Chapter 3 Physical Layer: Layer 1

1
Chapter 3 Physical Layer Layer 1

• MIS 430 9/E
• (This is a detailed chapter)

2
Overview Physical Connections

• Data (digital, analog)
• Circuits (physical, logical)
• Media (guided, wireless)
• Digital Transmission of Digital Data (coding,
  modes, Ethernet)
• Analog Transmission of Digital Data (modulation,
  modems)
• Digital Transmission of Analog Data (translating,
  CDs, telephones, IM)

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Digital and Analog Data

• Digital binary 0 or 1
• Analog continuously varying sine wave
• Translates between two formats
• Modem digital-gtanalog ... analog-gtdigital
• Codec analog-gtdigital digital-gtanalog
• Digital good for high speed, short distance,
  encrypted conversations, and is more efficient
• Analog good for long distance over an analog
  medium like telephone wires

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I. Circuits

• Circuit can refer to the logical connection or
  the physical connection
• Circuit Configuration
• Point-to-point see fig 3.1
• Uses all the capacity of one circuit between two
  ends
• Expensive where lots of nodes
• Multipoint see fig 3.2
• Circuit is shared over several nodes
• Reduces cable needed, especially over long
  distance
• Uses circuit capacity more efficiently

5
Data Flow

• Simplex 1 way transmission (TV, radio, some
  satellite, webcast)
• Half-Duplex
• 2-way transmission, 1 way at a time
• Think bridge under repair with stop lights
  requires control signals to reverse the flow
• Aka simplex in Europe
• Full-Duplex
• 2-way transmission, both ways simultaneous

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Multiplexing Mux

• Breaks one high speed circuit into lower speed
  channels to connect multiple nodes
• Type of multiplexor
• Frequency Mux FDM static allocation
• Each channel receives fixed frequency bandwidth
• Time Division Mux TDM static allocation
• Each channel receives fixed time slice of full
  bandwidth
• Statistical Time Divivision Mux stat mux
• Each channel is dynamic allocated time slice
  based on usage

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Multiplexing Mux (contd)

• Type of mux, contd
• Wavelength Division Multiplexing WDM
• In fiber optic, vary light color passed thru for
  different channels
• Inverse Mux
• Splits up a single large logical circuit into
  several lower speed physical circuits
• Ex television signal carried on 4 phone lines
  from Memorial Stadium

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How DSL Works (brief look)

• DSL splits your telephone wires via FDM
• Regular analog telephone line (tiny bandwidth)
• Upstream DSL digital (smaller bw)
• Downstream DSL digital (larger bw)
• DSL modem is installed in your home
• Pulls out the digital side
• Filters at each phone pulls out the telephone
  line signal to send to phone

9
Media Guided

• Guided Media (travels in a pipe)
• Twisted pair copper cable fig 3.9
• Number of pairs (telco 1 pair, LAN 2-4 pairs)
• Twisted to eliminate interference
• Coax(ial) copper cable (TV cable) fig 3.10
• Large central conductor wire, grounding shielding
• Cable modem uses coax input, but twisted pair out
• Fiber optic glass cable backbone choice
• Very high capacity
• Single mode (newer, better) vs multi-mode (old,
  lossy)
• Uses LEDs or laser to pulse light in on/off
  fashion
• Much more immune to RFI and hacking
• More immune to fires than copper wires

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Media Wireless (more in ch. 7)

• Radio (802.11a/b/g, cellular, Bluetooth)
• Infrared (line of sight TV remote, Palm PDA,
  laptop)
• Microwave (towers 20 miles apart)
• Satellite
• Geosynchronous orbit (22,280 miles stationary)
• Propagation delay (round trip is 45,000 miles,
  about ¼ second (speed of light 186,000 mi/sec)
• Raindrops, leaves can attenuate signal!
• Requires direct line of sight to satellite

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Media Selection Guided
Media Network Type Cost Trans. Distance Security Error Rates Speed
Twisted Pair LAN, modem Low Short Good Low Low-High
Coax Cable LAN Medium Short Good Low Low-High
Fiber Optic Any High Medium - Long Very good Very low High very high
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Media Selection Wireless
Media Network Type Cost Trans. Distance Security Error Rates Speed
Radio LAN Low Short Poor Medium Low to medium
Infrared LAN, BN Low Short Poor Medium Very Low
Micro- wave MAN, WAN Medium Long Poor Low-Medium Medium
Satellite WAN Medium Long Poor Low-Medium Medium to high!
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Tech Focus Wireless Yankees

• 50 Food service employee takes order in stands
  (limited to box seats)
• Has handheld terminal to enter order (cell
  phone)
• Directs orders to 3 kitchens, printed on 12 small
  receipt printers
• Server can authenticate credit cards
• http//jacksonville.bcentral.com/jacksonville/stor
  ies/1999/05/17/story8.html
• Also Philadelphia Eagles wireless E-Z Pass
  processing http//www.philly.com/mld/philly/busin
  ess/9258077.htm

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II. Digital Transmission of Digital Data

• Computers produce binary data BitBinary Digit
  0 or 1 (magnetic, optical, electrical two
  states)
• Hexadecimal numbers (Base 16) use 0-9, A-F
• Hex placeholders 163 162 161 160 or 4096 256
  16 1
• Ex D5 hex 13161 5160 213 decimal
• Hex is used to represent bits in compact fashion
• Binary placeholders 23 22 21 20 or 8 4 2
  1
• Ex 10108210 decimal A hex
• Ex 1111842115 decimal F hex
• Ex D hex 13 decimal 841 1101 bin
• Ex 8 hex 8 decimal 8 1000 binary
• Ex 47 hex 0100 0111 binary

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Base Conversions

• Base 2 to Base 10
• Multiply by powers of 2
• Base 10 to Base 16
• Divide by powers of 16
• Base 16 to Base 10
• Multiple by powers of 16
• Examples on the board!

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Computer Codes

• ASCII American Standard Code for Information
  Interchange
• 7-bit code 27128 unique codes
• 8-bit code 28256 unique codes
• EBCDIC Ext. Binary Coded Decimal Interchange
  Code (8 bit code)

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ASCII Code Pattern
A 41 Binary 0100 0001 1st
nybble081402014 a 61 Binary 0110
0001 1st nybble081412016
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Transmission Modes

• Parallel internal transfers, parallel port
• Separate lines for each of 8 bits
• 1 character sent at a time
• DB25 connector (Printer cable)
• Serial external stream of data sent
• Only 1 line 1 bit sent at a time, one after
  another
• Much slower than parallel, longer distances
• DB9 connector (COM port) to modem
• Ethernet, USB use serial transfers

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Digital Transmission

• Electricity 101
• DC vs. AC
• Amps electrical flow (volume)
• Volts pressure
• Watt volts amps (power)
• Digital Transmission see fig 3-12
• Unipolar (0v or 5v)
• Bipolar (-5v or 5v) fewer errors, more distinct
• Manchester encoding special unipolar where
  change from low to high 0, change from high to
  low 1
• -gt used for Ethernet transmissions

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III. Analog Transmission of Digital Data

• Predominant in telephone network POTS
• Like sound, uses sine wave fig 3-13
• Amplitude (height of the wave)
• Frequency (Hz, how many waves per second)
  Wavelength 1/Frequency
• Phase (where the wave begins along X axis)
• Our ears hear 20-14,000 Hz
• A above middle C on piano is 440 Hertz (ISO)
• Telephone circuits are from 0-4,000 Hz low
  fidelity ?

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Modulation

• We modulate the sine wave to carry the digital
  data
• AM amplitude modulation (0,1 by height) see
  fig 3-14
• FM frequency modulation (0,1 by wavelength)
  see fig 3-15
• PM phase modulation (0,1 by time wave arrives)
  see fig 3-16 most common

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Modulation, contd.

• Can send more than one bit per modulation (baud
  rate vs. bit rate)
• 2-bit AM see fig 3-17 uses 4 heights for 00,
  01, 10, 11 patterns
• 2-bit FM uses 4 frequencies
• 2-bit PM 4 wave points (0, 90, 180, 270o)
• QAM Quadrature AM sends 4 bits
• Uses 3 phases, 2 amplitudes
• 16 distinct signals, carries 4 bits 0000 to 1111
• TCM 6, 7, or 8 bits per signal

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Capacity of a Voice Circuit

• Bandwidth highest frequency lowest frequency
• Humans hear 20 14,000 thus bandwidth is 13,980
  Hz
• Voice grade phone line from 0 to 4000 Hz
• Noise reduces effective bandwidth
• Ideal Capacity
• AM 1 X 4000 4,000 bps
• QAM 4 X 4000 16,000 bps
• TCM 6 X 4000 24,000 bps

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How Modems Work

• Modem modulator/demodulator
• Data rate depends on
• Modem standard (both sides must agree)
• Compression
• Noise in that particular phone circuit

PC modem phone line modem
PC
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Old Analog Modem Standards
Standard Max Rate Signal Type Bits per symbol Data Rate
V.22 1200-2400 FM 1 1,200-2,400
V.32 2400 QAM 4 9,600
V.32bis 2400 TCM 6 14,000
V.34 3429 TCM 8.4 28,800
V.34 3420 TCM 9.8 33,600
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Modem Compression

• V.44 Lempel-Ziv encoding
• Repeating characters are replaced with a code and
  the count by the modems alone
• Programs little or negative compression
• Text 1.51 compression
• Graphics 501 or even higher compression
• Modems handshake to decide on connection
  particulars
• Newer modem standards V.90 and V.92 (later)

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IV. Digital Transmission of Analog Data

• Codec code/decode devices at each end
• Sample analog data to produce digital signals -
  see fig 3-19 for amplitude levels
• 7 bits 27 128 levels ok for human speech
• 16 bits 21665,536 levels ok for music
• Increase sampling interval improve sound
• CD 44,100 samples/sec using 16 bits 783,216,000
  bytes per CD in 74 minutes
• http//www.howstuffworks.com/cd.htm

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Telephones Carry Analog, Sorta

• Last mile (local loop) between your house and
  the local office is analog
• Rest of the telco system is digital, with codecs
  in the middle to translate
• PCM pulse code modulation in North America
• 8,000 samples/sec X 8 bits64,000 bps data for
  voice in digital form
• Happens to be same capacity as ISDN Integrated
  Systems Digital Network

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V.90, V.92 Modem Standards

• V.90 modem standard is analog and digital
• Upload is analog at V.34 or 33.6 Kbps
• Download is digital at 56K using PCM
• 8,000 samples/sec X 7 bits 56,000 bps
• Noise affects throughput 27128 levels
• However, power restrictions mean lt 56K actual
  speed
• Depends on having no more than 1 A?D conversion
  in the connection
• V.92 standard is digital both ways with very fast
  handshaking interval (but is it too late?)

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Multiplexing

• Mux combines several logical circuits into one
  physical circuit, to save .
• Transparent to the users they each have their
  own circuits
• Ex 4 terminals, one mux, one circuit, one mux at
  other end next to host fig 3-21
• Frequency Division Mux FDM
• Each logical circuit has a fixed fraction of the
  bandwidth, called channels

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Multiplexing

• Time Division Mux TDM
• Each user gets a of the time on the full
  circuit
• They take turns, so TDM must buffer sessions
• Statistical TDM Stat mux
• Each user gets a proportional share of the
  circuit time based on its current workload
• Much better if not all circuits use same
• Remember You cant put 10 pounds of olives in
  a 5 pound jar

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Multiplexing

• Inverse mux
• Where one very high speed logical circuit is
  split over several lower-speed physical lines to
  improve performance
• Ex TV feed from Memorial Stadium over telco
  needs several phone lines and the signal is
  reconstructed at other end
• However, satellite (1 circuit) is more common!

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Digital Subscriber Line (DSL)

• Very popular today for broadband ISP
• DSL splits twisted pair local loop into three
  channels
• Regular Telco analog line 4000 Hz bandwidth
• Digital portions one upload, one download
• Symmetric versus asymetric (ADSL) depends on up
  and down speed comparisons
• Split could be at demark point on back wall or at
  each phone using a filter
• DSL modem connects to Ethernet port in your
  computer or hub or router (or some have USB port)

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OLD Verizon DSL Packages
Package Speed Down/Up Price/month
Bronze Plus 768 Kbps 128 Kbps 50
Enhanced Bronze 1.5 M 128 K 60
Silver 384 K 384 K 70
Silver Plus 1.5 M 384 K 80
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Residential Verizon DSL Prices

• Initial First month is free, 34.95/month
  afterwards if ordered on the web
• Includes 9 email accounts, 10 MB web space
• Up to 1.5 Mbps download, 384 Kbps upload (HA!)
• 3 filters for regular phones necessary
• Plus 12.95 for shipping the kit (modem, filters,
  CDs, directions) to your home
• If combined with Verizon local and long distance
  calling package, 29.95/month
• Requires a voice phone line to home!

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Business DSL

• Assumes higher usage therefore costs more
• 59.95/month for 1.5M/384K deal with 1-year
  contract
• May order static IP address for extra
• Order online by 8/31 first three months free
  after rebate, waive 25 setup fee.
• EASY! Can use a router to split the signal and
  connect multiple users (residential, too)

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Cable Modem RoadRunner

• Broadband alternative to DSL http//www.rr.com/rdr
  un/
• Cable is a shared medium so the more neighbors
  that use it, the slower it will be
• Analogy 2-lane highway that clogs with
  increasing traffic
• Requires that the cable company install 2-way
  equipment in your neighborhood
• Generally is faster than DSL, especially at slack
  times
• Price is 44.95 per month, may be cheaper if
  ordered with cable TV. Plus installation not
  always free!
• Cable modem connects to
• coax input output is Ethernet or USB