• SLC means Single Level Cell and is the basic use of NAND cells.
• SLCs can store one bit of data per cell:
  • 0,
  • 1
• This requires two states to measure:
  • One state can be empty
  • The other state is NOT empty.
• Charging the battery (or writing the cell) is easy
  • Anything greater than empty changes the bit from 1 to 0.
• Checking the charge (or reading the cell) is also easy:
  • Checking for empty returns false
  • Cell must be not empty, so 0.

SLC is the fastest and most costly NAND.

• MLC means Multi-Level Cell and is a poorly named.
• MLCs can store two bits of data per cell:
  • 00, 01,
  • 10, 11
• MLC requires four states to measure:
  • Empty
  • Three charge charge ranges.
• Reading and writing is more complex than SLC:
  • Charging (writing) must be a precise charge
    • Otherwise the wrong bit sequence is saved.
  • Checking (reading) will take more than one attempt.

MLC 'costs less' than SLC, because an extra bit added 100% more storage per cell.
The actual cost is in performance, due to extra precision and checks.

• TLC stands for Triple Level Cell
• TLC can store three bits per cell:
  • 000, 001, 010, 011,
  • 100, 101, 110, 111
• TCL needs eight states to measure:
  • Empty
  • Plus seven charge ranges
• The charge states are smaller and closer together.
• Read and writing is more difficult than MLC:
  • Writing required accuracy
  • Reading requires more attempts to get the range (data).

TLC 'costs less' then MLC, because an extra bit added 33% more storage per cell.
The cost is more performance, due to more precision and checks.

(See the trend?)

• QLC stands for Quad Level Cell
• QLC can stores four bits per cell:
  • 0000, 0001, 0010, 0011, 0100, 0101, 0110, 0111
  • 1000, 1001, 1010, 1011, 1100, 1101, 1110, 1111
• QCL requires sixteen states to measure:
  • Empty
  • Plus fifteen charge ranges.
• Space between charge states small
  • Gaps separating them are even smaller.
• Reading and writing get even more difficult from TLC
  • Higher accuracy.
  • More Checks

Pros

• As bits are added, per cell, the higher capacity goes, while keeping the cost low too.
• Companies use the same amount of physical NAND, but you get a higher capacity.
• More layers give a lower price per GB.

Cons

• The performance and endurance goes down as you add bits.

• If you are looking for performance, less bits per cell is best.
• If you are looking for a low cost drive, the highest bits per cell is best.
  • Which also will be a higher capacity.