Data transmission modems - Introduction

Modem is a signal transformer: it contains a transmission unit, which transforms the signals arriving from the terminal through the DTE-DCE interface to signals with a format to be forwarded through the line, and it contains also a receiving unit, which transforms back the signals arriving through the line for a kind of format, which is appropriate for the terminal. The modem connection is nothing more, than a digital line section.

There are several data transmission modems depending on, that the transformation could be made from which DTE-DCE interface to what kind of line interface.

We review in this description modems and connections, where the line interface is a voice frequency subscriber interface.

Data transmission through a telephone channel was "buried" many times earlier, thinking, that the data transmission would be transacted by computer networks independent from the telecommunication networks. In spite of this, newer and newer modem constructions appeared on the market, which contained the all the achievements of the integrated circuit technique, the microprocessors, and the digital transmission technique (error correction coding, automatic equalizer) in small size, and on acceptable price.

    As a historical overview, we give a collection of ITU-TS (ITU-T) recommendations
    containing specifications of different modems useable on different rented and
    switched networks: 

    V.21          Switched network modem of 200 baud
    V.23          Switched network modem of 600/1200 baud
                      FSK modulation
    V.26          Rented four-wired 2400 bps modem
                      QPSK modulation
    V.26.bis      Switched 2400/1200 bps modem
    V.27          Rented four-wired 4800 bps modem with a manual equalizer 
    V.27.bis      Switched 4800 bps modem with an automatic equalizer
    V.27.ter      Switched 4800/2400 bps modem
    V.29          Rented four-wired 9600 bps modem
                      QAM modulation



V.21 - 300 bits per second duplex modem standardized for use in the general switched telephone network

V.22 bis - 2400 bits per second duplex modem using the frequency division technique standardized for use on the general switched telephone network and on point-to-point 2-wire leased telephone-type circuits

V.22 - 1200 bits per second duplex modem standardized for use in the general switched telephone network and on point-to-point 2-wire leased telephone-type circuits

V.23 - 600/1200-baud modem standardized for use in the general switched telephone network

V.26 bis - 2400/1200 bits per second modem standardized for use in the general switched telephone network

V.26 ter - 2400 bits per second duplex modem using the echo cancellation technique standardized for use on the general switched telephone network and on point-to-point 2-wire leased telephone-type circuits

V.26 - 2400 bits per second modem standardized for use on 4-wire leased telephone-type circuits

V.27 bis - 4800/2400 bits per second modem with automatic equalizer standardized for use on leased telephone-type circuits

V.27 ter - 4800/2400 bits per second modem standardized for use in the general switched telephone network

V.27 - 4800 bits per second modem with manual equalizer standardized for use on leased telephone-type circuits

V.28 - Electrical characteristics for unbalanced double-current interchange circuits

V.29 - 9600 bits per second modem standardized for use on point-to-point 4-wire leased telephone-type circuits

V.32 bis - A duplex modem operating at data signalling rates of up to 14 400 bit/s for use on the general switched telephone network and on leased point-to-point 2-wire telephone-type circuits

V.32 - A family of 2-wire, duplex modems operating at data signalling rates of up to 9600 bit/s for use on the general switched telephone network and on leased telephone-type circuits

V.33 - 14 400 bits per second modem standardized for use on point-to-point 4-wire leased telephone-type circuits

V.34 - A modem operating at data signalling rates of up to 33 600 bit/s for use on the general switched telephone network and on leased point-to-point 2-wire telephone-type circuits

V.38 - A 48/56/64 kbit/s data circuit-terminating equipment standardized for use on digital point-to-point leased circuits


    Modems used in measurement TTMER6: ELEKTRONIKA EM2 2400 bps modem.

    Literature: Berkes-Gonda: Adatátvitel Számítógépfelhasználóknak (Data
                transmission for computer users), pp. 166-213 (in Hungarian only)
                Sierra semiconductor SC11054 2400 bps modem chip description
                /it is at our disposal only at the measurement/