Client-server model

Roles within a network

Client

• sends out a request for a service to the server

Server

• responds to any incoming requests

Network protocols

• a set of agreed 'rules' or 'laws' for data exchange
  • e.g: the communication protocol is a set rule for exchanging messages between computers

• in many layers each handling a different part of the communication process
  • the collection of layers is known as a protocol stack

TCP - Transmission Control Protocol

• software that ensures a steady flow of data
• manages:
  • connecting the hosts
  • detecting when errors occur
  • retransmitting packets
  • detecting when a connection is broken

Client-server application

• an application that runs as a client and accesses an external server (two-tier client-server)

Examples

• web browsers
• email clients

Web application

• an application that runs inside the web browser (multi-tier)

Examples

• Gmail
• Google Apps

World Wide Web

Also known as WWW.

• a network interlinking web resources through individual URLs and hyperlinks

The Internet

• a global system of interconnected networks that uses TCP/IP to link devices together

PSTN

Also known as Public Switched Telephone Network.

Refers to all telephone networks in the world connected together by switching centres.

• allows any telephone to communicate with another
• internet connection via PSTN is known as 'dialup'
  • data is transferred using existing telephone lines
  • when data is being transmitted, the computer dials the network to set up a connection

Dedicated / lease lines

• a telecommunications cable between two points
• used to provide Internet access
  • this link allows hosting as well as browsing

Other details

• slowly replaced by public internet

Cellphone network

A wireless network spread over land areas; these are called 'cells'.

• covers a wide geographical area when connected together
• allows portable transceivers (e.g. mobile phones) to:
  • communicate with each other
  • access the internet via base stations

Cells

• each cell is served by at least one fixed-location transceiver known as a base stations
• uses a different set of frequencies to avoid interference with another cell

Copper cable

Advantages

• reputable and traditional

Disadvantages

• limited bandwidth
• hazards
• expensive

Fibre-optic cable

Advantages

• fast transmission
• are durable and safer

Disadvantages

• mildly expensive

Wireless

Encompasses radio waves and microwaves.

Advantages

• fast transmission
• cheaper

Disadvantages

• requires infrastructure to reach remote areas

Wireless

Encompasses radio waves and microwaves.

Advantages

• fast transmission
• long range
  • can reach remote areas

Disadvantages

• expensive
• hard to maintain

Bit streaming

Also called 'octet stream'.

• a contiguous sequence of bits, representing a stream of data
• transmitted continuously over a communications path, serially (one at a time)

Buffering of data

• data is downloaded and stored in a buffer before it is used
  • this ensures that there won't be any interruptions during processing

Broadband speed

• Broadband speed required depends on type of stream the user requires
  • better quality means a higher speed is needed

Real-time

An event is captured live.

• the video signal is encoded to streaming media files
• the encoded feed is then uploaded to a file server
• streaming servers duplicate the feed and send it to all clients requesting it in real time

Other details

• need faster broadband compared to on-demand

On-demand

• video is stored on a server as streaming media files
• a bit stream transmits the saved video when a client requests to view
• needs fast broadband speed
  • this is because the client has to download the bits and display it at the same time.

IP address

Also known as Internet Protocol address.

• a unique numerical label assigned to devices connected to a computer network that uses the Internet Protocol

Functions

• identification of host or network
• location addressing

IP address format

• consists of two versions, IPv4 and IPv6 which both have a different number of bits for addressing
• groups of hexadecimal digits are separated by colons

Public IP address

• a globally unique address that can be accessed over the internet
• logged by other servers when connecting through them

Private IP address

• an address within a private network that is not exposed to the Internet
  • e.g: a printer will have a private address so only the family devices may print from it

Protocol

http, also known as Hypertext Transfer Protocol.

• foundation of data communication for the World Wide Web
• provides hypertext documents that include hyperlinks to other resources
  • the user can easily access these by clicking

DNS

Also known as Domain Name Servers

• manages a large database of domain names to IP addresses
• converts a user friendly domain such as "https://sites.google.com" into an IP address: (192.0.2.235)

Loading a webpage

1. User specifies URL in client (browser)
2. Client sends DNS lookup request to convert the URL to an IP address and initiates a TCP connection with server
3. Server acknowledges TCP connection, client sends HTTP requests to retrieve content for the URL
4. Server replies with content for the webpage, the browser retrieves content from HTTP packets to render

Client-side script

• runs on the client (browser)
• written in a code supported by the browser
  • e.g: Javascript
• enables webpages to be scripted

Server-side script

• runs on the server
• written in a code supported by the server
  • e.g: PHP
• used to provide interfaces for client
• limits client access to databases