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371-1-0291: An Introduction to
Computer Networks
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http://www.cse.bgu.ac.il/Courses/course.asp?ID=63
Handout #1: Introduction
Additional Reading
Text book: Chaps. 1, 9.2.1
Lect-1: Introduction
Computer Networks
1
Outline
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An Introduction to the mail system
An Introduction to the Internet
Important concepts in Networking
Key Enablers of Large Networks
Lect-1: Introduction
Computer Networks
2
An Introduction to the mail system
Technion
Beer Sheva
Letter
Shimon
Reuven
Envelop
Admin
Admin
Delivery System
Lect-1: Introduction
Computer Networks
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Characteristics of the mail system
Each envelope is individually routed.
 No time guarantee for delivery.
 No guarantee of delivery in sequence.
 No guarantee of delivery at all !!!
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Things get lost
How can we acknowledge delivery?
Retransmission
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How to determine when to retransmit? Timeout?
Need local copies of contents of each envelope.
How long to keep each copy.
What if an acknowledgement is lost?
Each mailing functionality is carried out by
a specific network architecture layer
Lect-1: Introduction
Computer Networks
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An Introduction to the mail system
Technion
Beer Sheva
HOST
Application Layer
HOST
Shimon
Reuven
Transport Layer
Admin
Admin
Network Layer
Router
Lect-1: Introduction
Link Layer
Computer Networks
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An Introduction to the Internet
mail.technion.ac.il
bgumail.bgu.ac.il
Application Layer
HOST
HOST
Shimon
Reuven
Transport Layer
O.S.
D
Data
Router
Lect-1: Introduction
Data
O.S.
Header
Network Layer
H
Packet
Header
D
H
D
D
H
H
Computer Networks
D
D
H
H
Link Layer
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Characteristics of the Internet
Protocol (IP)
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Each packet is individually routed.
No time guarantee for delivery.
No guarantee of delivery in sequence.
No guarantee of delivery at all !!!
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
Packets get lost
Packet acknowledgements
Packet retransmission
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How to determine when to retransmit? Timeout?
Need local copies of contents of each packet.
How long to keep each copy?
End-to-end or hop-by-hop?
What if an acknowledgement is lost?
Lect-1: Introduction
Computer Networks
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Further Characteristics of the
Internet Protocol (IP)
No guarantee of integrity of data.
 Packets can be fragmented.
 Packets may be duplicated.

These need to be handled by the network
 So which layer does what ?

Lect-1: Introduction
Computer Networks
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An Introduction to the mail system
HOST
Application Layer
HOST
Shimon
Reuven
Transport Layer
Admin
Admin
Network Layer
Router
Lect-1: Introduction
Link Layer
Computer Networks
9
Layering in the Internet
Three out of seven

Transport Layer
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
Network Layer
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
Provides reliable, in-sequence delivery of data from endto-end on behalf of the application.
Provides “best-effort”, but unreliable, delivery of
“datagrams”.
Link Layer

Carries data over (usually) point-to-point links between
hosts and routers; or between routers and routers.
Lect-1: Introduction
Computer Networks
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Some questions about the mail system
How many sorting offices are needed and
where should they be located?
 How much sorting capacity is needed?
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How can we guarantee timely delivery?
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Should we allocate more for Rosh Hashana?
What prevents delay guarantees?
Or delay variation guarantees?
How do we protect against fraudulent mail
deliverers, or fraudulent senders?
Lect-1: Introduction
Computer Networks
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Outline




An Introduction to the mail system
An Introduction to the Internet
Important concepts in Networking
Key Enablers of Large Networks
Lect-1: Introduction
Computer Networks
12
Important Concepts in Networking
Multiplexing
 Buffering
 Encapsulation
 Socket API
 Common Applications

Lect-1: Introduction
Computer Networks
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Multiplexing
A method of sharing resources among users
 Provides a cost-effective resource sharing
 Packetizing messages enables Time Mux
 Routers enable Space Mux

Lect-1: Introduction
Computer Networks
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Buffering
The process of temporarily storing data until
a resource becomes available
 Enables to overcome random delays and
async transmission
 Handles temporarily overflow and reduces
loss of data

Lect-1: Introduction
Computer Networks
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Encapsulation
The process of embedding “data” in a new
“message format”
 The new message will consist of header+data
 Enables hierarchical layering where one layer
places its data into another layer format
 The latter is responsible to provide its end-toend service to the encapsulated data

Lect-1: Introduction
Computer Networks
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An Introduction to the mail system
HOST
Application Layer
HOST
Shimon
Reuven
Transport Layer
Admin
Admin
Network Layer
Router
Lect-1: Introduction
Link Layer
Computer Networks
17
Socket API
API: Application Programming Interface
 Socket API is a common interface by which
remote application can exchange messages
 Enables distributed development of
applications that can talk to each other

Lect-1: Introduction
Computer Networks
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Common Applications
A key factor for the Internet success
 Part of Internet suite
 Notable ones are:
 Domain Name Server (DNS)
 Mail
 Network File System (NFS)
 HTTP (one of the Web protocols)
 RTSP (Streaming – another Web protocol)

Lect-1: Introduction
Computer Networks
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Outline




An Introduction to the mail system
An Introduction to the Internet
Important concepts in Networking
Key Enablers of Large Networks
Lect-1: Introduction
Computer Networks
20
Three Key Enablers of Networks
Digitization of Signals
 Economies of Scale
 Network Externalities

Lect-1: Introduction
Computer Networks
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Digitization of Signals
Any information bearing signal can be
represented by a binary string with
arbitrarily high accuracy.
 Sample + quantize a Digital stream.
 Digital data is cheaper to store, manipulate
and, most importantly, reliably communicate.

Lect-1: Introduction
Computer Networks
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Economies of Scale
Cost of capacity grows slowly with
increased capacity.
 A network has fixed costs only (almost)
and zero marginal costs.

Cost
Capacity or # users
Lect-1: Introduction
Computer Networks
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Network Externalities

Value of the network increases as the
number of users increases, which in turn
attracts more users.
Benefit
tradeoff
Cost
# Users
Lect-1: Introduction
Computer Networks
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Next Lecture
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A Detailed FTP Example
Layering
Packet Switching and Circuit Switching
Some terms
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Data rate, “Bandwidth” and “throughput”
Propagation delay
Packet, header, address
Bandwidth-delay product, RTT
Additional Reading
Text book: Chaps. 1.2, 3.1
Lect-1: Introduction
Computer Networks
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