Network topologies and cellular systems: HSC Engineering Studies Telecommunications Engineering

In a star topology every node connects to a central hub or switch. Cabling cost is moderate (one cable per node) and faults are easy to isolate, but the central node is a single point of failure: if it fails, the whole network fails. In a full mesh every node connects directly to every other node, giving very high reliability (many redundant paths, no single point of failure) but at a steeply rising cabling cost, since $n$ nodes need $n(n-1)/2$ links. The engineering choice is reliability versus cost: star for typical LANs, mesh (or partial mesh) for critical backbones. Markers reward the description of each topology, the single-point-of-failure point for star, the redundancy point for mesh, and the cabling-cost contrast.

Frequency reuse divides the available spectrum into groups and assigns them to cells in a repeating pattern so that the same frequencies are reused in non-adjacent cells far enough apart that co-channel interference is acceptable. This multiplies the total system capacity, letting the network serve far more users than the raw spectrum alone would allow. Handover (handoff) transfers an active call from one base station to the next as the user crosses a cell boundary, the network monitoring signal strength and switching channels seamlessly so the call is not dropped. Smaller cells are used in dense urban areas because each cell can be reused more often across a given area, multiplying capacity where user density is highest, although this needs more base stations and more frequent handovers. Markers reward the reuse mechanism with the interference constraint, the handover description, and the capacity justification for small cells.

A star topology connects every node to a single central hub or switch. Its main weakness is a single point of failure: if the central hub fails, every node loses connectivity, even though the individual node-to-hub links are still intact.

Marking criteria: 1 mark for the correct structural definition, 1 mark for correctly naming the central hub as the single point of failure.

Any three of the following earn full marks.

• Geographic scope: a LAN covers one building or campus; a WAN spans cities or countries.
• Ownership: a LAN is typically owned and operated by the user organisation; a WAN is typically operated by telcos, with users paying for capacity.
• Typical technology: LANs commonly use Ethernet or Wi-Fi; WANs commonly use leased lines, fibre backbones, or satellite links.
• Speed: LANs typically run from 100 Mbps to 10 Gbps; WAN link speeds vary more widely, from a few Mbps to multi-gigabit.

Marking criteria: 1 mark per correct, clearly stated difference (maximum 3).

Cellular operators are allocated a limited total amount of spectrum. Frequency reuse divides that spectrum into a small number of frequency groups and assigns different groups to adjacent cells, so cells close together do not interfere. Because the same frequency groups can then be reused again once cells are far enough apart, the total number of simultaneous users the network can serve is multiplied well beyond what the raw spectrum allocation alone would allow at a single cell.

A smaller reuse factor (frequencies reused more often, i.e. reused in cells that are closer together) increases capacity further but raises co-channel interference, which can degrade call and data quality unless the network compensates with more careful power control, cell planning, or advanced interference-management techniques (used in modern OFDMA systems).

Marking criteria: 1 mark for stating spectrum is limited, 1 mark for correctly describing frequency groups assigned to non-adjacent cells, 1 mark for the capacity-multiplication consequence, 1 mark for a valid engineering cost of a smaller reuse factor (interference, cell-planning complexity).

(a) Bandwidth per group.

(b) Effective bandwidth from group A reused 4 times.

(c) Why this exceeds 14 MHz. The 14 MHz figure is the total spectrum the operator is licensed to use once. Reuse means the same 2 MHz slice of spectrum is transmitted independently and simultaneously in multiple geographically separated cells, because those cells are far enough apart that their signals do not meaningfully interfere with each other. The spectrum is not being duplicated physically; it is being spatially reused, so the effective capacity delivered to users across the whole network (8 MHz worth of group A traffic, plus similar multiples for the other six groups) is much larger than the single 14 MHz licence would suggest if it could only be used once.

Marking criteria: 1 mark for correct division (2 MHz per group), 1 mark for correct multiplication (8 MHz effective), 1 mark for explaining that reuse is spatial (not physical duplication of spectrum), 1 mark for linking non-adjacent cell spacing to acceptable interference, 1 mark for explicitly contrasting "licensed once" with "delivered many times over across the network".

A GEO satellite (about 36,000 km altitude) gives wide, stable coverage from a single satellite, well suited to broadcast television where a small one-way delay of around 240 ms does not disrupt one-way viewing. However, that same delay (around 480 ms round trip) is disruptive for interactive applications such as video calls, where the resulting lag makes conversation awkward.

A LEO constellation (roughly 500 to 2000 km altitude) gives round-trip latency in the tens of milliseconds, comparable to good terrestrial links, making it far better suited to video calls. The trade-off is that LEO coverage depends on a large, actively managed constellation (hundreds to thousands of satellites) rather than one fixed satellite, and can be more exposed to weather and terminal cost considerations.

A well-engineered solution for the community would likely use GEO or terrestrial broadcast for television and a LEO service (or terrestrial 4G/5G where available) for interactive video calling, rather than forcing one orbital regime to do both jobs well.

Marking criteria: 1 mark for correct GEO altitude/latency figures, 1 mark for correct LEO altitude/latency figures, 1 mark for correctly matching each service to its better-suited orbital regime, 1 mark for a justified overall recommendation rather than a single blanket answer.

This is a 6-mark EVALUATE: markers reward a judgement supported by contrasted engineering evidence across both the local topology and the WAN link, not a description of each option in isolation.

Local topology within the estate

A star (or tree of stars) topology from each house to a local distribution cabinet, then onward to an aggregation point, is the standard and most cost-effective choice: cabling cost scales linearly with the number of houses (n-1 links), and a fault on one house's link only affects that house. A full mesh between 400 houses would need n(n-1)/2, close to 80,000, individual links: completely uneconomic and unmanageable for a residential estate. The single point of failure at each distribution cabinet is an acceptable risk because a cabinet failure affects only its local group of houses, not the whole estate, provided cabinets are kept reasonably small.

WAN link to the exchange

Over 15 km, a fibre WAN link is now standard practice in Australia (NBN fibre-to-the-premises or similar) and gives high throughput with low latency, but a single fibre run is itself a single point of failure for the whole estate if it is cut. Because the estate also has a critical emergency operations centre, that link specifically should not depend on the same single fibre path: an additional backup path, for example a fixed-wireless (4G/5G) or satellite (LEO) link, adds resilience so the emergency centre and, ideally, essential estate services remain reachable if the primary fibre is damaged (e.g. by trenching work elsewhere).

Judgement

A star/tree topology is the right choice locally because reliability at the individual-house level does not justify mesh-level redundancy cost, but the emergency operations centre changes the calculus for the WAN link: its criticality justifies the extra cost of a secondary path (fixed wireless or satellite) that a purely cost-driven design would otherwise skip. This reflects the standard engineering trade-off of matching redundancy investment to the actual criticality of the service, not applying one topology rule everywhere.

Marker's note: top-band answers (1) correctly identify star/tree as appropriate for the estate with a cost justification using the mesh link-count formula, (2) correctly identify fibre as the appropriate WAN technology with its single-point-of-failure limitation, (3) specifically single out the emergency operations centre as justifying additional redundancy rather than treating the whole estate uniformly, and (4) end with an explicit judgement connecting redundancy cost to service criticality.