Data and Computer Communications

Introduction

Computer NetworkAn interconnected collection of autonomous computers.Two computers are said to be interconnected if they are able to exchange information.A system with one control unit and many slaves is not a network.

Computer Network (Cont.)Distributed Systems Computer

Network

The existence of multiple autonomous computers is transparent to the user.

User must explicitly do everything.

Allocation of jobs to processor and files to disks and all other system functions must be automatic.

Distributed system is a software system built on top of a network.

Overlap between distributed systems and Computer Network Example:More files around System can involve the User movement.

Computer Network (Cont.)Uses of Computer Network

Companies People Social Issues

Resource Sharing Access to remote information

News-groups

Geography Person To Person communication & e-mail

Bulletin Boards

High reliability: replication

Interactive Entertainment

Saving money on the flow

Client-server model

Scalability: Ability to increase system performance gradually as the workload grows.

A Communications ModelSource

Generates data to be transmittedTransmitter

Converts data into transmittable signalsTransmission system

Carries dataReceiver

Converts received signal into dataDestination

Takes incoming data

Simplified Communications Model - Diagram

Key Communications TasksTransmission system utilizationInterfacingSignal generationSynchronizationExchange managementError detection and correctionAddressing and routingRecoveryMessage formattingSecurityNetwork management

Network Hardware Transmission Technology

Broadcast Network Point – To – Point Network

Single communication channel that is shared by all the machines on the network.

Many connections between individual pairs of machines

All the others receive “Packets” in certain contexts, sent by any machine.

A packet may have to visit one or more intermediate machine.

An address field within the packet specifies for whom it is intended.

Routing algorithms play an important role in PTP networks.

Multicasting: transmission to a subnet of the machines.

Simplified Data Communications Model

NetworkingPoint to point communication not usually practical

Devices are too far apartLarge set of devices would need impractical number of connections

Solution is a communications network

Simplified Network Model

Local Area NetworksSmaller scope

Building or small campusUsually owned by same organization as attached devicesData rates much higherUsually broadcast systemsNow some switched systems and ATM are being introduced

Local Area Networks (Cont.)NETWORKS

LAN MAN WAN INTERNET

LAN CHARACTERISTICS

Size Transmission Technology Topology

Restricted in Size

Single Cable

10 to 100 Mbps

Low delay (ms)

Very few Errors

Megabits/Sec. (Unit)

BUS (Ethernet) Ring (Token ring)

MANMetropolitan Area Network Support data and voiceNo switching elements Standard: DQDB (Distributed Queue Dual Bus) Two unidirectional buses to which all the computers are connected. Each bus has a head-end, a device that initiates transmission activity. Traffic that is destined for a computer to the right of the sender uses the upper bus, traffics to the left uses the lower one.

Wide Area NetworksLarge geographical areaCrossing public rights of wayRely in part on common carrier circuitsAlternative technologies

Circuit switchingPacket switchingFrame relayAsynchronous transfer mode (ATM)

Wide Area Networks (Cont.) Host (end system). Subnet (communication subnet). WANs typically have irregular topologies.

WAN CONSISTS OF

Transmission Lines:- Circuits, Channels or Tanks

Switching Elements:- Specialized computers used to connect two or more transmission lines.

Internet Collection of interconnected networks.Example: A collection of LAN’s connected by a WAN.WAN : (router + hosts).SUBNET : (only routers).

Circuit SwitchingDedicated communications path established for the duration of the conversationE.G. Telephone network

Packet SwitchingData sent out of sequenceSmall chunks (packets) of data at a timePackets passed from node to node between source and destinationUsed for terminal to computer and computer to computer communications

Frame RelayPacket switching systems have large overheads to compensate for errorsModern systems are more reliableErrors can be caught in end systemMost overhead for error control is stripped out

Asynchronous Transfer ModeATM (cell relay)Evolution of frame relayLittle overhead for error controlFixed packet (called cell) lengthAnything from 10mbps to GbpsConstant data rate using packet switching techniqueOffers a constant data rate channel

Integrated Services Digital NetworkISDNDesigned to replace public telecom systemWide variety of servicesEntirely digital domainFirst generation ( narrowband ISDN )

64 kbps channel is the basic unitCircuit-switching orientationContributed to frame relay

Second generation ( broadband ISDN )100s of mbpsPacket-switching orientationContributed to ATM ( cell relay )

ProtocolsUsed for communications between entities in a systemMust speak the same languageEntities

User applicationsE-mail facilitiesTerminals

SystemsComputerTerminalRemote sensor

Protocol Hierarchies Organized as a series of layers or levels.The purpose of each layer is to offer certain services to the higher layers.Layer n on one-machine carries on a conversation with layer n on another machine.Protocol: is an agreement between the communicating parties on how communication is to proceed.Peers communicate using the protocol.In reality, no data directly transferred from layer n on one machine to layer n on another machine.

Protocol Hierarchies (Cont.)Each layer passes data and control information to the layer immediately below it.Between each pair of adjacent layers there is an “interface”.The design of layers helps in:

Minimizing the amount of information that must be passed between layersMake it simpler to reduce the implementation of one layer with a completely different one

Protocol stack: A list of protocol used by a certain system, one

protocol per layer.

Key Elements of a ProtocolSyntax

Data formatsSignal levels

SemanticsControl informationError handling

TimingSpeed matchingSequencing

Design Issues for the LayersAddressing.Data transfer.

Simplex communication.Half-duplex communication.Full-duplex communication.

Number and priorities of the logical connection channels. Many networks provide at least two logical channels per connection, one for normal data and one for urgent data.Error control.

Error detecting code.Error correcting code.

Design Issues (Cont.)How to receive data in order (sequence no.).How to keep a fast sender from swamping a slow receiver with data (flow control).Size of the message: disassembling >transmitting >reassembling messages.Routing: multiple paths between source and destination.

Protocol ArchitectureTask of communication broken up into modulesFor example file transfer could use three modules

File transfer applicationCommunication service moduleNetwork access module

Simplified File Transfer Architecture

A Three Layer ModelNetwork access layerTransport layerApplication layer

Network Access LayerExchange of data between the computer and the networkSending computer provides address of destinationMay invoke levels of serviceDependent on type of network used (LAN, packet switched etc.)

Transport LayerReliable data exchangeIndependent of network being usedIndependent of application

Application LayerSupport for different user applicationse.g. e-mail, file transfer

Interfaces and ServicesActive elements in each layer are called ENTITIES.Entity.

Software [example: process.].Hardware [example: intelligent I/O chip.].

The entities in layer n implement a service used by layer n+1.Layer n called service provider.Layer n + 1 called service user.Services are available at sap’s (service access points).Each SAP has an address that uniquely identifies it.

Interfaces and Services (Cont.) IDU: interface data unit.ICI: interface control info.SDU: service data unit.

At a typical interface, the layer n + 1 entity passes an IDU to the layer n entity through the SAP.In order to transfer the SDU, the layer n entity may have to fragment it into several pieces, each of which is given a header and send to as a separate PDU (protocol data unit) such as a packet.

Addressing Requirements

Two levels of addressing requiredEach computer needs unique network addressEach application on a (multi-tasking) computer needs a unique address within the computer

The service access point or SAP

Protocol Architectures and Networks

Protocols in Simplified Architecture

Protocol Data Units (PDU)At each layer, protocols are used to communicateControl information is added to user data at each layerTransport layer may fragment user dataEach fragment has a transport header added

Destination SAPSequence numberError detection code

This gives a transport protocol data unit

Network PDUAdds network header

Network address for destination computerFacilities requests

SERVICESConnection Oriented Connectionless

Modeled after the telephone system Modeled after posted system

Establish a connectionUse the ConnectionRelease the connection

Acts like a tube: receive data by the same order was sent

Messages could be received in different order than it was sent with

Reliable connection oriented service Unreliable connectionless service (not acknowledged) 

Request reply serviceSender transmits a single datagram containing a request, the reply contains the answer.Used to implement communication in the client-server model.

Operation of a Protocol Architecture

Service PrimitivesA service is formally specified by a set of primitives (operations) available to a user or other entity to access the service.Primitive tells the service to

Perform some action ORReport an action by a peer entity.

Example: Connection oriented service with 8 service primitives.

CONNECT.request – Request a connection to be established.CONNECT.indication – Signal the called party.

Example (Cont.)CONNECT.response – Used by the caller to accept/reject calls.CONNECT.confirm – Tell the caller whether the call was accepted.DATA.request – Request the data be sent.DATA.indication – Signal the arrival of data.DISCONNECT.request – Request that a connection be released.DISCONNECT.indication – Signal the peer about the request.Service Could be.

• Confirmed (Example: CONNECT).• Unconfirmed (Example: DISCONNECT).

Relationship of Services to Protocols

Service: is a set of primitives (operations) that a layer provides to the layer above it.Protocol.

A set of rules governing the format and meaning of the frames, packets, or messages that are exchanged by the peer entities within a layer.Entities use protocols in order to implement their service definitions.Entities are free to change their protocols, provided they do not change the service visible to their users.

REFERENCE MODELSOSI References Model TCP/IP Reference Model

TCP/IP Protocol ArchitectureDeveloped by the US defense advanced research project agency (DARPA) for its packet switched network (ARPANET).Used by the global internet.No official model but a working one.

Application layer.Host to host or transport layer.Internet layer.Network access layer.Physical layer.

Physical LayerPhysical interface between data transmission device (e.G. Computer) and transmission medium or networkCharacteristics of transmission mediumSignal levelsData ratesEtc.

Network Access LayerExchange of data between end system and networkDestination address provisionInvoking services like priority

Internet Layer (IP)Systems may be attached to different networksRouting functions across multiple networksImplemented in end systems and routers

Transport Layer (TCP)Reliable delivery of dataOrdering of delivery

Application Layer

Support for user applicationse.g. http, SMPT

TCP/IP Protocol Architecture Model

OSI ModelOpen systems interconnectionDeveloped by the international organization for standardization (ISO)Seven layersA theoretical system delivered too late!TCP/IP is the de facto standard

OSI References Model

International Standards Organization.OSI (Open Systems Interconnection).Reference model: deals with connecting open systems that are; Open for communication with other systems.

PrinciplesA layer should be created where a different level of abstraction is needed.Each layer should perform a well-defined function.The function of each layer should be chosen with an eye toward defining internationally standardized protocols.The layer boundaries should be chosen to minimize the information flow across the interfaces.The number of layers should be large enough that distinct functions need not be thrown together on the same layer out of necessity.

OSI LayersApplicationPresentationSessionTransportNetworkData linkPhysical

The Physical Layer Deals with transmitting raw bits over a communication channel.How many volts for 1 or 0.How many microseconds a bit lasts.Mechanics, electrical and procedural interfaces.

Data link Layer Break the input data up into data frames.Process the acknowledgement frames sent back by the receiver.Insert the frame delimiter.Solve the problems caused by damaged, lost and duplicate frames.Flow control.Full duplex transmission (piggybacking)Medium access sub layer deals with how to control access to the shared channel in broadcast networks.

Network Layer Routing packets from source to destination.Routes can be static or dynamicBottleneck, congestionConnect heterogeneous networks (different addressing method, larger packet service).In broadcast networks, routing problem is simple, so the network layer is thin.

Transport Layer Accept data from the session layer, split it up into smaller units if needed, pass these to the network layer and ensure that the all pieces arrive correctly at the other endUnder normal conditions, the transport layer creates a distinct network connection for each transport connection required by the session layerIf the transport connection requires a high throughput, the transport layer might create multiple network connections, dividing the data among the network connections to improve throughput

Transport Layer (Cont.)Transport layer determines what type of service to provide the session layer with and ultimately, the users of the entire networkThe transport layer is a true end-to-end layer, from source to destinationMultiple connections will be entering and leaving each host. There is a need to tell which message belongs to which connection (transport header)Establishing and deleting connections across the networkFlow control between hosts (as oppose between routers) so fast host cannot overrun a slow one

Session Layer Allows users on different machines to establish sessions between themA session might be used to allow a user to log into a remote timesharing system or to transfer a file between two machinesExample: token management. Only the side holding the token may perform the critical operation.Synchronization: insert a checkpoint.

Example: sending file for 20 hours. After a crash the portion after the checkpoint will be resend again.

Presentation Layer

Concerned with the syntax and semantics of the information transmitted.A typical example of a presentation service is encoding data in a standard agreed upon way. [Character strings, integers, floating-point numbers…].

Application Layer The application layer contains a variety of protocols that are commonly needed.Example: incompatible terminal type.One way to solve this problem is to define an abstract network virtual terminal that editor can be written to deal with. To handle each terminal type, a piece of s/w must be written to map the functions of the network virtual terminal onto the real terminal.Other application is file transfer(ftp).

TCP/IP and OSI Protocol Architectures

Example Of Networks Novell NETWARE.

Client-server model.IPX/SPX.Network layer runs IPX (internet packet exchange).IPX uses 10 byte address (IP uses 4 bytes) flat addressing.Transport protocol.

• NCP (network core protocol).• Transport service & other services.• SPX (sequenced packet exchange):• Just transport service.

Example Of Networks (Cont.)The application can choose between NCP & SPX

Transport control field counts how many networks the packet has traversed.About once a minute, each server broadcasts a packet giving its address and telling what services it offers.SAP (Service Advertising Protocol) is used for broadcastingRouters run some kind of special agent processes to construct databases of which servers are running.When a client is booted, it sends a request for a server. The agent on the local router machine sees this request, and matches up the request with the best server.

Example Of Networks (Cont.)The APRANET.

Packet switched network, consisting of subnet and host computers.IMPS (interface message processors) connected by transmission lines.Each IMP would be connected to at least two other imps.Each node consists of IMP and a host.Host sends messages of up to 8063 bits to its IMP.IMP breaks the message into packets of at most 1008 bits and forwards them independently toward the destination.56-kbps lines leased from telephone companies interconnect the IMPS.By 1990, the ARPANET had been overtaken by newer networks.

Example Of Networks (Cont.)NSFNET

By 1984 NSF Fig 1.26(the U.S. national science Foundation) began designing a high-speed successor to the ARPANET that would be open to all university research groups.By 1995 the NSFNET backbone was no longer needed to interconnect the NSF regional networks because numerous companies were running commercial IP Networks.

Example Of Networks (Cont.)The Internet.

In 1992, the internet society was set up, to promote the use of the internet.Four main applications.

Email.News.Remote login: telnet, rlogin.File transfer: FTP.

Example Of Networks (Cont.)Gigabit TESTBEDS.

The backbones operate at megabit speeds.Gigabit networks provide better bandwidth but not always much better delay.Example: sending a 1-kbit packet from NYC to san Francisco at (1 mbps) take.1 msec to pump the bits out and 20 msec for the delay, for a total of 21 msec. A 1-Gbps network can reduce this to 20.001 msec.For some applications, bandwidth is what counts, and these are the applications for which gigabit networks will make a big difference.Examples:- telemedicine & virtual meeting.

Example Data Communication Services

SMDSX.25FRAME RELAYBROADBAND ISDN AND ATM