Investigate network topologies (star, bus, ring, mesh, tree), local-area vs wide-area networks, and the engineering principles behind cellular networks (cells, frequency reuse, handover, generations 2G-5G) and satellite communications

What this dot point is asking

A telecommunications network connects many endpoints; its architecture determines reliability, cost, and how it scales. HSC Engineering Studies expects you to compare network topologies on their engineering trade-offs, understand the structural reasons cellular networks can serve billions of users, and place satellite communications in the same comparison framework.

The answer

A network is a graph of nodes (endpoints, switches, base stations) connected by links (cables, radio paths). The structural choice of how nodes connect is the topology; the technical choices about how the network grows and adapts are what cellular and satellite engineering add to that base.

Local network topologies

Star

All nodes connect to a central hub. Simple, cheap, easy to add or remove nodes. If the hub fails, the whole network fails. Most home and office LANs are star topologies (Wi-Fi access points or Ethernet switches act as the central hub).

Bus

All nodes share a single backbone cable. Cheap and simple in concept; common in early Ethernet (10BASE-2 coaxial). One cable break disrupts the whole network. Largely obsolete in modern wired networks.

Ring

Nodes connect in a closed loop. Each node receives data and passes it on. Token Ring (older IBM technology) and SONET (fibre rings in telco networks) use this. Predictable performance, but a single node failure can break the ring (although dual-ring SONET adds resilience).

Mesh

Many nodes connect to many other nodes. Full mesh has every node connected to every other. Highly resilient (multiple paths between nodes) but expensive (n(n-1)/2 links for n nodes). The Internet's backbone is a partial mesh.

Tree

Hierarchical, with a root node branching to children. Common in larger enterprise networks and cable TV distribution.

The choice of topology balances reliability (mesh wins), cost (bus/star win) and ease of management (star wins).

LAN vs WAN

• Local Area Network (LAN). Same building or campus. Typical: Ethernet (twisted pair or fibre), Wi-Fi. Speeds in the 100 Mbps to 10 Gbps range. Owned and operated by the user organisation.
• Wide Area Network (WAN). Spans cities or countries. Typical: leased lines from telcos, fibre backbones, satellite links. Speeds vary from a few Mbps to multi-gigabit. Operated by telcos; users pay for capacity.

The Internet is the world's largest WAN, built from interconnected ISP networks.

Cellular networks: the core innovation

Cellular networks solved the problem of serving many mobile users with limited radio spectrum. The key idea: divide service area into small geographic cells, each served by its own base station, then REUSE radio frequencies in non-adjacent cells.

• Cells. Each cell typically covers an area from a few hundred metres (dense urban) to several kilometres (rural). Hexagonal geometry is the idealised cell shape (real cells follow terrain, not geometry).
• Frequency reuse. Adjacent cells use different frequency bands to avoid interference; distant cells reuse the same frequencies. The reuse factor (typically 1/7 in older systems, 1/3 or 1/1 in modern OFDMA systems) determines spectrum efficiency.
• Handover (or handoff). As a mobile user moves between cells, the base station hands the call/data session to the adjacent cell's base station. Hard handover (break before make) was common in 2G; soft handover (multiple base stations simultaneously serving the user) is common in 3G+ systems.
• Base stations. Each base station has transmit/receive antennas, radio equipment, and backhaul (typically fibre) to the network core. Modern small cells and DAS (distributed antenna systems) supplement traditional cell towers in dense areas.

Cellular generations

• 1G (1980s). Analog voice (NMT, AMPS). Now retired in Australia.
• 2G (1990s). Digital voice + SMS (GSM in Australia and most of the world; CDMA in some networks). Retired in Australia (Telstra in 2017, others by 2018).
• 3G (2000s). Digital voice + data (UMTS, HSPA). Speeds up to a few Mbps. Largely retired or being retired in Australia by 2024-2025.
• 4G LTE (2010s). All-IP architecture, OFDMA, MIMO, peak speeds in the tens to hundreds of Mbps. The current dominant cellular network in Australia.
• 5G (2020s). Three deployment modes: low-band (similar coverage to 4G, modest speed gains), mid-band (the workhorse 5G band, around 3.5 GHz, substantial capacity gains), and mmWave (24+ GHz, very high speed but short range). Adds massive MIMO, beamforming, network slicing, and low-latency targets.

Each generation adds capabilities while typically reducing latency and increasing peak throughput. Modern devices typically support multiple generations simultaneously for backward compatibility.

Satellite communications

• Geostationary (GEO) satellites. Orbit at approximately 36,000 km altitude over the equator. Each satellite covers about a third of the Earth's surface. One-way path delay is around 240 ms (so round-trip latency is around 480 ms minimum). Used for broadcast TV, some long-distance telephony, NBN Sky Muster.
• Low Earth Orbit (LEO) satellites. Orbit at typically 500 to 2000 km altitude. Round-trip latency in the tens of milliseconds. Each satellite covers a small area but constellations (hundreds to thousands of satellites) provide continuous coverage. Examples: Starlink (SpaceX), OneWeb, Iridium. Growing rapidly in 2024-2026.
• Medium Earth Orbit (MEO). Between LEO and GEO. GPS and other navigation satellites use MEO.

Satellite is the engineering choice for remote-area coverage where terrestrial cabling or microwave is uneconomic, and for global broadcast. LEO constellations are eroding the latency disadvantage that traditionally favoured terrestrial links for interactive applications.

Examples in context

Example 1. Telstra's 3G retirement and the move to 4G/5G. Telstra retired its 3G network in 2024. Customers with 3G-only devices (older phones, some IoT devices, some medical alarms) had to migrate to 4G/5G or risk losing service. The decision freed up spectrum for 4G and 5G expansion. The pattern is repeating globally as carriers harvest legacy spectrum.

Example 2. NBN Sky Muster vs Starlink as a competitive comparison. NBN's Sky Muster service uses geostationary satellites with one-way latency around 240 ms (so 480 ms round-trip), limiting its suitability for video calls and gaming. Starlink (LEO) offers latency in the 30 to 50 ms range, comparable to NBN Fixed Wireless. Remote Australian users increasingly choose Starlink for the latency advantage despite cost. The example illustrates how a structural choice (orbital regime) creates very different user experiences.

Try this

Q1. Compare a star and a mesh network topology on reliability and cost. [4 marks]

• Cue. Star: cheap (few links, n-1 cables for n nodes), easy to manage, but the central hub is a single point of failure. Mesh: highly reliable (multiple paths between nodes), but expensive (up to n(n-1)/2 links for full mesh), and complex to manage. The trade-off is reliability vs cost; most real networks use partial mesh on the backbone and star on access.

Q2. Explain the role of frequency reuse in cellular networks. [4 marks]

• Cue. Spectrum is limited; cellular operators have only tens to hundreds of MHz of allocated bandwidth. Frequency reuse divides the service area into cells, with adjacent cells using different frequency bands to avoid interference and distant cells reusing the same frequencies. The reuse factor (typically 1/7 in older systems, 1/3 or 1/1 in modern OFDMA) determines spectrum efficiency. Without frequency reuse, cellular networks could only support a small fraction of the users they currently serve.

Q3. A regional Australian engineer is designing connectivity for a rural community 200 km from the nearest fibre point. Compare a 4G/5G fixed-wireless solution, a GEO satellite link, and a LEO satellite link on latency, throughput, and reliability. [6 marks]

• Cue. 4G/5G fixed wireless: lowest latency (10 to 30 ms), throughput up to tens of Mbps, dependent on tower line-of-sight. GEO satellite (Sky Muster): high latency (around 480 ms round-trip) limits interactive applications, throughput tens of Mbps, very reliable coverage but degraded in heavy rain. LEO satellite (Starlink): low latency (30 to 50 ms), throughput 50 to 250 Mbps, reliability depends on constellation density and weather, growing rapidly. The engineering choice depends on what the community needs most (latency for video calls argues against GEO; coverage continuity argues against terrestrial wireless).