Unit 6: Networks

A computer network is two or more computing devices linked by a transmission medium so they can exchange data and share resources under a common set of rules. The field rests on a handful of ideas the later sections keep invoking.

• Node: any addressable device on the network — a PC, server, printer, phone or router.
• Resource sharing: the core purpose — files, applications, hardware and an internet connection made available to many users.
• Protocol: an agreed set of rules governing how data is formatted, addressed, transmitted and acknowledged.
• Bandwidth: the data-carrying capacity of a link, measured in bits per second (bps, Mbps, Gbps).
• Client and server: a client requests a service; a server provides it (files, print, web, mail).
• Packet: a small unit of data with a header (addresses) and payload, routed independently across the network.

II. Sharing Data Any Time Anywhere

Anywhere access and its enablers

Modern networks let a user reach data regardless of physical location, decoupling information from any single machine.

• Remote access: logging into an organisation's resources from outside via a VPN (Virtual Private Network) that tunnels encrypted traffic over the public internet.
• Cloud storage: files held on internet-hosted servers (Google Drive, OneDrive, Dropbox) synchronised across devices, so an edit on a phone appears on a laptop.
• Mobility: wireless links (Wi-Fi, 4G/5G) free the user from a fixed cable, allowing access from any covered area.
• Anytime availability: servers run continuously, and replication across multiple sites keeps data reachable even if one server fails.
• Concurrency: many users read and write shared data at once; the server manages locking so simultaneous edits do not corrupt a file.

III. Uses of a Network

Why organisations build networks

Networks deliver value by pooling resources and enabling communication that standalone machines cannot.

• Hardware sharing: one printer, scanner or storage array serves many users, cutting cost — e.g. a single network printer for an office of 20.
• File and data sharing: a central file server holds documents any authorised user can open, avoiding duplicate copies.
• Application sharing: software installed on a server (e.g. a database) is used by many clients under a shared licence.
• Communication: email, instant messaging, voice-over-IP (VoIP) and video conferencing run over the network.
• Internet access sharing: one broadband connection is distributed to all nodes through a router.
• Centralised management: administrators apply security policies, backups and software updates from one point.

IV. Common Types of a Network and Hybrid Networks

A. Common types of a network

Networks are classified mainly by geographic scale.

• LAN (Local Area Network): covers a small area — a room, building or campus; high speed (100 Mbps–10 Gbps); privately owned.
• WAN (Wide Area Network): spans cities, countries or continents; the internet is the largest WAN; uses leased lines and public carriers, so it is slower and costlier per bit.
• MAN (Metropolitan Area Network): covers a city or large campus, larger than a LAN but smaller than a WAN — e.g. a city-wide cable TV network.
• PAN (Personal Area Network): very short range around one person, typically via Bluetooth — a phone paired with earbuds and a smartwatch.
• WLAN (Wireless LAN): a LAN using Wi-Fi radio instead of cables.

B. Hybrid networks

A hybrid network combines two or more distinct topologies or network types into a single system to gain the strengths of each.

• Definition: an interconnection that is not purely one topology — e.g. star networks joined by a bus backbone (a star-bus hybrid).
• Common form: several departmental LANs connected through a central backbone, producing a larger unified network.
• Advantages: scalable and flexible; a fault in one segment can be isolated without downing the whole network.
• Trade-off: more complex to design and manage, and costlier due to varied hardware.
• Example: a college where each floor is a star LAN, and the floors are linked by a fibre backbone forming the campus network.

V. How Networks are Structured

Client-server versus peer-to-peer

The structure describes how nodes relate in providing and consuming resources; two models dominate.

1. Client-server: dedicated servers provide resources to client machines.
   • Control: centralised security, accounts and backups managed on the server.
   • Scale: suited to large networks; performance depends on server capacity.
   • Cost: higher — needs powerful server hardware and administration.
2. Peer-to-peer (P2P): every node acts as both client and server, sharing its own resources directly.
   • Control: decentralised; each user manages their own machine's sharing.
   • Scale: best for small networks (typically under ~10 nodes).
   • Cost: low — no dedicated server required.

• Enterprise networks almost always use client-server for security and manageability; home and small-office setups often use peer-to-peer.

VI. Network Topologies and Protocols

A. Network topologies

Topology is the physical or logical arrangement of nodes and the links between them.

• Bus: all nodes share a single backbone cable with terminators at both ends.
  • Cheap and simple; a break in the backbone brings down the whole network.
• Star: every node connects to a central hub or switch.
  • Most common LAN layout; one cable failure affects only that node, but the central device is a single point of failure.
• Ring: nodes form a closed loop; data passes node to node in one direction.
  • Orderly access, but a single break can disrupt the ring unless it is dual-ring.
• Mesh: every node links to many or all others.
  • Highly fault-tolerant with redundant paths; expensive due to heavy cabling — used in WAN backbones.
• Tree (hierarchical): groups of star networks connected to a bus backbone, forming levels.

B. Protocols

A protocol is the rulebook that lets dissimilar devices communicate reliably.

• TCP/IP: the foundational suite of the internet.
  • IP (Internet Protocol): addresses and routes packets between networks.
  • TCP (Transmission Control Protocol): guarantees ordered, error-checked delivery by acknowledging each segment.
• HTTP/HTTPS: transfers web pages; HTTPS adds TLS encryption.
• FTP: transfers files between hosts.
• SMTP/POP3/IMAP: send and retrieve email.
• Ethernet (IEEE 802.3): governs framing and access on wired LANs.
• Wi-Fi (IEEE 802.11): governs wireless LAN communication.

Data flow (TCP/IP layers):
Application  -> HTTP, FTP, SMTP request
Transport    -> TCP splits into segments, adds ports
Internet     -> IP adds source/destination addresses
Network access -> Ethernet/Wi-Fi frames onto the medium

VII. Network Media

Guided versus unguided media

Media are the physical channels that carry signals; they split into cabled and wireless forms.

1. Guided (wired) media: signal is confined to a physical conductor.
   • Twisted-pair cable: copper pairs twisted to reduce interference; UTP Cat5e/Cat6 is the standard LAN cable, up to ~1–10 Gbps over 100 m.
   • Coaxial cable: a central conductor within shielding; used in older LANs and cable broadband.
   • Fibre-optic cable: transmits light through glass; immense bandwidth, immune to electromagnetic interference, spans long distances — used in backbones.
2. Unguided (wireless) media: signal propagates through free space.
   • Radio waves: Wi-Fi and Bluetooth; convenient but shorter range and shared spectrum.
   • Microwave: line-of-sight links between towers.
   • Infrared: very short range, line-of-sight (e.g. some remote controls).

• Selection factors: required bandwidth, distance, cost, and immunity to interference — fibre for speed and distance, twisted-pair for cheap short runs, wireless for mobility.

VIII. Network Hardware

Devices that build and connect networks

Physical hardware creates the links, directs traffic, and adapts devices to the medium.

• NIC (Network Interface Card): connects a device to the network and holds its unique MAC address (a 48-bit hardware identifier).

• Hub: a basic central connector that broadcasts incoming data to all ports; inefficient because it shares bandwidth and causes collisions.

• Switch: a smarter central device that forwards frames only to the destination port using MAC addresses, improving speed and reducing collisions.

• Router: connects different networks and chooses the best path for packets using IP addresses; joins a LAN to the internet.

• Modem: modulates and demodulates signals to carry digital data over an analogue line such as a telephone or cable connection.

• Access point (AP): provides Wi-Fi connectivity, bridging wireless devices to the wired network.

• Gateway: translates between networks using different protocols so they can interoperate.

• Repeater: regenerates a weakening signal to extend the transmission distance.

• Typical assembly: end devices with NICs connect to a switch (star topology), the switch connects to a router, and the router links through a modem to the internet service provider — a common small-office layout combining several of these devices.