---
title: "Modems or Magic"
type: "article"
slug: "modems-or-magic"
url: "http://localhost/article/modems-or-magic/"
markdown_url: "http://localhost/article/modems-or-magic.md"
published_at: "2026-11-09T22:32:59+00:00"
modified_at: "2026-11-09T22:32:59+00:00"
featured_image:
  url: "http://localhost/wp-content/uploads/2022/04/20230809-041332.jpg"
excerpt: "My previous article (Modems or Black Magic, UPDATE! Jan. 94) discussed the basic elements of communications by modem, but there is a lot more of this Magic. The field of the Information/Communications Super Highway is very complex and new developments appear at fast pace, such as an announcement by a company that they are developing…"
post_tag:
  - name: "Telecomputing"
    slug: "telecomputing"
    taxonomy: "post_tag"
    url: "http://localhost/tag/telecomputing/"
indiv:
  - name: "Abed Kahale"
    slug: "abed-kahale"
    taxonomy: "indiv"
    url: "http://localhost/indiv/abed-kahale/"
publication: "Update Magazine"
publication_r:
  id: 10283
  title: "Update Magazine"
  type: "periodical"
  url: "http://localhost/periodical/update-magazine/"
authors: "Abed Kahale"
issues_articles:
  - id: 26517
    title: "Update October 1994"
    type: "issue"
    url: "http://localhost/issue/ts-2068-up-date-october-1994/"
pages: "13-15"
pubdate: "October 1994"
archive_link: false
volumeissue: "vn"
---

# Modems or Magic

My previous article (Modems or Black Magic, UPDATE! Jan. 94) discussed the basic elements of communications by modem, but there is a lot more of this *Magic*. The field of the ***Information/Communications Super Highway*** is very complex and new developments appear at fast pace, such as an announcement by a company that they are developing a direct wire communications system that obsoletes modems. With the prevailing use of fiber-optics lines, communications will be in the millions of data bits transfer rate such as the cable TV system is now.

[![Graphic: "Information / Communications Super Highway" lettered in a circle](http://localhost/wp-content/uploads/2026/11/update-new-modems-or-magic-1-932x1024.png)](http://localhost/wp-content/uploads/2026/11/update-new-modems-or-magic-1.png)Graphic: "Information / Communications Super Highway" lettered in a circle

Computers normally process 16 or 32 bit words of digital information on their internal data buses. Even the computer’s parallel printer port offers a data path that is 8 bits wide. However there is only one transmitting wire (the other wire is for signal return) in a telephone line. Modems must break down (Modulate) each digital word into a sequence of audio signals, and send each signal in turn along the telephone line. An 8-bit data word is sent as eight 1-bit signals.

Most modems for computer communication are asynchronous or *un*synchronized. (synchronous modems are more expensive, they require complex circuitry.) Because two communicating modems are operating asynchronously, the receiving machine must know ***when*** a stream of data is being sent, ***where*** each stream of data starts and ends, and whether the stream is correct or not (see the previous article). To accomplish this feat, extra bits are added to the data to organize it into a standard sequence known as a dataframe having a start bit, a data bit, a parity bit and a stop bit.

### Port Connections and Signals

The translation from parallel data words to a serial data stream takes place within the computer’s serial port circuit in an IC known as a Universal Asynchronous Receiver/Transmitter (UART). Logic 1’s and 0’s are converted into bipolar signals. Logic 1’s are translated to – 5 to – 15 volt levels (mark), and logic 0’s are translated to + 5 to + 15 volt levels (space). Those bipolar signals are supplied to the TX (transmit) pin of a standard serial communications port. Those physical connections are also called RS-232C ports. (RS-232C is a standard of the Electronic Industry Association) In Europe, the RS-232C standard is called V.24.

An external modem connected to the serial port MOdulates the bipolar TX signal into an audio signal which is sent over the telephone line. Audio signals received by the modem and DEModulated into bipolar signals are returned to the serial port’s RX (receive) pin. An internal modem is a plug-in unit installed inside a computer containing a serial port. For most serial port connections, only TX , RX and ground wires are needed to establish a working port.

### Communications Software

All communications software must be able to operate in two distinct modes: the **command mode**, and the **terminal mode.** In the command mode, any commands issued control either the communications software, the host computer, or the modem connected to it. In the terminal mode, any commands issued are sent directly to the modem, or they are sent through the modem and to the remote computer modem.

Typical instructions that might be issued in the command mode include retrieving a telephone number from a data log, setting the proper communications protocol, dialing a number, and saving any data that is received to a disk.

In the terminal mode, all keyboard (or disk) input is sent directly to the modem. If the modem is *off-line* or not connected to a remote computer modem, the input is recognized as modem commands. if the modem is *on-line* and has an active connection to a remote computer modem, all inputs are sent through the modem to the remote computer unless certain special control characters precede the data.

The difference between the command mode (in which commands are acted on locally) and terminal mode (in which commands affect the remote computer) is probably the most confusing aspect of modem communications for beginners.

One common mistake made by newcomers, for example, is to initiate a file UPLOAD in terminal mode without issuing a corresponding command in the command mode. The first command is required to tell the computer to send a file. The second is necessary to instruct the host (remote) computer to receive it and store it on disk.

### Flow Control

To control the flow of data between two modems, software codes (such as XON and XOFF) are passed between them. Communications software interprets such codes and controls modem operation accordingly.

However, the flow of data between a computer and modem is not always controlled by software codes. Instead, additional signal lines in an RS-232 port allow for hardware flow control (or handshaking). The Request To Send (RTS) line tells a modem to prepare to receive data from the computer. Once the modem is ready to send, it will return a Clear To Send (CTS) signal to the computer. RTS and CTS signals act together to handle data transfer

When the computer is ready for operation (but not necessarily ready to send data), it asserts Its Data Terminal Ready (DTR) signal. DTR must remain active throughout the entire connection time. The modem sends a Data Set Ready (DSR) signal to the computer after the modem has been activated, and has finished self-tests or preparation for connection. The DTR and DSR signals establish the connection between modem and computer, but they do not control the flow of data between the two.

If a telephone ring signal is detected at the modem, a Ring Indicator (RI) signal is sent to the computer. When the modem subsequently picks up the ringing line and detects the presence of carrier, a Data Carrier Detect (DCD) signal is passed to the computer. Ring and carrier detect signals invoke the communication software to receive communication from a distant modem.

### Modulating the Signal

A modem transmits data by generating a carrier that is then modulated. Several different methods of signal modulation have been developed through the years to improve the efficiency of data transfers. Of course, two modems must be capable of the same modulation design for successful communications.

Two modems that are communicating with each other must generate different carrier frequencies. Each communication standard defines the transmitting and receiving carrier frequency. For example, for 300 BAUD communication, one modem must have a carrier center frequency of 1170 Hz, and the second must have a center frequency of 2125 Hz. By convention, the modem initiating the call, or the originate-mode modem, has the lower carrier frequency. The modem receiving the call, or the answer-mode modem, has the higher carrier frequency.

In the early days of modem communication, each transition of the audio carrier signal represented a single bit. Each transition is known as a BAUD, so the BAUD rate equaled the transmission rate in bits-per-second or BPS. Unlike those early modems (like the TS-2050), modern modem designs allow two, three, four, or more bits to be encoded into every audio signal transition (or BAUD). This means that modem throughput now equals two, three, or four times the BAUD rate being carried on the telephone line.

For example, a modem operating at 2400 BAUD (2400 audio signal transitions per second) can carry 4800 BPS if two bits are encoded onto every BAUD. The same 2400 BAUD modem can carry 9600 BPS if 4 bits are encoded onto every BAUD. Today, the modem’s BAUD rate rarely matches the modem’s bit rate in BPS. If the modem were operating at 4800 BAUD with 3-bit encoding, it would be transmitting 14,400 BPS (14.4 KBPS) V.32bis, and so on.

The concept of encoding is different from data compression. Encoding transfers all original data bits from one system to another. While data compression replaces repeating sequences of bits with much shorter bit sequences known as symbols or tokens. Encoding designs and data compression are described in more detail later in this article.

### Modulation Designs

All waveforms have three basic characteristics — amplitude, frequency, and phase. Each of those characteristics can be adjusted to represent a bit.

Frequency-Shift Keying(FSK) is similar To Frequency Modulation (FM) where only the frequency of a carrier is changed, and it is one of the oldest modulation design still in use. FSK sends a logic 1 at one particular frequency (usually 1750 Hz), and a logic 0 is sent at another discrete frequency (often 1080 Hz). Frequencies are typically sent at 300 BAUD, and each BAUD can carry one bit, so FSK can send 300 BPS. This early technique resulted in the classical “BAUD equals BPS” which still exists today. Bell 103 was the widely accepted modem standard — simple FSK modulation at 300 BAUD. This is the only standard in which the data rate matches the BAUD rate.

Phase-Shift Keying (PSK) is a close cousin of FSK, but the phase timing of a carrier wave is altered while the carrier’s frequency stays the same. A logic 1 or logic 0 is represented by the alteration of the carrier’s phase. Because phase can be shifted in several precise increments such as 0°, 90°, 180° and 270°. PSK can encode one, two, three, or more bits bit per BAUD. A 1200-BAUD modem using PSK can transmit 2400 BPS over an 1800 Hz carrier. PSK in conjunction with FSK can encode even more bits per BAUD.

In Quadrature-Amplitude Modulation (QAM), both the phase and amplitude of the wave are modulated to encode up to six bits onto every BAUD, although only four bits are usually reserved for data. Most QAM modems have a 1700-Hz or 1800-Hz carrier and a base rate of 2400 BAUD, so they can carry up to 9600 BPS.

Trellis-Coded Quadrature-Amplitude Modulation (TCQAM or TCM) also generates an 1800-Hz carrier at a 2400-BAUD base rate, but it uses the full 6-bit encoding capability of QAM to provide a rate of 14400 BPS. TCM is now the most popular modulation design for high-performance modems because data can be checked on-the-fly with high reliability.

As the speed increases, line noise becomes very critical to communications and ***error correction*** becomes vital.

### MNP Standards

The Microcom Networking Protocol (MNP) is a complete hierarchy of standards developed during the mid 1980s. They were designed to work with other modem technologies to provide ***error correction*** and ***data compression***. Originally developed by Microcom Inc., the protocol is now in the public domain.

MNP provides error control and data compression when one modem is communicating with another modem that supports MNP. MNP class 4 is specified by the ITU (International Telecommunications Union) V.42 as a backup error control design for LAPM (Link Access Procedure for Modems) in the event that V.24 cannot be invoked. Out of ten MNP levels, most modern modems support MNP2 to MNP5. Each MNP class has all the features of the previous class plus its own.

MNP *class 2: (stream mode):* Data is sent in both directions at the same time. That results in a speed about 84% as fast as data transmissions with no error correction.

MNP *class 3:* The sending modem strips the start and stop bits from a data block before sending it. The receiving modem then adds start and stop bits before passing the data to the receiving computer. It is about 8% faster than data transmissions with no error correction.

MNP *class 4*: A protocol (with some data compression) that checks telephone connection quality and uses adaptive packet assembly. On a noise free line, the modem sends larger blocks of data such as 1024 bytes. If the line is noisy, the modem sends smaller blocks of data such as 128 bytes so that less data must be resent in the event of an error.

MNP *class 5:* Provides data compression by detecting redundant data and converting it to fewer bits or tokens, thus increasing effective data throughput. A receiving modem decompresses the data before passing it to the receiving computer. MNP5 can speed data transmissions by as much as a factor of two compared with protocols having no data compression or error correction design.

MNP *class 6: Universal link negotiation* allows modems to obtain maximum performance from a line. The modems start at low speeds, then move to higher speeds until the best speed that both modems can work at is reached.

### The Vees — The Most Common ITU Standards

V.17—Describes an application-specific modulation design for Group 3 fax which provides two-wires half-duplex, trellis-coded transmission at 7200, 9600, 12000, and 14400 BPS. Despite its low number, this is a recently introduced standard.

V.21—Provides the specifications for 300 BPS FSK serial modems (based on BELL103).

V.22-Provides the specifications for 1200 BPS (600 baud) PSK modems (based upon BELL212A)

V.32—Defines the first of the truly modern modems as a 9600/4800-bps; QAM, full-duplex modem operating at 2400 baud. This standard also incorporates trellis coding and echo cancellation to produce a stable, reliable, high-speed modem.

V.32bis—A fairly new standard extending the V.32 specification to define a 4800/7200/9600/12000/14400 BPS TCQAM full-duplex modem operating at 2400 baud. Trellis coding, automatic transfer rate negotiation, and echo cancellation make this type of modem one of the most popular and least expensive for general communication.

V.32fast—The temporary name of a standard that the ITU has not yet completed. When finished, it will probably replace V.32bis with speeds up to 28,800 BPS. It is anticipated that this will be the last analog protocol, eventually giving way to all-digital protocols as local telephone systems become entirely digital.

V.34—The ratified version of V.32fast. It provides for data speeds up to 128 KBPS with transmission rates as high as 3429 BAUD.

V.42—Defines a two-stage process of detection and negotiation for LAPM error control.

V.42bis—Extends the V.42 standard to include data compression.

*Reference: [Stephen J. Bigelow](http://localhost/indiv/stephen-j-bigelow/), Electronics Now*