electromagnetic signals zfunction of time yanalog (varies smoothly over time) ydigital (constant...
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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