Bandwidth Numbers

The Divide-by-Eight Rule

Networks speak bits; payloads live in bytes. Every bandwidth estimate crosses this line:

 1 Gbps link = 1,000 Mbps = 125 MB/s     (÷8)
 10 Gbps     = 1.25 GB/s
 100 Mbps broadband = 12.5 MB/s

 forgetting ÷8 overstates capacity 8x — the most common estimation error in the field

Reference Capacities

Link	Throughput	What it means practically
Home fiber (typical)	100–1000 Mbps down	A 4K stream ≈ 25 Mbps — fine
Cloud instance NIC (typical)	1–25 Gbps	Per-box ceiling before scaling out
Cross-AZ / DC backbone	multi-Gbps, priced per GB	Internal chatter is cheap in speed, not in money
Mobile 4G/5G user	10–100 Mbps	Mobile-first payloads must respect this
Cross-region RTT-limited TCP	throughput bounded by window/RTT	Long fat pipes need tuning

The last row matters at scale: raw capacity is not achievable throughput across long distances — TCP windows and physics intervene.

Transfer-Time Math

 time = size ÷ effective throughput

 10 GB dataset over 1 Gbps (125 MB/s):   ≈ 80 s theoretical,
                                         ~2 min realistic (protocol overhead)
 1 TB migration over same link:          ≈ 2.3 hours realistic
 1 TB shipped on a physical device:      overnight. ← Snowball-class services
                                         exist precisely because math says so
 
 25 Mbps stream × 1 hour = ~11 GB per viewer-hour (video platform economics)

That last line drives entire industries: video platforms’ architecture IS this multiplication handled by CDNs.

Per-User Payload Budgets

Design targets translate to bandwidth budgets per interaction:

Interaction	Payload target
API JSON response	1–20 KB
Web page first load	under 500 KB ideal, often MBs reality
Image thumbnail / full	20 KB / 200 KB–2 MB
Audio stream	~1 Mbps (~7 MB/min)
1080p video	3–6 Mbps

Multiply by expected concurrency for system totals:

 50k concurrent viewers × 4 Mbps = 200 Gbps sustained egress
 → physically absurd from one region → CDN is the architecture (again)

Where Bandwidth Costs Concentrate

• Egress to internet: priced per GB; media systems’ dominant line item.
• Cross-AZ traffic: replicas and chatty services pay silently.
• Fan-out amplification: one write broadcast to N subscribers multiplies internal bytes.

Each has an architectural answer: CDN offload, replica-aware reads, and pub/sub trees respectively.

Interview Framing

Bandwidth fluency shows as one clean chain: payload × concurrency → Gbps → compared against a named ceiling (“that’s beyond one instance’s NIC” or “beyond one region’s sanity”) → architectural consequence. The divide-by-eight spoken aloud once inoculates against the classic slip; interviewers listen for it specifically.