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 1/8000 th 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 values This side 128 values This side 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 –Amplitude Intensity of the wave (height) –Wavelength Distance between comparable points on the wave –Frequency Number of up and down cycles per second (Hz) –Phase Relative state of the amplitude

Wave Characteristics Amplitude Wavelength

Amplitude Modulation

Frequency Modulation

Telecommunications System Main frame Main frame Front-End Processor Minicomputer Remote location multiplexer modems terminals

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 XXXXXXXXXXXX YYYYYYYYYYYY ZZZZZZZZZZZZ X X Y Y 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 Y Y Z Z XYZXYZXYZ -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 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?