The acronym is comprised of the primary two functions of the device: to MOdulate and DEModulate a stream of information for communication with another modem. In the sense of our topic, modulation is the conversion of the digital data within the computer to an analog form. An analog signal can be thought of as having an infinite number of levels within a range of amplitude, while the digital format has a discrete number of levels within that range-a handicap ramp vs. a flight of steps, or a rotary volume control on a radio vs. the up/down volume buttons on your TV remote control.

Most of us are aware that our computers run with binary data, i.e., a byte of information (8 bits) is comprised of 0's and 1's. These are the two states of binary-off/on, high/low, black/white-with nothing in between. The early modems (of the 300 bps [bits per second] family) simply converted the two states into two frequencies or tones. Within the computer, data is transmitted over buses - parallel pipelines of multiconductor ribbon cables. The "last mile" of your Internet connection has a single pipeline, the pair of copper conductors that is your telephone line. (Cable and DSL connections require special modems but the principle prevails.) The gobs of data on the multilane highways have to be funneled down to the serial port's two lane highway for transmission beyond your home. Information theory states that the possible amount of information correlates to bandwidth, which correlates to frequency-which brings us to another problem.

The human bandwidth for hearing is from 20 to 20,000 Hz (Hertz-we used to say cps, cycles per second, but that is now reserved as a unit of measurement for things mechanical). Devices employing this range of audio are referred to as HiFi (High Fidelity) systems. Developers of the telephone system realized early on that we only needed a small bandwidth for voice communications, around 600 Hz. The simple modulation of the 300 bps modems hit this barrier. How did we get around this limit? Compression and standards assure that modem will be able to talk to modem on the World Wide Web and other Internets.

It should be of no surprise to you that your computer is fast. The serial port (an intermediary path from the modem to the outside world) is capable of 115,200 bps. The present common modem for home use is the 56Kbps version running at slightly less (53Kbps) due to FCC regulation. This difference in speed gives the modem time to assemble the data into packets and apply compression algorithms more complicated than the above described simple modulation. Multiple pitches of sound, along with phase relationships, can represent combinations of bits, greatly reducing the volume of content to be transmitted. Compression algorithms look for redundancy in data streams. For example, if the data contains a string of four 0's, a single sound could be sent rather than the four bits. These algorithms are pretty smart. If the data to be transmitted has already been compressed, such as a JPEG image, the program knows not to compress it again.

All in all, modems are considerably slower than your PC. This requires a buffer, a special register to hold data while the modem catches up with the input from the PC. For this feature we need flow control. Flow control can be by software (Winmodem) or hardware. Software flow control uses commands, such as S to pause the PC and Q to resume sending. This occurs as the buffer is partially emptied and ready for more data. Hardware flow control uses two channels (separate from the data channel): RTS-Request To Send and CTS-Clear To Send. The hardware version is much preferred over the software version. Line noise or other interference can hamper communications; further, binary files cannot be sent over a modem using software flow control as the file could contain a sequence that would invoke the pause control.

We mentioned line noise as an impediment to communication. This is particularly prevalent in less densely populated areas (such as western North Carolina) where economics forced telephone companies to bundle widely separated neighbors onto multiplexed lines. Squirrels chewing on insulation also do not help. An error correcting algorithm reads the packets created by compression and produces a piece of data known as a checksum for the information to be transmitted. This number is then added to the packet before sending it. At the receiving modem, the packet is read by the same algorithm and another checksum is produced. Unless the checksums are identical, inferring no corruption of the data, the transmission is repeated.

Everything has to have a starting point, even the Big Bang. How does communication by computer get going? The first step, as in any communication, is a common language. Originally developed for the Hayes modem, the AT (for Attention) command set became the standard for modem operation. All commands start with AT; such as: ATDT (use Dial Tone) or ATDP for pulse dialing (rotary phones). Some other examples are: ATH, Hang up; ATA, Answer the phone; ATZ, reset (Zero out) to the default settings. Registers within the modem hold the values of settings, typically with numbers of 0 or 1, for Off or On, or integers for levels. Two speaker control registers are the L and M (for Loudness and Mute). ATM0 turns the speaker off and ATL3 sets the speaker's volume level to its highest.

Prior to the establishment of a modem-to-modem connection, all data to the modem in your PC is considered a command. A sort of boot-up routine-wake up, dial my ISP, check my ID and password, handshake with the ISP's server (that's what all that squealing and squawking is about-the devices are checking how fast they can talk to each other)-initiates your connection to the Internet. Now all bits to the modem are seen as data. What if you want to send one of those AT commands to the modem? As politeness does not apply to inanimate objects, we simply interrupt their conversation. We do so by sending an escape sequence of three plus signs (+++) which keeps the connection but stops the transmission. Using a program such as HyperTerminal, we await an "OK" from the modem and then can send a command to change the speaker level, to hang up, or to go back online-ATO.

The aforementioned handshake is a line probing protocol to check for line impairment- how much noise is present, is the interference cyclic (the modems can synchronize to avoid transmission during that period), and is more than one digital to analog conversion employed. In the last case, transmissions are restricted to no more than 33.6 Kbps. To briefly summarize the process, let's look at the connection phase:

• Command to modem: wake up-modem initializes to default settings.
• Dial my ISP-off-hook the phone line, wait for dial tone, generate the dial tones (perhaps add a digit to get an outside line) by the modem's DSP (Digital Signal Processor) and place the call.
• ISP's server modem answers with a carrier tone.
• The modems "talk" and agree on settings to optimize transmissions.
• The modems decide which compression table they will use.
• To terminate communications, one of the modems drops its carrier tone. Since they both monitor this signal at all times, the other modem then hangs up.

If your "call" is to access a web site, your browser software will ask your ISP's server to request a connection to the website's server. Now that we have power to communicate between two computers, your PC and your ISP's server, let's follow a "conversation."

1. Your PC checks that the modem is set to send a transmission.
2. The data then streams, a bit at a time, into the modem's buffer.
3. The modem looks at the buffer contents, groups bits of data together, runs the error correcting algorithm, and generates a checksum adding it to the packet.
4. If the data is compressible, the compression algorithm looks up the previously agreed upon table to encode the complex sounds for the data to be transmitted.
5. The DSP then generates the sound indicated.
6. The receiving modem checks the sounds against its compression table and decompresses the packet into a string of bits.
7. This string is then checked by the receiving modem's error correcting algorithm, comparing the checksums.
8. If the checksums are valid, the data is then passed on to your computer program.

Earlier, we mentioned compression and standards to assure compatibility among modems. Along with the specification of speed, there is another parameter: the V.nn of the ITU (International Telecommunications Union). The first 56Kbps modems came in two models: X2 from US Robotics and K56flex from the Open 56K forum that include Lucent Technologies and Rockwell Semiconductor Systems. Before buying a modem, you had to check with your ISP as to which system they employed. In February of 1998, the ITU created the universal V.90 standard. Implemented in September, it specified parameters common to both systems. Other specifications were: V.34, which set the 28.8Kbps standard with fallback to 24.4K and 19.2K; V.42, for better error correction; and the latest, V.92, incorporating a quicker connect time and Modem-on-Hold (MOH) to accommodate call waiting.

The devices can be external, internal, or PC Card modems. Regardless of form, a modem requires three basics:

• Interface-a power source, connection to the computer, and a communication line.
• DSP-the Digital Signal Processor to perform the modulation/demodulation, compression, and error checking routines.
• Flow control-hardware or software, a controller must be present to affect the flow of data between the DSP and the computer.

Perhaps you never realized how many modems we live with in today's world. The subject we have discussed is that of POTS (Plain Old Telephone Service) modems. Think about ATMs, gas stations' "pay outside," swiping a credit card at the supermarket, and even traffic light systems. Broadband modems (cable, DSL, satellite) are now available to many of us. But, until availability, reliability, and price are more attractive to the general populace, POTS modems will be with us for some time to come.

Speaking of broadband, Les Cottrell of SLAC recently set a land speed record on Internet II of 2.3 Gbps across the Atlantic Ocean. That is equivalent to getting 180 700MB-full CDs in 1 minute.

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