8051 is half duplex or full duplex

"Without connecting external hardware, how am i going to communicate serially??"

Of course, you can't!

But you entirely missed Per's point!
What he said in full was:

"It is only when you connect external hardware that connects the rx and tx signals..."

In other words, the 8051 is full duplex.
But, if you want your system to be full duplex, then you have to ensure that all parts of it can support full duplex - including your external hardware and your software.

"Without connecting external hardware, how am i going to communicate serially??"

Of course, you can't!

But you entirely missed Per's point!
What he said in full was:

"It is only when you connect external hardware that connects the rx and tx signals..."

In other words, the 8051 is full duplex.
But, if you want your system to be full duplex, then you have to ensure that all parts of it can support full duplex - including your external hardware and your software.