Modems in Data communication

Motivation on Modulation and Demodulation

If either analog or digital signals were used exclusively,

communications would be simplified. However, this is
impossible especially attempting to send signals across a
long distance. Digital signals cannot be transmitted far
without being converted to analog signals. Because
telephone system is an analog device, computer signals
must be converted to analog signals.
 Carrier
• Researchers found that a continuous, oscillating signal will
propagate farther than other signals.
• Instead of transmitting an electric current that only changes when
the value of a bit changes, long-distance communication systems
send a continuously oscillating signal, usually a sine wave, called
a carrier.
• The wave form of an analog signal carrier oscillates continuously
even when no signal is being sent.
 Data Modulation & Demodulation
• To send data, a transmitter modifies the carrier slightly.
• Collectively, such modifications are called modulation.
• The technique was originated for transmitting radio or TV signals.
• Generally speaking, modulation is the process to transform a digital
signal into an analog signal.
• At the receiving end, the analog signal is transformed back to digital
signals. The process is called demodulation.
• The device to perform modulation and demodulation is called a
modem. We will talk about modem later.
 Example of Data Modulation

The digital signal ’01’ is sent. The carrier is reduced to 2/3 full
strength to encode a 1 bit and 1/3 strength to encode a 0 bit.
 Baud Rate vs. Bit Rate
• Transmission speed can be measured in bits per second(bps).
• Technically, transmission is rated in baud, the number of
changes in the signal per second that the hardware generates.
• Baud rate determines the band width required to send the
signal.
• Baud rate = __________Bit rate_____
No. of Bits/Signal units

= Signal units/sec
• Bit rate = n * Baud rate, Where n is the number of signals in a string
• Baud rate < = Bit rate.
• Using RS-232 standard to communicate, bit rate = baud rate.
 Modulation Techniques
• Amplitude shift keying (ASK)
• Frequency shift keying (FSK)
• Phase shift keying (PSK)
 Amplitude Shift Keying

• Values represented by different amplitudes of carrier

• The amplitude (or height) of the sine wave varies to transmit the ones
and zeros
• Major disadvantage is that telephone lines are very susceptible to
variations in transmission quality that can affect amplitude
(Susceptible to sudden gain changes)
• Inefficient
 Example of ASK
Bit Values Amplitude
00 A1
01 A2
10 A3
11 A4
Amplitude Shifting Keying (four amplitudes),
two bits per baud
Amplitude Modulation

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 Phase Shift Keying
• Nyquist Theorem suggests that the number of bits sent per cycle can be increased
if the encoding scheme permits multiple bits to be encoded in a single cycle of the
carrier.
• ASK and FSK work well but require at least one cycle of a carrier wave to send a
single bit.
• PSK changes the timing of the carrier wave abruptly to encode data. Such change
is called a phase shift.
• Frequency and amplitude of the carrier signal are kept constant
• The carrier signal is shifted in phase according to the input data stream
• Each phase can have a constant value, or value can be based on whether or not
phase changes (differential keying)
Phase Modulation

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Differential Phase Shift Keying
(DPSK)

0 1 1 0

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 Frequency Shift Keying (FSK)

•Frequency of the carrier wave varies in accordance with the signal to

be sent
•Signal transmitted at constant amplitude
•More resistant to noise than ASK
•Less attractive because it requires more analog bandwidth than ASK
Frequency Modulation

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Frequency Shift Keying
 QAM
• Any of the simple techniques can be used with any number of
different signals.
• More signals means a greater bit rate with a given baud rate.
• The problem is that a higher bit rate requires more signals and
reduces the differences among them and makes the receiver’s job
more difficult.
• Another approach is to use a combination of frequencies,
amplitudes, or phase shifts, which allows us to use a larger group
of legitimate signals while maintaining larger differences among
them.(ASK+PSK)
• One technique is Quadrature Amplitude Modulation (QAM), in
which a group of bits is assigned a signal defined by its amplitude
and phase shift.
 Performance of Digital to Analog Modulation
Schemes
• Bandwidth
– ASK and PSK bandwidth directly related to bit rate
– FSK bandwidth related to data rate for lower frequencies,
but to offset of modulated frequency from carrier at high
frequencies
• In the presence of noise, bit error rate of PSK and QPSK are
about 3dB superior to ASK and FSK
 Analog-to-Digital Conversion
• Usually, A modem examines the incoming signals for
amplitude, frequencies, and phase shifts and generates digital
signals. This works for signals having constant characteristics.
• What about analog signals whose characteristics change
continually such as a voice signal?
 Pulse Code Modulation
• One way of making the signal truly digital is to assign
amplitudes from a predefined set to the sample signals.
• This process is called PCM.
Accuracy of PCM
The pulse amplitude is divided into eight values or 23 values.
1. The sampling frequency
2. The number of amplitudes chosen: in Fig 2.47, the
resulted signal becomes distorted.
 Modem

• Modem = modulator + demodulator

• A modem converts digital signals to analog signals before sending
them across a phone line.
• Another modem converts analog signals back to digital signals
before passing them to a receiver.
 Classification of Modems
According to directional capability:
» Half duplex Modem & Full duplex modem
According to Connectivity: 2 wire modem & 4 wire modem
Mode of transmission: Asynchronous modem & Synchronous modem
According to speed: Low speed :300/600/1200 bps
Medium speed:1200/2400/4800 bps
High speed : 9600/12000 bps
Wide band/ Group band:48 k/56 k/64 k/128 K/144 K and above
Types of line : dial up & lease line
According to configuration: point to point & multi-drop
According to coupling: electrical & acoustic
Simplex, half-duplex and full duplex
transmission

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Illustration of Dial-up Modem
Asynchronous (diagram)
 Asynchronous
• Data transmitted on character at a time (5 to 8 bits)
• Timing only needs maintaining within each character. The internal
timing pulses are synchronized repeatedly to leading age of start pulse.
• Resync with each character
• Asynchronous modems can handle data bytes with start and stop bits.
• In a steady stream, interval between characters is uniform (length of
stop element)
• In idle state, receiver looks for transition 1 to 0
• Then samples next seven intervals (char length)
• Then looks for next 1 to 0 for next char
• Simple
• Cheap
• Good for data with large gaps (keyboard)
 Synchronous - Bit Level
• Block of data transmitted without start or stop bits
• Clocks must be synchronized
• Can use separate clock line
– Good over short distances
– Subject to impairments
• Embeded clock signal in data
• Need to indicate start and end of block
• Use preamble and post amble
– e.g. series of SYN (hex 16) characters
– e.g. block of 11111111 patterns ending in 11111110
• More efficient (lower overhead) than async
Traditional Configurations
Point-to-point configuration

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Multipoint configuration

The advantage of multipoint is that it is cheaper and simpler to wire.

The disadvantage is that only one computer can use the circuit at a time.
 Modem Control Signals
• FROM DTE • FROM DCE
• TD: Transmit data • RD: Receive data
• DTR:DTE ready • DSR - Modem is ON, data set ready
• RTS : Request to send • DCD - connected
• *TT (Terminal Timing) • CTS (Clear to Send)
• RI (Ring Indication)
• *ST (Send Timing)
• *RT (Receive Timing)

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Serial Port Speed Buffering
• DTE port speed is fixed (115200 bps)
• DCE link speed is fixed (33600 bps)
• Data compression ratio is variable
• Flow control is required
• All modem applications must use hardware flow
control (RTS/CTS)

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 Digital interface
• front panel lights
• HS high speed (>2400 bps)
• AA auto answer on
• CD carrier detected
• OH off hook
• SD/RD sending/receiving data
• TR terminal ready
• MR modem ready
• RS request to send
• CS clear to send
• SYN synchronous mode
• ARQ data mode under error correction (MNP, V.42)

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Mechanical Specification
Local and Remote Loopback
 Test loops
• 1 Digital loop back : this loop is set up as close as possible to digital interface.
• 2.Remote Digital Loop Back : This loop check line and remote modem. It can be
used only in full duplex modem
• 3.local analogue loop back: the modulated carrier at the transmitter O/P of local
modem is looped backed to receiver I/P. The loopback may require some attenuator to adjust the
level.
• 4. remote analogue loop back: This is applicable for 4 wire connections only.The
two pair at distant end are disconnected from the modem and connected to each other
• 5.local digital loop back and loop forward: Local digital loop back is
provided for local modem and remote digital loopback is provided for remote modem.
• 6.local analogue loop back and loop forward: Local analogue loop back is
provided for local modem and remote analogue loopback is provided for remote modem.