Dark Fiber SLA Math: MTTR You Can Bank On
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Dark Fiber SLA Math: MTTR You Can Bank On

When everyone sells “five nines,” the only numbers that matter are the ones you can take to finance: hours down, dollars lost, and credits recovered. This guide turns dark-fiber SLAs into MTTR you can actually bank on, so procurement, ops, and finance all see the same math.

Why MTTR > “Five Nines” for Dark Fiber Buyers

Reality > marketing

Availability is a result; MTTR is a promise. An honest 4–8h MTTR with spares and crews beats a vague 99.99% with no restoration plan.

Finance-ready

MTTR converts directly into downtime hours/year and $/hour impact—numbers your CFO can model into TCO.

What to ask

  • Named MTTR by region (metro vs. long-haul)
  • Pre-positioned spares & crew locations
  • Escalation path + credit schedule by hour

The Three Equations Every Buyer Should Use

MetricFormulaWhat it tells you
Downtime (h/yr) incidents/year × MTTR (hours) Expected hours of impact per year.
Availability (%) 100 × [1 − downtime / hours measured] Real uptime on 24×7 (8760h) or business hours (2080h).
Dual-path availability 1 − (1 − A₁)(1 − A₂) (independent) Best case if routes are truly diverse; adjust for correlation.
Pick the right hours basisQuantify real incidentsModel credits vs. losses
1

Single vs. Dual Path — and the Correlation Trap

Independent failures (best case)

If paths are physically diverse (not the same duct/bridge/river crossing), combined availability multiplies—downtime plummets.

  • Affidavit + as-built KML for diversity
  • Named crossings to avoid (bridges, tunnels)

Correlated failures (real world)

Shared conduits, maintenance windows, or regional events mean both paths fail together. Your model must include a correlation factor.

  • Separate maintenance windows
  • Distinct ducts & POP entries
Demand a diversity exhibitSet correlation ≤ 0.2
2

SLA Credits vs. Real Downtime Cost

Credits are not cash flow

Credits reduce MRC; they don’t repay revenue loss. Model both lines: “credits recovered” and “downtime cost.”

Tiered policies

Many SLAs have thresholds (e.g., below SLA, below SLA−0.1%, below SLA−0.5%). Map them to annualized impact.

What to negotiate

  • Credits per hour past MTTR
  • Higher caps for repeated breaches
  • Incident reporting/OTDR data delivery
3

O&M Scope, Spares, and Restore Logistics

Scope clarity

Confirm who pays for locates, traffic control, permits, and municipal relocations. Hidden O&M can erase “cheap” MRCs.

Spares & crews

Named depots, spare reels, and on-call crews by region reduce MTTR variance. Ask for addresses, not promises.

Include in contract

  • Restore SLA by metro/long-haul
  • Penalties/credits per hour past MTTR
  • Planned maintenance notice windows
4

Acceptance Testing That Predicts MTTR

Characterization package

Demand OTDR, CD, PMD, and end-face images at turn-up. Dirty connectors and poor splices are future tickets.

Thresholds to set

  • Max span loss & connector count
  • PMD/CD envelopes for 400/800ZR
  • Remediation SLA if out of spec

Run the Numbers for Your Routes

Use the calculator below to convert incidents and MTTR into dollars. Model single vs. dual path, add a realistic correlation factor, and see how credits compare to true downtime cost.

SLA / MTTR Impact Calculator

Dual-path option (advanced)
SLA credit policy (optional)
Credits as % of monthly MRC. Adjust to match the provider’s SLA.

Notes: Availability = 1 − (downtime ÷ measured hours). Dual-path model blends independent failures with a correlation factor to reflect shared risks (e.g., bridge/duct cuts). Adjust credit thresholds to your provider’s SLA.

MTTR is where marketing ends and money begins. If your provider can state a number, show the spares, and sign for credits that scale by the hour, you can price risk and win budget. Use the calculator to set thresholds that make sense for your routes, then lock them into the contract with a diversity exhibit and restore guarantees.