Pilot symbols in IEEE 802.11a In IEEE 802.11a standards, there are many fields. Of which is the Data field which has 52 available sub-carriers used for transmission of coded bits. If the subcarrier locations are numbered from 32,-31,...0,1,...31.

I have read in the book that 

that 48 out of 52 subcarriers are used for transmission of data bits. While 4 pilots are used as pilots for phase and frequency tracking and training. The pilot subcarriers are located at -21,-7,7,21 with values 1 (at -21,-7,7) and -1 at 21.

I am just wondering, usually the preamble (fields before the data field) is used to estimate the channel by sending a pilot symbol (using ALL subcarriers). Then my questions are 

1) what is the purpose of sending these 4 pilot tones? 

2) I am guessing phase and frequency tracking is different from channel estimations?

3) My last question is do we send the pilot values on the predefined pilot subcarriers ALONG with the data bits? so basically is each OFDM symbol made up of pilot and data tones? This is different than the procedure in preamble where  ALL subcarriers are used to send one pilot symbol, right?

Thanks
A frequency offset $f_\mathrm{off}$ between the carrier frequency and the local oscillator (at the receiver) results in a linear phase, i.e. the received signal contains a factor $\mathrm{exp}(j2\pi f_\mathrm{off}/f_\mathrm{s}n)$, where $f_\mathrm{s}$ is the sampling frequency and $n$ is the discrete time. If the frequency offset is sufficiently low (e.g. because it has been estimated and compensated for in an earlier step) the linearly changing phase can be assumed constant during one OFDM symbol. This means that all samples of the received OFDM symbol have a constant phase offset that can be estimated using the pilot subcarriers. As the phase is changing slowly, this has to be done for every OFDM symbol individually. The pilot symbols can also be used to estimate phase noise.
Yes, see 1. Frequency and phase offsets are time varying effects while the channel is assumed to be constant for at least some OFDM symbols.
Yes, pilot tones are sent in every OFDM symbol, along with data subcarriers.