D-RATS was conceived as a way to use the low speed data channel of D-STAR radios. One scenario where D-RATS would be used is where there are multiple stations on a simplex frequency, and some of those stations are using portable radios with the power saving feature turned on.
Under those conditions, this is my understanding of the transfer settings in the Preferences>Radio>Transfers window:
When sending messages or transferring files, D-RATS sends some blocks of data and waits for an ACK before proceeding. Under strong signal conditions you might want to send a small amount of large blocks, which means waiting for fewer ACKs. Under weak signal conditions, sending a larger amount of smaller blocks means a smaller block has to be re-sent if an ACK is missed. The Block size and amount of Blocks in the Pipeline between ACKs is adjustable to suit signal conditions.
The Warmup settings are a method to insure portable radios using the power saving feature receive data. The warmup length is the amount of "fake" data that is sent before the "real" data so the receiving radio can be brought out of power saving mode and not miss any "real" data frames. Once the receiving radio has been awakened, it stays awake until it has determined that there is no longer any channel activity. The length of the stay awake time is adjustable in the radios settings. The warmup data need not be sent on every transmission as the receiving radio may still be awake. The warmup timeout can then be set to expire only after the receiving radio is expected to have gone back to sleep.
The forced transmission delay time is the amount of time D-RATS waits before sending data (real or not) to the radio to be transmitted. This is intended to reduce data collisions. If the value is positive, the delay time is fixed. If the value is negative, the delay time is a random value between x and zero.
In practice, I have found when sending D-RATS data through a repeater and gateway, warmup timeout should be set to zero so that the warmup blocks are sent with every transmission. The warmup length should be long enough so that a pathway can be established between the transmitting and receiving radios before data is sent.
This has been my experience using D-STAR radios. If you are using some other type of modem, I encourage you the experiment with different settings. I suspect the transmit delay settings will not make much difference and changing the block setting is where you will find the best throughput.
Finally, when you add a port in the Radio window, the Serial connection label states that it is for a D-STAR radio connected to a serial port but, the data sent to this port is just ASCII, and not formatted for any particular (like a TNC). If your modem simply turns on the transmitter when the buffer is full? (as some commercially made modems do), you can use the serial connection. The data rate can be as high as the serial port in your computer will support. Chat text is sent in one stream and can be and can be up to 4096 bytes long. Perhaps some of the new modulation schemes coming online (like VARA FM) can be adapted to work with D-RATS.
As one of my maker friends says: "Play, break and learn".
Happy experimenting.
Patrick (N3TSZ)
Patrick Connor