Electromagnetic Signals

Function of time Analog (varies smoothly over time) Digital (constant level over time,

followed by a change to another level)Function of frequency

Spectrum (range of frequencies) Bandwidth (width of the spectrum)

Periodic Signal Characteristics

Amplitude (A): signal value, measured in volts

Frequency (f): repetition rate, cycles per second or Hertz

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

Phase (Φ): relative position in time, measured in degrees

time(sec)

amp

litu

de

(vo

lts)

1 cycle

frequency (hertz)= cycles per second

phase difference

Analog Signaling

represented by sine waves

Digital Signaling

represented by square waves or pulses

time(sec)

amp

litu

de

(vo

lts)

1 cycle

frequency (hertz)= cycles per second

Digital Text Signaling

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 form: ASCII

ASCII Character Codes

Use 8 bits of data (1 byte) to transmit one character

8 binary bits has 256 possible outcomes (0 to 255)

Represents alphanumeric characters, as well as “special” characters

Digital Image Signaling

Pixelization and binary representation

Code: 0000000000111100011101100111111001111000011111100011110000000000

Why Study Analog?

Telephone system is primarily analog rather than digital (designed to carry voice signals)

Low-cost, ubiquitous transmission medium

If we can convert digital information (1s and 0s) to analog form (audible tone), it can be transmitted inexpensively

Voice 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

Bandwidth

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

bandwidth=3100HzGreater bandwidth leads to greater costsLimited bandwidth leads to distortionAnalog measured in Hertz, digital

measured in baud

BPS vs. Baud

BPS=bits per secondBaud=# of signal changes per

secondEach signal change can represent

more than one bit, through variations on amplitude, frequency, and/or phase

Transmission Media

the physical path between transmitter and receiver

design factors bandwidth attenuation: weakening of signal over

distances interference: number of receivers

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)

Transmission Impairments

Attenuation loss of signal strength over distance

Attenuation Distortion different losses at different frequencies

Delay Distortion different speeds for different

frequenciesNoise

Types of Noise

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

eliminatedIntermodulation

when different frequenciesCrosstalkImpulse noise

Less predictable

Transmission Media

two major classes conducted or guided media

use a conductor such as a wire or a fiber optic cable to move the signal from sender to receiver

wireless or unguided mediause radio waves of different frequencies and

do not need a wire or cable conductor to transmit signals

Guided Transmission Media

the transmission capacity depends on the distance and on whether the medium is point-to-point or multipoint

e.g., twisted pair wires coaxial cables optical fiber

Twisted Pair Wires

consists of two insulated copper wires arranged in a regular spiral pattern to minimize the electromagnetic interference between adjacent pairs

often used at customer facilities and also over distances to carry voice as well as data communications

low frequency transmission medium

Twisted Pair Wires

two varieties STP (shielded twisted pair)

the pair is wrapped with metallic foil or braid to insulate the pair from electromagnetic interference

UTP (unshielded twisted pair)each wire is insulated with plastic wrap, but

the pair is encased in an outer covering

Twisted Pair Wires

Category 3 UTP data rates of up to 16mbps are achievable

Category 5 UTP data rates of up to 100mbps are achievable more tightly twisted than Category 3 cables more expensive, but better performance

STP More expensive, harder to work with

Twisted Pair Advantages

inexpensive and readily availableflexible and light weight easy to work with and install

Twisted Pair Disadvantages

susceptibility to interference and noise

attenuation problem For analog, repeaters needed every 5-

6km For digital, repeaters needed every 2-

3kmrelatively low bandwidth (3000Hz)

Coaxial Cable (or Coax)

bandwidth of up to 400 MHzhas an inner conductor surrounded

by a braided meshboth conductors share a common

center axial, hence the term “co-axial”

Coax Layers

copper or aluminum conductor

insulating material

shield(braided wire)

outer jacket(polyethylene)

Coax Advantages

higher bandwidth 400 to 600Mhz up to 10,800 voice conversations

can be tapped easily (pros and cons)much less susceptible to interference

than twisted pair

Coax Disadvantages

high attenuation rate makes it expensive over long distance

bulky

Fiber Optic Cable

relatively new transmission medium used by telephone companies in place of long-distance trunk lines

also used by private companies in implementing local data communications networks

require a light source with injection laser diode (ILD) or light-emitting diodes (LED)

plastic jacket glass or plasticcladding

fiber core

Fiber Optic Layers

consists of three concentric sections

Fiber Optic Types

multimode step-index fiber the reflective walls of the fiber move the

light pulses to the receivermultimode graded-index fiber

acts to refract the light toward the center of the fiber by variations in the density

single mode fiber the light is guided down the center of an

extremely narrow core

fiber optic multimodestep-index

fiber optic multimodegraded-index

fiber optic single mode

Fiber Optic Signals

Fiber Optic Advantages

greater capacity (bandwidth of up to 2 Gbps)

smaller size and lighter weightlower attenuationimmunity to environmental

interferencehighly secure due to tap difficulty

and lack of signal radiation

Fiber Optic Disadvantages

expensive over short distancerequires highly skilled installersadding additional nodes is difficult

Wireless (Unguided Media) Transmission

transmission and reception are achieved by means of an antenna

directional transmitting antenna puts out focused beam transmitter and receiver must be aligned

omnidirectional signal spreads out in all directions can be received by many antennas

Wireless Examples

terrestrial microwave transmissionsatellite transmissionbroadcast radioinfrared

Terrestrial Microwave Transmission

uses the radio frequency spectrum, commonly from 2 to 40 Ghz

transmitter is a parabolic dish, mounted as high as possible

used by common carriers as well as by private networks

requires unobstructed line of sight between source and receiver

curvature of the earth requires stations (called repeaters) to be ~30 miles apart

Microwave Transmission Applications

long-haul telecommunications service for both voice and television transmission

short point-to-point links between buildings for closed-circuit TV or a data link between LANs

bypass application

Microwave Transmission Advantages

no cabling needed between siteswide bandwidth multichannel transmissions

Microwave Transmission Disadvantages

line of sight requirementexpensive towers and repeaterssubject to interference such as

passing airplanes and rain

Satellite Microwave Transmission

a microwave relay station in spacecan relay signals over long distancesgeostationary satellites

remain above the equator at a height of 22,300 miles (geosynchronous orbit)

travel around the earth in exactly the time the earth takes to rotate

Satellite Transmission Links

earth stations communicate by sending signals to the satellite on an uplink

the satellite then repeats those signals on a downlink

the broadcast nature of the downlink makes it attractive for services such as the distribution of television programming

dish dish

uplink station downlink station

satellitetransponder

22,300 miles

Satellite Transmission Process

Satellite Transmission Applications

television distribution a network provides programming from a

central location direct broadcast satellite (DBS)

long-distance telephone transmission high-usage international trunks

private business networks

Principal Satellite Transmission Bands

C band: 4(downlink) - 6(uplink) GHz the first to be designated

Ku band: 12(downlink) -14(uplink) GHz rain interference is the major problem

Ka band: 19(downlink) - 29(uplink) GHz equipment needed to use the band is still

very expensive

Satellite Advantages

can reach a large geographical areahigh bandwidthcheaper over long distances

Satellite Disadvantages

high initial costsusceptible to noise and interferencepropagation delay

Common Carriers

a government-regulated private company involved in the sale of infrastructure

services in transportation and communications

required to serve all clients indiscriminately

services and prices from common carriers are described in tariffs

Leased (or Dedicated) Lines

permanently or semi-permanently connect between two points

economical in high volume calls between two point

no delay associated with switching times

can assure consistently high-quality connections

Leased (or Dedicated) Lines

voice grade channels normal telephone lines in the range of 300 Hertz to 3300 Hertz

conditioning or equalizing reduces the amount of noise on the line,

providing lower error rates and increased speed for data communications

T-1 Carrieralso referred to as DS-1 signalingprovides digital full-duplex transmission

rates of 1.544Mbpsusually created by multiplexing 24 64-

Kbps voice or 56-Kbps data lineshigher speeds are available with T-3

(45Mbps) and T-4 services (274Mbps)in Europe, E-1 (2.048Mbps) is used instead

of T-1

Integrated Services Digital Network (ISDN)

all-digital transmission facility that is designed to replace the analog PSTN

basic ISDN (basic rate access) two 64Kbps bearer channels + 16Kbps

data channel (2B+D) = 144 Kbpsbroadband ISDN (primary rate access)

twenty-three 64Kbps bearer channels + 64 data channel (23B+D) = 1.536 Mbps

Past Criticism of ISDN

“Innovations Subscribers Don’t Need”“It Still Doesn’t Network”“It Still Does Nothing”Why so much criticism?

overhyping of services before delivery high price of equipment delay in implementing infrastructure incompatibility between providers'

equipment.

ISDN Channel Definitions

B (bearer) channels 64 kbps channels that may be used to

carry voice, data, facsimile, or imageD (demand) channels

mainly intended for carrying signaling, billing and management information to control ISDN services (out-of-band control messages)

may be either 16 or 64 kbps

Two Levels of ISDN Service

basic rate interface (BRI) 2B (64 kbps) + D (16 kbps) = 144 kbps

primary rate interface (PRI) 23B (64 kbps) + D (64 kbps) = 1.536

MbpsNorth American standard

30B (64 kbps) + D (64 kbps) = 1.984 MbpsEuropean standard