Fundamentals of Computer Networks: OSI Model, TCP/IP Stack, and Network Layers

What Is a Communication Network

Networks surround us constantly—telephone, telegraph, television, and computer networks; even biological systems like the nervous and digestive systems are networks. The most typical example is the computer network, which combines computer and communication technologies.

Evolution of Computer Networks

Early networks used low‑speed serial links such as X.25 and IBM's SNA based on host architectures.

Main Features of Computer Networks

Resource sharing

Information transmission and centralized processing

Load balancing and distributed processing

Integrated information services

Definitions of LAN, MAN, WAN

LAN (Local Area Network) : Typically spans a few kilometers, interconnecting computers, printers, modems, and other devices via a common medium.

MAN (Metropolitan Area Network) : Covers a medium‑scale area, usually within a city (about 10 km).

WAN (Wide Area Network) : Extends over long distances, using various serial connections to provide access across large geographic regions.

Common Network Topologies

Circuit Switching vs. Packet Switching

Circuit switching (based on telephone networks):

Advantages: low latency, transparent transmission.

Disadvantages: fixed bandwidth, low resource utilization, slow connection setup.

Packet switching (store‑and‑forward by packets):

Advantages: multiplexing, high resource utilization.

Disadvantages: higher latency, poorer real‑time performance, more complex devices.

Network Performance Metrics

Bandwidth

Describes the amount of data that can be transferred between two nodes within a given time, usually expressed in bps (e.g., Ethernet 10 Mbps, Fast Ethernet 100 Mbps).

Delay

Measures the time taken for data to travel from one node to another across the network.

Protocols and Standards

What Is a Network Protocol?

A set of rules and conventions that enable devices to communicate and exchange data.

Defines the format, timing, and error handling for information exchange.

Types of Standards

De facto standard: Widely used in practice without formal organization approval.

De jure standard: Officially endorsed by a standards body.

Standardization Organizations

ISO (International Organization for Standardization)

IEEE (Institute of Electrical and Electronics Engineers)

ANSI (American National Standards Institute)

EIA/TIA (Electronic Industries Alliance / Telecommunications Industry Association)

ITU (International Telecommunication Union)

IETF (Internet Engineering Task Force)

IRTF (Internet Research Task Force)

IANA (Internet Assigned Numbers Authority)

OSI Reference Model

The OSI model divides networking functions into seven layers, from the physical layer (Layer 1) up to the application layer (Layer 7). Lower layers (1‑3) handle data transmission and interconnection hardware, while upper layers (5‑7) provide host‑level services and ensure reliable data delivery.

TCP/IP Protocol Stack Overview

The TCP/IP suite originated in the late 1960s as a government‑funded packet‑switching research project and has become the dominant networking architecture worldwide. Like the OSI model, it is organized into layers, typically represented as a five‑layer model that merges OSI’s Application, Presentation, and Session layers.

TCP/IP Protocol Stack Details

IP provides an unreliable, connectionless best‑effort service, while TCP (and other transport protocols) add reliability, ordering, and flow control. All higher‑level protocols (TCP, UDP, ICMP, IGMP) are encapsulated within IP packets.

Physical Layer Functions

Specifies media type, connector type, signaling, and electrical/mechanical characteristics; defines how links are activated, maintained, and terminated; and sets limits for voltage levels, data rates, and maximum distances.

Typical physical‑layer media include coaxial cable, twisted pair, fiber optics, and radio waves; devices include repeaters and hubs.

Data Link Layer Functions

The MAC sub‑layer controls how data is placed onto the physical medium, while the LLC sub‑layer identifies protocol types and encapsulates data for transmission.

Data Link Layer Protocols

Common LAN and WAN protocols are illustrated below.

Typical data‑link devices are Ethernet switches.

Network Layer Functions and Devices

The network layer forwards packets between different networks using routers or layer‑3 switches.

Key responsibilities include addressing, routing, congestion management, and inter‑network connectivity.

Network Layer Protocols

Routers may run multiple routing protocols (e.g., RIP, OSPF) and support various network‑layer protocol suites such as TCP/IP and IPX.

Transport Layer Functions

The transport layer provides reliable end‑to‑end services to applications, handling segmentation, connection establishment, flow control, error detection, and ordered delivery.

Segmenting and reassembling data

Establishing logical connections

Ensuring reliable, in‑order delivery

Providing flow control to avoid buffer overflow

Transport Layer Protocol Comparison

Application Layer Functions

The application layer offers interfaces for user programs, handles data encryption/decryption, compression/decompression, and defines data representation standards.

For further reading, see the linked white‑papers on deterministic networking, white‑box switches, data‑center autonomous networks, and serverless edge computing.