CAT5e specification requires that ALL 4 PAIRS are capable of 100mhz at 100 meters.
If you have cable marked as CAT5e that doesn't meet those specifications, then you have out-of-spec cable that technically is not cat5e cable, and you should find a supplier that sells legitimate cable.
No, it is NOT common to use low quality pairs for the "last 2 pairs" in a CAT5e cable. Also, data for 100mbps is on pairs 1 and 3... NOT pairs 1 and 2.
Well. Where did I say that all 4 pairs aren't capable of 100mhz ? Like my first post, you are capable to take the ride with the bike. But the car makes the ride easier.
If you read spec in cat5e cable, its not possible to get 100mhz exactly in each pairs.. This is due to different crosstalk on the 4 different pairs. This will make the pairs capable of different mhz. To make it as close to 100mhz as possible, its common to hane 1 pair, lets say at 110mhz, 1 pair at 105, 1 pair of 95, and one pair of 90. To compensate, (both ways) its common to use cheeper AWG 24 in the 110 and 105mhz, thicker more expensive in the 90 and 95 pair. This will compensate so the total is as close as possible to the 100Mhz. This is in a LONG cable. If you find some "china made" preteminated cable, you could also find cheeper material, and smaller cable diameter. IF the cable is shorter. like a patch. ITs also some manufactures, like belden etc, that make 200mhz cables, but still call em cat5e
Cat5 compared to cat5e and cat6: (what's really differs)
Specified frequency range 1-100 MHz 1-100 MHz 1-250 MHz
Attenuation 24 dB 24 dB 21.7 dB(36 dB)
NEXT 27.1 dB 30.1 dB 39.9 dB(33.1 dB)
Power-sum NEXT N/A* 27.1 dB 37.1 dB(30.2 dB)
ACR 3.1 dB 6.1 dB 18.2 dB(-2.9 dB)
Power-sum ACR N/A 3.1 dB 15.4 dB(-5.8 dB)
ELFEXT 17 dB(new requirement) 17.4 dB 23.2 dB(15.3 dB)
Power-sum ELFEXT 14.4 dB (new requirement) 14.4 dB 20.2 dB(12.3 dB)
Return loss 8 dB* (new requirement) 10 dB 12 dB (8 dB)
Propagation delay 548 nsec 548 nsec 548 nsec (546 nsec)
Delay skew 50 nsec 50 nsec 50 nsec
But what's really important here, is that we talk about MINIMUM requirements. That is not the same as, this is what the actual performance and spec say. A 0,2$ china cable will work, but its not the same as a 10$ god made, spec X2 cable.