Improving Bit by Bit

To move signals of that frequency around efficiently within my transmitting station needed what can only be described as plumbing transmission lines made out of thin copper pipes and ceramic insulators—the alternative being expensive, specially constructed, but very high loss, coaxial cable. Yet here we are twenty years later happily shoving similar frequencies down twisted pairs without really thinking about the physics involved! Three factors about radio frequency signals are very important in understanding our problem:...
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Improving Bit by Bit Improving Bit by Bit . many installers and customers To move signals of that frequency around efficiently within AS DATA are finding that physical my transmitting station needed what can only be described R AT E S networks designed, installed and tested to the current Cat 5e as plumbing transmission lines made out of thin copper pipes and ceramic insulators—the alternative being RISE, or proposed Cat 6 standards simply won’t work properly at 100 Megabit Fast Ethernet, expensive, specially constructed, but very high loss, coaxial cable. Yet here we are twenty years later happily shoving similar frequencies down twisted pairs without really let alone at Gigabit speeds. Independent Management thinking about the physics involved! Consultant Philip Turtle explains some of the reasons why this is happening and how one manufacturer seems Three factors about radio frequency signals are very to have come up with the solution. important in understanding our problem: 1. They become attenuated very rapidly in a transmission There have been numerous reports during the latter half of line system such as our twisted pair, and the power loss 1999 about Cat 5e and “proposed” Cat 6 systems being increases with both frequency and line length. So the installed and passing category testing. Yet when presented signal you get at the receiving end is much, much less with real network traffic they have failed to pass data than you started with, a mere 0.005 watts out for every sensibly at all, even though the link lights on the network one-watt you put in at Cat 5 limits. interface cards (NICs) are “on,” both on the PC and the hub 2. They treat every wire as an antenna radiating energy or switch. into other conductors and receiving energy from them, too. Just think, you are trying to receive a signal which The practical solution in many cases has been to switch the is only five thousandths of the high power signal in an 100 Mb/s NICs from Full Duplex to Half Duplex, effectively adjacent wire, a fantastic recipe for the interference we halving the theoretical maximum bandwidth to 50 Mb/s know as “crosstalk.” and in fact, taking the practical bandwidth much lower. 3. They reflect or bounce back quite dramatically off any “discontinuity” that gets in their way, and any poor It’s all about radio waves. connections and impedance change or “mismatches” along the way prove to be very good reflectors. Every As a teenager, I was a keen ham radio practitioner and built bit of power that is reflected doesn’t get to the far end, a VHF radio station operating on the “two meter” band at reducing our power received to even less than five frequencies around 144 MHz. Why do I tell you this? thousandths of what we put in. And if that’s not Because Cat 5 systems use 100 MHz rated components and enough, the reflected signal interferes with and distorts cable, Cat 6 uses 200 MHz—both very close to my radio the signal which is going in the correct direction and station frequency. can often turn a “zero” into a “one” or vice versa.Figure 1 Attenuation, crosstalk, reflections NEXT and digital signal processing. Without the wizardry of digital signal processing, high speed NEXT data over twisted pairs just wouldn’t work. If you look at figure 1, you’ll see how the signal goes in and how it comes ...