Network Design and Engineering Practice
Design questions test whether you can turn requirements into a hierarchical topology, an addressing plan and a cabling specification that survives inspection. Numbers matter more than prose here.
Requirements and logical design
- Gather user counts per area, growth forecast, application traffic profiles, availability targets and budget; then size uplinks and choose redundancy (dual core, dual homing, link aggregation).
- Hierarchical model: access (density and PoE), distribution (policy, routing boundaries, VLAN aggregation), core (high-speed forwarding, minimal policy).
- Produce an addressing plan with summarisable blocks, reserved ranges for management, voice, wireless and future growth, and a VLAN-to-subnet mapping table.
Physical design and structured cabling
- Six subsystems: work area, horizontal cabling, backbone (vertical) cabling, telecommunications room, equipment room, entrance facility.
- Horizontal cable: permanent link ≤ 90 m plus patch cords, giving a 100 m channel. Fibre is used for building backbone and long or high-interference runs.
- Media choice: Cat5e for 1G to the desk, Cat6/6a for 10G, OS2 single-mode for campus backbone, OM3/OM4 multimode for within-building backbone.
- Consider PoE budget for APs and cameras, UPS sizing, rack space, cooling and separation from power cabling.
Documentation and acceptance
- Deliverables: requirements specification, logical and physical topology drawings, IP/VLAN plan, rack elevation, cable schedule with labelling scheme, configuration baseline, test plan and as-built documentation.
- Acceptance tests include certification of copper links (wire map, length, attenuation, NEXT), fibre loss testing, throughput and failover tests.
Worked example
A floor needs 96 outlets, and the telecommunications room sits at one corner. The farthest outlet measures 78 m of horizontal cable route plus 5 m of patch cord at each end. Permanent link 78 m ≤ 90 m, and the channel is 78 + 5 + 5 = 88 m ≤ 100 m, so the design passes. If the route measured 92 m, the permanent link would exceed the 90 m limit and a second telecommunications room or a fibre-to-the-desk design would be required.
Traps that cost marks
- Quoting 100 m as the horizontal cable limit instead of the channel limit.
- Confusing the equipment room (building-level, servers and core) with the telecommunications room (floor-level distribution).
- Designing a flat topology with no summarisation, making the routing table unmanageable.
- Omitting PoE and cooling calculations for wireless and camera deployments.