Computer NetworksModule 1: Introduction

Computer Networks, Andrew Tanenbaum 4e

Dr. Vikram Shete

St. Francis Institute of Technology

Administrative Trivia

• Marks Overview– Theory Exam (Univ.) = 80– Internal Exam = 20– Internal Termwork = 25

Type Based on Comments Total

Labs Experiments + Mini group project (ns2)

Average out of 10 + 5 for project

15

Assignments Two Internal 20 marks test*

Mapped to 5 marks each

5

Attendance Lectures + Labs 75% minimum 5

Grand total 25

Administrative Trivia

• Copying– From each other – From the web– From text book

• Paraphrase in your own words and cite the original sources– Unless you have developed significant theory

• Just because you are in India doesn’t mean you have to be corrupt

Administrative Trivia

• Penalty if found copying/cheating on assignments, labs and exams– 0 marks on that deliverable– Take great care of what you submit

• Meet me if you need help

Administrative Trivia

• Books– Computer Networks

• Andrew Tanenbaum & David Wetherall

– Computer Networks: A top down approach• James Kurose & Keith Ross

– Data Communications & Networking• Behrouz Forouzan

Why Computer Networks?

Why Study Computer Networks?

• Age of information

• Ever evolving

• Created jobs (both, Steve and paid!)

• Society has been affected by it

What is this course about?

• Fundamental concepts of networking

• Types of networks

• Features of networks

• Design issues involved in networking

• Will develop concepts which are at core of networking

What is this course NOT about?

• Not about a specific device or company

• May not be sufficient to get a job in itself

Historical Perspective

• December 23, 1947– William Shockley– Walter Brattain– John Bardeen

Source: http://www.computerhistory.org/

Historical Perspective

Source: Wikimedia Commons: http://en.wikipedia.org/wiki/File:Wafer_2_Zoll_bis_8_Zoll_2.jpg

Historical PerspectiveNetwork of 1861

Source: Wikimedia Commons

Historical Perspective

Where does it stands today?

Produced with permission from Chris Harrison- Carnegie Mellon University

Network Applications

Introduction

• Network Applications: – Business– Home– Mobile– Social

Payroll System UpdatesEmailSystem InformationCompany wide broadcastsTrainingRecruitmentTaxesData centersE-CommerceCollaborative work

Business Applications of Networks (Tanenbaum)

A network with two clients and one server.

Network Applications (Tanenbaum)

• Some forms of e-commerce.

Introduction

• Network Applications: – Business– Home– Mobile– Social

Access to remote information

Person-to-person communication

Interactive entertainment

Electronic commerce

“There is no reason for any individual to have a computer in his home” ~ Ken Olsen – President, Digital Equipment Corporation (# 2 computer vendor after IBM)

Home Network Applications

• In peer-to-peer system there are no fixed clients and servers. (eg: Napster)

Social Issues

• Ideas move beyond country boundaries• Countries have different cultural and legal

structures• Technical issues are not problematic• Issues escalate when people discuss

– Politics– Sex– Religion

Social Issues

• Censorship– Internet operators similar to phone operators– Cannot control what users say or do online– Can censor but defies freedom of expression

• Government censorship– Patriot Act of USA– PIPA and SOPA?

• Internet provides true freedom but– Brings forth many other unsolved issues

Network Hardware

Network Hardware

• Technology• Scale

• Broadcast links• Point-to-Point links

• Different from P2P networks

Network Hardware

• Technology• Scale

• Broadcast links• Point-to-Point links

• Different from P2P networks

PAN

LAN

MAN

WAN

Point-to-Point Line Configuration

Point-to-Point Line Configuration

Multipoint Line Configuration

Mesh Topology

Star Topology

Tree Topology

Bus Topology

Ring Topology

Hybrid Topology

Simplex

Half-Duplex

Full-Duplex

Network Hardware

• Technology• Scale

• Broadcast links• Point-to-Point links

• Different from P2P networks

PAN

LAN

MAN

WAN

Local Area Network

Local Area Network

Network Hardware (LAN)

• Local Area Networks (LAN)– Widely used for private network within

• Building, factory, campus etc.• Share common resources (printers, database etc)

– Small in size • Worst case transmission times known apriori• Tailor made designs are possible

Network Hardware (LAN)

– Wired LANs• Use cables, high speeds and low delays with no

errors• Traditional LANs operate at 10 Mbps to 100 Mbps• Newer ones can go upto 1Gbps

Network Hardware (LAN)

– LAN topologies• Bus

– At most one computer can transmit at a time– Rest must remain quiet– An arbitration method is required to resolve conflicts– IEEE 802.3 a.k.a Ethernet is a bus based broadcast

network. Operates at 10Mbps to 10Gbps

• Ring– IEEE 802.5, the IBM Token Ring protocol. Operates at 4

and 16 Mbps– Fiber Distributed Data Interface (FDDI) ring network

Local Area Networks

• Two broadcast networks• (a) Bus• (b) Ring

Metropolitan Area Network

Metropolitan Area Networks

• A metropolitan area network based on cable TV.

Figure 2-18

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Wide Area Network

Wide Area Networks

• Relation between hosts on LANs and the subnet.

Internetwork (Internet)

Network Hardware (Summary)

• Technology• Scale

• Broadcast links• Point-to-Point links

• Different from P2P networks

Network Software

• The start was hardware• Networks evolved • Network software is more structured now

than ever before• Layered architecture of network

Network Software

• Layered architecture– Reduces complexity

– Organized as a stack of layers

– Each layer has responsibilities and tasks

– Layers interact with the ones above and below

Network Software

• Layered Architecture– Each layer provides a service to the layer

above– Layer to layer communication in different

devices– The rules of communication are called

protocols

Network Software

• Data never moves directly between layers• Flows through the entire stack• Actual transmission at the lower most

layer– This is the physical medium

Network Software

• Set of layers and protocols together is called the network architecture

• Each layer uses a protocol• A set of protocols used by layers is called

a protocol suite/stack

Network Software

Protocol Hierarchies

Network Software

• Example information flow supporting virtual communication in layer 5.

Design Issues for Layers

• Addressing– Layers need to identify sources & destinations– Computers can have multiple processes

• Data transfer– Unidirectional flow– Bidirectional flow– Prioritized bidirectional flow

Design Issues for Layers• Error Control

– Physical connections are noisy– Common standard between receiver and

transmitter– Means to communicate errors have occurred

• Order of Data Received – Order may not preserved– Mechanism to detect out of order pieces – Mechanism to put those in order

Design Issues for Layers• Flow Control

– A sender may swamp the network with data– Feedback to reduce data transmission

• Length of Messages – Too long

• Disassemble, transmit and reassemble

– Too short• Assemble, transmit and disassemble

Design Issues for Layers• Multiplexing-Demultiplexing

– Inefficient to setup channels for each processes

– Layers will multiplex data streams from different processes

• Routing– Choosing a right path based on various

parameters• Privacy laws, costs involved, infrastructure etc.

Design Issues for LayersARE DATA FOrM

• Addressing

• Routing

• Error Control

• Data Transfer Mode

• Flow Control

• Order of Data Received

• Mux-Demux

Types of Services Offered by Layers to Layers Above Them

Connection-Oriented and Connectionless Services

Reference Models for Layers

Open Systems Interconnection

• OSI Reference Model– First introduced in 1970 by International

Standards Organization (ISO-OSI reference model)

– Aimed at enabling communication between two different systems

– No need to changed underlying hardware and technology

Open Systems Interconnection

• OSI Reference Model– An open system is a set of protocols– OSI model is not a protocol – Model allows designing a network architecture

which is: • Flexible• Robust• Interoperable

Open Systems Interconnection

• OSI Reference Model– Architecture is very general – Earlier protocols are rarely used today– Consists of 7 separate but related layers– Each layer defines a part of process in

moving data forward– Each layer is a group of closely related

functions– Each group is distinct from the other

OSI Layers (Forouzan)

Interface 7/6

Interface 6/5

Interface 5/4

Interface 4/3

Interface 3/2

Interface 2/1

Interface 7/6

Interface 6/5

Interface 5/4

Interface 4/3

Interface 3/2

Interface 2/1

Peer-to-Peer Protocol 7th Layer7

6

5

4

3

2

1

7

6

5

4

3

2

1

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

Presentation

Session

Transport

Network

Data Link

Physical

Network

Da.Li

Physical

Network

Da.Li

Physical

OSI Layers

• Layers belong to 3 subgroups– Network Support Layers (Layers 1,2,3)

• Electrical properties, physical connections, physical and logical addressing, transport timing and reliability

– User Support Layers (Layers 5,6,7)• Allow interoperability between unrelated software systems

– Link (Layer 4)• Ensures seamless communication between above 2 groups

OSI Layers

• Upper layers are always implemented in software

• Lower layers are a combination of hardware and software

• The physical layers is almost always hardware

An Exchange Using the OSI Model

Header

Hea

der +

Dat

afr

om P

revi

ous

laye

r

Encapsulation

Layers in Details

Physical Layer

Physical Layer

• Physical characteristics of interfaces and medium

• Representation of bits– electrical/optical encoding

• Data rate– Define duration of bit

Physical Layer

• Bit synchronization– Clock synchronization between sender and

receiver

• Line configuration– Point to point or multipoint

• Physical topology– Mesh, star etc

• Transmission mode– Simplex, half duplex or full duplex

Data Link Layer

Data Link Layer

• Transforms a raw transmission facility into a reliable service

• Physical layer appears error free to the network layer

Data Link Layer

• Broadly responsible for moving frames from one node to another

• Other functions include – Framing– Physical addressing– Flow control– Error control– Access control

Data Link Layer

• Framing – Divide stream of bits from network layer into

manageable units called frames

• Physical addressing– Sender’s and receiver’s address of the frame

within the network– If outside the network then to default gateway

Data Link Layer

• Flow control– Imposes flow control mechanism at the sender’s

end to avoid overwhelming the receiver

• Error control– Detect and retransmit damaged or lost frames– Recognize duplicate frames

• Access control– When 2 or more devices connect to same link,

data link layer protocols select the controlling device

Data Link Layer

Network Layer

Network Layer

• Logical addressing– Enables communication across networks

• Routing– Enable moving of packets over the network

Transport Layer

Transport Layer

• Responsible for Process to Process delivery

Transport Layer

• Other functions– Service-point addressing– Segmentation and reassembly– Connection control– Flow control– Error control

Transport Layer (Other Functions)

• Service-point addressing– Transport layer header contains the port

address– Network layer gets each packet to the correct

computer– Transport layer delivers it to the correct

process

Transport Layer (Other Functions)

• Segmentation and reassembly– Message is divided into segments each with a

sequence number– Sequence numbers allow reassembly

• Connection control– In connectionless each segment is treated

independently and delivered– In connection oriented service, a connection

is first setup and then segment is delivered

Transport Layer (Other Functions)

• Flow Control– Flow control is end to end and not link to link

like in data link layer

• Error Control– Control is performed process to process and

not on a link to link basis– Correction is achieved through retransmission

Transport Layer

Self Study

• Session • Presentation • Application

Summary of Layers

TCP/IP Protocol Suite

TCP/IP Protocol Suite

• TCP/IP was designed to have 4 layers• Compared to OSI it can be said to have 5

layers• Some of the functions in OSI are bundled

in TCP/IP

TCP/IP Protocol Suite

TCP/IP Protocol Suite

• 4 levels of addressing are used in internet using TCP/IP

TCP/IP Protocol Suite

TCP/IP v/s OSI

Comparison of OSI & TCP/IP

• Concepts central to OSI model– Services: Tells what a layer does– Interfaces: How to access services?– Protocols: Layers internal business to get the

job done

• Analogous to objects in OOP• TCP/IP does not clearly differentiate

between the above three

Comparison of OSI & TCP/IP

• Consequently protocols are well hidden in OSI compared to TCP/IP– Protocols can be changed as and when

technology changes– This is primary reason for a layered structure

Comparison of OSI & TCP/IP

• OSI model was before protocols– Hence is not protocol biased– Designers were inexperienced and did not

know “what functionality belonged to which layer”

• TCP/IP came after protocols– Model fits the existing protocols very well– Unsuitable for non-TCP/IP networks

Comparison of OSI & TCP/IP

• OSI model has 7 layers– Uneven distribution of functionalities– Very little in upper layers and transport and

network layers are overcrowded

• TCP/IP has 4 layers

Comparison of OSI & TCP/IP

• OSI supports connection oriented and connectionless in the network layer – Only connection oriented in the transport layer– Transport layers services visible to user

• TCP/IP supports connectionless in network layer– But both in transport layer– Users get to choose between either