Was just wondering how can we control speed of BLDC using wired control system.

i think u are asking how can we control brushless motor using esc without using any reciever by controlling it wired
lets try answering it
if u power the esc with battery and use a computer controlled servo card[which add 3 wire servo inputs to computer using pci ports],then u can control it
i saw servo control card in some lhs website-funnhobby as much i  remember

Can be done easily by a pulse generator where the pulse width can be varied between 1 & 2 mS. No need whatsoever for a computer  and/or card.

Try This.............you need to modify wiring to use ESC.
use only white pin (Signal) of ESC
(Red and black/ brown and red) can be used to power the circuit.

thanks for detailed answer .