Choosing a telephone network’s topology means trading link count, hop count and what one failure takes down. For nine villages a full mesh needs 36 links, a star 8, and a ring or a three-group hierarchy 9. Of the cheap layouts only the ring leaves everyone connected after any single cut or lost node, while a star puts every village behind one hub. Pick the layout from the failures you can afford, not from the link count alone.
Part of Telecommunications
Star, mesh, ring and hierarchical layouts for joining exchanges, compared by links, hops and failure behaviour.
The Layouts Differ By What They Cost And What One Fault Takes Away
Topology Comparison recommends a switched star with hierarchical extension for almost every new network, treats full mesh as impractical beyond 5 to 10 nodes except for backbones, and notes that topology is very expensive to change once cabled. The descriptions of each shape are in Network Topologies and Star Topology; this article works the telephone case. For telephones the “links” are trunk routes: pole lines, repeaters and the circuits they carry, so every link is costly.
Worked Example: Nine Villages, Four Layouts
Let the villages be A to I.
Full mesh. Every pair joined: 9 × 8 / 2 = 36 links. Every call is one hop, and any single link fails without isolating anyone. The Network Topologies article gives 5 nodes as 10 links and 10 nodes as 45.
Star. One village is the hub and the other eight each connect to it: 8 spokes. Any two villages are two hops apart. If a spoke is cut, one village is isolated. If the hub fails, every village is isolated from every other.
Ring. Each village is joined to two neighbours: 9 links. The farthest pair is 4 hops apart (the shorter way round is at most 4 of the 9 links). A single cut isolates nobody, because the other way round still works; Dynamic Routing’s ring of four, where cutting one link turns a one-hop route into three hops (Dynamic Routing), shows the price. A second cut splits the ring.
Three-group hierarchy. Groups {A, B, C}, {D, E, F}, {G, H, I} with hubs A, D and G. Spokes: B–A, C–A, E–D, F–D, H–G, I–G, which is 6. Hub triangle: A–D, D–G, G–A, which is 3. Total 9. The longest route, B to H, is B–A–G–H: 3 hops. A cut on a hub-to-hub link isolates nobody. A cut on a spoke isolates one village. If hub A fails, B and C, whose only route is through A, are cut off from everyone, and A’s own traffic is lost.
| Layout | Links | Longest route | One link cut | One hub or node lost |
|---|---|---|---|---|
| Full mesh | 36 | 1 hop | no one isolated | no one else isolated |
| Star | 8 | 2 hops | 1 village isolated | all isolated from each other |
| Ring | 9 | 4 hops | no one isolated | rest stay connected |
| Hierarchy | 9 | 3 hops | 0 or 1 village isolated | that hub’s group isolated |
Counts are derived from the layouts above and apply to this example only.
Telephones Add Three Costs To The Graph
- Each hop is an exchange. A call that passes through several exchanges passes through each one’s switching, trunk and blocking probability. Fewer hops means fewer chances to be blocked (see Network Architecture).
- Each trunk is a long wire. Past roughly 8 km on 0.5 mm wire a loop needs thicker wire, loading or repeaters (Long-Distance), so a link that looks short on the map may be expensive.
- A hub is a place where everything is visible. The exchange at the centre sees all inter-village traffic, and the issues in Manual Exchange apply at settlement scale.
Choosing
- List the failures you must survive. Is one cut pole line acceptable, or must the clinic stay reachable?
- Start with a star or hierarchy. It is cheapest and matches how exchanges grow.
- Add one ring-closing link at the weakest spot. Joining the last two ends of a chain turns a single point of failure into two routes.
- Do not mesh beyond 5 to 10 nodes. The cost climbs as N × (N − 1) / 2.
- Route the second path separately. Two links on the same pole line fail together; see Reliability Design.
What You Need
- A map with route lengths and terrain
- A list of critical lines, such as clinic, mill and watch posts
- A cost per kilometre for pole line, from Pole Line Building
- A note of who owns each hub
Common Mistakes
- Counting links but not hops. A ring of nine saves wire and costs up to four exchanges in a row.
- Treating the hub as free. A star is as strong as its hub.
- Closing the ring on one pole line. Two paths on the same poles are one path.
- Meshing early. Link count grows faster than traffic.
Safety
Trunk routes
A long trunk is a lightning antenna and crosses power lines in places; keep clear of power conductors and fit protectors (Lightning Protection). Any voltage found on a pair is an emergency to clear first (Short Circuit Test).
What’s Next
- Learn next: Reliability Design, redundant paths and fault detection
- Combine with: Network Architecture for sizing the links you choose