150- W STP-A Noise and Interference

W STP A Noise and Interference

There are three major sources of noise and interference in a high-speed 150- W STP-A system:

1. Far-end reflected noise
2. Near-end crosstalk (NEXT)
3. Skew

Before we look at the major noise and interference effects, I'd like to say a word about some effects that don't show up in this list: near-end reflections, alien crosstalk , and far-end crosstalk . These problems don't tend to arise in 150- W STP-A systems because of the way typical applications use 150- W STP-A.

All known 150- W STP-A LAN applications use the pairs unidirectionally: one pair for data traveling in one direction and the other pair for data traveling in the opposite direction. As a result, problems with near-end echoes don't arise, because when you transmit on one pair, you are not also listening on that same pair. Similarly, because both pairs are completely used, leaving no room for other alien applications within the same cable jacket, alien crosstalk doesn't matter. Lastly, far-end crosstalk doesn't arise because it requires two parallel transmission channels working in the same direction ”something that doesn't happen with 150- W STP-A.

Why is 150- W STP-A always used in the unidirectional mode? The reason has to do with the typical development scenario for most LAN-type products. Early in their life, several high-speed LAN standards historically endorsed 150- W STP-A, because the bandwidth of the cable is so great that it is easy to make it work. The early adopters that purchase new LAN standards are often willing to undergo the pain of 150- W STP-A installation in order to gain the benefits of bleeding-edge LAN technology. Later, as the standard gains credibility and users want to deploy it in larger quantities , manufacturers invest in the development of UTP-style transceivers. The UTP transceivers are undoubtedly more complex and more costly, but the extra expense is more than compensated by the reduced installation cost associated with UTP. Examples of LAN standards that were originally available in 150- W STP-A format include 10 Mb/s Ethernet, Token Ring, FDDI, 100 Mb/s Ethernet, and 1000 Mb/s Ethernet. In all cases the UTP versions of these transceivers, once available, conquered the market.

The subjects of far-end reflections and near-end crosstalk are so similar to the UTP case that I have incorporated them into Section 8.3.2, "UTP: Near-End Reflections" and Section 8.3.4, "UTP: Near-End Crosstalk".

POINT TO REMEMBER

• When 150- W STP-A is used in a unidirectional mode it is not subject to near-end reflections, alien crosstalk, or far-end crosstalk.