Transmission Media

Chapter 3

Knowledge Checkpoints

Media and their characteristicsCategories of twisted pairsApplications for wire, cable,

fiberEmerging applications for

radiated media

Important Terms Fiber Frequency spectrum Amplifiers OTDR Twisted pair Coax Cat UTP, STP DSLAM Dark fiber

BRI, PRI DSL HFC Oxygen layer Line of sight Footprint EMI, RFI Symmetrical &

asymmetrical Propagation delay

Physical Layer: Architectures, Devices

and Circuits

Analog Transmission In analog transmission, the state of line can

vary continuously and smoothly among an infinite number of states• States can be signal strengths, voltages, or

other measurable conditions

Time

Strength

Digital Transmission In digital transmission, time is divided into

periods of fixed length called clock cycles• For modems, a few thousand clock cycles per

second• For LANs, millions of clock cycles per second

Clock CycleTime

Strength

Bits and Baud Baud Rate = Number of clock cycles/sec

• 4 baud (not 4 bauds/second)

Bit Rate = Number of bits/second• 8 bits/second

00

01

10

01

1 Second

Possible Change Not Made

Equations

For Each Clock Cycle• 2Bits per clock cycle = Number of possible states • Overall• Bit rate = Baud Rate * Bits per clock cycle

Example• Baud rate of 10,000 with four possible states

• Bits per clock cycle = 2 (22=4) • Bit rate = 10,000 * 2 (Eq. 2)• Bit rate = 20,000 bps

Transmission Speeds

Bit: A single 1 or 0

Bits per second (bps)

• Factors of 1,000 (not 1,024 as in memory)• kilobits per second (kbps)--Note lower case k• megabits per second (Mbps)• gigabits per second (Gbps)• terabits per second (Tbps)

Occasionally given in bytes per second (Bps)• Bits per second / 8• Uncommon

100101001 ...

Communication Media

The media is the matter or substance that carries voice, video or data transmission

There are two basic types of media: Guided media - those in which the message flows

through a physical media Radiated media (unguided) - Those in which the

message is broadcast through space

Guided Media

Twisted Pair Wire - insulated pairs of wire, twisted to minimize electromagnetic interference between wires

Coaxial Cable - wire with a copper core and an outer cylindrical shell for insulation

Fiber Optic Cable - high speed streams of light pulses from lasers or LEDs carry information inside hair-thin strands of glass or plastic called optic fibers

Important Facts about Twisted Pair

Lowest frequency spectrum –a max of 1MHz Susceptible to noise and high error rate Distance between repeaters (cost issue) Security is poor Low acquisition cost Foundation of PSTN Standards govern categories (maximum data rate) Applications include: telephone, PBX, desktop,

LANs, local loop

Twisted Pair Media Twisted-pair wires are classified by:

American wire gauge (AWG) rating Shielding, either unshielded twisted-pair (UTP) or shielded

twisted-pair (STP) Categories that define the wire’s rated acceptable speed

and error characteristics AWG Rating

• The AWG rating is a measure of the thickness of the copper conductor in the cable

• The higher the AWG rating, the smaller the diameter of the wire

• Twisted-pair wiring for LANs have an AWG rating of 22-26

Wire Propagation Effects: Cross-Talk Interference

Cross-Talk Interference• Multiple wires in a bundle and each radiates

some of its signal• Causes “cross-talk” interference in nearby

wires

Wire Propagation Effects:Cross Talk

Wire is Twisted• Several twists per inch• Interference adds to signal over half twist, subtracts

over other half• Roughly cancels out• Simple but effective

Single Twist

Interference- +

Signal

Wire Propagation Effects:Cross Talk

Terminal Cross-Talk Interference• Wire must be untwisted

at ends to fit into connectors

• Cross-talk interference is high at termination

• Problems severe if untwist more than about 1.25 cm (1/2 inch)

• Usually the biggest propagation effect

TerminalCross Talk

Characteristics of Common LAN Media

Unshielded twisted-pair

Shielded twisted-pair

Coaxial Cable

Fiber optic cable

Broadcast radio

Spread spectrum radio

Microwave radio

Infrared light

Common Speeds Common Speeds (Mbps)(Mbps)

Medium Medium TypeType

1, 4, 10, 16, 100, 1000

1, 4, 10, 16, 100, 1000

10, 16, 50

10, 16, 50, 100, 1000, 2000

2

2, 10, 16

5.7

4, 10, 16

Less capable than other conducted media

Better than unshielded; less capable than fiber optic or coaxial cables

Good; less capable than fiber optic cable

Excellent

Subject to interference

Good

Subject to interference

Objects can block transmission

Error CharacteristicsError Characteristics

Practical Issues in Propagation Effects

Distance limits in standards prevent serious propagation effects• For instance, usually 100 meters maximum for

ordinary copper wire

• Stay within limits, usually no serious problems

Problems usually occur at connectors• Crossed wires

• Poor connections

• Cross-talk interference

Wire Media: UTP Unshielded Twisted Pair (UTP)

• Ordinary copper wire

• Twisted several times per inch to reduce interference

• Pair of wires needed for a complete electrical signal

• Unshielded: nothing but plastic coating No protection from interference

Wire Media: UTP Unshielded Twisted Pair (UTP)

• Business telephone wiring traditionally comes in 4-pair UTP wire bundles

• Used in LAN wiring to use existing building wiring technology

Wire Propagation: RJ-45

RJ-45 connector terminates a UTP bundle

• Slightly wider than RJ-11 residential connector

• Width needed for 8 wires

Wire Media: UTP to the Desktop

UTP• Dominant for line from desktop to first hub or switch

• Inexpensive to buy and install

• Rugged: can take punishment of office work

• Easily 100 Mbps, 1 Gbps with careful insulation

UTP

First Hub or Switch

ISDN

Two B Channels are 64 kbps

• Original idea: one for voice, one for data

1101

BChannel

BChannel

ISDN Multiplexes (mixes) three channels on one UTP wire pair

to the desktop BRI & PRI

1101

BChannel

BChannel

2B+D

MultiplexedOnto One

Set of Wires

ISDNModem

D Channel:control signals

ISDN Costs

Line is expensive• $60-$80/mo plus installation fee• ISP charge is separate; may charge more

for ISDN access

Dial Up: Not always connected• Do not have to pay for full-time use

Need “ISDN modem” (expensive)

ISDN Modem

ISDN “Modem” is a Misnomer• Modem is for digital device on an analog line

• ISDN line is digital

ISDN modem really contains

• Codec to link analog telephone to digital ISDN line

• Data service unit (DSU) to translate between PC digital format and ISDN digital format (voltage levels, timing, etc.,)

Twisted-Pair Wire Category Summary

1

2

3

4

5

Maximum Data RateCategory

1 Mbps

4 Mbps

10 Mbps

16 Mbps

100, 155, and 1,000 Mbps

Telephones

Token Ring LANs

Ethernet LANs

Token ring LANs

Ethernet, fast ethernet, and gigabit ethernet LANs, CDDI LANs and asynchronous transfer mode (ATM)

Typical UseCost (Relative to Category 1)

1

1.5

2

3

4

DSLs

Digital Subscriber Lines

Digital Subscriber Lines (DSLs)

Offered by Telephone Companies• Lines to customer premises are subscriber

lines, which connect subscribers to the telephone system--These are digital

• “Digital subscriber line”

TelephoneNetwork

TelephoneNetwork ISPISPDSL

Modem

DSLModem

DSL

Digital Subscriber Lines (DSLs)

Several types of digital lines for subscribers: for homes, others for businesses• For residential customers, usually

multiplexes regular phone, high-speed data Use existing phone line coming into house Use Internet without tying up phone

TelephoneNetwork

TelephoneNetwork ISPISPDSL

Modem

DSLModem Existing

Phone Line

Digital Subscriber Lines

Most common for home is Asymmetric DSL (ADSL)

• Upstream and downstream speeds are different (asymmetrical)

• Upstream at 64 kbps or more

• Downstream at 256 kbps to a few Mbps

• Asymmetric speed is good for web surfing

• About $50 per month incl. ISP; more for faster service

TelephoneNetwork

TelephoneNetwork ISPISPDSL

Modem

DSLModem

ADSL

64 or more kbps

256 kbps-a few Mbps

Digital Subscriber Lines G.Lite (G.992.2) Standard (ADSL1)

• New ADSL standard from ITU-T

• Up to 1.5 Mbps downstream speed

• No carrier installation is necessary

• Not being widely adopted by ADSL vendors yet

TelephoneNetwork

TelephoneNetwork ISPISPDSL

Modem

DSLModem

ADSL

64 or more kbps

Up to 1.5 Mbps

Digital Subscriber Lines Digital Subscriber Line Access Multiplexer

• Telco must install a DSLAM at end switching office (CO)

TelephoneNetwork

TelephoneNetwork ISPISPDSLAMDSLAM

DSL

DSL

DSL

Digital Subscriber Lines Splitting Voice and Data

• Voice and data are split at home by the DSL modem

• Voice and data are also split at the telephone company’s first switching office

DSLModem

DSLSplitter

DSL

DSLAM

VoiceNetwork

Computer

Phone

Important Facts about Coax

Greater frequency spectrum Capable of multiple channels Better performance (lower error rate & better bandwidth) Amplifiers every 1.5 miles Requires a bus topology Noise at connection points & susceptible to lightening Applications include: interoffice trunks, LANs,

international lines, cable TV, local loop Bi-directional upgrade required High installation cost

Wire Media: Coax Coaxial Cable

• Used in cable TV, VCRs

• Central wire, external concentric cylinder

• Outer conductor wrapped in PVC

Screw-On Connector

InnerWire

Outer Conductor Wrapped in PVC

Wire Media: Coaxial Cable

Coaxial Cable

• Installed widely today in old 10 Mbps Ethernet LANs

• Not being used in new installations

Optical fiber more cost-effective for long links

UTP more cost-effective for desktop links

A Single Conductor Coaxial Cable

Outer Insulation Mesh Shielding Insulation Conductor

Guided Media

Cable Modem Service

Broadband Internet Access

Cable Modem Service

Service of Cable Television Companies

• Deliver 10 Mbps downstream to the home

• Capacity is shared by multiple subscribers, so real speed is more limited

• Limited to about 64 kbps to 256 kbps upstream

• Does not tie up telephone line--Always available

Cable TVNetwork

Cable TVNetwork

Also ISPFunctions

Also ISPFunctions

CableModem

CableModem

Cable Modem Sharing in Perspective

Sharing is Not as Extreme as it May First Appear• Shared within Blocks of 500 Houses• Only Some Households in Block will Subscribe• Only Some Subscribers will be Online at Any

Moment• Only Some of Online Subscribers will Send and

Receive at Any Moment• Only these will Share the Capacity

So Sharing Does Not Reduce Speed Unless the Adoption Rate is Large

Cable Modem Connection

Cable Modem Service

Cost is about $50 per Month

• Includes ISP service!

• Installation usually costs $100 to $150 and includes a cable modem and a network interface card

• Best alternative today beyond V.90 modem

• Cost-competitive with adding a second phone line to handle your modem communication

Views of a Fiber Optic Cable

Plastic Covering

Glass Cladding

Glass Conductor

Important Facts about Fiber

Greater distance between repeaters (500 miles) Single and multi-mode fibers Applications include: backbone for public and

private networks, the Internet, LANs High bandwidth Elastic traffic carrying capacity Low error rate Secure transmission High installation cost

Guided Media

The earliest fiber optic systems were multimode, (light could reflect inside the cable at many different angles)

Single mode fiber optic cables transmit a single direct beam of light through a cable that ensures the light only reflects in one pattern

Fiber is more secure than coax or wire

Guided Media

Optical Fiber Optical Fiber

• Glass core, surrounding glass cladding• Light source turned on/off (pulses) for 1/0• Total internal reflection at boundary• Almost no attenuation

LightSource

Cladding

Core

Reflection

Optical Fiber Multimode Fiber

• Wide core makes easy to splice (50 or 62 microns)

• Many angles for rays (modes)

• Short propagation distance (usually 200 m to 500 m)

LightSource

Wire Media: Optical Fiber

Single Mode Fiber

• Narrow core difficult to splice (5 or 8 microns)

• Only one angle for rays (one mode), so (almost) no distortion

• Longer propagation distance (usually up to 2 km for LAN fiber, longer for long-distance fiber)

• Narrow core makes fiber fragile and difficult to splice

Wire Media: Optical Fiber

Optical Fiber• High speeds over long distances

200 m to 2 km

• Costs more than UTP, but worth it on long runs

• Good for all links between hubs and switches within and between buildings in a site network

OpticalFiber

Wire Media: UTP and Optical Fiber

The emerging pattern: UTP from first hub or switch to desk, fiber everywhere else

Optical Fiber Limited by Distortion

• Light entering at different angles travels different distances (different number of reflections)

• Called different modes

• Light from successive bits becomes mixed over long distances

LightSource

Radiated Media

Radio (wireless) data transmission uses the same basic principles as standard radio transmission

Infrared Transmission uses low frequency light waves to carry data through the air on a direct line-of-sight path between two points

Radiated Media A microwave is an extremely high frequency

radio communication beam that is transmitted over a direct line-of-sight path between two points

Transmission via GEO satellite is similar to transmission via microwave except, instead of transmitting to another nearby microwave dish antenna, it transmits to a satellite 22,300 miles in space

Radiated Media

Radiated Media A disadvantage of satellite transmission is the

delay that occurs because the signal has to travel out into space and back to Earth (propagation delay)

A problem associated with some types of satellite transmission is raindrop attenuation (some waves at the high end of the spectrum are so short they can be absorbed by raindrops)

Radiated Media

Ku-band satellites use waves that are so short they can be caught and concentrated in much smaller dish antennas, called very small aperture terminals (VSAT)

The larger Earth dish hubs can cost as much as several hundred thousand dollars

Radio Propagation

Broadcast signal• Not confined to a wire• Moves through the air• Security risk

Radio Waves

When Electron Oscillates, Gives Off Radio Waves• Single electron gives a very weak signal

• Many electrons in an antenna are forced to oscillate in unison to give a practical signal

Radio Propagation Problems

Wire Propagation is Predictable• Signals go through a fixed path: the wire• Propagation problems can be easily

anticipated• Problems can be addressed easily

Radio Propagation is Difficult• Signals begin propagating as a simple sphere• But they can be blocked• There are shadow zones

ShadowZone

Radio Propagation Problems

Radio Propagation is Difficult

• Signals are reflected

• May arrive at a destination via multiple paths

• Signals arriving by different paths can interfere with one another

• This is called multi-path interference

Radio Propagation: Service Bands

Service Bands

• Divide spectrum into bands for services

• A band is a contiguous range of frequencies

• FM radio, cellular telephone service bands

0 Hz

Cellular Telephone

FM Radio

AM Radio

ServiceBands

Radio Propagation: Channels and Bandwidth

Service Bands are Further Divided into Channels• Like television channels

• Bandwidth of a channel is highest frequency minus lowest frequency

0 Hz

Channel 3

Channel 2

Channel 1

ServiceBand

ChannelBandwidth

Radio Propagation: Channels and

Bandwidth Example• Highest frequency of a radio channel is 43 kHz

• Lowest frequency of the radio channel is 38 kHz

• Bandwidth of radio channel is 5 kHz (43-38 kHz)

0 Hz

Channel 3

Channel 2

Channel 1

ServiceBand

ChannelBandwidth

Radio Propagation Problems

Wire Propagation is Predictable• Signals go through a fixed path: the wire• Propagation problems can be easily anticipated• Problems can be addressed easily

Radio Propagation is Difficult• Signals begin propagating as a simple sphere• But they can be blocked• There are shadow zones

ShadowZone

Radio Propagation: Waves

Waves

Amplitude(strength)

Wavelength(meters)

Frequency in hertz (Hz)Cycles per Second

One Second7 Cycles

1 Hz = 1 cycle per second

Radio Propagation: Frequency Spectrum

Frequency Spectrum• Frequencies vary (like strings in a harp)

• Frequencies measured in hertz (Hz)

• Frequency spectrum: all possible frequencies from 0 Hz to infinity

0 Hz

Frequencies Metric system

• kHz (1,000 Hz) kilohertz; note lower-case k

• MHz (1,000 kHz) megahertz

• GHz (1,000 MHz) gigahertz

• THz (1,000 GHz) terahertz

Radio Propagation: Channels and

Bandwidth Shannon’s Equation

• W is maximum possible (not actual) transmission sped in a channel

• B is bandwidth of the channel: highest frequency minus lowest frequency

• S/N is the signal-to-noise ratio

W = B Log2 (1 + S/N)

Radio Transmission: Broadband

Speed and Bandwidth• The wider the channel bandwidth (B), the faster

the maximum possible transmission speed (W)

• W = B Log2 (1+S/N)

MaximumPossible

Speed

Bandwidth

Broadband

Two Uses of the Term “Broadband”

Technically, the signal is transmitted in a single channel AND the bandwidth of the channel is large

• Therefore, maximum possible transmission speed is high

Popularly, if the signal is fast, the system is called “broadband” whether it uses channels at all

Media Selection

Network Transmission ErrorMedia Type Cost Distance SecurityRates Speed

Twisted Pair LAN Low Short Good Low Low-highCoaxial Cable LAN Mod. Short Good Low Low-highFiber Optics any High Mod.-long V. Good V.Low High-V.High

Network Transmission ErrorMedia Type Cost Distance Security Rates Speed

Radio LAN Low Short Poor Mod LowInfrared LAN, BN Low Short Poor Mod LowMicrowave WAN Mod Long Poor Low-Mod ModSatellite WAN Mod Long Poor Low-Mod Mod

Guided Media

Radiated Media

Important Figures to Study

P. 55 Table 3.1 P. 68 Figure 3.3

Chapter Questions Why is coax more secure than twisted pair? Why is fiber the best choice for maximum

security? How does physical lay-out of a building influence

your selection? What does this statement mean?

• The frequency spectrum in which a medium operates directly relates to the bit rate you can obtain with the medium.