Introduction

NetworksNetworks - What are They

What is a network - a bit difficult to defineWe use the word all the time and seem to understand its meaning

Can talk of networks ofPeopleRoadsCompaniesTelephones

Here we’re going to talk about networks of computers

In the early days of computingHad one big computerPainted blue Kept in an air conditioned room and fed punch cardsAll the data and information kept in one place

Cheaper computersAllowed more computersMoved them out of the air conditioned room onto the desktopStuff suddenly got distributedPeople still needed to share the stuff

Sharing stuffAt the start sharing stuff meant

Writing the stuff to be shared to a tape or big floppy diskWalking over to the person who needed the stuffCopying the files to the new computer

Synchronizing stuff became an immediate problem

Major breakthroughRS232

Someone got the bright idea Let’s connect the computers together using a couple of wires

Pins 2, 3, and 7 that’s all you need - trust meThat was the start and the endThe rest is history

StuffWhat is stuff

Can mean a lot of thingsLet’s start at the bottom

We start with a collection of symbols or marksThese are our alphabet

In isolation they have no meaning

We next associate a meaning with the symbolsThis gives us numbers letters or other such primitivesStill these have no real meaning

We next begin grouping the symbols or associating a contextWe now have dataThis is the first step towards something usefulThe symbol 1 may mean a single buffalo or a biological needThe symbols leaf may indicate a tree appendage or a new set of clothes

Applying additional groupings and context to dataGives informationNote

The word information will be used throughout

Convey the general notion of StuffMoving stuff

Different from meaning hereIntent should be evident from the context

Continuing gives knowledgeThis gives a hierarchy of the form

Each level provides A richer expressive powerGreater flexibility

HistoryPrimitive with

Word of mouthDrums

Mark

Symbol

Relation

Data

Relation

Information

Relation

Knowledge

Relation

SmokePolished GlassMessengersPony express

Passing message from person to personPrimitive routers

AlexanderObjective

Move information from one place to anotherAs amount and importance of information increased

Complexity and sophistication of the network Increased correspondingly

ElementsPiece of information - Message

May be encodedRepresented some way

Means by which to send it - Transport mechanismHardware / software / whatever

Place or places to send from and to

Hardware

Can offer a number of different viewsObserve the views are not orthogonalCan have combinations

Types by Transmission TechnologiesBroadcastMulticastPoint to Point

Types by Information FlowSerial

BitCharacterWord

ParallelBitCharacterWord

Types by TopologyStarRingTreeFull or complete connectivityAd hocNetwork can be

Interconnected collection of nodes and arc Easily modeled using graph theoretic techniques

SimpleEach node in the graph

Is a vertexEach connection between two nodes

Is an edgeCollection of sub-networks

Need interconnections between sub-networksSuch connections provide for inter network communication

Referred to as inter-net connectionsMost familiar is the Internet

Provides interconnection between networks all over the world

Types by DistanceLocal Area Networks - LAN

Small to medium geographical areaAutomobile or aircraft have several LANsBoundary scan test technology uses integrated on chip networksA computer may internally connect constituents using a LAN

Single room with a couple of devicesSeveral buildings several hundred devicesSingle city to several cities

Wide Area Networks - WANMedium to large geographical areaSingle city to several citiesSingle CountrySeveral Countries

Types by TechnologySneaker netWire

Simple as a couple of wiresTwisted pairsCo-axial cable

Fiber OpticsWireless

Radio waves of one form or anotherMay be direct digital formTraditional modem through cellular telephone

Easy to installExcellent in areas where wire infrastructure is not in place

Cheaper than wire based

Typically slower than wire typeMore prone to errors

EnvironmentInterference

Types by MobilityMobile computing

Typically wireless but not necessaryStationary

Typically wired but not necessaryInterconnecting Networks

Software

Hardware provides The physical means by which data is moved from one place to another

Software includesDataAddressing and control mechanism

Virtual networksMost contemporary communications networks can be viewed as

Hierarchy of virtual networksHave talked about the hardware

This is the lowest layerOften called the physical layer

Above the hardwareVarying number of software layers or levelsAt each level

A different language is spokenReferred to as a protocol

The function is to provide services for the level aboveRelationship

Service providerService consumer

Physical Layer

Layer 1 Layer 1Layer 1 Protocol

Layer 2 Layer 2Layer 1 Protocol

Layer n-1 Layer n-1Layer n-1Protocol

Layer n Layer nLayer n Protocol

Entire collection called A network architecture

Set of protocols used by a machine calledA protocol stack

Information sent on each level calledA message or messagesIt is possible that message on higher level

Composed of several lower level messagesWill discuss in more detail shortly

Graphically this looks like

Familiar schemeWe should be familiar with such techniquesConsider a computer

Lowest level is the physical machineAbove the hardware

Machine CodeAssemblerHigher level languages C C++ Basic

At each level we have a virtual machineEach machine has its own languageEach has its own set of instructions

Bits and BytesAt the lowest level

Communication is simply a collection of 1’s and 0’sAs we move to higher levels

We place (different) meaningful interpretations on Information on the level below

ProtocolsA protocol is simply an interpretation placed on

Bits and bytes to be transmitted or receivedAs noted earlier we’re transferring messages Message

Is comprised of a collection of bitsSome are interpreted as

Data Some are interpreted as

Control informationOften called a header

DataThis is the actual stuff that is to be sentMoving the data is the goal of the communication

Control Control information is the overhead

Getting the data from one place to another

Two kinds of controlHeader informationScheme for executing the data transfer

Header Information

Potential header elements might beAddress or identifier information

Provides routing and destination informationIdentifies the senders and receivers of the message

Indication of the message Start and EndSize

Message type or structurePadding or fill bits

To ensure proper sizeSeparator

Error informationDetection onlyDetection and correction

Transfer scheme

ConsiderationsData movement

One direction - SimplexTwo directions

One direction at a time - half-duplexBoth directions simultaneously - full-duplex

Number of channelsSpeed or Timing

Individual bitsFlow control

Message orderingSub messages are not always sent in sequential order

HandshakingUse or not useLevel

Complete messageSub messages

Connection and Connectionless Two kinds of services

Connection OrientedConnectionless

Connection Oriented

Transaction protocolEstablish the connection

Transact the businessTerminate or release the connection

Messages Enter one end Extracted from the other end

Ordering preserved

Connectionless

Each (sub)message carries full address informationIf a message is composed of several sub messages

The sub-messages may not be in the order sentMessages sent as

Datagram serviceMessage sent - receiver does not acknowledge

Acknowledged datagram serviceMessage sent - receiver acknowledges

Request-reply serviceSender transmits a datagram containing a requestReceiver returns a datagram containing the answerFrequently used in the client-server model

Services and Service PrimitivesServices

Collection of primitive actions available to the user Request some action to be performedReport on the action taken

These provide the public interface to the service Similar to

Function members in C++Methods in Smalltalk

TypesServices may be

ConfirmedTransaction protocol

request - for an actionindication - the action occursresponse - receiver responds to the actionconfirm - the receiver confirms the action

UnconfirmedTransaction protocol

request - for an actionindication - the action occurs

Service PrimitiveImplementation of a particular action

ModelsLet’s examine several different network models in current use

Each is implemented as a collection of layers as discussedWe generally have two kinds of systems

OpenDesigned to communicate with other systems and vice versa

ClosedCommunication is limited to the confines of the system

OSI Model

Model is called theOpen Systems Interconnection modelNot an architecture but a model

Exact services and protocols are not specifiedProposed and developed by

The International Standards OrganizationISOAn international body similar to ANSI

American National Standards InstituteTheir larger task is to try to establish international standards

Many objects that affect our daily livesThe is becoming essential with the internationalization of trade and economies

OSI ModelComprised of 7 layers

As described in our earlier modelA standard has been defined for each layer

Physical Layer

Moves collections of bits (1’s and 0’s) Over a communications channelThere is no meaning or structure

At this level concern is forMechanical and electrical interfacesIntegrity of bitsPhysical characteristics of the bits

Logical 1 or 0Number of volts for each

The width of each bitDetermines how quickly bits can be moved through the channel

Transfer scheme

Layer Unit Transmitted

Physical BitData Link FrameNetwork PacketTransport TDPUSession SPDUPresentation PPDUApplication APDU

Uni-directionalBi-directional

Connectors and cablingHow connection established and released

Data Link Layer

Moves collections of bits aggregated as framesSender

Breaks data stream into data framesReceiver

Acknowledges reception via an acknowledgment frameData link layer must

Create and recognize frame boundariesAccomplished by surrounding a frame with delimiters

Header and trailerDelimiters must be distinguishable from the data frame

Handle frame re-transmission in the case of corruptionMust accommodate duplicate re-transmission

If acknowledge frame is lostHandle duplex transmission and acknowledgmentHandle broadcast traffic including shared channel access

Network Layer

Manages the routing of a transmission from the source to the destinationRouting alternatives

Fixed path via static tablesPredetermined and integrated into the network

Determined at the time of the connectionDynamically modified throughout the session

May be done to relieve congestionAccommodate network failureFind ‘shortest’ path

Must accommodateDifferent characteristics between or among networks

AddressingMessage sizeProtocols

Manage accounting for network useAt the network layer and below

Activities are directed toward Managing the networkPhysical movement of data

Bits are collected into manageable packetsAbove the network layer

Collection of virtual machines

These have the task of managing the sessionThe toplevel message is broken down into manageable packets

The interface occurs at the transport layerBelow is the subnet Above is the session

Transport Layer

Among the tasks of the transport layer Accept data from the session layer

Immediately aboveSubdivide into packets that are compatible with the network layer

Immediately belowDesire transactions to be implemented such that

Hardware appears invisible to the higher layersMust accommodate

Speed demandsHigh speed

Subdivide problem into multiple parallel transmissionsLow speed

Implement as a single pathPrice demands

Maximally use the transport mechanism is the transaction cost is high

Determine the type of servicePoint to point in order sentTransport of individual message componentsBroadcast to multiple destinations

Control the flow of informationPrevent over / under run

Session Layer

Permits users on separate or different machines to communicateLike transport layer - supports movement of data between machinesOffers richer set of features and capabilities

Analogous to moving to a higher level languageCan do a job in assemblerEasier to do in language like C

Potential servicesNote

Moving from must dos to offers to doManage dialog control

For single direction transmissionTrack turns to send

Manage tokens in token passing protocolsExample

IBM token ring

Like a relay raceCannot use the network until you have been given permissionPermission is in form of a token passed around

Synchronize transactionsReassemble message if necessaryIf complete transfer cannot be completed in single session

Major errorLine dropMachine crash

Rather than retransmit complete messageResume from point of last correct reception

Presentation Layer

Moving up another layer to richer set of toolsGoal of presentation layer is to offer generic set of solutions to common problemsPotential services

Map information - types, structures, encoding etc.From source computer representationTo network representationFrom network representation to destination representation

Similar to p-code generated from some compilersAt the p-code level can be moved between machinesAt the machine code level cannot be moved

Application Layer

This level also deals with incompatibilities between Systems at opposite ends of the network

HardwareSome accommodation at this levelUsually done at a lower level

SoftwarePrimary focus

Potential incompatibilitiesFile systemsTerminal typesMail systemsRemote procedure execution

TCP/IP Model

HistoryOriginated with the U.S. Department of Defense

Sponsored and developed as link between UniversitiesGovernment Installations

Contractors DARPA

Defense Advanced Research Projects AgencyEstablished ARPA NetPrivatized with the end of the cold warNow has become the familiar Internet

Means to exchange information over standard telephone linesPrimary goal

Seamless interconnectionExtreme reliability

As long as source and destination machines operableTransmission would / could continueEntailed sophisticated routing and re-routing schemes

ResultImplemented a packet switching networkConnectionless inter-network layer

This layer is key to the complete designConfiguration

Implemented with 4 layers rather than 7 as in OSI modelMissing are

PresentationSession

Data and PhysicalCombined into a single layer

Host to Network

No significant requirements specified at this levelHost system must

Be able to connect to the networkTransmit / Receive

IP (Internet protocol) packets

Internet Layer

As notedKey layer

Defines the official Packet formatProtocol

IP - Internet ProtocolObjective

OSI TCP/IPPhysical Host to NetworkData LinkNetwork InternetTransport TransportSession Not PresentPresentation Not PresentApplication Application

Get message comprised of packetsFrom point A to point B

No requirement placed on Packet ordering during transmissionRoute a packet may take

All packets may not take the same route

Transport Layer

Equivalent to the OSI transport layerSimilar responsibilities

Two communication protocols are defined for the layerTCPUDP

TCPTCP - Transmission Control ProtocolVery reliableConnection oriented protocolEnsures data stream

Originating on one machineDelivered to any other machine on the internet

Process Disassembles the byte stream into packetsPasses them to the internet level

Handles flow control

UDPUDP - User Datagram ProtocolUnreliable

In the sense that messages are not inherently acknowledgedConnectionless protocolAlternative to TCP

Designed for hosts who want to implement their ownPacket sequencingFlow control

Finds application in Request - response type applications

Client serverTrade speed for accuracy

Application Layer

Supports / Contains all the high level protocolsThree basic ones in support of the original intent of the development

Virtual Terminals - TELNETRecall the discussions of OSI applications layer

File Transfer - FTPFile transfer protocol

Electronic Mail - SMTPSimple mail transfer protocol

Later additionsMapping of host name to network address - DNS

Domain Name ServiceMoving news articles - NNTP

Network news transfer protocolInterface to the WWW - HTTP

Hypertext transfer protocol

Data Communication ServicesWe’ve looked at

HardwareSoftwareSeveral reference models

Recall network serviceSet of primitives or operations

A layer offers to its usersRepresents a set of tools to get a job done

Let’s briefly examine several communication servicesIntroducing the acronyms - Gotta do that

DQDB - Distributed Queue Dual BusSMDS - Switched Multimegabit Data ServiceX.25 - X.25Frame Relay - Frame RelayB-ISDN - Broadband Integrated Services Digital NetworkATM - Asynchronous Transfer Mode

DQDB - Distributed Queue Dual BusMentioned earlierComputers are interconnected via two unidirectional buses

Message traffic to the Right proceeds on the upper busLeft proceeds on the lower bus

SMDS - Switched Multimegabit Data ServiceDesigned by Bellcore as an internet service

Computer NComputer 3Computer 2Computer 1

Interconnect multiple local area networks - LANs

Broadband network - High Speed Data ServiceBroadband as many interpretations

Telephony Anything wider than 4KHz

Generally assumed to support higher frequency transmissionUp to 10’s to 100’s of megahertz

SMDS - 45 Mbps

Switched ServiceLines not dedicated but switched as needed

For above systemTo support full interconnectivity among all LANs

Require 6 dedicated linesUnless high utilization

The cost of dedicated system does not make senseAlternative

Connection on demandWhen needed - make connection

LAN via dedicated line to switchAbove with SMDS requires 4 lines

Eliminate 2 linesSuch a system is designed to handle bursts of data

Connectionless service with packet transmissionPacket order not guaranteedPacket format

Upto 10K bytes16 address bytes

8 byte destination address8 byte source address

LAN 3LAN 1

LAN 4LAN 2

Dedicated Lines

LAN 3

Switch

LAN 1

LAN 4LAN 2

Format4 bit code15 decimal digit telephone number

Country codeCity / area codeLocal numberEach digit is encoded in a 4 bit field

9188 data bytesMaximumNo restrictions on the content of the data

Bytes may contain any arbitrary protocol

Dynamic Message LengthEach access point contains a counter

Continuously incremented at constant rateWhen a packet arrives

Length in bytes compared to counter valueIf less

TransmitSubtract length from the count

ElseDiscard

ExampleLet counter increment at 10 ms rateEvery second will accumulate 100,000 countsAverage

Gives an average throughput of 100,000 bytes/secBurst

Assume a short message - 100 to 1000 bytesCan assure full speed (45 mbs) if transmit every 1ms to 10ms

X.25International standard developed in Europe early 70s by CCITT

Comite Consultatif International Telegraphique et TelephoniqueDesigned to provide an interface between

Public packet switched networksUsers

Digital transmission rather than analogMost telecommunication signaling is analog basedHas slowed acceptance

Connection oriented service with packet transmissionPacket order guaranteedTransmission commences after establishment of a (virtual) circuit

Packets up to 128 bytes

Circuit ConnectionSwitched virtual

Source - Make a request to communicateConnection establishedCommunication proceedsLink relinquished

Permanent virtualConnection established in advanceNot relinquished when transaction completeNot a physical nor dedicated connectionDesigned for burst data transmission

Frame RelayLow end service

Provides a means to identify Start and End of a frame

Error detectionDiscard on errorError recover is user’s responsibility

No flow control

Connection oriented service with packet transmissionPacket order guaranteedReception not acknowledged

Permanent virtualConnection established in advanceUsually between 2 pointsCan have a one to many configuration

Each virtual circuit identified by a 10 bit numberIncluded in each transmitted frame

Packets up to 1600 bytes

ATM - Asynchronous Transfer ModeAs the name suggests

ATM is the opposite of STMSynchronous Transfer Mode

Error ManagementNo packet acknowledgmentNo retransmission on errorCan be implemented on higher layer

STM

Most commonly used for reliable long distance Voice and data transmission

Works as followsBandwidth of the link divided into time slots or bucketsBuckets are collected together in a group like a linked listThus

On the channelThe group is repeated every T secThere can be up to m different groups

Such a configuration gives a total of n*m time slotsWhen a connection is established

It is assigned one of the time slotsFor the duration of the transmission

Data for that connection placed in that time slotTime slot occupied even if no data transmitted

Maximum number of connection thereforen*m

Values of T, m, and n Set up by international standards committees

The telecommunications companies are investigating Fiber optic cross country and cross oceanic links

With Gigabit/sec speedsWould like to carry

Real time traffic Voice and hi-resolution video

Which can tolerate some loss but not delayNon real time traffic

Computer data and file transfer Which may tolerate some delay but not loss

Problem Peak bandwidth requirements

May be quite high As in high-resolution full motion video

The duration for which the dataMay be quite small

That isThe data comes in bursts Must be transmitted at the peak rate of the burst

Tn-1T2T1T0

Average arrival time between bursts May be quite large and randomly distributed.

For bursty connectionsWaste of bandwidth

To reserve them a bucket at their peak bandwidth rate

When on the average Only 1 in 10 buckets may actually carry the data.

STM becomes inefficient with increasingPeak bandwidth of the link, Peak transfer rate of the trafficOverall burstiness of the traffic

Expressed as a ratio of peak/average

ATMInstead of always identifying a connection by bucket number

Carry the connection identifier along with the data Keep the size of the bucket small

If any bucket got dropped Not too much data lost,Some cases could easily be recovered

Similar idea to packet switching

Fixed packet sizeArose out of motivation

To sustain the same voice quality as in STM networksBut in the presence of some lost packets

Two end points are associated with each other via Virtual Circuit Identifier - VCI label

Instead of by a time-slot or bucket numberVCI is carried in the header portion of a packet

The packet is carried in the same type of bucket as STM

The terms fast packet, cell, and bucket Used interchangeably refer to the same thing.

Fast packet switching attempts to solve Unused bucket problem of STM

TechniqueStatistically multiplex several connections on the same link

Based on their traffic characteristics

If a large number of connections are very bursty Assign all to the same linkTheory

Statistically they will not all burst at the same timeIf some of them do burst simultaneously

There will sufficient elasticity that the burst Can be buffered up Put in subsequently available free buckets

Called statistical multiplexing

Connection oriented service with packet transmissionPacket order guaranteedReception not acknowledgedPacket format

Up to 53 bytes5 header bytes

3 byte VCI label1 Control field1 byte Header checksum4 byte Adaptation layer

Optional44 or 48 data bytes - based upon adaptation layer bytes

MaximumNo restrictions on the content of the data

Bytes may contain any arbitrary protocol

ISDN - Integrated Services Digital NetworkDesigned with the goal of replacing most of the existing

Analog networksProvide a means to transmit higher demand signals

High resolution / full motion videoComputer data and large files

Key technology is the ATM transmissionThe ISDN reference model is different from those examined thus far

ISDN ModelThree layers - three dimensionsBroken down very well in an object centered wayTasks and responsibilities are collected into related

LayersPlanes

LayersPhysical Layer

Physical medium including voltages and timingPlaces no restrictions on the actual hardware

Copper OKAimed towards fiber implementation

ATM LayerConcerned with

Packets and packet transportManagement of virtual connectionsFlow and congestion control

ATM Application LayerAccommodates data groupings larger than the specified packet sizeSource

Breaks data into appropriately sized packetsDestination

Reassemble back into original

Physical and Application layersDivided into two pieces

Interface to higher layerCollection of routines to do the work of the layer

User and Control PlanesThe tasks of managing the data transport and connection process

Separated into to groups that sit above or to the sideThe reference hierarchy

Denoted the User plane and the control planes

User PlaneData transportFlow controlError management

Control PlaneManage the connection / disconnection processes

SummaryHave looked at a general overview of networksExamined

HardwareSoftware

Reference ModelsNetwork ServicesStabdardsLearned some of the jargon and acronyms

Will now examine all in much greater detail

The reference model we will useComposite / distillation / abstraction of those currently in use

Physical LayerData Link LayerMedium Access SublayerNetwork LayerTransport Layer

Application Layer

The model Uses the layer paradigmIncludes all the functionality

Is not a model in actual use.