Modem Standards

Modems have been around for more than 30 years, so as you might imagine, a lot of standards have evolved. Table 15-3 summarizes some of the modem standards.

Table 15-3 Modem Standards

Standard

Speed

Comments

V.22

1200 bps (600 baud)

Mainly used outside the United States

V.22bis*

2400 bps (600 baud)

First widely deployed worldwide standard

V.32

4800/9600 (2400 baud)

Adjusts speed based on line quality

V.32bis*

14.4kbps (2400 baud)

Backward compatible with V.32

V.34

28.8 kbps

Backward compatible with V.32bis and V.32

V.42

28.8 kbps

Same speed as V.34, but with error-correction features

V.90

56 kbps (downstream), 33 kbps (upstream)

Created from two earlier competing standards, X2 and K56Flex

V.92

56 kbps/33 kbps (downstream/ upstream) or 48 kbps (each direction)

Connects and finds correct speed more quickly than V.90; allows "modem-on-hold"

*"bis" simply means "version 2."

*"bis" simply means "version 2."

Note that for some standards, the speed differs depending on the direction of transmission. Most applications today cause a lot more data to be sent toward the client side of the connection. For instance, when you sit at a PC and browse a web page, the web server sends many more bytes to you than you send to it. By using modem standards that use asymmetric rates, the maximum rate can be increased for the direction of data that needs the additional bandwidth.

V.92, the latest of these standards, has some very interesting features. You can configure it to transfer data at symmetric (48-kbps) rates or asymmetric rates equivalent to V.90's 56 kbps downstream and 33 kbps upstream. It also allows the modem to recognize "call waiting" signals from the telco, letting you take or make a call while keeping your modem connection up for a short time. Technically, you are not sending data and talking at the same time because data transmission is put "on hold," but it is a very convenient feature.

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