Between two given points in an integrated services digital network (ISDN,) the time between the moment that the first bit of a data unit, such as a frame or block, passes the first given point and the moment that bit passes the second given point, plus the transmission time of the data unit. Note: Transit delay is defined only between pairs of boundaries. Transit delay of a FPDU states at the time t1 at which the first bit of the FPDU crosses the first boundary, and ends at the time t2 at which the last bit of the FPDU crosses the second boundary. Transit delay = t2 -t1.