Transcript Chapter 1

Basic Concepts and
Principles
Chapter 1
Copyright 2001 Panko
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for the chapter or module
Ch 5
Mod N
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From Chapter 5
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From Module N
Error
–
Corrects error in the original text
Box
–
Material in box within chapter
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Network

A Network is an Any-to-Any Communication
System
– Can connect any station to any other
Network
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Network

Each Station has a Unique Network Address
–
–
To connect, only need to know the receiver’s address
Like telephone number
GHI
DEF
ABC
MNO
“Connect to GHI”
JKL
Data Communications

Originally, There was a Sharp Distinction:
–
Voice and Video Communication versus
–
Data Communications, in which one or both
parties is a computer
 Database
 Electronic
mail
 World Wide Web
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Distinction is fading because voice and video
communication are increasingly computerbased
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Voice and Video Networks

Telephone Network
–
–
Customer premises (home or office)
Local loop (access line) connects customer premises to
first switching office
Switching Office
Local Loop
(Access Line)
Connection
Customer
Premises
Customer
Premises
Voice and Video Networks

Hierarchy of switches

Trunk lines connect switches
Trunk Line
Switch
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Voice and Video Networks
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Circuit
–
–
–
–
End-to-End Connection between Phones
May pass through multiple switches
And trunk lines
Reserved (guaranteed)
capacity during call
Circuit
Circuit
Voice and Video Networks
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Reserved Circuit Capacity is Expensive
–
Good for voice, because conversations are fairly
constant
–
Bad for data, because most data transmission is bursty;
e.g., in World Wide Web, download, then stare at screen
for average of 60 seconds between brief downloads
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Capacity is wasted between bursts; still must pay of
capacity
Packet-Switched Data Networks
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Packet Switching
–
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Circuit switching is expensive due to reserved capacity
Packet switching breaks transmissions into messages
–
Messages are short (averaging a few hundred bytes)
because switches handle short messages efficiently
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Messages are called packets (sometimes, frames or
other names)
Message
Packets
Packet-Switched Data Networks
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Packet Switching Decision
–
–
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When a packet arrives at a switch, the switch must
decide which of several ports (connections) to use to
send the packet back out
Complex
B
Made at each packet switch
B?
D?
Switch A
D
Packet
C?
C
Packet Switched Data Networks
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Multiplexing
–
–
–
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Packets from many conversations are mixed
(multiplexed) over each trunk line
Only pay for the trunk line capacity used
Dramatic trunk line cost savings
The reason for packet switching
Multiplexing on
Trunk Line
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Analog Transmission

In analog transmission, the state of the line can
vary continuously, rising and falling smoothly in
intensity among an infinite number of states
–
–
New
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State may be voltage, frequency or another line signal
characteristic
The human voice is like this
When we speak into a telephone, we generate
analogous (similar) electrical signals, hence the name
Strength
Time
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Digital Transmission

In digital transmission, time is divided into periods of
fixed length called clock cycles

Line is in one state (voltage level, etc.) during each clock
cycle; at end of cycle, stays same or changes abruptly.
State 1
Abrupt Change
Stays Same
State 2
Clock Cycle
Time
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Digital Transmission

Only a few possible states
–
–
New
2, 4, 8, 16, 32, rarely more
If a signal is near one state but a bit off, the receiver
will still read it correctly as being at that state
Strength
Clock Cycle
Time
Digital Versus Binary Transmission

Digital transmission: a few states (2, 4, 8, 16, etc.)

Binary transmission: exactly two states
– One state represents 1, the other 0
– Binary is a type of digital transmission
Few States (4)
Two States
1
0
Digital
Binary
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Digital Communication
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Modems
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Computers have digital output
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Telephone network assumes analog input
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Modem translates between digital device and analog
line for data transmission over the phone system
Digital
Signal
Analog
Signal
Modem
LANs and WANs

Networks Have Different Geographical Scopes

Local Area Networks (LANs)
–
–
–
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Small Office
Office Building
Industrial Park / University Campus
Wide Area Networks (WANs)
–
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Connect corporate sites or
Connect corporate sites with sites of customers and
suppliers
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Elements of a Simple LAN
Hub or Switch
Hub or Switch connects
all stations
Wiring
Wiring is standard
business telephone wiring
(4 pairs in a bundle)
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Elements of a Simple LAN
Client PC
Client PCs are used by
ordinary managers and
professionals; receive service
Have network interface cards (NICs)
NIC
Servers provide services to client PCs
Also have NICs
There usually are multiple servers
Have special server operating system
Have service software (e-mail, etc.)
Server
Client PC
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Terminal-Host Systems

Created in the 1960s
–
–
–
Central host computer does all the processing
Terminal is dumb--only a remote screen and keyboard
Largest hosts are mainframes
 Dominate host business use today
Terminals
Host
Terminal-Host Systems
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Many Mainframe Applications Were Created in
the 1960s through 1980s
–
Legacy systems--systems created by your predecessors
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Would not use the same platform today if built new
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But too expensive to rewrite all legacy applications at
once
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Must live with many host legacy applications for now
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Older networks can be legacy systems as well; “legacy
system” is not just limited to mainframe applications
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PC Networks

The Most Common Platform in Organizations
Allows PCs to share resources
Both Wintel (Windows/Intel) PCs and Macintoshes
–
–
Client
PC
PC Server
Network
PC Server
Client
PC
PC Network Components
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File Servers
–
–
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Store files (data files and programs)
The most common type of server in PC networks
Almost all file servers are themselves PCs
File Server
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File Server Program Access
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File Server Program Access is the Most Common
Way to Execute Programs in PC Networks
–
–
–
Program files are stored on the file server
But downloaded (copied) to client PC for execution
Executed on client PC, not on file server
Executed
on
Client PC
Stored
on the
File
Server
Download
Client PC
File Server
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File Server Program Access
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PC Processing Power Limits FSPA Programs
–
–
Client PCs do not get very large
Only programs small enough to operate on limited
client PCs can be used
Executed on the
Client PC
Client PC
File Server
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Client/Server Processing

Client and Server Machines
–
–
–
Neither has to be a PC (Although the client usually is)
 Platform independence
Two programs: client and server programs
Example: browser and webserver application program
Client Machine
Client
Program
Server
Program
Server
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Client/Server Processing

Cooperation Through Message Exchange
–
Client program sends Request message, such as a
database retrieval request
–
Server program sends a Response message to deliver
the requested information or an explanation for failure
Server
Program
Client Program
Request
Response
Client Machine
Server
C/S Servers often are Workstation
Servers
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Workstations
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Look like PCs but…
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More powerful (and expensive) than PCs
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Do not use standard Intel PC microprocessors
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Usually run the UNIX operating system
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Client and server workstations
Special Microprocessor
UNIX Operating System
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Program Functionality (Size)
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High program functionality requires large program
size
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File Server Program Access
–
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Client/Server Processing
–
–
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Poor: client PCs are small, can only execute small
programs
Good: not limited to client PC processing power
Heavy work can be done on the server machine
Terminal-Host Systems
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Good: Hosts can be very large
Platform Independence
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File Server Program Access
–
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Poor: Only works with PC clients and PC file servers
Client/Server Processing
–
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Excellent: use any server you want, also any client
Terminal-Host Systems
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Poor: Hosts require terminals and only work with a few
terminal types
Scalability
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Ability to grow as demand grows
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File Server Program Access
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Poor: client PCs do not get very large
Client/Server Processing
–
–
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Very good: Platform independence allows servers to be
larger than PCs
To grow, leave client machine the same, increase the
size of the server machine
Terminal-Host Systems
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Excellent: have an enormous range of processing power
User Interface
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File Server Program Access
–

Client/Server Processing
–

Very good: uses local PC processing power
Very good: uses local PC processing power for user
interface
Terminal-Host System
–
–
Poor: Relies on distant hosts; user interface quality
limited by high long-distance transmission costs
Monochrome, text-only screen; no animation
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Response Time (When User Hits a Key)

File Server Program Access
–
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Very good: uses local PC processing power
Client/Server Processing
–
–
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Very good: local PC processing power for user interface
But retrievals from the server can cause delays
Terminal-Host System
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Poor: Relies on distant hosts; long delays if overloaded
Accessing the WWW from Home
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A Common and Important Situation
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Must be understood
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Good way of introducing networking concepts
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The Internet

The Internet is a Worldwide Group of Networks
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Not a single network
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Individual networks on the Internet are called subnets
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The Internet
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Routers
–
–
Connect the Internet’s individual networks (subnets)
Cooperate to give an end-to-end route for each packet
(message)
Routers
Route
The Internet
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Hosts
–
–
–
Host
Any computer attached to the Internet is a host
Webservers are host
Desktop and notebook PCs are hosts too
Host
The Internet
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Network deliver messages based on network
addresses
–
Host
The Internet has two addressing systems for hosts
Host
 IP addresses
 Host names
The Internet
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Host IP addresses
–
–
–
–
Strings of 32 ones and zeros
Usually represented by four number segments separated
by dots: dotted decimal notation
For example, 128.171.17.13
Official addresses for hosts
127.18.47.145
127.47.17.47
The Internet
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Dotted Decimal Notation
–
IP addresses are really strings of 32 bits (1s and 0s)
 10000000101010100001000100001101
–
To convert this to dotted decimal notation, first, divide
them into four bytes (also called octets)
 10000000 10101010 00010001 00001101
 Note: Conversion is for human convenience;
computers work with 32-bit sequences, not dotted
decimal notation
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Both octets and bytes are collections of eight bits
 But “octet” is used in networking
The Internet
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Dotted Decimal Notation
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Convert each binary (Base 2) octet into decimal (Base
10)
 10000000 binary is 128 decimal
 10101011 binary is 171 decimal
 00010001 binary is 17 decimal
 00001101 binary is 13 decimal
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Binary to Decimal Conversion
Position
7
Position
0
Binary
10100011
=
Decimal
163
Note: Starts with 0
Position
(N)
Value
(2N)
7
6
5
4
3
2
1
0
128
64
32
16
8
4
2
1
Bit Decimal
1
0
1
0
0
0
1
1
128
0
32
0
0
0
2
1
163
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The Internet

Host Names
CNN.COM
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The other network addressing system on the Internet
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Easy to remember
 www.microsoft.com
 voyager.cba.hawaii.edu
 Two or more text “labels” separated by dots
 No relationship between segments and labels
–
Not official names of hosts
 Like nicknames
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Usually, only servers have host names
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The Internet

Internet Service Providers (ISPs)
–
–
–

You must have an account with an ISP
Connects you to the Internet
May provide other services (e-mail account, etc.)
Carrier Access Line
–
–
–
Usually provided by local telephone carrier
Connects you to the ISP
You pay for this separately from your ISP charges
Carrier Access Line
ISP
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The Internet

The Internet is Not Free
–
–
–

You pay your ISP around $20 per month, sometimes
more
Part of this pays for ISP expenses
Part of this pays the Internet backbone to carry your
messages
You Usually Also Pay the Telephone Carrier
Separately for the Carrier Access Line
Carrier Access Line
ISP
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The Internet

Internet Backbone
–
Itself consists of many competing but interconnected
backbone carriers
–
Sometimes, backbone carriers are also ISPs
Internet Backbone
ISP
Carriers
ISP
Standards
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
Standards are rules of operation that most or all
vendors follow

Open standards are created and owned by public
standards organizations
–
No single vendor controls these standards
Standards Are Layered

For Internet Access to a Webserver, standards are
set at five layers
–
–
–
–
–

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Application
Transport
Internet
Data Link
Physical
Together, these standards provide all that is needed
for application programs on different hosts on
different networks to work together
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Internet Standards

Messages are Exchanged at Multiple Layers
App
HTTP
App
Trans
TCP
Trans
Int
IP
Int
IP
Int
DL
PPP
DL
?
DL
Phy
Modem
Phy
?
Phy
User PC
Router
Webserver
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Internet Standards

Application Layer Standards
–
Standards at the application layer specify how two
application programs communicate
–
For example, browser on user PC and webserver
application program on webserver
 Follow the HyperText Transfer Protocol (HTTP)
standard if webservice application
Browser
Webserver
Application program
HTTP if webservice application
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Internet Standards

Transport Layer Protocols
–
Standards at the transport layer specify how two host
computers will work together, even if they are of
different platform types (PCs, workstations,
mainframes, and so forth)
–
Gives platform independence
PC
PC or
Other Computer
Internet Standards

HTTP Requires the Use of the TCP Transport
Standard at the transport layer
–
–
Transmission Control Protocol
TCP messages are called TCP segments
TCP if webservice application (HTTP)
TCP Segment
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Internet Standards

Internet Layer Protocols
– Standards at the internet layer specify how hosts and
routers will act to route packets end to end, from the
source host to the destination host, across many single
networks (subnets) connected by routers
Route
Host
Host
Single Network
(Subnet)
Internet Standards

The Internet Protocol (IP) is the Main Protocol
for Routing Packets Across the Internet
–
–
–
The IP in “TCP/IP”
IP messages are called packets
All internet layer messages are called packets
Packet
IP
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Internet Standards

Subnets
–
–
A subnet is a single networks (LANs, WANs, point-topoint link) on the Internet
A packet will pass through several subnets along its
route across the Internet
Subnet 3
Subnet 1
Subnet 2
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Internet Standards

Different Subnets Can Have Different Subnet
Protocols
–
IP at the internet layer routes across different protocols
at the subnet layer
LAN Subnet
Protocol 3
Point-to-Point Subnet
Protocol 1
WAN Subnet
Protocol 2
Subnets Standards

Subnets are Single Networks on the Internet

Subnet Standards Divided into Two Layers
–
Physical layer standards govern the transmission of
individual bits within a subnet
–
Data Link layer standards govern the transmission of
messages within a subnet
 Organize
individual bits into structured messages
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Internet Standards

Data Link Layer Standards
–
–
Standards at the data link layer specify how to transmit
messages within a single network
Messages at the data link layer are called frames
Frame
10010001001
Data Link
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Internet Standards

Data Link Layer Standards
–
For accessing the Internet from home via telephone
modem, use the Point-to-Point Protocol (PPP)
–
Only used between home and ISP!
–
Other subnets connecting routers are likely to use
different subnet protocols!
?
PPP
ISP
Internet Standards

62
Physical Layer Standards
–
–
While the data link layer is concerned with the
organization and transmission of organized messages,
standards at the physical layer specify how to transmit
single bits one at a time
Work bit by bit; no frame organization
63
Internet Standards

Physical Layer Standards in Internet Access from
Home
–
–
–
–
Telephone jack (RJ11)
Telephone wire
Serial port connection to external modem
Modem
Serial
Port
External Modem
Telephone
Wire
Wall Jack
Internet Standards

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Subnet Versus Internet Layer Standards
–
–
Internet layer provides routing across multiple subnets
Subnet layer standards (data link and physical) provide
for transmission within a single network
Internet
Layer
Subnet Layer
Internet Standards

Analogy for Subnet versus Internet
–
–
–
Take a vacation
Route from beginning to end (like internet layer)
For different parts, may travel by car, airplane, or boat
(like subnet layer)
Internet
Layer
Subnet Layer
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Standards Organizations and Architectures

Architecture is a Design for Standards Creation
–
Specifies what types of standards are needed
(application, transport, etc.)
–
After architecture is designed, individual standards of
each type are created
–
Analogy: architecture of house specifies what rooms
will be needed and their relationships.
–
After architecture is settled, individual rooms are
designed
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Standards Organizations and Architectures

TCP/IP Architecture
–
Under the Internet Engineering Task Force (IETF)
–
TCP/IP is the architecture, while TCP and IP are
individual standards
 There are other TCP/IP standards
–
IETF standards dominate in corporations at the
application, transport, and internet layers
 However, application, transport, and internet
standards from other architectures are still used to
some degree
67
Standards Organizations and Architectures

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OSI Standards
–
Reference Model of Open Systems Interconnection
–
Created by the International Telecommunications
Union-Telecommunications Standards Sector (ITU-T)
–
And the International Organization for
Standardization (ISO)
–
OSI standards dominate the data link and physical layers
 Other
architectures specify the use of OSI standards at
these layers
69
Internet Standards

5-Layer Hybrid TCP/IP-OSI Architecture
–
–
Most widely used architecture in organizations today
Used on the Internet
Application
TCP/IP
Transport
TCP/IP
Internet
TCP/IP
Data Link
OSI
Physical
OSI
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Internet Standards

Recap: Accessing the WWW from Home with a
Telephone Modem
App
HTTP
App
Trans
TCP
Trans
Int
IP
Int
IP
Int
DL
PPP
DL
?
DL
Phy
Modem
Phy
?
Phy
User PC
Router
Webserver
TCP/IP versus OSI

71
Box
Lowest Four Layers are Comparable in
Functionality
TCP/IP
OSI
Application
Application
Presentation
Session
Transport
Network
Data Link
Physical
Transport
Internet
Data Link (use OSI)
Physical (use OSI)
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OSI Divides the Application Layer
Box

OSI Session Layer
–
–
Sets up a connection between two application programs
on different machines
Manage streams of transactions (session); if there is a
break, can resume at the last roll-back point
Transactions
73
OSI Divides the Application Layer
Box

OSI Presentation Layer
–
Handles presentation differences between the two
machines (how data are stored and represented)
–
Two presentation layer processes select and use a
common data format for exchanging data
Uses
Data
Format
A
Application Data
Exchange in
Common Format C
(Transfer Syntax)
Uses
Data
Format
B
OSI Divides the Application Layer
Box

OSI Application Layer
–
Governs application-to-application communication
freed from concerns about presentation format and
transaction management
74
Quality of Service (QoS)

Want network to have good quality of service
(QoS)—work well!

Congestion
–

–

When too many transmissions are on a network, traffic
will slow down; this is congestion
Latency
–
Latency is the amount of time that packets or frames
are delayed because of congestion. Measured in
milliseconds (ms),
Want guarantees of worst-case latency
Throughput
–
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Throughput is the guaranteed speed in bits per second
Quality of Service (QoS)

Reliability Measures

Availability
–
–

Availability is the percentage of time the network is
available to users.
Telephone system has 99.999% availability
Error Rate
–
–
Error rate is the percentage of bits or messages that
contain errors
3% - 6% of all packets are lost on the Internet
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77
Security

A Growing Problem

Encryption for Confidentiality
–
–
New
–
47
Sender encrypts messages before sending them so that
anyone intercepting them en route cannot read them
Receiver can decrypt encrypted messages and read
them
Have confidentiality (unreadability by interceptors)
Encryption
101101
Decryption
47
Security

Authentication
–
–

78
Sender of a message must prove their identity
To thwart impostors who impersonate people
Access Control
–
Prohibits or authorizes access to various resources
(files, programs, etc.)
–
Needs authentication but also more
–
Access control lists for resources specify what
resources the authorized person may use and how they
may use them