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3.21 What do all of the modem terms mean?


This item is from the PC Hardware FAQ, by Willie Lim and Ralph Valentino with numerous contributions by others. (v1.25).

3.21 What do all of the modem terms mean?

[From: arnoud@ijssel.hacktic.nl (Arnoud Martens)]

A modem (MOdulator-DEModulator) is a device capable of converting digital data from your computer into an analog signal that is suitable for transmission over low band width telephone lines. A modem thus makes it possible to connect two computers over a telephone line and exchange data between them.

Basically a modem picks up the phone, and dials a number. A modem on the other side will pick up the phone and the two modems will negotiate which protocol to use. When they agree the actual transmission of data can begin.

The major feature of a modem is the speed that it can achieve connecting to other modems. This speed is often expressed in baud or bits per second. The first is a feature of the line and specifies how much of the bandwidth of the phone channel is used and is fixed to 2400 baud. A baud is defined as the number of lines changes per second. Bits per second is the actual amount of data transmitted in one second. Most modems are capable of sending more than one bit per line transition by using very intelligent signal modulation techniques. So the bps can be eight times higher compared to the baud rate.

The modulation techniques that a modem uses are standarized by the ITU-T ( former CCITT), so that modems of different brands can connect to each other as they use the same modulation schemes. These standards are often incorporated in a protocol definition that is referred to by the letter V followed by a number. The most common protocols are:

    V21: (300 baud)
    V22bis: (2400 baud)
    V32: (9600 baud)  
    V32bis: (14400 baud)

A modem is often advertised only by its fastest protocol, most of these modems "speak" slower protocols as well.

There are also standards on using data compression by the modem, such as MNP5 and V42bis, and error control protocols (V42 and MNP4). These standards can reduce the transmitted data by a factor four, by using advanced compression techniques.

To give you an idea a how fast fast is in modem technology: V32bis transmits somewhat like 1600 characters per second (that is ~33% of 1 page of text). Transferring a file of 1Mb takes about 12 minutes. Using V42bis can speed up transmission to 4000 characters per second for uncompressed data.

Apart from these standardized protocols there are also faster protocols which are supported by some modem manufacturers. But remember anything faster than 14k4 is *not yet* standarized, and often different manufacturers use their own modulation scheme that allows only their own modems communicate at that speed. The most common high speed protocols are:

V32 terbo (19200 baud) V34 (28800 baud) or Vfast.

The standard for V34 is being worked on, it will be released somewhere in 1994. Some modem manufacturers already sell modems with the (prelimenary) V34 standard. If you are serious about buying a fast modem, upgradability to this standard should be provided by the manufacturer.

When you use your modem it is important to differentiate between command status and connect status of your modem. When you are connected to an another modem everything you send to the modem, will be transmitted to the other modem. In command mode everything you type will be recieved and interpreted by the modem. Command mode allows you to change the default settings for your modem.

In command mode it is likely that your modem will respond to the Hayes AT command set. "AT commands" all have prefix AT, and can be used to change the (default) settings of your modem. To check if your modem works, fire up a terminal program (such as kermit), connect to your modem (in kermit c [ENTER]) and issue AT [ENTER], if your modem works it should respond with OK. For a list of all "AT commands" see the manual of your modem, as most AT commands are modem specific.

If you buy a fax-modem, you should pay attention to a couple of things. First the modem must support Class 2 fax commands, furthermore automatic fax mode selection is a big pro. That means if you receive a call the modem is capable of detecting a fax message or a modem connection and act properly (start up a fax receiving program or spawn something like a login process on the connection).

Finally there is no best modem to get, brands and qualities change very fast, as do the prices. If you are interested in buying one, subscribe to the newsgroup comp.dcom.modems, most postings in this group are very brand oriented and you will recognize fast enough which users are satisfied over their modems and which are not.


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