Osi models

EC2352- COMPUTER

NETWORKS

UNIT I PHYSICAL LAYER

Data Communications – Networks - Networks

models – OSI model – Layers in OSI model – TCP /

IP protocol suite – Addressing – Guided and

Unguided Transmission media

Switching: Circuit switched networks – Data gram

Networks – Virtual circuit networks. Cable networks

for Data transmission: Dialup modems – DSL –

Cable TV – Cable TV for Data transfer.

TEXT BOOKS:

1. Behrouz A. Foruzan, “Data communication and

Networking”, Tata McGraw-Hill, 2006: Unit I-IV

2. Andrew S. Tannenbaum, “Computer Networks”,

Pearson Education, Fourth Edition, 2003: Unit V

CHAPTER 1

Data Communications Components

Data Representation

Data Flow

Networks Network criteria

Physical topology

Categories of Networks

Protocols Syntax

Semantics

Timing

DATA COMMUNICATIONS

Purpose of communication- to share information

Sharing Information can be local or remote.

The term telecommunication means communication

at a distance.

The word data refers to information presented in a

form agreed by the parties creating and using the

data

Data communications are the exchange of data

between two devices via some form of transmission

medium such as a wire cable.

COMPONENTS

DATA REPRESENTATION

Text - bit pattern (a sequence of 0’s and 1’s)

Numbers – bit pattern (number converted to binary)

Images – bit pattern

Audio – sound or music

Video – picture or movie

DATA FLOW

NETWORKS

A network is a set of devices (often referred to as

nodes) connected by communication links.

A node can be a computer, printer, or any other

device capable of sending and/or receiving data

generated by other nodes on the network.

A link can be a cable, air, optical fiber, or any

medium which can transport a signal carrying

information.

NETWORK CRITERIA

Performance

Depends on Network Elements

Reliability

Failure rate of network components

Security

Data protection against corruption/loss of data due to:

Errors

Malicious users

PHYSICAL STRUCTURES

Type of Connection

Point to Point - single transmitter and receiver

Multipoint - multiple recipients of single transmission

PHYSICAL STRUCTURES

How devices

are connected

to make a n/w.

MESH TOPOLOGY:

A physical topology in which every device has adedicated point to point link to every other device.

In mesh topology we need n(n-1)/2 links.

Advantages:

It eliminating traffic problems.

Fault identification is easy.

Mesh topology is robust.

Disadvantages:

It is expensive and difficult to install a mesh n/w.

Example:

Connection of telephone regional offices to everyother regional offices.

STAR TOPOLOGY:

Each device has a dedicated point to point link

only to central controller usually called a hub.

Advantages:

Hub provides Fault identification is easy.

Less expensive than mesh.

It include robustness.

Disadvantages:

The failure of the central hub can bring down the

entire n/w.

Example:

Used in LANs

BUS TOPOLOGY:

Bus topology is multipoint.

One long cable acts as a backbone to link all thedevices in a n/w.

Advantages:

Easy to add more workstations.

Inexpensive to install.

Bus topology works well for small n/ws

(2-10 devices)

Disadvantages:-

If the backbone breaks, the n/w goes down.

It is difficult to isolate where a problem may be.

Adding new devices may require modification orreplacement of the backbone.

Example:

Used in Ethernet LANs.

RING TOPOLOGY:

Each device has a dedicated point to point link

with only the two devices on either side of it.

Advantages:

Easy to install and reconfigure.

No collisions.

Disadvantages:

A break in the ring can disable the entire n/w.

CATEGORIES OF NETWORKS

Local Area Networks (LANs)

Limited to a few kilometers

Ex: single office, building or campus

Wide Area Networks (WANs)

Long distances

Provide connectivity over large areas ie,

country, continent

Metropolitan Area Networks (MANs)

Size b/w LAN and WAN

Inside city ie, cable tv n/w, telephone n/w

PROTOCOLS

A protocol is synonymous with rule. It consists of a set ofrules that govern data communications.

It determines what is communicated, how it iscommunicated and when it is communicated.

Elements of protocol:

Syntax

Structure or format of the data

Indicates how to read the bits - field

Semantics

Interprets the meaning of the bits

Knows which fields define what action

Timing

When data should be sent and what

Speed at which data should be sent or speed at which it is being received.

THE OSI MODEL

An ISO standard that covers all aspects of network

communications is the Open Systems

Interconnection (OSI) model.

An Open System is a set of protocols that allows

any two different systems to communicate

regardless of their underlying architecture.

SEVEN LAYERS OF THE OSI MODEL

A mnemonic for remembering the layers of the OSI

model is

“Please Do Not Touch Steve’s Pet

Alligator”Please - Physical Layer

Do - Data Link Layer

Not – Network Layer

Touch – Transport Layer

Steve’s – Session Layer

Pet – Presentation Layer

Alligator – Application Layer

AN EXCHANGE USING THE OSI MODEL

PHYSICAL LAYER:

The physical layer is responsible for movements of individual bits from

one hop to the next.

Responsibilities:

Data rate : The number of bits sent each second.

Synchronization of bits : The sender and receiver clocks must

be synchronized.

Line configuration : Connection of device to the media.

Physical topology : How devices are connected to make a n/w.

Transmission mode : Direction of transmission b/w two

devices.

PHYSICAL LAYER

DATA LINK LAYER:

The Data link layer is responsible for moving frames from one hop to

the next.

Responsibilities:

Framing : Divides the stream of bits into manageable units

called frames.

Flow control : A technique to control the rate of flow of frames.

Error control : To handling errors in data transmission and to

detect and retransmit lost or damaged frames.

Access control : To determine which device has control over

the link at any given time.

Physical addressing : To handle the addressing problem

locally.

DATA LINK LAYER

HOP-TO-HOP DELIVERY

NETWORK LAYER:

The Network layer is responsible for the delivery of individual

packets from the source host to the destination host.

Responsibilities:

Logical addressing : The n/w layer adds a header to the

packet coming from upper layer that includes the logical

addresses of the sender and receiver.

Routing : When n/w are connected to create a large n/w, the

connecting devices route the packets to their final destination.

NETWORK LAYER

SOURCE-TO-DESTINATION DELIVERY

TRANSPORT LAYER:

The Transport layer is responsible for the delivery of a message from

one process to another.

Responsibilities:

Segmentation and reassembly : A message is divided into

transmittable segments and to reassemble the message

correctly upon arriving at the destination.

Connection control : It can be either connectionless or

connection oriented

Flow control : A technique to control the rate of flow of frames.

Error control : To handling errors in data transmission and to

detect and retransmit lost or damaged frames.

TRANSPORT LAYER

SESSION LAYER

Responsibilities:

Dialog Control :

It allows communication b/w

two processes to take place

in either half or full duplex

mode.

Synchronization :

It allows a process to add

checkpoints to a stream of

data.

PRESENTATION LAYER

Responsibilities:

Translation : Theprocesses in two systemsare usually exchanginginformation in the form ofcharacter strings, numbersand so on.

Encryption : Originalinformation to another form

Compression : Reducesthe number of bitscontained in the information.

APPLICATION LAYER

Services:

File Transfer, access and

management

Mail Services

SUMMARY OF LAYERS

TCP/IP AND OSI MODEL

TCP/IP AND OSI MODEL

PHYSICAL & DATA LINK LAYERS:

TCP/IP does not define any specific protocol.

It support all the standard protocols.

NETWORK LAYER:

TCP/IP supports the Internetworking Protocol.

IP uses four supporting protocols

ARP, RARP, ICMP IGMP.

TRANSPORT LAYER:

It supports three protocols.

UDP, TCP ,SCTP.

APPLICATION LAYER:

Many protocols are defined at this layer.

SMTP, FTP, HTTP, DNS, SNMP, TELNET

ADDRESSES IN TCP/IP

RELATIONSHIP OF LAYERS AND ADDRESSES

IN TCP/IP

PHYSICAL ADDRESSES:

The physical address also known as the link address, is the

address of a node as defined by its LAN or WAN.

LOGICAL ADDRESSES:

Logical Addresses are necessary for universal communications.

A Universal addressing system is needed in which each host can

be identified uniquely.

A logical address in the internet is currently a 32-bit address.

The physical addresses will change from hop to hop, but the

logical addresses usually remain the same.

PORT ADDRESSES :

In TCP/IP architecture, the label assigned to a process is called

a port address.

A port address in TCP/IP is 16 bits in length.

SPECIFIC ADDRESSES:

User friendly addresses that are designed for that specific

address.

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