This post is based on the textbook Computer Networking: A Top-Down Approach (6th Edition) by James Kurose and Keith Ross.

• 1. The Internet: A Nuts-and-Bolts Description
  • 1.1 Hosts or End Systems
  • 1.2 Communication Links and Packet Switches
  • 1.3 Packet Switches
  • 1.4 ISP (Internet Service Provider)
  • 1.5 Protocols and Standards
• 2. The Internet: A Services Description
• 3. What Is a Protocol?
  • 3.1 Analogy with Human Protocols
  • 3.2 Definition of a Network Protocol
• 4. Network Edge
  • 4.1 Role of End Systems
  • 4.2 Client-Server Model vs P2P Model
• 5. Access Networks
  • 5.1 Home Access: DSL, Cable, FTTH
    • DSL (Digital Subscriber Line)
    • Cable Internet
    • FTTH (Fiber To The Home)
  • 5.2 Enterprise Access: Ethernet and WiFi
  • 5.3 Wide-Area Wireless Access: 3G/4G/5G
• 6. Physical Media
  • 6.1 Guided Media
    • Twisted Pair Details
    • Fiber Optic Details
  • 6.2 Unguided Media
• 7. Summary
• 8. Review Questions

1. The Internet: A Nuts-and-Bolts Description

The Internet is a computer network connecting billions of computing devices worldwide.

1.1 Hosts or End Systems

All devices connected to the Internet are called hosts or end systems. This includes not only traditional desktop PCs and servers but also smartphones, tablets, IoT devices, and more.

1.2 Communication Links and Packet Switches

End systems are connected by a network of communication links and packet switches.

• Communication links: Composed of various physical media such as coaxial cable, copper wire, optical fiber, and radio spectrum
• Transmission rate: Varies by link, measured in bps (bits per second)

When transmitting data, the sending end system divides the data into segments and adds a header to each segment to create packets.

1.3 Packet Switches

A packet switch receives packets arriving on input communication links and forwards them on output communication links. The two most prominent types of packet switches are:

• Router: Used in the network core
• Link-layer switch: Used in access networks

1.4 ISP (Internet Service Provider)

End systems access the Internet through ISPs.

End system -> Access ISP -> Regional ISP -> Tier-1 ISP -> Internet backbone

Each ISP is an independently managed network of packet switches and communication links.

1.5 Protocols and Standards

All activity on the Internet is governed by protocols. The two most important protocols are:

• TCP (Transmission Control Protocol)
• IP (Internet Protocol): Specifies the format of packets exchanged between routers and end systems

Internet standards are developed by the IETF (Internet Engineering Task Force), and the standards documents are called RFCs (Request for Comments).

2. The Internet: A Services Description

The Internet can be viewed as an infrastructure that provides services to applications.

Distributed applications (email, web, social networks, streaming, etc.) run on end systems that exchange data with each other. The Internet provides a socket interface that enables programs to deliver data to other end systems.

The socket interface is a set of rules that a sending program must follow. Using a postal system analogy, it is like the rules for placing a letter in an envelope, writing the address, affixing a stamp, and dropping it into a mailbox.

3. What Is a Protocol?

3.1 Analogy with Human Protocols

Even human conversations follow protocols:

Person A: "Hello"               -> Message sent
Person B: "Hello"               -> Response message
Person A: "What time is it?"    -> Request
Person B: "It's 2:30"           -> Response

3.2 Definition of a Network Protocol

A network protocol defines the format and order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.

Example: TCP connection establishment process

Client                        Server
   |--- TCP connection request -->|
   |<-- TCP connection reply -----|
   |--- GET /index.html -------->|
   |<-- HTTP response data ------|

4. Network Edge

4.1 Role of End Systems

End systems are located at the edge of the network, which is why they are also called edge devices. End systems serve two roles:

• Client: The side that requests services (desktops, smartphones, etc.)
• Server: The side that provides services (located in data centers)

4.2 Client-Server Model vs P2P Model

5. Access Networks

The network that connects an end system to the first router (edge router) is called the access network.

5.1 Home Access: DSL, Cable, FTTH

DSL (Digital Subscriber Line)

Uses existing telephone lines. The telephone company also serves as the ISP.

Home ---- DSL modem ---- Phone line ---- DSLAM ---- ISP
                                           |
                                      Telephone network

• Upstream: 1-2.5 Mbps (typical)
• Downstream: 10-24 Mbps (typical)
• Asymmetric transmission

Cable Internet

Uses an HFC (Hybrid Fiber Coax) network, leveraging cable TV infrastructure.

Home ---- Cable modem ---- Coaxial cable ---- Fiber node ---- CMTS ---- ISP

• Downstream: 42.8 Mbps (shared)
• Upstream: 30.7 Mbps (shared)
• Shared broadcast medium: Multiple users share bandwidth

FTTH (Fiber To The Home)

Optical fiber is connected directly to the home.

• AON (Active Optical Network): Based on Ethernet switches
• PON (Passive Optical Network): Based on splitters (more common)

5.2 Enterprise Access: Ethernet and WiFi

End system -- Ethernet switch -- Institutional router -- ISP
                  |
          Wireless access point (AP) -- WiFi devices

• Ethernet: 100 Mbps to 10 Gbps
• WiFi (802.11): 11 Mbps to several Gbps (depending on the standard)

5.3 Wide-Area Wireless Access: 3G/4G/5G

Access is provided through mobile carrier base stations. Usable within a range of tens of kilometers.

6. Physical Media

Physical media are broadly divided into guided media and unguided media.

6.1 Guided Media

Twisted Pair Details

• UTP (Unshielded Twisted Pair): Commonly used for LANs within buildings
• Performance varies by category: Cat 5e (1 Gbps), Cat 6 (10 Gbps), etc.

Fiber Optic Details

• Immune to electromagnetic interference
• Very low attenuation, suitable for long-distance transmission
• Used for undersea cables

6.2 Unguided Media

Electromagnetic waves propagate through free space.

• Terrestrial radio channels: WiFi, cellular
• Satellite radio channels: Geostationary orbit satellites (GEO), Low-earth orbit satellites (LEO)

Terrestrial radio channel classification:
  +-- Short range (1-2m): Bluetooth, personal devices
  +-- Local area (tens to hundreds of m): WiFi (802.11)
  +-- Wide area (tens of km): Cellular (3G/4G/5G)

7. Summary

Two perspectives for understanding the Internet:

1. Component perspective: A network composed of hosts, routers, links, and protocols
2. Service perspective: An infrastructure providing services to distributed applications

Key concepts:

• Protocols define the format, order, and actions of messages
• End systems are at the network edge; routers and switches are in the core
• Access networks connect end systems to the network core
• Physical media are divided into guided (copper, fiber) and unguided (wireless) media

8. Review Questions