程序代写代做代考 scheme IOS Chapter 1. Introduction to Data Communications

Chapter 1. Introduction to Data Communications

Networks, Security, and Privacy
158.235

A/Prof Julian Jang-Jaccard

Massey University

Physical Layer

Reading: Chapter 3 in the prescribed textbook

Physical Layer

• Layer 1 in the Internet model

• Focus on transmission over
circuits

• Types of Circuits

– Physical circuits connect
devices & include wires

– Logical circuits refer to the
transmission characteristics of
the circuit

– Physical and logical circuits may
be the same or different. For
example, in multiplexing, one
physical wire may carry several
logical circuits.

Internet Model

Application

Transport

Network

Data Link

Physical

Outline

• Media

• Digital Transmission of Digital Data

• Analog Transmission of Digital Data

• Digital Transmission of Analog Data

Media

• Physical matter that carries (the voice or data)

transmission

• Guided media:

• Transmission flows along a physical guide

• e.g. twisted pair, coaxial cable and fiber optic cable

• Wireless media (radiated media)

• the transmission flows through the air or space

• e.g. Examples radio such as microwave and satellite

Guided Media

• Twisted-pair (TP) cable

– Insulated pairs of wires bundled together

– Wires twisted to reduce electromagnetic interference

– Some times use additional shielding (STP)

– Commonly used for telephones, LANs

– Characteristics

• Price – inexpensive

• Distance – typically up to 100m

• Use – Telephones, LANs

Guided Media

• Coaxial cable

– Has a single copper core, plus

outer insulation, shielding, and

inner insulation

– Less prone to interference

– Characteristics

• Price – inexpensive (but

more costly than TP)

• Distance – up to 2 km (1.2

miles)

• Use: Cable TV / Internet

Guided Media

• Fiber optic cable
– Optical core made of glass or

plastic

– Data transmitted using light from
lasers or LEDs

– Resistant to interference and
corrosion

– Extremely fast data rates

– Characteristics

• Price: Expensive

• Distance: 500m – 100km

• Use: Trunk line / Backbone,
long distance circuits (e.g.,
undersea cables)

Guided Media

• Fiber optics

– Multimode (about 50 micron core)

– Graded index multimode

– Single mode (about 5 micron core)

Wireless Media

• Radio

– Wireless transmission of electrical waves through

air

– Each device on network has a radio transceiver

operating at a specific frequency range

– Enables mobile network communication

– Characteristics

• Distance: depends on frequency and power

• Use: Wireless LANs, cellular and cordless

phones, baby monitors

Wireless Media

• Microwave
– High-frequency radio

communication

– Requires line of sight which may
require large antennas and towers

– Affected by weather

– Characteristics

• Distance: ~60 km (due to
curvature of earth

• Use: Trunk line / Backbone, long
distance

• Satellite
– Special form of microwave

communication

– Long distance leads to
propagation delays

Factors Used in Media Selection

• Type of network

– LAN, WAN, or Backbone

• Cost

– Always changing; depends on the distance

• Transmission distance

– Short: up to 300 m; medium: up to 500 m

• Security

– Wireless media is less secure

• Error rates

– Wireless media has the highest error rate (interference)

• Transmission speeds

– Constantly improving; Fiber has the highest

Media Summary

Outline

• Media

• Digital Transmission of Digital Data

• Analog Transmission of Digital Data

• Digital Transmission of Analog Data

Types of Data Transmitted

• Analog data

– Produced by telephones

– Sound waves, which vary continuously over time,
analogous to one’s voice

– Can take on any value in a wide range of
possibilities

• Digital data

– Produced by computers, in binary form

– Information is represented as code in a series of
ones and zeros

– All digital data is either on or off, 0 or 1

Types of Transmission

• Analog-Analog transmissions

– Analog data transmitted in analog form

– Examples of analog data being sent using analog transmissions
are broadcast TV and radio

• Digital-Digital transmissions

– Computer networks send digital data using digital transmissions

• Analog  Digital Transmissions

– Modem (modulator/demodulator): used when digital data is sent
as an analog transmission

– Codec (coder/decoder): used when analog data is sent via digital
transmission

Data Type vs. Transmission Type

Analog

Transmission

Digital

Transmission

Analog

Data

AM and FM Radio,

Broadcast TV

Pulse code

modulation.

MP3, CDs, iPOD,

VoIP

Digital Data Modems – sending

email from your

house using

telephone line

Data transmitted as

ASCII/EBCDIC over

Ethernet LANs,

printer

• Coding scheme needed to

ensure sender and receiver

understand messages (e.g.,

ASCII, Unicode, etc.)

• A character is represented by a

group of bits

Digital Data-Digital Transmission

Digital Transmission of Digital Data

• Sender and receiver must agree upon:

– Set of symbols

• How bits are encoded as voltages or light

pulses

• e.g., +5V might be encodes as a “1”

– Symbol rate

• How many symbols are sent per second

• e.g., with a symbol sent at every clock cycle. 64

kilohertz (kHz) = 64,000 symbols/sec

Digital Transmission of Digital Data

• Five types of signaling techniques

1. Unipolar – voltage is 0 or positive representing

binary bits (in some circuits, 0 and negative

voltage could be used)

Digital Transmission of Digital Data

• Five types of signaling techniques

2. Bipolar NRZ – voltage is positive or negative,

but not zero

• Fewer errors than unipolar because signals

are more distinct

Digital Transmission of Digital Data

• Five types of signaling techniques

3. Bipolar RZ – voltage is positive or negative,

returning to zero between each bit

• Fewer synchronization errors than bipolar

NRZ

Digital Transmission of Digital Data

• Five types of signaling techniques

4. Bipolar AMI – voltage is 0, positive, or negative,

returns to zero between each bit, and alternates

between positive and negative voltage

Digital Transmission of Digital Data

• Five types of signaling techniques

5. Manchester – voltage is positive or negative and

bits are indicated by a mid-bit transition

Ethernet uses it – less susceptible to bit errors to

going unnoticed

Outline

• Media

• Digital Transmission of Digital Data

• Analog Transmission of Digital Data

• Digital Transmission of Analog Data

Analog Transmission of Digital Data

• Telephone system built for analog data

– Electrical signals mimic sound waves (i.e., voice)

– Analog transmissions take on range of values (vs.

discrete values of digital transmissions)

– Need a modem (modulator/demodulator) to

convert from analog to digital and vice versa

Analog Transmission of Digital Data

• Three characteristics of waves

1. Amplitude: height of wave (decibels)

2. Frequency: waves per second (hertz)

• Wavelength is the inverse of frequency

3. Phase: wave direction (degrees) or the point

at which the wave begins

Analog Transmission of Digital Data

• Carrier wave is basic wave transmitted

through a circuit

• Modulation is the modification of a carrier

wave’s fundamental characteristics in order to

encode information

• Three ways to modulate a carrier wave:
1. Amplitude Modulation (AM) or Amplitude Shift Keying (ASK)

2. Frequency Modulation (FM) or Frequency Shift Keying (FSK)

3. Phase Modulation (PM) or Phase Shift Keying (PSK)

Analog Transmission of Digital Data

• Amplitude
Modulation

• Frequency
Modulation

• Phase
Modulation

Analog Transmission of Digital Data

• Symbol: One or more modifications to a carrier

wave used to encode data

• Can send 1 bit by defining two different symbols

(e.g., amplitudes, frequencies, etc.)

• Can send multiple bits by defining more than two

symbols

– Need more complicated information coding schemes

– 1 bit of information  2 symbols

– 2 bits of information  4 symbols

– 3 bits of information  8 symbols

– n bits of information  2
n
symbols

Analog Transmission of Digital Data

• Two-bit Amplitude Modulation

– With 4 levels of amplitude defined as symbols, 2

bits can be transmitted per symbol

Analog Transmission of Digital Data

• Data rate (or bit rate) is the number of bits

transmitted per second

• Symbol rate: number of symbols transmitted per

second

Data rate = symbol rate × (# bits/symbol)

• Example

Symbol rate = 16,000 symbols/sec

#bits/symbol = 4 bits/symbol

Data rate = 16,000 symbols/sec × 4 bits/symbol

= 64,000 bits/sec = 64Kbps

Outline

• Circuits and Data Flow

• Multiplexing

• Media

• Digital Transmission of Digital Data

• Analog Transmission of Digital Data

• Digital Transmission of Analog Data

Digital Transmission of Analog Data

• Codecs (COde, DECode) is a device or software

that converts an analog signal (e.g., voice) into

digital form and the reverse

• Pulse-Code Modulation (PCM) converts analog

to digital by:
1. Sampling the analog signal at regular intervals

2. Measuring the amplitude of each sample

3. Encoding (quantizing) the amplitude as binary data

• Quantizing Error is the difference between the

original analog signal and the approximated,

digital signal

PAM – Measuring Signal

• Sample analog waveform across time and measure

amplitude of signal

• In this example, quantize the samples using only 8 pulse

amplitudes or levels for simplicity

• Our 8 levels or amplitudes can be depicted digitally by using

0’s and 1’s in a 3-bit code, yielding 2
3
possible amplitudes

PAM – Encoding and Sampling

000 – PAM Level 1

001 – PAM Level 2

010 – PAM Level 3

011 – PAM Level 4

100 – PAM Level 5

101 – PAM Level 6

110 – PAM Level 7

111 – PAM Level 8

• For digitizing a voice signal, it is typically 8,000 samples per second

and 8 bits per sample

• 8,000 samples x 8 bits per sample  64,000 bps transmission rate

needed

• 8,000 samples then transmitted as a serial stream of 0s and 1s

111 110 011 …… ………… 111 …..

Minimize Quantizing Errors

• Increase number of amplitude levels

– Difference between levels minimized  smoother signal

– Requires more bits to represent levels  more data to

transmit

– Adequate human voice: 7 bits  128 levels

– Music: at least 16 bits  65,536 levels

• Sample more frequently

– Will reduce the length of each step  smoother signal

– Adequate Voice signal: twice the highest possible frequency

(4Khz x 2 = 8000 samples / second)

– RealNetworks: 48,000 samples / second

Digital Transmission of Analog Data

Wired and Wireless

Local Area Networks
Reading: Chapter 6 in the prescribed textbook

Why Use a LAN?

Information sharing
 Improved decision making

 May reduce data duplication and inconsistency

Resource sharing
 Devices such as printers can be shared by many

clients

Software sharing
 Some software can be purchased on a per-seat

basis and resides on server

 Reduces costs, simplifies maintenance and

upgrades

Device Management
 Software updates and configuration are easier

LAN Components

1. Clients
2. Servers
3. Network interface cards

(NICs)
4. Network cables

5. Hubs / switches / access
points

6. Software

LAN Components

1. Clients

– Devices on the network that request information from
servers

2. Servers

– Devices on the network that deliver information or
provide services to clients

3. Network interface cards (NIC)

– Also called network cards and network adapters

– Operate at layers 1 and 2

– Commonly built into motherboards

– Ethernet NICs contain unique MAC address

LAN Components

4. Network Cables

Name Type
Maximum

Data Rate Used by

Category 3 UTP 10 Mbps 10BASE-T

Category 5 UTP/STP 100 Mbps 100BASE-T

Category 5e UTP/STP 1 Gbps 1000BASE-T

Category 6/6a UTP/STP 10Gbps 10GBASE-T

OM1 (62.5/125 µm) Fiber 1-10 Gbps* 1000BASE-SX

OM3 (50/125 µm) Fiber 10-100 Gbps* 10GBASE-SR

* Speed depends on circuit length

LAN Components

5. Hubs and switches
 Link cables from different devices, sometimes

more than one type of cabling

 Act as repeaters, reconstructing and

strengthening incoming signals

LAN Components

5. Access points (APs) use radio waves to

connect wireless clients to the wired network

(instead of connecting using hubs/switches)

– Many APs use power over Ethernet (PoE) for

electricity

– No external power is needed

– Power flows over unused twisted pair wires

– Also used by some IP cameras and phones

LAN Components

LAN Components

6. Software

– Network Operating System (NOS)

• Runs on devices and manage networking functions
• E.g., Novel NetWare, Microsoft Windows Server,

Linux
• E.g., Cisco IOS or JUNOS on routers

– Clients devices typically have network software
components included with OS installation

• E.g., TCP/IP included in Windows, OS X, and Linux
• Allows clients to view and access available network

resources
– Provides directory services about LAN resources

– Network profiles specify resources that devices and
users can access

WIRED ETHERNET

Wired Ethernet

• Used by almost all LANs today

• Originally developed by a consortium of

Digital Equipment Corp., Intel and Xerox

• Standardized as IEEE 802.3

• Layer 2 protocol, but physical layer must

meet protocol requirements

Topology

Topology: Basic geographic layout of a

network

Types

 Logical: How the network works conceptually

 Physical: How the network is physically installed

Hub-based (Shared) Ethernet

Hub-based Ethernet

 Also called shared or traditional Ethernet

 Logical bus topology means that all devices

receive every frame as if they were connected to

the same circuit

 The hub is a multiport repeater

Hub-based (Shared) Ethernet

Hub-based Ethernet uses physical star

topology

A

B

C
D

E

Message

for C

Messag

e for C

Messag

e for C

Messag

e for C

Messag

e for C

Message

for C

Swtich-based Ethernet

 Logical star topology means that only the

destination receives the frame

– Switch reads destination address of the frame and

only sends it to the interface (physical port)

connected to a circuit

– Uses forwarding tables (also called MAC or CAM

tables), which are similar to routing tables

– Breaks up the collision domain

 Physical star topology

Switch-based Ethernet

Switch-based Ethernet

A

B

C
D

E

Message

for C

Message

for C

Switch Operation

 Switches learn which MAC address is
associated with an interface (physical port)
by reading the source address on a frame

 When a new frame is received, the switch
reads the destination MAC address

 Looks up destination address in the
forwarding table

– If found, forwards frame to the corresponding
interface

– If not found, broadcasts frame to all devices (like
a hub)

Forwarding table

Switch-based Ethernet
Switch Forwarding Table

A

B

C D

1 2

3 4

00-22-69-13-EA-3E 00-22-69-13-EA-3A

00-22-69-13-EA-01

00-22-69-13-EA-6C

MAC Port

00-22-69-13-EA-3E 1

00-22-69-13-EA-3A 2

00-22-69-13-EA-01 3

00-22-69-13-EA-6C 4

Learning Switch Operation

• Switch starts by working like a simple hub

– With an empty forwarding table

• It gradually fills its forwarding

table by learning about the

nodes

– Reads the source MAC address of the incoming frame

and records it to the corresponding port number

– Reads the destination MAC address. If not in the Table

then it broadcasts the frame to all ports

– Waits for the destination computers to respond, and

repeats the first step

Media Access Control (MAC)

• Uses a contention-based protocol called

CSMA/CD (Carrier Sense Multiple Access /

Collision Detect)

• Frames can be sent by two computers on

the same network at the same time

• They will collide and destroy each other

• Can be termed as “ordered chaos”

• Tolerates, rather than avoids, collisions

CSMA/CD

• Carrier Sense (CS):

– A computer listens to the bus to determine if another

computer is transmitting before sending anything

– Transmit when no other computer is transmitting

• Multiple Access (MA):

– All computers have access to the network medium

• Collision Detect (CD):

– Declared when any signal other than its own detected

• Normally occurs before the transmission of 512th bits

– If a collision is detected

• To avoid a collision, both wait a random amount of

time and then resend message

WIRELESS ETHERNET

Wireless Ethernet

• Commonly called Wi-Fi

• A family of standards developed by IEEE

formally called 802.11

• Uses radio frequencies to transmit signals

through the air (instead of cables)

• Wi-Fi has many benefits

– Provides network connections where cabling is

impossible or undesirable

– Allows device and user mobility

– Potentially more economical than wired networks

Wireless Ethernet

• Components

– Access points (APs)

• Antenna type

– Omnidirectional

– Directional

• Association with AP

– Active vs. passive scanning

– Wireless NICs

• Topology

Physical star

Logical bus

Association with AP

AP 2 AP 1

H1

BBS 2 BBS 1

1

2
3

1

passive scanning:
(1) beacon frames sent from APs

(2) association Request frame sent:

H1 to selected AP

(3) association Response frame sent

from selected AP to H1

AP 2
AP 1

H1

BBS 2 BBS 1

1
2 2

3
4

active scanning:
(1)Probe Request frame

broadcast from H1

(2)Probe Response frames sent

from APs

(3)Association Request frame

sent: H1 to selected AP

(4)Association Response frame

sent from selected AP to H1

WLAN Media Access Control

• Uses CSMA/CA

– CA  collision avoidance (before collision happens)

– A station waits until another station is finished

transmitting plus an additional random period (i.e. back-

off timer) before sending anything

• collisions harder to detect on wireless Ethernet

(‘over the air’), so more effort is put into avoidance

• Contrast with CSMA/CD

– detect collision, stop transmission, wait, and re-transmit

– after collision

MAC Techniques

• May use two MAC techniques simultaneously

– Distributed Coordination Function (DCF)

• Also called “Physical Carrier Sense Method”

– Point Coordination Function (PCF)

• Also called “Virtual Carrier Sense Method”

• Optional: (can be set as “always”, “never”, or “just

for certain frame sizes”)

Distributed Coordination Function

• Relies on the ability of computers to physically listen before
they transmit

– When a node wants to send a message:

• First listens to make sure that the transmitting node has
finished, then

• Waits a period of time longer

• Each frame is sent using stop-and-wait ARQ

– By waiting, the listening node can detect that the sending node has
finished

• ACK/NAK sent a short time after a frame is received, (hence,
ensuring no collision occurring) shorter than the wait time
required for other nodes to start transmitting

• DCF Suffers from the hidden node problem

Sender

1 if sense channel idle for DIFS then

transmit entire frame (no CD)

2 if sense channel busy then

start random backoff time

timer counts down while channel idle

transmit when timer expires

if no ACK, increase random backoff interval,

repeat 2

Receiver

– if frame received OK

return ACK after SIFS (ACK needed due to

hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK

Distributed Coordination Function

Hidden Node Problem

A
B

C

Hidden terminal/node problem

 B, A hear each other

 B, C hear each other

 A, C can not hear each other
means A, C unaware of their
interference at B

A B C

A’s signal
strength

space

C’s signal
strength

Signal attenuation:

 B, A hear each other

 B, C hear each other

 A, C can not hear each other
interfering at B

Point Coordination Function

• Hidden Node problem

– Two computers can not detect each other’s signals

• A computer is near the transmission limits of the AP
at one end and another computer is near the
transmission limits at the other end of the AP’s range

• Cannot sense each other’s transmission signals

– DCF method will not work

• Solution: PCF

– First send a Request To Send (RTS) signal to the AP

• Request to reserve the circuit and duration

– AP responds with a Clear To Send (CTS) signal,

• Also indicates duration that the channel is reserved

– Computer wishing to send begins transmitting

Point Coordination Function

AP
A B

time

DATA (A)
defer

802.11 Frame

Address 2: source MAC address

Address 1: AP

to receive this frame
Address 3: dest. MAC

address

Address 4: used only

in ad hoc mode

Addres

s 1

(6

bytes)

FCS

(6 bytes)

Frame

Control

(2 bytes)

Addres

s 2

(6

bytes)

Sequen

ce

Control

(2

bytes)

Data

(46-2312

bytes)

Duratio

n / ID

(2

bytes)

Addres

s 3

(6

bytes)

Addres

s 4

(6

bytes)

Address 2: source MAC

address

• Includes four address fields

Two addresses have the same meaning as in
wired Ethernet, the others are used
communicating with APs and other devices

Frequency Ranges

• WiFi devices transmit and receive within frequency ranges

– These frequency ranges are divided into “channels”

• Frequency ranges

– 2.4 GHz range

• 2.412-2.462 Ghz

• 3 non-overlapping channels

– 5 GHz range

• 5.180-5.320 and 5.745-5.825 Ghz

• 12 non-overlapping channels

• Larger frequency range → higher potential bandwidth

• Higher frequency → greater attenuation (i.e., shorter range)

• Overlapping channels should be minimized

Types of Wi-Fi

Type
Date

Published

Max Tx

Speed

Frequency

(Ghz)
Official Status

802.11a 1999 54 Mbps 5, 3.7 Obsolete

(Superseded)

802.11b 1999 11 Mbps 2.4 Obsolete

(Superseded)

802.11g 2003 54 Mbps 2.4 Obsolete

(Superseded)

802.11n 2009 600 Mbps 2.4/5 Obsolete

(Superseded)*

802.11ac 2013 6.77 Gbps 2.4,5 Current

802.11ad 2012 ~7 Gbps 2.4, 5 Current

802.11ax Est. 2019 ? 2.4, 5 In-Progress

*Still widely used in 2014

END