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http:\\ciscocertifications.info
It is important that you read and study the "CCNA Concepts" portion of this study guide. We have identi-fied important "KEYPOINTS" in this section. Please ensure that you absolutely know and understandthese. You will find them in double lined boxes throughout the text.CCNA ConceptsOSI Reference
The OSI Model is the most important concept in the entire study guide, memorize it!! Many of the test
questions will probably be based upon your knowledge about what happens at the different layers.
OSI MODELLayerNameFunction
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Application Layer
Provides network services to user applications. Establishes program-to-
program communication. Identifies and establishes the availability of the
intended communication partner, and determines if sufficient resources
exist for the communication.
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Presentation Layer
Manages data conversion, compression, decompression, encryption, and
decryption. Provides a common representation of application data while
the data is in transit between systems. Standards include MPEG, MIDI,
PICT, TIFF, JPEG, ASCII, and EBCDIC.
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Session Layer
Responsible for establishing and maintaining communication sessions be-
tween applications. In practice, this layer is often combined with the Trans-
port Layer. Organizes the communication through simplex, half and full
duplex modes. Protocols include NFS, SQL, RPC, AppleTalk Session
Protocol (ASP) and XWindows.
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Transport Layer
Responsible for end-to-end integrity of data transmission. Hides details of
network dependent info from the higher layers by providing transparent
data transfer. The "window" works at this level to control how much in-
formation is transferred before an acknowledgement is required. This layer
segments and reassembles data for upper level applications into a data
stream. Port numbers are used to keep track for different conversations
crossing the network at the same time. Uses both connection-oriented and
connectionless protocols. Supports TCP, UDP and SPX.
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Network Layer
Routes data from one node to another. Sends data from the source network
to the destination network. This level uses a 2 part address to establish and
manages addressing, track device locations, and determines the best path to
use for moving data on the internetwork. Responsible for maintaining
routing tables. Routers operate at this level.
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Data Link Layer
Responsible for physically transmission of data from one node to another.
Handles error notification, network topology, flow control. Translates
messages from the upper layers into data frames and adds customized
headers containing the hardware destination and source address. Bridges
and switches operate at this layer.
Logical Link Control Sublayer Acts as a managing buffer between the
upper layers and the lower layers. Uses Source Service Access Points
(SSAPs) and Destination Service Access Points (DSAPs) to help the lower
layers talk to the Network layer. Responsible for timing, and flow control.
Media Access Control Sublayer Builds frames from the 1's and 0's that
the Physical layer picks up from the wire as a digital signal, and runs Cyclic
Redundancy Checksum (CRC) to assure that nothing was damaged in tran-
sit.
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Physical Layer
Manages putting data onto the network media and taking the data off.
Sends and receives bits. Communicates directly with communication me-
dia. Provides electrical and mechanical transmission capability.