Data Communication Equipment- describes the hardware that allows digital terminal equipment (DTE)
to access a transmission facility, such as a metallic or optical fiber cable. Two types of Data Communication Equipment 1. Modem- allows DTE to access and interface analog transmission facilities. It converts digital signal to analog and vice versa. With these, modem includes ADC and DAC. ADC is used for mo dem’s receiver and
DAC for modem transmitter 2. Channel Service Unit (CSUs) and Digital Service Unit (DSUs)- two devices that allow DTE to access
and interface with digital transmission facilities. They have diff function but often combined in one unit. -used to terminate a digital circuit at a subscriber’s location and allow the s ubscriber to connect to a local central telephone office. -allows devices connected to LAN to access and connect digital transmission facilities leased from service providers. a. DSU- converts unipolar digital signal from a local area network’s digital ter minal into self-clocking
bipolar digital signal. b. CSU- serves as the demarcation point between t he digital station equipment and telecommunications
lines. It converts digital signals to a format more suitable for transmission over digital transmission facility. >>insert pic 15.1 VOICE-BAND DATA COMMUNICATIONS MODEMS
The most common type of DCE is the data communications modem. It is also called as datasets, dataphones or simply modems. The word modem is a contractio derived from the word ‘modulator and
demodulator’. Primary purpose – to interface computers, computer networks, and other digital terminal equipment to analog communications facilities. Types of Modems
1. Telephone-loop modems or POTS modems- connects the telephone company through the same local loops used for standard voice telephone circuits. 2. Broadband modems or cable modems- more sophisticated. Capable of transporting data at much higher bit rates over wide-band communication channels (optical fiber, cox cables, microwave radio and satellite communications systems) >>insert pic 15.2
A modem is, in essence, a transparent (doesnt convert or change information contained in data) repeater that converts electrical signa;s in digital form to electrical signal in analog form and vice versa. It is physically located between DTE and analog communication channel. Modem Transmitter- is a digital-to-analog converter (DCA), as it converts digital pulses to analog
representation. The analog signals are outputted onto an analog communications channel where they are transported through the syste to a distant receiver. Modem Receiver- Is a analog-to-digital converter (ADC). The demodulated digital pulses are outputted
onto a serial digital interface and tr ansported to the DTE.
Bit Rate versus Baud
1. Bit rate- rate of change of the digital input data 2. Baud- rate of change of output analog signal.
VOICE-BAND MODEM BLOCK DIAGRAM >>insert pic 15.3
1. Serial Interface Circuit- interfaces the modem TX and RX to the serial interface. a. Transmit section - accepts digital information for serial interface, converts it to the appropriate voltage levels, and then directs the information to modulator b. Receive section – receives digital information from demodulator circuit, converts it to appropriate voltage level =s, and then directs information to the serial interference. 2. Modulator circuit- the digital information modulate an analog carrier producing a ‘digitally modulated analaog signa l.’ 3. Bandpass filter and Equalizer Circuit- present in transmitter andd receiver section of modems.
a. Transmit bandpass filter- limits the BW of digitally modulated analog signals to a BW apropriate for transmission over a standard telephone cicuit. b. Receive Bandpass filter- limit the BW of the signals allowed to reach the demodulator circuit (reduce noise,improve system performance) c.
Equalizer circuits- compensate for BW and gain imperfection.
4. Telco interface Circuit- to match impedance of the modem to the impedance of the telephone line and regualte the amplitude of the transmit signal.
5. Demodulator Circuit- receives odulated signals from the bandpass filter and equalizer circuit and converts the digitally modulated analog signal to digital signal.
6. Carrier and clock generation circuit- generates appropriate clock and timing signals required for performing transmit and receive funstions in an orderly and timely fashion. VOICE-BAND MODEM CLASSIFICATIONS
a. Asynchronous or synchronous b. Digital modulation Schemes – amplitude-shift keying (ASK), frequency-shift keying (FSK), phase-shift keying (PSK) or quadrature amplitude modulation (QAM) c.
Internal or external
d. Low-speed, Medium-speed, High-speed or Very high speed e. Wideband or voice band f.
Personal or commercial
Regardless how modem is classified they all share a common goal – to convert digital pulses to analog signals in the TX and analalog signal to digital pulses in the RX.
ASYNCHRONOUS VOICE-BAND MODEM
It can be generally classified as low-speed voice-band modem, as they are typically used to transport asynchronouse data.This type of modems use relatively simple modulation schemes, such as ASK or FSK , and are relatively low-speed application less than 2400bps such as te lemetry and caller ID. A popular modem that use this is the Bell System 103. Synchronous data transported by asynchronous modems is called isochronous transmission. >>insert pic 15.4
Clocking information is not required because FSK modulators are typically voltage-controlled oscillators, which react to voltage level independent of any timing reference.
BELL SYSTEM 103-COMPATIBLE MODEM
It is capable of full-duplex operation over a two -wire telephone line at bit rates up to 300bps. Two Data Channels
1. Low-band channel(originate band) - occupies bandwidth from 300Hz to 1650Hz A modem that originates a call transmits on the low-band (originate) frequencies and receives on the high-band frequencies. *Mark frequency – 1270Hz *Space frequency – 1070Hz
2. High-band channel (answer band) - occupies bandwidth from 1650Hz to 3000Hz
The modem that answers the call transmits on the high-band (answer) frequencies and receives on the low-band frequencies. *Mark frequency – 2225Hz *Space frequency – 2025Hz
*Ideal BW of standard voice-grade telephone circuit – 0Hz to 4000Hz. *Frequency-division Multiplexing (FDM) allows full transmission over two-wire circuit, as signal can propagate in both directions at the same t ime without interfering with each other.
BELL SYSTEM 202- COMPATIBLE MODEM 202T modem- specifies four-wire, full-duplex operation, utilized on four-wire private-line data circuits 202S modem- specifies two-wire, half-duplex operation, designed for the two-wire switched public
telephone network.
SYNCHRONOUS VOICE-BAND MODEMS
It uses phase-shift keying (PSK) or quadrature amplitude modulation (QAM) to transport synchronous data at transmission rate between 240bps and 56 000bps over standard voice-grade telephone lines. It is
more complicated because of clock and carrier recovery circuits and more expensice compared to asynchronous. Phase-Shift Keying (PSK) – medium speed , operating between 2400 bps and 4800 bps.
a.
QPSK- 2400-bps modems
b. 8-psk- 4800-bps modems 16-QAM – high speed, operate at 9600bps
MODEM SYNCHRONIZATION
During request-to-send/clear-to-send (RTS/CTS), a transmit modem outputs a special, internally generated bit pattern called a training sequence. This pattern is used to synchronize the re ceive modem at the distant end of the communication channel. 1. Modem equalizer- compensation for phase delay distortion and amplitude distortion inherently present on telephone communications channels. a. Compromise equalizer- located in the transmit senction of the modem, provides preequalization ( they shape the transmitted signal by altering its delay and gain characteristics before the signal reaches the telephone line) b. Adaptive equalizer (automatic)- located at the receiver section of a modem, provides post equalization. Automatically adjust their gain and delay characteristics to compensate for phase and amplitude impairements encountered on the communication channel. >>insert fig 15.7 LOW-SPEED SYNCHRONIZATION
Generally asynchronous and use noncoherent FSK, the tr ansmit carrier frequency and clock frequency are nit recovered in the receive modem.scrambler and descrambler circuits are unnecessary.
MEDIUM- and HIGH-SPEED MODEM SYNCHRONIZATION
Are used for transission rates of 240obps to higher. Requires the receive carrier oscillators to be at least frequency coherent ( and possi ble phase coherent) with the transmit modem’s carrier oscillator. Clock recovery must also be accomplished in the receive modem. It contain scrambler and descrambler circuits and adaptive equalizer. a.
Training- type of modulation and encoding technique used is a modem determines the number
of bits required in the training sequence and, therefore, the duration of the training sequence.
b. Clock recovery-It must be maintained for duration of transmission. A better me thod would be to scramble the data in the transmit modem before modulation and descramble the data in the receive modem efter demodulation. c.
Scrambler and Descrambler circuits Scrambler circuit- used to detect undesireable sequences of 1s and 0s and convert them to a
sequence more helpful to clock recovery. Pseudorandomize data and alter it in logical manner. It detect occurence of an undesireable bit sequence and convert it to a more acceptabe pattern. Descrambler- used to detect scrambled data and convert it back to its original sequence >>insert picture 15.10
