Dear John,
Both the CAT5 and CAT5e have a max frequency of 100 Mhz. CAT6 is 250 Mhz.
As the categorized ratings go up (CAT3,4,5,5e,6) the specifics like: insertion loss, NEXT, ACR, ELFEXT, Return Loss, Delay and Delay Skew get more focused to allow higher speeds. The standards are based on repeatable results (i.e., your test and my test will correlate):
Example:
CAT5 (link type-BL)
NEXT = 29.3 dB
ELFEXT = 17.0 dB
Return Loss = 10.1 dB
Delay Skew = 45 nSec
CAT5e (link type-PL ..was originally BL)
NEXT = 32.3 dB
ELFEXT = 18.6 dB
Return Loss = 12.0 dB
Delay Skew = 44 nSec
In a sense, your system will run at the "lowest common" speed. If you mix CAT5, CAT5e and CAT6 cables, say CAT6 from switch to workststation and CAT5 to other switches, your system throughput will look like CAT5 (100 Mhz)cable only.
Regards
Peter Buitenhek
Profit Developer.com