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Transmission Media. Media. Basic function of media – carry flow of information in form of bits through a LAN In a copper based network, bits will be electrical signals In a fiber based network, bits will be light pulses Media considered to be Layer 1 component of a LAN

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media
Media
  • Basic function of media – carry flow of information in form of bits through a LAN
    • In a copper based network, bits will be electrical signals
    • In a fiber based network, bits will be light pulses
    • Media considered to be Layer 1 component of a LAN
  • Physical path between transmitter and receiver
  • Wired and Wireless
  • Communication is in the form of electromagnetic waves
  • Characteristics and quality of data transmission are determined by characteristics of medium and signal
  • In wired media, medium characteristics is more important, whereas in wireless media, signal characteristics is more important
transmission media1
Transmission Media
  • Physical path between transmitter and receiver
  • Wired and Wireless
  • Communication is in the form of electromagnetic waves
  • Characteristics and quality of data transmission are determined by characteristics of medium and signal
  • In wired media, medium characteristics is more important, whereas in wireless media, signal characteristics is more important
b asic limitations
Basic limitations
  • Attenuation
  • Delay Distortion
  • Noise
    • Thermal/White Noise
    • Intermodulation Noise
    • Crosstalk
    • Echo
    • Impulse Noise
design factors for transmission media
Design Factors for Transmission Media
  • Bandwidth: All other factors remaining constant, the greater the band-width of a signal, the higher the data rate that can be achieved.
  • Transmission impairments. Limit the distance a signal can travel.
  • Interference: Competing signals in overlapping frequency bands can distort or wipe out a signal.
  • Number of receivers: Each attachment introduces some attenuation and distortion, limiting distance and/or data rate.
classes of transmission media

Transmission media

Guided

Unguided

Twisted-pair

cable

Coaxial

cable

Fiber-optic

cable

Classes of Transmission Media
  • 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 media
    • use radio waves of different frequencies and do not need a wire or cable conductor to transmit signals
wire conductors
Wire Conductors
  • Wire types: single conductor, twisted pair, & shielded multiconductor bundles.
  • Large installed base.
  • Reasonable cost.
  • Relatively low bandwidth, however, recent LAN speeds in the 100 Mbps range have been achieved.
  • Susceptible to external interference.
  • Shielding can reduce external interference.
  • Can transmit both analog and digital signals. Amplifier required every 5 to 6 km for analog signals. For digital signals, repeaters required every 2 to 3 km.
wired twisted pair
Wired - Twisted Pair
  • The oldest, least expensive, and most commonly used media
  • Pair of insulated wires twisted together to reduce susceptibility to interference : ex) capacitive coupling, crosstalk
  • Skin effect at high frequency
  • Up to 250 kHz analog and few Mbps digital signaling ( for long-distance point-to-point signaling)
  • Need repeater every 2-3 km (digital), and amplifier every 5-6 km (analog)
twisted pair
Twisted Pair
  • 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
  • Telephone (subscriber loop: between house and local exchange)
  • High-speed (10 - 100 Mbps) LAN :
    • token ring, fast -Ethernet
types of twisted pair
Types of Twisted Pair
  • 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
ratings of twisted pair
Ratings of Twisted Pair
  • Category 3
    • UTP cables and associated connecting hardware whose transmission characteristics are specified up to 16 MHZ.
    • data rates of up to 16mbps are achievable
  • Category 4
    • UTP cables and associated connecting hardware whose transmission characteristics are specified up to 20 MHz.
  • Category 5
    • UTP cables and associated connecting hardware whose transmission characteristics are specified up to 100 MHz.
    • data rates of up to 100mbps are achievable
    • more tightly twisted than Category 3 cables
    • more expensive, but better performance
  • Category 5 enhanced, Cat 6, cat 7
    • Fast and giga-ethernet
  • STP
    • More expensive, harder to work with
twisted pair advantages
Twisted Pair Advantages
  • Advantages
    • Inexpensive and readily available
    • Flexible and light weight
    • Easy to work with and install
  • Disvantages
    • Susceptibility to interference and noise
    • Attenuation problem
      • For analog, repeaters needed every 5-6km
      • For digital, repeaters needed every 2-3km
    • Relatively low bandwidth (MHz)
wired transmission media
Wired Transmission Media
  • Coaxial Cable
    • Most versatile medium

=> LANs, Cable TV, Long-distance telephones, VCR-to-TV connections

    • Noise immunity is good
    • Very high channel capacity

=> few 100 MHz / few 100 Mbps

    • Need repeater/amplifier every few kilometer or so (about the same as with twisted pair)
  • Has an inner conductor surrounded by a braided mesh
  • Both conductors share a common center axial, hence the term “co-axial”
coaxial cable
Coaxial cable
  • Signal and ground wire
    • Solid center conductor running coaxially inside a solid (usually braided) outer circular conductor.
    • Center conductor is shielded from external interference signals.
properties of coaxial cable
Properties of coaxial cable
  • Better shielding allows for longer cables and higher transfer rates.
  • 100 m cables
    • 1 to 2 Gbps feasible (modulation used)
    • 10 Mbps typical
  • Higher bandwidth
    • 400 to 600Mhz
    • up to 10,800 voice conversations
  • Can be tapped easily: stations easily added (pros and cons)
  • Much less susceptible to interference than twisted pair
  • Used for long haul routes by Phone Co.
    • Mostly replaced now by optical fiber.
  • High attenuation rate makes it expensive over long distance
  • Bulky
  • Baseband vs. broadband coax.
wired transmission media1
Wired Transmission Media
  • Optical Fiber
    • Flexible, thin (few to few hundred m), very pure glass/plastic fiber capable of conducting optical rays
    • Extremely high bandwidth : capable of  2 Gbps
    • Very high noise immunity, resistant to electromagnetic interference
    • Does not radiate energy/cause interference
    • Very light
    • Need repeaters only 10’s or 100 km apart
    • Very difficult to tap : Better security but multipoint not easy
    • Require a light source with injection laser diode (ILD) or light-emitting diodes (LED)
wired transmission media2
Wired Transmission Media
  • Optical Fiber (Cont’d)
    • Need optical-electrical interface (more expensive than electrical interface)
wired transmission media3
Wired Transmission Media

Optical Fiber

  • Principle of optical fiber transmission: Based on the principle of total internal reflection
  • If >, medium B (water) has a higher optical density than medium A (air)
  • In case the index of refraction<1 (>), if  is less than a certain critical angle, there is no refracted light i.e., all the light is reflected. This is what makes fiber optics work.
fiber optics physics 101
Fiber optics & Physics 101
  • Refractive indexmaterial = (Speed of light in vacuum)/(Speed of light in material)
  • Light is bent as it passes through a surface where the refractive index changes. This bending depends on the angle and refractive index. Frequency does not change, but because it slows down, the wave length gets shorter, causing wave to bend.
  • In case of fiber optic media, refractive index of core > refractive index of cladding thereby causing internal reflection.

cladding

core

fiber optic layers

plastic jacket

glass or plastic

cladding

fiber core

Fiber Optic Layers
  • consists of three concentric sections
modes of fiber
Modes of fiber
  • Fiber consists of two parts: the glass core and glass cladding with a lower refractive index.
  • Light propagates in 1 of 3 ways depending on the type and width of the core material.
    • Multimode stepped index fiber
      • Both core and cladding have different but uniform refractive index.
      • Relies on total internal reflection; Wide pulse width.
    • Multimode graded index fiber
      • Core has variable refractive index (light bends as it moves away from core).
      • Narrow pulse width resulting in higher bit rate.
    • Singlemode fiber (> 100 Mbs)
      • Width of core diameter equal to a single wavelenth.
slide30

Mode

Multimode

Single mode

Step index

Graded-index

fiber optic types
Fiber Optic Types
  • multimode step-index fiber
    • the reflective walls of the fiber move the light pulses to the receiver
  • multimode 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
types of optical fiber
Types of optical fiber
  • Modes, bundles of light rays enter the fiber at a particular angle
  • Single-mode
    • Also known as mono-mode
    • Only one mode propagates through fiber
    • Higher bandwidth than multi-mode
    • Longer cable runs than multi-mode
    • Lasers generate light signals
    • Used for inter-building connectivity
types of optical fiber1
Types of optical fiber
  • Multi-mode
    • Multiple modes propagate through fiber
    • Different angles mean different distances to travel
      • Transmissions arrive at different times
      • Modal dispersion
    • LEDs as light source
    • Used for intra-building connectivity
fiber optic signals
Fiber Optic Signals

fiber optic multimode

step-index

fiber optic multimode

graded-index

fiber optic single mode

fiber optic
Fiber Optic

Advantages

  • greater capacity (bandwidth Gbps)
  • smaller size and lighter weight
  • lower attenuation
  • immunity to environmental interference
  • highly secure due to tap difficulty and lack of signal radiation

Disvantages

  • expensive over short distance
  • requires highly skilled installers
  • adding additional nodes is difficult
fibe r channel requirements
Fiber Channel Requirements
  • Full duplex links with 2 fibers/link
  • 100 Mbps – 800 Mbps
  • Distances up to 10 km
  • Small connectors
  • high-capacity
  • Greater connectivity than existing multidrop channels
  • Broad availability
  • Support for multiple cost/performance levels
  • Support for multiple existing interface command sets
components of an optical transmission system
Components of an optical transmission system
  • 3 components

1. Light source

2. Transmission medium

3. The detector

  • Light means a 1 bit, no light means a 0 bit.
  • Transmitter LED or injection laser diode.
  • Detector (photodiode or photo transistor) generates an electrical pulse when light falls on it.
  • Unidirectional data transmission system.
    • Electrical signal to light signal and back again.
fiber cables
Fiber cables
  • Multimode: diameter of core is ~50 microns.
    • About the same as a human hair.
  • Single mode: diameter of core 8-10 microns.
  • They can be connnected by connectors, or by splicing, or by fusion.
fiber vs copper
Fiber vs. copper
  • Fiber (pros)
    • Higher bandwidth,
    • Lower attenuation,
    • Immune to electromagnetic noise and corrosive chemicals,
    • Thin and lightweight,
    • Security (does not leak light, difficult to tap).
  • Fiber (cons)
    • Not many skilled “fiber engineers,”
    • Inherently unidirectional,
    • Fiber interfaces are expensive.
wireless unguided media transmission
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
the radio spectrum

l

f

The Radio Spectrum
  • Radio wave
    • Wavelength l = c/f
    • Speed of light c=3x108 m/s
    • Frequency: f

[V|U|S|E]HF = [Very|Ultra|Super|Extra] High Frequency

f = 900 MHz  l = 33 cm

atmospheric transmission media
Atmospheric Transmission Media
  • Infrared Transmission
    • Infrared networks use infrared light signals to transmit data
    • Direct infrared transmission depends on transmitter and receiver remaining within line of sight
    • In indirect infrared transmission, signals can bounce off of walls, ceilings, and any other objects in their path
atmospheric transmission media1
Atmospheric Transmission Media
  • RF Transmission
    • Radio frequency (RF) transmission relies on signals broadcast over specific frequencies
    • Narrowband concentrates significant RF energy at a single frequency
    • Spread spectrum uses lower-level signals distributed over several frequencies simultaneously
infrared

Wireless Transmission

Infrared
  • For short-range communication
  • Remote controls for TVs, VCRs and stereos
  • IRD port
  • Indoor wireless LANs
  • Do not pass through solid walls
  • Better security and no interference (with a similar system in adjacent rooms)
  • No government license is needed
  • Cannot be used outdoors
infrared1
Infrared
  • Transceivers must be within line of sight of each other (directly or via reflection)
  • Unlike microwaves, infrared does not penetrate walls
  • Fairly low bandwidth (4 Mbps).
  • Uses wavelengths between microwave and visible light.
  • Uses transmitters/receivers (transceivers) that modulate noncoherent infrared light.
  • No frequency allocation issue since not regulated.
  • Uses include local building connections, wireless LANs, and new wireless peripherals.
infrared waves
Infrared Waves
  • Short range communication.
    • e.g. Remotes on VCR’s and TV’s.
  • Directional.
  • Do not pass through walls.
    • Behaves more like visible light.
  • Can be used for LANs
    • indoors only.
  • Can just use visible unguided light (lasers).
wireless transmission
Wireless Transmission

Frequencies

  • 2GHz to 40GHz
    • Microwave
    • Highly directional
    • Point to point
    • Satellite
  • 30MHz to 1GHz
    • Omnidirectional
    • Broadcast radio
  • 3 x 1011 to 2 x 1014
    • Infrared
wireless transmission1
Wireless Transmission

Terrestrial Microwave

  • Parabolic dish
  • Focused beam
  • Line of sight
  • Long haul telecommunications
  • Higher frequencies give higher data rates
terrestrial microwave
Terrestrial Microwave
  • used for long-distance telephone service
  • uses radio frequency spectrum, from 2 to 40 Ghz
  • parabolic dish transmitter, mounted high
  • used by common carriers as well as private networks
  • requires unobstructed line of sight between source and receiver
  • curvature of the earth requires stations (repeaters) ~30 miles apart
radio transmission
Radio Transmission
  • Radio waves
    • Easy to generate, travel long distances, and penetrate buildings easily.
    • Omnidirectional.
    • Low frequencies
      • Pass through obstacles well,
      • Quick power drop off (e.g. 1/r3 in air).
    • High frequencies
      • Travel in straight lines and bounce off obstacles.
      • Absorbed by rain.
    • Subject to electrical interference
media broadcast radio
Media: Broadcast Radio
  • Covers 30MHz to 1 GHz
  • Omindirectional
  • Enables mobile communication & computing!
  • Broadcast mechanisms: cellular radio, radio nets, & low-orbit satellites.
  • Low bandwidth.
  • Lack of security.
  • Susceptible to interference (primarily multipath interference).
  • Reallocation of limited frequencies may be required for wireless communication growth.
microwave transmission
Microwave transmission
  • Microwave waves
    • Travel in straight lines and thus can be narrowly focused.
      • Easy to avoid interference with other microwaves.
    • Parabolic antenna is used to concentrate the energy (improves SNR).
    • More popular before fiber.
    • Waves do not pass through buildings.
    • Multiple towers used as repeaters.
media terrestrial microwave
Media: Terrestrial Microwave
  • High bandwidth (~45 Mbps).
  • No cabling between sites.
  • Clear line-of-sight required (30 miles).
  • Susceptible to radio interference.
  • Attenuation increases with rainfall
  • Lack of security.
  • Up-front investment in towers & repeaters.
  • Low power used to minimize effects on people
  • line of sight requirement
  • expensive towers and repeaters
  • subject to interference such as passing airplanes and rain
satellite microwave

Wireless Transmission

Satellite Microwave
  • Satellite is relay station
  • Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency
  • Requires geo-stationary orbit
    • Height of 35,784km
  • Optimum transmission in 1 - 10 GHz range;
  • Bandwidth of 100’s MHz
  • Significant propagation delay (270 ms)
  • Application: Television, long distance telephone, Private business networks
satellite transmission process
Satellite Transmission Process

satellite

transponder

dish

dish

22,300 miles

uplink station

downlink station

satellite transmission applications
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
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
physical media
physical link: transmitted data bit propagates across link

guided media:

signals propagate in solid media: copper, fiber

unguided media:

signals propagate freelye.g., radio

Twisted Pair (TP)

two insulated copper wires

Category 3: traditional phone wires, 10 Mbps ethernet

Category 5 TP: 100Mbps ethernet

Physical Media
physical media coax fiber
Coaxial cable:

wire (signal carrier) within a wire (shield)

baseband: single channel on cable

broadband: multiple channel on cable

bidirectional

common use in 10Mbs Ethernet

Physical Media: coax, fiber

Fiber optic cable:

  • glass fiber carrying light pulses
  • high-speed operation:
    • 100Mbps Ethernet
    • high-speed point-to-point transmission (e.g., 5 Gbps)
  • low error rate
physical media radio
signal carried in electromagnetic spectrum

no physical “wire”

bidirectional

propagation environment effects:

reflection

obstruction by objects

interference

Physical media: radio

Radio link types:

  • microwave
    • e.g. up to 45 Mbps channels
  • LAN (e.g., waveLAN)
    • 2Mbps, 11Mbps, 54 Mbps
  • wide-area (e.g., cellular)
    • e.g. CDPD, 10’s Kbps
  • satellite
    • up to 50Mbps channel (or multiple smaller channels)
    • 270 msec end-end delay
    • geosynchronous versus LEOS
cables at least 16 types described
Cables, at least 16 types described
  • Cat 3, 5, 6, 7
  • Screened and unscreened
  • USB cable
  • IEEE 1394 cable
  • Plastic Optical Fiber
  • 50/125, 62.5/125 and singlemode fibre
  • 75 ohm 3-GHz coax
  • speaker wire - two grades
ieee 1394 firewire
IEEE 1394 ‘Firewire’

22 AWG

28 AWG

IEEE 1394b

800, 1600, 3200 Mb/s over POF

3.2 Gb/s over glass fibre

100 Mb/s over UTP

400 Mb/s over 4.5 m

universal serial bus
Universal Serial Bus

1.5, 12 or 480 Mb/s, up to 5 m, cascade 5 devices up to 30 m

choosing the right transmission media
Choosing the Right Transmission Media
  • Areas of high EMI or RFI
  • Corners and small spaces
  • Distance
  • Security
  • Existing infrastructure
  • Growth
media selection criteria
Media: Selection Criteria
  • Cost (Initial, Expansion, & Maintenance)
  • Speed (Data Rate & Response Time)
  • Availability
  • Expandability
  • Error Rates
  • Security
  • Distance (Geography & Number of Sites)
  • Environment
  • Application-Specific Constraints
  • Maintenance
from signals to packets
From Signals to Packets

Analog Signal

“Digital” Signal

0 0 1 0 1 1 1 0 0 0 1

Bit Stream

0100010101011100101010101011101110000001111010101110101010101101011010111001

Packets

Header/Body

Header/Body

Header/Body

Packet

Transmission

Sender

Receiver

modulation
Modulation
  • Sender changes the nature of the signal in a way that the receiver can recognize.
    • Similar to radio: AM or FM
  • Digital transmission: encodes the values 0 or 1 in the signal.
    • It is also possible to encode multi-valued symbols
  • Amplitude modulation: change the strength of the signal, typically between on and off.
    • Sender and receiver agree on a “rate”
    • On means 1, Off means 0
  • Similar: frequency or phase modulation.
  • Can also combine method modulation types.
amplitude and frequency modulation
Amplitude and FrequencyModulation

0 0 1 1 0 0 1 1 0 0 0 1 1 1 0 0 0 1 1 0 0 0 1 1 1 0

0 1 1 0 1 1 0 0 0 1

assumptions
Assumptions
  • We use two discrete signals, high and low, to encode 0 and 1
  • The transmission is synchronous, i.e., there is a clock used to sample the signal
    • In general, the duration of one bit is equal to one or more clock ticks
encoding
Encoding
  • Goal: Send bits from one node to another node on the same physical media
  • Problem: Specify a robust and efficient encoding scheme to achieve this goal
encoding schemes
Encoding Schemes
  • Non Return to Zero (NRZ)
  • Non Return to Zero Inverted (NRZI)
  • Manchester Encoding
  • 4B/5B Encoding
modulation1
Modulation
  • Non-Return to Zero (NRZ)
    • Used by Synchronous Optical Network (SONET)
    • 1=high signal, 0=low signal
    • Long sequence of same bit cause difficulty
      • DC bias hard to detect – low and high detected by difference from average voltage
      • Clock recovery difficult
modulation2
Modulation
  • Non-Return to Zero Inverted (NRZI)
    • 1=inversion of current value, 0=same value
    • No problem with string of 1’s
    • NRZ-like problem with string of 0’s
modulation3
Modulation
  • Manchester
    • Used by Ethernet
    • 1=low to high transition, 0=high to low transition
    • Transition for every bit simplifies clock recovery
    • Not very efficient
      • Doubles the number of transitions
      • Circuitry must run twice as fast
modulation4
Modulation
  • 4b/5b
    • Used by FDDI
    • Uses 5bits to encode every 4bits
    • Encoding ensures no more than 3 consecutive 0’s
    • Uses NRZI to encode resulting sequence
    • 16 data values, 3 “special” illegal values, 6 “extra” values, 7 illegal values
chapter summary
Chapter Summary
  • Information can be transmitted via analog or digitally
  • Both signals suffer attenuation
  • Throughput is the amount of data a medium can transmit during a given period of time
  • Costs depend on many factors
  • Three specifications dictating networking media
  • Length of a network segment is limited due to attenuation
  • Connectors connect wire to the network device
  • Coaxial cable consists of central copper core surrounded by an insulator and a sheath
  • In baseband transmission, digital signals are sent through direct current pulse applied to the wire
chapter summary1
Chapter Summary
  • Twisted-pair cable consists of color-coded pairs of insulated copper wires, twisted around each other and encased in plastic coating
  • The more twists per inch in a pair of wires, the more resistant to noise
  • STP cable consists of twisted pair wires individually insulated and surrounded by a shielding
  • UTP cabling consists of one or more insulated wire pairs encased in a plastic sheath
  • UTP comes in a variety of specifications
  • Fiber-optic cable contains one or several glass fibers in its core
  • On today’s networks, fiber is used primarily as backbone cable
chapter summary2
Chapter Summary
  • Best practice for installing cable is to follow the TIA/EIA 568 (see “structured cabling”) specifications and manufacturer’s recommendations
  • Wireless LANs can use radio frequency (RF) or infrared transmission
  • Infrared transmission can be indirect or direct
  • RF transmission can be narrowband or spread spectrum
  • To make correct media transmission choices, consider, throughput, cabling, noise resistance, security/flexibility, and plans for growth
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