Introduction

Tags: #◇Computer Science #◇CSNETWK #◇Introduction #◇Conceptual

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◇Overview

The Internet is an interconnected "network of networks" that impacts daily life. It consists of billions of connected computing devices known as hosts (or end systems) located at the network's "edge." The "core" is made up of packet switches (routers and switches) that forward chunks of data (packets) through various communication links (fiber, copper, radio, satellite) characterized by their transmission rate (bandwidth).

Network communication is strictly governed by protocols, which define the format, order of messages sent and received, and actions taken upon transmission or receipt. Prominent Internet protocols include HTTP, TCP, IP, WiFi, and Ethernet. Internet standards are formalized via RFCs (Requests for Comments) published by the IETF (Internet Engineering Task Force).

Network Edge

The edge includes hosts (clients and servers, often in data centers) and access networks. Cable-based Access: Uses frequency division multiplexing (FDM) over a shared hybrid fiber coax (HFC) to transmit data and TV at different frequencies. Asymmetric speeds: 40 Mbps to 1.2 Gbps downstream, 30-100 Mbps upstream. Digital Subscriber Line (DSL): Uses existing telephone lines to connect to a DSLAM in the central office, carrying voice and data at different frequencies. Dedicated speeds: 24-52 Mbps downstream, 3.5-16 Mbps upstream. Home and Wireless Access Networks: Combines modems, routers, NAT, firewalls, and Wi-Fi access points. Wireless LANs (WLANs/WiFi) provide 11 to 450 Mbps locally, while cellular networks (4G/5G) cover tens of kilometers at tens of Mbps. Enterprise and Data Center Networks: Utilize high-speed Ethernet (100 Mbps to 10 Gbps) and Wi-Fi to interlink switches and routers, scaling up to hundreds of Gbps for inter-server connections.

Physical Media Guided media: Solid transmission mediums like twisted pair copper wire (Category 5/6), coaxial cable (broadband, bidirectional), and fiber optic cables (high-speed light pulses, low error rate, immune to noise). Unguided media: Electromagnetic signals (wireless radio) affected by reflection, obstruction, and noise. Examples include Wi-Fi, Bluetooth, 4G/5G, terrestrial microwave, and satellite.

Network Core

The core is a mesh of interconnected routers implementing packet switching. Hosts break application messages into smaller packets, which are forwarded hop-by-hop. Store-and-forward: An entire packet must be received at a router before it can begin transmission on the next link. Queueing and Loss: Packets queue if the arrival rate exceeds the link's transmission rate. If the router's memory (buffer) is full, packets are dropped (lost). Forwarding vs Routing: Forwarding (switching) is the local action of moving an arriving packet to the appropriate output link based on its header. Routing is the global action of determining the end-to-end path using routing algorithms.

Circuit Switching

An alternative to packet switching used in traditional telephone networks. Resources (bandwidth) are allocated and reserved end-to-end for a "call," ensuring guaranteed performance without sharing. The link's capacity is divided via Frequency Division Multiplexing (FDM) or Time Division Multiplexing (TDM). While packet switching handles bursty data efficiently with on-demand allocation, it can suffer from congestion, unlike circuit switching's reliable but less scalable dedicated reservations.

Internet Structure

Access ISPs connect edge devices to the Internet. Connecting millions of access ISPs directly does not scale ($O(N^2)$). Instead, a hierarchical structure is used: Tier 1 Commercial ISPs (e.g., AT&T, Sprint) and massive Content Provider Networks (e.g., Google) sit at the center. Regional ISPs and Internet Exchange Points (IXPs) connect lower-tier access ISPs to the core.

◇Important Formulas

Packet Transmission Delay:
$d_{trans} = \frac{L}{R}$
Where $L$ is the packet length (in bits) and $R$ is the link transmission rate or bandwidth (in bits/sec).

◇Worked Examples

Example 1: Single-hop Packet Transmission Delay
Given a packet size $L = 10 \text{ Kbits}$ and a link transmission rate $R = 100 \text{ Mbps}$.
Calculate the transmission delay for this single hop.
$d_{trans} = \frac{10 \times 10^3 \text{ bits}}{100 \times 10^6 \text{ bps}} = \frac{10,000}{100,000,000} \text{ seconds} = 0.0001 \text{ seconds} = 0.1 \text{ msec}$

◇References & Navigation

• Next Topic: ◇Layered Network Design
• Related Notes: ◇Internetworking