People use the words internet and web as if they meant the same thing. They do not: one is the hardware, the other is a service that runs on it. This lesson separates them, looks at the hardware that carries the internet to your home, and explains how video and music are streamed.
The internet is the global network of interconnected networks: the physical infrastructure of cables, routers and links. The World Wide Web is a collection of web pages and resources, linked by hyperlinks and reached using HTTP(S) and a browser; it is just one service that runs over the internet. The internet is supported by modems, the PSTN, dedicated lines and the cell phone network. Bit streaming sends audio or video as a continuous stream of bits that is played as it arrives, from a buffer, either real-time (live) or on-demand (pre-recorded); the connection's bit rate must be at least the stream's bit rate.
Fig. 1: a streamed file is played from a buffer while the rest of it is still arriving
Section 01
Internet or web?
In the lab, send a message across a network of routers. That network of routers and links is the internet; a web page is just one kind of data it carries.
The internet
The World Wide Web (WWW)
a global network of interconnected networks
a collection of linked web pages, documents and resources
hardware and infrastructure: cables, routers, servers, satellites
content and a service: web pages written in HTML, identified by URLs
uses the TCP/IP protocols to move packets
uses HTTP and HTTPS, accessed with a web browser
carries many services: email, file transfer, VoIP, streaming, online games and the web
is one of the services that runs over the internet
Examiner trap
"The internet is the web" and "you use a browser to get on the internet" both lose marks. Email, for example, uses the internet but not the web.
Section 02
Internet hardware.
The syllabus names four things that connect homes, businesses and mobile users to the internet.
Hardware
What it is and does
Modem (modulator-demodulator)
converts the computer's digital signals into analogue signals for transmission over a telephone line, and converts received analogue signals back into digital
PSTN (public switched telephone network)
the worldwide network of telephone lines and exchanges originally built for voice calls; it also carries internet data, from slow dial-up to broadband over the same copper lines into homes
Dedicated (leased) line
a permanent, private connection between two sites, rented from a telecoms company; not shared, so it has a guaranteed bit rate and is more secure, but it is expensive
Cell phone network
an area is divided into cells, each with a base station (mast); mobile devices connect by radio to the nearest mast, and the network hands the connection on as the user moves; 4G and 5G give mobile internet access
Data then travels across the internet's backbone of fibre-optic cables, undersea cables, satellite and microwave links, passed on by routers.
An ISP (internet service provider) connects users to this backbone.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
Which statement is correct?
Quick check+10 XP
In the lab, why can packets of one message take different routes?
Quick check+10 XP
What does a modem do?
Quick check+10 XP
Which is an example of real-time bit streaming?
Section 03
Bit streaming.
Streaming lets you start watching before the whole file has arrived. Whether it plays smoothly depends on the bit rate.
Bit streaming: audio or video is sent as a continuous sequence of bits, compressed, and played as it arrives. The file does not have to be downloaded first.
Buffer: incoming data is held in a buffer in memory and played from there. While the buffer holds enough data, short drops in connection speed are not noticed. If it empties, playback pauses (buffering).
Real-time (live) streaming: the event is encoded and sent as it happens, e.g. a live match or a video call. It cannot be fast-forwarded and the buffer is kept small to reduce the delay behind the live event.
On-demand streaming: pre-recorded files are stored on a server and streamed when the user chooses, e.g. a film or a podcast. The user can pause, rewind and skip, and a larger buffer can be built up.
Bit rate: the number of bits sent per second (e.g. Mbps). The broadband connection's bit rate must be at least the bit rate of the stream; higher-quality video needs a higher bit rate. Several users sharing one connection share its bit rate.
Worked example Data used by a stream
A film is streamed at 5 Mbps for 1 hour. How much data is transferred, in gigabytes? (1 Mbps = 1 000 000 bits per second; 1 GB = 1 000 000 000 bytes.)