What Is a Communication Network? Key Concepts, Layers, and Protocols Explained

What Is a Communication Network

We are constantly surrounded by networks such as telephone, telegraph, television, and computer networks; even our bodies contain networks like the nervous and digestive systems. The most typical example is the computer network, which combines computer and communication technologies.

Evolution of Computer Networks

Early simple links were low‑speed serial links based on host architecture, 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

LAN, MAN, and WAN Definitions

LAN (Local Area Network) : Typically covers a few kilometers and interconnects computers, printers, modems, and 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 large distances, using various serial connections to provide access over a wide geographic area.

Common Network Topologies

Circuit Switching vs. Packet Switching

Circuit switching (used in 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 devices

Packet switching units are called cells or Layer‑2 frames.

Performance Metrics

Bandwidth : The amount of data that can be transferred from one node to another within a given time, usually expressed in bps (e.g., Ethernet 10 Mbps, Fast Ethernet 100 Mbps).

Delay : The time it takes for data to travel from one node to another across the network.

Protocols and Standards

A network protocol is a set of rules and conventions that enable devices to communicate and understand each other.

Data communication standards are of two types:

De facto standards: widely used and accepted without formal organization approval.

De jure standards: officially approved by standards bodies.

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, allows modular structure, and eases implementation and maintenance.

The lower layers (1‑3) handle data transmission and are often implemented in hardware; the upper layers (5‑7) ensure correct data delivery and are typically software‑based.

TCP/IP Protocol Stack Overview

Originating from a late‑1960s U.S. government project, TCP/IP became the dominant networking suite by the 1990s and forms the basis of the Internet.

Like the OSI model, the TCP/IP model is layered; the five‑layer model integrates aspects of both OSI and TCP/IP.

TCP/IP Stack Details

The IP protocol provides best‑effort, connectionless delivery; it does not guarantee packet arrival or order. Reliability is achieved 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 functional requirements for establishing, maintaining, and terminating physical links, including voltage levels, data rates, maximum distances, and connectors.

Typical media: coaxial cable, twisted pair, fiber optic, radio waves.

Typical devices: repeaters, hubs.

Data Link Layer Functions

MAC Sub‑layer : Controls how data is placed on the physical medium.

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

Data Link Layer Protocols

LAN and WAN protocols; Ethernet switches are typical data‑link devices.

Network Layer Functions and Devices

Responsible for forwarding packets between different networks; devices include routers and layer‑3 switches.

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

Network Layer Protocols

Transport Layer Functions

Provides reliable, end‑to‑end services to applications, including segmentation, connection establishment, data transfer, and ordered, error‑checked delivery.

Core functions:

Segmenting and reassembling data between application and network layers.

Establishing logical end‑to‑end connections.

Ensuring correct, reliable transmission with flow control and checksums.

Maintaining order and preventing duplication.

Main transport protocols: TCP (reliable, connection‑oriented) and UDP (unreliable, connectionless).

Transport Layer Protocol Comparison

Application Layer Functions

Provides user interfaces and handles specific application tasks such as encryption, decryption, compression, decompression, and defining data representation standards.