CompTIA® Network+ Exam Notes : Osi Models And Its Functions

1. Networking Concepts

1.1 OSI model and its function

The OSI model (Open System Interconnection) model defines a computer networking framework to implement protocols in seven layers. A protocol in the networking terms is a kind of negotiation and rule in between two networking entities. The figure below shows the OSI reference model and the seven layers.

OSI Model

Interface between layers

Interface is responsible for passing the data and network information down through the layers of the sending device and back up through the layers of the receiving device.

The 7 layers of OSI model are briefly explained below:

1. The Application Layer (Layer-7): 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. Application layer (layer 7) is the protocol layer that provides interface to the users or other programs. Most of the applications such as Telnet and FTP operate at layer 7 of the OSI stack.

2. The Presentation Layer (Layer-6): 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 formats the data such that the upper layers are able to understand the data.

3. The Session Layer (Layer-5): 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 Sessions layer is responsible for maintaining communication sessions.

4. The Transport Layer (Layer-4): 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. The Transport layer is responsible for maintaining end-to-end communications so that the information is transported reliably over the network.

5. The Network Layer (Layer-3): 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 per-determined methods. Routers work at Network layer. The Network layer is responsible for routing packets through the Internet.

In other words, Layer 3 protocols correspond to the network layer and use packets as unit of exchange. IP Security (IPSec) is an example of Layer 3 tunneling protocol. IPSEC encapsulates IP packets in an additional IP header before sending them across an IP network.

  • The Packet Filters work at Network Layer of OSI model.
  • The Application Layer Proxy works at the Application Layer of OSI model
  • Network Address Translation (NAT) is primarily used to hide internal network from external network, such as the Internet. A NAT basically translates the internal IP addresses to external IP addresses and vice-versa. This functionality assures that external users do not see the internal IP addresses, and hence the hosts.
  • A Firewall implemented with stateful technology (like Checkpoint Firewall) works at all layers of the OSI model.
  • Gateway is used to translate protocols, as it works at the application layer.

6. The Data Link Layer (Layer-2): Data Link Layer is layer 2 of OSI reference model. The Datalink layer is responsible for transmission and reception of frames. It does error detection and correction and uses frame headers for this purpose.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 as Token passing or Ethernet.

Examples of data-link layer protocols that use point to point dedicated connection are:

  • PPP
  • HDLC
  • LAPB

Examples of data-link layer protocols that use packet switching are:

  • Frame Relay
  • ATM
  • X.25

7. Physical Layer (Layer-1): The actual flow of bits take place through Physical layer. At Physical layer, the interface between the DTE and DCE is determined. Physical layer is responsible for bit synchronization. Physical layer, as the name implies, transmits the physical signals over the media. The media may be electrical (such as 10BaseT) or optical (such as FDDI).

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).

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Segments are associated with Transport layer,

Packets are associated with Network Layer, and

Frames are associated with Data Link Layer.

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Repeaters work at Physical layer (Layer 1),

Bridges and simple switches work at Data Link Layer (Layer 2),

Routers work at Network Layer (Layer 3) of ISO-OSI Reference Model.

A Wireless Access Point (WAP) works at the physical layer of OSI model.

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