The Rural Library's Shared 8 Mbps Link
A rural community library runs a computer lab with 12 desktop computers connected to one ADSL router. The library pays for an 8 Mbps downstream and 1 Mbps upstream package. During after-school hours, learners complain that educational videos take a very long time to start, while the same videos play well at 6 a.m. A technician tests one computer: ping to the router is 2 ms, ping to a national website is 260 ms, and a 250 MB training video downloads at roughly 0.6 Mbps. One computer, PC-07, cannot reach the internet at all and shows the address 169.254.18.4 with subnet mask 255.255.0.0. The librarian has a budget for only one upgrade this year and is considering either a fibre connection or one wireless access point so that learners can use their phones.
1. Using the measurements given, explain why the videos are slow in the afternoon but acceptable at 6 a.m. Calculate how long the 250 MB video takes in each case.
Read the worked answer
Convert the file size
- 250 MB x 8 = 2000 Mbit
Alone on the link (about 6 a.m.)
- 2000 / 8 Mbps = 250 s, about 4 minutes. This matches the learners' report that the same video plays well early in the day.
Shared by 12 machines (afternoon)
- 8 / 12 = about 0.67 Mbps per machine if the traffic is spread evenly
- 2000 / 0.67 = about 3000 s, roughly 50 minutes
Interpretation The fault is contention for a fixed WAN link, not faulty computers. The 2 ms ping to the router proves the local network and cabling are healthy; the 260 ms ping outside shows packets queueing at a saturated or congested link. A 480p stream needs about 1-1.5 Mbps continuously, which 0.67 Mbps cannot supply, so the player buffers instead of playing. The 1 Mbps upstream is also a warning sign for cloud backup and video calling, which this calculation does not cover.
2. Diagnose PC-07 and describe the checks you would carry out, in order, before changing any settings.
Read the worked answer
Diagnosis 169.254.18.4 with mask 255.255.0.0 lies in the link-local range 169.254.0.0 to 169.254.255.255. This is an APIPA address: the computer assigned it to itself because no DHCP server offered a lease. So it has a working network interface but no valid configuration, and cannot route to the internet.
Checks in a sensible order
- Physical layer: check link lights, reseat the patch cable, move the cable to a known-good switch port, and swap in a cable from a working PC.
- Release and renew the lease (ipconfig /release then /renew) and see whether an offer arrives.
- On the router, confirm DHCP is enabled and that the address pool is not exhausted. With 12 PCs and staff or learner phones joining, a small pool can run out, and exhausted leases produce exactly this symptom on one or two machines.
- Confirm the adapter is set to obtain an address automatically, and check for an address conflict or an accidental second DHCP server. A temporary static address in the free range is a useful test, but it is a workaround, not the repair.
3. The librarian can fund only one upgrade. Recommend either the fibre connection or the wireless access point, justifying the choice with the evidence available.
Read the worked answer
Recommendation: the fibre connection.
Reasoning
- The measured bottleneck is the WAN link. Twelve machines share 8 Mbps, so each receives about 0.67 Mbps in the afternoon, and that figure alone explains a 50-minute download.
- An access point adds no internet bandwidth. It would place learners' phones onto the same 8 Mbps, taking capacity away from the 12 desktops and making the afternoon complaints worse. It would improve convenience, not speed.
- Fibre increases the shared capacity that every device draws on, which is the quantity the complaints actually track.
Conditions and caveats
- Check the upstream speed in the fibre package: 1 Mbps upstream handicaps uploads, cloud backup and video calls.
- Ask about contention ratio, data cap and installation cost before committing the whole budget.
- Pair the upgrade with free measures: schedule large downloads off-peak, cache the most-used videos on a local server, and filter or throttle non-educational traffic. These reduce demand and cost nothing, and they are worth doing even before the fibre arrives.