Each QPI comprises two 20-lane point-to-point data links, one in each direction (full duplex), with a separate clock pair in each direction, for a total of 42 signals. Each signal is a differential pair, so the total number of pins is 84. The 20 data lanes are divided onto four "quadrants" of 5 lanes each. The basic unit of transfer is the 80-bit flit, which is transferred in two clock cycles (four 20-bit transfers, two per clock.) The 80-bit flit has 8 bits for error detection, 8 bits for "link-layer header", and 64 bits for data. QPI bandwidths are advertised by computing the transfer of 64 bits (8 bytes) of data every two clock cycles in each direction.