Computer Networks Chapter 2: Network Models

Computer Networks - Chapter 2: Network Models

• Layered Architecture: The concept of layers is used in daily life (e.g., sending a letter through the post office). Layers provide a hierarchy of tasks, with higher layers interacting with middle layers, then lower layers. Each layer relies on the services of the layer below. Reference models like OSI and TCP/IP make network implementations more manageable and understandable by dividing complex functions into simpler components. They improve vendor interoperability and aid in data transfer understanding.

OSI Reference Model Architecture

• OSI Model Objectives: The Open Systems Interconnection (OSI) model, an ISO standard, defines a seven-layer architecture for network communications and aims to allow different devices to communicate regardless of their architecture. It standardizes components, facilitates communication between hardware and software, prevents layering interference, and facilitates learning.

• OSI Seven Layers: The OSI model comprises these seven layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. Each layer performs specific tasks in the network.

• Peer-to-Peer Communication: Layer n in one machine interacts with layer n in another, providing a service for layer n+1. Entities in corresponding layers communicate to perform actions.

• Encapsulation/Decapsulation: Encapsulation adds control information as data moves down layers. The data portion gets headers and/or trailers added at each layer. Decapsulation is the reverse process of removing headers and trailers as data moves up layers. Each layer's protocol data unit (PDU) has a different name (e.g., data, segment, packet, frame, bits) at each stage.

TCP/IP Model and Protocol Suite

• TCP/IP Model History: The TCP/IP model's predecessor, ARPANET, emerged in 1969. Robert Kahn and Vinton Cerf developed TCP in 1973, eventually leading to the TCP/IP suite. This suite evolved with additions like Telnet, FTP, DNS and others.

• TCP/IP Model versus OSI Model: The TCP/IP model has four layers, Network Access Layer, Internet Layer, Transport Layer, and Application Layer, compared to the OSI model's seven layers. Their transport and network layers show similarities.

• TCP/IP Protocol Suite: The suite includes protocols for diverse tasks (e.g., email, file transfer, remote access). It's an open standard, overseen by the IETF Working Groups, using RFCs for standards.

Functions of Each Layer in OSI Model

• Application Layer: Provides services for the user to interact with the network. Examples include email (SMTP, POP3), web browsing (HTTP, HTTPS), file transfer (FTP), remote access (Telnet, SSH), and DNS.

• Presentation Layer: Deals with common representation of data between application layers, coding/conversion, compression/decompression and encryption/decryption.

• Session Layer: Provides services to the presentation layer to organize dialogs and manage data exchange, enabling session establishment, management, termination, and dialogue control.

• Transport Layer: Provides reliable and efficient end-to-end transport of packets, handling connection control, service port addressing, segmentation, reassembly, sequencing, error control, and flow control (e.g., TCP, UDP).

• Network Layer: Delivers individual packets from the source to the destination host, using functions like logical addressing (e.g., IPv4, IPv6, IPX) and routing to determine the best path. (e.g., RIP, OSPF, IS-IS, EIGRP, BGP).

• Data Link Layer: Moves frames between hops, implementing functions like framing, physical addressing (e.g., MAC addresses in Ethernet), hop-to-hop error controls, hop-to-hop flow control and access control (e.g., HDLC, PPP).

• Physical Layer: Responsible for moving individual bits over a physical medium, specifying physical characteristics of interfaces, representation of bits, data rate, synchronization, line configuration and physical topology. It details communication links (e.g., twisted-pair cable, coaxial cable, optical fiber, radio, satellite).

This quiz covers the concepts of layered architecture and the OSI reference model as discussed in Chapter 2 of Computer Networks. Learn how the OSI model defines a seven-layer architecture and its objectives in enabling communication between different systems. Enhance your understanding of network models and their applications.