Networks 1 Intro

8/14/2019 Networks 1 Intro

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Chapter Outline

Introduction (slides and 7.A) Layered Architecture (slides and 7.B &

7.D)

Routing (slides and 7.D) Reliable Transmission & Flow Control

(slides and read 7.E)

Congestion Control (slides and read 7.F)


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This Lecture

What is a network? Sharing the infrastructure

Circuit switching

Packet switching Best Effort Service

Analogy: the mail system Internets Best Effort Service


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Networks Why they are interesting?

Overcome geographic limits Access remote data Separate clients and server

Goal: Universal Communication (any to any) Design the cloud

Network


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Connectivity

LinkDSL, T1, T3, ...Characterized by

Capacity or bit-rate (1.5 Mb/s, 100Mb/s, )

Propagation delay (10us, 10ms, 100ms, ..) Transfer time on a link = #bit/bit-rate + propagation delay


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Connectivity

A mesh requires N2 links too costly


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This Lecture

What is a network? Sharing the infrastructure

Circuit switching




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Two ways to share

Circuit switching (isochronous)Packet switching (asynchronous)


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Circuit Switching

DATA

Caller Callee

BostonSwitch LASwitch

propagation

delay

between

caller and

and Boston

switch

processing delay at switch

Its the methodused by thetelephone network

A call has threephases:

1. Establish circuit fromend-to-end (dialing),

2. Communicate,3. Close circuit (tear

down). If circuit not

available: busysignal

(1)

(2)

(3)


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Switch

Circuit Switching:Multiplexing/Demultiplexing

Time divided into frames and frames divided into slots Relative slot position inside a frame determines which

conversation the data belongs to E.g., slot 0 belongs to the red conversation Need synchronization between sender and receiver

Frames

0 1 2 3 4 5 0 1 2 3 4 5Slots =

One way for sharing a circuit is TDM:

Lecture notes use the word frame for slot


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Circuit Switching

Assume link capacity is C bits/sec Each communication requires R bits/sec #slots = C/R Maximum number of concurrent communications is C/R

What happens if we have more than C/R communications? What happens if the a communication sends less/more

than R bits/sec?

Design is unsuitable for computer networks wheretransfers have variable rate (bursty)


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Internet Traffic Is Bursty

Daily traffic at an MIT-CSAIL router

Max In:12.2Mb/s Avg. In: 2.5Mb/s

Max Out: 12.8Mb/s Avg. Out: 3.4 Mb/s


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Packet Switching Used in the Internet

Data is sent in Packets(header contains controlinfo, e.g., source anddestination addresses)

Per-packet routing At each node the entire

packet is received,stored, and thenforwarded (store-and-forward networks)

No capacity is allocated

Header Data

Packet 1

Packet 2

Packet 3

Packet 1Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

processing

delay of

Packet 1at Node 2

propagationdelay

between

Host 1 &

Node 2transmission

time of

Packet 1

at Host 1

Host 1 Host 2

Node 1 Node 2


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Router

Packet Switching:Multiplexing/Demultiplexing

Multiplex using a queue Routers need memory/buffer

Demultiplex using information in packet headerHeader has destination Router has a routing table that contains information about

which link to use to reach a destination

Queue


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Queues introduce Variable Delay

Delay = Queuing delay + propagation delay + transmissiondelay + processing delay

Losses When packets arrive to a full queue/buffer they are

dropped


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Packet switching also show reordering

Host A

Host BHost E

Host D

Host C

Node 1Node 2

Node 3

Node 4

Node 5

Node 6 Node 7

Packets in a flow may not follow the same path (depends

on routing as we will see later)

packets may bereordered


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This Lecture

What is a network? Sharing the Infrastructure

Circuit switching




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The mail system

Dina Nick

MIT Stanford

Admin Admin


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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! Things get lost How can we acknowledge delivery? Retransmission

How to determine when to retransmit? Timeout?

If message is re-sent too soon duplicates


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The mail system

Dina Nick

MIT Stanford

Admin Admin


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The Internet

Dina Nick

Nms.csail.mit.edu Leland.Stanford.edu

O.S. O.S.HeaderData HeaderData

Packet


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Characteristics of the Internet Each packet is individually routed No time guarantee for delivery No guarantee of delivery in sequence

No guarantee of delivery at all! Things get lost Acknowledgements Retransmission

How to determine when to retransmit? Timeout?

If packet is re-transmitted too soonduplicate


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Best Effort

No Guarantees: Variable Delay (jitter) Variable rate

Packet loss Duplicates Reordering

(notes also state maximum packet length)


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Differences Between Circuit & Packet Switching

Circuit-switching Packet-Switching

Guaranteed capacity No guarantees (best effort)

Capacity is wasted if data isbursty

More efficient

Before sending dataestablishes a path

Send data immediately

All data in a single flowfollow one path Different packets mightfollow different paths

No reordering; constantdelay; no pkt drops

Packets may be reordered,delayed, or dropped


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This Lecture

We learned how to share the networkinfrastructure between manyconnections/flows

We also learned about the implications ofthe sharing scheme (circuit or packetswitching) on the service that the

traffic receives

Documents

Networks 1 Intro