RF is effected in Environment by a number of ways.
2. Frequency shifting and
3. Timing shifting
Equalizer is used to cater for these shifts to ensure correct decoding of symbols. so we decode the signal right by sampling it at the right moment.
Equilization is usually of two broad types blind and assisted / aided.
In GSM as far as I know its the asisted equilization in which the transmitter transmits a known sequence of bits in a fixed part of the burst. The receiver already knows the bit sequence and its location and tunes its equilizer waits to recover the desired signal. then assuming that the environmental effect on these bits would be similar to that on the rest of the signal it applies the same weights to the other signal.
PIX could confirm that in Alcatel it is called TSC training sequence code and is taken as BCC (if I remember correctly).
It is transmitted in the middle of a normal burst between the flag bits.