Quick Links:
   Modem Transmission
   Modem Reception
   Auto Call Unit (ACU)
   Phone Lines
   Full or Half Duplex Transmission Modes
   Asynchronous and Synchronous Serial Data Transmission

[Note: This information was extracted from EHB 6-520, Section 5-5.]

The purpose of a modem (modulator demodulator) is to link the serial I/O ports of Data Terminal Equipment (DTE) via the telephone lines. (DTE is classified as any equipment that sends and/or receives digital data such as computers, printers, and terminals).

Modem Transmission

To transmit data over the phone lines, it is necessary to change the digital output of DTE into an analog signal. The analog signal must have a bandwidth narrow enough to fit into the 300 Hz to 3400 Hz frequency range of normal phone lines. To do this, the modem modulates a carrier signal (usually 1700 Hz) with the serial data from the DTE. The modulation may be amplitude, frequency, phase, or some combination of the three.

The type of modulation used will determine the maximum rate of data transferred for a given bandwidth. The more complex the modulation, the faster the data can be transferred. On the other hand, complex modulation schemes are more susceptible to noise and error, so fast modems must have sophisticated error handling systems. Regardless of the type of modulation, the carrier will change only in discrete steps, thus if frequency modulation is used, a 1 could be 1.8 kHz and a zero could be 2.1 kHz.

In general, the various values of phase frequency or amplitude that the carrier may take on are called states. Each state (or event) on the output of the modem is called a baud and the rate at which the states change is called the baud rate. The bandwidth of the modem output signal is determined by its baud rate.

For the modem used, the max baud rate is 2400 baud/sec. The type of modulation used in the modems in this system is called Quadrature Amplitude Modulation (QAM).

In QAM, the carrier is modulated in both phase and amplitude. Incoming digital data is encoded up to 4 bits at a time to represent one of 16 possible phase/amplitude combinations. In this way 4 bits of data can be represented by a single transition in the state of the carrier. Thus, for a baud rate of 2400, 9600 bits per second (bps) can be transferred over the phone lines.

The 16 phase/amplitude combinations are referred to as a signal-set constellation. The constellation is chosen so that signals are spaced close together. This reduces the amount of power needed for transmission. To reduce the error rate introduced by packing the signals in a dense formation, a coding/encoding technique known as trellis coding is used. This technique restricts the sequences of signals used so that the signals within a sequence are far apart.

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Modem Reception

In order to receive analog data and convert it back to digital, the modem must demodulate the carrier and convert the various different states back into digital data. To facilitate this, the modem that is transmitting sends a signal which contains frequency, phase, and amplitude references.

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Auto Call Unit (ACU)

Each dial-up line modem contains an internal ACU that operates under CCITT specification V.25 bis. The ACU allows the Data Terminal Equipment (DTE) to initiate calls on the Public Switched Telephone Network. The DTE transfers serial ASCII data to the ACU in a bit-synchronous, HDLC-like protocol outlined in the V.25 built-in synchronization (bis) protocol.

1. The ACU uses the Data Terminal Ready (DTR), Ring Indicator (RI), Clear To Send (CTS), Data Carrier Detection (DCD), Data Set Ready (DSR), and Request To Send (RTS) signals to signify various states of call establishment.

2. When the modem is disconnected from the telephone line and the DTE has set the DTR signal off, the devices are idle. If the modem detects an incoming call, it alerts the DTE via the RI signal. To answer an incoming call, the DTE must turn the DTR signal on and enter the dialog state. The modem responds by turning the CTS signal on to enter the dialog state.

3. When the modem is in dialog state, it accepts V.25 built-in synchronization commands from the DTE. These commands may be entered by the operator at the keyboard. The Connect Incoming Call (CIC) command instructs the modem to immediately answer an incoming call. The Call Requested Number (CRNn) command instructs the modem to dial the number (n) that follows it. The Call Request with Address (CRSa) command instructs the modem to dial a number from a specific address (a) in modem memory. The Disregard Incoming Call (DIC) command instructs the modem to disregard an incoming call.

4. When an incoming call comes in while the modem is in dialog state, the modem answers the call after it receives the CIC command from the DTE. If a CIC command is not issued, the modem answers the call automatically after two rings.

   When the modem answers the call it enters the connecting state. The modem turns the CTS signal off and does not accept any commands unless the call fails or the call is disconnected. If the call fails, the modem turns the CTS signal on and enters dialog state.

5. When a connection is successful, the modem enters the data state. The modem turns on the DSR and DCD signals and inbound data is accepted from the remote site. When the DTE turns the RTS signal on, the modem turns the CTS signal on and outbound data transmission takes place. If the remote modem disconnects, the local modem drops the connection and enters dialog state. The DTE disconnects the call by turning the DTR signal off.

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Phone Lines

Telephone lines are either 2-wire or 4-wire. Dial lines are 2-wire and leased lines are either 2-wire or 4-wire. Two-wire lines constitute a single signal path and are usually a twisted pair. A 4-wire line is two twisted pairs.

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Full or Half Duplex Transmission Modes

Data transmission can be half-duplex, simulated half-duplex, and full-duplex.

1. In half-duplex transmission, data flows in both directions, but only in one direction at a time.

2. Full-duplex permits simultaneous transmission in both directions. This is accomplished either by transmitting data at one frequency in one direction and another frequency in the opposite direction, by using 4-wire lines, or by using echo canceling. Echo canceling cleans up the received signal by removing interference and echoes caused by the transmitted signal. It is another method of full duplex modulation over two lines.

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Asynchronous and Synchronous Serial Data Transmission

In asynchronous transmission, characters travel individually through the line as they occur. The receiving data terminal must recognize when to start sampling the data or it will misinterpret it. For this reason, start and stop bits are added to every character to identify the beginning and end. The receiving DTE will now recognize each character, instead of reading a continuous stream of unintelligible bits.

1. Parity bits are sometimes added to asynchronous characters for error checking. The transmitting data terminal adds bits to make the "1" bits of each character total either an odd or even number. If the receiving data terminal counts the number of bits in a character and finds it is even when it should have been odd, or vice versa, an error has occurred during transmission.

2. In asynchronous transmission, the modem and terminal must agree on transmission rate and the total number of bits in each character. This number includes the bits making up each character as well as start, stop, and parity bits.

In synchronous transmission entire blocks of data, rather than individual characters, are transmitted. These blocks of data are accompanied by timing signals which synchronize the receiving and transmitting DTEs. The data block and timing signals are called frames.

Procedures called protocols are usually required to initiate and maintain data exchange during synchronous data communication. These protocols protect against errors. Rather than transmitting individual characters framed by start and stop bits as with asynchronous transmission, data is held in a buffer until a block is created. Then, the entire block is transmitted. The transmitted block is prefixed by SYN or synchronization characters and terminated by end of block characters and block check characters. The number of synchronization characters and their makeup is determined by the protocol used.