Cisco® CCNA Exam Cram Notes : OSI and DoD Models

X. Appendix

2. OSI and DoD Models

2.1 OSI Model 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.

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.

6. The Data Link Layer: Data Link Layer is layer 2 of OSI reference model. This layer is divided into two sub-layers:
1. Logical Link Control (LLC) sub-layer.
2. 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.

5. 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).

2.2. DOD Model and its function

DOD Model maps to OSI model as below:

IP is the protocol that works at Internet layer. It forwards the packets based on the network address (IP address) in the packets and the routing tables.

The four layers of DoD model from Top to Bottom are:

1. Process Layer: Responsible for implementation of user-level applications such as mail delivery, file transfer, and remote login.

2. Host-to-Host Layer: Responsible for flow control,and re transmission of lost packets. TCP, and UDP protocols work at this layer.

3. Internet Layer: Responsible for routing data across a network of networks. Routed protocols such as IP work at this layer. Routing protocols are also associated with this layer.

4. Network Access Layer: Responsible for the delivery of data segments over a given particular hardware media. Different protocols such as CSMA/CD work at this layer.

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