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CCNA Study Notes1(1) |
| 来源:www.stu88.com 时间:2006-04-01 |
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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 -- Multiplexing 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: 共10页: 1 [2] [3] [4] [5] [6] [7] [8] [9] [10] 
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