Understanding Network Layers: Functions and Protocols

Posted on Feb 23, 2025 in Computer Engineering

Network Layers Explained

The data link layer is responsible for establishing error-free communication. It ensures reliable data transfer to the upper layers. This layer operates at the physical level, managing data packets and ensuring their integrity. It divides messages into manageable fragments.

The data link layer handles tasks such as establishing and maintaining data flow, notifying users of errors, managing re-transmission requests, discarding duplicate data, and ensuring rapid transmission. It also manages data formatting, addressing, transmission order, error detection, and recovery. Standards like Ethernet and Token Ring are defined at this level.

Network Layer

The primary function of the network layer is routing. It determines the optimal path for data packets to travel. This layer manages data transfer, administration, and broadcast messages. Protocols like the Internet Protocol (IP) and Internetwork Packet Exchange (IPX) are utilized at this level.

Transport Layer

The transport layer ensures reliable information transfer, even in the presence of failures in the lower layers. It handles error detection, system dropouts, speed matching between transmission and reception, and alternative route discovery. Key protocols at this level include Transmission Control Protocol (TCP) and Sequenced Packet Exchange (SPX).

Session Layer

The session layer manages dialogues between applications. It establishes, maintains, and terminates connections. This layer enhances the transport layer by organizing functions that allow users to communicate effectively. It also includes security features like user authentication and system administration.

Presentation Layer

The presentation layer deals with the syntax and semantics of transmitted information. It handles data representation, ensuring compatibility between different systems. For example, it converts data from ASCII to EBCDIC. It also manages data encryption and compression, and controls devices like printers.

Application Layer

The application layer is the highest layer in the OSI model. It defines the protocols used by applications and user processes. It uses communication protocols to facilitate dialogues and manages information exchange between users and the operating system. This includes file transfer and application program interactions.

Initially, the ISO referenced five groups of application protocols:

  • System Protocols: Oriented towards the management of interconnected computers in a network.
  • Application Management Protocols: Control the execution of processes, manage access, and allocate resources.
  • System Protocols: Manage operating system defects, file access, communication flaws, and remote defect notifications.
  • Specific Application Protocols: Dependent on the specific needs of applications.

Bridges

Bridges operate using the following steps:

  1. Memory storage of received data for analysis.
  2. Error checking to ensure data integrity. Erroneous data is discarded.
  3. Format adaptation, adding or removing fields to match the destination segment.
  4. Forwarding data if the destination is within an accessible network segment.

Bridges operate at Layer 2 (data link) and make decisions based on MAC addresses, without affecting routing protocols.