William Stallings
Data and Computer
Communications
Chapter 4
Transmission Media
1

Overview
 Guided - wire
 Unguided - wireless
 Characteristics and quality determined by medium and signal
 For guided, the medium is more important
 For unguided, the bandwidth produced by the antenna is more important
 Key concerns are data rate and distance
2

Design Factors
 Bandwidth
 Higher bandwidth gives higher data rate
 Transmission impairments
 Attenuation
 Interference
 Number of receivers
 Major factor in guided media
 More receivers (multi-point) introduce more attenuation
3

Electromagnetic Spectrum
4

Guided Transmission Media
 the transmission capacity depends on the distance and on whether the medium is pointto-point or multipoint
 e.g.,
 Twisted Pair
 Coaxial cable
 Optical fiber
5

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
6

Twisted Pair - Applications
 Most common medium
 Telephone network
 Between house and local exchange (subscriber loop)
 Within buildings
 To private branch exchange (PBX)
 For local area networks (LAN)
 10Mbps or 100Mbps
7

Twisted Pair - Pros and Cons
 Cheap
 Easy to work with
 Low data rate
 Short range
8

Twisted Pair - Transmission
Characteristics
 Analog
 Amplifiers every 5km to 6km
 Digital
 Use either analog or digital signals
 repeater every 2km or 3km
 Limited distance
 Limited bandwidth (1MHz)
 Limited data rate (100MHz) using different modulation & signaling techniques
 Susceptible to interference and noise
9

Unshielded and Shielded TP
 Unshielded Twisted Pair (UTP)
 Ordinary telephone wire
 Cheapest
 Easiest to install
 Suffers from external electromagnetic interference
(EM)
 Shielded Twisted Pair (STP)
 the pair is wrapped with metallic foil or braid to insulate the pair from electromagnetic interference
 More expensive
 Harder to handle (thick, heavy)
10

UTP Categories
 Cat 3
 up to 16MHz
 Voice grade found in most offices
 Twist length of 7.5 cm to 10 cm
 Cat 4 (least common)
 up to 20 MHz
 Cat 5
 up to 100MHz
 Commonly pre-installed in new office buildings
 Twist length 0.6 cm to 0.85 cm
11

Twisted Pair Advantages
 inexpensive and readily available
 flexible and light weight
 easy to work with and install
12

Twisted Pair Disadvantages
 susceptibility to interference and noise
 attenuation problem
 For analog, repeaters needed every 5-6km
 For digital, repeaters needed every 2-3km
 relatively low bandwidth
13

Coaxial Cable
14

Coaxial Cable Applications
 Most versatile medium
 Television distribution
 Aerial to TV
 Cable TV
 Long distance telephone transmission
 Can carry 10,000 voice calls simultaneously
 Being replaced by fiber optic
 Short distance computer systems links
 Local area networks
15

Coaxial Cable - Transmission
Characteristics
 Analog
 Amplifiers every few km
 Closer if higher frequency
 Up to 500MHz
 Digital
 Repeater every 1km
 Closer for higher data rates
16

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
17

Coax Disadvantages
 high attenuation rate makes it expensive over long distance
 bulky
18

Optical Fiber
19

Optical Fiber - Benefits
 Greater capacity
 Data rates of hundreds of Gbps
 Smaller size & weight
 Lower attenuation
 Electromagnetic isolation
 Greater repeater spacing
 10s of km at least
20

Optical Fiber - Applications
 Long-haul trunks
 Metropolitan trunks
 Rural exchange trunks
 Subscriber loops
 LANs
21

Optical Fiber - Transmission
Characteristics
 Act as wave guide for 10 14 to 10 15 Hz
 Portions of infrared and visible spectrum
 Light Emitting Diode (LED)
 Cheaper
 Wider operating temp range
 Last longer
 Injection Laser Diode (ILD)
 More efficient
 Greater data rate
 Wavelength Division Multiplexing
22

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
23

Optical Fiber Transmission
Modes
24

Fiber Optic Signals fiber optic multimode step-index fiber optic multimode graded-index fiber optic single mode
25

Fiber Optic Advantages
 greater capacity (bandwidth of up to 2 Gbps)
 smaller size and lighter weight
 lower attenuation
 immunity to environmental interference
 highly secure due to tap difficulty and lack of signal radiation
26

Fiber Optic Disadvantages
 expensive over short distance
 requires highly skilled installers
 adding additional nodes is difficult
27

Wireless Transmission
 Unguided media
 Transmission and reception via antenna
 Two techniques are used:
 Directional
 Focused beam
 Careful alignment required
 Omnidirectional
 Signal spreads in all directions
 Can be received by many antennas
28

Frequencies
 2GHz to 40GHz
 Microwave
 Highly directional
 Point to point
 Satellite
 30MHz to 1GHz
 Omnidirectional
 Broadcast radio
 3 x 10 11 to 2 x 10 14
 Infrared
 Local
29

Wireless Examples
 terrestrial microwave transmission
 satellite transmission
 broadcast radio
 infrared
30

Terrestrial Microwave
 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
31

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
32

Microwave Transmission
Advantages
 no cabling needed between sites
 wide bandwidth
 multichannel transmissions
33

Microwave Transmission
Disadvantages
 line of sight requirement
 expensive towers and repeaters
 subject to interference such as passing airplanes and rain
34

Satellite Microwave
 a microwave relay station in space
 Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency
 geostationary 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
35

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
36

Satellite Transmission Process satellite transponder dish uplink station dish
22,300 miles downlink station
37

Satellite Transmission
Applications
 television distribution
 a network provides programming from a central location using direct broadcast satellites (DBS)
 long-distance telephone transmission
 high-usage international trunks
 private business networks
38

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
39

Satellite Advantages
 can reach a large geographical area
 high bandwidth
 cheaper over long distances
40

Satellite Disadvantages
 high initial cost
 susceptible to noise and interference
 propagation delay
41

Broadcast Radio
 Omnidirectional
 FM radio
 UHF and VHF television
 Requires line of sight
 Suffers from multipath interference
 Reflections
42

Infrared
 Achieved using tranceivers that modulate noncoherent infrared light
 Requires line of sight (or reflection)
 Blocked by walls
 e.g. TV remote control, Infrared port
43

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
44

Leased (or Dedicated) Lines
 permanently or semi-permanently connect between two points
 economical in high volume calls between two points
 no delay associated with switching times
 can assure consistently high-quality connections
45

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
46

T-1 Carrier
 also referred to as DS-1 signaling
 provides digital full-duplex transmission rates of
1.544Mbps
 usually created by multiplexing 24 64-Kbps voice or 56-Kbps data lines
 higher speeds are available with T-3 (45Mbps)
[sometimes referred to a DS-3 lines; can be multiplexed into 28 T-1 signals; T-3 consists of
672 individual channels, each of which supports
64-Kbps] and T-4 services (274Mbps)
 in Europe, E-1 (2.048Mbps) is used instead of T-1
47

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 Kbps
 broadband ISDN (primary rate access)
 twenty-three 64Kbps bearer channels + 64 data channels (23B+D) = 1.536 Mbps
48

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.
49

ISDN Channel Definitions
 B (bearer) channels
 64 kbps channels that may be used to carry voice, data, facsimile, or image
 D (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
50

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 Mbps
 North American standard
 30B (64 kbps) + D (64 kbps) = 1.984 Mbps
 European standard
51