Modems: Theory of Operation

Modems are devices that let computers transmit information over ordinary telephone lines. The word explains how the device works: modem is an acronym for “modulator/demodulator.” Modems translate a stream of information into a series of tones (modulation) at one end of the telephone line, and translate the tones back into the serial stream at the other end of the connection (demodulation). Most modems are bidirectional —every modem contains both a modulator and a demodulator, so a data transfer can take place in both directions simultaneously.

Modems have a flexibility that is unparalleled by other communications technologies. Because modems work with standard telephone lines, and use the public telephone network to route their conversations, any computer that is equipped with a modem and a telephone line can communicate with any other computer that has a modem and a telephone line, anywhere in the world. Modems thus bypass firewalls, packet filters, and intrusion detection systems.

What’s more, even in this age of corporate LANs, cable modems, and DSL links, dialup modems are still the single most common way that people access the Internet. This trend is likely to continue through the first decade of the 21st century because dialup access is dramatically cheaper to offer than high-speed, always-on services.

Serial Interfaces

Information inside most computers moves in packets of 8, 16, 32, or 64 bits at a time, using 8, 16, 32, or 64 individual wires. When information leaves a computer, however, it is often organized into a series of single bits that are transmitted sequentially. Often, these bits are grouped into 8-bit bytes for purposes of error checking or special encoding. Serial interfaces transmit information as a series of pulses over a single wire. A special pulse called the start bit signifies the start of each character. The data is then sent down the wire, one bit at a time, after which another special pulse called the stop bit is sent (see Figure 10-1) .

Figure 10-1. A serial interface sending the letter K (ASCII 75)

Because a serial interface can be set up with only three wires (transmit data, receive data, and ground), it’s often used with terminals. With additional wires, serial interfaces can be used to control modems, allowing computers to make and receive telephone calls.

The RS-232 Serial Protocol

One of the most common serial interfaces is based on the RS-232 standard. This standard was developed to allow individuals to use remote computer systems over dialup telephone lines with remote terminals. The standard includes provisions for a remote terminal that is connected to a modem that places a telephone call, a modem that answers the telephone call, and a computer that is connected to that modem. The terminal can be connected directly to the computer, eliminating the need for two modems, through the use of a special device called a null modem adapter. Sometimes this device is built directly into a cable, in which case the cable is called a null modem cable.

Tip

Universal Serial Bus (USB), Firewire, and even Ethernet are all high-speed serial systems that use low-level serial protocols to transport packets from which higher-level protocols are built. This chapter does not concern itself with these serial interfaces.

The basic configuration of a terminal and a computer connected by two modems is shown in Figure 10-2.

Figure 10-2. A terminal and a computer connected by two modems

The computer and terminal are called data terminal equipment (DTE), while the modems are called data communication equipment (DCE). The standard RS-232 connector is a 25-pin D-shell type connector; only 9 pins are used to connect the DTE and DCE sides together.

Of these nine pins, only transmit data (pin 2), receive data (pin 3), and signal ground (pin 7) are needed for directly wired communications. Five pins (2, 3, 7, 8, and 20) are needed for proper operation of modems (although most also use pins 4 and 5). Frame ground (pin 1) was originally used to connect electrically the physical frame (chassis) of the DCE and the frame of the DTE to reduce electrical hazards and static.

Because only 8 pins of the 25-pin RS-232 connector are used, the computer industry has largely moved to smaller connectors that follow the 9-pin RS-232-C standard. Most PCs are equipped with this 9-pin RS-232-C connector, shown in Figure 10-3.

Figure 10-3. The standard 9-pin RS-232-C connector

The pinouts for the 25-pin RS-232 and 9-pin RS-232-C are both summarized in Table 10-1.

Table 10-1. RS-232 pin assignments for a 25-pin connector

25-pin RS-232 location	9-pin RS-232-C location	Code	Name	Description
1	n/a	FG	Frame Ground	Chassis ground of equipment. (Note: this pin is historical; modern systems don’t connect the electrical ground of different components together because such a connection causes more problems than it solves.)
2	3	TD (or TxD)	Transmit Data	Data transmitted from the computer or terminal to the modem.
3	2	RD (or RxD)	Receive Data	Data transmitted from the modem to the computer.
4	7	RTS	Request to Send	Tells the modem when it can transmit data. Sometimes the computer is busy and needs to have the modem wait before the next character is transmitted. Used for “hardware flow control.”
5	8	CTS	Clear to Send	Tells the computer when it’s OK to transmit data. Sometimes the modem is busy and needs to have the computer wait before the next character is transmitted. Used for “hardware flow control.”
6	6	DSR	Data Set Ready	Tells the computer that the modem is turned on. The computer should not send the modem commands if this signal is not present.
7	5	SG	Signal Ground	Reference point for all signal voltages.
8	1	DCD	Data Carrier Detect	Tells the computer that the modem is connected by telephone with another modem.Unix may use this signal to tell it when to display a `login`: banner.
20	4	DTR	Data Terminal Ready	Tells the modem that the computer is turned on and ready to accept connections. The modem should not answer the telephone—and it should automatically hang up on an established conversation—if this signal is not present.
22	9	RI	Ring Indicator	Tells the computer that the telephone isringing.

A number of nonstandard RS-232 connectors are also in use. The Apple Macintosh computer uses a circular 9-pin DIN connector, and there are several popular (and incompatible) systems for using RJ-11 and RJ-45 modular jacks.

In general, you should avoid using any RS-232 system that does not carry all eight signals between the data set and the data terminal in a dialup environment.

Originate and Answer

Modern modems can both place and receive telephone calls. After a connection between two modems is established, information that each modem receives on the TD pin is translated into a series of tones that are sent down the telephone line. Likewise, each modem takes the tones that it receives through its telephone connection, passes them through a series of filters and detectors, and eventually translates them back into data that is transmitted on the RD pin.

To allow modems to transmit and receive information at the same time, different tones are used for each direction of data transfer. By convention, the modem that places the telephone call runs in originate mode and uses one set of tones, while the modem that receives the telephone call operates in answer mode and uses another set of tones.

High-speed modems have additional electronics inside them that perform data compression before the data is translated into tones. Some high-speed standards automatically reallocate their audio spectrum as the call progresses to maximize signal clarity and thus maximize data transfer speed. Others allocate a high-speed channel to the answering modem and a low-speed channel to the originating modem, with provisions for swapping channels should the need arise.

Baud and bps

Early computer modems commonly operated at 110 or 300 baud, transmitting information at a rate of 10 or 30 characters per second, respectively. Today most analog modems sold deliver the theoretical maximum download speed of 56 Kbps.^[101] Special modems on digital ISDN lines can deliver 128 Kbps.

Five to twelve bits are required to transmit a “standard” character, depending on whether we make upper-/lowercase available, transmit check-bits, and so on. A multibyte character code may require many times that for each character. The standard ISO 8859-1 character set requires eight bits per character, and simple ASCII requires seven bits. Computer data transmitted over a serial line usually consists of one start bit, seven or eight data bits, one parity or space bit, and one stop bit. The number of characters per second (cps) is thus usually equal to the number of bits per second divided by 10.

The Origin of Baud

Baud is named after the 19th-century French inventor, J. M. E. Baudot. He invented a method of encoding letters and digits into bit patterns for transmission. A 5-bit descendent of his code is still used in today’s TELEX systems. Baud is not an abbreviation for “bits-audio,” although that is a commonly used equivalent.

The word “baud” refers to the number of audible tokens per second that are sent over the telephone line. On 110- and 300-bits-per-second (bps) modems, the baud rate usually equals the bps rate. On 1,200-, 2,400-, and higher bps modems, a variety of audible encoding techniques are used to cram more information into each audible token. TDD phone devices for the deaf generally use a lower-speed modem than modern computers usually do.

We have seen some writers refer to this unit of measure as the bawd. In addition to being a sad statement about the level of proofreading involved, we cannot help but wonder if this is not actually a measure of how fast pornography is downloaded on their modems.

^[101]The 56 Kb speed is the maximum theoretical speed because the U.S. phone system digitizes voice calls at 56 Kb samples per second. Thus, this is the maximum, but this is often not achievable in practice.

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