She puts, say, 100 bitcoin in 2-of-2 multi-sig account C. When Alice wants to pay Bob, she gives Bob a single funding transaction which sends money from C, 1 Bitcoin to Bob and 99 to Alice. To build a bidirectional channel, one can have two unidirectional channels but they have a limitation, once Alice has exhausted her channel, she cannot send more money to Bob even if Bob has sent Alice money via the other channel. Alternatively, one funding channel with time lock of 100 can be forked into another funding channel of time lock of 99, which can further be forked into funding channels with the smaller time locks.