Transcript Chapter 3 Physical Layer - Indiana State University

Chapter 3
Physical Layer: Layer 1
MIS 430 9/E
(This is a detailed chapter)
Chapter 3
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Overview: Physical Connections
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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
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Digital binary: 0 or 1
Analog: continuously varying sine wave
Translates between two formats
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Modem: digital->analog ... analog->digital
Codec: analog->digital … digital->analog
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
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Circuit can refer to the logical connection or
the physical connection
Circuit Configuration
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Point-to-point; see fig 3.1
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Uses all the capacity of one circuit between two ends
Expensive where lots of nodes
Multipoint; see fig 3.2
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Circuit is shared over several nodes
Reduces cable needed, especially over long distance
Uses circuit capacity more efficiently
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Data Flow
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Simplex: 1 way transmission (TV, radio,
some satellite, webcast)
Half-Duplex
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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
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2-way transmission, both ways simultaneous
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Multiplexing: Mux
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Breaks one high speed circuit into lower
speed channels to connect multiple nodes
Type of multiplexor
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Frequency Mux: FDM static allocation
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Time Division Mux: TDM static allocation
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Each channel receives fixed frequency bandwidth
Each channel receives fixed time slice of full bandwidth
Statistical Time Divivision Mux: stat mux
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Each channel is dynamic allocated time slice based on
usage
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Multiplexing: Mux (contd)
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Type of mux, contd
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Wavelength Division Multiplexing WDM
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In fiber optic, vary light color passed thru for
different channels
Inverse Mux:
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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)
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DSL splits your telephone wires via FDM
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DSL modem is installed in your home
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Regular analog telephone line (tiny bandwidth)
Upstream DSL digital (smaller bw)
Downstream DSL digital (larger bw)
Pulls out the digital side
Filters at each phone – pulls out the
telephone line signal to send to phone
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Media: Guided
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Guided Media (travels in a “pipe”)
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Twisted pair copper cable – fig 3.9
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Coax(ial) copper cable (TV cable) – fig 3.10
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Number of pairs (telco 1 pair, LAN 2-4 pairs)
Twisted to eliminate interference
Large central conductor wire, grounding shielding
Cable modem uses coax input, but twisted pair out
Fiber optic glass cable: backbone choice
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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
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(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
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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 Cost
Type
Trans.
Security
Distance
Error
Rates
Speed
Twisted
Pair
LAN,
modem
Low
Short
Good
Low
LowHigh
Coax
Cable
LAN
Medium Short
Good
Low
LowHigh
Fiber
Optic
Any
High
Very
good
Very low High –
very
high
Medium
- Long
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Media Selection: Wireless
Media
Network Cost
Type
Trans.
Distance
Security Error
Rates
Speed
Radio
LAN
Low
Short
Poor
Medium
Low to
medium
Infrared
LAN, BN
Low
Short
Poor
Medium
Very Low
Microwave
MAN,
WAN
Medium
Long
Poor
LowMedium
Medium
Satellite
WAN
Medium
Long
Poor
LowMedium
Medium
to high!
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Tech Focus: Wireless Yankees
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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/stories/1999/05
/17/story8.html
Also Philadelphia Eagles wireless “E-Z Pass” processing
http://www.philly.com/mld/philly/business/9258077.htm
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II. Digital Transmission of
Digital Data
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Computers produce binary data: Bit=Binary Digit: 0
or 1 (magnetic, optical, electrical – two states)
Hexadecimal numbers (Base 16) use 0-9, A-F
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Hex placeholders: 163 162 161 160 or 4096 256 16 1
Ex: D5 hex= 13*161 + 5*160 = 213 decimal
Hex is used to represent bits in compact fashion
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Binary placeholders: 23 22 21 20 or 8 4 2 1
Ex: 1010=8+2=10 decimal = A hex
Ex: 1111=8+4+2+1=15 decimal = F hex
Ex: D hex = 13 decimal = 8+4+1 =1101 bin
Ex: 8 hex = 8 decimal = 8 = 1000 binary
Ex: 47 hex = 0100 0111 binary
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Base Conversions
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Base 2 to Base 10
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Base 10 to Base 16
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Divide by powers of 16
Base 16 to Base 10
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Multiply by powers of 2
Multiple by powers of 16
Examples … on the board!
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Computer Codes
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ASCII: American Standard Code for
Information Interchange
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7-bit code 27=128 unique codes
8-bit code 28=256 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 nybble=0*8+1*4+0*2+0*1=4
a: 61 Binary = 0110 0001
1st nybble=0*8+1*4+1*2+0*1=6
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Transmission Modes
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Parallel: internal transfers, parallel port
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Separate lines for each of 8 bits
1 character sent at a time
DB25 connector (Printer cable)
Serial: external stream of data sent
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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
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Electricity 101
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DC vs. AC
Amps: electrical flow (volume)
Volts: pressure
Watt = volts * amps (power)
Digital Transmission: see fig 3-12
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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
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-> used for Ethernet transmissions
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III. Analog Transmission of
Digital Data
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Predominant in telephone network: POTS
Like sound, uses sine wave: fig 3-13
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Our ears hear 20-14,000 Hz
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Amplitude (height of the wave)
Frequency (Hz, how many waves per second);
Wavelength = 1/Frequency
Phase (where the wave begins along X axis)
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
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We modulate the sine wave to “carry”
the digital data
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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.
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Can send more than one bit per modulation
(baud rate vs. bit rate)
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QAM: Quadrature AM sends 4 bits
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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)
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
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Bandwidth = highest frequency – lowest
frequency
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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:
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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
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Modem = modulator/demodulator
Data rate depends on
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PC
Modem standard (both sides must agree)
Compression
Noise in that particular phone circuit
modem
phone line
modem
Chapter 3
PC
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Old Analog Modem Standards
Standard
Max Rate
V.22
Signal
Type
1200-2400 FM
Bits per
symbol
1
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
Chapter 3
Data Rate
1,2002,400
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Modem Compression
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V.44 – Lempel-Ziv encoding
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Repeating characters are replaced with a code and
the count by the modems alone
Programs: little or negative compression
Text: 1.5:1 compression
Graphics: 50:1 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
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Codec: code/decode devices at each end
Sample analog data to produce digital signals
- see fig 3-19 for amplitude levels
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7 bits: 27= 128 levels ok for human speech
16 bits: 216=65,536 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
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http://www.howstuffworks.com/cd.htm
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Telephones Carry Analog,
Sorta
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“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
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PCM pulse code modulation in North America
8,000 samples/sec X 8 bits=64,000 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
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V.90 modem standard is analog and digital
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Upload is analog at V.34+ or 33.6 Kbps
Download is digital at 56K using PCM
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8,000 samples/sec X 7 bits = 56,000 bps
Noise affects throughput: 27=128 levels
However, power restrictions mean < 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
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Mux combines several logical circuits into one
physical circuit, to save $$$.
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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
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Each logical circuit has a fixed fraction of the
bandwidth, called channels
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Multiplexing
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Time Division Mux: TDM
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Statistical TDM: Stat mux
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Each user gets a % of the time on the full circuit
They take turns, so TDM must buffer sessions
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 can’t put 10 pounds of
olives in a 5 pound jar”
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Multiplexing
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Inverse mux
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Where one very high speed logical circuit is
split over several lower-speed physical lines
to improve performance
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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)
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Very popular today for broadband ISP
DSL splits twisted pair local loop into three channels
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Regular Telco analog line: 4000 Hz bandwidth
Digital portions: one upload, one download
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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
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Initial: First month is free, $34.95/month
afterwards if ordered on the web
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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
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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
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Broadband alternative to DSL
http://www.rr.com/rdrun/
Cable is a shared medium so the more neighbors that
use it, the slower it will be
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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
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coax input; output is Ethernet or USB
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