Doppler Shift

What is the maximum Doppler shift for the GSM mobile cellular system on the ``down-link'' from the base station to the mobile (935 to 960 MHz)? What is it on the ``up-link,'' or mobile to base direction (890 to 915 MHz)? Assume that the mobile is driven at a speed of 180km/h on the autobahn. Comment on the validity of the assumption that the channel is constant during the transmission of a burst (approximate duration 0.5 ms).