CCNA study notes on Open Systems Interconnection (OSI) Model

OSI Model

1.The 7 layers of OSI model are:

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

• 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 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.

• The Transport Layer: The Transport Layer is responsible for multiplexing upper-layer applications, session establishment, and tearing-down of virtual circuits. This layer is responsible for flow control, to maintain data integrity.

• 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 routing methods. Routers work at Network layer.

• 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: The LLC sub-layer handles error control, flow control, framing, and MAC sub-layer addressing.

  • Media Access Control (MAC) sub-layer: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.

• Physical Layer: The actual flow of signals 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: EIA/TIA-232, EIA/TIA-449,V.24,V.35,X.21,G.703,HSSI (High Speed Serial Interface).

2. SONET defines interface standards at the physical layer of the OSI seven-layer model. The standard defines a hierarchy of interface rates that allow data streams at different rates to be multiplexed. SONET establishes Optical Carrier (OC) levels from 51.8 Mbps (OC-1) to 9.95 Gbps (OC-192).

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