TTI interval

PS: This question is related to 3GPP Specification, but ofcourse has
relevance to Basic communication fundas.

I was wondering how the TTI interval is arrived at in 3GPP spec? I know
that, the ratio of the Transport block size (in bits) to the TTI
interval would provide a measure of the throughput, but I reckon the
TTI interval (10ms, 20ms etc in UMTS Release 99 and 2ms in HSDPA and
may be even smaller values in future evolutions).
My question is specifically on the origin of these TTI figures.

I guess this may have come from the overall delay requirements of the
system (processing delay, transmission delay, network delay and so on).
Can some one throw some light on the exact parameters (requirement per
se) and how the 10ms figure for the TTI is arrived at?

TTI- Transmission time interval

Thanks in advance

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