Understand No Restriction
This document was uploaded by user and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this DMCA report form. Report DMCA
Overview
Download & View Understand No Restriction as PDF for free.
More details
- Words: 3,966
- Pages: 20
Lesson 3
Understanding the Hostto Host Communications Model Overview The 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 and describe 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 hosttohost 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 deencapsulation
n
Describe how peertopeer communication works
n
List the purposes and functions of the TCP/IP suite in data communications
Understanding HosttoHost Communications Hosttohost communications requires a consistent model. The model addresses hardware, software, and data transmission. This topic describes the purpose of this model.
Understanding HosttoHost Communications
§ Older model – Proprietary – Application and combinations software controlled by one vendor § Standardsbased model – Multivendor software – Layered approach
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—12
The early development of networks was chaotic in many ways. The original hosttohost 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 singlevendor 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 physicallevel 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.
144
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
The OSI Reference Model The 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 interoperable technology § Accelerates evolution § Simplifies teaching and learning
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—13
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 were introduced. By the mid1980s, 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 systems—those 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. © 2007 Cisco Systems, Inc.
Building a Simple Network
145
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:
146
n
Reducing complexity: The OSI model breaks network communications into smaller, simpler parts.
n
Standardizing interfaces: The OSI model standardizes network components to allow multiplevendor 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.
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
The OSI Model Layers and Their Functions Each 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
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—14
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.
© 2007 Cisco Systems, Inc.
Building a Simple Network
147
The Seven Layers of the OSI Model (Cont.)
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—15
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.
148
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
The Seven Layers of the OSI Model (Cont.)
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—16
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.
© 2007 Cisco Systems, Inc.
Building a Simple Network
149
The Seven Layers of the OSI Model (Cont.)
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—17
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 dataflow 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.
150
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
The Seven Layers of the OSI Model (Cont.)
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—18
Layer 5: The Session Layer The session layer establishes, manages, and terminates sessions between two communicating hosts. The session layer also synchronizes dialog between the presentation layers of the two hosts and manages their data exchange. For example, web servers have many users, so there are many communication processes open at a given time. It is important, then, to keep track of which user communicates on which path. In addition to session regulation, the session layer offers provisions for efficient data transfer, class of service (CoS), and exception reporting of session layer, presentation layer, and application layer problems.
© 2007 Cisco Systems, Inc.
Building a Simple Network
151
The Seven Layers of the OSI Model (Cont.)
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—19
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.
152
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
The Seven Layers of the OSI Model (Cont.)
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—110
Layer 7: The Application Layer The application layer is the OSI layer that is closest to the user. This layer provides network services to the applications of the user, such as email, file transfer, and terminal emulation. The application layer differs from the other layers in that it does not provide services to any other OSI layer, but only to applications outside the OSI model. The application layer establishes the availability of intended communication partners and synchronizes and establishes agreement on procedures for error recovery and control of data integrity.
© 2007 Cisco Systems, Inc.
Building a Simple Network
153
Encapsulation and DeEncapsulation Information 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 deencapsulation of data. This topic describes the encapsulation and deencapsulation processes.
Data Encapsulation
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—111
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:
154
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.
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
Step 4
The session layer adds the session layer header (Layer 5 header) to the data. This then becomes the data that is passed down to the transport layer.
Step 5
The transport layer adds the transport layer header (Layer 4 header) to the data. This then becomes the data that is passed down to the network layer.
Step 6
The network layer adds the network layer header (Layer 3 header) to the data. This then becomes the data that is passed down to the data link layer.
Step 7
The data link layer adds the data link layer header and trailer (Layer 2 header and trailer) to the data. A Layer 2 trailer is usually the frame check sequence (FCS), which is used by the receiver to detect whether the data is in error. This then becomes the data that is passed down to the physical layer.
Step 8
The physical layer then transmits the bits onto the network media.
Example: Sending a Package through a Postal Service Encapsulation works very similarly to sending a package through a postal service. The first step is to put the contents of the package into a container. Next, you write the address of the location to which you want to send the package on the outside of the container. Then you put the addressed package into the postal service collection bin, and the package begins its route toward its destination.
© 2007 Cisco Systems, Inc.
Building a Simple Network
155
Data DeEncapsulation
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—112
DeEncapsulation 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 datalink 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 datalink header.
Step 4
The data link layer strips the datalink 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 deencapsulation. Each subsequent layer performs a similar de encapsulation process.
Example: Receiving a Package The deencapsulation 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.
156
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
PeertoPeer Communication So 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 peertopeer communication.
PeertoPeer Communication
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—113
During the process of peertopeer 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. Each layer 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 datalink frame into a pattern of 1s and 0s (bits) for transmission on the medium (usually a wire) at Layer 1.
© 2007 Cisco Systems, Inc.
Building a Simple Network
157
TCP/IP Suite The TCP/IP suite—whose 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.
TCP/IP Stack
§ Defines four layers § Uses different names for Layers 1 through 3 § Combines Layers 5 through 7 into single application layer
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—114
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 as follows: n
158
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.
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
n
Application layer: The application layer provides applications for file transfer, network troubleshooting, and Internet activities and supports network application programming interfaces (APIs) that allow programs that have been created for a particular operating system to access the network.
Note
© 2007 Cisco Systems, Inc.
Although this course refers to the TCP/IP stack, it has become common in the industry to shorten this term to “IP stack.”
Building a Simple Network
159
TCP/IP Stack vs. the OSI Model
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—115
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
160
Because the TCP/IP network access layer contains both the OSI data link and physical layers, it has become common to modify the classic fourlayer TCP/IP module into a five layer module. In this course, the fivelayer 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 besteffort 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 of features related to applications.
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
Summary This topic summarizes the key points that were discussed in this lesson.
Summary § The OSI reference model defines the network functions that occur at each layer. § The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. § The data link layer defines how data is formatted for transmission and how access to the physical media is controlled. § The network layer provides connectivity and path selection between two host systems that may be located on geographically separated networks.
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—116
Summary (Cont.) § 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. § The session layer establishes, manages, and terminates sessions between two communicating hosts. § The presentation layer ensures that the information sent at the application layer of one system is readable by the application layer of another system. § The application layer provides network services to the applications of the user, such as email, file transfer, and terminal emulation.
© 2007 Cisco Systems, Inc. All rights reserved.
© 2007 Cisco Systems, Inc.
ICND1 v1.0—117
Building a Simple Network
161
Summary (Cont.) § 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. § 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. This process is referred to as deencapsulation.
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—118
Summary (Cont.) § TCP/IP is now the most widely used protocol for a number of reasons, including its flexible addressing scheme, its usability by most 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 and function, with correlation at the physical, data link, network, and transport layers. The OSI model divides the application layer of the TCP/IP stack into three separate layers.
© 2007 Cisco Systems, Inc. All rights reserved.
162
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
ICND1 v1.0—119
© 2007 Cisco Systems, Inc.
Understanding the Hostto Host Communications Model Overview The 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 and describe 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 hosttohost 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 deencapsulation
n
Describe how peertopeer communication works
n
List the purposes and functions of the TCP/IP suite in data communications
Understanding HosttoHost Communications Hosttohost communications requires a consistent model. The model addresses hardware, software, and data transmission. This topic describes the purpose of this model.
Understanding HosttoHost Communications
§ Older model – Proprietary – Application and combinations software controlled by one vendor § Standardsbased model – Multivendor software – Layered approach
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—12
The early development of networks was chaotic in many ways. The original hosttohost 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 singlevendor 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 physicallevel 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.
144
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
The OSI Reference Model The 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 interoperable technology § Accelerates evolution § Simplifies teaching and learning
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—13
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 were introduced. By the mid1980s, 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 systems—those 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. © 2007 Cisco Systems, Inc.
Building a Simple Network
145
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:
146
n
Reducing complexity: The OSI model breaks network communications into smaller, simpler parts.
n
Standardizing interfaces: The OSI model standardizes network components to allow multiplevendor 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.
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
The OSI Model Layers and Their Functions Each 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
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—14
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.
© 2007 Cisco Systems, Inc.
Building a Simple Network
147
The Seven Layers of the OSI Model (Cont.)
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—15
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.
148
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
The Seven Layers of the OSI Model (Cont.)
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—16
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.
© 2007 Cisco Systems, Inc.
Building a Simple Network
149
The Seven Layers of the OSI Model (Cont.)
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—17
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 dataflow 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.
150
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
The Seven Layers of the OSI Model (Cont.)
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—18
Layer 5: The Session Layer The session layer establishes, manages, and terminates sessions between two communicating hosts. The session layer also synchronizes dialog between the presentation layers of the two hosts and manages their data exchange. For example, web servers have many users, so there are many communication processes open at a given time. It is important, then, to keep track of which user communicates on which path. In addition to session regulation, the session layer offers provisions for efficient data transfer, class of service (CoS), and exception reporting of session layer, presentation layer, and application layer problems.
© 2007 Cisco Systems, Inc.
Building a Simple Network
151
The Seven Layers of the OSI Model (Cont.)
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—19
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.
152
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
The Seven Layers of the OSI Model (Cont.)
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—110
Layer 7: The Application Layer The application layer is the OSI layer that is closest to the user. This layer provides network services to the applications of the user, such as email, file transfer, and terminal emulation. The application layer differs from the other layers in that it does not provide services to any other OSI layer, but only to applications outside the OSI model. The application layer establishes the availability of intended communication partners and synchronizes and establishes agreement on procedures for error recovery and control of data integrity.
© 2007 Cisco Systems, Inc.
Building a Simple Network
153
Encapsulation and DeEncapsulation Information 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 deencapsulation of data. This topic describes the encapsulation and deencapsulation processes.
Data Encapsulation
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—111
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:
154
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.
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
Step 4
The session layer adds the session layer header (Layer 5 header) to the data. This then becomes the data that is passed down to the transport layer.
Step 5
The transport layer adds the transport layer header (Layer 4 header) to the data. This then becomes the data that is passed down to the network layer.
Step 6
The network layer adds the network layer header (Layer 3 header) to the data. This then becomes the data that is passed down to the data link layer.
Step 7
The data link layer adds the data link layer header and trailer (Layer 2 header and trailer) to the data. A Layer 2 trailer is usually the frame check sequence (FCS), which is used by the receiver to detect whether the data is in error. This then becomes the data that is passed down to the physical layer.
Step 8
The physical layer then transmits the bits onto the network media.
Example: Sending a Package through a Postal Service Encapsulation works very similarly to sending a package through a postal service. The first step is to put the contents of the package into a container. Next, you write the address of the location to which you want to send the package on the outside of the container. Then you put the addressed package into the postal service collection bin, and the package begins its route toward its destination.
© 2007 Cisco Systems, Inc.
Building a Simple Network
155
Data DeEncapsulation
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—112
DeEncapsulation 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 datalink 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 datalink header.
Step 4
The data link layer strips the datalink 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 deencapsulation. Each subsequent layer performs a similar de encapsulation process.
Example: Receiving a Package The deencapsulation 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.
156
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
PeertoPeer Communication So 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 peertopeer communication.
PeertoPeer Communication
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—113
During the process of peertopeer 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. Each layer 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 datalink frame into a pattern of 1s and 0s (bits) for transmission on the medium (usually a wire) at Layer 1.
© 2007 Cisco Systems, Inc.
Building a Simple Network
157
TCP/IP Suite The TCP/IP suite—whose 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.
TCP/IP Stack
§ Defines four layers § Uses different names for Layers 1 through 3 § Combines Layers 5 through 7 into single application layer
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—114
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 as follows: n
158
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.
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
n
Application layer: The application layer provides applications for file transfer, network troubleshooting, and Internet activities and supports network application programming interfaces (APIs) that allow programs that have been created for a particular operating system to access the network.
Note
© 2007 Cisco Systems, Inc.
Although this course refers to the TCP/IP stack, it has become common in the industry to shorten this term to “IP stack.”
Building a Simple Network
159
TCP/IP Stack vs. the OSI Model
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—115
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
160
Because the TCP/IP network access layer contains both the OSI data link and physical layers, it has become common to modify the classic fourlayer TCP/IP module into a five layer module. In this course, the fivelayer 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 besteffort 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 of features related to applications.
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
© 2007 Cisco Systems, Inc.
Summary This topic summarizes the key points that were discussed in this lesson.
Summary § The OSI reference model defines the network functions that occur at each layer. § The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. § The data link layer defines how data is formatted for transmission and how access to the physical media is controlled. § The network layer provides connectivity and path selection between two host systems that may be located on geographically separated networks.
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—116
Summary (Cont.) § 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. § The session layer establishes, manages, and terminates sessions between two communicating hosts. § The presentation layer ensures that the information sent at the application layer of one system is readable by the application layer of another system. § The application layer provides network services to the applications of the user, such as email, file transfer, and terminal emulation.
© 2007 Cisco Systems, Inc. All rights reserved.
© 2007 Cisco Systems, Inc.
ICND1 v1.0—117
Building a Simple Network
161
Summary (Cont.) § 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. § 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. This process is referred to as deencapsulation.
© 2007 Cisco Systems, Inc. All rights reserved.
ICND1 v1.0—118
Summary (Cont.) § TCP/IP is now the most widely used protocol for a number of reasons, including its flexible addressing scheme, its usability by most 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 and function, with correlation at the physical, data link, network, and transport layers. The OSI model divides the application layer of the TCP/IP stack into three separate layers.
© 2007 Cisco Systems, Inc. All rights reserved.
162
Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.0
ICND1 v1.0—119
© 2007 Cisco Systems, Inc.
Related Documents
Understand No Restriction
June 2020 0
Part 1 No Restriction
December 2019 7
Nokia 6610 Rus No Restriction
June 2020 0
Restriction Enzymes
November 2019 18
Restriction Enzyme.ppt
May 2020 3