Thank you for your spending time to reply my post. It was close to it, Ronnie. It was about the IP address mistakenly set on the TFTP server. When setting the static IP address on the domain controller server ( on which WDS was installed) virtual machine, I did not check it against the used IP addresses. My physical machine also had the same IP address as that of this domain controller server virtual machine. These two machines (virtual and physical) had the same IP addresses, silly me. Consequently the DHCP kept wondering which IP address was actually assigned to TFTP server before directing PXE boot client to the right TFFP server. Mainly the issue was resolved.
Next time, I try to replicate the same silly scenario (both physical and virtual machine with the same IP address) and ping to itself to see if anything interesting comes up, given that both machines still had the access to the Internet at the time of error occurrence.
For learning purpose, I found the screenshot (see below) for those who learn by visual aid and experiments, including me, and might run into all sorts of problem with WDS. it may serve as a road-map (a starting point) to figure out solution or to rule out false assumption and to pinpoint the true cause of the problem. It is better to understand logic/process by pictorial presentation with clear explanation to each of the components involved and to how one interacts with one another in sequential order. Microsoft notoriously came up with an abundant pool of headache terminology and often give learners much of difficulties and unnecessary challenges. One who is well familiar with Microsoft technical terms either has worked in the related field for years or must have gigabyte-sized memory chip planted in his/her brain to memorize a great amount of Microsoft terminology and its abbreviation.
Of course, it is still valuable to have a backup from MCSE certified professionals as a guide to assisting learners to have a clear mental image of how a particular service work and its related components within Microsoft OS)