"Computer Networks" By A. Tannenbaum says that Bandwidth delay product is determined by the bandwidth in bits/sec multiplied by the one-way transit time, and then multiplied the bandwidth of 50kbps with half of 500msec round trip propagation delay ($50kbps \times 250msec$), Whereas I'm finding that the Formula for Bandwidth delay product is $Bandwidth \times RTT$RTT. Which one is correct?