Saturday, March 24, 2012

Cisco CCNA: OSI Reference Model

Cisco CCNA: OSI Reference Model

This series of courses is designed to help you prepare for the Cisco Certified Network Associate certification exam. Each of the 60 objectives on the CCNA exam will be covered, and the questions will be indicative of the types of questions on the exam.
Effectively managing large networks, made up of varied technology, can be frustrating. However, even in today's fast-paced environment, network design principles and the OSI reference model provide guidance to administrators and manufacturers alike.
This unit will give you a brief overview of the history of networking, show you the three main types of client-server networks, and explain the principles of network design.


After completing this unit, you should be able to:

  • Discuss the history of networking

  • Identify three types of client-server networks

  • Explain the differences between three types of client-server networks

  • List the four principles of network design


This unit does not address any specific Cisco objectives. However, it does provide background information that is essential for the CCNA exam.
In the course index, questions about background information are indicated with the abbreviation BCK and a short description of the question subject matter.

Topic 1.1: A Brief History of Networking

*Centralized Computing
Centralized computing environments first arose in the 1960s and 1970s in the form of mainframe environments.

Topic 1.1.1: Mainframe

*Mainframe Computing
In a mainframe computing environment, terminals are connected to a single host or mainframe. The mainframe contains most of the computing power, including memory and file-sharing capabilities.

*Dumb Terminals
The terminals in mainframe networks are called "dumb" terminals because they rely on the mainframe to complete tasks such as printing and data sharing.

*Security Measures
In a mainframe environment, access to applications, data, and other computing processes is tightly controlled.

Topic 1.1.2: Personal Computers

*A Revolution
The personal computer phenomenon revolutionized centralized computing.
Computing power increasingly resided in individual terminals, while the connection between them was used to share expensive hardware, such as printers and hard drives.

Topic 1.1.3: Client-Server

*Clients and Servers
A network environment where computing power rests in individual terminals or PCs, while resources and data are shared, is called a client-server environment.

*LANs and WANs
LANs (Local Area Networks) and WANs (Wide Area Networks) are examples of client-server environments.

*Different Developments
The evolution of client-server architecture did not eliminate the use of mainframe networks. However, the communication protocols and applications developed for each remained separate.

*Communication Protocols
Mainframe networks use a host-based method of communication, while many client-server networks use protocols such as Novell's IPX/SPX (Internetwork Packet Exchange/Sequenced Packet Exchange) or TCP/IP (Transmission Control Protocol/Internet Protocol)

Question 1

Topic 1.2: Local Area Networks

*LAN Structure
LANs (Local Area Networks) are networks which are structured to operate in a limited geographical area, such as a building or the floor of a building.

*Multiple User Access
Bandwidth, the rating of a connection's maximum throughput, can vary between devices on a network. Using a LAN, multiple users can access high-bandwidth connections at the same time.

*Reliability
LANs can be privately controlled and normally provide access to resources available on the local network.
Loss of service due to shutdown is unusual.

*Connecting to the Network
Each PC connected to a LAN has a NIC (Network Interface Card) which allows it to connect to the network. LAN networks may also contain hubs and routers that help control the flow of data.

*Multiple LANs
As bandwidth demands have grown along with the computing power of PCs, it is increasingly common to connect several smaller LANs together, rather than create a single large LAN.

Question 2

Topic 1.3: Wide Area Networks

*Building Larger Networks
LANs are the building blocks for larger networks. By connecting multiple LANs together, you can create a network that spans a large geographic area.

*Creating a MAN
An example of a larger network is a MAN (Metropolitan Area Network). MANs are usually the result of LANs which have been connected together to span an area the size of a city or college campus.

*Creating a WAN
WANs (Wide Area Networks) encompass a much larger geographical area than LANs or MANs, extending over several cities, states, and countries.

*WANs Rely on Carriers
WANs typically operate through public telecommunications carriers.
They are generally used to connect LANs that are separated by distance.

*Accessing Different Amounts of Bandwidth
Using WANs, multiple users can access different amounts of bandwidth usually over a serial connection, where data is transmitted sequentially over a single communication path, or channel.

*Restricted Access
While WANs are reliable, access for connected workstations may be restricted by the network administrator. This restriction is usually imposed because of the high cost of telecommunications services used for the connection.

*Public Data Networks
In some areas, government agencies or private organizations have created networks to allow the public access to computer communications. These networks are called PDNs (Public Data Networks), and are often used by small organizations to create a WAN without the cost of long-distance circuits.

Question 3

Topic 1.4: Internetworks

*Defining Internetworks
Internetworks are designed for the global economy of the 1990s. Internetworks are simply an assortment of individual networks connected by networking devices. These individual networks can be LANs, WANs, mainframe networks, or other computing resources.

*Internetworks as a Solution
Internetworks evolved to solve three key problems inherent in isolated networks:
  • No information sharing
  • Duplication of resources
  • Difficult administration

Topic 1.4.1: Sharing Information

*Disadvantages of Isolated Networks
Isolated networks don't allow information to be shared outside the network. Furthermore, isolated networks encourage duplication of resources because the same hardware and software must be provided to each network.

Topic 1.4.2: Administration Issues

*Administrative Efficiency
Centralized administration of a single, larger network rather than localized management of multiple smaller networks is also more efficient.

Topic 1.4.3: Transmission Methods

*Different Computing Platforms
Internetworks use a variety of transmission methods and contain both new and old technology. This mix of computing platforms creates a unique challenge at a time when it is imperative for networks to be reliable, flexible, and scalable to meet the demands of business.

*The Role of Routers
Routers are a vital part of internetworks, because they provide the means for networks to communicate with each other.

*Enterprise Networks
Internetworks customized for the needs of a particular company, agency or other enterprise are referred to as enterprise networks. Enterprise networks can connect a corporation to all its branches, customers, and business partners.

Question 4


* Exercise 1
Try determining which type of network is best suited to your needs.


Examine the following table
Step Action
1 Familiarize yourself with the different types of networks available.
Use library, Internet, and company resources to learn how mainframe, LAN, WAN, and internetworks are implemented.
2 Talk to the network administrator of your company.
What types of networks does the company use? Why? What are the strengths and weaknesses of the networks used?
3 Consider what type of network you would like to help administer.
What types of companies are likely to use this type of network?
4 Familiarize yourself with those companies to see if their environment will suit you.


Topic 1.5: Network Design Goals

*Design Goals
Regardless of its size, a network is simply a way to connect hardware and software together. To satisfy the needs of the organization relying on the network, it is important to keep in mind the four basic goals of network design:
  • Connectivity
  • Reliable performance
  • Management control
  • Scalability


Topic 1.5.1: Connectivity

*Network Connectivity
It seems obvious, but the fundamental purpose of any network is to connect separate terminals and other resources together.

Topic 1.5.2: Reliable Performance

*Reliability
As companies and other organizations become increasingly dependent upon the network to provide file sharing and other computing resources, reliability becomes a factor.
The ability to update software and backup data falls under the category of reliability.

Topic 1.5.3: Management Control

*Network Maintenance
Management control is simply network maintenance.
Once the network is designed and operational, there must be a way to monitor network resources and troubleshoot problems as they arise.

Topic 1.5.4: Scalability

*Supporting Growth
The ability of a network to support growth, or scalability, is fundamental.
As businesses strive to succeed in a global economy, the network must be able to expand smoothly and efficiently.

Question 5


* Exercise 2
Try applying the four principles of network design.

Examine the following table
Step Action
1 List the four principles of network design.
Think about the challenges you would face in designing a network that has a satisfactory design.
List them.
2 Talk to a network administrator.
Discuss the challenges faced in designing and maintaining her particular network.
3 Evaluate how your list and the network administrator's list compare.
Did you accurately anticipate the challenges of network design?


Topic 1.6: Unit 1 Summary

In this unit, you learned how today's array of large, complex networks arose from the first mainframe computing environments developed in the 1960s and 1970s.
You learned about three main types of networks — LANs, WANs, and internetworks — as well as the characteristics of each. You also learned the importance of applying the four principles of network design when planning a network.
In the next unit, you'll learn about the upper layers of the OSI reference model.

Unit 2. The Upper OSI Layers



In this unit, you learn about the OSI (Open Systems Interconnection) reference model. You see what the model is, and learn about the functions of the upper layers of the model — the Application, Presentation, and Session layers.
Learning the roles of the upper layers will give you a better understanding of the basic aspects of network behavior.

After completing this unit, you should be able to:
  • Define the OSI reference model

  • List the tasks associated with the Application layer

  • Identify the functions handled by the Presentation layer

  • Name the protocols associated with the Session layer


This unit does not address any specific Cisco objectives. However, it does provide background information that is essential for the CCNA exam.
In the course index, questions about background information are indicated with the abbreviation BCK and a short description of the question subject matter.

Topic 2.1: Understanding the Model

*OSI Reference Model
The OSI (Open Systems Interconnection) reference model was developed by the ISO (International Organization for Standardization).
The model provides a framework for the tasks and protocols associated with networking.

*Layer Functions
The OSI reference model breaks networking into specific tasks which are then assigned to one of seven layers within the model.
Taken together, the layers show how data moves from software on one system across hardware to software on another system.

*Layer Identification
The seven layers of the OSI reference model are represented by a stack of layers with the lowest layer (Physical) numbered as layer 1. The seven layers are:
  • Application
  • Presentation
  • Session
  • Transport
  • Network
  • Data Link
  • Physical

*The Upper Layers
The Application, Presentation, and Session layers of the OSI reference model are often referred to as the upper layers.
These layers primarily handle tasks associated with applications, while the lower layers deal with data transport and hardware configuration issues.

Topic 2.2: The Application Layer

*Application Layer
The Application layer, or Layer 7, of the OSI reference model is called the "top" layer because it is closest to the user.
This layer is responsible for providing network services to user applications.

*Organizing Resources
The Application layer determines the resources required for applications to communicate, and ensures those resources are available.

*Synchronization
The Application layer also synchronizes applications which are located on the server and client components of a network.

*Reducing Errors
Finally, the Application layer maintains data integrity and error control between two communicating applications.

*Servicing by the Application Layer
The applications serviced by the Application layer are network applications, not computer applications such as spreadsheets and word processors.

Topic 2.2.1: Network Applications

*Examples of Network Applications
Network applications work primarily within a local area or enterprise network. Examples include:
  • Client-server processes
  • Electronic mail
  • File transfer
  • Remote access capabilities


*Using Network Services
For a computer application to use network services, it must have a network application installed.
For example, to share spreadsheet data across a network, the spreadsheet must contain a file transfer component.

Topic 2.2.2: Internetwork Applications

*Examples of Internetwork Applications
Internetwork applications designed to move information between networks are also handled by the Application layer. Examples include:
  • electronic data interchange (EDI)
  • e-mail gateways
  • world wide web (WWW)
  • special interest bulletin boards
  • financial transaction services


Question 6

Question 7

Question 8

Topic 2.3: The Presentation Layer

*Presentation Layer
The layer below the Application layer is the Presentation layer.
The Presentation layer ensures the Application layer will be able to read the data when it is presented. Thus, this layer is responsible for data formatting, conversion, and transferring syntax between Presentation layer entities.

Topic 2.3.1: Manipulating Data

*Data Handling
The Presentation layer also handles data encryption, decryption, compression, and decompression.

*Modifying Data
The Presentation layer is the only layer of the OSI reference model where data can be changed as part of the networking process.

Topic 2.3.2: Data Conversions

*Converting Data
When data is transferred between systems whose text or data character representations are different, the Presentation layer converts the data to a generic format for transmission, and back into a native format for reading and manipulation.

*Conversion Examples
An example of the Presentation layer converting data syntax is changing EBCDIC to ASCII.
HTML (Hypertext Markup Language), which determines the appearance of web pages, also operates at the Presentation layer.

*Multimedia Conversions
Conversions between PICT, TIFF, JPEG, and GIF graphics formats are handled by the Presentation layer.
Audio and video formats handled by the Presentation layer include MIDI, MPEG, and QuickTime.

Question 9

Question 10

Topic 2.4: The Session Layer

*Session Layer
The last of the upper layers of the OSI reference model is the Session layer.
This layer controls all communication sessions between applications on different network devices by establishing, managing, and terminating a session.

*Coordinating Services
The Session layer is the communications coordinator in the OSI reference model.
It ensures that service requests and responses between network devices are properly handled.

*Three Modes
There are three modes of communication which are available at the Session layer:
  • simplex
  • half-duplex
  • full-duplex


Topic 2.4.1: Simplex Communication

*Simplex Mode
Simplex communication is a session where communication flows only one way.
In this type of a session, one device transmits while the other receives.

Topic 2.4.2: Half-duplex Communication

*Half-duplex Mode
Half-duplex communication is a session where communication flows only one way at a time.
In this type of a session, one device transmits while the other receives. At the end of the transmission, the receiver can then begin a transmission of its own.

Topic 2.4.3: Full-duplex Communication

*Full-duplex Mode
Full-duplex communication is a session where communication flows in both directions simultaneously.

Topic 2.4.4: Communication Stages and Protocols

*Three Stages
Communication sessions are split into three distinct stages by the Session layer. The stages are:
  • Connection establishment
  • Data transfer
  • Connection release


*Protocols and Languages
Examples of protocols and languages that manage communication sessions are:
  • SQL (Structured Query Language)
  • RPC (Remote Procedure Call)
  • ASP (AppleTalk Session Protocol)
  • DNA SCP (Digital Network Architecture Session Control Protocol)


Question 11

Question 12

Question 13

Question 14


* Exercise 1
Try testing your knowledge of the OSI reference model.

Examine the following table
Step Action
1 List the seven layers of the OSI reference model.
Be sure they are in the correct order.
2 What are the tasks and protocols associated with the upper layers of the model?
List them with the appropriate layer.
Remember, knowing the layers is not enough, you must know the functions each performs.
3 Talk to a network administrator or other networking experts.
Ask them about the OSI reference model. How important is it in their work? How does it help or hinder them?


Topic 2.5: Unit 2 Summary

In this unit, you learned about the upper layers of the OSI reference model.
You learned the distinction between network applications and other kinds of applications. You also learned about the file formats and protocols associated with the Presentation layer, and the responsibilities of the Session layer.
In the next unit, you'll learn about the remaining layers of the OSI reference model.

Unit 3. The Lower OSI Layers



You have already learned about the upper layers of the OSI reference model. The upper layers primarily handle tasks associated with network applications.
In this unit, you learn about the lower layers of the OSI reference model. The lower layers — Transport, Network, Data Link, and Physical — primarily handle tasks associated with data transport and hardware configuration.

After completing this unit, you should be able to:
  • List the operations of the Transport layer

  • Name the functions handled by the Network layer

  • Identify the services performed by the Data Link layer

  • Explain how the Physical layer relates to hardware configuration


This unit provides information that is relevant to the following CCNA exam objective:
  • List the key internetworking functions for the OSI Network layer


Topic 3.1: The Transport Layer

*Transport Layer
The Transport layer is one of the four bottom layers of the OSI reference model whose assigned tasks primarily deal with data transfer.

*Transmitting Data
The Transport layer is responsible for the reliable transmission of data between two communicating devices. This layer has services that can segment and reassemble data from several upper-layer applications onto one Transport-layer data stream.

*Four Features
The Transport layer controls the reliable transmission of data with the help of four basic features:
  • Multiplexing
  • End-to-end transport services
  • Data integrity
  • Reliable transport


Topic 3.1.1: Multiplexing

*Sharing Connections
Multiplexing is essentially a networking ability that allows several upper-layer applications to share the same connection by segmenting and reassembling data into a data stream.
Each segment includes extra bits that provide the message type, originating program, and protocols used.

Topic 3.1.2: Transport Services

*Logical Connections
Logical connections allow access to internetworks, overlooking the real physical connections that make the connection possible.
End-to-end transport services allow the Transport layer to establish a logical connection between transmitting and receiving devices.

Topic 3.1.3: Data Integrity

*Data Flow Control
Data integrity is maintained, in part, by controlling data flow.
If the data flow overwhelms a receiving device, the Transport layer will slow the data stream and retransmit as needed to maintain data integrity.

Topic 3.1.4: Reliable Transport

*Securing Delivery
Reliable transport simply means that the Transport layer is designed to transmit data in the proper order, between devices, without loss or duplication of data.

Question 15

Question 16

Topic 3.2: The Network Layer

*Network Layer
The Network layer is located below the Transport layer. This layer determines the path on which data will be transferred or routed between networks.

Topic 3.2.1: Network Map

*A Network Map
Network-layer protocols create and maintain a "network map" by blending information about the connections available to each network device. Network devices then use this map for processes like determining a path through the network and switching packets.

*Choosing a Path
Using the network map, the Network layer can determine the best path for information to take to arrive at a particular destination.

Topic 3.2.2: Addressing

*Network Addresses
The Network layer manages the network addresses that are used to identify each network link. Routers exchange information about the paths available on a network using these addresses.

Question 17

Question 18

Topic 3.3: The Data Link Layer

*Data Link Layer
The layer below the Network layer is the Data Link layer.
The Data Link layer focuses on the transmission of data using physical addressing across a physical link.

Topic 3.3.1: Functions

*Data Link Functions
The tasks handled by the Data Link layer are:
  • Physical addressing
  • Error notification
  • Topology of the network
  • Control of data flow


*Converting Data
The Data Link layer converts information from the Network layer into frames.
The Physical layer then transmits the data along the physical media.

*Data Link Operations
Ethernet and Token Ring are examples of network topologies that offer the functionality defined on the Data Link layer.
Bridges also operate at this layer.

Topic 3.3.2: Sublayers

*Two Sublayers
The Data Link layer is divided into two sublayers — Logical Link Control and Media Access Control.

Question 19

Question 20

Topic 3.4: The Physical Layer

*Physical Layer
The Physical layer is the bottom layer of the OSI reference model, closest to the network interface.
It activates and maintains the physical link between devices.

Topic 3.4.1: Elements

*Physical Elements
Elements of the Physical layer include wiring, repeaters, and signals.

*Encoding and Decoding
The Physical layer encodes and decodes digital bits to communicate directly with network devices.

Topic 3.4.2: Specifications

*Physical Specifications
Specifications for the Physical layer include items such as maximum transmission distance, physical connectors, and voltage levels.
These specifications may be LAN or WAN specific.

Question 21

Question 22

Question 23

Question 24

Question 25


* Exercise 1
Try working with the OSI reference model.

Examine the following table
Step Action
1 List the seven layers of the OSI reference model.
2 Which networking tasks are associated with each layer?
Place each task with its appropriate layer.
3 Examine the model as a whole. Notice how the networking tasks progress as you move up and down the reference model.
Remember, knowledge of the OSI reference model is integral to understanding data transport and networking tasks.
4 Now that you are familiar with all of the layers in the model, list as many protocols as you can.
Put each protocol with its associated layer.
5 For more information on the OSI reference model, visit Cisco's Web site or do an Internet search.


Topic 3.5: Unit 3 Summary

In this unit, you learned about the lower layers of the OSI reference model which deal with data transport and hardware configuration. These lower layers are Transport, Network, Data Link, and Physical.
You learned the tasks and protocols associated with each of these layers, and saw how each layer provides services required for the networking process.
In the next unit, you'll learn why the OSI reference model is so important and why the industry uses a layered model.

Unit 4. Using a Layered Model



The previous units in this course covered what makes up the OSI reference model. You saw that the model is divided into upper layers which service applications and lower layers which handle data transport.
In this unit, you learn why the industry developed the OSI reference model. You see advantages of a layered model as presented in theory, and the limitations of a layered model as implemented in practice.

After completing this unit, you should be able to:
  • Discuss the advantages of using a layered model

  • List five reasons the industry uses a layered model

  • Explain the limitations of a layered model in practice


This unit provides information that is relevant to the following CCNA exam objective:
  • Identify at least three reasons why the industry uses a layered model


Topic 4.1: Developing a Framework

*Development Timeline
Developing a framework for dividing network functions began around the same time the first mainframe networks came into use.

Topic 4.1.1: Communication Needs

*Essential Communication
As more and more vendors began to develop networking devices, it became necessary for these devices to be able to communicate with each other.

Topic 4.1.2: Using the OSI Model

*A Conceptual Model
The OSI reference model itself does not provide a way for devices to communicate. Instead, it is a conceptual model which allows for the development of protocols which enable devices to communicate.

Topic 4.1.3: Protocols

*What is a Protocol?
A protocol is nothing more than a set of rules that spell out how devices communicate. Protocols can be implemented on one or more layers of the OSI reference model.

*Layer Management
Understanding the tasks each layer completes and the way layers interact with each other is crucial to understanding the networking process and being able to troubleshoot problems as they arise.

*Layer Interfacing
Each layer in the OSI works with the layer immediately above and below it.
Each layer is also able to interface with its peer layer on another system.

Topic 4.1.4: Services

*Providing Services
Layers interact to provide services to each other.
Each layer is capable of handling several service requests simultaneously.

Question 26

Topic 4.2: Advantages of a Layered Model

*Layer Advantages
The advantages of using a layered model are:
  • Specialization
  • Independence
  • Interoperability
  • Generalization
  • Troubleshooting
Let's look at each one.

Topic 4.2.1: Specialization

*Focusing on Functions
Using a layered model assists in the development of networking products. As each product is developed, developers know which functions are handled by each layer.
Thus, they can focus on the tasks of a single layer, knowing other functions will be handled by different layers. This focus encourages specialization.

Topic 4.2.2: Independence

*Separating Tasks
The OSI reference model assigns tasks to a particular layer. Developers are then free to develop protocols which accomplish those tasks.
Separating tasks and protocols by layer allows developers to change protocols associated with one layer without affecting protocols associated with another layer.

Topic 4.2.3: Interoperability

*A Modular Design
The OSI reference model is modular in nature. This modular design encourages developers to adhere to the specifications of each layer.
Using the specifications of the model allows developers to create devices and protocols which are compatible with other networking products.

Topic 4.2.4: Generalization

*Encouraging Growth
The OSI reference model outlines the tasks associated with networking. It does not specify the protocols or devices which must be used to accomplish those tasks.
By sticking to generals rather than specifics, the OSI reference model encourages continued growth and creativity.

Topic 4.2.5: Troubleshooting

*Simplification
The OSI reference model breaks down the complex, interrelated tasks of networking into smaller subsets. The smaller subsets make it easier to learn and develop networking devices and protocols. It also makes it easier to troubleshoot problems when they arise.

Question 27

Question 28

Topic 4.3: Implementing a Layered Model

*A Concept Only
While the OSI reference model has a clear separation of networking duties, it is a conceptual model.
In reality, not all networking technologies fit neatly into a single layer.

*Thinking Outside the Model
For example, TCP/IP works on both the Network and Transport layers.
In addition, products developed prior to or during the evolution of the OSI reference model may not conform precisely to each layer's assigned tasks.

*A Development Guideline
While some of the protocols used in networking don't fit neatly into the model, most vendors have agreed to use the OSI reference model as a guideline when developing networking products and services.

Question 29

Question 30


* Exercise 1
Try working with the concept of a layered model.

Examine the following table
Step Action
1 List 5 reasons why the industry uses a layered model.
2 Match the reasons for using a layered model with the principles of network design?
Do the two work to achieve the same goal?
3 Talk to a network administrator and a developer.
Discuss with them how using a layered model helps/hinders their work.
How much does an understanding of the model help them achieve their goals?
4 For more information on using a layered model, visit the ISO or Cisco Web site.


Topic 4.4: Unit 4 Summary

In this unit, you learned why the industry uses a layered model. Reasons for using a layered model include specialization, independence, generalization, interoperability, and troubleshooting.
You also learned that not all networking technologies fit neatly into a single layer. However, while some of the protocols used in networking don't conform precisely into the model, it still provides a framework for developing networking technology.
This course explored the OSI reference model and the attributes of its layers. This information will be integral as you continue in this Cisco series.

 

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