chapter 2 the osi model by dr.sukchatri prasomsuk school of ict, university of phayao

Chapter 2

The OSI Model

By

Dr.Sukchatri PRASOMSUK

School of ICT,

University of Phayao

Objectives

• On completion of this chapter, you will be able to perform the following tasks:– Describe how data traffic is exchanged

between source and destination devices– Identify the roles and functions of a hub,

switch, and router, and where they best fit in the network

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Contents

• 2.1 Introduction & Definition• 2.2 The Model : Layered Architecture• 2.3 Functions of the Layers• 2.4 TCP/IP Protocol Suite

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2.1 Introduction & Defining Components of the Network

2.1 Introduction & Defining Components of the Network

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Main OfficeBranch Office

Home Office

Mobile Users

Internet

Defining Components of the NetworkDefining Components of the Network

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Floor 2

Floor 1

Server Farm

Branch

Office

Telecommuter

ISDN

Remote

Campus

Network Structure Defined by Hierarchy

Network Structure Defined by Hierarchy

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Distribution Layer

Core Layer

AccessLayer

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Access Layer CharacteristicsAccess Layer Characteristics

End station entry point to the network

Access Layer

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Distribution Layer Characteristics

Distribution Layer Characteristics

• Access Layer Aggregation Point

• Routes traffic • Broadcast/Multicast

Domains• Media Translation• Security• Possible point for remote access

Distribution Layer

Core Layer Characteristics Core Layer Characteristics

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• Fast transport to enterprise services

• No packet manipulation

Core Layer

2.2 The OSI Model10/80

2.3 Functions of the Layers– Layer 7 : Application Layer– Layer 6 : Presentation Layer– Layer 5 : Session Layer– Layer 4 : Transport Layer– Layer 3 : Network Layer– Layer 2 : Data Link Layer– Layer 1 : Physical Layer

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OSI Model OverviewOSI Model Overview

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Application (Upper) Layers

Session

Presentation

Application

OSI Model OverviewOSI Model Overview

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Data Flow Layers

Transport Layer

Data Link

Network Layer

Physical

Application (Upper) Layers

Session

Presentation

Application

Role of Application LayersRole of Application Layers

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TelnetHTTP

User Interface

EXAMPLES

Application


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TelnetHTTP

ASCIIEBCDICJPEG

User Interface

• How data is presented

• Special processing such as encryption

EXAMPLES

Presentation

Application


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TelnetHTTP

ASCIIEBCDICJPEG

Keeping different applications’ data separate

User Interface

•How data is presented

•Special processing such as encryption

Operating System/Application Access Scheduling

EXAMPLES

Session

Presentation

Application


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Keeping different applications’ data separate

User Interface

•How data is presented

•Special processing such as encryption

TelnetHTTP

ASCIIEBCDICJPEG

Operating System/Application Access Scheduling

Transport

Data Link

Network

Physical

EXAMPLES

Session

Presentation

Application

Role of Data Flow LayersRole of Data Flow Layers

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EIA/TIA-232V.35

EXAMPLES

Physical • Move bits between devices• Specifies voltage, wire

speed and pin-out cables


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802.3 / 802.2HDLC

EIA/TIA-232V.35

EXAMPLES

Data Link

Physical

• Combines bits into bytes and bytes into frames

• Access to media using MAC address

• Error detection not correction• Move bits between devices• Specifies voltage, wire



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802.3 / 802.2HDLC

EIA/TIA-232V.35

IPIPX

EXAMPLES

Network

Data Link

Physical



• Error detection not correction• Move bits between devices• Specifies voltage, wire


Provide logical addressing which routers use for path determination


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TCPUDPSPX

802.3 / 802.2HDLC

EIA/TIA-232V.35

IPIPX

EXAMPLES

Transport

Data Link

Physical

• Reliable or unreliable delivery

• Error correction before retransmit



• Error detection not correction• Move bits between devices• Specifies voltage, wire speed

and pin-out cables

Network Provide logical addressing which routers use for path determination


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TCPUDPSPX

802.3 / 802.2HDLC

EIA/TIA-232V.35

IPIPX

Presentation

Application

SessionEXAMPLES

• Reliable or unreliable delivery

• Error correction before retransmit



• Error detection not correction• Move bits between devices• Specifies voltage, wire speed

and pin-out cables

Transport

Data Link

Physical

Network Provide logical addressing which routers use for path determination

Layer 1 : Physical layer

◦ Lowest layer of OSI architecture provides services to the link layer, acquiring, maintaining and disconnecting the physical circuits that form the connecting communications path.

◦ Handles the electrical and mechanical interface as well as the procedural requirements of the interconnection medium.

◦ Responsible for bit synchronization and the identification of a single element as a one or a zero.

◦ This layer includes mechanical, electrical, functional and procedural specifications.

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Layer 1 : Physical layer▫The physical layer is the rough equivalent of

the traditional data-terminal-equipment (DTE) to data-communications-equipment (DCE) interface.

▫Typical protocols at the physical layer include the RS-232, the RS-449 family, CCITT X.25 and X.21 facility interfaces, other CCITT (V) and (X) series recommendations, and the physical aspects of the IEEE 802.X media access protocols for Local Area Networks.

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Physical Layer Functions

Physical Layer Functions

Defines

• Media type

• Connector type

• Signaling type

Eth

ern

et

802.3 V.3

5

Ph

ysic

al

EIA

/TIA

-232

Physical Layer: Ethernet/802.3Physical Layer: Ethernet/802.3

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Hub

Hosts

Host

10Base2—Thick Ethernet10Base5—Thick Ethernet

10BaseT—Twisted Pair

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Hubs Operate at Physical layer

Hubs Operate at Physical layer

A B C D

Physical

• All devices in the same collision domain• All devices in the same broadcast domain• Devices share the same bandwidth

Hubs: One Collision DomainHubs: One Collision Domain

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• More end stations means more collisions

• CSMA/CD is used

Layer 2 : Data link layer

◦ Link layer services relate to the reliable interchange of data across a point-to-point or multipoint data link that has been established at the physical layer.

◦ Link layer protocols manage establishment, control and termination of logical link connection. They control the flow of user data, supervise recovery from errors and abnormal conditions, and acquire and maintain character and block or frame synchronization.

◦ It attempts to add reliability, flow and error control, and communication management.

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Layer 2 : Data link layer▫Data link control protocols include character-

oriented Binary Synchronous Communication (BSC), ANSI X3.28m,

▫ the more recent bit-oriented ADCCP (Advanced Data Communications Control Procedure) and its international counterpart HDLC, X.25, LAPB, ISDN, LAPD and IEEE 802.X logical link control.

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Data Link layer Functions

Data Link layer Functions

Defines• Physical source and destination addresses

• Higher layer protocol (Service Access Point)associated with frame

• Network topology • Frame sequencing• Flow control• Connection-oriented or connectionless

Data

Lin

kP

hysic

al

EIA/TIA-232v.35

Eth

ern

et

Fra

me R

ela

y

HD

LC802.2

802.3

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DataSource add FCSLengthDest. add

Variable266 4

0000.0C xx.xxxx

Vendor assigned

IEEE assigned

MAC Layer - 802.3

Data Link Layer FunctionsData Link Layer Functions

Preamble

Ethernet II uses “Type” here and does not use 802.2.

MAC Address

8# Bytes

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DataDest SAP

Source SAP

DataSource add FCSLengthDest add

Variable11

802.2 (SAP)

MAC Layer - 802.3

Data Link Layer FunctionsData Link Layer Functions

Ctrl

1 or 2

3 2

Preamble

DataDest SAPAA

Source SAPAA

Variable

11

802.2 (SNAP)

Ctrl

03

1or2

OR

OUI ID

Type

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• Each segment has its own collision domain• All segments are in the same broadcast domain

Data Link

Switches and Bridges Operate at Data Link Layer

Switches and Bridges Operate at Data Link Layer

OR1 2 3 1 24

SwitchesSwitches

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• Each segment has its own collision domain

• Broadcasts are forwarded to all segments

Memory

Switch

Layer 3 : Network layer

Responsible for providing communication between two hosts across a communication network. Services include routing, switching, sequencing of data, flow control and error recovery.

It provides the interface such that higher layers need not know about the underlying topology.

It provides connection management, routing, and error and flow control.

The CCITT X.25 packet layer is the best known network layer protocol for packet- switched networks. X.21 is used for circuit-switched networks.

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Layer 3 : Network layer

DoD has developed the IP Internet control protocol.

Other examples of network protocols include the CCITT Q.931 network layer and the ISO 8473 connectionless inter-network protocol.

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Network Layer Functions

Network Layer Functions

• Defines logical source and destination addresses associated with a specific protocol

• Defines paths through network

• Interconnects multiple data links

Netw

ork

IP, IPX

Data

Lin

kP

hysic

al

EIA/TIA-232v.35

Eth

ern

et

Fra

me R

ela

y

HD

LC802.2

802.3

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DataSourceaddress

Destination address

IP

Network Layer FunctionsNetwork Layer Functions

Header

172.15.1.1

NodeNetwork

• Logical Address

Network Layer End Station Packet


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11111111 11111111 00000000 00000000

10101100 00010000 01111010 11001100

BinaryMask

BinaryAddress

172.16.122.204 255.255.0.0

172 16 122 204

255

Address Mask

255 0 0

Network Host

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Routing TableNET INTMetric124

S0S0E0

100

1.0 4.0

1.3E0

4.3

S0

2.2

E0

2.1

S0

4.1

4.2

1.1

1.2

Routing TableNET INT Metric

124

E0S0S0

001

• Logical addressing allows for hierarchical network• Configuration required• Uses configured information to identify paths to networks


Routers: Operate at the Network Layer

Routers: Operate at the Network Layer

• Broadcast control

• Multicast control

• Optimal path determination

• Traffic management

• Logical addressing

• Connects to WAN services

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Using Routers to Provide Remote AccessUsing Routers to Provide Remote Access

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Internet

Telecommuter

Branch Office

Modem or ISDN TA

Mobile User

Main Office

Layer 4 : Transport layer

Highest layer directly associated with the movement of data through the network.

It provides a universal transparent mechanism for use by the higher layers that represent the users of the communications service.

The transport layer is expected to optimize the use of available resources while meeting user requirements.

Responsible for the end-to-end integrity of the edit exchange and must bridge the gap between services provided by the underlying network and those required by the higher layers.

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Layer 4 : Transport layer• Classes of transport protocols have been developed that

range from extremely simple to very complex. • Simple transport layers can be used when the network

provides a high quality, reliable service. • A complex transport protocol is used when the

underlying service does not, or is assumed to be unable to, provide the required level of service.

• The ISO has promulgated International Standard 8073 as a transport protocol. This standard defines five (5) classes of protocols, ranging from a simple Class (0) to a complex Class (4).

• Another transport protocol example is the Transmission Control Protocol (TCP) developed by the DoD and now finding wide application in commercial environments.

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Transport Layer Functions

Transport Layer Functions

• Distinguishes between upper layer applications

• Establishes end-to-end connectivity between applications

• Defines flow control

• Provides reliable or unreliable services for data transfer

Netw

ork

IPXIP

Tran

sp

ort

SPXTCP UDP

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Encapsulating DataEncapsulating Data

Transport

Data Link

Physical

Network

Upper Layer Data

Upper Layer DataTCP Header

DataIP Header

DataLLC Header

0101110101001000010

DataMAC Header

Presentation

Application

Session

Segment

Packet

Bits

Frame

PDU

FCS

FCS

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Upper Layer Data

De-encapsulating DataDe-encapsulating Data

LLC Hdr + IP + TCP + Upper Layer Data

MAC Header

IP + TCP + Upper Layer Data

LLC Header

TCP+ Upper Layer Data

IP Header

Upper Layer Data

TCP Header

0101110101001000010

Transport

Data Link

Physical

Network

Presentation

Application

Session

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Reliable Transport Layer FunctionsReliable Transport Layer Functions

Synchronize

Acknowledge, Synchronize

Acknowledge

Data Transfer

(Send Segments)

Sender Receiver

Connection EstablishedConnection Established

Layer 5 : Session layer

• A session binds two application processes into a cooperative relationship for a certain time.

• The session layer provides an administrative service that handles the establishment (binding) and release (unbinding) of a connection between two presentation entities.

• Sessions are established when an application process requests access to another application process.

• Session protocols include ISO 8327, CCITT X.25, ECMA 75 and CCITT T.62 which is intended for use in teletex services.

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Layer 6 : Presentation layer• These services allow an application to properly interpret the

information being transferred. • This includes translation, transformation, formatting and

syntax of the information. • These functions may be required to adapt the information-

handling characteristics of one application process to another.

• Examples include code translation, structuring of data for display on screen, format control and virtual terminal protocols.

• The syntactical representation of data has been defined in DIS 8824 and 8825.

• CCITT has described the presentation protocol for message-handling systems in X.409 and for Telex in X.61.

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Layer 7 : Application layer

This layer provides management functions to support distributed applications utilizing the OSI environment.

It is the window through which the applications gain access to the services provided by the communications architecture.

These include identification of the cooperating processes, authentication of the communicant, authority verification, agreement on encryption mechanisms, determination of resource availability and agreement on syntax, e.g. character set, data structure.

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Layered protocol

• Functions: communication, sharing of media, leased line and public-switched connections,orderly interleaving of control and data packets, error-free communication, data integrity, connection-control, point-to-point and multi-point connections

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Layered protocol

• layered protocol configuration:▫point-to-point (Non-switched) ▫Point-to-point (switched)▫multipoint (non-switched)▫multipoint (switched)▫loop system

• Features of layered protocols: decomposition,service access point, modularity, flexibility, survivability and security

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2.4 TCP/IP Protocol Suite

Transmission Control Protocol/ Internetworking Protocol (TCP/IP)

Used in the Internet, was developed prior to the OSI model.

Consists of 5 layers :- Application Layer- Transport Layer- Network Layer (Internet Layer)- Data Link Layer- Physical Layer

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TCP/IP Protocol StackTCP/IP Protocol Stack

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7

6

5

4

3

2

5

4

3

2

Application

Presentation

Session

Transport

Network

Data Link

Physical

1

Application

Transport

Internet

Data Link

Physical

1

Application Layer OverviewApplication Layer Overview

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* Used by the router

Application

Transport

Internet

Data Link

Physical

File Transfer- TFTP *- FTP *- NFS

E-Mail- SMTP

Remote Login- Telnet *- rlogin *

Network Management- SNMP *

Name Management- DNS*

File Transfer- TFTP *- FTP *- NFS

E-Mail- SMTP

Remote Login- Telnet *- rlogin *

Network Management- SNMP *

Name Management- DNS*

Transport Layer OverviewTransport Layer Overview

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Transmission ControlProtocol (TCP)

User Datagram Protocol (UDP)

Transmission ControlProtocol (TCP)

User Datagram Protocol (UDP)

Application

Transport

Internet

Data Link

Physical

Connection- Oriented

Connectionless

TCP Segment FormatTCP Segment Format

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Source port (16) Destination port (16)

Sequence number (32)

Headerlength (4)

Acknowledgement number (32)

Reserved (6)Code bits (6) Window (16)

Checksum (16) Urgent (16)

Options (0 or 32 if any)

Data (varies)

20Bytes

Bit 0 Bit 15Bit 16 Bit 31

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Application

Presentation

Session

Transport

Network

Data Link

Physical

OSI Model

PDU Functional Responsibilities

Examples

Written Exercise: OSI Model Written Exercise: OSI Model

Basics of Subnetting

– Subnetworks or subnets– Why Subnets : – A primary reason for using subnets is to reduce the size of a

broadcast domain. – Broadcasts are sent to all hosts on a network or subnetwork. – When broadcast traffic begins to consume too much of the

available bandwidth, network administrators may choose to reduce the size of the broadcast domain

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– Addressing without subnets

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- Addressing with subnets

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- The 32 bits binary IP Address

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- Subnet Mask :To determine the subnet mask for a particular subnetwork IP address follow these steps.

(1) Express the subnetwork IP address in binary form.

(2) Replace the network and subnet portion of the address with all 1s.

(3) Replace the host portion of the address with all 0s.

(4) As the last step convert the binary expression back to dotted-decimal notation

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– The AND Function

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• Creating a Subnet : To create subnets, you must extend the routing portion of the address. The Internet knows your network as a whole, identified by the Class A, B, or C address, which defines 8, 16, or 24

routing bits (the network number).

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Address Class Size of Default Host Field Maximum Number of Subnet Bits

A 24 22

B 16 14

C 8 6

EX.


•Determining subnet mask size• Subnet Masking :

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Basics of Subnetting•Class B Subnet Planning Example :• Ex 1 :10001100.10110011.11011100.11001000 140.179.220.200 IP Address 11111111.11111111.11100000.00000000 255.255.224.000 Subnet Mask --------------------------------------------------------------------------------------- 10001100.10110011.11000000.00000000 140.179.192.000 Subnet Address 10001100.10110011.11011111.11111111 140.179.223.255 Broadcast addr•Ex 2 :

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• Class C

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•Private Address Space : There are certain addresses in each class of IP address that are not assigned. These addresses are called private addresses. Private addresses might be used by hosts that use network address translation (NAT), or a proxy server, to connect to a public network; or by hosts that do not

connect to the Internet at all. •Private Subnets : There are three IP network addresses reserved for private networks. The addresses are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.

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Example : Class C network number of 200.133.175.0 You want to utilize this network across multiple small groups within an organization. You can do this by subnetting that network with a subnet address.

•We will break this network into 14 subnets of 14 nodes each. This will limit us to 196 nodes on the network instead of the 254 we would have without subnetting, but gives us the advantages of traffic isolation and security. To accomplish this, we need to use a subnet mask 4 bits long. Recall that the default Class C subnet mask is •255.255.255.0 (11111111.11111111.11111111.00000000 binary)•Extending this by 4 bits yields a mask of •255.255.255.240 (11111111.11111111.11111111.11110000 binary)

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Basics of Subnetting•This gives us 16 possible network numbers, 2 of which cannot be used:

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Basics of Subnetting•Allowed Class A Subnet and Host IP addresses

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Basics of Subnetting•Allowed Class B Subnet and Host IP addresses

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Basics of Subnetting•Allowed Class C Subnet and Host IP addresses

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SummarySummary

•After completing this chapter, you should be able to perform the following tasks:– Describe how data moves through a network– Identify the roles and functions of routers,

switches and hubs, and specify where each device best fits in the network

– How to calculate and organize Subnet for each class.

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Assignments/ Q&A

1. Written exercise : OSI Model.

2.What are some of the advantages of using the OSI model in a networking environment?

3. Describe the encapsulation process.

4. How many broadcast and collision domains are on a hub?

5. Practice Lab Activity : 10.4.1, 10.6.6, 10.7.5, 10.7.7

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chapter 2 the osi model by dr.sukchatri prasomsuk school of ict, university of phayao

Documents

application layer layer

network layer layer

presentation layer layer

session layer layer

layers layer

transport layer layer

data link layer layer

physical layer