What Is a Computer Network? A Complete Guide to Layers, Protocols, and Topologies

What Is a Communication Network

Everyday life is surrounded by various networks such as telephone, telegraph, television, and computer networks; even the human body contains networks like the nervous and digestive systems. The most typical example is the computer network, which combines computer technology and communication technology.

Evolution of Computer Networks

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

Main Characteristics of Computer Networks

Resource sharing

Information transmission and centralized processing

Load balancing and distributed processing

Integrated information services

Definitions of LAN, MAN, and WAN

LAN (Local Area Network) : Typically covers a few kilometers and interconnects computers, printers, modems, or other devices via a medium.

MAN (Metropolitan Area Network) : Covers a medium scale, between LAN and WAN, usually within a city (about 10 km).

WAN (Wide Area Network) : Spans long distances, using various serial connections to provide access over a larger geographic area.

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 establishment

Packet switching (store‑and‑forward):

Advantages: multiplexing, high resource utilization

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

Packets are encapsulated in frames at the data link layer.

Network Performance Metrics

Bandwidth

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

Delay

Describes 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 formats and agreements that both parties must follow.

Types of Data Communication Standards

De facto standard: Widely used and accepted without formal organization approval.

De jure standard: Officially recognized and defined 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 simplifies network operations, provides compatibility and standard interfaces, promotes standardization, and allows modular implementation.

The lower layers (1‑3) handle media and data transmission; the upper layers (5‑7) ensure correct data delivery.

TCP/IP Protocol Suite Overview

TCP/IP originated from a US government‑funded packet‑switching research project in the late 1960s and became the dominant networking model by the 1990s. It is an open system with freely available specifications.

TCP/IP Stack

The IP protocol provides best‑effort, connectionless delivery without guaranteeing reliability. Reliability is offered by the upper‑layer TCP protocol. All TCP, UDP, ICMP, and IGMP traffic is encapsulated in IP packets.

Physical Layer Functions

Defines media types, interface types, and signaling; specifies electrical, mechanical, and procedural requirements for establishing, maintaining, and terminating links; and sets parameters such as voltage levels, data rates, maximum distance, and connector types.

Supports synchronous and asynchronous serial ports (e.g., V.24/V.35/X.21, RS‑232 up to 115.2 kbps) and interfaces like G.703 E1/T1.

Physical Media and Devices

Media: coaxial cable, twisted pair, fiber optic, radio waves.

Devices: repeaters, hubs.

Data Link Layer Functions

MAC sub‑layer : Controls access to the physical medium.

LLC sub‑layer : Identifies protocol types and encapsulates data for transmission.

Data Link Layer Protocols

LAN and WAN protocols are illustrated below, with Ethernet switches as typical devices.

Network Layer Functions and Devices

Responsible for forwarding packets between different networks using routers or layer‑3 switches.

Key tasks include addressing, routing, congestion management, and interconnecting heterogeneous networks.

Addressing: Assigns identifiers to nodes, forming the basis for routing.

Routing: Determines optimal paths and forwards packets; routers compute routing tables.

Congestion control: Manages overload to prevent loss and delay.

Inter‑network connectivity: Supports multiple link types and media.

Network Layer Protocols

When a host sends a packet to a remote network, the local router receives the frame, strips the data‑link header, processes the network‑layer header, looks up the routing table, and forwards the packet via the appropriate outbound interface. Routers can maintain multiple routing protocols (e.g., RIP, OSPF, IPX) and support various network stacks.

Transport Layer Functions

The transport layer provides reliable, end‑to‑end services to applications, handling segmentation, connection establishment, data transfer, and ordering.

Segmentation and reassembly of application data.

Establishing logical end‑to‑end connections.

Ensuring ordered, reliable delivery with flow control and error checking.

Key protocols include TCP (reliable, connection‑oriented) and UDP (unreliable, connectionless).

Transport Protocol Comparison

Application Layer Functions

The application layer provides interfaces for users, handles encryption, compression, and defines data representation standards.

Provides user interfaces and specific application processing.

Performs data encryption/decryption, compression/decompression.

Defines standards for data representation.