Data and Computer Communications

Introduction

Computer NetworkA collection of computers that communicate with one another over transmission lines.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.)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 transmitted

TransmitterConverts data into transmittable signals

Transmission systemCarries data

ReceiverConverts received signal into data

DestinationTakes incoming data

Simplified Communications Model - Diagram

Key Communications Tasks

Transmission 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

1. 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.)

NETWORKSLAN 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)  

2. 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.

3. Wide Area Networks

Large 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.

Subnet (ISP) the collection of routers and communication lines that moved packets from the source host to the destination host.

Criteria for Comparing WANs

Criteria Category

Criteria Description

Cost Initial Setup Transmission line; equipment setup feeds, including labor and training costs

Operational Fees for leases of lines and equipment; ISP and other service fees; ongoing training

Maintenance

Periodic maintenance costs; problem diagnosis and repair costs; mandatory upgrade costs

Performance Speed Line and equipment speed

Latency Delays during busy periods

Availability Frequency of service outage

Criteria Category

Criteria Description

Performance Loss rate Frequency retransmission required

Transparency

User involvement in operation

Performance guarantees

Vendors agree to cost penalties if levels of service not met

Other Growth potential

How difficult to upgrade when service needs or capacity increase?

Commitment periods

Length of leases and other agreements

Management time

How much management activity is required?

Risk, Financial

How much is at stake if system not effective?

Technical If using new technology, what is the likelihood of failure?

4. Internet

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

Circuit Switching

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

Packet Switching

Data 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 Relay

Packet 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 Mode

ATM (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 Network

ISDNDesigned 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 Layer

Exchange 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 Layer

Reliable 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 PDU

Adds network headerNetwork address for destination computerFacilities requests

SERVICES

Connection 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 service

Sender 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 MODELS

OSI References Model TCP/IP Reference Model

TCP/IP Protocol Architecture

Developed 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 Layer

Exchange 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 Model

Open 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

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OSI Model vs. TCP/IP Model OSI TCP/IP

Developer ISO DARPA

Planning Model first (Proactive) Protocol first (Reactive)

Layers 7 4

Strength Widely used to classify protocol stacks

Interoperable, widely used protocol implementation

Weakness No widely accepted protocol implementation

Model fits no other protocol stack

Critique of the OSI Model and Protocols

1. Bad timing : The competing TCP/IP protocols were already in widespread use by research universities by the time the OSI protocols appeared

2. Bad Technology: The model is extraordinarily complex;

Addressing, flow control, and error control, reappear again and again in each layer are

unnecessary and inefficient.

3. Bad ImplementationsThe initial implementations were huge, unwieldy, and slow because the model and the protocols are complex;Bad image;

4. Bad Politics thought to be the creature of the European telecommunication ministries, the European Community, and later the U.S. Government

Critique of the TCP/IP Model and Protocols

First, does not clearly distinguish the concepts of service, interface, and protocol;Second, is not at all general and is poorly suited to describing any protocol stack other than TCP/IP

Critique of the TCP/IP Model and Protocols cont…

Third, is not really a layer but an interfaceFourth, does not distinguish (or even mention) the physical and data link layers

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.)

NSFNETBy 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

Write a paper, regarding:

Topology of NetworksTransmission Media (wired and wireless)How does encryption work? What is the purpose of Firewall?What is a VPN, Why is it so important?

USING YOUR KNOWLEDGESuppose you manage a group of seven employees in a small business. Each of your employees wants to be connected to the internet. Consider two alternatives:

Alt A: Each employee has his own modem and connects individually to the internetAlt B: The employees’ computers are connected using a LAN and the network uses a single modem to connect

a) Sketch the equipment and lines required for each alternative.

b) Explain in the actions you need to take to create each alternative

c) Compare the alternatives using the criteria for comparing WANs

d) Which of these two alternatives do you recommend?

Consider the situation of a company that has two offices at physically separated sites. Suppose each office has a group of 15 computers.

a. If the two offices are retail art galleries, what is likely to be the most common type of interoffice communication? Given your answer, what type of WAN do you think is most appropriate?

b. Suppose the two offices are manufacturing sites that communicate via email and that regularly exchange large drawings and plans. What are the advantages and disadvantages of each of the four WAN types for these offices? Under what circumstances would you recommend a leased-line WAN?

c. Suppose the two offices are the same as described in part b, but that in addition each has salespeople on the road who need to connect to the office computers. How would your answer to part b change?

d. Would you change your answer to part c if both offices are located in the same building? Why or why not?

e. What additional factors would you need to consider if one of the offices in part c was in Los Angeles and the other was located in Singapore?