Describe data transmission methods (fibre optic, wireless)

Communications Technology (Cambridge IGCSE / A‑Level IT 9626)

1. Overview of Networks

Networks interconnect devices so that data, resources and services can be shared. The syllabus distinguishes several network types, each with characteristic bandwidth, latency, media and typical uses.

1.1 Network Types – characteristics table

Network Type	Typical Bandwidth	Typical Latency	Common Media	Typical Uses
Local Area Network (LAN)	100 Mbps – 10 Gbps (Cat 6a, fibre)	≤ 1 ms	UTP, fibre‑optic, Wi‑Fi	School computer labs, office floor, data‑centre interconnect
Metropolitan Area Network (MAN)	1 Gbps – 100 Gbps	≈ 5 ms	Fibre‑optic, microwave links	City‑wide campus networks, ISP aggregation
Wide Area Network (WAN)	10 Mbps – 400 Gbps (backbone fibre, satellite)	10 ms – 500 ms (satellite)	Fibre‑optic, leased lines, satellite	Internet connectivity, inter‑branch links
Personal Area Network (PAN)	≤ 3 Mbps (Bluetooth), 600 Mbps (Wi‑Fi 802.11ax)	≤ 10 ms	Bluetooth, Infra‑red, Wi‑Fi	Connecting a laptop to a printer, headphones, smartphones
Mobile (Cellular) Network	5 Mbps – 1 Gbps (4G/5G)	≈ 30 ms – 100 ms	Cellular radio (UHF, microwave, mm‑wave)	Hand‑held devices, IoT, mobile broadband

1.2 Network Topologies

Topologies describe how devices are physically or logically linked. The syllabus expects knowledge of the four basic forms and their advantages/disadvantages.

Topology	Physical / Logical Example	Advantages	Disadvantages
Star	All devices connect to a central switch or hub (most modern LANs)	Easy to add/remove devices; failure of one link does not affect others	Central device is a single point of failure; more cabling required
Bus	Devices share a single coaxial or twisted‑pair backbone (early Ethernet)	Simple, inexpensive cabling	Collision domain is large; a break in the backbone disables the whole network
Ring	Each device connects to two neighbours forming a closed loop (Token Ring, FDDI)	Predictable access method; can be fault‑tolerant with dual rings	Failure of one link can break the ring unless a secondary ring is present
Mesh	Multiple redundant paths between devices (e.g., WAN backbone, Wi‑Fi mesh)	High reliability and load‑balancing	Complex, costly cabling and configuration

1.3 Network Architectures

• Client‑Server – Dedicated servers host resources (files, web pages, applications). Clients request services.
• Peer‑to‑Peer (P2P) – Every node can act as both client and server, sharing resources directly (e.g., file‑sharing apps).
• Virtual Private Network (VPN) – Creates an encrypted “tunnel” over a public network so remote users appear as if they are on the private LAN.

2. Network Components

Component	Primary Function	Typical Placement
Network Interface Card (NIC)	Provides physical & data‑link connection to the medium.	Inside every end‑device (PC, laptop, printer).
Hub	Repeats incoming electrical signals to all ports (OSI Layer 1).	Legacy small LANs; now largely replaced by switches.
Switch	Forwards frames based on MAC addresses (OSI Layer 2).	Core of modern LANs; connects end‑devices and APs.
Router	Routes packets between different IP sub‑networks (OSI Layer 3).	Between LAN and WAN, or between multiple LANs.
Bridge	Connects two LAN segments and filters traffic by MAC address.	Often integrated into switches; used in older designs.
Access Point (AP)	Provides wireless connectivity to a wired LAN.	Strategically placed to cover a building or campus.
Gateway	Translates between different network protocols (e.g., LAN ↔ Internet).	At the edge of a network, often combined with a router.

3. Network Addressing

3.1 MAC Addressing (Data‑Link Layer)

• 48‑bit hexadecimal identifier burned into NICs (e.g., 00‑1A‑2B‑3C‑4D‑5E).
• Globally unique – used by switches to forward frames within a LAN.

3.2 IPv4 Addressing (Network Layer)

• 32‑bit binary address written in dotted decimal (e.g., 192.168.1.10).
• Divided into network and host portions using a subnet mask (e.g., 255.255.255.0 → /24).
• Classful legacy (A, B, C) is no longer taught; focus on CIDR notation.

3.3 IPv6 Addressing

• 128‑bit address written in hexadecimal groups (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334).
• Provides virtually unlimited address space and built‑in features such as auto‑configuration.

3.4 Subnetting Example (A‑Level)

Given network 192.168.10.0/24 and a requirement for 4 sub‑nets, the subnet mask becomes 255.255.255.192 (/26). Resulting sub‑nets:

192.168.10.0  – 192.168.10.63
192.168.10.64 – 192.168.10.127
192.168.10.128 – 192.168.10.191
192.168.10.192 – 192.168.10.255

4. Routing Concepts

4.1 Static vs. Dynamic Routing

• Static routing – Administrator manually enters routes. Simple, predictable, but does not adapt to failures.
• Dynamic routing – Routers exchange information using routing protocols; tables update automatically.

4.2 Routing Tables

Each router maintains a table of destination networks, next‑hop addresses, and metrics (cost, hop count, bandwidth). The best match (longest prefix) is chosen for forwarding.

4.3 Interior Gateway Protocols (IGP)

Protocol	Typical Metric	Common Use
RIP (Routing Information Protocol)	Hop count (max 15)	Small, simple LAN/WANs
OSPF (Open Shortest Path First)	Link cost (bandwidth‑based)	Large enterprise networks

4.4 Exterior Gateway Protocol (EGP)

• BGP (Border Gateway Protocol) – Path‑vector protocol used between autonomous systems (e.g., Internet service providers).

5. Network Security

5.1 Firewalls

• Device or software that filters traffic based on a set of rules (source/destination IP, ports, protocol).
• Typical placement: between the LAN and the Internet (perimeter firewall) or between VLANs (internal firewall).
• Basic rule‑set example:

  ALLOW  TCP  192.168.1.0/24  →  0.0.0.0/0  port 80,443   (web traffic)
  DENY   ALL  any            →  any                     (default deny)
  ALLOW  ICMP any            →  any                     (ping for troubleshooting)
          

5.2 Virtual Private Networks (VPN)

• Creates an encrypted tunnel over a public network (usually the Internet).
• Common tunnelling protocols:
  • IPSec – operates at Layer 3; provides authentication, confidentiality, and integrity.
  • SSL/TLS VPN – uses HTTPS (Layer 7) for remote‑access clients.
  • L2TP/IPSec – combines Layer 2 tunnelling with IPSec security.
• Security considerations: strong encryption (AES‑256), mutual authentication (certificates or pre‑shared keys), and proper key management.

6. Cloud Computing

6.1 Service Models

Model	What is provided?	Typical Example
IaaS – Infrastructure as a Service	Virtual machines, storage, networking.	Amazon EC2, Microsoft Azure VMs
PaaS – Platform as a Service	Runtime environment, databases, development tools.	Google App Engine, Heroku
SaaS – Software as a Service	Complete applications delivered via a web browser.	Google Workspace, Microsoft 365

6.2 Deployment Models

Model	Ownership & Access	Typical Use‑case
Public Cloud	Owned & operated by a third‑party provider; shared resources.	Small schools using Google Workspace.
Private Cloud	Dedicated infrastructure for a single organisation (on‑premise or hosted).	University data centre offering internal services.
Hybrid Cloud	Combination of public and private clouds, with data and applications moving between them.	Backup of on‑site servers to a public cloud storage service.

7. Data Transmission Media

7.1 Copper Cabling

• Unshielded Twisted Pair (UTP) – Cat 5e, Cat 6, Cat 6a. Supports 1 Gbps (Cat 5e) to 10 Gbps (Cat 6a) over ≤ 55 m for 10 Gbps.
• Coaxial Cable – Used for cable TV and some broadband (DOCSIS). Bandwidth up to 1 Gbps; attenuation ≈ 0.5 dB/100 m at 1 GHz.

7.2 Fibre‑Optic Transmission

Light pulses travel through a glass or plastic core, offering very high capacity and immunity to EMI.

• Principle: Laser diode or LED emits light; “light on” = binary 1, “light off” = binary 0.
• Key characteristics:
  • Bandwidth: up to several Tb/s on a single strand.
  • Attenuation: ≈ 0.2 dB/km for modern single‑mode fibre.
  • EMI immunity and high security – tapping is difficult and detectable.
• Types of fibre:
  • Single‑mode – core ≈ 9 µm; long‑distance (tens‑to‑hundreds of km) high‑speed links.
  • Multimode – core 50–62.5 µm; short‑reach (≤ 2 km) within buildings or data‑centres.
• Typical applications: Backbone networks, data‑centre interconnects, submarine cables, high‑definition video streaming.

7.3 Wireless Transmission

• Principle: Antenna converts electrical signals to RF/microwave radiation; a receiving antenna reconverts them to electrical form.
• Key characteristics:
  • Mobility – users can move within the coverage area.
  • Bandwidth varies with frequency band, modulation scheme and environment.
  • Susceptible to interference, attenuation, multipath fading.
  • Security – encryption (WPA3 for Wi‑Fi, LTE/5G encryption) is essential.
• Frequency bands (examples):
  • LF (30–300 kHz) – long range, very low data rates (e.g., RFID).
  • VHF (30–300 MHz) & UHF (300 MHz–3 GHz) – TV, FM radio, early mobile phones.
  • Microwave (3–30 GHz) – Wi‑Fi (2.4/5 GHz), radar, satellite links.
  • Millimetre‑wave (30–300 GHz) – 5G back‑haul, high‑capacity point‑to‑point links.
• Common wireless technologies (A‑Level focus):
  • Wi‑Fi – IEEE 802.11ac/ax (2.4 GHz & 5 GHz, up to 9.6 Gbps).
  • Bluetooth – short‑range, low‑power (up to 2 Mbps, BLE 5.0).
  • LTE/5G – cellular broadband (LTE up to 300 Mbps downlink, 5G > 1 Gbps).
  • Satellite – GEO or LEO constellations for remote broadband.

7.4 Satellite Links

• Geostationary (GEO) satellites at ≈ 36 000 km give coverage of a large region but introduce latency of 500 ms–1 s.
• Low‑Earth‑Orbit (LEO) constellations (e.g., Starlink) sit at 500–1 200 km, offering latency < 30 ms and bandwidth up to several hundred Mbps.
• Used where terrestrial cabling is impractical – rural schools, maritime communication.

7.5 Impact on Streaming & Latency

• Bit‑rate vs. video quality (typical values): 1080p ≈ 5 Mbps, 4K ≈ 25 Mbps, 8K ≈ 80 Mbps.
• Latency sources: propagation delay (≈ 5 µs/km in fibre, 3 µs/km in air), processing delay in routers/switches, and queuing delay.
• Real‑time applications (online gaming, video conferencing) require latency ≤ 30 ms; high‑bandwidth, low‑latency fibre is preferred.

8. Network Performance & Quality of Service (QoS)

Metric	Definition	Typical Units / Target
Throughput	Amount of data successfully transferred per unit time.	Mbps or Gbps; aim for ≥ 80 % of link capacity.
Latency	Time taken for a single packet to travel from source to destination.	Milliseconds; ≤ 5 ms for LAN, ≤ 30 ms for WAN gaming.
Jitter	Variation in packet delay – critical for voice/video.	≤ 30 ms for VoIP.
Packet loss	Percentage of packets that never reach the destination.	≤ 1 % for most applications; ≤ 0.1 % for video streaming.
QoS mechanisms	Prioritisation (DSCP, VLAN tagging), traffic shaping, policing.	Ensures latency‑sensitive traffic (VoIP, video) gets priority.

9. Network Protocols (OSI / TCP‑IP)

Protocol	OSI / TCP‑IP Layer(s)	Purpose / Typical Use
TCP	Transport (Layer 4)	Reliable, connection‑oriented data transfer (web pages, file transfers).
UDP	Transport (Layer 4)	Connection‑less, low‑latency transmission (streaming, online games).
IP	Network (Layer 3)	Addressing and routing of packets across networks.
ICMP	Network (Layer 3)	Diagnostics and error messages (e.g., ping, traceroute).
ARP	Data Link (Layer 2)	Maps IPv4 addresses to MAC addresses on a LAN.
DHCP	Application (Layer 7)	Dynamic allocation of IP addresses and network parameters.
HTTP / HTTPS	Application (Layer 7)	Web page transfer; HTTPS adds TLS encryption.
FTP	Application (Layer 7)	File transfer between client and server.
SMTP	Application (Layer 7)	Sending email messages.
POP3 / IMAP	Application (Layer 7)	Retrieving email from a server.
TLS / SSL	Presentation (Layer 6)	Encrypts data for secure transport (used by HTTPS, email).
OSPF, RIP, BGP	Network (Layer 3)	Routing protocols – OSPF/RIP for interior routing, BGP for inter‑autonomous‑system routing.

10. Fundamental Capacity Limit – Shannon‑Hartley Theorem

The maximum theoretical data rate C of a communication channel is:

C = B · log₂(1 + S/N)

• B = bandwidth of the channel (Hz).
• S/N = signal‑to‑noise ratio (linear, not in dB).

The theorem applies to both fibre‑optic and wireless links; fibre typically offers a very high B and excellent S/N, whereas wireless links have lower bandwidth and are more affected by noise and interference.

11. Summary Diagram (suggested)