ECIS469: Lecture 2

Fundamentals of Networking

Data Communications

• exchange of digital information between two devices using an electronic transmission medium

Converting Analog to Digital

• Pulse Code Modulation (PCM)– Like getting a ticker quote every 10 minutes– Approximates the actual signal curve– In PCM

• Measure the signal height every 1/8000th of a second• 8 bits used to report the height at each

measurement• 8*8000=64,000 bits per second to provide

approximation of analog signal• 64Kbps represents a single voice line in digital

telecommunications

Pulse Code Modulation

Sampling Interval = 1/8000 second

1111 1111

0000 0000

128 valuesThis side

128 valuesThis side

Value transmitted

Value transmitted

How about a CD?

• Lasers etches lands and pits on the surface of a CD

• Uses 16 bits to measure height of signal

• Samples 44,100 times per second for each of two channels

• 16*44100*2 = 176,000 bps• One hour of music requires 633Mb

Digital to Analog Conversion

• Needed to transmit computer signals over telephone lines

• Analog signal characteristics– AmplitudeAmplitude

• Intensity of the wave (height)

– WavelengthWavelength• Distance between comparable points on the wave

– FrequencyFrequency• Number of up and down cycles per second (Hz)

– PhasePhase• Relative state of the amplitude

Wave Characteristics

Amplitude

Wavelength

Amplitude Modulation

Frequency Modulation

Telecommunications System

MainframeMainframe

Front-EndProcessor

MinicomputerMinicomputer

Remote locationRemote location

multiplexermultiplexer

modemsmodems

terminalsterminals

Multiplexing

• Allows multiple signals to be sent over same medium at same time

• Modes of multiplexing– Frequency Division (FDM)– Time Division (TDM)

Frequency Division Multiplexing

X X X X X X

Y Y Y Y Y Y

Z Z Z Z Z Z

X X X X

Y Y Y Y

Z Z Z Z

-originally designed so multiple voice streams could be placed on same telephone line

-Multiple analog signals superimposed but on different frequency spectra

-Involves pair of multiplexers

Time Division Multiplexing

X X X X

Y Y Y Y

Z Z Z Z

X Y Z X Y Z X Y Z

-Each signal allotted a time slot- Creates a composite stream with slots dedicated to data sources-If data source is not sending, slot goes unused – wasteful- Instead, use statistical TDM in which slots are dynamically allocated-If there is big demand, buffers are used.

Transmission Media

• the physical path along which the data is carried

• Types– twisted pair– coaxial– fiber optics and free space– satellite – terrestrial

Transmission Media

• Twisted Pair– pair of wires twisted along entire length– usually copper with an insulating coat– Unshielded Twisted Pair (UTP) popular

with LANs• CAT3 (voice) and CAT5 are common• CAT5 used for both voice and data

– 100Mbs transmission speed– Limited segment length – signals needs

regeneration every 100 meters

Transmission Media

• Coaxial cable– thick insulated copper wire– Longer segment lengths– can carry up to 200 Mb/second– less interference due to shielding– Uses FDM to transmit 1000s of voice

channels and 100s of TV channels– Not popular in LANS

• More difficult to work with than UTP

Transmission Media• Fiber Optics cable

– thousands of little fiber optic strands• May be glass or plastic• Thickness of a human hair• Inner core surrounded by glass (cladding) • Can be single mode or multimode• Single mode

– Expensive, bigger capacity, long segment length– 8/125

• Multimode– Cheaper, less capacity– 62.5/125

– Data transmitted as pulses of light– 500 Kb/sec to several GB/sec

A typical optic fiber

- Core made of silica and germania- Optic cladding is pure silica- Mix of different refractive indices allows for total internal reflection

Point-to-point fiber optic system

Advantages of fiber optics

• Nearly infinite capacity– Single fiber can carry 40000 telephone calls or

250 channels of television

• High transmission rates at greater distances• Immune to interference and electricity• Does not corrode (being glass)• Smaller and lighter than coaxial or twisted

pair• Extremely secure

Wireless Transmission

• Directional– Focuses electromagnetic beam in direction of

receiver• Terrestrial microwave• Satellite microwave

• Omni directional– Spreads the electromagnetic signal in all

directions• AM and FM radio• 3G networks• Smart watches

Terrestrial Microwave

• Parabolic dish antenna sends signal to receiving dish

• Line-of-sight• Typically on towers to avoid

obstacles• Frequencies in the gigahertz range

What is a telecommunications satellite?

Telecommunications satellites

• Space-based cluster of radio repeaters (called transponders)

• Link – terrestrial radio transmitters to

satellite receiver (uplink)– Satellite transmitters to terrestrial

receivers (downlink)

Orbits

• Mostly geostationary (GEO)– Circular orbit– 22,235 miles above earth– Fixed point above surface – Almost always a point on Equator

• Must be separated by at least 4 degrees

Satellite services• Wide Area Broadcasting

– Single transmitter to multiple receivers

• Wide Area Report-Back– Multiple transmitters to a single receiver– Example VSATs (very small aperture terminals)

• Also have microwave transmitters and receivers– Allows for spot-beam transmission (point- to-point data

communications)

• Can switch between beams upon request (Demand Assigned Multiple Access –DAMA)

• Multi-beam satellites link widely dispersed mobile and fixed point users

Earth-based equipment• Original

microwave transmitters and receivers were large installations– Dishes measuring

100 feet in diameter

• Modern antennas about 3 feet in diameter

A Modern GEO satellite (IntelSat 900 series)

• May have more than 72 separate microwave transponders

• Each transponder handles multiple simultaneous users (protocol called Time Division Multiple Access)

• Transponder consists of– Receiver tuned to frequency of uplink– Frequency shifter (to lower frequency to that

of transmitter)– Power amplifier

IntelSat 902 (launched August 30, 2001)

Frequency ranges

• Most transponders operate in 36MHz bandwidth

• Use this bandwidth for– voice telephony (400 2-way

channels/transponder)– Data communication (120Mbs)– TV and FM Radio

C-band, Ku-band, Ka-band• Most GEO satellites operate in the C-Band

frequencies– Uplink at 6 GHz– Downlink at 4 GHz

• Ku-band also used– Uplink at 14 GHz– Downlink at 11 GHz

• Above bands best suited for minimal atmospheric attenuation

• Few slots left… forcing companies to look at Ka band (uplink:30 GHZ , downlink: 20 GHz)

Companies on the forefront: Teledesic

• Offer “Internet-in-the-Sky”• Main shareholders Craig McCaw

and Bill Gates• McCaw also has taken over ICO

Global Communications• Wanted Iridium but has backed out

Teledesic

• Again, series of LEO satellites• 24 pole orbiting satellite rings, 15 degrees apart• 12 satellites in each ring (total = 288 LEO

satellites)• Worldwide switching.. Satellites pass on data

through laser• Will map IP packets on latitudes and longitudes ..

Average will be 5 satellite hops in 75 ms• Supposed to start in 2002; offer 2Mbps Internet

access from terminals starting at $1000 each– Postponed to 2005

Optical Transmission

• Cutting edge• Uses modulated monochromatic light

to carry data from transmitter to receiver

• Optical wavelengths are suited for high rate broadband communications

• Laser-based (up to 1000 times faster than coaxial)

Research Question for Next Class

• What is Abilene?