Week 2
Things you want to know
Week 2
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Describe the differences between AC/DC
Define amplitude and frequency
List 3 transmission flaws
Describe uses of repeaters and amplifiers
List 3 ways of encoding data
Differences between bandwidth and throughput
Describe the differences between AM and FM
Describe two types of multiplexers
Describe the bits in a T-1
Electric Current
• Electric current - the controlled movement of an electrical
charge (or electrons) along the atoms of a conductor.
• Circuit - a closed connection between an electric source (such
as a battery) and a load (such as a lamp) over which current
may flow.
• Signal - occurs when current manipulated to transmit
information.
Direct and Alternating Current
• Direct current (DC) an electrical charge
flows steadily in one
direction over the
conductor.
Direct and Alternating Current
• Alternating current
(AC) - the electrical
charge flows in one
direction first, then in
the opposite direction,
then back in the first
direction, and so on, in
an alternating fashion
over the conductor.
Figure 1–5
Abstract depiction of a typical waveform generated by human speech.
Figure 1–6
Voltage is used to measure the signal strength of various amplitudes. The greater the amplitude, the louder the sound
and the stronger the signal.
Figure 1–10
Measuring any of these sounds using a dB meter would result in values similar to those listed.
Figure 1–11 Milliwatt values compared with dBm values. For example 0.000001 mW and –60 dBm represent the same value. Using –60 dBm to
identify the strength of a signal is much simpler than using 0.000001 mW. For the human ear to discern a change in volume the signal strength
must be doubled or halved!
Figure 1–12
Correlation between loss of dBm and frequency. The greater the frequency, the greater the loss. Other variables also
affect loss of circuit, such as the gauge of the cable.
Figure 1–13 End-to-end circuit loss using dBm as the measurement value. The total end-to-end loss of the circuit on this one-way
termination is –8.5 dBm. At position C, the signal is regenerated, thus canceling out any loss up to that point.
Direct and Alternating Current
Direct and Alternating Current
Measuring Electricity
Measuring Electricity
Measuring Electricity
Analog Transmission
• Analog electromagnetic
signals that
continuously vary in
their strength and
speed.
Transmission Flaws
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Noise - unwanted interference from external sources, which can degrade or
distort a signal.
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Attenuation - the loss of a signal’s strength as it travels away from its source.
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Amplifier - an electronic device that increases the voltage, or power, of the
signals.
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Regeneration - when digital signals are repeated, they are actually
retransmitted in their original, pure form, without any noise.
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Repeater - a device that regenerates a digital signal.
Transmission Flaws
Transmission Flaws
Noise Effects on Communications
• Data
– Satisfactory in the presence of white noise but
impulse noise will destroy a data signal
– BER (Bit Error Rate) performance measure in
digital systems
• Voice
– White noise (continuous disturbance) can be
bothersome to humans but impulse noise
acceptable for speech communications
– SNR (Signal-to-Noise Ratio) is used as a
performance measure in analog systems
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Noise
• External Noise: Originates in the
communication medium
– Man-made noise
• Generated by equipment such as motors
– Atmospheric noise (also called static)
• Dominates at lower frequencies and typical solution
involves “noise blanking”
– Space noise (Mostly solar noise)
• Dominates at higher frequencies and can be a serious
problem in satellite communications
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Noise
• Internal Noise: Originates in the
communication equipment
– Thermal noise (also called white noise)
• Is produced by random motion of electrons in a
conductor due to heat
• Noise Power in watts is directly proportional to
Bandwidth in Hz, and the temperature in degrees
Kelvin
– Shot noise
– Excess noise (same as flicker noise or pink noise)
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Signal-to-Noise Ratio (SNR)
• Signal-to-Noise Ratio (SNR)
– Is expressed in decibels
SNR dB   10 log 10
where:
 PS

 PN



PS is the signal power in watts
PN is the noise power in watts
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Hartley-Shannon Theorem:
Significance of SNR
• Hartley-Shannon Theorem (also called
Shannon’s Limit) states that the
maximum data rate for a
communications channel is determined
by a channel’s bandwidth and SNR.
• A SNR of zero dB means that noise power
equals the signal power.
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Noise Ratio (NR)
and
Noise Figure (NF)
NR 
SNRinput
SNRoutput
NF = 10 log (NR)
NF (dB) = (SNR)input (dB) – (SNR)output (dB)
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Encoding and the Numbering System
• Encoding - the process of modifying data so that it can be
interpreted by the receiver.
• Methods for encoding data include:
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The Decimal System
The Binary System
Hexadecimal System
EBCDIC
ASCII
UNICODE
Measuring Data
Throughput and Bandwidth
• Throughput - the amount of data that a communications
channel can carry during a given period of time.
– The physical nature of every communications channel determines its
potential throughput.
• Bandwidth - a measure of the difference between the
highest and lowest frequencies that a media can transmit.
Introduction
• Electromagnetic (E/M) Spectrum
– Ranges from 30 Hz to several GHz
– FCC jurisdiction over the use of this spectrum
• Block diagram of an electronic
communications system
Transmitter
Receiver
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E/M Spectrum
For a nice colorful chart see: www.ntia.doc.gov/files/ntia/publications/spectrum_wall_chart_aug2011.pdf
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Freq allocation chart
• Freq allocation chart
www.ntia.doc.gov/files/ntia/publications/spec
trum_wall_chart_aug2011.pdf
Communications System Parameters
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Type of Information
Bandwidth
Broadband versus Baseband
Synchronous versus Asynchronous
Simplex, Half-Duplex and Full-Duplex
Serial versus Parallel
Analog versus Digital
Noise
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Type of Information
• Data, Voice and Video, each have specific
transmission requirements
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Bandwidth
• Range of frequencies that can be transmitted with
minimal distortion
• Measure of transmission capacity of the
communications medium
• Hartley’s law states that the amount of
information that can be transmitted is directly
proportional to bandwidth and transmission time
I = ktBW
• Analog: BW is expressed in Hz
• Digital: BW is expressed in bps
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Broadband versus Baseband
• Broadband
– Simultaneous transmission of multiple channels
over a single line
– Originated in the CATV industry
• Baseband
– Digital transmission of a single channel
– Advantages: Low-cost, Ease of installation, and
High transmission rates
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Synchronous versus Asynchronous
• Asynchronous
– Transmission of a single character
– Incorporates framing bits (start and stop bits)
– More cost-effective but inefficient
• Synchronous
– Transmission of a block of data
– Requires a data clock
– SYN bits transmitted at the beginning of a data block
– Expensive and complex but extremely efficient
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Efficiency of Transmission
M
Efficiency 
100%
M C
M 

Overhead  1 
 100%
 M C 
where: M = Number of message bits
C = Number of control bits
Efficiency % = 100 – Overhead %
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802.3 Frame
Preamble
SOF
Mac Dest
MAC
Source
Type
Payload
CRC
7 Octets
1
6
6
2
46-1500
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Overhead26 Octets
Payload
46-1500 Octets
Runts
< 72 Octets
Giants > 1572 Octets
Error correction throws out “runts and giants”
Efficient?
• 1500/1500 +26 * 100 = 0.982
• 46/46+26 *100= 0.638
• Compare this to ATM where the fixed frame
size is 53 bytes 48 data and 5 bytes for
overhead. Is it more or less efficient than
ethernet?
Simplex, Half-Duplex Full-Duplex
• Simplex
– In only one direction from transmitter to
receiver
• Half-Duplex
– Two-way communications but in only one
direction at a time
• Full-Duplex
– Simultaneous two-way communications
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Serial versus Parallel
• Serial
– Transmitting bits one after another along a
single path
– Slow, cost-effective, has relatively few errors,
practical for long distances
• Parallel
– Transmitting a group of bits at a single instant
in time, which requires multiple paths
– Fast but expensive, practical for short distances
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UART
• Universal Asynchronous Receiver Transmitter
(UART):
• Parallel to Serial converter
– Transmit section
• Parallel data is put on an internal data bus, then stored
in a buffer storage register from where it is sent to a shift
register, which adds start and stop bits, and a parity bit.
The data is then transmitted one bit at a time to a serial
interface.
– Receive section
• Serial data is shifted into a shift register where start, stop
and parity bits are stripped off. The remaining data is
transferred to a buffer storage register and then on to
the internal data bus.
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Analog versus Digital
• Analog
– Continuously varying quantities
• Digital
– Discrete quantities
– Most commonly binary
– All information is reduced to a stream of 0s and 1s
which enables the use of a single network for voice,
data and video
– Digital circuits are cheaper, more accurate, more
reliable, have fewer transmission errors and are
easier to maintain than analog circuits
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Analog-to-Digital Conversion
• Analog-to-Digital conversion device is also referred to as a
codec (coder-decoder).
• An everyday example of such a device is the modem
(modulator/demodulator), which converts digital signals that
it receives from a serial interface of a computer into analog
signals for transmission over the telephone local loop, and
vice versa.
• Which has better sound reproduction an analog recording or a
digital master?
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Modulation
• Modulation
– Means of controlling the characteristics of a
signal in a desired way
• Fourier Analysis
– Time domain
• Graph of voltage against time
• An oscilloscope display
– Frequency domain
• Graph of amplitude or power against frequency
• A spectrum analyzer display
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Modulation Schemes for
Radio Broadcast
• Amplitude Modulation (AM)
– Oldest and simplest forms of modulation used for analog
signals
– Amplitude changes in accordance with the modulating
voice signal
– Transmits longer distance
• Frequency Modulation (FM)
– Frequency changes in accordance with the modulating
signal, which makes it more immune to noise than AM
– The amount of bandwidth necessary to transmit an FM
signal is greater then that needed for AM
– Transmits shorter distance – Line of sight ~50 miles
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Frequency Shift Keying (FSK)
• Frequency Shift Keying (FSK)
– Popular implementation of FM for data
applications
– Was used in low-speed modems
– Carrier is switched between two frequencies, one
for mark (logic 1) and the other for space (logic 0).
For full-duplex, there are two pairs of mark and
space frequencies
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FSK Technique
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Phase Modulation (PM)
• Phase Modulation (PM)
– Amount of phase-shift changes in accordance with
the modulating signal. In effect, the carrier
frequency changes, and therefore, PM is
sometimes referred to as “indirect FM”
– Advantage of PM over FM is that in PM, the carrier
can be optimized for frequency accuracy and
stability. Also, PM is adaptable to data applications
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Examples of Phase Shift
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PSK and QAM
• Phase Shift Keying (PSK)
– Most popular implementation of PM for data
– In BPSK (Binary PSK): one bit per phase change
– In QPSK: two bits per phase change (symbol)
Bit Rate = Baud rate x Bits per Symbol
• Quadrature Amplitude Modulation (QAM)
– Uses two AM carriers with 90o phase angle between
them, which can be added so that the amplitude
and phase angle of the output can vary continuously
– Implemented in V.32bis and V.90 modems
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Modulation Techniques for Modems
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Pulse Modulation
• Pulse Modulation
– Used for both analog and digital signals
– Analog signals must first be converted to digital
signals, which involves “sampling”
• First step is low-pass filtering of the analog signal
• Second step is sampling the analog signal at the Nyquist
rate (at least twice the maximum frequency component
in the waveform)
• Third step is transforming the pulses into a digital signal
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Pulse Modulation Schemes
• PAM (Pulse Amplitude Modulation)
– First important step in Pulse Code Modulation
• PPM (Pulse Position Modulation)
– Random arrival time makes PPM unsuitable for
transmission
• PWM (Pulse Width Modulation)
– Unsuitable for transmission because of varying
pulse width
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Pulse Code Modulation
• Pulse Code Modulation (PCM)
– Only technique that renders itself well to transmission,
and most commonly used
– Transmitted information is coded by using a character
code such as the ASCII
– T-1 uses PCM
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Allotted bandwidth per voice channel is 4 kHz
Therefore, the Nyquist sampling rate is 8 kHz
Eight bits per sample are coded
Thus, each PCM channel is 64 kbps
24 channels gives an aggregate of 1.536 Mbps, with
additional 8 kbps for synchronization, giving 1.544 Mbps
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Multiplexing
• Multiplexing:
– Two or more signals are combined for
transmission over a single communications path
– FDM (Frequency Division Multiplexing)
• Each signal is assigned a different carrier frequency
– TDM (Time Division Multiplexing)
• Digital transmission that is protocol insensitive
• Used in T-1s where each of the 24 channels is assigned
an 8-bit time slot
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TDM
• Conventional TDM
– Bit-interleaved
• A single bit from each I/O port is output to the aggregate
• Simple, efficient, and requires no buffering of I/O data
– Byte-interleaved
• One byte from each I/O port is output to the aggregate
• Fits well with the microprocessor-driven byte-based environment
• Statistical TDM
– Allocates time slices on demand
– Additional overheads (for example, station address)
– Aggregate channel BW is less than the sum of individual
channel BWs
– I/O protocol sensitive
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T-1
• T-1 - basic block of the telephony system.
• 24 voice/data of 64kbps channels
• 8 bit samples (each channel sampled 8 X 24 channels
= 192
• 1 bit for error correction (Parity) = 192 + 1 =193
• 8000 samples per second (2 x 4k bandwidth)
193 x 8000 =1.544mbps.
• Europe uses 30 channels (E-1) their block is 2.048
mbps.
WDM
• WDM (Wavelength Division Multiplexing)
– Cost-effective way to increase fiber capacity
– Each wavelength of light transmits information and WDM
multiplexes different wavelengths
• DWDM (Dense WDM) System
– Invention of the flat-gain wideband optical amplifier
increased the viability of DWDM
– Typically employed at the core of carrier networks
– Affords greater bandwidth in pre-installed fibers
– Can carry different types of data (IP, ATM, SONET)
– Can carry data at different speeds
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DWDM System Components
• Transmitter:
– Semiconductor laser
• Modulator/Demodulator and MUX/DeMUX:
– Electro-optical device
• Receiver:
– Photodetector and Optical amplifier
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