2 Communication and Information Technology



  • Overview of the chapter:

    • 2.1 Transmission Media
    • 2.2 The internet
    • 2.3 Internet supporting hardware
    • 2.4 The World Wide Web
    • 2.5 Client-Server architecture
    • 2.6 Bit-streaming
    • 2.7 IP Addressing
    • 2.8 Domain name
    • 2.9 HTML, JavaScript, and PHP

    2.1 Transmission Media

    Data could be transmitted by cable or wireless.

    No matter which method is used, the aims are always the same:

    • Aim1: Fast transmission
      • Bandwidth
    • Aim2: Data integrity
      • High-frequency attenuation
      • Interference
    • Aim3: Low cost
      • Cost
      • Require for repeaters
      • Maintenance

    2.1.1 Cable transmission

    There are three types of cable to transmit data.

    Twisted pair Coaxial Fibre-optic
    Structure Copper<br/>Different twist rate to reduce interference<br/>Bundled lines Copper<br/>Unbundled lines Optic Fibres
    Usage 1. Connection of telephone handset to line<br/>2. High-speed local area networks 1. Old long-distance telephone cables<br/>2. Cable televisions 1. Long distance phone line<br/>2. Long distance cabling
    An Image

    You need to remember:
    a) The different sector of the measurement.
    b) Relative performance of the three cables in each of the sector.

    • Fibre-optic performs the best across all sectors, yet it is the most expensive.
    • Twisted pair is the worst, except for "high frequency attenuation".

    Modern is a kind of analog-to-digital convertor. By setting up one at each end, people could connect to the internet through phone lines. (Through POTS / PTSN).

    Typically organizations will lease lines for LAN / WAN.

    PSTNs (Public Switched Telephone Network) provides internet connection through: Traditional networks lines, public hotspot and cellular connection.

    2.1.2 Wireless transmission

    Through electromagnetic radiation. Options are radio, microwave and infrared.

    • Most applications: Microwaves
      • Because of band-width limitations of Radio waves
    • Indoor applications: Infrared

    2.1.3 Wire vs Wireless

    Pros and cons

    Wire Wireless
    Permission Need landowner permission Need government permission
    Interference Greater
    Repeaters More required

    Applications

    Wire Wireless
    Home / Small offices Equally efficient.
    Phone calls Traditional phones Cell phones
    Long-range Uses undersea cables. Uses satellites<br /><br />GEO: Long distance phone and internet connection. <br />MEO: For GPS. <br />LEO: Supplement Earth Phone services. Can connect distant components at low cost.<br /><br />There will be delays in transmission.

    2.2 The Internet

    Network Structure

    LAN < WAN < ISP < Internet.

    • LAN: Local Area Network

      • Short distance communication, e.g. In a same building

      • WLAN: Wireless Local Area Network

    • WAN: Wide Area Network

      • Long distance communication, e.g. The whole country
    • ISP (Internet Service Provider)

      • An ISP is a network.

      • Connections between ISP are managed by IXPs. (Internet Exchange Points.

      • The Tier 1 ISPs are the major content providers.

      • Functions of ISPs

        • Initially, functions of ISP is to provide access to Internet. Now these ISP are known as "access ISPs".
        • They connect to regional ISPs then Tier 1 ISPs.
    • Internet

      • The internet is not a WAN. It is the biggest internetwork in existence.
      • It hasn't been designed, and there's no definition of its structure.

    The World Wide Web is not a network. It is a distributed application available on the Internet. WWW is not Internet.

    WWW contains an enormous collection of websites / multimedia.
    Characteristics of WWW:

    • Uses HTTP to transfer files in HTML formats
    • URL specifies location of website.
    • They can contain Hyperlinks -- when clicked, gives direct access to other web pages.

    2.4 Internet Supporting hardware

    Router

    "A device that acts as a node on the Internet."

    The center of a star topology can be a hub, switch, or a router. When a information is sent to the center:

    * A hub broadcasts it to every end-system

    * A switch is more intelligent. It can track the address of the devices. So it can sent to the specified end-system in a unicast.

    * A router is the most intelligent. It can decide which specific device to broadcast to.

    Routers are the backbones of the Internet. A router can also function as a gateway and network address translation box and firewall.

    Chapter 17.03, A2 Computer Science textbook.

    Switch a LAN

    Gateway

    "A device that connects networks of different underlying technologies."

    When different networks needs to connect to the Internet, a Gateway will be needed.

    Gateways can be combined with Firewalls and Routers.

    Gateway and Switch in a LAN

    Server

    "A device that provides services via a network."

    There are different types of Servers:

    • Application Server
    • Web Server
    • Domain Name Server (DNS)
    • File Server
    • Proxy Server
    Web Server Domain name Server File Server Proxy Server
    Function Provides access to a web application (See Section 2.9) <br />A middleware is needed for the transmission of scripting through the network (See Section 2.5) <br />The client is the web browser. Stores server files<br/>Can be used as cloud storage Can act as a Firewall (->Chapter 8)<br/>Stores a copy of web page as cache. When a page request comes, it, instead of the web server, can respond.<br/>Therefore the web server doesn’t need to search through its file storage every time.
    Location Server Farms, Tier 1 components providers

    2.5 Client-server Architecture

    In a distributed computer system:

    • The client carries out part of the processing
    • A server carries out the other part
    • A middleware helps them to cooperate.

    A web page will be "dynamic" under this architecture. (The information displayed is determined by client's request.)

    Many also require a web-enabled database to be installed on the server. (See Chapter 8: database)


    2.6 Bit Streaming

    Any reference to "streaming media" would imply bit streaming is used.

    Process of bit streaming:

    1. Server gains data
      1. For live-streaming data, the main server first sends the data to multiple content servers. They are responsible to send data to users.
      2. The data is compressed. A bit stream is generated.
      3. The bit stream is sent to the user.
    2. Delivery Process:
      1. The incoming data is stored in the buffer.
      2. If the volume of data in the buffer reaches the high water mark, the user's computer will send a control signal to the server, asking for less data. Vice versa for low water mark.
    3. Playing Process:
      1. The user's media machine reads the data from the buffer and plays it.

    User experience

    • The bit rate user receives should match that of the original file on the server.
      • Transmission speed must be sufficient. (Between server, buffer and player)
      • Transmission to buffer should be faster than file bit rate, to fill up the buffer -- avoids unexpected delays.
    • Buffer size should be big enough.

    2.7 IP Addressing

    IP address is a component of TCP/IP protocol. (-> Chapter 17)

    It defines from where and to where data is transmitted.

    2.7.1 IPv4

    "32-bit long, hierarchical address of a device on the internet."

    Components

    NetID and Host ID.

    • NetID defines the network the device belongs.
    • HostID defines the device in that network.

    The first 3 bit determines class of the IP address.

    First 3 bits First digit range (denary) Net ID Host ID
    Class A 0xx 0~126 First 1 byte Last 3 bytes
    Class B 10x 127~191 First 2 bytes Last 2 bytes
    Class C 110 192~255 First 3 bytes Last 1 byte

    2.7.2 Classless Inter-domain Routing (CIDR)

    On the base of IPv4, CIDR adds a furthermore 8 bits to the end. The 8 bits define which bits are for NetID. Like this:

    Example of an CIDR: 
    11000011000011000000011000001110 / 00010101
    
    Sample Questions:
    Q1. Decode following CIDR to IPv4
    Solution: convert each 8-bit binary to denary. 
    11000011 00001100 | 00000110 00001110 / 00010101
    195     .   12    .    6    .    14. /   21
    
    Final Answer: 195.12.6.14/21
    
    Q2. Find the NetID of this IP address. 
    The suffix is 21, meaning :first 21 bits are netID.
    
    110000110000110000000 | 11000001110 / 00010101
    |                     |              |           |
    |  <--- NetID --->   | <- HostID -> | <-Suffix->|
    
    Solution: the NetID is 110000110000110000000
    

    2.7.3 Sub-netting

    Based on class C of IPv4.

    Instead of giving each LAN a separate network ID, sub-netting combines all LANs to a router. They all share the same, and only one network ID as the router.

    Structure of the IP address also changes.


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