Fundamentals of Computer Networks: Concepts, Models, and Protocols

What Is a Communication Network

We are constantly surrounded by various networks such as telephone, telegraph, television, and computer networks; even the human body contains many network-like systems 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 spans a few kilometers and interconnects computers, printers, modems, or other devices via a common medium.

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

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

Common Network Topologies

Circuit Switching and 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 by packets):

Advantages: multiplexing, high resource utilization

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

Packets are called cells; the second‑layer unit is a frame.

Performance Metrics of Computer Networks

Bandwidth

Describes the amount of data that can be transferred from one node to another within a given time period, 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 formats and agreements that allow different devices in a network to communicate.

A normative description of rules defining how network devices exchange information.

Two Types of Data Communication Standards

De facto standard: widely used and accepted in practice without formal organization endorsement.

De jure standard: officially recognized and defined by a standards organization.

Standardization Organizations

International Organization for Standardization (ISO)

Institute of Electrical and Electronics Engineers (IEEE)

American National Standards Institute (ANSI)

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

International Telecommunication Union (ITU)

Internet Engineering Task Force (IETF)

Internet Research Task Force (IRTF)

Internet Assigned Numbers Authority (IANA)

OSI Reference Model

The OSI Reference Model (OSI RM) provides a layered framework for network communication.

Simplifies network operations.

Provides compatibility and standard interfaces between devices.

Facilitates standardization work.

Allows modular separation of functions.

Easy to implement and maintain.

Layers 1‑3 constitute the lower (media) layer, handling data transmission and interconnection devices. Layers 5‑7 form the upper (host) layer, ensuring correct data delivery via software.

TCP/IP Protocol Stack Overview

The TCP/IP suite originated in the late 1960s as a government‑funded packet‑switching research project and became the dominant networking model by the 1990s. It is an open system whose protocols are publicly documented and freely implementable, forming the foundation of the global Internet.

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

TCP/IP Protocol Stack

The IP protocol provides best‑effort (unreliable, connectionless) packet delivery, without guaranteeing delivery or ordering. Reliable, connection‑oriented services are offered by the TCP protocol above IP. All TCP, UDP, ICMP, IGMP traffic is ultimately encapsulated in IP packets.

Physical Layer Functions

The physical layer defines medium types, interface types, and signaling; it specifies electrical, mechanical, and procedural requirements for establishing, maintaining, and terminating links, as well as characteristics such as voltage levels, data rates, maximum distance, and connector types.

Synchronous serial ports can act as DCE or DTE and support protocols such as V.24/V.35/X.21; asynchronous ports support RS‑232 up to 115.2 kbps. G.703 defines E1/T1 interfaces.

Physical Media and Devices

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

Devices: repeaters, hubs.

Data Link Layer Functions

The MAC sub‑layer (Media Access Control) controls how data is transmitted over the physical medium and interacts with the physical layer. The LLC sub‑layer (Logical Link Control) identifies protocol types and encapsulates data for transmission.

Data Link Layer Protocols

LAN and WAN protocols operate at this layer.

Typical data‑link devices include Ethernet switches.

Network Layer Functions and Devices

Functions: forwarding packets between different networks.

Devices: routers, layer‑3 switches.

The network layer selects appropriate paths and forwards packets, providing addressing, routing, congestion management, and inter‑network connectivity across heterogeneous links.

Addressing: assigns network addresses to nodes.

Routing: determines optimal paths and forwards packets via routers.

Congestion management: controls traffic overload to avoid loss or delay.

Inter‑networking: enables communication across diverse link types and media.

Network Layer Protocols

When a host needs to send a packet to a remote network, the local router receives the frame, strips the link‑layer header, processes the network‑layer header, looks up the routing table, and forwards the packet out the appropriate interface. The router may support multiple routing protocols (e.g., RIP, OSPF) and multiple network‑layer protocol suites.

Transport Layer Functions

The transport layer provides reliable, end‑to‑end services to applications, handling segmentation, connection establishment, data transfer, and ensuring ordered, error‑free delivery.

Segmentation and reassembly of data.

Establishment of logical end‑to‑end connections.

Flow control and error checking via checksums.

Guaranteeing in‑order, loss‑free delivery.

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

Transport Layer Protocol Comparison

Application Layer Functions

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

Provides user interfaces and specific application processing.

Performs data encryption, decryption, compression, and decompression.

Defines standards for data representation.