TEL205 Week 14

Stallings Chapter 15:Data Transmission

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Electromagnetic Signals

• Analog Signal • signal intensity varies in a smooth fashion over time.

In other words, there are no breaks or discontinuities in the signal

• Digital Signal • signal intensity maintains a constant level for some

period of time and then changes to another constant level

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Analog Sine Wave

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Digital Square Wave

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Periodic Signal Characteristics

• Peak Amplitude (A)• Maximum signal value, measured in volts

• Frequency (f)• Repetition rate• Measured in cycles per second or Hertz (Hz)

• Period (T)• Amount of time it takes for one repetition, T=1/f

• Phase ()• Relative position in time, measured in degrees

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s(t) = (4/) (sin (2ft) + (1/3) sin (2(3f)t))

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Frequency Domain Concepts

• Spectrum of a signal is the range of frequencies that it contains

• Absolute bandwidth of a signal is the width of the spectrum

• Effective bandwidth contained in a relatively narrow band of frequencies, where most of signal’s energy is found

• The greater the bandwidth, the higher the information-carrying capacity of the signal

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Bandwidth

• Width of the spectrum of frequencies that can be transmitted• if spectrum=300 to 3400Hz, bandwidth=3100Hz

• Greater bandwidth leads to greater costs• Limited bandwidth leads to distortion

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Analog Signaling

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Voice/Audio Analog Signals

• Easily converted from sound frequencies (measured in loudness/db) to electromagnetic frequencies, measured in voltage

• Human voice has frequency components ranging from 20Hz to 20kHz

• For practical purposes, the telephone system has a narrower bandwidth than human voice, from 300 to 3400Hz

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Image/Video: Analog Data to Analog Signals

• Image is scanned in lines; each line is displayed with varying levels of intensity

• Requires approximately 4Mhz of analog bandwidth

• Since multiple signals can be sent via the same channel, guardbands are necessary, raising bandwidth requirements to 6Mhz per signal

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Digital Signaling

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Digital Text Signals

• Transmission of electronic pulses representing the binary digits 1 and 0

• How do we represent letters, numbers, characters in binary form?

• Earliest example: Morse code (dots and dashes)

• Most common current forms: ASCII, UTF

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Transmission Media

• Physical path between transmitter and receiver (“channel”)

• Design factors affecting data rate• bandwidth• physical environment• number of receivers• impairments

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Impairments and Capacity

• Impairments exist in all forms of data transmission

• Analog signal impairments result in random modifications that impair signal quality

• Digital signal impairments result in bit errors (1s and 0s transposed)

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Transmission Impairments:Guided Media

• Attenuation• loss of signal strength over distance

• Attenuation Distortion• different losses at different frequencies

• Delay Distortion• different speeds for different frequencies

• Noise• distortions of signal caused by interference

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Transmission Impairments:Unguided (Wireless) Media

• Free-Space Loss• Signals disperse with distance

• Atmospheric Absorption• Water vapor and oxygen contribute to signal loss

• Multipath• Obstacles reflect signal creating multiple copies

• Refraction• Thermal Noise

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Types of Noise

• Thermal (aka “white noise”)• Uniformly distributed, cannot be eliminated

• Intermodulation• When different frequencies collide (creating

“harmonics”)

• Crosstalk• Overlap of signals

• Impulse noise• Irregular spikes, less predictable

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Channel Capacity

• The rate at which data can be transmitted over a given path, under given conditions

• Four concepts• Data rate• Bandwidth• Noise• Error rate

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Shannon Equation

• C = B log2 (1 + SNR)

• B = Bandwidth • C= Channel• SNR = Signal-to-noise ratio

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Stallings Chapter 16:Data Communication Fundamentals

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Data Communication Components• Data

• Analog: Continuous value data (sound, light, temperature)

• Digital: Discrete value (text, integers, symbols)

• Signal• Analog: Continuously varying electromagnetic wave• Digital: Series of voltage pulses (square wave)

• Transmission• Analog: Works the same for analog or digital signals• Digital: Used only with digital signals

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Analog DataSignal Options

• Analog data to analog signal• Inexpensive, easy conversion (eg telephone)• Data may be shifted to a different part of the

available spectrum (multiplexing)• Used in traditional analog telephony

• Analog data to digital signal• Requires a codec (encoder/decoder)• Allows use of digital telephony, voice mail

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Digital DataSignal Options

• Digital data to analog signal• Requires modem (modulator/demodulator)• Allows use of PSTN to send data• Necessary when analog transmission is used

• Digital data to digital signal• Requires CSU/DSU (channel service unit/data

service unit)• Less expensive when large amounts of data are

involved• More reliable because no conversion is involved

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Transmission Choices

• Analog transmission• only transmits analog signals, without regard for

data content• attenuation overcome with amplifiers• signal is not evaluated or regenerated

• Digital transmission• transmits analog or digital signals• uses repeaters rather than amplifiers• switching equipment evaluates and regenerates

signal

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Data

Signal

TransmissionSystem

A

DD

DA

A

Data, Signal, and Transmission Matrix

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Advantages of Digital Transmission

• The signal is exact• Signals can be checked for errors• Noise/interference are easily filtered out• A variety of services can be offered over one

line• Higher bandwidth is possible with data

compression

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Why Use Analog Transmission?

• Already in place• Significantly less expensive• Lower attenuation rates• Fully sufficient for transmission of voice

signals

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Analog Encoding of Digital Data

• Data encoding and decoding technique to represent data using the properties of analog waves

• Modulation: the conversion of digital signals to analog form

• Demodulation: the conversion of analog data signals back to digital form

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Modem

• An acronym for modulator-demodulator• Uses a constant-frequency signal known as

a carrier signal• Converts a series of binary voltage pulses

into an analog signal by modulating the carrier signal

• The receiving modem translates the analog signal back into digital data

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Methods of Modulation

• Amplitude modulation (AM) or amplitude shift keying (ASK)

• Frequency modulation (FM) or frequency shift keying (FSK)

• Phase modulation or phase shift keying (PSK)

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Amplitude Shift Keying (ASK)

• In radio transmission, known as amplitude modulation (AM)

• 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

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1 0 0 1

ASK Illustration

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Frequency Shift Keying (FSK)

• In radio transmission, known as frequency modulation (FM)

• 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

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1 1 0 1

FSK Illustration

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Phase Shift Keying (PSK)

• Also known as phase modulation (PM)• 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)

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0 0 1 1

PSK Illustration

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0 1 1

Differential Phase Shift Keying (DPSK)

0

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Cable Modems• Permits Internet access over cable television networks. • ISP is at or linked by high-speed line to central cable

office • Cables used for television delivery can also be used to

deliver data between subscriber and central location• Upstream and downstream channels are shared among

multiple subscribers, time-division multiplexing technique

• Splitter is used to direct TV signals to a TV and the data channel to a cable modem

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Cable Modem Layout

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Asymmetric DigitalSubscriber Line (ADSL)

• New modem technology for high-speed digital transmission over ordinary telephone wire.

• Telephone central office can provide support for a number of ISPs,

• At central office, a combined data/voice signal is transmitted over a subscriber line

• At subscriber’s site, twisted pair is split and routed to both a PC and a telephone• At the PC, an ADSL modem demodulates the data signal for the PC.

• At the telephone, a microfilter passes the 4-kHz voice signal.

• The data and voice signals are combined on the twisted pair line using frequency-division-multiplexing techniques (Chapter 17)

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DSL Modem Layout

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Digital Encoding of Analog Data

• Evolution of telecommunications networks to digital transmission and switching requires voice data in digital form

• Best-known technique for voice digitization is pulse-code modulation (PCM)

• The sampling theorem: If a signal is sampled at regular intervals of time and at a rate higher than twice the significant signal frequency, the samples contain all the information of the original signal.

• Good-quality voice transmission can be achieved with a data rate of 8 kbps

• Some videoconference products support data rates as low as 64 kbps

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Converting Samples to Bits

• Quantizing• Similar concept to pixelization• Breaks wave into pieces, assigns a value in

a particular range• 8-bit range allows for 256 possible sample

levels• More bits means greater detail, fewer bits

means less detail

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Codec

• Coder/Decoder• Converts analog signals into a digital form

and converts it back to analog signals• Where do we find codecs?

• Sound cards• Scanners• Voice mail• Video capture/conferencing

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Digital Encodingof Digital Data

• Most common, easiest method is different voltage levels for the two binary digits

• Typically, negative=1 and positive=0• Known as NRZ-L, or nonreturn-to-zero level,

because signal never returns to zero, and the voltage during a bit transmission is level

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Differential NRZ

• Differential version is NRZI (NRZ, invert on ones)

• Change=1, no change=0• Advantage of differential encoding is that it is

more reliable to detect a change in polarity than it is to accurately detect a specific level

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Problems With NRZ

• Difficult to determine where one bit ends and the next begins

• In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses

• Timing is critical, because any drift results in lack of synchronization and incorrect bit values being transmitted

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Biphase Alternatives to NRZ

• Require at least one transition per bit time, and may even have two

• Modulation rate is greater, so bandwidth requirements are higher

• Advantages• Synchronization due to predictable transitions• Error detection based on absence of a transition

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Manchester Code

• Transition in the middle of each bit period• Transition provides clocking and data• Low-to-high=1 , high-to-low=0• Used in Ethernet

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Differential Manchester

• Midbit transition is only for clocking• Transition at beginning of bit period=0• Transition absent at beginning=1• Has added advantage of differential

encoding• Used in token-ring

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Digital Encoding Illustration

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Digital Interfaces

• The point at which one device connects to another

• Standards define what signals are sent, and how

• Some standards also define physical connector to be used

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Analog Encoding of Analog Information

• Voice-generated sound wave can be represented by an electromagnetic signal with the same frequency components, and transmitted on a voice-grade telephone line.

• Modulation can produce a new analog signal that conveys the same information but occupies a different frequency band• A higher frequency may be needed for effective

transmission• Analog-to-analog modulation permits frequency-

division multiplexing

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Asynchronous and Synchronous Transmission

• For receiver to sample incoming bits properly, it must know arrival time and duration of each bit that it receives

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Asynchronous Transmission

• Avoids timing problem by not sending long, uninterrupted streams of bits

• Data transmitted one character at a time, where each character is 5 to 8 bits in length.

• Timing or synchronization must only be maintained within each character; the receiver has the opportunity to resynchronize at the beginning of each new character.

• Simple and cheap but requires an overhead of 2 to 3 bits per character

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Synchronous Transmission

• Block of bits transmitted in a steady stream without start and stop codes.

• Clocks of transmitter and receiver must somehow be synchronized• Provide a separate clock line between transmitter and

receiver; works well over short distances,

• Embed the clocking information in the data signal.

• Each block begins with a preamble bit pattern and generally ends with a postamble bit pattern

• The data plus preamble, postamble, and control information are called a frame

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Error Control Process

• All transmission media have potential for introduction of errors

• All data link layer protocols must provide method for controlling errors

• Error control process has two components• Error detection• Error correction

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Error Detection: Parity Bits

• Bit added to each character to make all bits add up to an even number (even parity) or odd number (odd parity)

• Good for detecting single-bit errors only• High overhead (one extra bit per 7-bit

character=12.5%)

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Error Detection: Cyclic Redundancy Check (CRC)

• Data in frame treated as a single binary number, divided by a unique prime binary, and remainder is attached to frame

• 17-bit divisor leaves 16-bit remainder, 33-bit divisor leaves 32-bit remainder

• For a CRC of length N, errors undetected are 2-N

• Overhead is low (1-3%)

Chapter 17:Data Link Controland Multiplexing

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Flow Control

• Necessary when data is being sent faster than it can be processed by receiver

• Computer to printer is typical setting• Can also be from computer to computer,

when a processing program is limited in capacity

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Error Correction

• Two types of errors• Lost frame• Damaged frame

• Automatic Repeat reQuest (ARQ)• Error detection• Positive acknowledgment• Retransmission after time-out• Negative acknowledgment and retransmission

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Data Link Control

• Specified flow and error control for synchronous communication

• Data link module arranges data into frames, supplemented by control bits

• Receiver checks control bits, if data is intact, it strips them

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High-Level Data Link Control

• On transmitting side, HDLC receives data from an application, and delivers it to the receiver on the other side of the link

• On the receiving side, HDLC accepts the data and delivers it to the higher level application layer

• Both modules exchange control information, encoded into a frame

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HDLC Frame Structure• Flag: 01111110, at start and end• Address: secondary station (for

multidrop configurations)• Information: the data to be

transmitted• Frame check sequence: 16- or

32-bit CRC

• Control: purpose or function of frame• Information frames:

contain user data• Supervisory frames:

flow/error control (ACK/ARQ)

• Unnumbered frames: variety of control functions (see p.131)

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HDLC Operation

• Initialization: S-frames specify mode and sequence numbers, U-frames acknowledge

• Data Transfer: I-frames exchange user data, S-frames acknowledge and provide flow/error control

• Disconnect: U-frames initiate and acknowledge

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HDLC Examples

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Multiplexing

• Shared use of communication capacity• Commonly used in long-haul communications,

on high-capacity fiber, coaxial, or microwave links

• Multiplexer combines data from n input lines and transmits over a higher-capacity data link

• Demultiplexer accepts multiplexed data stream, separates the data according to channel, and delivers them to the appropriate output lines.

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Multiplexing Diagram

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Motivations for Multiplexing

• The higher the data rate, the more cost-effective the transmission facility• cost per kbps declines with an increase in the data rate

of the transmission facility• cost of transmission and receiving equipment, per

kbps, declines with increasing data rate.

• Most individual data communicating devices require relatively modest data rate support

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Frequency Division Multiplexing (FDM)

• Requires analog signaling & transmission• Total bandwidth = sum of input bandwidths +

guardbands• Modulates signals so that each occupies a

different frequency band• Standard for radio broadcasting, analog

telephone network, and television (broadcast, cable, & satellite)

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Wavelength Division Multiplexing• Form of FDM used when multiple beams of

light at different frequencies are transmitted on the same optical fiber

• Most WDM systems operate in the 1550-nm range. In early systems, 200 MHz was allocated to each channel, but today most WDM systems use 50-GHz spacing

• dense wavelength division multiplexing (DWDM) connotes the use of more channels, more closely spaced (≤200Ghz), than ordinary WDM

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FDM Example: ADSL

• ADSL uses frequency-division modulation (FDM) to exploit the 1-MHz capacity of twisted pair.

• Asymmetric because ADSL provides more capacity downstream (from the carrier’s central office to the customer’s site) than upstream (from customer to carrier).

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3 Elements of ADSL Strategy

• Reserve lowest 25 kHz for voice, known as POTS

• Use echo cancellation or FDM to allocate a small upstream band and a larger downstream band

• Use FDM within the upstream and downstream bands, using “discrete multitone”

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Echo Cancellation

• Entire frequency band for the upstream channel overlaps the lower portion of the downstream channel

• Advantages• The higher the frequency, the greater the attenuation. • More flexible for changing upstream capacity

• Disdvantages• Need for echo cancellation logic on both ends of line

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Discrete Multitone (DMT)• Uses multiple carrier signals at different

frequencies, sending some of the bits on each channel.

• Transmission band (upstream or downstream) is divided into a number of 4-kHz subchannels.

• Modem sends out test signals on each subchannel to determine the signal to noise ratio; it then assigns more bits to better quality channels and fewer bits to poorer quality channels.

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Synchronous Time-Division Multiplexing (TDM)

• Used in digital transmission• Requires data rate of the medium to exceed data rate of

signals to be transmitted• Signals “take turns” over medium• Slices of data are organized into frames• Used in the modern digital telephone system

• US, Canada, Japan: DS-0, DS-1 (T-1), DS-3 (T-3), ...• Europe, elsewhere: E-1, E3, …

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Digital Carrier Systems

• Long-distance carrier system designed to transmit voice signals over high-capacity transmission links (e.g. optical fiber, coaxial cable, and microwave)

• Evolution of these networks to digital involved adoption of synchronous TDM transmission structures

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DS-1 Transmission Format

• Multiplexes 24 channels• Voice transmission

• Frame contains 8 bits per channel plus a framing bit for 24 8 + 1 = 193 bits

• Signal digitized with PCM at 8000 samples/second

• Data rate of 8000 193 = 1.544 Mbps

• Data transmission• 23 channels of data are provided• Last channel position reserved for special sync byte

• Mixed voice and data uses all 24 channels

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DS-1 Illustration

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T-1 Facilities

• Transmission facilities supporting DS-1• Often used for leased dedicated transmission

between customer premises• Private voice networks• Private data network• Video teleconferencing• High-speed digital facsimile• Internet access

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SONET/SDH

• SONET (Synchronous Optical Network) is an optical transmission interface proposed by BellCore and standardized by ANSI.

• Synchronous Digital Hierarchy (SDH), a compatible version, has been published by ITU-T

• Specifications for taking advantage of the high-speed digital transmission capability of optical fiber.

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SONET/SDH Signal Hierarchy

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STS-1 and STM-N Frames