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01-introduction-to-computer-networks.md

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Introduction to Computer Networks

Internet

  • Internet is a network of networks - interconnected ISPs (Internet Service Providers);
    • Infrastructure that provides services to users and applications;
  • Protocols define 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;
    • e.g., HTTP, SMTP, FTP, TCP, IP, Ethernet, 802.11 (Wi-Fi);
  • Internet Standards define the protocols used in the Internet;
    • Request for Comments (RFCs) are the documents that describe the standards and protocols used in the Internet;
    • Internet Engineering Task Force (IETF) is the organization that develops and promotes Internet standards;
  • Network has the following structure:
    • Network Edge - hosts (end systems) that run application programs;
    • Access Networks, Physical Media - communication links that connect end systems to the network core;
    • Network Core - interconnected routers that forward packets.

Internet Structure

  • Tier-1 ISPs are the top-level ISPs that connect to each other to form the Internet backbone;
  • Tier-2 ISPs are the middle-level ISPs that connect to Tier-1 ISPs to provide Internet access to end users;
  • Tier-3 ISPs are the bottom-level ISPs that connect to Tier-2 ISPs to provide Internet access to end users;
  • Peering is the relationship between two ISPs that allows them to exchange traffic without charging each other;
  • Internet Exchange Points (IXPs) are the locations where ISPs connect to each other to exchange traffic;
  • Regional ISPs are the ISPs that provide Internet access to a specific region;
  • Content Providers are the companies that provide content and services over the Internet.

Network Edge

  • End Systems are the devices that run application programs;
    • e.g., PCs, smartphones, web servers;
  • Hosts are end systems that are connected to the Internet;
    • Clients are hosts that request services from servers;
    • Servers are hosts that provide services to clients - often located in data centers.

Access Networks

  • Access Networks connect end systems to the network core;
  • Bandwidth is the transmission rate of a communication link in bits per second (bps);
  • Throughput is the actual transmission rate of a communication link in bits per second (bps);
  • There are different types of access networks:
    • Digital Subscriber Line (DSL) - uses existing telephone lines to provide high-speed data connections;
      • Uses modems to convert digital data to analog signals for transmission over the telephone lines;
      • Dedicated Line - provides a direct connection between the customer and the telephone company's central office;
      • < 2.5 Mbps upstream transmission rate;
      • < 24 Mbps downstream transmission rate;
    • Cable Network - uses existing cable television infrastructure to provide high-speed data connections;
      • Uses cable modems to convert digital data to analog signals for transmission over the cable television infrastructure;
      • Frequency Division Multiplexing (FDM) is used to separate the television channels from the data channels;
      • Hybrid Fiber-Coax (HFC) is used to connect the cable headend to the neighborhood;
      • Shared Line - provides a shared connection between the customer and the cable company's headend;
    • Fiber - uses optical fibers to provide high-speed data connections;
      • Passive Optical Network (PON) is used to connect the optical line terminal to the optical network units;
      • GPON is a type of PON that uses Gigabit Ethernet to connect the optical line terminal to the optical network units;
        • Up to 2.5 Gbps upstream transmission rate;
        • Up to 1.25 Gbps downstream transmission rate;
      • Shared Line - provides a shared connection between the customer and the company's headend;
    • Wireless - uses radio waves to provide high-speed data connections;
      • Wireless Access Points (WAPs) are used to connect wireless devices to the network;
    • Ethernet - uses Ethernet cables to provide high-speed data connections;
      • Ethernet Switches are used to connect Ethernet cables to the network.

Physical Media

  • Physical Media are the communication links that connect end systems to the network core;
    • Bit is the smallest unit of data propagated through a network;
    • Physical link is the connection between the transmitter and the receiver;
    • Guided Media are the physical links that guide the electromagnetic signals along a specific path;
      • e.g., twisted pair, coaxial cable, fiber optic cable;
    • Unguided Media are the physical links that allow the electromagnetic signals to propagate freely;
      • e.g., radio waves, microwaves, infrared;
  • Twisted Pair is a type of guided media that uses twisted pairs of copper wires to transmit data;
    • Unshielded Twisted Pair (UTP) is a type of twisted pair that does not have a shield around the copper wires;
    • Shielded Twisted Pair (STP) is a type of twisted pair that has a shield around the copper wires;
    • Category 5e (Cat 5e) is a type of twisted pair that can transmit data at a rate of 1000 Mbps;
    • Category 6 (Cat 6) is a type of twisted pair that can transmit data at a rate of 10 Gbps;
  • Coaxial Cable is a type of guided media that uses a copper core surrounded by a shield to transmit data;
    • Broadband Coaxial Cable is a type of coaxial cable that can transmit data at a rate of 1 Gbps;
  • Fiber Optic Cable is a type of guided media that uses glass or plastic fibers to transmit data;
    • Single-Mode Fiber is a type of fiber optic cable that uses a single mode of light to transmit data;
    • Multi-Mode Fiber is a type of fiber optic cable that uses multiple modes of light to transmit data;
    • Broadband Fiber Optic Cable is a type of fiber optic cable that can transmit data at a rate of 10 Gbps;
  • Radio Waves are a type of unguided media that uses radio waves to transmit data;
    • e.g., Wi-Fi, Bluetooth.

Network Core

  • Network Core is the interconnected routers that forward packets;
  • Packet Switching is the transmission technology that divides messages into small packets that are transmitted independently over the network;
    • Store-and-Forward is the packet-switching technique that stores the entire packet in the router's memory before forwarding it;
    • Queueing Delay is the delay that occurs when packets are stored in a router's memory, in a queue, before being forwarded - when arival rate > transmission rate;
      • Packet Loss occurs when the queue is full and the router drops packets;
      • Packet delay d is the sum of the transmission delay d_trans, propagation delay d_prop, queueing delay d_queue and processing delay d_proc: d = d_proc + d_queue + d_trans + d_prop;
  • There are two key network-core functions:
    • Routing is the process of determining the best path for a packet to take from the source to the destination;
    • Forwarding is the process of moving a packet from the router's input port to the appropriate output port;
  • Circuit Switching is the transmission technology that establishes a dedicated communication path between two devices before the transmission of data;
    • Time Division Multiplexing (TDM) is the circuit-switching technique that divides the communication path into time slots;
    • Frequency Division Multiplexing (FDM) is the circuit-switching technique that divides the communication path into frequency bands;
    • Packet Switching is more efficient than circuit switching because it does not require the establishment of a dedicated communication path.

Protocol Layers

  • Protocol Layers are the abstractions that allow the separation of concerns in the design of a network;
    • Each layer provides a service to the layer above it;
    • Each layer uses the services provided by the layer below it;
    • Each layer communicates with the corresponding layer on the other end of the communication link;
  • The Internet Protocol Stack is the set of protocols used in the Internet - also known as the TCP/IP Protocol Stack:
    • Application Layer is the top layer that provides services to applications - e.g., HTTP, SMTP, FTP;
    • Transport Layer is the layer that provides end-to-end communication services - e.g., TCP, UDP;
    • Network Layer is the layer that provides routing services - e.g., IP;
    • Link Layer is the layer that provides data-link services - e.g., Ethernet, 802.11;
    • Physical Layer is the bottom layer that provides physical services;
  • The OSI Model is the model that defines seven layers of protocols, adding the Presentation Layer and the Session Layer to the Internet Protocol Stack - this layers are in the Application Layer of the Internet Protocol Stack.

Protocol Layers

  • Encapsulation is the process of adding headers and trailers to the data at each layer of the protocol stack;
    • Data is the information that is transmitted over the network;
    • Header is the information that is added to the data at each layer of the protocol stack;
    • Trailer is the information that is added to the data at each layer of the protocol stack;
    • Packet is the data, header, and trailer at each layer of the protocol stack.

Wireshark is a network protocol analyzer that captures and displays the packets traveling through a network.