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CCNA Study Notes1(1) |
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| CCNA Study Notes1(1) |
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作者：佚名 文章来源：不详更新时间：2006-5-30 8:37:50  |
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1. Identify and describe the functions of each of the seven layers of the OSI reference model.
Physical Layer
The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems. Physical layer specifications define such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and the physical connectors to be used.
Data Link Layer
The data link layer provides reliable transit of data across a physical network link. Different data link layer specifications define different network and protocol characteristics, including the following:
Physical addressing -- Physical addressing (as opposed to network addressing) defines how devices are addressed at the data link layer.
Network topology -- Data link layer specifications often define how devices are to be physically connected (such as in a bus or a ring topology).
Error notification -- Error notification involves alerting upper layer protocols that a transmission error has occurred.
Sequencing of frames -- Sequencing of data frames involves the reordering of frames that are transmitted out of sequence.
Flow control -- Flow control involves moderating the transmission of data so that the receiving device is not overwhelmed with more traffic than it can handle at one time.
The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the data link layer into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC).
Network Layer
The network layer provides routing and related functions that allow multiple data links to be combined into an internetwork. This is accomplished by the logical addressing (as opposed to the physical addressing) of devices. The network layer supports both connection-oriented and connectionless service from higher-layer protocols.
Transport Layer
The transport layer implements reliable internetwork data transport services that are transparent to upper layers. Transport layer functions typically include the following:
Flow control -- Flow control manages data transmission between devices so that the transmitting device does not send more data than the receiving device can process.
Multiplexing -- Multi
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plexing allows data from several applications to be transmitted onto a single physical link.
Virtual circuit management -- Virtual circuits are established, maintained, and terminated by the transport layer.
Error checking and recovery -- Error checking involves various mechanisms for detecting transmission errors. Error recovery involves taking an action (such as requesting that data be retransmitted) to resolve any errors that occur.
Some examples of transport layer implementations follow:
Transmission Control Protocol (TCP), Name Binding Protocol (NBP), OSI transport protocols
Session Layer
The session layer establishes, manages, and terminates communication sessions between presentation layer entities. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer. Some examples of session layer implementations follow:
Apple ZIP, DEC SCP, NFS, SQL, RPC, X Windows, ASP
Presentation Layer
The presentation layer provides a variety of coding and conversion functions that are applied to application layer data. These functions ensure that information sent from the application layer of one system will be readable by the application layer of another system. Some examples of presentation layer coding and conversion schemes follow:
Common data representation formats -- The use of standard image, sound, and video formats allow the interchange of application data between different types of computer systems.
Conversion of character representation formats -- Conversion schemes are used to exchange information with systems using different text and data representations (such as EBCDIC and ASCII).
Common data compression schemes -- The use of standard data compression schemes allows data that is compressed at the source device to be properly decompressed at the destination.
Common data encryption schemes -- The use of standard data encryption schemes allows data encrypted at the source device to be properly unencrypted at the destination.
Presentation layer implementations are not typically associated with a particular protocol stack. Some well known standards follow:
Data: ASCII, EBCDIC, Encryption
Visual Imaging: PICT, TIFF, GIF, JPEG
Vi
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deo: MIDI, MPEG, QuickTime
Application Layer
The application layer interacts with software applications that implement a communicating component. Application layer functions typically include the following:
Identifying communication partners -- The application layer identifies and determines the availability of communication partners for an application with data to transmit.
Determining resource availability -- The application layer must determine whether sufficient network resources for the requested communication are available.
Synchronizing communication -- Communication between applications requires cooperation that is managed by the application layer.
The application layer is the OSI layer closest to the end user. That is, both the OSI application layer and the user interact directly with the software application. Some examples of application layer implementations follow:
TCP/IP applications -- TCP/IP applications are protocols in the Internet Protocol suite, such as Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).
OSI applications -- OSI applications are protocols in the OSI suite such as File Transfer, Access, and Management (FTAM), Virtual Terminal Protocol (VTP), and Common Management Information Protocol (CMIP).
2. Describe connection-oriented network service and connectionless network service and identify the key differences between them.
Connection-Oriented Network Service
Connection-oriented service involves three phases:
Connection establishment -- During the connection establishment phase, a single path between the source and destination systems is determined. Network resources are typically reserved at this time to ensure a consistent grade of service (such as a guaranteed throughput rate).
Data transfer -- During the data transfer phase, data is transmitted sequentially over the path that has been established. Data always arrives at the destination system in the order in which it was sent.
Connection termination -- During the connection termination phase, an established connection that is no longer needed is terminated. Further communication between the source and destination systems requires that a new connection be established.
Connection-oriented service has two significant disadvantages as compared to connectionless network service:
Static p
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ath selection -- Because all traffic must travel along the same static path, a failure anywhere along that path causes the connection to fail.
Static reservation of network resources -- A guaranteed rate of throughput requires the commitment of resources that cannot be shared by other network users. Unless full, uninterrupted throughput is required for the communication, bandwidth is not used efficiently.
Connection-oriented services are useful for transmitting data from applications that are intolerant of delays and packet re-sequencing. Voice and video applications are typically based on connection-oriented services.
Connectionless Network Service
Connectionless network service does not predetermine the path from the source to the destination system, nor are packet sequencing, data throughput, and other network resources guaranteed. Each packet must be completely addressed because different paths through the network might be selected for different packets, based on a variety of influences. Each packet is transmitted independently by the source system and is handled independently by intermediate network devices. Connectionless service offers two important advantages over connection-oriented service:
Dynamic path selection -- Because paths are selected on a packet-by-packet basis, traffic can be routed around network failures.
Dynamic bandwidth allocation -- Bandwidth is used more efficiently because network resources are not allocated bandwidth that they are not going to use.
Connectionless services are useful for transmitting data from applications that can tolerate some delay and re-sequencing. Data-based applications are typically based on connectionless service.
3. Describe data link addresses and network addresses and identify the key differences between them.
Data Link Layer Addresses
A data link layer address uniquely identifies each physical network connection of a network device. Data link addresses are sometimes referred to as physical or hardware addresses. Data l[1] [2] [3] 下一页
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