56 kbit/s modem

V.90 is a recommended by the ITU data transmission method for 56-kbit/s-Telefonmodems.

Principle of operation

In the analog telephone network, where the transmission bandwidth is limited to 3.1 kHz, according to the Shannon theorem, the maximum data transfer rate is at normal line quality to 30 to 40 kbit / s limited ( ITU V.34 standard 33.6 kbit / s).

Since the telephone lines are almost always transmitted digitally in today's telephone network on transmission lines, or between two exchanges, there will be no real analog data transmission between two more telephone modems. On the way from a PC via modem, switch and vice versa, the data are at least twice each from digital to analog and then converted from analog to digital signals.

For V.90 The V.90 host modem used by the transmitter must have a digital connection and the signal generated by this PCM digital signal must exactly once by an digital-to- analog converter (usually by the line circuit in the digital exchange to the receiver is connected ), the analog data generated by the converter are fed in the network clock, whereby a unidirectional high -speed transmission is made possible.

The standard V.90 client modems for subscriber-side connection to analogue connections dominate exclusively receiving the V.90 coded signals; in the transmit direction they use the conventional analog modulation methods such as V.34 with max. 33.6 kbit / s

The transmission rate in the digital telephone network in North America (as opposed to the usual in Europe 64 kbit / s) specified on 56 kbit / s. Therefore, the standard for 56K modems to the same maximum rate was defined. As additional modulation information must be transmitted, but not more than 56 kbit / s are possible here as well. In addition, the least significant (eighth ) bit sporadically used in some digital networks for the purposes of the network operator. This is not noticeable in telephone conversations, but it makes the use of all eight bits for data transmission impossible. Therefore, 56k sampling rate of 8 kbits and 7 useful bits per PCM symbol (actually voice sample ).

Is the attenuation of the telephone line between the client modem and the digital-to- analog converter is too strong, the signal is distorted and there is data corruption. Therefore, it is negotiated when a connection between the modem and digital switching center, how many bits per symbol can be error-free encoding. The Leitungsbedigungen change during an existing connection, the modulation is re- negotiated ( Retrain ). Typically, the achievable speed is between 40 and 50 kbit / s On a poor or very heavily damped, long distance analog line via the PCM coding achievable data rate to 33.6 kbit / s or limited under may V.90 benefits nothing. An additional multiplexer connecting cables connected analog connections work V.90 usually not.

Dissemination

The predominant use of V.90 telephone modem connections, the connection of analog subscriber connections by means V.90 client modems to digital V.90 dial-in node of dial-in Internet access providers.

For residential customers, the digital connection to the telephone network is given only for ISDN connections. Since operable because V.90 host modems are relatively expensive, ISDN offers better all-digital transmission method between two ISDN lines and ISDN in many European countries is widespread, is usually used ISDN for remote data transmission via a direct phone connection between private customers in Europe.

V.92

The V.92 standard increases the theoretically possible transmission speed of the user-side client modems at 48 kbit / s (using PCM Upstream ). Here, as already in the V.90 connection from the host to the client modem, the PCM encoding in network clock harnessed - the simultaneous use of PCM encoding in both transmit and receive direction, however, is not possible technically. With an active upstream PCM, therefore, the modulation is changed in the receiving direction of the client modem in a conventional analog modulation method. The switching of PCM coding between the transmit and the receive direction is realized dynamically by the current workload of the transmit and receive channel.

Furthermore, it is shortened by the V.92 for dial-up ( handshake) time required (Quick Connect); finally was with V.92 also compared to the previous method improved V.42bis compression standard V.44 created, as well as a waiting call answered implemented the possibility of holding a V.92 modem connection while the participants. Although the detection of Call Waiting works with most newer modems, keeping the connection is, however, not supported by any well-known dial-in ISP in Germany.

Most dial- backbone operators in Germany have so far not implemented V.92; Exceptions are the dial- backbones of BT and Freenet AG, in which individual V.92 functions such as Quick Connect and V.44 compression are implemented. PCM upstream is not offered in Germany. The pure Call Waiting detection without being implemented by the ISP, but works with any corresponding V.92 modem that has this feature built.

History

Precursor of V.90 were the transmission method of X2 3Com/US Robotics and K56flex (also known as K56 ) Rockwell. Both methods were about the same time on the market, however, were not compatible with each other. In 1998, this was developed to a common standard V.90.

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