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ModemReview of the modemThe telephone system, landline communication systems and radio systems cannot directly transport digital information without some distortion in the signal due to the bandwidth limitation inherent in the connecting medium. The reason for the difficulty in transferring digital information over a telephone network, for example, is the limited bandwidth inherent in the communication media, such as telephone cable with capacitance and inductance.a schematic view of the modems place in the communications hierarchy.

An example of what a digital signal would look like at the far end of a cable if it were injected directly into the cable is given

There are two types of modem available today: Dumb (or non-intelligent) modems depend on the computer to which they are connected, to instruct the modem when to perform most of the tasks such as answering the telephone. Smart modems have an on-board microprocessor enabling them to perform such functions as automatic dialing and the method of modulation to use.

Synchronous or asynchronousIn asynchronous communications each character is encoded with a start bit at the beginning of the character bit stream and a parity and stop bit at the end of the character bit stream. The receiver then synchronizes with each character received by looking out for the start bit. Once the character has been received, the communications link returns to the idle state and the receiver watches out for the next start bit (indicating the arrival of the next character).

Synchronous communication relies on all characters being sent in a continuous bit stream. The first few bytes in the message contain synchronization data allowing the receiver to synchronize onto the incoming bit stream. Hereafter synchronization is maintained by a timing signal or clock. The receiver follows the incoming bit stream and maintains a close synchronization between the transmitter clock and receiver clock.

Modes of operationModems can operate in two modes: Half-duplex Full-duplex

A simplex system in data communications is one that is designed for sending messages in one direction only and has no provision for sending data in the reverse direction. A duplex system in data communications is one that is designed for sending messages in both directions. Duplex systems are said to be half-duplex when messages and data can flow in both directions but only in one direction at a time.Duplex systems are said to be full-duplex when messages can flow in both directions simultaneously. Full-duplex is more efficient, but requires a communication capacity of at least twice that of half-duplex.

Components of a modem

Modem receiverFilter and amplifier Noise is removed from the signal and the resultant signal is then amplified.

EqualizerThis minimizes the effect of attenuation and delay on the various components of the transmitted signal. A predefined modulated signal (called a training signal) is sent down by the transmitting modem.

Demodulator This retrieves the bit stream from the analog signal.

Descrambler (synchronous operation only)This restores the data to its original serial form after it has been encoded in the scrambler circuit to ensure that long sequences of 1s and 0s do not occur. Long sequences of 1s and 0s are difficult to use in synchronous circuits because of the difficulty of extracting clocking information.

Data decoder The final bit stream is produced here in true RS-232 format.

Modem transmitterData encoderThis takes the serial bit stream and uses multilevel encoding (where each signal change represents more than one bit of data) to encode the data. Depending on the modulation technique used the bit rate can be two or four (or more) times the baud rate.

Scrambler (synchronous operation only)The bit stream is modified so that long sequences of 1s and 0s do not occur (with consequent problems for the receiver not being able to extract the clock rate).

ModulatorThe bit stream is converted into the appropriate analog form using the selected modulation technique. Where initial contact is established with the receiving modem, a carrier is put on the line initially.

AmplifierThis increases the level of the signal to the appropriate level for the telephone line and matches the impedance of the line.

The RS-232/RS-422/RS-485 interface standardsRS-232, RS-422 and RS-485 form the key element in transferring digital information between the RTUs (or operator terminals), and the modems, which convert the digital information to the appropriate analog, form suitable for transmission over greater distances.These standards were designed primarily to transport digital data from one point to another. The RS-232 standard was initially designed to connect digital computer equipment to a modem where the data would then be converted into an analog form suitable for transmission over greater distances. The RS-422 and RS-485 standards can perform the same function but also have the ability of being able to transfer digital data over distances of over 1200 m. The RS-232-C interface standard for serial data communicationThe RS-232 interface standard was developed for a single purpose clearly stated in its title, and defines the interface between data terminal equipment (DTE) and data communications equipment (DCE) employing serial binary data interchange.

Electrical signal characteristicsThe RS-232 interface standard is designed for the connection of two devices called: DTE data terminal equipment (e.g. a computer or printer) A DTE device communicates with a DCE device. A DTE device transmits data on pin 2 and receives data on pin 3. DCE data communications equipment Also called data circuit-terminating equipment in RS-232-D/E (e.g. a computer or modem). A DCE device transmits data between the DTE and a physical data communications link (e.g. telephone system). A DCE device transmits data on pin 3 and receives data on pin 2.

The RS-232 standard defines twenty-five (25) electrical connections, which are each described later. The electrical connections are divided into the four groups shown below: Data lines Control lines Timing lines Special secondary functions.

Interface mechanical characteristicsAlthough not specified by RS-232-C, the DB-25 connector (25-pin, D-type) has become so closely associated with RS-232 that it is the de facto standard. The DB-9 connector de facto standard (9-pin, D-type) is also commonly used

Functional description of the interchange circuitsThe EIA circuit functions are defined, with reference to the DTE, as follows: a. Pin 1 protective ground (shield) A connection is seldom made between the protective ground pins at each end. Their purpose is to prevent hazardous voltages, by ensuring that the DTE and DCE chassis are at the same potential at both ends. But, there is a danger that a path could be established for circulating earth currents. So, usually the cable shield is connected at one end only. b. Pin 2 transmitted data (TXD) This line carries serial data from Pin 2 on the DTE to Pin 2 on the DCE. The line is held at MARK (or a negative voltage) during periods of line idle.c. Pin 3 received data (RXD) This line carries serial data from Pin 3 on the DCE to Pin 3 on the DTE. d. Pin 4 request to send (RTS) See clear to send. e. Pin 5 clear to send (CTS) When a half-duplex modem is receiving, the DTE keeps RTS inhibited. When it becomes the DTEs turn to transmit, it advises the modem by asserting the RTS pin. When the modem asserts the CTS it informs the DTE that it is now safe to send data. The procedure is reversed when switching from transmit to receive. f. Pin 6 data set ready (DSR) This is also called DCE ready. In the answer mode, the answer tone and the DSR are asserted two seconds after the telephone goes off hook. g. Pin 7 signal ground (common) This is the common return line for the data transmit and receive signals. The connection, Pin 7 to Pin 7 between the two ends, is always made. h. Pin 8 data carrier detect (DCD) This is also called the received line signal detector. Pin 8 is asserted by the modem when it receives a remote carrier and remains asserted for the duration of the link.i. Pin 20 DTE ready (or data terminal ready) DTE ready enables, but does not cause, the modem to switch onto the line. In originate mode, DTE ready must be asserted in order to auto dial. In answer mode, DTE Ready must be asserted to auto answer. j. Pin 22 ring indicator This pin is asserted during a ring on the line. k. Pin 23 data signal rate selector (DSRS) When two data rates are possible, the higher is selected by asserting Pin 23.

The sequence of asynchronous operation of the RS-232 interface

Asynchronous operation is arguably the more common approach when using RS-232 and will be examined in this section using the more complex half duplex data interchange. It should be noted that the half-duplex description is given as it encompasses that of full- duplex operation. Figure 6.13 gives a graphical description of the operation with the initiating user terminal (or DTE) and its associated modem (or DCE) on the left of the diagram and the remote computer and its modem on the right

Flow control

These techniques are widely used in modem communications to ensure that there will be no overflow of data by the device receiving a stream of characters, which it is temporarily unable to process, or store. The receiving device needs a facility (called flow control) to signal to the transmitter to temporarily cease sending characters down the line.There are three mechanisms of flow control described below. The first two techniques are software based and the last, RTS/CTS, is hardware-based handshaking. XON/XOFF signalingWhen the modem decides that it has too much data arrivi