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

What Is a Communication Network

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

Evolution of Computer Networks

Simple links based on host architecture used low‑speed serial links, X.25 and IBM's SNA.

Main Characteristics of Computer Networks

Resource sharing

Information transmission and centralized processing

Load balancing and distributed processing

Comprehensive information services

Definitions of LAN, MAN, WAN

LAN (Local Area Network) : Typically within a few kilometers, a collection of computers, printers, modems, or other devices interconnected by some medium.

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

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

Common Network Topologies

(Image illustrating typical 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

Performance Metrics of Computer Networks

Bandwidth

Describes 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

Describes the time it takes 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.

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

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 simplifies network operations, provides compatibility and standard interfaces between devices, promotes standardization work, allows modular separation, and eases implementation and maintenance.

Layers 1‑3 are the lower (media) layers, handling data transmission; layers 5‑7 are the upper (host) layers, ensuring correct data delivery.

OSI model consists of seven layers from bottom to top.

TCP/IP Protocol Stack Overview

The TCP/IP suite originated from a late‑1960s US government‑funded packet‑switching research project and became the dominant networking model by the 1990s. It is an open system with freely available specifications and implementations, forming the foundation of the global Internet.

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

TCP/IP Protocol Stack

IP is a best‑effort, connectionless protocol that provides unreliable, unacknowledged delivery; it does not guarantee packet arrival or order. Reliable, connection‑oriented services are provided by the upper‑layer TCP protocol. All TCP, UDP, ICMP, IGMP traffic is ultimately encapsulated in IP packets.

Physical Layer Functions

Specifies medium type, interface type, signaling; defines electrical, mechanical, procedural, and functional requirements for establishing, maintaining, and terminating physical links; sets levels, data rates, maximum distances, and connector characteristics.

Supports synchronous DCE/DTE interfaces (e.g., V.24/V.35/X.21) and asynchronous RS‑232 (max 115.2 kbps). Includes G.703 E1/T1 interfaces.

Physical Media and Devices

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

Devices: repeaters, hubs.

Data Link Layer Functions

MAC Sub‑layer (Media Access Control) defines how data is transmitted over the physical medium and interacts with the physical layer.

LLC Sub‑layer (Logical Link Control) identifies protocol types and encapsulates data for network transmission.

Data Link Layer Protocols

Includes LAN and WAN protocols; typical equipment is Ethernet switches.

Network Layer Functions and Devices

Function: forward packets between different networks.

Devices: routers, layer‑3 switches.

The network layer selects appropriate paths and forwards packets so they reach the destination correctly.

Addressing: assigns identifiers to each node, forming the basis for routing.

Routing: determines the path for packet delivery; routers compute routes and forward packets accordingly.

Congestion Management: controls traffic overload to prevent loss or delay.

Inter‑network Connectivity: works across diverse link and media types to provide end‑to‑end communication.

Network Layer Protocols

When a host needs to send a packet to another network, the local router receives the frame, strips the link‑layer header, processes the network‑layer data, looks up the routing table, and forwards the packet via the appropriate outbound interface. The process repeats at each hop until the packet reaches the destination network, where the final router delivers it to the target host.

Routers can support multiple routing protocols (e.g., RIP, OSPF, IPX) and multiple protocol stacks (TCP/IP, IPX) simultaneously.

Transport Layer Functions

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

Segmenting upper‑layer data and reassembling received segments.

Establishing logical end‑to‑end connections.

Transferring data between hosts with checksum verification and flow control.

Guaranteeing in‑order, loss‑free delivery.

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

Transport Layer Protocol Comparison

(Image comparing TCP, UDP, and other transport protocols)

Application Layer Functions

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

Provides user interfaces and processes specific application logic.

Performs data encryption, decryption, compression, and decompression.

Defines standards for data representation.

(Image illustrating application‑layer concepts)

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