1 CSE 422 Computer Networks CSE 422. 2 Technology Over The Centuries 18th Century: Mechanical Systems Accompanying The Industrial Revolution 19th Century:

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CSE 422CSE 422Computer Computer NetworksNetworks

CSE 422

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Technology Over The Centuries

• 18th Century: Mechanical Systems Accompanying The Industrial Revolution

• 19th Century: Age of The Steam Engine• 20th Century: Information Gathering,

Processing, and Distribution e.g.,– Worldwide Telephone Network – Invention of Radio and TV– Computer Industry– Launching of Communication Satellites

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0M

5M

10M

15M

20M

25M

30M

1982 1984 1986 1988 1990 1992 1994 1996 1998

Internet Growth (by Number of Computers)

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Computer Networks:

• Definition: Interconnected Collection of Autonomous Computers

• Goals – Resource Sharing– Lower Communication Costs– Client-Server Model– High Reliability – Communication Medium Among Widely Separated

People– Smooth System Growth– Simpler Software Design

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Applications of Networks

• Access to Remote Programs– Simulation– Computer Aided Ed.,– Medical Diagnosis

• Access to Remote Data Bases– Reservations For Hotels, Airplanes– Home Banking– Automated Newspaper– Automated Library– Access to Information System: (e.g. World Wide Web)

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Applications of Networks (cont.)

• Communication Medium– Electronic Funds Transfer System– Electronic Mail– Teleconferencing– Worldwide Newsgroups– International Contacts by Humans

• Entertainment Industry– Video On Demand– Multiperson real-time simulation games– Selecting any movie/TV program ever made– Live TV may becomes interactive with audience

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Social Issues

• Views on politics, religion, sex, etc. distributed– Newsgroups debate sensitive issues

– Network operators risk being sued for contents

– Rights to free speech may be violated

– Anonymous messages can be desirable, but ...

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Classification of interconnected processors by physical size

0.1 m Circuitboard Data flow machine1m System Multiprocessor10m Room100m Building Local Network1km Campus10km City Metropolitan Area100km Country (Wide Area) Network1,000km Continent10,000km Planet The Internet

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Network Structure

Communication Subnet (Subnet)– Switching Elements (Routers)– Transmission Lines (Circuits)

Boundary of the Communication subnet

Routers

Hosts

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Types of Design For Subnets

Point-to-Point Circuits (Channels)– Example of Topologies

Some possible topologies for a point-to-point subnet(a) Star (b) Loop (c) Tree (d) Complete (e) Intersecting loops (f) Irregular

(a) (b) (c)

(d) (e) (f)

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Types of Design For Subnets (cont.)

Broadcast Channels– Examples of Topologies

Communication subnet using broadcasting(a) Bus (b) Satellite or Radio (c) Ring

(a) (b) (c)

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Types of Design For Subnets (cont.)

Note: Broadcast Subnets May Allocate Channel By:

1. Static Methods• TDMA

2. Dynamic Methods• Centralized

• Decentralized

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Summary of Network types -LANs, MANs, & WANs

• Local area networks (LANs)-are privately owned networks within a single building or campus of up to a few kilometers in size

• LANs-have three distinguished characteristics: (1) size, (2) transmission technology, & (3) topology

• Metropolitan area networks (MANs)-basically a larger version of LANs, and uses similar technology

• MAN-has just one or two cables and contains no switching elements

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Network Types (Cont.)-LANs, MANs, & WANs

• MAN standard-Distributed Queue and Dual Bus (DQDB), consists of two unidirectional buses (cables) to which all computers are connected

• WAN-spans a large geographical area; it consists of several hosts, connected to a subnet, which in turn is connected via transmission lines and switching elements

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Network Types (Cont.)-LANs, MANs, & WANs

Architecture of DQDB metropolitan area network

1 2 3 NComputer

Bus A Direction on flow on bus A

Direction on flow on bus B

Architecture of DQDB metropolitan area network

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Network Types (Cont.)-Wireless Networks

• Mobile computing, (e.g., notebook computers & portable digital assistants (PDA) is growing at a rapid rate)

• Users want network connectivity in cars, airplanes, & other remote sites

• The use of a portable computer capable of wireless networking will very likely revolutionize the way we use computers

• Possible uses: portable office, fleets of trucks, taxis, buses, and repairpersons (keeping in contact with home)

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Network Types (Cont.)-Wireless Networks

• Other uses: workers at disaster sites (fires, floods, etc.) where telephone system is destroyed; military operations

• Some disadvantages: low bandwidth (1-2 Mbps), high error rates, & frequent disconnections

• Wireless networks communicate via modulating radio waves or pulsing infrared light

• Wireless communication; linked to wired network infrastructure by transceivers

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Network Types (Cont.)-Wireless Networks

• Cell- area cover by an individual transceiver's signal; the cell sizes vary widely

• Wireless networks comes in many forms. Some universities have installed antennas all over campus to allow students to access the library card catalog, while sitting under the trees

• Security is a problem, because connection to wireless is so easy; challenge for software designers

• Address migration also presents a challenge

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Examples of Networks

• Commercial Networks– DECNET– SNA

• National Network– ARPANET– NREN– EDUNET– USENET

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Examples of Networks (cont.)

• Local Area Networks– NOVELL NETWARE

– MAP and TOP

• Packet Carriers– TYMNET– TELENET

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The Internet Emerges-funded by ARPA

• Need to interconnect LANs, MANs, and WANs• Initially interconnected: NSFNET and ARPANET• Results: Internet, with TCP/IP Software• Growth continues exponentially, doubles each yr.• Main applications: Email, Remote Login, News,

File Transfer• New application: WWW, with Internet Explorer,

further increased the Internet usage

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Data Communications Organizations

ISO CCITT

ANSI State Dept.

EIA Carriers Other NTIA

NCS Org.

Government Agencies

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A Simplified Architecture for File Transfer

Computer X Computer Y

File transferapplication

CommunicationsService module

Network accessModule

File transferapplication

CommunicationsService module

Network accessModule

CommunicationsNetwork

Network interfacelogic

Network interfacelogic

File and file transfer command

Communications-related data units

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Network Architectures

• Protocols

• Layers

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The ISO Reference Model (Basic Principles)

1. A layer should be created where a different level of abstraction is needed.

2. Each layer should perform a well defined function.

3. The function of each layer should be chosen with an eye toward defining internationally standardized protocols.

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The ISO Reference Model (Basic Principles) (cont.)

4. The layer boundaries should be chosen to minimize the information flow across the interfaces.

5. The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity, and small enough that the architecture does not become unwieldy.

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Design Issues For The Layers:

• Mechanism For Connection Establishment• Mechanism For Connection Termination• Rules for Data Transfer

– Simplex– Half Duplex– Full Duplex

• Error Control• Properly Sequencing Messages

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Design Issues For The Layers: (cont.)

• Flow Control

• Routing

• Multiplexing Conversations

• Mechanism For Handling Arbitrarily Long Messages

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Layer 7

Layer 1

Layer 2

Layer 3

Layer 4

Layer 5

Layer 6

Layer 7

Layer 1

Layer 2

Layer 3

Layer 4

Layer 5

Layer 6

Layer 7 protocol

Layer 4 protocol

Layer 3 protocol

Layer 2 protocol

Layer 1 protocol

Layer 5 protocol

Layer 6 protocol

Physical Medium

Layer 6/7 interface

Layer 1/2 interface

Layer 2/3 interface

Layer 3/4 interface

Layer 4/5 interface

Layer 5/6 interface

Layers, protocols, and Interfaces

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Example information flow supporting virtual comm. in layer 7.

m

M

M

H4

m

M

M

M1

H4 M1

H4 M1

H3

H3 T2H2

H4 M2

H4 M2

H4 M2

H3

H3 T2H2

H4 M1

H4 M1

H4 M1

H3

H3 T2H2

H4 M2

H4 M2

H4 M2

H3

H3 T2H2

Transmitted

6/7 interface

5/6 interface

6/7 interface

5/6 interface

Source machine Destination machine

Layer2protocol

Layer3 protocol

Layer4 protocol

Layer5 protocol

Layer6 protocol

Layer7 protocol

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Physical Layer

• Concerned with Transmitting Raw Bits over a Communication Channel.

• Design Issues:– Mechanical, Electrical, Procedural Interfacing to

Subnet• Implemented in Hardware

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Data Link Layer

• Takes a Raw Transmission Facility & Transforms It To a Line Which Appears Free of Transmission Errors to The Network Layer.

• Breaks Input Data Into Frames, Transmitting Frames Sequentially, & Process Acknowledgment Frames.

• Design Issues:– Solve Problems Caused By Damaged, Lost, or Duplicate

Frames.– How to Keep Fast Transmitters From Drowning Slow

Receiver.

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Network Layer --- Communication Subnet Layer• Determines Chief Characteristics of IMP Host

Interface & How Packets Are Routed Within The Subnet.

• Software Accepts Messages From The Source Host, Converts Them To Packets, & See That Packets Are Routed Correctly.

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Network Layer --- Comm. Subnet Layer (cont.)• Design Issues

– The Division of Labor Between The IMPs & The Host (i.e., Who Should Ensure That All Packets Are Correctly Received at Their Destination, & in Proper Order.)

– How The Route is Determined? By Using Static Tables, Dynamic Tables, or ?

• Implemented in Host by I/O Drivers

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Transport Layer --- Host to Host Layer

• Provides a flow of data between two hosts, for the application layer above.

• Accepts Data From Session Layer, Splits It Into Smaller Units, If Needed, Passes to Network Layer, & Ensures That All Pieces Arrive Correctly at Other End.

• Determines The Type of Service Provided to The Session Layer. e.g.,– Error --- Free (Virtual) Point-to-Point Channel That

Delivers Messages in The Order They Were Sent.

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Transport Layer --- Host to Host Layer (cont.)– Transport of Isolated Messages With No Guarantee

About The Order of Delivery.

– Broadcasting of Messages to Multiple Destinations.

• Design Issues– Mechanism to Regulate The Flow of Information From

One Host to Another.

– Determine Which Message Belongs to Which Connection.

• Implemented as Part of The Host OS.

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Session Layer --- User Interface Layer

• User Negotiate to Establish a Connection with a Process on Another Machine.

• Manages The Session Once It Has Been Set Up, (e.g., If Transport Connections are Unreliable, The Session Layer May Be Required To Recover From Broken Transport Connections.)

• Implemented as Part of The OS.

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Presentation Layer

• Represents Information to Communication Application-Entities In a Way That Preserves Meaning While Resolving Syntax Differences. Typical Functions Include:

• Text Compression– Encryption for Security– Syntax Selection– Conversion Between Character Codes (e.g.,

ASCII to EBCDIC)

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Application Layer

• Based on Request From User, This Layer Selects Appropriate Services To Be Supplied From Lower Layers. e.g.– Identification of Intended Communication

Partners & Their Availability & Authenticity.– Determination of Cost Allocation Methodology.– Establishment of Error Recovery Responsibility.– Agreement on Required Privacy.

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Application Layer (cont.)

• Design Issues– Problem of Partitioning to Gain Maximum

Advantage of Network.– Questions of Network Transparency, Hiding The

Physical Distribution of Resources From The User.

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Layer OSITCP/IP

Protocol Suite SNA

7

1

2

3

4

5

6

Application

Presentation

Physical

Data Link

Network

Transport

Session

Process/Application

Host-Host

NetworkAccess

Internet

Transaction Services

PresentationServices

Data Flow Control

Transmission Control

Path Control

Data Link Control

Physical Control

Approximate correspondences between the various networks

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A Critique of the OSI Model and Protocols

• Bad timing

• Bad technology

• Bad implementation

• Bad Politics

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A Critique of the TCP/IP Reference Model

• Does not distinguish concepts of service, interface, and Protocol clearly

• Not at all general, & poorly suited to describing any other protocol stack

• Host-to-network layer is not really a layer, but an interface between the network and the data-link layers

• Does not distinguish between physical and data link layers

• IP and TCP were well thought out, but the other protocols were ad hoc

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In order to transfer the SDU, the layer N entity may have to fragmentit into several pieces, each of which is given a header and sent as a separate PDU such as a packet

Relation Between Layers at an Interface

ICI SDU

ICI SDU

SAP

IDULayer N+1

Interface

Layer N

SAP = Service Access PointIDU = Interface Data UnitSDU = Service Data UnitPDU = Protocol Data UnitICI = Interface Control Information

Layer N entitiesExchange N-PDUsIn their layer Nprotocol

SDU

Header N-PDU

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Layering of TCP/IP-based protocols

NFSRPC

HTTP FTP TELNET DNS SNMP

transportlayer

networklayer

data linklayer

TCP UDP

IP

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Six different types of services

Service Example

Reliable message stream Sequence of pagesReliable byte stream Remote loginUnreliable connection Digitized voice

Unreliable datagram Electronic junk mailAcknowledged datagram Registered mailRequest-reply Database query

Connection- oriented

Connection- less

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Service Primitives

requestLayer N+1

Layer N

response24

1 3confirm indication

Physical channelHost 1 Host 2

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Service Primitives (cont.)

• Request: An entity wants the service to do some work

• Indication: An entity is to be informed about an event

• Response: An entity wants to respond to an event• Confirm: An entity is to be informed about its

request

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Service Primitives (cont.)

• To make the concept of a service more concrete, let us consider as an example a simple connection-oriented service with eight service primitives as follows:

1. CONNECT.request --- Request a connection to be established.

2. CONNECT.indication --- Signal the called party.3. CONNECT.response --- Used by the callee to

accept/reject calls.4. CONNECT.confirm --- Tell the caller whether the

call was accepted.

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Service Primitives (cont.)

5. DATA.request --- Request that data be sent.

6. DATA.indication --- Signal the arrival of data.

7. DISCONNECT.request --- Request that a connection be released.

8. DISCONNECT.indication --- Signal the peer about the request.

• In this example, CONNECT is a confirmed service (an explicit response is required) whereas DISCONNECT is unconfirmed (no response).

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Example Data Communication Services

• Switched Multimegabit Data Services (SMDS)-connecting LANs

• SMDS is designed to handle bursty traffic• SMDS service: simple connectionless packet

service• A useful feature of SMDS is broadcasting• Another useful feature is address screening on both

incoming & outgoing packets

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Example Data Communication Services (cont.)

LAN 1 LAN 3

LAN 2Leasedlines LAN 4

SMDS

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Data Commun. Services (Cont.)- X.25 Networks

• Standard developed during the 1970s by CCITT• Provides an interface between public packet

networks & their customers• X.25 comprises the physical layer, the data link

layer & the network layer• X.25 is connection-oriented & supports both

switched virtual circuits & permanent ones• Provides ACKs and flow control

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Data Commun. Services (Cont.)-X.25 Networks

• Note: some older terminals still do not speak X.25 & need yet another way to connect (Packet Assembler Disassembler)

• Multiplexing & switching of logical connections take place in layer 3

• Call control signaling is carried on the same logical connection as user data

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Data Commun. Services (Cont.)-Frame Relay

• An absolute connection-oriented service• Goal: move bits from A to B at reasonable speed &

low cost• Can be thought as a virtual leased line• Does not provide ACKs or flow control• Variable size packets (Frames) may be up to 1600

bytes• Designed to operate at user data rates of up to 2

Mbps

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Data Commun. Services (Cont.)-Frame Relay

• Lower delay & higher thru put, since internal processing is reduced, as is the protocol functionality at the user-network interface

• Call control signaling is on a separate logical connection from user data

• Multiplexing & switching of logical connections take place in layer 2

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Data Commun. Services (Cont.)- B-ISDN and ATM

• Asynchronous Transfer Mode (ATM), Universal information carrier: voice, data, & video

• ATM networks are connection-oriented• Example services: video on demand, live TV from

many sources, full motion multimedia E-mail, CD-quality music, high-speed data transport, LAN interconnection

• Small fixed-sized packets (cells), 53 bytes long, of which 5 bytes are header & 48 bytes are payload

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Data Commun. Services (Cont.)- B-ISDN and ATM

• ATM is called cell relay- a cell-switching technology• Cell delivery is not guaranteed, but the order is• Cell-switching: highly flexible, & can handle both

VBR & CBR traffic, digital switching of cell is easy via fiber optics, facilitates TV distribution broadcasting

• Normal speed for ATM networks is 155 Mbps, 622 Mbps, and gigabit speed later

• The ATM Forum: an international group that guides the future of ATM

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Data Commun. Services (Cont.)- B-ISDN and ATM

Plane management

Layer management

Control plane User plane

Upper layersUpper layers

Physical layer

ATM adaption layer

ATM layer

CSSAR

TCPMD

CS: Convergence sublayerSAR: Segmentation and reassembly sublayerTC: Transmission convergence sublayerPMD: Physical medium dependent sublayer

Sub layer

Sub layer

Sub layer

Sub layer

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Data Commun. Services (Cont.)- B-ISDN and ATM

The ATM layers and sublayers, and their functionsOSIlayer

ATMlayer

ATMsublayer Functionality

3/4

2/3

2

1

AALCS

SAR

Providing the standard interface(convergence)

Segmentation and reassembly

Flow controlcell header generation/extractionvirtual circuit/path managementCell multiplexing/demultiplexing

ATM

Cell rate decouplingHeader checksum generation and verificationCell generationPacking/unpacking cells from the enclosing envelopeFrame generation

Bit timing Physical network access

TC

PMD

Physical

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Data Commun. Services (Cont.)- B-ISDN and ATM

Different networking services.

Issue DQDB SMDS X.25 Frame relay ATM

Connection oriented Yes No Yes Yes Yes

Normal speed(Mbps) 45 45 .064 2 155

Switched No Yes Yes No Yes

Fixed-size payload Yes No No No Yes

Max payload 44 9188 128 1600 48

Permanent VCs No No Yes Yes Yes

Multicasting No Yes No No Yes

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Gigabits Testbeds

• Michigan State University: High-Speed Networking & Performance Research Laboratory (HSNP)

• ARPA & NSF financed a number of university-industry gigabit testbeds– MIT, U of Penn., IBM Watson Lab, and Bellcore;

Aurora- (a testbed linking four sites in the Northeast)

– AT\&T Bell Labs, Berkley, the U of Wis; Blanca- (research issues: protocols, host interfaces, etc)

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Gigabit Testbeds Cont.

– Cal Tech, JPL, Los Alamos, & San Diego Super Computer Center; CASA- (aimed at doing research on super computer applications

– CMU; Nectar- (an experimental MAN from CMU to Pittsburgh, interested in applications involving chemical process flowsheeting & Oper. Res.)

– U of NC, NC State U, IBM Res. Triangle Park; VISTAnet- research focuses on 3D images to plan radiation therapy for cancer patients)