Which network topology was used in older telecom networks?

Question

BlackTom AI · Accepted Answer

Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) implementations used ring topology for metro and long-haul telecom networks because rings provide predictable paths and fast protection switching.

1. Relevant knowledge points and concepts
- Ring topology: nodes connected in a closed loop so data travels around the ring; often implemented with dual counter-rotating rings for redundancy.  
- Protection/restoration: telecom rings (e.g., SONET/SDH) can automatically reroute traffic the opposite way when a link/node fails, giving very fast recovery times.  
- Other common topologies: bus (single shared medium), star (central hub with spokes), mesh (multiple interconnections between nodes).

2. Step-by-step reasoning process
- Identify the topology characteristics needed for telecom backbones: reliability, predictable latency, and fast failover.  
- Compare each option to those requirements:
  - Bus: single shared medium is simple but has single points of failure and scalability limits. Not typical for carrier transport.  
  - Mesh: very resilient but expensive and complex; used increasingly in modern IP backbones, not the classical older telecom design.  
  - Star: useful for access networks (many subscribers to a central exchange) but the central hub is a single point of failure for backbone traffic.  
  - Ring: offers a closed-loop that can be designed with protection switching (dual rings) so a single break does not isolate nodes.

3. Intermediate evidence/details
- SONET/SDH ring architectures (e.g., Unidirectional Path Switched Ring, Bi-directional Line Switched Ring) were standard designs in legacy carrier networks because they supported sub-50 ms protection switching and orderly multiplexing of circuits.

4. Why the correct answer is correct
- Ring topology directly matches the design goals of older telecom networks: deterministic paths, built-in redundancy via opposite-direction rerouting, and efficient use of transport equipment. Historical carrier deployments frequently used ring architectures for metro/regional transport.

5. Why other options are incorrect
- Bus: used in early LAN coax, not telecom backbones (single point of failure, poor scalability).  
- Mesh: adopted later for IP backbones because of high cost/complexity compared to rings at the time.  
- Star: common for subscriber access to a central office, but not as the primary backbone topology for older telecom transport.

6. Conclusion
Given the need for fast protection and the historical use of SONET/SDH rings, the correct answer is Ring Topology, which matches the provided answer.

Which network topology was used in older telecom networks?单项选择题

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