Comprehensive Overview of Computer Networks: History, Architecture, Protocols, and Layer Functions

Preface

Computer networking is a fundamental subject, but the material can be dense for self‑learners; the article aims to guide readers through the entire topic patiently.

Prerequisite Knowledge

The textbook "Computer Networks" by Xie Xiren is widely used; the first chapter provides a concise overview of the field.

Brief History of the Internet

Stage 1 (1950s): Early data‑communication research and network theory.

Stage 2 (1960s): ARPANET and packet‑switching.

Stage 3 (mid‑1970s): Standardization of network architecture and protocols.

Stage 4 (1990s onward): Rapid growth of the Internet, high‑speed, wireless, mobile, and security technologies.

Development of the Internet

The development can be divided into seven phases, including batch processing, time‑sharing systems, computer communication technology, the emergence of computer networks, widespread Internet adoption, the era of ubiquitous connectivity, and the era of network security.

Network Performance Metrics

Bit : Basic unit of data, also called bit rate when referring to transmission speed.

Bandwidth : Maximum data rate a link can carry, measured in bits per second.

Throughput : Actual amount of data transferred per unit time.

Delay : Includes transmission delay and propagation delay.

Delay‑Bandwidth Product : Number of bits that can be stored in a link.

RTT (Round‑Trip Time) : Time from sending a packet to receiving its acknowledgment.

Utilization : Ratio of time a channel or network is actively used.

Essential Knowledge

Network Classification

LAN (Local Area Network) : Small geographic area, high speed.

MAN (Metropolitan Area Network) : Connects multiple LANs within a city.

WAN (Wide Area Network) : Connects distant locations, usually via leased lines.

Network Topology

Bus : Low cost, easy to extend, but low reliability.

Ring : Token‑controlled, real‑time, but maintenance is difficult.

Star : High reliability and easy management, but central node must be highly reliable.

Computer Network Structures

Three common layering models are presented:

OSI 7‑Layer Model

Five‑Layer Model

TCP/IP Model

Physical Layer

Explains the difference between digital and analog signals and the need for conversion (e.g., PCM). Discusses guided media (coaxial cable, fiber, twisted pair) and non‑guided media (wireless).

Channel

Describes wired, wireless, and storage channels, and introduces Shannon’s formula for channel capacity.

Channel Multiplexing

Time‑Division Multiplexing (TDM)

Frequency‑Division Multiplexing (FDM)

Statistical TDM

Data Link Layer

Ethernet Frame

Components include destination MAC, source MAC, type field, payload, and FCS for error detection.

Error Detection

Discusses parity check and CRC; CRC is used to generate the FCS field.

Adapter

Analogous to power adapters, they convert serial transmission on the medium to parallel processing inside a computer.

CAM Table

Switches maintain a Content‑Addressable Memory table mapping MAC addresses to ports; the table is populated via learning and updated on traffic.

CSMA/CD Protocol

Describes carrier‑sense multiple access with collision detection, its use in bus‑topology Ethernet, and variations of non‑persistent, 1‑persistent, and p‑persistent CSMA.

Network Layer

IP Protocol

Explains the purpose of IP addresses for identification and routing, and why both MAC and IP addresses are needed.

IP Datagram

Key fields: version, header length, TOS, TTL, protocol identifier, etc.

Subnet Mask and IP Address

Shows how subnet masks separate network and host portions of an address.

ICMP Protocol

Provides error reporting and diagnostic messages for IP.

ARP Protocol

Maps IP addresses to MAC addresses within a LAN.

Interior Gateway Protocols

RIP: Distance‑vector protocol, simple but limited to ~15 hops.

OSPF: Link‑state protocol using Dijkstra’s algorithm.

NAT Protocol

Describes static NAT, dynamic (pooled) NAT, and NAPT (port‑level NAT) for conserving public IPv4 addresses.

IPv6 Protocol

IPv6 expands address space to 128 bits; transition mechanisms include dual‑stack and tunneling.

Transport Layer

Stop‑and‑Wait Protocol

Explains basic ARQ, retransmission on loss, and handling of delayed acknowledgments.

UDP

Connectionless, unreliable, low‑overhead datagram service.

TCP

TCP Overview

Connection‑oriented, reliable, full‑duplex, byte‑stream service.

TCP Segment

Fields: source/destination ports, sequence number, acknowledgment number, data offset, window, checksum, flags (SYN, ACK, FIN, RST, PSH).

Sliding Window

Mechanism to control flow and improve throughput.

Flow Control

Receiver advertises a window size to limit sender’s rate.

Congestion Control

Slow start

Congestion avoidance

Fast retransmit

Fast recovery

Three‑Way Handshake

Establishes a reliable TCP connection using SYN, SYN‑ACK, and ACK.

Four‑Way Termination

Closes a TCP connection with FIN, ACK, FIN, ACK sequence.

TCP vs UDP Use Cases

TCP for reliable, connection‑oriented tasks (file transfer, email, remote login); UDP for low‑latency, connection‑less scenarios (voice, video, live streaming).

Application Layer

HTTP Protocol

Client‑server request/response model built on TCP; HTTPS adds SSL/TLS for security.

FTP Protocol

File transfer service operating over TCP.

DNS Protocol

Translates human‑readable domain names to IP addresses.

DHCP Protocol

Automatically assigns IP configuration parameters to hosts.

Source: juejin.im/post/6850037269244575757