Lect-1: Introduction Computer Networks 1

371-1-0291: An Introduction to Computer Networks

Handout #1: Introduction

Additional Reading Text book: Chaps. 1, 9.2.1

Homepagehttp://www.cse.bgu.ac.il/Courses/course.asp?ID=63

Lect-1: Introduction Computer Networks 2

An Introduction to the mail system An Introduction to the Internet Important concepts in Networking Key Enablers of Large Networks

Outline

Lect-1: Introduction Computer Networks 3

An Introduction to the mail system

Shimon Reuven

Beer Sheva Technion

Admin Admin

Letter

Envelop

Delivery System

Lect-1: Introduction Computer Networks 4

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 !!!

Things get lost How can we acknowledge delivery? Retransmission

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

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An Introduction to the mail system

Admin Admin

Application Layer

Transport Layer

Network Layer

Link Layer

Shimon Reuven

Beer Sheva Technion

Router

HOST HOST

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An Introduction to the Internet

Network Layer

Link Layer

Application Layer

Transport Layer

O.S. O.S.HeaderData HeaderData

HD

HD

HD

HD HD

HD

bgumail.bgu.ac.il mail.technion.ac.il

Shimon Reuven

Router

HOST HOST

Packet

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Characteristics of the Internet Protocol (IP)

Each packet is individually routed. No time guarantee for delivery. No guarantee of delivery in sequence. No guarantee of delivery at all !!!

Packets get lost Packet acknowledgements Packet retransmission

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?

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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 ?

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An Introduction to the mail system

Admin Admin

Application Layer

Transport Layer

Network Layer

Link Layer

Shimon Reuven

Router

HOST HOST

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Layering in the Internet

Transport Layer Provides reliable, in-sequence delivery of data from

end-to-end on behalf of the application.

Network Layer 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.

Three out of seven

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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? Should we allocate more for Rosh Hashana?

How can we guarantee timely delivery? What prevents delay guarantees? Or delay variation guarantees?

How do we protect against fraudulent mail deliverers, or fraudulent senders?

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An Introduction to the mail system An Introduction to the Internet Important concepts in Networking Key Enablers of Large Networks

Outline

Lect-1: Introduction Computer Networks 13

Important Concepts in Networking

Multiplexing Buffering Encapsulation Socket API Common Applications

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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

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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

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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-to-end service to the encapsulated data

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An Introduction to the mail system

Admin Admin

Application Layer

Transport Layer

Network Layer

Link Layer

Shimon Reuven

Router

HOST HOST

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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

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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 20

An Introduction to the mail system An Introduction to the Internet Important concepts in Networking Key Enablers of Large Networks

Outline

Lect-1: Introduction Computer Networks 21

Three Key Enablers of Networks

Digitization of Signals Economies of Scale Network Externalities

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Digitization of Signals

Any information bearing signal can be represented by a binary string with arbitrarily high accuracy.

Sample + quantize Digital stream. Digital data is cheaper to store,

manipulate and, most importantly, reliably communicate.

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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.

Capacity or # users

Cost

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Network Externalities Value of the network increases as the

number of users increases, which in turn attracts more users.

Cost

# Users

tradeoff Benefit

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Next Lecture• A Detailed FTP Example• Layering• Packet Switching and Circuit Switching• Some terms

– Data rate, “Bandwidth” and “throughput”– Propagation delay– Packet, header, address– Bandwidth-delay product, RTT

Additional Reading Text book: Chaps. 1.2, 3.1