TTI, Transmission Time Interval, is a parameter in UMTS (and other digital telecommunication networks) related to encapsulation of data from higher layers into frames for transmission on the radio link layer.
The length of time required to transmit one such block determines the TTI.
In order to be able to adapt quickly to the changing conditions in the radio link a communications system must have shorter TTIs.
In order to benefit more from the effect of interleaving and to increase the efficiency of error-correction and compression techniques a system must, in general, have longer TTIs.
In 1xEV-DO technology, a slot, which is not quite the same thing as the TTI, but which still fulfills a somewhat similar function, is 1.667 ms.