Computer Communication & Networks

Lecture # 01Introduction

Course Instructor:Engr. M.Zeeshan Sarwar

Grading Policy

Final Exam: 40% Mid term Exam 20% Assignments 5% Quizzes: 10% Labs 25%

Quizzes may be announced or unannounced. Exams are closed-book and extremely time

limited. Exams consist of design questions,

numerical, maybe true-false and short answer questions.

Reading Text book:

Data Communications and Networking, 4/e B.A. Forouzan, McGraw-Hill, 2003,

ISBN 0-07-292354-7. Reference books:

Computer Networking, a top-down approach featuring the Internet (3rd edition), J.K.Kurose, K.W.Ross,Addison-Wesley, 2005,

ISBN 0-321-26976-4. Computer Networks, A Systems Approach

L. Peterson & Davie Data and Computer Communication by William

Stallings (7th Edition) Prentice Hall.

Data Communications

The term term telecommunicationtelecommunication means means communication at a distance. The word communication at a distance. The word datadata refers to information presented in whatever refers to information presented in whatever form is agreed upon by the parties creating form is agreed upon by the parties creating and using the data. and using the data. Data communicationsData communications are are the exchange of data between two devices the exchange of data between two devices via some form of transmission medium such via some form of transmission medium such as a wire cable. as a wire cable.

Fundamental Characteristics

The effectiveness of a data communication system depend on four fundamental characteristics: Delivery Accuracy Timelines Jitter

Five Components of Data Communication

1. Message

2. Sender

3. Receiver

4. Medium

5. Protocol

Direction of data flow

Simplex

Half Duplex

Full Duplex

Network design

Before looking inside a computer network, first agree on what a computer network is

Computer network ?

Set of serial lines to attach terminals to mainframe ?

Telephone network carrying voice traffic ?

Cable network to disseminate video signals ?

Specialized to handle:

Keystrokes

Voice

Video

What distinguishes a Computer network ?

Generality Built from general purpose

programmable hardware Supports wide range of applications Not optimized for special purpose

application like making phone calls or delivering television signals

Information, Computers, Networks Information: anything that is represented in bits

Form (can be represented as bits) vs Substance (cannot be represented as bits)

Properties: Infinitely replicable Computers can “manipulate” information Networks create “access” to information

Networks

Potential of networking: move bits everywhere, cheaply, and with desired

performance characteristics Network provides “connectivity”

What is “Connectivity” ?

Direct or indirect access to every other node in the network

Connectivity is the magic needed to communicate if you do not have a direct pt-pt physical link. Tradeoff: Performance characteristics worse than true physical

link!

Building Blocks

Nodes: PC, special-purpose hardware… hosts switches

Links: coax cable, optical fiber… point-to-point

multiple access…

Why not connect each node with every other node ? Number of computers that can be

connected becomes very limited Number of wires coming out of each

node becomes unmanageable Amount of physical hardware/devices

required becomes very expensive Solution: indirect connectivity using

intermediate data forwarding nodes

A Network

A network can be defined recursively as

two or more nodes connected by a physical link

Or

two or more networks connected by one or more nodes

Switched Networks

two or more nodes connected by a link

white nodes (switches) implement the network

colored nodes (hosts) use the network

A network can be defined recursively as...

Switched Networks

two or more networks connected by one or more nodes: internetworks

white nodes (router or gateway) interconnects the networks

a cloud denotes “any type of independent network”

A network can be defined recursively as...

Switching Strategies

Circuit switching: carry bit streams

a. establishes a dedicated circuit

b. links reserved for use by communication channel

c. send/receive bit stream at constant rate

d. example: original telephone network

• Packet switching: store-and-forward messagesa. operates on discrete

blocks of datab. utilizes resources

according to traffic demand

c. send/receive messages at variable rate

d. example: Internet

What next ?

Hosts are directly or indirectly connected to each other Can we now provide host-host connectivity ?

Nodes must be able to say which host it wants to communicate with

Addressing and Routing

Address: byte-string that identifies a node usually unique

Routing: forwarding decisions process of determining how to forward messages

to the destination node based on its address Types of addresses

unicast: node-specific broadcast: all nodes on the network multicast: some subset of nodes on the network

Wrap-up

A network can be constructed from nesting of networks

An address is required for each node that is reachable on the network

Address is used to route messages toward appropriate destination

What next ?

Hosts know how to reach other hosts on the network

How should a node use the network for its communication ?

All pairs of hosts should have the ability to exchange messages: cost-effective resource sharing for efficiency

Multiplexing Physical links and nodes are shared among users

(synchronous) Time-Division Multiplexing (TDM) Frequency-Division Multiplexing (FDM)

L1

L2

L3

R1

R2

R3Switch 1 Switch 2

Multiple flows on a single link

Do you see any problem with TDM / FDM ?

What Goes Wrong in the Network?

Reliability at stake

Bit-level errors (electrical interference) Packet-level errors (congestion)

distinction between lost and late packet Link and node failures

distinction between broken and flaky link distinction between failed and slow node

What Goes Undesirable in the Network?

Required performance at stake

Messages are delayed Messages are delivered out-of-order Third parties eavesdrop

The challenge is to fill the gap between application expectations and hardware capabilities

Networks: key issues

Network criteria Performance

Throughput Delay

Reliability Data transmitted are identical to data received. Measured by the frequency of failure The time it takes a link to recover from a failure

Security Protecting data from unauthorized access

Terminology

The throughput or bandwidth of a channel is the number of bits it can transfer per second

The latency or delay of a channel is the time that elapses between sending information and the earliest possible reception of it

Network topologies

Topology defines the way hosts are connected to the network

Network topology issues

1. high throughput (bandwidth)

2. low latency

a goal of any topology

Bandwidth and Latency

1. telecommunications: range of radio frequencies: a range of radio frequencies used in radio or telecommunications transmission and reception

2. computing: communications capacity: the capacity of a communications channel, for example, a connection to the Internet, often measured in bits per second

3. a data transmission rate; the maximum amount of information (bits/second) that can be transmitted along a channel

Latency

A synonym for delay, is an expression of how much time it takes for transmission from one designated point to another

Bandwidth

Categories of Topology

Mostly used network topologies

mesh

star

bus

ring

A hybrid topology: a star backbone with three bus networks

Hierarchical organization of the Internet

LAN, WAN & MAN

Network in small geographical Area (Room, Building or a Campus) is called LAN (Local Area Network)

Network in a City is call MAN (Metropolitan Area Network)

Network spread geographically (Country or across Globe) is called WAN (Wide Area Network)

Layering & Protocol Stacks

What’s a protocol?

human protocols: “what’s the time?” “I have a question” introductions

… specific msgs sent

… specific actions taken when msgs received, or other events

network protocols: machines rather than humans all communication activity in Internet governed by protocols

Protocol

protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt

a human protocol and a computer network protocol:

Hi

Hi

Got thetime?

2:00

TCP connection req.

TCP connectionreply.

Get http://gaia.cs.umass.edu/index.htm

<file>

time

Standard

Essential in creating and maintaining an open and competitive market for equipment manufacturers

Guaranteeing national & international interoperability of data & telecommunication technology & process.

Layered TasksAn example from the everyday life

Hierarchy?

Services

Why layered communication?

To reduce complexity of communication task by splitting it into several layered small tasks

Functionality of the layers can be changed as long as the service provided to the layer above stays unchanged makes easier maintenance & updating

Each layer has its own task Each layer has its own protocol

Reference Models

OSI reference model TCP/IP

OSI Reference model

Open System Interconnection 7 layers

1. Crate a layer when different abstraction is needed

2. Each layer performs a well define function

3. Functions of the layers chosen taking internationally standardized protocols

4. Number of layers – large enough to avoid complexity

Seven layers of the OSI model

Exchange using OSI Model

Issues, to be resolved by the layers Larger bandwidth at lower cost Error correction Flow control Addressing Multiplexing Naming Congestion control Mobility Routing Fragmentation Security ....

Applications

E-mail Searchable Data (Web Sites) E-Commerce News Groups Internet Telephony (VoIP) Video Conferencing Chat Groups Instant Messengers Internet Radio

Research areas in Networking Routing Security Ad-hoc networks Wireless networks Protocols Quality of Service …

Readings

Chapter 1: 1.1, 1.2 Computer Networks, A Systems Approach

L. Peterson & Davie Chapter 1 (B. A Forouzan)

Section 1.1, 1.2, 1.3,1.4

Chapter 2 (B.A Forouzan) Section 2.1