrier tone associated with the fixed speed set by the F command (or, for K56Flex modems, the AT+MS command).

In answer mode after sending the answer tone, the modem generates a single carrier associated with the fixed speed until connection is achieved or the time-out period set in S7 is exceeded.

The important point to note about fixed mode operation is that a set line speed incompatible with the remote system results in no connection being established.