The Scaling Problem

Ethereum processes roughly 15-30 transactions per second on L1. Visa’s network handles around 1,700 TPS on average, with peak capacity up to ~24,000 — not 65,000, that figure is often misquoted. The point stands: blockchains need to scale by orders of magnitude for general use.

Layer 2 solutions process transactions off the main chain while inheriting its security.

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Layer 1 vs Layer 2

	Layer 1 (Ethereum)	Layer 2
Security	Direct	Derived from L1
Speed	~30 TPS	1,000-10,000+ TPS
Cost	$5-50/tx	$0.01-0.50/tx
Finality	~15 min	Seconds (soft) → L1 (hard)

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Types of Layer 2

1. Rollups

Rollups bundle transactions and post data to L1:

Type	Proof	Finality
Optimistic	Fraud proofs (challenge period)	~7 days
ZK	Validity proofs (cryptographic)	Minutes to hours

2. State Channels

Direct peer-to-peer channels:

• Fast for repeated interactions
• Limited to participants
• Example: Lightning Network

3. Sidechains

Separate chain with own consensus:

• Fast and cheap
• Less secure (own validator set)
• Example: Polygon PoS

4. Validium

ZK proofs but data stored off-chain:

• Very cheap
• Data availability risk
• Example: StarkEx

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Optimistic Rollups

Assume transactions are valid, allow challenges.

Challenge period: Anyone can submit fraud proof if batch is invalid.

Rollup	Status	TVL
Arbitrum	Live	$8B+
Optimism	Live	$5B+
Base	Live	$2B+

Pros & Cons

Pros	Cons
EVM compatible	Long withdrawal time (7 days)
Mature technology	Fraud proof game theory
High throughput	Centralized sequencer (current)

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ZK Rollups

Prove validity using zero-knowledge proofs.

Rollup	Status	EVM?
zkSync Era	Live	Yes
StarkNet	Live	No (Cairo)
Polygon zkEVM	Live	Yes
Scroll	Live	Yes

Pros & Cons

Pros	Cons
Fast finality	Complex technology
Cryptographic security	Expensive proof generation
Faster withdrawals	Less mature

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Security Comparison

Solution	Security Model	Data Availability
Optimistic Rollup	L1 + fraud proofs	On L1 (calldata)
ZK Rollup	L1 + validity proofs	On L1
Validium	Validity proofs	Off-chain
Sidechain	Own consensus	Own chain

Key insight: Rollups inherit L1 security. Sidechains don’t.

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Using Layer 2

Bridging Assets

1. Connect wallet to bridge
2. Deposit ETH/tokens on L1
3. Wait for confirmation
4. Assets available on L2

Important: Each L2 has its own bridge. Withdrawals may take time (especially optimistic rollups).

Gas on L2

L2s still use gas, but much cheaper:

Operation	Ethereum	Arbitrum
ETH transfer	$2-10	$0.01-0.05
Swap	$20-100	$0.10-0.50
NFT mint	$30-150	$0.20-1.00

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Practice Exercises

Exercise 1: Calculate Savings (Beginner)

You do 100 swaps/month.

• Ethereum: $30/swap
• Arbitrum: $0.30/swap

Monthly savings?

Answer

Ethereum: 100 × $30 =$3,000 Arbitrum: 100 × $0.30 =$30 Savings: $2,970/month

Exercise 2: Security Analysis (Intermediate)

Compare withdrawing from:

• Arbitrum (optimistic rollup)
• zkSync (ZK rollup)
• Polygon PoS (sidechain)

What are the risks and timeframes?

Answer

• Arbitrum: 7 day dispute period, but L1 secured
• zkSync: Fast (hours), proof verified on L1
• Polygon PoS: Fast, but own validator set-bridge exploits possible

Exercise 3: Design Question (Advanced)

Why do optimistic rollups have a 7-day challenge period? What would happen with 1 day?

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Knowledge Check

1. What problem does Layer 2 solve?

2. What’s the difference between optimistic and ZK rollups?

3. Why are sidechains less secure than rollups?

4. What is the 7-day challenge period for?

5. Where is transaction data stored in rollups?

Answers

1. Scalability. L1 can’t handle enough transactions cheaply. L2 processes off-chain, posts proofs to L1.

2. Optimistic assumes valid, allows fraud challenges. ZK proves validity cryptographically-faster finality.

3. Own consensus. Sidechains don’t inherit L1 security. Rollups post data/proofs to L1.

4. Fraud proof window. Anyone can challenge an invalid batch. After 7 days, it’s finalized.

5. On Layer 1 (as calldata). This ensures data availability-anyone can reconstruct state.

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Summary

L2 Type	Security	Finality	EVM
Optimistic Rollup	L1 inherited	7 days	Yes
ZK Rollup	L1 inherited	Hours	Varies
Sidechain	Own validators	Fast	Yes

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What’s Next?

• What Is MEV? — MEV on L2
• Gas Fees — L2 economics