Network Topologies: Mapping the Network

Network Topologies: Mapping the Network

What you'll learn

• Define what a topology is and read simple topology diagrams
• Compare bus, ring, star and mesh using the classic road analogies
• Explain why nearly every modern LAN is a star or a hierarchical star
• Recognize the three wireless topologies: ad hoc, BSS and ESS

This lesson follows Chapter 1 of Network Essentials, which teaches topology with a map-maker's eye. When geographers use the word, they mean the lay of the land — where the rivers run relative to the mountains and the plains. A network topology is the same idea applied to connections: the layout that shows how every device links to every other, the overall picture of the network's shape. Dr. Al-Doori's course builds the whole comparison on roads, and the analogies are so effective that we will retell each one. (Wireless shapes draw on Chapter 1 too, with their full treatment coming in Module 6.)

Bus: the desert highway

In a bus topology, every device attaches to one shared cable. The book's image: a lone highway through the desert. There is only one road, and every stop — every fuel station and diner — sits directly on it. Historically the "road" was a single coaxial cable snaking from machine to machine.

The shape dictates the behavior. One shared medium means one conversation at a time: while any device is transmitting, every other device must wait, and all of them hear everything (the link is half duplex — traffic flows both ways, but never simultaneously). Two devices transmitting at once produce a collision — the whole bus is a single collision domain, and a single broadcast domain too. Worst of all, the shared cable is a single point of failure with many points of access: break the highway anywhere and no through traffic moves; the whole network goes down from one damaged connector.

Ring: the beltway

In a ring topology, each device connects to the next, and the last connects back to the first, forming a closed loop. The book's image: the beltway that circles a large city. To deliver something from one point on the loop to another, you get on and travel in the ring's one direction of traffic until you reach your exit. And to reply, the destination does not turn around — it keeps going the same direction until the loop brings it back to the sender.

Like the bus, a classic ring carries one conversation at a time; rings of this era usually enforced order by passing around a small "permission slip" (a token) that granted the right to transmit — a mechanism you will meet again as history in Module 3. And like the bus, the shape is fragile: one broken link or dead device interrupts the loop for everyone, unless a second, counter-rotating ring is added as a backup — an expensive remedy that real ring networks of the era did use.

Star: the traffic hub

In a star topology, every device connects by its own cable to one central device. The book's image: a traffic hub — a city center from which roads radiate outward in every direction.

The star's behavior depends entirely on what sits at the center. With the old shared-medium hub (obsolete) at the center, a star still behaved like a bus — one conversation at a time. With a switch at the center — the modern reality — the star comes alive: the switch builds a private lane per conversation (Lesson 1's classroom pairs), so many pairs of devices can talk simultaneously. The book's dry qualifier is worth keeping: everyone can talk at the same time — more or less. (Broadcasts still reach everyone, and the lanes share the switch's internal capacity, so the freedom has fine print — Module 3 reads that fine print with you.)

Failure behavior is the star's other virtue. Cut one cable and you lose exactly one device; the rest of the network never notices. The trade-off is honest and visible: the center itself is now the critical point. If the switch dies, the whole star goes dark — which is why the center is where organizations spend their money and their redundancy.

Sketch your Lesson 1 home inventory again from memory: a central box with lines radiating to every device. You have been living in a star all along.

Hierarchical star and mesh: how big networks are built

One star can only grow so large — a switch has a finite number of ports, and a building has many floors. The natural next step is to connect stars to stars: each floor's devices connect to that floor's switch, the floor switches connect to a building switch, and so on upward. This hierarchical star (a tree of stars) is the actual shape of nearly every office, campus and school network you will ever see.

At the other extreme sits the mesh: instead of one center, many direct interconnections. The book offers two images — a city built as a grid, where every block touches its neighbors and you can travel from anywhere to anywhere without passing through a single central point; or, better still, having a direct flight between every pair of cities. In a full mesh, every node links directly to every other: for n devices that is n(n−1)/2 links — 10 links for 5 nodes, 45 for 10 — which makes mesh the costliest of the four shapes, bought only where it is genuinely needed. Where failure is not an option — the connections between core routers, or the provider networks whose interlinking is the Internet (Lesson 2's mesh of WAN links) — the expense buys the ability to lose any single link and still reach everything by another path. Most real meshes are partial: extra paths where they matter, not everywhere.

The chapter's practical conclusion still holds word for word in 2026: real networks combine topologies, and the most common combination is star at the edges with mesh at the core.

Wireless topologies: ad hoc, BSS, ESS

Radio changes the medium but not the need for shape. Chapter 1 closes its topology tour with the three wireless arrangements:

• Ad hoc — two or more devices talk radio-to-radio directly, with no access point between them (the standards call this an Independent Basic Service Set, or IBSS): improvised as needed (the name is Latin for "for this purpose"). Quick file-sharing links between two laptops or phones work this way.
• BSS (Basic Service Set) — one wireless access point (AP) anchors the network, and every wireless device talks through it. Look at the shape: it is a star, with the AP at the center and radio in place of cables. A single-router home Wi-Fi is a BSS.
• ESS (Extended Service Set) — multiple access points, joined together, presenting one network. Walk across a campus or an airport and your phone hops from AP to AP without you noticing, because every AP advertises the same network name. Coverage larger than one radio circle requires exactly this.

A 2026 wrinkle the book could not have named: consumer "mesh Wi-Fi" systems — the multi-pod kits sold for homes — are, in these terms, an ESS whose access points happen to interconnect wirelessly, forming a small mesh among themselves. The marketing word "mesh" describes the pods' links to each other; what your devices experience is an extended service set. Being able to take apart a product name with vocabulary from a 2007 textbook is a good sign the vocabulary was worth learning.

From the textbook to 2026: bus and ring are history — star and mesh won

The four shapes are not equally alive, and honesty about that is part of this course's method.

Bus. Original: coax-cable bus LANs were early Ethernet's standard build, and the topology explains their era's behavior — collisions, shared capacity, fragility. What changed: twisted-pair cabling and switches made star wiring cheaper and better; bus LANs vanished completely, and no one has built one this century. 2026 practice: you will never install a bus (obsolete as a wired LAN) — but you learn it because its physics never left. Every Wi-Fi channel is a bus in the air: a shared medium all nearby devices contend for. The vocabulary of the desert highway explains your Wi-Fi's worst evenings.

Ring. Original: token-passing ring LANs (Token Ring, and the fiber ring FDDI) were the star's serious 1990s rival, and the beltway analogy taught their delivery model. What changed: switched Ethernet won the LAN completely; physical ring LANs are museum pieces (obsolete in the LAN). 2026 practice: the ring idea survives where its geometry pays — some metro and carrier fiber loops still run as rings, because a loop gives every site two directions to reach every other, and industrial networks use ring arrangements for the same resilience. Worth knowing, never worth building at the office (legacy — still worth knowing).

Star and mesh. Original: presented as two options among four. What changed: they absorbed everything. The switch made star the universal LAN shape; scale made hierarchical star the universal building shape; and the need for fault tolerance made mesh the shape of cores, data centers and the Internet itself. 2026 practice: wired LAN = hierarchical star around switches; backbone = mesh; home Wi-Fi = a BSS or ESS star over radio. When you design your capstone network in Module 9, these are the shapes you will choose from — the other two you will simply be able to recognize in stories, exams and legacy corners.

Activity: Draw the four topologies — and find yours

Paper design work: drawing the shapes fixes them in memory far better than reading about them.

Objective. Produce labeled diagrams of bus, ring, star and mesh, analyze how each behaves when a link fails, and identify which topology your own home or school network uses.

Setup. Paper and pen or any drawing app. Your sketches from Lessons 1 and 2. For each drawing, use five devices labeled A through E.

Steps.

1. Bus: draw one long horizontal line; attach A–E to it by short stubs. Label the long line "shared cable."
2. Ring: draw A–E as the corners of a pentagon and connect them in a closed loop. Add an arrowhead showing the single direction of travel.
3. Star: draw a small square in the center labeled "switch," place A–E around it, and connect each device to the center with its own line.
4. Full mesh: place A–E in a circle. Before drawing, predict the number of links using n(n−1)/2. Then connect every pair, counting as you go.
5. On each finished diagram, mark one link with an X (a failure) and shade every device that loses some or all connectivity because of it.
6. Now open your Lesson 1–2 sketches and write a two-sentence verdict: which topology (or combination) is your network, and why do you think it was built that way?

Expected result. Four labeled diagrams; the mesh has exactly 10 links. Failure shading: bus — potentially everyone (the shared cable is severed); ring — everyone, because the loop is broken; star — only the one device on the cut cable; mesh — nobody loses reachability, traffic detours around the X.

Verify. Check your mesh link count against the formula (5 × 4 ÷ 2 = 10). Check your failure shading against the behavior described in this lesson's sections. Your home verdict should almost certainly be: a star (or an ESS star over radio) centered on the router — matching what your Lesson 1 sketch already looked like.

Questions.

1. Which topology tolerated the failed link best, and which worst?
2. How many links would a full mesh of 10 devices need? Would you pay for that in an office of 10 PCs?
3. Which of the four shapes is your Wi-Fi, once you treat radio links as invisible cables?
4. Why do you think your school or office wires every wall port back to a closet, rather than daisy-chaining room to room?

If it goes wrong.

• Your mesh drawing turns into spaghetti. Keep the five nodes on a circle: draw the outer pentagon first, then the five inner diagonals, and count as you draw.
• You are unsure of your school's topology. Look at the evidence: many wall ports, each running individually back to a wiring closet, is star wiring; stacked closet switches feeding a building switch is the hierarchical star.
• Your failure shading for the bus feels ambiguous. Both are defensible: a cleanly severed cable can partition the bus (each side keeps talking within itself) or, in real coax networks, kill the whole segment. Shade it as "everyone affected" — that reflects the practical experience that made the topology extinct.

Check yourself

1. In an old workshop, one damaged connector takes every machine offline at once. In a modern office, a cut cable takes down exactly one desk. Name each topology from its failure symptom.
2. A company wants to connect 6 branch routers in a full mesh. How many links is that? Name one reason they might settle for a partial mesh instead.
3. Two laptops exchange files over Wi-Fi with no router or access point anywhere nearby. Which wireless topology is this?
4. Walking from the check-in hall to gate B22, your phone stays connected to "Airport-WiFi" the whole way. Which wireless topology is this, and what makes the seamless walk possible?
5. A bus and a classic ring both allow only one conversation at a time. Why is that acceptable on a city beltway but fatal in a modern office network — and which Lesson 1 idea gives the star-with-a-switch its advantage?
6. A hospital demands that the failure of any single link between its four core switches leaves every switch still reachable. Which of the four classic topologies does the requirement force, and what will it cost?

Answers

1. The workshop is a bus — one shared cable, so one fault can silence everyone. The office is a star — each desk has its own cable to the center, so one cut isolates one device.
2. 6 × 5 ÷ 2 = 15 links. A partial mesh saves money (and router ports) by providing extra paths only between sites that genuinely need the redundancy or carry the most traffic.
3. Ad hoc — devices connected directly by radio with no access point (an Independent Basic Service Set, or IBSS); two or more can take part.
4. An ESS (Extended Service Set): multiple access points joined into one network with the same name, so the phone hops from AP to AP as you move between their coverage circles.
5. A beltway serves drivers who each occupy the road briefly and can wait their turn; an office generates many simultaneous, continuous conversations. The switch at a star's center creates a private lane per conversation — the micro-segments of Lesson 1 — so simultaneous talking replaces turn-taking.
6. A (full) mesh among the four cores: every switch links to every other (6 links), so any single failure leaves alternate paths. The cost is the extra links and ports — the classic mesh trade: money for fault tolerance.

Key terms

• Topology — the layout of a network's connections; its shape.
• Bus — all devices share one cable; one conversation at a time (obsolete as a wired LAN).
• Half duplex — traffic in both directions, but only one direction at a time.
• Ring — devices connected in a closed loop, traffic traveling in one direction (obsolete in the LAN; survives in carrier/metro loops).
• Token — the circulating "permission slip" classic rings used to grant transmission rights.
• Star — every device has its own link to a central device; the modern LAN shape.
• Hierarchical star — stars connected to stars; the shape of real buildings and campuses.
• Mesh — many direct interconnections; full mesh links every pair (n(n−1)/2 links), partial mesh only where needed.
• Wireless access point (AP) — the device that anchors a wireless network.
• Ad hoc — a direct device-to-device wireless connection with no AP.
• BSS (Basic Service Set) — one AP and its connected devices; a radio star.
• ESS (Extended Service Set) — multiple APs presenting one network, enabling roaming.

Summary

• A topology is the shape of a network's connections, and shape dictates behavior: who can talk when, what a failure costs, what growth costs.
• Bus (the desert highway) and ring (the beltway) share their medium — one conversation at a time and fragile under failure — and both are history in the LAN.
• Star (the traffic hub) gives every device its own link; with a switch at the center, conversations run simultaneously and one cut cable costs one device.
• Hierarchical star — stars of stars — is how every real building and campus network is built.
• Mesh (a direct flight between every pair of cities) buys fault tolerance with link count: n(n−1)/2 for full mesh; the Internet's core is the world's great partial mesh.
• Wireless has shapes too: ad hoc (direct), BSS (one AP — a radio star), ESS (many APs, one network — how campuses and home "mesh Wi-Fi" kits work).
• Real networks combine shapes: star at the edge, mesh at the core — exactly as the textbook taught.

Next lesson

You now know networks by size and by shape. The last question of Module 1 is about power: who controls the resources — every computer for itself, or a central authority? Peer-to-peer versus client-server, and the story of a law firm that outgrew its own goodwill.

Sources and further study

• Network Essentials, Dr. Tahseen Al-Doori — Chapter 1, "Introduction to Networking": the topology comparisons, the road analogies, and the wireless service-set definitions this lesson retells.
• GeeksforGeeks — "Types of Network Topology": a current reference walkthrough of the same shapes with additional diagrams.
• Cisco Networking Academy — Networking Basics (free course): topology vocabulary as used in industry training, including BSS/ESS.