The idea
Every topic so far in this module has assumed a channel exists, with some bandwidth, carrying some signal. This page is about what that channel is actually made of, and how the devices using it are physically arranged.
The first split is guided vs unguided. Guided media physically direct the signal along a path — a copper wire, a glass fibre. Unguided media broadcast through space, with no fixed path at all — radio, microwave, satellite.
Guided media
How it works
Twisted pair, coaxial, fibre optic
Twisted pair carries a frequency range of 100 Hz to 5 MHz and consists of two insulated copper wires wrapped around each other. UTP (Unshielded Twisted Pair) has no metal braid; Ethernet and Token Ring run on UTP grades 1 to 5, with the top grade, Cat 5, rated for data transmission up to 100 Mbps. STP (Shielded Twisted Pair) adds a metal braid around the pair to reduce outside electrical interference.
Coaxial cable carries a frequency range of 100 kHz to 500 MHz. It is built
from an inner conducting wire, wrapped in insulation, surrounded by a hollow
outer conducting wire, covered by a plastic jacket. Two varieties: standard
(thick) coax, 10Base5, and thin coax, 10Base2.
Fibre-optic cable is built from a core, cladding, and buffer coating. Light stays inside the core through total internal reflection — the core has a higher refractive index than the cladding, and that difference is what keeps light propagating down the fibre instead of escaping through the sides.
Aside
The 100 Hz–5 MHz figure above is the lecture’s own generic twisted-pair frequency range — it was not independently confirmed as specifically the Cat 5 rating, which is a separate, better-attested figure (100 Mbps data rate, 100 MHz per the TIA/EIA-568-A standard). Treat the two numbers as describing different things: one is generic twisted-pair, the other is Cat 5 specifically.
Where marks get lost
Twisting fixes crosstalk, not attenuation or bandwidth
The exam phrasing is specific: twisting minimises interference and crosstalk between adjacent pairs, because both wires end up equally exposed to whatever noise is nearby, and a receiver reading the difference between the two wires cancels out interference that affects them equally. It does not increase bandwidth or reduce signal attenuation — those are separate properties, and STP’s braid is what addresses outside electrical interference specifically, not the twisting itself.
Fibre — advantages, disadvantages, and why cladding matters
Cladding’s job, stated plainly, is to propagate the signal down the fibre core: the refractive-index difference at the core-cladding boundary causes total internal reflection, trapping light inside the core rather than letting it leak out.
Advantages of fibre: higher bandwidth, less signal attenuation, immunity to electromagnetic interference, resistance to corrosive material, light weight, and greater immunity to tapping — a meaningful security property, since intercepting a fibre signal is far harder than tapping copper. Fibre also runs much further without needing regeneration: up to 50 km, against roughly 5 km typical for twisted pair and coaxial.
Disadvantages: installation and maintenance are harder, light only propagates in one direction so a full duplex link needs two fibres, and cost is higher.
| Twisted pair | Coaxial | Fibre optic | |
|---|---|---|---|
| Frequency range | 100 Hz – 5 MHz | 100 kHz – 500 MHz | Optical, well above either |
| Typical regeneration distance | ~5 km | ~5 km | Up to 50 km |
| Main weakness | Susceptible to EM interference (STP mitigates it) | Bulkier, still copper-based | Cost, installation, unidirectional light |
| Main strength | Cheap, easy to install | Better shielding than twisted pair | Bandwidth, distance, immunity to EMI and tapping |
Unguided media
The lecture names four categories: terrestrial microwave, satellite communications (geosynchronous satellites), cellular telephony, and radio frequency allocation and usage. Beyond naming and classifying them, the slides give no further specification for any of the three named technologies — no frequency bands, link budgets or orbital details. Do not expect the exam to ask for numbers specific to terrestrial microwave, satellite, or cellular; what it does ask about is covered below.
| Unguided media — advantages | Unguided media — disadvantages |
|---|---|
| High mobility — users connect while moving | Weak security — signals are easily intercepted |
| No cable layout — saves material and labour | High interference — vulnerable to weather and physical obstructions |
| Fast deployment — speeds up setting up a network | Lower bandwidth than guided media |
| Overcomes terrain that would block a cable run | Fast attenuation — signal strength drops rapidly |
| Easy scalability — new users need no new cabling |
How it works
Sky wave vs line-of-sight propagation
Sky-wave (ionospheric) propagation. Higher-frequency radio waves, in the 2–30 MHz range, travel upward and bounce off the Earth’s ionosphere back down. It depends on the ionosphere itself, which changes with time of day and weather — this is why some radio links work at night and not during the day, or vice versa.
Line-of-sight (space/tropospheric) propagation. Very high frequency signals travel in a straight line, and the transmitting and receiving antennas must be able to “see” each other. This is limited by the Earth’s curvature to about 50 km for ground-based stations.
Check yourself
A microwave link needs its two antennas mounted on towers so they can see each other, and stops working past about 50 km between ground stations. Which propagation mode is this, and what physically limits its range?
Line-of-sight propagation. The limit is the Earth’s curvature — beyond roughly 50 km, the ground itself blocks the direct path between two ground-based antennas.
Physical topologies
How it works
Bus, star, ring, mesh, tree
The lecture names and describes five physical topologies:
- Bus — every device taps onto one shared line.
- Star — every device connects to a central hub.
- Ring — every device connects to exactly two neighbours, forming a closed loop.
- Mesh — every device connects directly to every other device.
- Tree — a hierarchy of star-like clusters connected through a root.
Aside
Bus and star both end up looking like “Ethernet” in practice — a switch turns
the logical shared-bus behaviour into a physically star-shaped wiring layout,
so the two are less different in a modern network than the diagrams suggest.
Mesh is the topology that does not scale: n nodes need n(n-1)/2 links, so
doubling the node count roughly quadruples the cabling.
Where marks get lost
The lecture's material does not clearly define a sixth topology
Some course material for this unit refers to six physical topologies, but the slides only name and describe five: bus, star, ring, mesh, tree. Hybrid appears as a separate mention — combinations of the topology types above — without being given a specific structure of its own the way the other five are.
If a question asks for six, hybrid is the most likely candidate for the missing one, and the honest answer is that it means “more than one topology type combined,” not a single fixed shape. Do not present a specific hybrid diagram as if the lecture defined it — it did not.
In the exam
- Why twisted pair is twisted. Minimises interference and crosstalk by exposing both wires equally to outside noise.
- UTP vs STP. The braid, and nothing else, is the difference.
- Purpose of cladding. Traps light in the core via total internal reflection, caused by the core’s higher refractive index.
- Fibre’s advantages, by name. Bandwidth, attenuation, EMI immunity, corrosion resistance, weight, tap resistance, and distance — up to 50 km versus roughly 5 km for copper.
- Sky wave vs line-of-sight. Frequency range, what each depends on (ionosphere vs a clear line of sight), and what limits each (time of day and weather vs Earth’s curvature at about 50 km).
- Unguided media, generally. Know the advantages/disadvantages table — mobility and fast deployment against weak security and fast attenuation. Do not expect numeric detail on terrestrial microwave, satellite, or cellular specifically.
- The five topologies, plus the hybrid caveat. Name and describe bus, star, ring, mesh, tree. If a question wants a sixth, say hybrid means combining topology types, and that the slides do not define one structure for it.
- Mesh link count.
n(n-1)/2— be ready to compute it for a givenn.