Identifier: C2T6L5 | Revision: 2026-10-26

Decision and objectives

A mortgage-backed security can repay principal earlier when rates fall and later when rates rise. That timing asymmetry can hurt even if borrowers ultimately pay every unit of principal. Evaluate prepayment, extension, credit enhancement and exit liquidity separately. None is summarized adequately by the original contractual final maturity.

Objectives: Calculate weighted-average principal life; Stress timing and credit buffers; Distinguish credit enhancement from liquidity support.

Prerequisites: C2T6L4.

Mechanics

Prepayment returns principal ahead of schedule, reducing the future interest base and creating reinvestment risk. Extension means principal arrives later than the expected schedule, increasing funding or sale risk. Weighted-average life measures average principal repayment time weighted by principal amounts, without discounting; it differs from duration's present-value weighting of all payments.

Credit enhancement is a buffer such as subordinated claims, overcollateralization, reserve accounts or an identified guarantee, with defined rules. Liquidity support addresses timing gaps under its terms and may not cover ultimate asset losses. A reserve used to make today's payment may leave less protection tomorrow. The same supporting cash cannot be counted twice.

Price and return drivers

Weighted-average life = sum(time × principal repaid at time) / total principal. It is scenario-dependent when prepayment changes. Credit-buffer loss allocation needs a separate waterfall. Prepayment is not always beneficial: a premium-priced security repaid at par can lose premium and require reinvestment at lower rates, while an extended discount-priced security may not supply needed cash.

Worked case

Hypothetical restricted amortizing pool, principal TRY 1,000,000. Base principal receipts: TRY 300,000 at year one, 400,000 at year two, 300,000 at year three. Weighted-average life = (1×300,000 + 2×400,000 + 3×300,000)/1,000,000 = 2.0 years. Fast prepayment schedule 600,000; 300,000; 100,000 has life 1.5 years. Slow schedule 100,000; 300,000; 600,000 has life 2.5 years. All repay the same principal but fund different dates. Coupons, prices, tax and costs are excluded to isolate principal timing.

Separate credit buffer: junior support TRY 100,000 plus a reserve TRY 50,000 absorbs pool losses before the senior claim under the supplied rule. A TRY 120,000 loss uses the junior 100,000 and reserve 20,000; senior principal loss is zero; reserve remaining 30,000.

Adverse case

A TRY 180,000 pool loss exceeds the combined 150,000 buffer and reaches senior holders by TRY 30,000. If the investor needs TRY 500,000 after year one, the base principal schedule leaves a TRY 200,000 timing gap; the slow schedule leaves TRY 400,000. Credit support may keep eventual principal intact while failing to fund that date. A secondary sale at a large discount or no executable quote can turn a timing problem into a realized loss. Prepayment schedules are contingent expected-cash scenarios, not known pre-expiry market values.

Practical use

Retrieve pool performance and prepayment reports, delinquency definitions, servicer advances, reserve rules, triggers and whether support is committed, capped or cancellable. Confirm access restrictions for the actual issue and execution liquidity for the desired size. A listed security is not automatically saleable quickly at its valuation mark. Investor custody, spread, commission, tax and any premium amortization records belong in the final comparison.

Decision boundaries

Use a range of principal schedules and cash buffers rather than one average-life projection. Decline a security for a fixed near-term liability if even a moderate extension state creates an unfunded payment and the investor cannot tolerate forced sale. Do not mistake a liquidity facility for a guarantee against final credit loss or assume senior status eliminates timing risk.

Practice and references

Exercise: hypothetical principal 600,000 repays 100,000 at year one, 200,000 at year two and 300,000 at year three. Answer: weighted-average life = (100,000+400,000+900,000)/600,000 = 2.333333 years. If junior support is 60,000, reserve 20,000 and pool loss 95,000, senior loss under the supplied first-loss rule is max(95,000−60,000−20,000,0) = TRY 15,000.

Takeaway: test both ultimate losses and the date on which usable principal reaches the investor.

Reference guide: securitized-debt, weighted-average-life, credit-enhancement-loss, Prepayment, Extension risk, Credit enhancement, Weighted-average life.

Next: C2T7L1. Topic test: C2T6.

Primary sources (checked 2026-10-26; changing terms require a publication recheck): SPK Communique III-58.1 on asset-backed or mortgage-backed securities — Articles 1–4; fund assets, issuance and servicing provisions; SPK Communique III-59.1 on covered securities — Articles 1–4; collateral register and asset matching provisions.

Shared reference guide | Class examination