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Physical Infrastructure of the Internet

Global Network Topology​

The internet is physically a hierarchical structure of interconnected networks:

Tier 1 Networks (Internet Backbone)
    ↓
Tier 2 Networks (Regional ISPs)
    ↓
Tier 3 Networks (Local ISPs)
    ↓
End Users

Network Tiers​

Tier 1 Networks​

  • Definition: Networks that can reach every other network on the internet without paying for transit
  • Characteristics:
    • Peer with each other for free
    • Form the internet backbone
    • Global reach
  • Examples:
    • AT&T
    • Verizon
    • Level 3 (now Lumen)
    • NTT Communications
    • Telia Carrier

Tier 2 Networks​

  • Definition: Regional networks that peer with some networks but purchase transit from Tier 1
  • Characteristics:
    • Serve specific geographic regions
    • Mix of peering and paid transit
  • Examples: Regional ISPs, medium-sized carriers

Tier 3 Networks​

  • Definition: Local ISPs that purchase all internet access from Tier 2 or Tier 1
  • Characteristics:
    • Serve end customers
    • Buy all transit
  • Examples: Your local cable/DSL provider

Physical Transmission Media​

1. Fiber Optic Cables​

Most important backbone technology

How They Work​

  • Transmit data as pulses of light
  • Core surrounded by cladding (reflects light)
  • Total internal reflection keeps light inside

Types​

  • Single-mode: Long distance, single light path
  • Multi-mode: Shorter distance, multiple light paths

Advantages​

  • Extremely fast (up to 100+ Gbps per fiber)
  • Low latency
  • Long distance without signal degradation
  • Not affected by electromagnetic interference

Where Used​

  • Undersea cables connecting continents
  • Backbone connections between cities
  • Data center interconnects
  • Last-mile in some areas (FTTH - Fiber to the Home)

2. Submarine Cables​

Connect continents across oceans

Key Facts​

  • Over 400 submarine cable systems globally
  • Carry 99% of intercontinental data traffic
  • Thousands of kilometers long
  • Laid on ocean floor by specialized ships

Major Routes​

  • Trans-Atlantic: USA ↔ Europe
  • Trans-Pacific: USA ↔ Asia
  • Europe ↔ Middle East ↔ Asia
  • Americas interconnections

Famous Cables​

  • TAT-14: USA to Europe
  • FASTER: USA to Japan
  • SEA-ME-WE 3: Southeast Asia to Middle East to Western Europe

Structure​

[Core Fiber Optics]
[Copper/Power]
[Protective Layers]
[Steel Wire Armor]
[Outer Sheath]

3. Copper Cables​

Twisted Pair (Ethernet - Cat5e, Cat6, Cat7)​

  • Speed: Up to 10 Gbps (Cat6a/7)
  • Distance: Up to 100 meters
  • Use: LANs, home networks, office buildings

Coaxial Cable​

  • Speed: Up to 1 Gbps
  • Use: Cable internet, older TV networks
  • Structure: Central conductor, insulation, shield, jacket

4. Wireless Transmission​

Wi-Fi (802.11)​

  • Standards: 802.11a/b/g/n/ac/ax (Wi-Fi 6)
  • Frequency: 2.4 GHz, 5 GHz, 6 GHz
  • Range: 30-100 meters
  • Use: Local area networks

Cellular Networks​

  • Generations: 3G, 4G/LTE, 5G
  • Coverage: Wide area (tower-based)
  • Use: Mobile internet access

Satellite​

  • Types:
    • Geostationary (35,000 km altitude)
    • Low Earth Orbit - LEO (500-2,000 km)
  • Use: Remote areas, ships, aircraft
  • Examples: Starlink, OneWeb, traditional satellite internet
  • Use: Point-to-point connections
  • Common: Between buildings, in mountainous areas

Network Hardware​

1. Routers​

  • Function: Forward packets between networks
  • Layer: Network layer (Layer 3)
  • Role:
    • Determine best path for data
    • Connect different networks
    • Internet backbone and home networks

2. Switches​

  • Function: Forward frames within a network
  • Layer: Data link layer (Layer 2)
  • Role:
    • Connect devices in a LAN
    • Use MAC addresses
    • More intelligent than hubs

3. Hubs (Legacy)​

  • Function: Broadcast data to all connected devices
  • Layer: Physical layer (Layer 1)
  • Status: Largely replaced by switches

4. Modems​

  • Function: Modulate/demodulate signals
  • Purpose: Convert digital to analog (and vice versa)
  • Types: DSL, cable, fiber modems

5. Network Interface Cards (NICs)​

  • Function: Connect device to network
  • Contains: MAC address
  • Types: Ethernet, Wi-Fi adapters

Internet Exchange Points (IXPs)​

Physical locations where networks interconnect

Purpose​

  • Allow networks to exchange traffic directly
  • Reduce costs (avoid paying for transit)
  • Reduce latency (shorter paths)
  • Improve reliability

How They Work​

[ISP A] ─┐
[ISP B] ─┼─ [IXP Switch] ─ Peering
[ISP C] ─┘

Major IXPs​

  • DE-CIX (Frankfurt, Germany)
  • AMS-IX (Amsterdam, Netherlands)
  • LINX (London, UK)
  • JPNAP (Japan)
  • Any2 (Multiple locations)

Data Centers​

Facilities housing servers and networking equipment

Components​

  1. Servers: Compute resources
  2. Storage: Data storage systems
  3. Networking: Switches, routers, load balancers
  4. Power: UPS, generators, redundant feeds
  5. Cooling: HVAC systems to prevent overheating
  6. Security: Physical and digital security measures

Types​

  • Colocation: Rent space for your equipment
  • Cloud: Rent virtual resources (AWS, Azure, GCP)
  • Enterprise: Company-owned data centers
  • Edge: Smaller facilities closer to users

Tiers (Uptime Institute)​

  • Tier 1: 99.671% uptime (28.8 hours downtime/year)
  • Tier 2: 99.741% uptime (22 hours downtime/year)
  • Tier 3: 99.982% uptime (1.6 hours downtime/year)
  • Tier 4: 99.995% uptime (26 minutes downtime/year)

Content Delivery Networks (CDNs)​

Distributed networks of servers for faster content delivery

How CDNs Work​

  1. Content replicated to multiple locations
  2. User requests routed to nearest server
  3. Reduces latency and load on origin server

Major CDN Providers​

  • Cloudflare
  • Akamai
  • Amazon CloudFront
  • Fastly
  • Azure CDN

Benefits​

  • Faster load times
  • Reduced bandwidth costs
  • Better reliability
  • DDoS protection

The "Last Mile"​

Connection from ISP to end user

Technologies​

  1. DSL (Digital Subscriber Line)

    • Uses existing phone lines
    • Speed: 1-100 Mbps

  2. Cable Internet

    • Uses coaxial cable (TV infrastructure)
    • Speed: 50-1000+ Mbps

  3. Fiber (FTTH/FTTP)

    • Fiber directly to home
    • Speed: 100 Mbps - 10 Gbps

  4. Fixed Wireless

    • Radio signals from tower to home
    • Speed: 25-100 Mbps

  5. Satellite

    • From satellite to dish
    • Speed: 25-150 Mbps (higher with Starlink)

  6. Cellular (4G/5G)

    • Mobile network for home internet
    • Speed: 25-1000+ Mbps (5G)

Network Redundancy​

Why Redundancy Matters​

  • Hardware failures
  • Cable cuts
  • Natural disasters
  • DDoS attacks

Redundancy Strategies​

  1. Multiple paths: Data can take alternate routes
  2. Backup systems: Failover capabilities
  3. Geographic diversity: Spread across locations
  4. Protocol resilience: TCP retransmission, routing protocols