Reference Series Table of Contents For This Issue

How Computers Work, Part I
August 2001• Vol.5 Issue 3
Page(s) 66-71 in print issue

The Driving Force Behind Modems
Analyze Their Ability To Propel Our Travels & Transmissions Online
You may know what service a modem provides, but you probably haven’t given much thought to how they work. The word “modem” is actually an acronym standing for modulator-demodulator, which succinctly describes what modems do.

Before going into too much detail, we must first explain that all computers store data digitally. To send that digital information over your analog phone line, the modem modulates, or converts, the data into an analog format. At the other end of the phone line, the receiving modem demodulates, or converts, the data back into its original digital form.

That’s the short of it. Once you get past the basics of the modem, issues such as modem speed crop up. Will a 56Kbps (kilobits per second) modem give you the speed you need? How does modulation work? Modulation alone will only give a modem a top speed of 600bps (bits per second). How do modems achieve faster speeds? Compression. This article explains how this all works, what flow control and line probing are, how error correction keeps data from becoming garbled, how PCs control modems, and the importance of standards such as V.90.

  Modulation-Demodulation. How does modulation convert digital data into analog form? It depends on the modem. Primitive modems, the 300bps modem for example, simply used changes in pitch, or frequency, to send data.

Data is usually stored in binary form as a string of 0s and 1s. Zero means “off” and 1 means “on.” Each 0 or 1 stands for 1 bit of datum. The modems used two tones to represent 0s and 1s. The modem sending the data generated the tones while the receiving modem listened and interpreted the tones as either 0s or 1s.

Computers send data to your modem much faster than your modem can transmit it over the phone line. For example, the top speed of most PC serial ports is 115,200bps, whereas the top speed of most modems is 56,000bps.

This speed difference means the modem has time to assemble the data into larger chunks before sending it. The data is then sent using more complicated sounds than the old, simplistic on/off tones. Each distinct sound represents a certain combination of bits of data. For example, a single sound can now represent four data bits, dramatically increasing speed.

There are many different possible combinations for bits of data. Those with “more complicated sounds” often involve multiple frequencies and phase relations and need compression to travel quickly through the Internet. Compression is any method of condensing information so it can be stored in less space or transmitted in less time. Although data compression happens in many ways, a compression program generally looks for redundancies in a file and then compresses the identical pieces of data into one representative token. For example, if the data your computer is sending has a string of four 0s, one signal could be used to represent all four bits.

Not all data needs compressed, however. Some files, especially graphics files or files that have already been compressed, such as Zip file archives, don’t benefit from compression. Plain text or word processing files, however, are generally very compressible.

On the other hand, compressing already compressed data can actually slow data transmission. Therefore, compression schemes in today’s modems analyze what type of data they’re sending and turn compression on or off depending on the data.

  Moderating Flow Control. As we noted before, your computer can send data to your modem faster than the modem can process it.

Modems have a buffer to store information in while compressing it, but that buffer’s not infinite. When the buffer fills up, the modem sends a signal to the computer telling it to stop sending information until more room clears in the buffer. As space opens in the buffer, the modem sends another signal to the computer to let it know that the modem’s ready to receive data again. This process repeats continually as the buffer fills and empties.

Either hardware or software moderates the flow control. Software flow control uses a control signal, such as control-S, to tell the computer to pause, and another, such as control-Q, to tell the computer to resume. Hardware flow control uses two channels in the modem’s connection to the computer separate from the data channel (through which the modem sends and receives data and commands). The two channels are designated as RTS(request to send) and CTS (clear to send). The modem sends a signal on the RTS channel when the buffer is full (in response to the computer’s request about sending data), or it sends a signal on the CTS channel when ready to receive data.

Hardware flow control is the preferred and more reliable method of the two. Line noise can generate the software flow control pause command, causing it to discontinue the transmission until the resume command is sent. Also, binary files cannot be sent over modems that have software flow control because the binary file might contain in it the pause command. If that is the case, that part of the binary file would fail to transmit; when it would be recognized, that part would be interpreted as a pause command.

  Error Correction Importance. No matter the speed of your modem, it’s not much use to you if any hint of line noise cor rupts the data. At best, the corrupted data means the information you see on-screen is slightly garbled, with weird characters and letters appearing where they shouldn’t be. At worst, corrupt data can make an entire downloaded file useless.

Error correction, then, is as important to modems as compression. Thanks to compression, data is already broken into blocks. Error correction looks at these blocks of data and runs an algorithm (a list of instructions that can be used to accomplish a task) that represents the combination and sequence of bits in that block of data as a number, or checksum. The checksum is then attached to the block of data and transmitted with it.

At the receiving end, the modem takes each block of data it receives and runs the same algorithm that the transmitting modem did. If the receiving modem discovers that its checksum doesn’t match the one originally attached to the block of data, it orders the transmitting modem to resend that block of data.

  Speak A Command Language. So how does your computer tell the modem to pick up the phone and dial a number or hang up? It does this through the modem’s command language. All modems use the same basic command language, called the AT command set. In communications, the AT command set represents a group of instructions that activate certain functions on a modem. Although the AT command set was originally developed for the Hayes modem, it has now become the standard for all modems produced today.

Having one standard modem language is vital. Otherwise, software developers would have to write programs to work with hundreds of different types of modems. Something as simple as the command to dial a number could differ from modem to modem. With a common language, however, all modems perform the basic commands the same.

The AT (attention) command precedes all commands. An example of an AT command is the command ATDT, which initiates touch-tone dialing. ATDP, on the other hand, initiates dialing using the pulse system (of rotary phones). Other common commands include ATH (hang up), ATA (answer the phone), and ATZ (reset the modem to its default settings).

A host of registers change the modem’s settings, as well. This can include everything from adjusting the speaker volume to choosing what type of flow control to use. The settings are changed by sending the AT command, followed by the register, then the setting. Settings are usually represented by a number, such as 0 for off and 1 for on. The speaker control registers, for example, are L and M; L sets the loudness, M sets whether the speakers are on or off. The command ATM0 turns the speaker off and the command ATL3 sets the speaker’s volume at its highest.

How does your modem know the difference between the data you want to send to another modem versus a modem command? Simply put, until your modem connects to another modem, all the data you send the modem is interpreted as a command. For example, if your computer tries to send a request to retrieve a Web address and your modem isn’t connected online, the modem initially tries to read the request as a modem command. However, because it isn’t recognizable as such, it simply returns an error message.

Conversely, if the modem is connected online or to another modem, you can’t send it a command until you get its attention, and that’s where an escape sequence enters the equation. The escape sequence that all compatible modems use is three plus (+) signs in a row. Once the modem sees +++, it stays connected to the other modem, but stops sending data. And if you use a terminal program, such as HyperTerminal, you see the response from your modem as “OK” while it awaits another command from you. Then, you can change the settings, hang up (ATH), or go back online (ATO).

  Line Breakdown. Modems use line probing to test the quality of telephone line connections. This feature was introduced with 28,800bps modems and has dramatically improved with the standardization of 56Kbps V.90 modems.

As two modems negotiate a connection, line probing is initiated when your modem directs the receiving modem to transmit a line probe signal. The receiving modem complies by sending the signal, which your modem then analyzes to see what line impairments are present.

Line impairment, as the name implies, refers to interference on a phone line that impairs the transmission of data from one modem to another. By analyzing the line probe, your modem can work out a connection that avoids the interference. For example, if your modem recognizes interference that repeats at a regular interval, it can synchronize itself with the other modem so that no data is transmitted during the interference.

Line probing also determines if there are too many digital-to-analog conversions. If there is more than one conversion present, your modem will be limited to a top speed of 33,600bps. With most phone connections, however, consumers won’t need to worry about this aspect because there should be only one digital-to-analog conversion: at the telephone company’s switching station. But, if you’re in an office building with its own switch, or in a neighborhood where the phone company added an extra switch due to heavy phone usage, a second digital-to-analog conversion will occur. For a visual perspective of this whole process, see our two-page graphic within this article.

  The Evolution Of Modems. The first modems, developed in the 1950s and 1960s, connected computers to mainframe computers. Banks frequently needed modems to connect terminals to the bank’s main computer. Later, airlines used modems for their reservation system. In addition, news wire services, such as the Associated Press, also discovered the convenience of modems. The top speed of these modems ran from 75bps to 110bps, inconceivably slower than today’s 56Kbps modems.

In 1975, the MITS Altair computer, the first personal computer, was introduced. The introduction of personal computers was important for modems because it meant there was also a consumer market to boost their popularity. At the time, standard modems ran 110bps and 300bps. These modems did not plug directly into the telephone line; instead, users needed to dial a certain number and place the headset on an acoustic coupler.

Moreover, these modems weren’t as “smart” as today’s modems. Users couldn’t send a command from the computer to tell the modem to dial a number or disconnect. Users controlled modems by using switches and buttons, each of which had to be set correctly in order to make a connection.

Today, thanks to modular phone jacks that plug directly into the modem and the universal acceptance of the AT command set, we take the ease-of-use of modems for granted. And, as much as we may complain about transmission speed, today’s 56Kbps modems are 180 times faster than the 300bps modems that were common just 20 years ago.

  Beyond Speed. Modem speed isn’t the only convenience spoiling us these days. Just a couple of years ago, different modem manufacturers developed their own proprietary standards. As a result, their modems couldn’t communicate with modems made by other manufacturers—at least not at top speed.

Without the ITU(International Telecommunications Union) allocating universal standards, such as V.34, V.42, and V.90, we’d have a virtual tower of Babel running across the wires. Modems from different manufacturers would each have their proprietary modulation, compression, and error correction methods. As a result, U.S. Robotics modems could only connect to other U.S. Robotics modems and not to those from another manufacturer.

This happened with 56,000bps modems, and before that, with 14,400bps modems. In fact, 56Kbps modems, available since early 1997, have a quite a history behind them. Due to conflicting standards and other related problems, very few of the 56Kbps modems actually came close to reaching their promised speeds. While other modem speeds usually fall a little short, the 56Kbps modems fell well below their advertised speeds, often in the 40Kbps area.

Until 1998, there was never an overall industry standard established for 56Kbps. The two incompatible standards available at that time constantly struggled for control: x2 from U.S. Robotics and K56flex from the Open 56K forum, which included Lucent Technologies and Rockwell Semiconductor Systems.

This competition led to other problems for consumers as well. Users had to be careful when shopping for modems and ISPs (Internet service providers) because they needed to make sure their modem and ISP were compatible with one another. If they already had an ISP, they needed to ask which brand of 56Kbps modem their ISP used before heading to the store and buying the brand of modem their ISP supported. If users got fed up with their ISP and decided to switch, they might have been required to get another modem or find an ISP that supported the modem brand they already had. Needless to say, shopping for both a modem and ISP was more complicated than it is today.

All of that changed in February of 1998 when the ITU declared that a single unified standard for 56Kbps would be officially implemented in September of that year. The cooperation between 3Com Corporation (which bought U.S. Robotics in 1997 and spun it back off as its own company in September 2000) and Rockwell led to the unification of the K56flex and x2 technologies and the creation of the new universal standard, which is named V.90.

Today, although the V.90 standard has united 56Kbps modems to the point where they are theoretically capable of reaching a speed of 56,000bps, one more aspect still holds them back. The FCC (Federal Communications Commission) has specific restrictions on telephone line voltage that limit 56Kbps modems to a top speed of 53,000bps.

  Modem Variations. Not all modems fit into the internal mold. External modems are virtually identical to their inside counterparts and share the same components and design (with the exception of their physical structure and how they connect to the computer).

External modems connect through the computer’s serial port instead of through channels in an internal ISA (Industry Standard Architecture) or PCI (Peripheral Component Interconnect) slot. They also draw power from an adapter that plugs into a wall outlet, not from the computer itself as internal modems do. USB (Universal Serial Bus) external modems are similar, but they connect to the PC by drawing power through the USB port.

PC Card modems, another modem variation, are used in notebooks and include all of the standard modem components in a much smaller package. PC Card modems resemble internal modems, except they’re much smaller, have a slightly different connection to the computer, and are easily removed.

All these modems have three key components in common: the interface, the DSP (digital signal processor), and the controller. To understand the role of each component, see the sidebar “Three Key Components.”

Some modems only have the interface component, but these modems place such a heavy burden on the computer that they usually don’t perform as well as regular modems. These modems, called Winmodems or host-based modems, are relatively inexpensive because they use the PC’s processor to do all the work that other modems’ processors would otherwise have to do.

In addition to the physical variations among modems, variations exist regarding their purpose. The most common application for modems, of course, is using them to connect to the Internet through an ISP. However, modems have been used for other tasks in the past, and as long as computers of all shapes and sizes need to communicate with each other, new uses will be found for modems in the future. Today, numerous specialized computers need customized modems to communicate with other computers. These types of computers range from the credit card terminal at a department store to traffic light controllers and automatic teller machines.

  Future Forecast. What can we expect in the future when regular modems are already as fast as they can get? (By using the term “regular modems,” we refer to modems that use POTS[Plain Old Telephone Service], the same phone system you use for voice telephone calls.)

After all, with FCC restrictions on phone line voltage imposing a 53,000bps limitation on 56Kbps modems, can consumers really expect to blaze online at the speed of light someday? Well, that day hasn’t arrived yet, but there are new types of technologies and modems available that promise to break the 56Kbps barrier.

The two main challengers are cable modems and DSL (Digital Subscriber Line) routers (often referred to as DSL modems). Both offer huge improvements in speed and boast several added conveniences. For example, if you’re online using a regular modem, you can’t call anyone or receive phone calls while using the modem (unless you have “call waiting” and don’t mind being kicked offline when someone calls you). Cable and DSL, however, allow users to go online without tying up their phone lines.

These two high-speed Internet access options, as well as the ISDN (Integrated Services Digital Network ) method, are known as broadband services. Broadband transmission lets several data streams transmit simultaneously over common communications lines and offer incredible speed and other advantages over the traditional analog modem setup.

The major disadvantage of each of these broadband alternatives is that they aren’t readily accessible nationwide. Due to various complications involving old or faulty communications lines that need to be replaced or providers that require users to be within a certain number of feet of a central office, some U.S. residents will have to live without broadband services for now.

As of April 2001, 18% of U.S. households subscribed to broadband Internet services, according to a study by research firm TNSIntersearch. However, an additional 13%of those surveyed said they had planned to subscribe to broadband within six months.

When those surveyed were asked why they had yet to subscribe to the new services, the participants cited high prices and low availability as the main reasons for not making the step up. Twenty-two percent responded saying cable modems and DSLs cost too much while 15% said these services were not even offered as an option to them.

The best way to find out whether you qualify for any of these services is to visit the Web sites of your local telephone company, your area cable company, and national providers.

  Take The Broadband Route Online. So what will it be? Do you want to continue using the 56Kbps modem you already have or should you consider using cable modems, xDSL, or ISDN?

According to the most recent statistics, Web surfers are willing to invest in speedy Internet connections. However, until the overall price drops and broadband services become more readily available, the analog, dial-up modem won’t pack its bags for Florida just yet. If past experience in the computer industry is any indication of what intense competition can accomplish, consumers can look forward to falling prices and increased connection options for traveling online.  

by John Lalande

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Three Key Components

All modems, regardless of whether they are internal, external, or PC Card modems, include three key components in order to function.

1.The interface, which connects the modem to the computer, the power source, and a communications or telephone line.

2.The DSP (digital signal processor), which performs the modulation-demodulation, compression, and error correction tasks.

3.The controller, which operates the modem by carrying out commands as they are sent from the computer and controls the flow of data between the computer and DSP.

Terms To Know

AT command set— In communications, a group of instructions that activate certain functions on a modem. Although the AT command set originally was developed for the Hayes modem, it has become the standard for all modems.

checksum—A calculated value transmitted with data to check for errors in the transmission.

CTS (clear to send)—A hardware signal that indicates the receiving unit is ready to accept the information from the sending unit.

demodulation—The process of interpreting the variations of a modulated carrier signal into digital transmissions.

modulate—In a communications program, the process of changing a signal prior to transmission so signals are translated into meaningful information. For example, a frequency modulator decodes signals over a telephone line into data displayed on a computer screen.

RTS (request to send)—Communication between two devices in which one asks if it is all right to start transmitting data.

V.90—The modem standard reached by the ITU(International Telecommunications Union) on Feb. 5, 1998, for 56Kbps (kilobits per second) modems. The standard didn’t become official until sometime in the fall of 1998.

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© Copyright by Sandhills Publishing Company 2001. All rights reserved.