Block Time can be decided by difficulty

Sybil resistance/ Chain selection

Nakamoto consensus = POW +longest chain rule

Miner: The first to figure out get paid - Block award, 51% attack

ETHERUM 2.0: POS - stake in each account, Validators. Choose - Validate

Randomness: RANDAO， choose a node randomly

-Pros: Only ONE node to calculate,

-Cons: not decentralized enough

SCALABILITY: a limit of system

SHARDING(ROLLUP): main chain to coordinate (solution to scalability issue of LAYER 1)

LAYER 1: bitcoin, etherum

LAYER 2: App add on LAYER 1; ARBITRIUM,,OPTIMISM, ROLLUP

SIDE CHIAN