Week 2

Things you want to know

Week 2

• 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

• Noise - unwanted interference from external sources, which can degrade or distort a signal.

• Attenuation - the loss of a signal’s strength as it travels away from its source.

• Amplifier - an electronic device that increases the voltage, or power, of the signals.

• Regeneration - when digital signals are repeated, they are actually retransmitted in their original, pure form, without any noise.

• Repeater - a device that regenerates a digital signal.

Transmission Flaws

Transmission Flaws

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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

where: PS is the signal power in watts

PN is the noise power in watts

N

S10 P

P log 10 dB SNR

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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)

NF = 10 log (NR)

NF (dB) = (SNR)input (dB) – (SNR)output (dB)

output

input

SNR

SNRNR

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:– 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.

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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 SpectrumFor a nice colorful chart see: www.ntia.doc.gov/files/ntia/publications/spectrum_wall_chart_aug2011.pdf

Freq allocation chart

• Freq allocation chart www.ntia.doc.gov/files/ntia/publications/spectrum_wall_chart_aug2011.pdf

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Communications System Parameters

• 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

%100

CM

MEfficiency

%1001

CM

MOverhead

where: M = Number of message bitsC = Number of control bits

Efficiency % = 100 – Overhead %

802.3 Frame

Preamble SOF Mac Dest MAC Source

Type Payload CRC

7 Octets 1 6 6 2 46-1500 4

Overhead26 OctetsPayload 46-1500 OctetsRunts < 72 OctetsGiants > 1572 OctetsError 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?

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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 forRadio 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)

• 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

Bit Rate = Baud rate x Bits per Symbol

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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• 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

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.

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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