chapter 31 chapter 3 physical layer: layer 1 mis 430 9/e (this is a detailed chapter)
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Chapter 3 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)
Chapter 3 3
Digital and Analog Data Digital binary: 0 or 1 Analog: continuously varying sine wave Translates between two formats
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
Chapter 3 4
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
Chapter 3 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
Chapter 3 6
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
Chapter 3 7
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
Chapter 3 8
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
Chapter 3 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
Chapter 3 10
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
Chapter 3 11
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
Chapter 3 12
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!
Chapter 3 13
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/stories/1
999/05/17/story8.html
Also Philadelphia Eagles wireless “E-Z Pass” processing http://www.philly.com/mld/philly/business/9258077.htm
Chapter 3 14
II. Digital Transmission of Digital Data Computers produce binary data: Bit=Binary 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= 13*161 + 5*160 = 213 decimal
Hex is used to represent bits in compact fashion 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
Chapter 3 15
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!
Chapter 3 16
Computer Codes
ASCII: American Standard Code for Information Interchange 7-bit code 27=128 unique codes 8-bit code 28=256 unique codes
EBCDIC: Ext. Binary Coded Decimal Interchange Code (8 bit code)
Chapter 3 17
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
Chapter 3 18
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
Chapter 3 19
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 -> used for Ethernet transmissions
Chapter 3 20
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
Chapter 3 21
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
Chapter 3 22
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
Chapter 3 23
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
Chapter 3 24
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
Chapter 3 25
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
Chapter 3 26
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: 1.5:1 compression Graphics: 50:1 or even higher compression
Modems handshake to decide on connection particulars
Newer modem standards: V.90 and V.92 (later)
Chapter 3 27
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: 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 http://www.howstuffworks.com/cd.htm
Chapter 3 28
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 bits=64,000 bps data
for voice in digital form Happens to be same capacity as ISDN –
Integrated Systems Digital Network
Chapter 3 29
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: 27=128 levels However, power restrictions mean < 56K actual speed
Depends on having no more than 1 AD conversion in the connection
V.92 standard is digital both ways with very fast handshaking interval (but is it too late?)
Chapter 3 30
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
Chapter 3 31
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 can’t put 10 pounds of
olives in a 5 pound jar”
Chapter 3 32
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!
Chapter 3 33
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)
Chapter 3 34
OLD Verizon DSL Packages
Package Speed Down/Up
Price/month
Bronze Plus 768 Kbps 128 Kbps
$50
Enhanced Bronze
1.5 M128 K
$60
Silver 384 K384 K
$70
Silver Plus 1.5 M384 K
$80
Chapter 3 35
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!
Chapter 3 36
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)
Chapter 3 37
Cable Modem: RoadRunner 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 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