Why is the 100base-TX maximum distance unchanged from the original specification on better cable?

Question

I understand that the 100base-TX Ethernet standard is from 1995, and was originally designed for older and poorer grade CAT-5 cabling than what is now commonly available.

With the introduction of the newer and faster ethernet speeds, and higher grade cables to support those speeds, why hasn't the maximum specified signalling distance for 100base-T increased when it is used on these same higher grade cables?

It seems like 100base-TX should be fully reliable for considerably further distance than is specified by the standards when it is run on, for example, CAT-5e, CAT-6 or CAT-7 cable.

Answer 1

The distance limitation is based on minimum frame size (64bytes) and the propagation time to all nodes so collision detection (the CD part of CSMA/CD) can happen. 100B-TX exists on half-duplex hubs. That part of the standard still applies to even 1000base specs -- although, if the signal can make it further, it can work in out-of-spec situations.

[Note: no one uses hubs anymore, but the spec is still the spec. Those devices still work... if you have them, and I do.]

Answer 2

Newer/faster standards utilize much higher frequencies in the cable where certain negative characteristics of the cable will have more of an effect, such as NEXT (near end cross talk). Newer cable grades are primarily designed to address these issues.

Other characteristics of the cable standards do not change significantly with higher grade cable and have more of an impact on Examples of such characteristics:

• attenuation/insertion loss - the difference between Cat5e and Class FA (also sometimes referred to as "Cat7A") is only 3.7dB, compared to the difference in NEXT which is almost 35dB.
• propagation delay - (mentioned in Ricky's answer as well) consistent across all cable categories
• return loss - a 2dB difference between Cat5e and the rest of the standards

These latter characteristics have more to do with the limitations of sending low voltage electrical signals on copper wires. As such, they won't ever change much even with other improvements to the cable and these are also the factors that primarily go into determining the "distance" a signal can be sent on a link.

So even with a better grade of cable, that doesn't really have much impact on the distance a signal can be sent reliably on a cable used for networking.

Answer 3

Further to Ricky's point, you can run out of spec in situations where you are on a switch because then there are no collisions to contend with. I personally have run 500 feet without issues. Signal attenuation is the limiting factor when there are no collision domains.