Data transfer and flow control

If you are seeing lost data (that is, when file sizes on the receiving end do not match files sizes on the sending end), you may be experiencing a flow control problem.

Flow control occurs when the receiving system tells the sending system to slow down (in reality, stop) because it has nowhere to put the data being transmitted at the moment. This slows the flow of data, and, in most cases, no data loss occurs.

However, flow control sometimes causes chain reactions. For example, imagine computer A is receiving data from computer B via modem (that is, there are two modems, with one modem on each end of the connection). Computer A says stop to its modem. This modem might have some space to save the data still coming from the other side, but when that space is exhausted it will have to tell the other modem to stop as well. Of course this modem may at some point have to tell Computer B to stop. This is end-to-end flow control.

If end-to-end flow control isn't working properly, the result is usually the loss of data, since you expect the other side to obey your request to stop, and you would discard any data sent shortly after the stop request.

Two types of flow control are supported:

• Hardware flow control uses two of the wires that are connected between your machine and the modem to tell the other side to stop. These wires are called CTS (Clear To Send) and RTS (Request To Send).

• Software flow control uses an exchange of control bytes to tell the other side to stop.

The device name is used to select the type of flow control needed to communicate on the line. The /dev/ttynn series devices and the /dev/ttynns series devices are the software flow control devices, and the /dev/ttynnh series devices (and the /dev/term/nnh series devices) are the hardware devices.

Since most modems come up with hardware flow control already turned on, if you select COM1 or COM2 from the Modem Manager, you will get /dev/tty00h for COM1 and /dev/tty01h for COM2 respectively. When you need COM3, COM4, and so on, you would click on other to manually select a device. (When manually selecting a device, you typically should use a /dev/ttynnh series device.)

If you know that you need to use software flow control on a particular line, do not select COM1 and COM2. Instead, select other and enter /dev/tty00s or /dev/tty01s.