Asset-Backed Securities

While hedge funds pool investor capital, ABS pool underlying debt obligations and repackage their cash flows into tradeable securities. This section covers how that pooling process works and how the two major tranche structures, CMOs and CDOs, split risk differently among investors.

What Is Securitization?

An asset-backed security (ABS) is a debt instrument backed by a pool of underlying assets: mortgages, auto loans, credit card receivables, student loans, or other financial obligations.

The process of pooling these assets and issuing securities backed by the pool is called securitization:

Originator creates loans → Loans pooled into a trust → Trust issues securities (tranches) → Investors buy tranches and receive cash flows

Think of it this way: A bank has thousands of individual car loans sitting on its books. No one wants to buy a single car loan, but bundle 10,000 of them together and slice the bundle into pieces, and now investors can buy a share of the cash flows. That bundling-and-slicing process is securitization.

What Structural Features Do All ABS Share?

All asset-backed securities share certain structural features:

• Indenture: The trust indenture governs payment priority, events of default, and trustee duties
• Maturities: Vary by tranche; senior tranches typically have shorter expected maturities, subordinated tranches have longer maturities
• Form of ownership: Typically book-entry form (no physical certificates)
• Interest payments: Usually monthly (mortgage-backed) or quarterly; follows the cash flow pattern of the underlying collateral
• Call features: Clean-up calls allow the servicer to retire remaining securities when the pool balance falls below a threshold (typically 10%); some ABS also carry other optional or mandatory call provisions beyond the clean-up call
• Put features: A minority of ABS carry a put (tender) feature, letting the holder require the issuer to redeem the security at a stated price on specified dates; typical amortizing ABS do not have this feature
• Accrued interest: Mortgage-backed securities use the 30/360 day-count convention

Characteristics That Vary by ABS Type

• Maturity: Stated maturity vs. expected maturity (weighted average life); actual maturity depends on prepayment behavior
• Type of collateral: Determines cash flow pattern and risk profile (mortgages behave differently from auto loans or credit cards)
• Priority of claim: Senior tranches are paid first; subordinated tranches absorb losses first
• Call provisions: Clean-up calls, optional redemption, scheduled amortization

How Do CMO Tranches Divide Timing Risk?

A collateralized mortgage obligation (CMO) is a type of mortgage-backed security that divides a pool of mortgages into multiple tranches (slices), each with different maturities, cash flow priorities, and risk characteristics.

Key CMO Fundamentals

• Issued by government agencies (Ginnie Mae, Fannie Mae, Freddie Mac) or private entities
• Unlike pass-through securities (which distribute all principal and interest pro rata), CMOs redirect cash flows to different tranches according to a predetermined payment schedule
• CMOs do not have a fixed maturity date; they have an expected average life based on prepayment assumptions
• CMO tranches divide timing risk (when you get your money back), not credit risk

CMO Tranche Types

Exam Tip: Gotchas

• IO strips move INVERSELY to most bonds. Their value increases when rates rise because prepayments slow down, meaning more interest is collected over a longer period.
• PO strips behave like typical bonds. Their value increases when rates fall because principal is returned faster.

PAC vs. TAC vs. Companion: A Closer Look

Here is how PAC, TAC, and companion tranches compare:

• PAC tranches are protected against both prepayment and extension risk within a specified prepayment band (e.g., 100-300 PSA)
• If actual prepayments fall outside the band, even PAC holders face risk
• Companion tranches act as shock absorbers: their average life shortens dramatically when rates fall (prepayments accelerate) and extends dramatically when rates rise

What Are Prepayment Risk and Extension Risk?

These two risks are the defining features of mortgage-backed securities.

Think of it this way: You lent money expecting to be repaid over 20 years. If rates drop and everyone refinances, you get your money back in 5 years but can only reinvest at lower rates (prepayment risk). If rates rise and nobody refinances, you are stuck waiting 30 years for your money while missing out on higher rates elsewhere (extension risk).

Prepayment Risk (Contraction Risk)

• When interest rates fall, homeowners refinance or pay off mortgages early
• Principal is returned sooner than expected
• Investors must reinvest at lower prevailing rates (reinvestment risk)
• The security's average life shortens (contracts)

Extension Risk

• When interest rates rise, homeowners hold their mortgages longer (no incentive to refinance)
• Principal payments are delayed beyond expectations
• The security's average life extends
• Investor's capital is locked into below-market rates for longer

Who Bears the Risk?

Exam Tip: Gotchas

• Which tranche has the MOST prepayment risk? The companion tranche. When rates drop, companion tranches absorb the surge in prepayments (shortened average life), while PAC tranches remain stable within their band.

How Do CDO Tranches Divide Credit Risk?

A collateralized debt obligation (CDO) is a structured finance product backed by a diversified pool of debt instruments: corporate bonds, loans, mortgage-backed securities, or other ABS.

CDO Tranche Structure

CDOs divide their cash flows into tranches based on credit risk, not timing:

The Waterfall Structure

Cash flows and losses move in opposite directions:

• Cash flows (payments): Senior → Mezzanine → Equity (top-down)
• Losses (defaults): Equity → Mezzanine → Senior (bottom-up)

Think of it this way: Picture a three-story building in a flood. The ground floor (equity) gets soaked first. The second floor (mezzanine) stays dry unless the water rises past the first floor. The top floor (senior) only floods in a catastrophe. Equity investors accept this risk because they earn the highest yield when the water stays low.

CMO vs. CDO: Key Differences

Exam Tip: Gotchas

• CMO tranches divide TIMING risk (when you get your money back). CDO tranches divide CREDIT risk (whether you get your money back). This is the key distinction.

What Should You Check on Exam Day?

• Securitization pools loans into a trust that issues tranches; investors buy those tranches for a share of the underlying cash flows.
• CMOs redirect principal and interest to tranches by a predetermined schedule instead of paying pro rata like pass-throughs, so they have an expected average life, not a fixed maturity.
• Know the tranche order from safest to riskiest: PAC (protected within its band), TAC (protected against prepayment only, not extension), companion (absorbs the excess), Z-tranche (accrues, paid last), and the speculative IO/PO strips.
• IO strips gain value when rates rise (prepayments slow, more interest collected); PO strips gain value when rates fall (prepayments accelerate, principal returned faster). This inverse/direct pairing is a frequent exam trap.
• Prepayment (contraction) risk shortens average life when rates fall; extension risk lengthens it when rates rise. Companion tranches absorb both.
• CDOs tranche by credit risk, not timing: cash flows pay senior first, then mezzanine, then equity; losses hit equity first, then mezzanine, then senior.