Size a hybrid connection and cost the alternatives

applied · 45 min · Objective 1.3

Task

Decide how a company should connect its datacentre to a cloud provider, by working out how much bandwidth the workload needs and comparing a VPN over the internet against a dedicated connection. Connectivity choice is a scenario question, and the scenario always contains the numbers that decide it.

Steps

  1. Take the workload: a nightly backup of 900 GB that must finish inside a six-hour window, plus 40 Mbps of steady daytime application traffic between on-premises and cloud.
  2. Convert the backup into a sustained bit rate. Watch the units — 900 GB is gigabytes, the link is rated in bits per second, and the factor of eight is where most people go wrong.
  3. Add the daytime traffic and decide whether they overlap. If the backup runs overnight they do not, so size the link on the larger of the two rather than the sum.
  4. Compare the two options against that figure. A site-to-site VPN over an existing internet circuit costs nothing extra but shares the circuit and offers no latency guarantee. A dedicated connection costs a monthly fee plus a cross-connect but gives consistent latency and does not consume the internet circuit.
  5. Write a two-sentence recommendation, naming the number that decides it and the risk you are accepting.

Verify

python3 -c "
gb, hours, day_mbps = 900, 6, 40
bits = gb * 8 * 1000**3
mbps = bits / (hours * 3600) / 1_000_000
print('backup needs', round(mbps,1), 'Mbps sustained')
print('daytime needs', day_mbps, 'Mbps')
print('size on the larger, not the sum:', round(max(mbps, day_mbps),1), 'Mbps')
print('add 30 percent headroom ->', round(max(mbps, day_mbps)*1.3,1), 'Mbps')
for name, cap in (('100 Mbps circuit',100), ('200 Mbps circuit',200), ('1 Gbps circuit',1000)):
    print(name, 'sufficient' if cap >= max(mbps, day_mbps)*1.3 else 'too small')
"

The backup needs about 333 Mbps sustained, which is the number that decides the whole question. A 100 Mbps circuit cannot do it in six hours no matter how the contract is worded.

Notes

The unit conversion is the examinable skill here, and it is worth doing slowly once: bytes to bits is times eight, and a six-hour window is 21,600 seconds. Getting either wrong changes the answer by a factor that would buy the wrong circuit.

The other lever worth naming in your recommendation is incremental backup. Dropping 900 GB nightly to 90 GB of changes turns a 333 Mbps requirement into 33 Mbps, and the cheapest bandwidth is the traffic you do not send.