ARCHIVED: What are some common modem terms?

Baud rate

Baud rate refers to modem speed (see below), measuring how fast the modem can transmit and receive the electrical pulses that carry data across a phone line. In older, slower modems, the baud rate is the same as the number of bits per second (see below); but newer, faster modems have ways of sending more than one bit of data with each pulse, so the number of bits per second may be higher than the baud rate.

Bits per second (bps)

Bps measures how much data can be transmitted in one second. Dividing bps by eight yields the number of bytes transmitted per second. One double-spaced typewritten page is equivalent to approximately 1,024 bytes, or one kilobyte.

Data compression

Simply put, data compression allows you to transfer more information in less time. Repetitive data patterns are represented by shorter codes. The modem transmits the codes instead of the data itself. Since fewer bits are transmitted, transfer speeds are faster. This is most effective with text files, which some compression schemes can compress at a ratio as high as four to one. Binary files such as programs and images are harder, sometimes impossible, to compress. V.42bis and MNP 5 are examples of common data compression protocols.

Error correction

Error correction is required for high speeds (14,400+ bps) and effective data compression. Error correction circuitry checks to make sure that the data received on one end is the same as the data sent on the other. Without error correction circuitry, modems could not compensate for even the smallest inconsistencies in phone lines. V.42 and MNP 1-4 are examples of common error correction protocols.

Integrated Services Digital Network (ISDN)

ISDN, digital telephone service, requires a special phone line and an Internet service provider (ISP) that supports ISDN. Indiana University currently does not offer ISDN, but a few of the local ISPs do for a monthly fee.

Modem speed

A modem's speed is determined by the number of kilobits of data transmitted per second (Kbps). It's a good rule of thumb to buy the fastest modem you can comfortably afford. Higher speed modems will still support connection with slower modems. Common modem speeds are:

14.4Kbps/14,400bps

These modems are fast enough to read email but are barely adequate for browsing the World Wide Web via a PPP connection.

28.8Kbps/28,800bps

Until recently, 28.8Kbps modems were the acceptable standard for modems. They will allow access to email and the World Wide Web.

33.6Kbps/33,600bps

This is still a common speed for older modems, but most new modems sold today are faster.

56Kbps/56,000bps

The 56Kbps connection speed is only theoretical in the United States, because FCC power restrictions limit real speed to about 52Kbps. Also, even though modems are marketed as being capable of 56Kbps, that speed applies only to downloads. Downloading is the data transfer going from your ISP to your computer. Uploading from your computer to your ISP takes place at the traditional 28.8 or 33.6Kbps speeds. This difference between downloading and uploading speeds is not a problem, since most of the time the only information you upload to an ISP would be keystrokes or mouse clicks. The vast majority of data transfer takes place as downloads.

Not all phone lines can handle 56Kbps connection speeds. The technology that allows connections faster than 33.6Kbps relies on there being only one conversion from digital to analog then back to digital on the phone line between your computer and your ISP. While this applies to a majority of phone lines in the United States, there is no guarantee that your phone line will support this faster communication rate.

Even with a single digital-analog-digital conversion on your phone line, you can realistically expect to achieve connection speeds only in the high 30s to low 50s Kbps. Any amount of noise present on a phone line can severely reduce a modem's transmission rate.

Also, if you use two 56K modems together (for example, to access your office computer from home using a product such as pcAnywhere), you will be making two analog-to-digital conversions, so you will be able to connect at only up to 33.6Kbps.

Point-to-Point Protocol (PPP)

PPP connects your personal computer to the Internet with a high-speed (14,400 bps or faster) modem. PPP programs give you the ability to run network applications like web browsers, telnet, and FTP. At IU, these programs would otherwise be usable only by computers that are connected directly to IU's network by Ethernet.

Types of modems

Internal or external

External modems are usually more expensive, but they are easier to install and troubleshoot and move to a new computer.

Internal modems cost less and don't take up desk space, but tend to be more difficult to install and configure. They also take up an expansion slot on your motherboard. If you are using an older IBM compatible (pre-1992), it may not support external modems faster than 2,400bps; check with the computer's manufacturer. Also, there are no third-party internal modems for desktop Macintoshes recommended by UITS.

Winmodems

Winmodems have less hardware than traditional modems. The product costs less to manufacture, but the design has liabilities. To make up for having less hardware, a winmodem requires more of your computer's CPU than a traditional modem. Also, as the name implies, a winmodem will work only with certain versions of Windows. It will not work with DOS, OS/2, or Windows NT.

PCMCIA/Cardbus/PC Card modems

PCMCIA (Personal Computer Memory Card International Association) modems, also known as PC Card modems, are small, thin modems plugged into special slots in many laptop computers. These modems are generally expensive compared to their non-PCMCIA counterparts, but in the case of a laptop, there is usually no other choice. PC Card modems are usually available in the standard PCMCIA 16-bit variety, or in the currently more common Cardbus 32-bit variety.