Introduction To Computer Networks

Introduction to Computer Networks

INTRODUCTION TO COMPUTER NETWORKS

Muhammad Adil Raja

Roaming Researchers, Inc.

August 25, 2014


OUTLINE

1 USES OF COMPUTER NETWORKS

2 NETWORK HARDWARE

3 NETWORK SOFTWARE

4 REFERENCE MODELS

5 CONCLUSIONS


Uses of Computer Networks

USES OF COMPUTER NETWORKS I

Business Applications.Resource Sharing.VPNs (Virtual Private Networks).Client-Server Model,Web Application.Communication Medium Between Employees.IP Telephony.Voice over IP (VoIP).Desktop Sharing.E-Commerce (Electronic Commerce).

Home Applications.Internet access provides home users with connectivity toremote computers.Personalized information.Peer-to-peer Communication.



USES OF COMPUTER NETWORKS II

Instant Messaging.Twitter.Social Networks.FacebookCollaborative Websites – Wikis, Wikipedia.IPTV (IP Television).Ubiquitous Computing.Power-line Networks.RFID (Radio Frequency IDentification).

Mobile Users.Connectivity.Wireless Hotspots.Fixed Wireless and Mobile Wireless.Text Messaging.Texting.Short Message Service.



USES OF COMPUTER NETWORKS III

Smart Phones.GPS (Global Positioning System).m-commerce (mobile-commerce).NFC (Near Field Communication).Sensor Networks.Wearable computers.

Social Issues.Profiling users.Cookies.Gmail is read by Google.Spam.Content intended for criminal behavior.Phishing.USe of CAPTCHAs.


Network Hardware

NETWORK HARDWARE I

PANs (Personal Area Networks).Bluetooth.

LANs (Local Area Networks).When LANs are used by companies, they are calledenterprise networks.Access Point (Wireless Routers).Base Station.Wireless LANs – IEEE 802.11/ WiFi.Wired LANs – IEEE 802.3/ Ethernet.Switched Ethernet.VLANs.Classic Ethernet.Power-line Networks.Metropolitan Area Network (MAN) – covers a city.IEEE 802.16 – WiMAX.


Network Hardware

NETWORK HARDWARE II

Wide Area Networks (Wide Area Networks).A WAN covers a country or a continent.Subnet – carries messages from host to host.Transmission Lines – copper, optical, radio.Switching Elements – connect two or more transmissionlines.Routers.Internetworks.VPN (Virtual Private Networks.Routing Algorithms.Forwarding algorithms.

Internetwork.Internetwork or internet – a collection of interconnectednetworks.Gateway – a machine that connects two or more networks.


Network Software

NETWORK SOFTWARE I

Protocol HierarchiesLayered architectures.Protocol – A set of rules for communication betweenmachines.Peers in hierarchies.Network architecture – A set of layers and protocols.Protocl Stack – A list of protocols used by a certain system,one protocol per layer.

Design Issues for the Layers.Error correction.Routing.Protocol layering.Internetworking.Scalability.Statistical multiplexing.


Network Software

NETWORK SOFTWARE II

Flow Control.Congestion.Real-time Services.Quality of Service.Confidentiality Authentication.Integrity.

Connection-Oriented Versus Connectionless Service.


Reference Models

REFERENCE MODELS I

The OSI Reference Model.

OSI (Open Systems Interconnection).

OSI model.

The TCP/IP Reference Model.


Reference Models

REFERENCE MODELS IISEC. 1.4 REFERENCE MODELS 41

Layer k

Layer k + 1

Layer k - 1

Protocol

Service provided by layer k

Layer k

Layer k + 1

Layer k - 1

Figure 1-19. The relationship between a service and a protocol.

1.4 REFERENCE MODELS

Now that we have discussed layered networks in the abstract, it is time to lookat some examples. We will discuss two important network architectures: the OSIreference model and the TCP/IP reference model. Although the protocols associ-ated with the OSI model are not used any more, the model itself is actually quitegeneral and still valid, and the features discussed at each layer are still very im-portant. The TCP/IP model has the opposite properties: the model itself is not ofmuch use but the protocols are widely used. For this reason we will look at bothof them in detail. Also, sometimes you can learn more from failures than fromsuccesses.

1.4.1 The OSI Reference Model

The OSI model (minus the physical medium) is shown in Fig. 1-20. Thismodel is based on a proposal developed by the International Standards Organiza-tion (ISO) as a first step toward international standardization of the protocols usedin the various layers (Day and Zimmermann, 1983). It was revised in 1995 (Day,1995). The model is called the ISO OSI (Open Systems Interconnection) Ref-erence Model because it deals with connecting open systems—that is, systemsthat are open for communication with other systems. We will just call it the OSImodel for short.

The OSI model has seven layers. The principles that were applied to arrive atthe seven layers can be briefly summarized as follows:

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

2. Each layer should perform a well-defined function.

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

FIGURE: The relationship between a service and a protocol.


Reference Models

THE OSI REFERENCE MODEL I

A Seven Layered Architecture.1 Application.2 Presentation – Concerned With the syntax and semantics

of the information transmitted.3 Session – Dialog control, token management,

synchronization.4 Transport.5 Network.6 Datalink Layer.7 Physical Layer.

The Application Layer.HTTP (HyperText Transfer Protocol).FTP (File Transfer Protocol).


Reference Models

THE OSI REFERENCE MODEL II

Transport Layer.Accept data from the layer above it.Split it up into smaller units if need be.Pass these to the network layer.Ensure that the pieces all arrive correctly at the other end.

The Network Layer.Controls the operation of the network.Determine how packets are routed from the source to thedestination.

The Data Link LayerThe main task of the data lik layer is to transform a rawtransmission facility into a line that appears free ofundetected transmission errors.It masks the real errors so that the network layer cannotsee them.


Reference Models

THE OSI REFERENCE MODEL III

Accomplishes this task by having the sender break theinput data into data frames.If the service is reliable, the receiver confirms correctreceipt of each frame by sending back anacknowledgement frame.How to control access to the shared channel?The medium access control sublayer.

The Physical LayerConcerned with transmitting raw bits over a communicationchannel.The design issues: To receive a 1 as a 1 and a 0 as a 0.Lifetime of bits (nanoseconds).Can simultaneous transmission happen in both directions?How to establish the initial connection.How to tear it down when both sides are finished?How many pins the network connector has?


Reference Models

THE OSI REFERENCE MODEL IV

What is each pin used for?These design issues largely deal with mechanical, electricaland timing interfaces.It also deals with the physical transmission medium, whichlies below the physical layer.


Reference Models

THE OSI REFERENCE MODEL V42 INTRODUCTION CHAP. 1

Layer

Presentation

Application

Session

Transport

Network

Data link

Physical

7

6

5

4

3

2

1

Interface

Host A

Name of unitexchanged

APDU

PPDU

SPDU

TPDU

Packet

Frame

Bit

Presentation

Application

Session

Transport

Network

Data link

Physical

Host B

Network Network

Data link Data link

Physical Physical

Router Router

Internal subnet protocol

Application protocol

Presentation protocol

Transport protocol

Session protocol

Communication subnet boundary

Network layer host-router protocolData link layer host-router protocolPhysical layer host-router protocol

Figure 1-20. The OSI reference model.

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

5. The number of layers should be large enough that distinct functionsneed not be thrown together in the same layer out of necessity andsmall enough that the architecture does not become unwieldy.

Below we will discuss each layer of the model in turn, starting at the bottomlayer. Note that the OSI model itself is not a network architecture because it doesnot specify the exact services and protocols to be used in each layer. It just tellswhat each layer should do. However, ISO has also produced standards for all thelayers, although these are not part of the reference model itself. Each one hasbeen published as a separate international standard. The model (in part) is widelyused although the associated protocols have been long forgotten.

FIGURE: The OSI reference model.


Reference Models

THE TCP/IP REFERENCE MODEL I

1 The Application Layer (HTTP/FTP/TELNET etc.)2 The Transport Layer (TCP/UDP).3 The Internet Layer (IP).4 The Link Layer.


Reference Models

THE TCP/IP REFERENCE MODEL II

46 INTRODUCTION CHAP. 1

words, the DoD wanted connections to remain intact as long as the source anddestination machines were functioning, even if some of the machines or transmis-sion lines in between were suddenly put out of operation. Furthermore, since ap-plications with divergent requirements were envisioned, ranging from transferringfiles to real-time speech transmission, a flexible architecture was needed.

The Link Layer

All these requirements led to the choice of a packet-switching network basedon a connectionless layer that runs across different networks. The lowest layer inthe model, the link layer describes what links such as serial lines and classic Eth-ernet must do to meet the needs of this connectionless internet layer. It is notreally a layer at all, in the normal sense of the term, but rather an interface be-tween hosts and transmission links. Early material on the TCP/IP model has littleto say about it.

The Internet Layer

The internet layer is the linchpin that holds the whole architecture together.It is shown in Fig. 1-21 as corresponding roughly to the OSI network layer. Itsjob is to permit hosts to inject packets into any network and have them travel in-dependently to the destination (potentially on a different network). They mayeven arrive in a completely different order than they were sent, in which case it isthe job of higher layers to rearrange them, if in-order delivery is desired. Notethat ‘‘internet’’ is used here in a generic sense, even though this layer is present inthe Internet.

TCP/IPOSI

Application

Presentation

Session

Transport

Network

Data link

Physical

7

6

5

4

3

2

1

Application

Transport

Internet

Link

Not presentin the model

Figure 1-21. The TCP/IP reference model.

The analogy here is with the (snail) mail system. A person can drop a se-quence of international letters into a mailbox in one country, and with a little luck,

FIGURE: The TCP/IP reference model.


Reference Models

THE TCP/IP REFERENCE MODEL III48 INTRODUCTION CHAP. 1

Link Ethernet802.11SONETDSL

IP ICMP

HTTP RTPSMTP DNS

TCP UDP

Internet

Transport

Layers Protocols

Application

Figure 1-22. The TCP/IP model with some protocols we will study.

include the Domain Name System (DNS), for mapping host names onto their net-work addresses, HTTP, the protocol for fetching pages on the World Wide Web,and RTP, the protocol for delivering real-time media such as voice or movies.

1.4.3 The Model Used in This Book

As mentioned earlier, the strength of the OSI reference model is the model it-self (minus the presentation and session layers), which has proven to be ex-ceptionally useful for discussing computer networks. In contrast, the strength ofthe TCP/IP reference model is the protocols, which have been widely used formany years. Since computer scientists like to have their cake and eat it, too, wewill use the hybrid model of Fig. 1-23 as the framework for this book.

5 Application

4 Transport

3 Network

2 Link

1 Physical

Figure 1-23. The reference model used in this book.

This model has five layers, running from the physical layer up through thelink, network and transport layers to the application layer. The physical layerspecifies how to transmit bits across different kinds of media as electrical (orother analog) signals. The link layer is concerned with how to send finite-lengthmessages between directly connected computers with specified levels of reliabil-ity. Ethernet and 802.11 are examples of link layer protocols.

FIGURE: The TCP/IP model with some protocols we will study.


Reference Models

THE TCP/IP REFERENCE MODEL IV

48 INTRODUCTION CHAP. 1

Link Ethernet802.11SONETDSL

IP ICMP

HTTP RTPSMTP DNS

TCP UDP

Internet

Transport

Layers Protocols

Application

Figure 1-22. The TCP/IP model with some protocols we will study.

include the Domain Name System (DNS), for mapping host names onto their net-work addresses, HTTP, the protocol for fetching pages on the World Wide Web,and RTP, the protocol for delivering real-time media such as voice or movies.

1.4.3 The Model Used in This Book

As mentioned earlier, the strength of the OSI reference model is the model it-self (minus the presentation and session layers), which has proven to be ex-ceptionally useful for discussing computer networks. In contrast, the strength ofthe TCP/IP reference model is the protocols, which have been widely used formany years. Since computer scientists like to have their cake and eat it, too, wewill use the hybrid model of Fig. 1-23 as the framework for this book.

5 Application

4 Transport

3 Network

2 Link

1 Physical

Figure 1-23. The reference model used in this book.

This model has five layers, running from the physical layer up through thelink, network and transport layers to the application layer. The physical layerspecifies how to transmit bits across different kinds of media as electrical (orother analog) signals. The link layer is concerned with how to send finite-lengthmessages between directly connected computers with specified levels of reliabil-ity. Ethernet and 802.11 are examples of link layer protocols.

FIGURE: An alternative reference model.


Conclusions

CONCLUSIONS

The figures have been taken from, Computer Networks,Andrew S. Tanenbaum, 5th Edition.

http://computernetworks5e.org/blogs/

http://computernetworks5e.org/blogs/

Introduction To Computer Networks

Engineering

Transcript of Introduction To Computer Networks