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8/14/2019 Understand No Restriction

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Lesson 3

Understanding the Host-to-Host Communications Model

OverviewThe Open Systems Interconnection (OSI) reference model was created to help define how

network processes function in general, including the various components of networks and

transmission of data. Understanding the structure and purpose of the OSI model is central to

understanding how one host communicates with another. This lesson introduces the OSI model

and describes each of its layers.

Objectives

Upon completing this lesson, you will be able to describe the layers of the OSI model anddescribe how to classify devices and their functions according to their layer in the OSI model.

This ability includes being able to meet these objectives:

n Identify the requirements of a host-to-host communications model

n Define the purpose of the OSI reference model

n Define the characteristics, functions, and purposes of each of the OSI layers

n Describe the process of encapsulation and de-encapsulation

n Describe how peer-to-peer communication works

n List the purposes and functions of the TCP/IP suite in data communications


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1-44 Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0 2007 Cisco Systems, Inc.

Understanding Host-to-Host CommunicationsHost-to-host communications requires a consistent model. The model addresses hardware,

software, and data transmission. This topic describes the purpose of this model.

2007 Cisco Systems, Inc. All rights reserved. ICND1 v1.01-2

Understanding Host-to-Host

Communications

Older model

Proprietary

Application and combinations software controlled by onevendor

Standards-based model

Multivendor software

Layered approach

The early development of networks was chaotic in many ways. The original host-to-host

communications models were proprietary with each vendor controlling its own application and

embedded communication software. An application written by one vendor would not function

on a network developed by another vendor.

Business drivers and technology advances led a push for a multivendor solution. The first step

was to separate application software from communications software. This allowed new

communications technologies to be implemented without requiring new applications, but it still

required a single-vendor solution for communications software and hardware.

It became apparent that a multivendor solution for communications software and hardware

would require a layered approach with clearly defined rules for interlayer interaction. Within a

layered model, hardware vendors could design hardware and software to support emerging

physical-level technologies (that is, Ethernet, Token Ring, Frame Relay, and so on) while other

vendors could write software to be used by network operating systems that control host

communications.


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2007 Cisco Systems, Inc. Building a Simple Network 1-45

The OSI Reference ModelThe OSI reference model provides a means of describing how data is transmitted over a

network. The model addresses hardware, software, and data transmission. This topic describes

the purpose of the OSI model.


Why a Layered Network Model?

Reduces complexity

Standardizes interfaces

Facilitates modular engineering

Ensures interoperabletechnology

Accelerates evolution

Simplifies teaching and learning

The early development of networks was chaotic in many ways. The early 1980s saw

tremendous increases in the number and sizes of networks. As companies realized that they

could save money and gain productivity by using networking technology, they added networks

and expanded existing networks as rapidly as new network technologies and products wereintroduced.

By the mid-1980s, companies began to experience difficulties from all of the expansions they

had made. It became more difficult for networks using different specifications and

implementations to communicate with one another. The companies realized that they needed to

move away from proprietary networking systemsthose systems that are privately developed,

owned, and controlled. In the computer industry, proprietary is the opposite of open.

Proprietary means that one company or a small group of companies controls all use of the

technology. Open means that use of the technology is available free to the public.

To address the problem of networks being incompatible and unable to communicate with one

another, the International Organization for Standardization (ISO) researched different network

schemes. As a result of this research, the ISO created a model that would help vendors create

networks that would be compatible with, and operate with, other networks.

The OSI reference model, released in 1984, was the descriptive scheme that the ISO created. It

provided vendors with a set of standards that ensured greater compatibility and interoperability

between the various types of network technologies produced by companies around the world.

Although other models exist, most network vendors today relate their products to the OSI

reference model, especially when they want to educate customers on the use of their products.


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The OSI model is considered the best tool available for teaching people about sending and

receiving data on a network.

The OSI reference model separates network functions into seven categories. This separation of

networking functions is called layering. The OSI reference model has seven numbered layers,

each illustrating a particular network function. The OSI model defines the network functions

that occur at each layer. More importantly, the OSI model facilitates an understanding of how

information travels throughout a network. In addition, the OSI model describes how data

travels from application programs (for example, spreadsheets) through a network medium, to

an application program located in another computer, even if the sender and receiver are

connected using different network media.

The OSI reference model provides a number of benefits in understanding how networks

function, by doing the following:

n Reducing complexity: The OSI model breaks network communications into smaller,

simpler parts.

n Standardizing interfaces: The OSI model standardizes network components to allow

multiple-vendor development and support.

n Facilitating modular engineering: The OSI model allows different types of network

hardware and software to communicate with one another.

n Ensuring interoperable technology: The OSI model prevents changes in one layer from

affecting the other layers, allowing for quicker development.

n Accelerating evolution: The OSI model provides for effective updates and improvements

to individual components without affecting other components or having to rewrite the

entire protocol.

n Simplifying teaching and learning: The OSI model breaks network communications into

smaller components to make learning easier.


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The OSI Model Layers and Their FunctionsEach OSI layer has a specific function and associated software or devices. This topic describes

each layer and its functions.


The Seven Layers of the OSI Model

Layer 1: The Physical Layer

The physical layer defines the electrical, mechanical, procedural, and functional specifications

for activating, maintaining, and deactivating the physical link between end systems.

Characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum

transmission distances, physical connectors, and other similar attributes are defined by physical

layer specifications.


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The Seven Layers of the OSI Model (Cont.)

Layer 2: The Data Link Layer

The data link layer defines how data is formatted for transmission and how access to the

physical media is controlled. This layer also typically includes error detection and correction to

ensure reliable delivery of the data.


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Layer 3: The Network Layer

The network layer provides connectivity and path selection between two host systems that may

be located on geographically separated networks. The growth of the Internet has increased the

number of users accessing information from sites around the world, and the network layer is the

layer that manages this connectivity.


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Layer 4: The Transport Layer

The transport layer segments data from the system of the sending host and reassembles the data

into a data stream on the system of the receiving host. For example, business users in large

corporations often transfer large files from field locations to a corporate site. Reliable delivery

of the files is important, so the transport layer will break down large files into smaller segments

that are less likely to incur transmission problems.

The boundary between the transport layer and the session layer can be thought of as the

boundary between application protocols and data-flow protocols. Whereas the application, presentation, and session layers are concerned with application issues, the lower four layers are

concerned with data transport issues.

The transport layer shields the upper layers from transport implementation details. Specifically,

issues such as reliability of transport between two hosts are assigned to the transport layer. In

providing a communication service, the transport layer establishes, maintains, and properly

terminates virtual circuits. Transport error detection and recovery and information flow control

ensure reliable service.


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Layer 6: The Presentation Layer

The presentation layer ensures that the information sent at the application layer of one system is

readable by the application layer of another system. For example, a PC program communicates

with another computer, one using extended binary coded decimal interchange code (EBCDIC)

and the other using American Standard Code for Information Interchange (ASCII) to represent

the same characters. If necessary, the presentation layer translates between multiple data

formats by using a common format.


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Encapsulation and De-EncapsulationInformation that is to be transmitted over a network must undergo a process of conversion at

both the sending end and the receiving end of the communication. That conversion process is

known as encapsulation and de-encapsulation of data. This topic describes the encapsulation

and de-encapsulation processes.


Data Encapsulation

Encapsulation

The information sent on a network is referred to as data or data packets. If one computer wants

to send data to another computer, the data must first be packaged by a process called

encapsulation. Encapsulation wraps data with the necessary protocol information before

network transit. As the data moves down through the layers of the OSI model, each OSI layer

adds a header (and a trailer, if applicable) to the data before passing it down to a lower layer.

The headers and trailers contain control information for the network devices and receiver to

ensure proper delivery of the data and to ensure that the receiver can correctly interpret the

data.

The figure illustrates how encapsulation occurs. It shows the manner in which data travels

through the layers. The following steps occur to encapsulate data:

Step 1 The user data is sent from an application to the application layer.

Step 2 The application layer adds the application layer header (Layer 7 header) to the user

data. The Layer 7 header and the original user data become the data that is passed

down to the presentation layer.

Step 3 The presentation layer adds the presentation layer header (Layer 6 header) to the

data. This then becomes the data that is passed down to the session layer.


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Data De-Encapsulation

De-Encapsulation

When the remote device receives a sequence of bits, the physical layer at the remote device

passes the bits to the data link layer for manipulation. The data link layer performs the

following steps:

Step 1 The data link layer checks the data-link trailer (the FCS) to see if the data is in error.

Step 2 If the data is in error, it may be discarded, and the data link layer may ask for the

data to be retransmitted.

Step 3 If the data is not in error, the data link layer reads and interprets the control

information in the data-link header.

Step 4 The data link layer strips the data-link header and trailer, and then passes the

remaining data up to the network layer based on the control information in the data-

link header.

This process is referred to as de-encapsulation. Each subsequent layer performs a similar de-

encapsulation process.

Example: Receiving a Package

The de-encapsulation process is similar to that of reading the address on a package to see if it is

for you, and then removing the contents of the package if it is addressed to you.


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Peer-to-Peer CommunicationSo that data packets can travel from the source to the destination, each layer of the OSI model

at the source must communicate with its peer layer at the destination. This topic describes the

process of peer-to-peer communication.


Peer-to-Peer Communication

During the process of peer-to-peer communication, the protocols at each layer exchange

packets of information called protocol data units (PDUs) between peer layers.

These data packets originate at a source on a network and then travel to a destination. Eachlayer depends on the OSI layer below it to provide a service. To perform its service function,

the lower layer uses encapsulation to put the protocol data unit (PDU) from the upper layer into

lower layer data field. Each layer then adds whatever headers the layer needs to perform its

function. As the data moves down from Layer 7 through Layer 2 of the OSI model, additional

headers are added.

The network layer provides a service to the transport layer, and the transport layer presents data

to the network subsystem. The network layer moves the data through the Internet by

encapsulating the data and attaching a header to create a packet (the Layer 3 PDU). The header

contains information required to complete the transfer, such as source and destination logical

addresses.

The data link layer provides a service to the network layer by encapsulating the network layer

packet in a frame (the Layer 2 PDU). The frame header contains the physical addresses

required to complete the data link functions, and the frame trailer contains the FCS.

The physical layer provides a service to the data link layer, encoding the data-link frame into a

pattern of 1s and 0s (bits) for transmission on the medium (usually a wire) at Layer 1.


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TCP/IP SuiteThe TCP/IP suitewhose name is actually a combination of just two individual protocols,

Transmission Control Protocol (TCP) and Internet Protocol (IP)is divided into layers, each of

which performs specific functions in the data communication process. This topic describes how

the layers of TCP/IP are organized into a stack.


Defines four layers

Uses different names for Layers 1through 3

Combines Layers 5 through 7 into

single application layer

TCP/IP Stack

The TCP/IP suite was developed at approximately the same time as the OSI model. Like the

OSI model, the TCP/IP suite is a means of organizing components in an order that reflects their

functions in relation to one another. The components, or layers, of the TCP/IP stack are asfollows:

n Network access layer: This layer covers the same processes as the two lower OSI layers:

Physical layer: The physical layer defines the electrical, mechanical, procedural,

and functional specifications for activating, maintaining, and deactivating the

physical link between end systems. Characteristics such as voltage levels, timing of

voltage changes, physical data rates, maximum transmission distances, physical

connectors, and other similar attributes are defined by physical layer specifications.

Data link layer: The data link layer defines how data is formatted for transmission

and how access to the network is controlled.

n Internet layer: This layer provides routing of data from the source to the destination by

defining the packet and the addressing scheme, moving data between the data link and

transport layers, routing packets of data to remote hosts, and performing fragmentation and

reassembly of data packets.

n Transport layer: The transport layer is the core of the TCP/IP architecture, providing

communication services directly to the application processes running on network hosts.


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TCP/IP Stack vs. the OSI Model

Both the OSI model and the TCP/IP stack were developed, by different organizations, at

approximately the same time as a means to organize and communicate the components that

guide the transmission of data. The layers of the TCP/IP stack correspond to the layers of the

OSI model:

n The TCP/IP network access layer roughly corresponds to the OSI physical and data link

layers and is concerned primarily with interfacing with network hardware and accessing the

transmission media.

Note Because the TCP/IP network access layer contains both the OSI data link and physicallayers, it has become common to modify the classic four-layer TCP/IP module into a five-

layer module. In this course, the five-layer model is used.

n The TCP/IP Internet layer corresponds closely to the network layer of the OSI model and

deals with the addressing of and routing between network devices.

n The TCP/IP transport layer, like the OSI transport layer, provides the means for multiple

host applications to access the network layer, either in a best-effort mode or through a

reliable delivery mode.

n The TCP/IP application layer addresses applications that communicate with the lower

layers and corresponds to the separate application, presentation, and session layers of the

OSI model. The additional layers of the OSI model provide some additional organization offeatures related to applications.


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SummaryThis topic summarizes the key points that were discussed in this lesson.


Summary

The OSI reference model defines the network functions that occurat each layer.

The physical layer defines the electrical, mechanical, procedural,and functional specifications for activating, maintaining, anddeactivating the physical link between end systems.

The data link layer defines how data is formatted for transmissionand how access to the physical media is controlled.

The network layer provides connectivity and path selectionbetween two host systems that may be located on geographicallyseparated networks.


Summary (Cont.)

The transport layer segments data from the system of the sendinghost and reassembles the data into a data stream on the systemof the receiving host.

The session layer establishes, manages, and terminates sessionsbetween two communicating hosts.

The presentation layer ensures that the information sent at theapplication layer of one system is readable by the applicationlayer of another system.

The application layer provides network services to theapplications of the user, such as e-mail, file transfer, and terminalemulation.


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Summary (Cont.)

The information sent on a network is referred to as data or datapackets. If one computer wants to send data to another computer,the data must first be packaged by a process called

encapsulation. When the remote device receives a sequence of bits, the physical

layer at the remote device passes the bits to the data link layer formanipulation. This process is referred to as de-encapsulation.


Summary (Cont.)

TCP/IP is now the most widely used protocol for a number ofreasons, including its flexible addressing scheme, its usability bymost operating systems and platforms, its many tools and utilities,and the need to use it to connect to the Internet.

The components of the TCP/IP stack are the network access,Internet, transport, and application layers.

The OSI model and the TCP/IP stack are similar in structure andfunction, with correlation at the physical, data link, network, andtransport layers. The OSI model divides the application layer ofthe TCP/IP stack into three separate layers.

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