Consider a protocol that uses piggybacking, a sending window size of 4, and 400-bit frames. This protocol is used to transfer data over a 200 kbps channel with a 4 msec one-way propagation delay. Unfortunately, the receiver has no data to send back. It needs to send its acknowledgements in separate frames. What is the maximum amount of time the receiver can wait before sending, such that the bandwidth efficiency does not drop below 50%?