Module 1: Introduction · 5 SMD123 Computer Communications 9 Examination • Redo of assignments and quizzes is not permitted Exceptions: Prior permission for late submission or for

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Module 1: Introduction

Kaustubh S. PhanseDepartment of Computer Science and Electrical Engineering

Luleå University of Technology

SMD123 Computer Communications

SMD123 Computer Communications 2

Lecture Objectives

• Define course objectives

• Discuss the syllabus, course structure and otherlogistics

• Introduction to computer networks and the InternetMotivation and overviewDefinitions and abstractionsNetwork architecture

Layered modelOSI reference model

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Course Objectives

• Understand the fundamental concepts and principlesunderlying computer networks, protocols, and architectures

• Gain knowledge about widely-used and emergingnetworking technologies

• Be able to assess and compare the alternate protocols and technologies (… and design new ones!)


Course Objectives

• Enable study of advanced topics in the futureSMD144 Communication NetworksSMD143 Network AlgorithmsSMD147 Network Projects…

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Course Information

• Course examiner

• Teaching assistant

• Course webpage: http://www.sm.luth.se/csee/courses/smd/123/

Dr. Kaustubh PhanseOffice: A3409E-mail: [email protected]: 0920-491428

Mr. Johan NykvistOffice: A3205E-mail: [email protected]: 0920-492898


Course Information

• PrerequisitesUnderstanding of basics of computer systems and organizationAbility to understand program code and write simple programs in modern high-level language like C and Java

• Course textbookComputer Networking: A Top-Down Approach Featuring the Internet by James Kurose and Keith Ross, 3rd edition (June 2004), Addison-Wesley, ISBN: 0321269764

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Course Structure• Theoretical

1. Introduction2. Network Applications3. Transport Layer4. Network Layer5. Routing6. Link Layer and Local Area Networks7. Multimedia Networking8. Computer Network Security

• LaborationsThree lab assignments


Examination• Continuous examination NO FINAL EXAM!

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Examination• Redo of assignments and quizzes is not permitted

Exceptions:Prior permission for late submission or for taking the quiz at another timeEmergency, e.g., medical reasonsIf you need 25 points or less to pass the course

• Homework solutions to be prepared and submittedindividually

• For labs, groups of no more than two students

• Code of conduct


Grading

• Maximum achievable number of points is 500 (350 theoretical + 150 laborations)

• Homework 30% (six assignments with 5% each)• Quiz 40% (two quizzes with 20% each)• Labs 30% (three assignments with 10% each)

• To pass the course, you need to have passed both the theoretical and practical parts of the course

At least 175 theory points to pass the theory partAt least 75 lab points to pass the laborations part

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Grading

• Grade scale: U, 3, 4, 5

• If you pass both theory and lab parts and getat least 250 points ~ grade 3at least 325 points ~ grade 4at least 425 points ~ grade 5

Introduction to Computer Networks and the Internet

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Computer networks...they are everywhere!

Blah...blah...blah...

Yada...yada...yada...


Network components

Intermediate devices (routers, switches)

Host A

End system

Host B

End system

Communication link

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The Internet: network of networks

• Roughly hierarchical• “Tier-1” ISPs (national/international coverage) at the

centreTreat each other as equals

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

Tier-1 providers interconnect (peer) privately

NAP

Tier-1 providers also interconnect at public network access points (NAPs)



• “Tier-2” ISPs: smaller (often regional) coverageConnect to one or more tier-1 ISPs, possibly other tier-2 ISPs

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet

tier-2 ISP is customer oftier-1 provider

Tier-2 ISPs also peer privately with each other, interconnect at NAP

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The Internet: network of networks• “Tier-3” ISPs and local ISPs

Last hop “access” network (closest to end systems)

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP



Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier-3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet


The Internet: network of networks• Data may pass through many networks!

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP



Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

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EdgeCore


Bandwidth and throughput• Indicates the amount of data (e.g., bits) that can be

transferred in some time unit (e.g., second)1 megabits per second (Mbps) = bits per second

• End-to-end bandwidth is as good as the “weakest” link• “Throughput” is used to measure the performance of a

systemThe amount of data that was actually transferred in some time unit

610

1 10 0seconds = 1 microsecond is the “bit width”

610−

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Network edge• End systems (hosts)

Run applications and servicesWeb, e-mail, multimedia, …

• Connection-oriented services (TCP-based)Reliable data transfer; congestion control; flow controlExamples: Web (HTTP), file transfer (FTP), e-mail (SMTP), …

• Connectionless services (UDP-based)Unreliable data transfer; no congestion control and flow controlExamples: Internet telephony, teleconferencing, directory service (DNS), …


Access technologies and physical media • Access Networks

Residential accessDial-up, DSL, cable, …

Company accessEthernet, Gigabit Ethernet, optical fiber, …

Wireless accessIEEE 802.11x, 802.16x, 3G, …

• Physical MediumGuided media

Signals propagate in solid media: copper, fiber, coaxial cable, …Unguided media

Signals propagate freely, e.g., radio, infra-red, …

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Network core

• Mesh of interconnected routers• Support data transfer between end systems

Circuit switchingPacket switching


Circuit switching• Bit stream carried over a path established during call

set-up• Resources are reserved for the duration of the call

Service guarantee• Inefficient use of resources• Example: Traditional telephone networks

Circuit switches

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Circuit switching: FDM and TDMExample: 4 users

frequency

time

Frequency Division Multiplexing (FDM)

frequency

timeframe

Time Division Multiplexing (TDM)


Numerical

In a network using TDM, the link bandwidth is 8,000 frames per second and each slot consists of 8 bits. Each circuit is allocated one slot in a frame. What is the bandwidth of each circuit?

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Numerical

How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network?

•All links are 1.536 Mbps•Each link uses TDM with 24 slots (one slot per circuit)•500 ms to establish end-to-end circuit


Packet switching• Data transmitted as packets = data + control information• Resources not necessarily reserved in advance

Statistical multiplexing• Store and forward architecture• Example: The Internet

Packet switches, e.g., routers

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Packet switching

• Aggregate demand may exceed available resourcesCongestion and delay: packets waiting in queue to use the link

A

B

C10 Mb/sEthernet

1.5 Mb/s

D E

statistical multiplexing

queue of packetswaiting for output

link


Packet switching: datagram networks

• No connection- or flow-state information is maintained

• Packet forwarding decisions based on a packet’s destination host address and not on the connection or application flow to which the packet belongs

• Analogous to the postal serviceHierarchical network-wide addressing, e.g., 130.240.60.72

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Packet switching: virtual circuit (VC) networks

• Virtual connection set-up and maintained between a source and destination

State information maintained at every intermediate switch• Each VC identified by a VC ID• Switching based on the “VC ID to outbound link”

mapping and not on the source and destination end system addresses


Numerical: circuit switching vs. packet switching

Link bandwidth = 1 Mbps; total number of users = 35Each user transmits at 100 Kbps; probability that a user is active is 0.1

Circuit switchingMaximum number of users that can be supported = 1 Mbps/100Kbps = 10

Packet switchingProbability that 11 or more users (out of 35) are active is 0.0004In other words, the probability of 10 or less users being active is 0.9996!

So, many more users can be “accommodated” (This is often termed as oversubscription…made possible due to statistical multiplexing)

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Numerical: circuit switching vs. packet switching

How long does it take to send a file of 640,000 bits from host A to host B over a packet-switched network?

•All links are 1.536 Mbps(a) If 24 users are transmitting the filesimultaneously (assume equal sharing of bandwidth)(b) If a single user is transmitting the file


Summary: circuit switching vs. packet switching

• Packet switchinggreat for bursty dataon-demand resource sharingsimple, no call set-upCan lead to congestion: queuing delays and packet loss!

Protocols needed for congestion control and reliable data transfer

• Service guarantees with packet switching (module 7)Needed for real-time applicationsQuality of service (QoS) mechanisms

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Traffic intensity

• Dependence of average queuing delay on traffic intensity

La/R ~ 0: average queuing delay smallLa/R 1: delay becomes largeLa/R > 1: more “work” arriving than can be serviced, average delay infinite!

• Traffic intensity = La/RR = link bandwidth (bps)L = packet length (bits)a = average packet arrival rate


What is a "protocol"?

Tjena

Hejsan

Hur är laget?

Bra, tack

Hej då

Hej då

Tjena

What?

Talar du svenska?

I don’t understand!

Hej då

.

.

The Internet Engineering Task Force (IETF) handles standardization of protocols used in the Internet (http://www.ietf.org)

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Layering• Networks are typically modeled as a set of layered,

cooperating processes• Decomposes system into simpler, manageable

components• Layer N provides services to layers N+1 and above

and uses services offered by layers N-1 and belowApplication Programs

Process-to-Process Channels

Host-to-Host Connectivity

Networking Hardware

Laye

rs


Open Systems Interconnect (OSI) ModelThe International Standards Organization (ISO) has developed the seven-layer OSI model

Source: ECE 4984 Wireless and Mobile Systems Design course by Prof. Luiz A. DaSilva, Virginia Tech

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Source: http://www.searchnetworking.com


Encapsulation and decapsulationmessage

segmentdatagramframe

sourceapplicationtransportnetwork

linkphysical

HtHnHl MHtHnHl MHtHn MHtHn MHt MHt M

MM

destinationapplicationtransportnetwork

linkphysical

HtHnHl MHtHnHl MHtHn MHtHn MHt MHt M

MM

networklink

physical

networklink

physical

linkphysical

linkphysical

HtHnHl MHtHnHl MHtHn MHtHn M

HtHnHl MHtHnHl MHtHn MHtHn M

HtHnHl MHtHnHl M HtHnHl MHtHnHl M

router

switch

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Advantages of layering• Data structures and algorithms, etc. in a layer are not visible to

other layers

• Modular design – complex systems can be decomposed into more easily understood and realizable pieces

• System can evolve since layers can be changedAlternate services can be offered at layer N+1 that share the services of layer NA layer can be modified or omitted if some or all of its services are not needed

• Confidence in correct operation enhanced by testing each layer independently


Disadvantages of layering

• Some network functions need to access and operate at multiple layers

• Poorly conceived layers can lead to awkward and complex interfaces

• There may be performance penalties due to extra overhead of layers

• Design of (an older) layer N+1 may be sub-optimal given the properties of (a new) layer N

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Delay components

• Processing delay (dproc)Delay incurred due to processing of the packet at a nodeDepends on the node’s processing power

• Queuing delay (dqueue)Delay incurred by packet as it waits to be transmitted onto a linkDepends on how congested a link is

proptransqueueprocnodal ddddd +++=


Delay components

• Transmission delay (dtrans)The time to push an entire packet (all its bits) onto a link

Packet length is L bitsTransmission rate (or link bandwidth) is R bits per secondTransmission delay (or store-and-forward delay) is L/R

• Propagation delay (dprop)The time required to propagate from one end of the link to the other

Physical distance between two ends of a link is dPropagation speed is s (e.g., speed of light)Propagation delay is d/s

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Some more thoughts on delay...

• End-to-end latency or delay is the sum of all nodaldelays

• Round-trip time (RTT) is the amount of time it takes to send a message from one end of the network to the other and back


Numerical

R R RL

It takes L/R seconds to transmit a packet of L bits on to a link with bandwidth R bps. Entire packet must arrive at router before it can be transmitted on next link, i.e., store and forward.

Ignoring propagation, queuing and processing delay, end-to-end delay = ?

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Numerical

R R R

LLL

Ignoring propagation, queuing and processing delay, end-to-end delay = ?

A

S1 S2

B


Bandwidth-delay product• The amount of data required to fill the “pipe” i.e., the

amount of data a sender can transmit before the receiver receives the first bit

• For feedback-based protocols like TCP, we are interested in the RTT than one-way latency

Bandwidth-delay product is expressed as BW x RTT

Delay

Bandw

idth (BW

)

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Internet timeline

SOURCE: http://www.isoc.org/internet/history/brief.shtml


Internet hosts

SOURCE: http://www.zakon.org/robert/internet/timeline/

Documents

Module 1: Introduction · 5 SMD123 Computer Communications 9 Examination • Redo of assignments and quizzes is not permitted Exceptions: Prior permission for late submission or for