04 Media

Transmission Media: Wires, Cables, Fiber Optics, and Microwaves Based on Chapter 4 of William Stallings, Data and Computer Communication, 7th Ed.

Kevin Bolding Electrical Engineering Seattle Pacific University Seattle Pacific University

Transmission Media

No. 1

Transmission Media • A signal must be transmitted through some medium • Guided Media determine the path of the signal • Wires (cables, twisted pair, coax) • Fiber Optics • Other things… • Signals Propagate in all directions in Unguided Media • The medium is usually free space (air), but the signal type gets the name • Refers to transmitting signals through passive media that does not change the signal’s direction • Microwaves, broadcast radio waves • Lasers, Infrared Seattle Pacific University

Transmission Media

No. 2

Media Issues • Frequency range • Some media support higher frequencies than others • Impairments • Different media deform signals differently • Some are more susceptible to noise and distortion • Cost • We’re in the real world… • Number of receivers • Broadcast vs. point-to-point

Seattle Pacific University

Transmission Media

No. 3

How Fast/How Far can a Signal be Sent? • The question: • Given a source signal with a given power, how far can it go before it is attenuated so much that the SNR is too low to be usable?

• As far as media is concerned, the main issue is attenuation • Attenuation increases with distance. Usually expressed in dB/m, dB/100ft, etc.

• Attenuation usually increases with frequency. • A graph or table showing attenuation/length vs. frequency is common.

Seattle Pacific University

Transmission Media

No. 4

Attenuation Curves Attenuation per 100ft for UTP/Coax

Attenuation per 100ft (dB)

25 20 Cat-5 UTP RG58 Coax RG6 Coax

15 10 5 0 1

10

100

1000

MHz

Attenuation is very dependent on conductor size At higher frequencies, other issues, such as crosstalk, matter more

Seattle Pacific University

Transmission Media

No. 5

Frequency of various signals Frequency (Hz) 102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015

Power/ Telephone

Radio

Microwave Infrared

Visible Light

Twisted Pair Coax AM Radio FM Radio/TV Microwave Trans.

Optical Fiber

106 105 104 103 102 101 100 10-1 10-2 10-3 10-4 10-5 10-6

Wavelength (Meters) Source: Stallings, Fig. 4.1

Seattle Pacific University

Transmission Media

No. 6

Guided Media • Guided media control the path of the signal wave • Electrical – Signal needs conductor and ground • Differences are in how ground/conductor interact • Twisted pair • Coax • Striplines on PCBs

• Optical – Signal is sent using internal reflection • Differences are in light sources and fiber diameter

Seattle Pacific University

Transmission Media

No. 7

Electrical Cables • The issue is electromagnetic transmission/reception • Loops make great antennas • Antenna strength proportional to the area inside of the loop • Worse for shorter wavelengths

• Common ground systems (such as PCBs with ground planes) • Return path directly below signal • Minimizes loop area Seattle Pacific University

signal Interference prop. to area

return signal return

Better…

Trace on PCB

Ground return

Transmission Media

No. 8

Twisted Pair Cables l

n ur et R

together • Loop size proportional to twist size • Adjacent twists are 180 degrees out of phase • Tend to cancel out • Varying the twist size helps to minimize crosstalk

a gn Si

• Twist the signal and ground

Adjacent Loops Out of phase

• Data rates • Over long distances, about 1-3 Mbps • Short distances: 100Mbps, sometimes 1Gbps

Seattle Pacific University

Transmission Media

No. 9

Shielding

• Twisted pair usually comes bundled with several pairs in a

cable • Unshielded – Just a plastic (teflon) jacket • For distances of around 100m • Cat-3 UTP: <16Mbps, Cat-5 UTP: <100Mbps, Cat-5e UTP: 100+ Mbps

• Shielded – Includes a grounded shield

(source: Microsoft Networking Essentials)

Seattle Pacific University

Transmission Media

No. 10

Coaxial Cables • Concentric mesh wire for ground • Acts as an excellent shield • Very little interference or radiation

• Carries much higher frequencies and data rates • 1-500MHz spectrum • Data rates in 100s of Mbps

• The downside • Expensive to manufacture • More difficult to install

Seattle Pacific University

Transmission Media

No. 11

Optical Fiber • Relies on total internal reflection • Light waves bounce of edge of fiber • Channels waves to (Source: Stallings, Fig. 4.4) destination • Varieties • Multi-mode (wide fiber) • Light waves bounce off at different angles • Some have shallow angles (straight path), while others have steeper angles (crooked path) • Results in pulse spreading • Single-mode (narrow fiber) • Only a straight shot down the middle is allowed • Requires a laser source Seattle Pacific University

Transmission Media

No. 12

Fiber has its advantages • Advantages • No electromagnetic interference • Very little attenuation • Extremely high bandwidth (THz) • Small, lightweight • Disadvantages • More expensive transceivers • More difficult to install

Seattle Pacific University

Transmission Media

No. 13

Wireless (Unguided) Media • Omnidirectional • Signal radiates in all directions • Good for broadcast • Inexpensive antenna • Directional • Signal radiates in a single direction • Usually requires parabolic (dish) antenna • 2-40 GHz (microwave) • Also works with lasers

Seattle Pacific University

Transmission Media

No. 14

Terrestrial Radio (Line of Sight) •

Limited to line-of-sight for most signals (more or less) • Max distance (m):

d = 7140 Kh h = height (in meters) K = fudge factor (around 4/3) •

Attenuation prop. to square of distance traveled • Free space, isotropic* antenna:

Ptrans (4πd ) 2 (4πfd ) 2 = = 2 Prcv λ c2

Pt (4πfd ) 2 10 log10 = 10 log10 Pr c2

f = frequency d = distance (m) λ= wavelength (m) c = speed of light

4πfd loss (dB ) = 20 log10 = 20 log10 f + 20 log10 d − 147.56dB c Seattle Pacific University

Transmission Media

No. 15

Terrestrial Radio (All forms) • Ground-wave propagation follows the curvature of the earth • Frequencies below 2MHz • AM radio (550-1600KHz)

• Sky-wave propagation relies on the ionosphere and the surface of the earth to refract waves back-and-forth • Frequencies 2MHz-30MHz • Short-wave Radio, HAM radio

Ionosphere

• Line of site is point-to-point in a nearly straight line

• Frequencies 30MHz and up • FM radio, TV, Mobile phones, etc. Seattle Pacific University

Transmission Media

No. 16

Satellite Radio • Requires satellite in geosynchronous orbit • 35,784 km • Delay of ¼ second (round-trip) • Satellites spaced 4 degrees apart •

Above 10GHz, signal is attenuated by atmosphere

• Higher frequencies use smaller dishes, though

•

Nice try:

• “Constellations” of low-orbit satellites

http://www.mike-willis.com/Tutorial/gases.htm

Seattle Pacific University

Transmission Media

No. 17