Transcript Basic Concepts

Basic Concepts and
Principles
Chapter 1
Copyright 2001 Panko
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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
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Originally, There was a Sharp Distinction:
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Voice and Video Communication versus
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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
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–
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
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Hierarchy of switches
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Trunk lines connect switches
Trunk Line
Switch
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Voice and Video Networks
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Circuit
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End-to-End Connection between Phones
May pass through multiple switches
And trunk lines
Circuit
Circuit
Voice and Video Networks
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Reserved Capacity
– Circuit capacity is reserved during duration of
each call
– At each switch
Reserved
– On each trunk line
Capacity
Reserved
Capacity
Circuit
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Voice and Video Networks

Reserved Circuit Capacity Guarantees Throughput
–
–
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Never get less than reserved capacity
Nothing like congestion on the Internet
Reserved Circuit Capacity is Expensive
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–
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Pay for it whether you use it or not
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 a long time until next download
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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Switching Decision
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–
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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 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 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
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–
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State may be voltage, frequency or another line signal
characteristic
Voice and video analog
Telephone network assumes analog input signals
Strength
Time
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Digital Transmission

In digital transmission, time is divided into
periods of fixed length called clock cycles
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The line is kept in one of only a few possible
states (conditions) during each clock cycle
Strength
Clock Cycle
Time
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Digital Transmission

At the end of each clock cycle, the line may
change abruptly to another of these few states
– Can also stay the same
Abrupt
Change
Strength
Stays Same
Clock Cycle
Time
Digital Versus Binary Transmission

Digital transmission: a few states (2, 4, 8, 16, etc.)
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Binary transmission: exactly two states
– One state represents 1, the other 0
Few States
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
(1101) Modem
Analog
Signal
LANs and WANs
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Networks Have Different Geographical Scopes
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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
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Client PC
Client PCs are used by
ordinary managers and
professionals; receive service
Servers provide services
to client PCs
Server
Server
Server
Client PC
Elements of a Simple LAN
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Client PC
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Begin with stand-alone PC
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Add a network interface card (NIC) to
deal with the network
Networks have many client PCs
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Server
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Most PC nets have multiple servers
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