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

Working principle

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

Since in today's telephone network the telephone connections are almost entirely digitally transmitted on long-distance lines or between two exchanges , there is no longer any real, i.e. continuously analog, data transmission between two telephone modems. On the way from a PC via modem, exchange and vice versa, the data is converted at least twice from digital to analog and again from analog to digital signals.

For V.90 that the need transmitter used V.90 host modem via a digital connection available and from that generated PCM - digital signal must exactly once a digital-to-analog converter through (usually by the subscriber circuit in the digital exchange , to which is connected to the receiver), whereby the generated analog data are fed in by the converter in the network cycle, which enables a higher unidirectional transmission speed.

The commercially available V.90 client modems for connection to analog connections on the subscriber end can only receive the V.90-coded signals; in the sending direction, 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 is specified at 56 kbit / s (in contrast to the 64 kbit / s common in Europe). Therefore, the standard for 56k modems was defined at the same maximum rate. Since additional modulation information has to be transmitted, no more than 56 kbit / s is possible here either. In addition, in some digital networks the least significant (eighth) bit is used sporadically for the purposes of the network operator. This is not noticeable in telephone calls, but makes it impossible to use all eight bits for data transmission. Hence 56k from 8k Bd sampling rate and 7 usable bits per PCM symbol (actually voice sample ).

If the attenuation of the telephone line between the client modem and the digital-to-analog converter is too strong, the signal will be falsified and data errors will occur. Therefore, when the connection is established between the modem and the digital exchange, it is negotiated how many bits per symbol can be encoded without errors. If the line conditions change during an existing connection, the modulation is renegotiated (retrain). The achievable speed is usually between 40 and 50 kbit / s. On an inadequate or very strongly attenuated, long analog line, the data rate that can be achieved via PCM coding can be limited to 33.6 kbit / s or less, V.90 is of no use here. V.90 usually does not work on analog connections switched via multiplexer connection lines .


The predominant application of V.90 telephone modem connections, the connection of analog subscriber terminals by means of V.90 client modem to the digital V.90 dial-in nodes of Dialin - internet access providers .

For private customers, the digital connection to the telephone network is only available with ISDN connections . Since V.90 host modems that can be operated on it are relatively expensive, ISDN offers better fully digital transmission methods between two ISDN connections and ISDN is widespread in many European countries, ISDN is mostly used for remote data transmission via direct telephone connections between private customers in Europe.


The V.92 standard increases the theoretically possible transmission speed of the client modem on the subscriber side to 48 kbit / s (using PCM upstream ). As with the V.90 connection from the host to the client modem, the PCM coding is made usable in the network cycle - the simultaneous use of the PCM coding in both the send and receive directions is not technically possible. When the PCM upstream is active, the modulation in the receiving direction of the client modem is therefore switched to conventional analog modulation methods. The changeover of the PCM coding between the sending and receiving directions takes place dynamically according to the current utilization of the sending and receiving channels.

Furthermore, V.92 shortens the time required for dialing in (handshake) (quick connect); Finally, with V.92, an improved compression standard V.44 was created compared to the predecessor method V.42bis, as well as the possibility of maintaining a V.92 modem connection while the subscriber answers a waiting connection. The detection of call waiting while works with most newer modems, call hold, however, by any well-known dial-in - ISP supported in Germany.

Most dial-in backbone operators in Germany have not yet implemented V.92; Exceptions are the dial-in backbones from BT and Freenet AG , where individual V.92 functions such as Quickconnect and V.44 compression are implemented. PCM upstream is not offered in Germany. The pure call-waiting detection does not need to be implemented by the ISP, but works with any corresponding V.92 modem that has this function built in.


V.90 was preceded by the X2 transmission method from 3Com / US Robotics and K56flex (also called K56 +) from Rockwell. Both methods came on the market around the same time, but were not compatible with each other. In 1998 these were further developed into the common standard V.90.

Web links


  1. ITU-T Recommendations V.92: Enhancements to Recommendation V.90