Cisco® CCNA Exam Cram Notes : OSI And DOD Models

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1. Networking Fundamentals

1. OSI and DoD Models

1.1 OSI Models and its function

The 7 layers of OSI model are:

1. The Application Layer: Application layer is responsible for identifying and establishing the availability of intended communication partner and verifying sufficient resources exist for communication. Some of the important application layer protocols are: WWW, SMTP,FTP,etc.

2. The Presentation Layer: This layer is responsible for presenting the data in standard formats. This layer is responsible for data compression, decompression, encryption, and decryption. Some Presentation Layer standards are: JPEG, MPEG, MIDI, PICT, Quick Time, TIFF. The presentation layer of the OSI model handles the conversion of data between a Standards-based or platform independent formats to a format understood by the local application. This allows for data to be transported between devices and still be understood by applications on either end of the communication link.

The presentation layer performs the following functions:

  • Communication with the application layer above.
  • Translation of data conforming to cross-platform standards into formats understood by the local machine.
  • Communication with the session layer below.

Examples of Presentation Layer Functions

  • Conversion of a Sun .RAS raster graphic to JPG.
  • Conversion of ASCII to IBM EBCDIC
  • Conversion of .PICT on a MAC to .jpg
  • Conversion of .wav to .mp3

3.The Session Layer: Session Layer is responsible for co-ordinating communication between systems/nodes. The following are some of the session layer protocols and interfaces: a) Network File System (NFS),SQL,RPC(Remote Procedure Call), X-Windows, ASP, DNA SCP.

4. The Transport Layer: The Transport Layer is responsible for multiplexing upper-layer applications, session establishment, and tearing-down of virtual circuits. This layer does "flow control" to maintain data integrity. Flow Control prevents the problem of a sending host on one side of the connection overflowing the buffers in the receiving host.

5. The Network Layer: There can be several paths to send a packet from a given source to a destination. The primary responsibility of Network layer is to send packets from the source network to the destination network using a pre-determined methods. Routers work at Network layer.

Functions of Network Layer :

  • It translates logical network address into physical address. Concerned with circuit, message or packet switching.
  • Routers and gateways operate in the network layer. Mechanism is provided by Network Layer for routing the packets to final destination.
  • Connection services are provided including network layer flow control, network layer error control and packet sequence control.
  • Breaks larger packets into small packets.

6. The Data Link Layer: Data Link Layer is layer 2 of OSI reference model. This layer is divided into two sub-layers:

  • Logical Link Control (LLC) sub-layer.
  • Media Access Control (MAC) sub-layer.
  • The LLC sub-layer handles error control, flow control, framing, and MAC sub-layer addressing.
  • The MAC sub-layer is the lower of the two sub-layers of the Data Link layer. MAC sub-layer handles access to shared media, such a Token passing or Ethernet.

Functions of Data Link Layer :

  • Framing: Frames are the streams of bits received from the network layer into manageable data units. This division of stream of bits is done by Data Link Layer.
  • Physical Addressing: The Data Link layer adds a header to the frame in order to define physical address of the sender or receiver of the frame, if the frames are to be distributed to different systems on the network.
  • Flow Control: A flow control mechanism to avoid a fast transmitter from running a slow receiver by buffering the extra bit is provided by flow control. This prevents traffic jam at the receiver side.
  • Error Control: Error control is achieved by adding a trailer at the end of the frame. Duplication of frames are also prevented by using this mechanism. Data Link Layers adds mechanism to prevent duplication of frames.
  • Access Control: Protocols of this layer determine which of the devices has control over the link at any given time, when two or more devices are connected to the same link.

7. Physical Layer: The actual flow of bits take place through Physical layer. At Physical layer, the interface between the DTE and DCE is determined. The following are some of the standard interfaces are defined at Physical layer: A> EIA/TIA-232, EIA/TIA-449,V.24,V.35,X.21,G.703,HSSI (High Speed Serial Interface).

Functions of Physical Layer :

  • Representation of Bits: Data in this layer consists of stream of bits. The bits must be encoded into signals for transmission. It defines the type of encoding i.e. how 0's and 1's are changed to signal.
  • Data Rate: This layer defines the rate of transmission which is the number of bits per second.
  • Synchronization: It deals with the synchronization of the transmitter and receiver. The sender and receiver are synchronized at bit level.
  • Interface: The physical layer defines the transmission interface between devices and transmission medium.
  • Line Configuration: This layer connects devices with the medium: Point to Point configuration and Multipoint configuration.
  • Topologies: Devices must be connected using the following topologies: Mesh, Star, Ring and Bus.
  • Transmission Modes: Physical Layer defines the direction of transmission between two devices: Simplex, Half Duplex, Full Duplex.
  • Deals with baseband and broadband transmission.

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