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V.21 CCITT Standard

Note that this document is not actually the V21 standard or a copy thereof. I have designed this document to not violate copyright but to make the content of the standards available to a wide audience.

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.


This standard describes a modem. It is designed for simple equipment, for use on the voice telephone network and is supposed to be simple to operate. The data has a low rate. The data rate is slow enough to connect up the equipment available in 1964, which included electromechanical equipment such as teletypes.

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.


  1. Possibility

    It is possible to set up a low rate data transmission on switched and leased telephone lines.
  2. Full Duplex

    The circuit is full duplex and can transmit data in both directions simultaneously at a rate of up to 300 bits per second.

    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.

  3. Frequencies

    V.21 specifies the frequencies used in terms of a mean frequency and an offset.

    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.

  4. Synchronicity

    Asynchronous or synchronous data transmission is permitted. The modems have to provide the synchronisation or clocking when synchronous transmission is used. This standard has nothing to say about how this is done.
  5. Echo Suppression

    Refer to V.25 for echo suppression disabling.
  6. Power

    The output power of the modem is limited to 1 milliWatt or 0 dBm. The power level at an entry to an international line is also specified to be at or below -13 dBm. Which is 0.05 milliWatts. This comes from reccomendation V.2 paragraph 2. In practice this means that to gain approval the modem must put below 0.05 mW onto the phone line, in case it finds that there is no loss.
  7. Channel Assignment

    1. When both data channels are in use, that is full duplex, the originator uses the low frequency channel number 1. The answer end uses the high frequency channel number 2.
    2. Even if the reverse channel is used for checking, service signal and error control purposes only this makes no difference to the calling end using the channel 1 and the answering end using channel number 2.
    3. The channel assignment rules are designed to allow any two telephone subscribers using V.21 to communicate whatever, including data or service signals etc. There are special cases where there is no caller or answerer, in which case it is up to the correspondants to agree on the assignment of the channel. This could happen in a simplex transmission for example or on a leased line or multidrop line.
  8. Interface Circuits

    1. The interfaces on the digital side of the modem.

      The digital interface is documented by using an interchange circuit number and designation. The designations differ slightly from current use. Three options are listed, switched telephone network, point to point leased line, and multipoint leased line.

      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.

    2. Data carrier detect (DCD) is called data channel received line signal indicator and is circuit 109. The DCD response time is the time from when a tone appears or dissapears till when the modem changes the condition on te circuit. The test is done using a tone representing a 1 or a mark level.
    3. Clear to Send (CTS) is called Ready for sending and is circuit 106. All modem options must supply this. The CTS response time is the differnect in time between asserting RTS (if its available) and the appearance of CTS. Alternately it is the time difference between DCD appearing and CTS appearing.
    4. CTS response time for ON to Off is less than 2 milliseconds. For OFF to ON for multipoint of non-voice lines is 20 to 50 milliseconds. For OFF to ON for lines which may also carry voice it is 400 to 1000 milliseconds.
    5. DCD response time for loss of carrier is 20 to 80 milliseconds. When detecting carrier when there is no voice the time till appearance of DCD shall be no more than 20 milliseconds. For lines which may also carry voice the DCD response time should be between 300 and 700 milliseconds.
    6. Data carrier detect threshold
    7. If the received signal from the line is greater than -43 dBm DCD should be ON.

      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!

    8. Fault conditions.

      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.

      1. DSR failure ==> OFF
      2. RTS failure ==> OFF
      3. DTR failure ==> OFF
      4. other circuit failure ==> 0 or 1
      This will prevent operation if the DCE or DTE fail, for example are powered off.
  9. Electrical characteristics of digital interface circuits.

    Use V.28 for the electrical characteristics.I guess this is like RS-232

    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

  10. Manufacturer Assistance

    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.