Functions and Limitations of the OSI Model

Layer 7: Application Layer

The Application layer of the OSI model is the layer closest to the end-user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implements a communicating component.

Functions of the Application Layer

  1. Responsible for identifying and establishing the availability of the intended communication partner and determining if sufficient resources for the intended communication exist

Security Challenges of the Application Layer

  1. Being able to verify the genuineness of the particular application
  2. Availability of internetworking applications required for variety of tasks.


Layer 6: Presentation Layer

The presentation layer establishes context between application-layer entities, in which the application-layer entities may use different syntax and semantics if the presentation service provides a big mapping between them. This layer provides independence from data representation by translating between application and network formats. It translates the data into the form that the application accepts.

Functions of the Presentation Layer

  1. It provides coding and format conversion functions
  2. Handles data compression and decompression
  3. Handles data encryption and decryption
  4. Also involve in multimedia operations

Security Challenges of the Presentation Layer

  1. Handling of all the available data formats may not easily be achieved


Layer 5: Session Layer

The Session Layer is responsible for setting up, managing and then terminating sessions between Presentation Layer entities.

Functions of the Session Layer

  1. Handles setup and termination of connection between two Presentation Layer entities
  2. Provides dialog between devices or nodes.
  3. Coordinates communications between systems and serves to organize their communication by offering three different modes: simplex, half-duplex and full-duplex
  4. Keeps different applications’ data separate from other applications’ data

Security Challenges of the Session Layer

  1. Multiplicity of Session Layer Protocols may lead to security challenges


Layer 4: Transport Layer

The transport layer provides the functional and procedural means of transferring variable-length data sequence from a source to a destination host via one or more networks, while maintaining the quality of service functions. Services located in the Transport layer both segment and reassemble data from upper-layer application and unite them into a single data stream.

Functions of the Transport Layer

  1. Provides end-to-end data transport services and can establish a logical connection between sending host and destination host on an internetwork
  2. Provides mechanisms for multiplexing upper-layer applications
  3. It hides details of any network-dependent information from the higher layers by providing transparent data transfer.
  4. Maintaining flow control

Security Challenges of the Transport Layer

  1. Ensuring data integrity
  2. Data loss might result from overflow of the buffer of the receiving host


Layer 3: Network Layer

The Network Layer is responsible for routing through an internetwork and for network addressing. This means that the Network layer is responsible for transporting traffic between devices that are not locally attached.

Functions of the Network Layer

  1. Translation of logical machine address into physical machine address
  2. Responsible for routing traffic between two different networks.
  3. Handles logical addressing
  4. Responsible for uniquely identifying each network

Security Challenges of the Network Layer

  1. Loss of packet if the entry for the destination network is not found in the routing table.
  2. Message delivery at the network layer is not necessarily guaranteed to be reliable


Layer 2: Data Link Layer

The Data Link layer ensures that messages are delivered to the proper device and translates messages from the Network layer into bits for the Physical layer to transmit.

It provides node-to-node data transfer-a link between two directly connected nodes

Functions of the Data Link Layer

  1. It defines protocols to establish and terminate a connection between two physically connected devices.
  2. It defines a protocol for flow control between the two end-points.
  3. Formatting of the message into data frames and adds a customized header containing the hardware destination and the source address
  4. Formats messages into frames
  5. It detects and possibly corrects errors that may occur in the physical layer
  6. Responsible for uniquely identifying each device on a local network
  7. Provides flow control and sequencing control bits

Security Challenges of the Data Link Layer

  1. Cannot handle broadcasts


Layer 1: Physical Layer

The physical layer is the layer that contains physical medium of data transfer such as the cables and the interface cards

Functions of the Physical Layer

  1. It defines the electrical and physical specification of the data connection
  2. It defines transmission modes ie simplex, half-duplex and full-duplex
  3. It defines network topology such as bus, mesh or ring
  4. Encoding of the bits is done in this layer
  5. It determines whether encoded bits will be transmitted by baseband(digital) or broadband(analog) signal
  6. It mostly deals with raw data
  7. Communicates with various types of communication media
  8. Specifies the electrical, mechanical and functional requirements for each medium
  9. Identifies the Data Terminal Equipment(DTE) and the Data Communication Equipment(DCE)
  • Allows for disparate systems to communicate

Security Challenges of the Physical Layer

  1. Interconnectivity between network equipment from different vendors may be a challenge.

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