Chapter 31 Chapter 3 Physical Layer: Layer 1 MIS 430 9/E (This is a detailed chapter)

Chapter 3 1

Chapter 3 Physical Layer: Layer 1

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

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