Post on 21-Dec-2015
transcript
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
Lect-1: Introduction Computer Networks 6
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 ?
Lect-1: Introduction Computer Networks 9
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
Lect-1: Introduction Computer Networks 17
An Introduction to the mail system
Admin Admin
Application Layer
Transport Layer
Network Layer
Link Layer
Shimon Reuven
Router
HOST HOST
Lect-1: Introduction Computer Networks 18
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)
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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 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