Layer 2 is an overloaded term. Rollups execute transactions elsewhere and post data or proofs to a base layer. Payment and state channels (including application-specific state channels) keep activity between participants off-chain and use the base layer only to open, close or settle disputes. Sidechains run their own consensus behind a bridge. Saying a chain has no layer 2 must name which family is meant.
Key idea. Rollups continuously publish to the base layer; channels only touch it at the edges; sidechains have their own security.
Example
A payment-channel network routes payments across chains of two-party channels; a rollup posts its transaction data or validity proofs to the base layer it settles to; an application-specific state channel settles a game or contract between its participants. None of these is interchangeable.
Keep in mind
- Beta channel systems are labelled beta and are not presented as equivalent to mature networks without evidence.
- Channels require participants or watchers to stay available to contest a stale close; rollups add sequencer and proof-system assumptions.
Sources
- Scaling (ethereum.org developer docs) (external site)
- State channels (ethereum.org developer docs) (external site)
- lnd: Lightning Network Daemon (external site)