BCS1110-7 Computer Networks

Lecture notes

• Lecture materials
• Lecture contents
• Introduction to Networks
• Introduction to the internet
• TCP/IP Protocol
• Application Layer Protocols
• Transport Layer Protocols
• Internet Protocols & IP Addresses
• Domain names and DNS
• Network Layer
• Internet Governance and infrastructure
• Hardware for Computer Networks
• Application and Services

Lecture materials

• High school networks notes (in Italian)

Lecture contents

• Introduction to Networks
• Understanding the Internet
• Application and Services
• Developing for the Web

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Introduction to Networks

• Communication networks → to exchange data and information

  • Data → database of values
  • Information → what is extracted from processed data

• Shannon’s Communication Model → Devised by Claude Shannon in 1948

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• Classification of networks by extension

  • PAN (Personal area network) → Serves a single individual wirelessly
  • LAN (Local area network) → connects machines in a single building
  • WAN (Wide area network) → large areas, are made up by smaller networks

• Communication channels → medium for information transmission

  • Wired channels → wires or cables
    • Pros → Shileded, dependable and secure
    • Cons → costly, limited mobility and easy to damage
  • Wireless channels
    • Used means → Radio signals, microwave
    • Radio Frequency → Sent by transceivers through antennas
    • Wireless devices and transceivers → send and receive data and include antennas

• Security in wired channels → Wired connections are secure

• Bandwidth in communication channels

  • Bandwidth → transmission capacity
  • Broadband → at least 25 Mbps
  • Narrowband → Slower than 25 Mbps

• Topology of Networks → structure and layout of network components

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  • Point-to-point → connects peripheral devices to a host
  • Star topology → Connects devices to a central device
  • Mesh topology → All the devices are connected with each other

• Network nodes

  • Network node → any device on a network
  • Data Terminal Equipment → stores or generates data
  • Data Communication Equipment → controls data speed, signal conversion, error checking and routing

• Devices as Network Nodes

  • Router → controls data flow, gateway, communication between multiple networks
  • Modem → converts signals for communication channels
  • Switch → communication within a single network

Introduction to the internet

• Internet → global system of computers connected together
  • Redundancy → The internet is redundant (Redundancy of paths)
  • Hierarchical → internet is organized into levels/groups
• Birth of the internet → 1957 project at Stanford to connect computing machines at a distance as a part of the ARPA project. This later expanded to connect all universities in the US and include multiple services.
• Client-Server Architecture
  • Checkout https://traceroute-online.com

    tracerout [website]
    

Uniform Resource Locator

• Protocol → Rules for the information
• Domain name → human readable, gets converted to an IP Address via a Domain Name Server
• IP Address → Computer readable unique address for a machine in a network
• Hierarchy → each byte of the address gets more specific

Link to original

• Challenges of the internet
  • How does the receiving computer interpret the response
  • How to communicate across multiple operating systems
  • how to ensure all information was transferred
  • how to handle multiple requests and responses to the same router
  • How do routers know where to send information

TCP/IP Protocol

• Defines rules for sending and receiving information between devices
• First developed by the US Department of Defense, now used widely
• Layers of the protocol:
  1. Network Layer → Captures the physical aspects of data transmission and hardware related protocols.
  2. Internet Layer → Looks after the logical transmission of data. Defines an IP Address for each device connected to the network.
  3. Transport Layer → Responsible for end-to-end communication and handles error-free transfers (TCP and UDP protocols). Some data has higher importance than other and is thus handled differently (e.g. bank info)
  4. Application Layer → Where the server defines networking preferences and safety (SSL)

Application Layer Protocols

• Hypertext Transfer Protocol → for web browsing, defines how web browsers and web servers communicate
  • 200s → all good
  • 400s → client errors
  • 500s → server errors
• Languages: HTML, CSS, JavaScript

Transport Layer Protocols

• Responsible for end-to-end communication and data integrtiy
• Handles data segmentation, sequencing, error correction and flow control
• Transmission Control Protocol → ensures reliable communication, confirms data receipt and retransmits lost data
• User Datagram Protocol → Faster but less reliable, suitable for real-time application and video-streaming

Internet Protocols & IP Addresses

• Devices connected to the internet need to adhere to the IP protocol

IP Address

• The ISP provides the IP Addresses → 4 byte (4 * 8 bits = 32 bits) address
  • IPv4 and IPv6 standards
• Static IP → IP Address does not change (e.g. for web services)
• Dynamic IP → IP addresses change frequently (e.g. phone, laptop and personal devices)
• Private IP → Valid only within a local network (unique within the network)
• Public IP → Accessible anywhere on the internet (assigned to router)

• Format: XX.XX.XX.XX
• Hierarchy → First byte corresponds to the larger network, then the following is more and more specific
• IPv6 → Expanding to 6bytes instead of only 4 bytes (but lacking infrastructure)

Link to original

Domain names and DNS

• Domain name → easy to remember
  • Top-level domains → .it, .com, .org, .nl
• Domain Name System → associates domain names with the corresponding IP Addresses

• DNS Spoofing → unauthorized changes to the hierarchy

Network Layer

• Handles the physical aspect of communication
• Specifies and defines cable types, signal standards and data rates
• Ethernet → wired connection with twisted-pair or coaxial cables
• Wi-Fi → standard for wireless connections, using radio frequency tech

Internet Governance and infrastructure

• Internet Corporation for Assigned Names and Numbers (ICANN) → Supervises internet
• ISP (Internet Service Providers) → Maintain the infrastructure
• Tier 1 ISP networks form the backbone of the internet (around 16 globally)
• Tier 2 Service Providers (e.g. Ziggo, Vodafone, KPN)

Hardware for Computer Networks

• Router → forwards packets and helps in allocation and deallocation of Private IP addresses
  • Gateway → each router connects one network to another
• Wireless Access Point → Connected to a router, allow wireless devices to participate in the network
• Content Distribution Network (CDN) → Used in handling high traffic
  • Redistribution → redirects requests and responses through routers to avoid traffic
  • Caching → saves frequently requested resources
  • Load balancing → stores resources across multiple servers to manage load

Distributed Systems

• Aim at solving the high expenses of memory and CPU usage of certain processes
• Linking “cheaper” computers into a bigger machine → each smaller machine solves or stores part of the problem
• Challenges → Computer crashes

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Exam information

All the content after this is irrelevant for the examination

Application and Services

• Client-Server Model →
  • Server → Runs on a server to cater to requests from clients
    • Web Servers
    • EMail Servers
    • Game Servers
    • Realtime Comm. Servers
    • Application Server
  • Client → Limited process, makes requests to the servers
• Databases for Information Storage

Exam style questions

• What does error 404 mean?
• Why do we need standardisation to make the internet work?
• What does the application/transport/internet/network layer do?

Key takeaways and questions

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Lecture slides