.......yes

Oh yeah, as shockneck said, no way will any 5.3 media boot on your system.
When you boot AIX it loads a mini install kernel and then tries to configure devices.
No way will the 5.3 even load the config / install kernel on a 7248.
5.3 requires at least a chrp system with firmware late anough to support 5.3 and the 7248 is not a chrp system.
Get it booting a 5.1 diag CD and then go find a copy of AIX 5.1 install media to install AIX - if it will even support 5.1!
I cannot find any docs for the 7248 any more so 4.3.3 could be your limit.

i tried the diag cd's version 5.1 and 5.2. neither of them boot.
there is a lot of activity on the cdrom drive when i either hit F5 or set the boot device to the cdrom, but no boot. does that mean that all the 5.x versions of aix do not like my 7248? i can only hardly believe this, because i believe in the company where i work there are such machines with aix 5.

any more suggestions?

If at all it will boot 5.1 only with one of the latest microcodes for that workstation, perhaps only the last one IBM made available.
Starting point for your search: http://www14.software.ibm.com/webapp/set2/firmware/gjsn
With an old firmware you might be stuck with some 4.x oslevel.

I don't think the F5 thing works on the 7248, too old.
Do you have the SMS diskette?