. The signaling rate refers to the maximum physical speed of the interface, that is, 480 million transitions (bits) can be reliably detected per second over the interface. Data transfer rate is the amount of usable data (probably peak) that can be transferred in a second. 53 MB x 8 = 424 mb, less than the signaling rate because the signaling rate includes packet overhead information. Think of the start, stop and parity bits in RS-232 data. They are overhead bits that help with the communication channel, but aren't part the overall (useful) data rate.
