Communication

Paper 1 · Unit 2. Networks, topologies, Ethernet and IP addressing.

1. Why networks, LAN and WAN

Networking devices lets users share files, printers and an internet connection, communicate, and manage security and backups centrally.

	LAN	WAN
Area	A small geographical area, e.g. one building or site	A large geographical area, e.g. a country or the world
Ownership	Usually owned by one organisation	Uses communication links owned by others (e.g. telecom companies)
Links	Cables and Wi-Fi owned by the organisation	Leased lines, PSTN, satellites, the internet

2. Client-server and peer-to-peer

	Client-server	Peer-to-peer
Roles	Servers provide services (files, email, web, print); clients request them	Every computer has equal status and can share its own resources
Benefits	Central security, backups and management; scales to many users	Cheap and simple to set up; no single point of failure
Drawbacks	Expensive servers and skilled staff; the server is a single point of failure	Hard to manage security and backups; performance falls as it grows
Suits	Schools, businesses, websites	Small homes and offices, file sharing

A thin client relies on a server for most processing and storage (e.g. a terminal or browser-based app). A thick client does most processing and storage itself, and works partly offline.

3. Topologies

Topology	Description	Benefit	Drawback
Bus	All devices share one backbone cable; data is sent along it to all devices and the addressed one accepts it	Cheap, little cable	Collisions; if the backbone fails the whole network fails
Star	Each device connects to a central switch, which forwards data only to the destination	A failed cable affects one device; fast and secure	The switch is a single point of failure; more cable
Mesh	Devices connect to several others; data can take several routes	Very reliable: alternative routes	Expensive and complex
Hybrid	A combination of two or more topologies	Flexible for large sites	Complex to manage

4. Cloud computing

Cloud computing provides computing services (storage, servers, software) over the internet. A public cloud is shared infrastructure run by a provider for many customers; a private cloud is dedicated to one organisation.

Benefits	Drawbacks
Access from anywhere with an internet connection	Needs a reliable internet connection
No need to buy or maintain servers; pay for what you use	Ongoing costs; reliance on the provider
Easy to scale up or down	Data security and privacy depend on the provider
Provider handles backups and updates	Data may be stored in other countries with different laws

5. Wired and wireless media

Medium	Characteristics
Copper cable (twisted pair, coaxial)	Cheap and easy to install; affected by interference; lower bandwidth over distance
Fibre-optic cable	Uses light; very high bandwidth over long distances; immune to electrical interference; more expensive
Radio waves (Wi-Fi)	No cables, devices can move; shorter range, lower speeds and less secure than wired
Microwaves	Line-of-sight point-to-point links between towers; high bandwidth; affected by weather and obstacles
Satellites	Cover huge areas including remote places; high latency (delay)

6. LAN hardware and routers

Device	Role
Switch	Connects devices in a star LAN and forwards data only to the device it is addressed to (using MAC addresses)
Server	Provides services such as file storage, printing or email to clients
NIC / WNIC	Network interface card (wired / wireless) that connects a device to the network; has a MAC address
WAP	Wireless access point: lets wireless devices join a wired network
Bridge	Connects two LAN segments using the same protocol
Repeater	Regenerates a weakened signal so it can travel further
Router	Connects different networks (e.g. a LAN to the internet); forwards packets towards their destination using IP addresses; often assigns private IP addresses

7. Ethernet and CSMA/CD

Ethernet is the common protocol for wired LANs. On a shared medium, two devices may transmit at once and their signals collide. CSMA/CD handles this:

1. Carrier sense: a device listens and only transmits when the channel is idle.
2. Collision detection: while transmitting it checks for a collision.
3. If a collision is detected, it stops and sends a jam signal.
4. Each device waits a random time (back-off) before trying again.

8. Bit streaming

Bit streaming sends audio or video as a continuous stream of bits that plays while it downloads, using a buffer to smooth out gaps.

• Real-time: a live event is streamed as it happens.
• On-demand: pre-recorded files are streamed when the user chooses.

The bit rate needed depends on quality; if the broadband speed is lower than the bit rate, the buffer empties and playback pauses.

9. The internet, WWW and IP addresses

The internet is the global network infrastructure; the WWW is the collection of web pages accessed over it. Internet hardware includes modems, the PSTN, dedicated (leased) lines and the cell phone network.

Topic	Key points
IPv4	32 bits, four denary numbers 0–255 separated by dots
IPv6	128 bits, eight groups of four hex digits separated by colons; vastly more addresses
Subnetting	Splits a network into smaller subnets; part of the address is the network ID, the rest the host ID; improves management and security
Public vs private	Public addresses are unique on the internet; private addresses (e.g. 192.168.x.x) are used inside a LAN and hidden behind the router, which adds security
Static vs dynamic	Static never changes (good for servers); dynamic is allocated by DHCP each time a device connects

10. URLs and DNS

A URL gives the protocol, domain name and path of a resource, e.g. https://revisionhub.online/coding-lab/. The browser asks a DNS server to translate the domain name into an IP address; if that server does not know, it asks other DNS servers in the hierarchy. The browser then requests the resource from the web server at that IP address.

11. Examiner tips

1. Justify your choice of model or topology for the scenario, not in general.
2. A switch uses MAC addresses; a router uses IP addresses and connects different networks.
3. CSMA/CD: listen, transmit, detect collision, jam signal, random wait, retry.
4. Private IP addresses are not routed on the internet; the router’s public address is used instead.
5. Bit streaming needs a buffer; explain what happens when broadband speed is below the bit rate.
6. Fibre: mention light, bandwidth, distance and immunity to interference.

12. Practice questions

13. Topic test: predict your grade