The title of the standard is 300 bits per second duplex modem standardized for use in the general switched telephone network.
It was originally approved in 1964, and must have been written before then. It has been ammended every 4 years since then.
The data transmission is full duplex, which means that data can be sent in both directions at the same time. An alternative is to send data in one direction and use the reverse channel for error control signalling. The bandwidth of the mdoem signals is designed to fit in a normal voice channel and to avoid interfering with analogue telephone signalling.
A telephone call makes a connection between the two communicating parties and data transmission can start automatically or by operators switching in the modems.
Frequency shift keying is used. There are two states of the signal, so this is a binary modulation method. This means that there are two different frequencies. The modulation rate is equal to the data rate.
Some modems may have an upper limit of 200 bits per second, but otherwise use this standard.
first channel 1180 for 0, 980 Hz for 1
second channel 1850 for 0, 1750 Hz for 1
You can see that the frequency shift is 200 Hz in each case. The zero bit takes on the high frequency and the one bit takes on the low frequency so that this is inverted FSK (Frequency Shift Keying).
These frequencies are the nominal frequencies. The transmitter is allowed to be out by 6 Hz above or below. The receiver must allow for another 6 Hz of frequency offset on the transmission line. So the receiver has to tolerate a plus or minus 12 Hz offset.
For a switched telephone line the modem must provide the following circuits: signal ground, transmit data, receive data, clear to send, dataset ready, data terminal ready, data carrier detect, and ring indicator.
For a point to point leased line the modem must provide the following circuits: signal ground, transmit data, receive data, clear to send, request to send (if no voice), data set ready, data terminal ready, and data carrier detect.
For a switched telephone line the modem must provide the following circuits: signal ground, transmit data receive data, request to send, clear to send, data set ready, data terminal ready, data carrier detect and select transmit frequency.
If the received signal from the line is less than -48 dBm, DCD should be OFF.
The OFF to ON signal transition level must be at least 2 dB higher than the level for an ON to OFF transition, otherwise the behaviour between -48 dBm and -43 dBm is not specified.
Modems may also be desensitised!
Sometimes it may be possible to distinguish a fault in a circuit from the ON or OFF condition. This is how they are to be interpreted.
Use ISO 2110 for connector and pin assignments. I guess this is a DB 25 plug
There is a statement that the interface is being restandardised to use less pins and to use a balanced iterface. I guess this is X.21
For and end to end connection between two subscribers the expected range of attenuation is from 5 to 30 dB in the 800 to 1000 Hz range. At 1750 Hz, expect upto 35 dB attenuation.
The modem operator is not allowed to have a control for send level or receive sensitivity.