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Understanding cabling limits for high-bandwidth systems

Apple's Thunderbolt cable may be a bit pricey, but the high-bandwidth Thunderbolt technology will require a good cable.

With more of Apple's Mac line slated to include Thunderbolt and the availability of some Thunderbolt devices, Apple has been releasing Thunderbolt-related software and firmware updates to tackle bugs. In addition, Apple has made available a Thunderbolt cable, which is required but not included with some Thunderbolt devices like the Promise Pegasus RAID drive. Some people may be curious about the need for the special cable, and they may be taken aback by its steep $49 price tag.

When a signal is transferred digitally over an electrical wire, it is done in individual bits (ones and zeros) that are basically detectable voltage differences between the signal and "ground" wires in the cable: when the signal is a 1 or "on," there is a detectable voltage difference (like a light bulb being turned on), and when it is a 0 or "off" there is no detectable voltage difference (the light bulb is off). The receiving system then makes sense of the patterns of "ons" and "offs" that it detects in the cable.

This setup has been the case for all digital systems and seems simple enough, but wire has physical properties that will prevent this process from happening at high speed over long distances. Just imagine clapping your hands on a windy day--people close to you will hear the signal, but it may be lost in the wind for people farther away.

When it comes to transmitting a signal via a cable, as with the clapping example there is a limit to how far the signal can be sent before it decays to the point where the receiving system cannot make use of it. This is because the cable itself will always impede on the signal based on four physical properties of the cable:

  1. Conductance: Some metals are far superior conductors than others and therefore will offer less resistance through the length of the cable.
  2. Wire shape: The size, shape, twist and weave patterns, and the uniformity of these down the length of the wire also have a major impact on the cable's conductance.
  3. Insulation: Proper insulation around cable components will keep the signal contained within the conductor and thereby help prevent signal decay down the length of the wire.
  4. Shielding: All cables will be exposed to electromagnetic noise, even from other components of the cable itself (the different signal channels in the cable). Having proper shielding to deflect or ground electromagnetic noise will prevent interference with the signal.

With these in mind, when making a good-quality cable an attempt is made to lessen the impact that both the cable and external noise have on the signal by using special conductive alloys in very uniform weave patterns and strand sizes. In addition the insulating wrapping for the wires will be made of a high-quality material and will be uniform in size around the wire, and the cable will have good-quality shielding.

Even if a cable is made of good-quality materials and manufactured well, it will still have limits based on how the system is using it. With high-throughput digital signaling, the system has to distinguish exceptionally small and sharp voltage changes in the electrical signal in the wire, at very high rates of speed. Any small amount of noise or signal decay will cause the signal to be lost and require the sending system to resend the information.

When you turn on a light bulb and turn it off again you can perceive it as being either completely on or off, even if you change its state at a rate of twice per second. However, if you manage to rapidly turn it on and off at 100 times per second then your eye may see just a continuous light, and not be able to detect any change in intensity. This is not only because of your eye, but also because of the properties of the lightbulb itself. When you turn off the power the light's filament will continue glow for a split-second. Therefore if you wait longer than that split-second to turn the light back on then you will perceive the light as having been off. If you wait a shorter time than that split-second then the light will not completely turn off before you give it power again and instead may only dim a little. However, with a high-quality filament that immediately shuts off with no delay, the light will shut off faster so the ability to distinguish rapid ons and offs will be easier.

What this all means is that for any cable system there is a limit to not only how far a signal can be sent, but also how fast the signal can be sent, and these limits become even more pronounced the higher the bandwidth of the system is. Therefore, the required throughput of the Thunderbolt technology's cable will be a lot more important than it has been for USB and FireWire connections.

Even though Apple's $49 price tag for its cable may seem a bit high, do not expect a good-quality cable to be much cheaper than that. If you find a Thunderbolt cable being sold for under $10 (especially for longer ones), then you likely are going to get much lower bandwidth than with a well-made and more expensive cable. As a high-bandwidth system, Thunderbolt will be much more sensitive to cabling details, and you will benefit from using well-made and appropriate cables.

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