I've had a good deal of experience with Cat 5, Cat 5e and Cat 6 cabling. The point about all four pairs being used for gigabit is a good one. I worked with some of the older Cat 5 cable which had differing amounts of twist on the pairs. The "data pairs", orange and green, had a fairly tight twist, while the other pairs had a loose twist. I tried using this cable with one of the early Cat 5 KVM extenders, and found that even on a 100' run to a monitor running [EMAIL PROTECTED] the colors misconverged. The propagation was different for the loose pairs vs. the tightly twisted ones. This propagation skew would make the cable completely unusable for gigabit networking.
At 100 Mbps the blue & brown pairs are used for overall "balance" in the cable and are intended to be grounded at both ends by the NICs. At 10 Mbps you could get away with running two Ethernet circuits in a single Cat 5 cable, not so much at 100 Mbps. Ethernet cabling has a chacteristic impedance of 100 ohms, balanced. Electrically, the difference between the various categories of cable is to what frequency the impedance of the cable remains within about 15 percent of that 100 ohm spec. There's also an attenuation spec, but a quick search didn't find the reference I was looking for. Keep in mind that Ethernet is NRZ encoding, (non-return to zero), so at 350 MHz it's 700 Mbits of data on a pair, more than enough for gigabit. Each "cycle" of the waveform on the cable represents two bits. So how do you keep the impedance flatter throughout the frequency response of the cable? Tighter twisting is good, because it makes the twisted pair more nearly round and thus uniform. Twisting the four pairs around each other along the cable is another technique that keeps the relative spacing of pairs to other pairs more constant - these are characteristics of Cat 5e cable. The Cat 6 cable I've worked with has a plastic separator in the middle, in the shape of a plus (+). There's a channel for each of the four pairs, and the plastic separator keeps the pair spacing very precisely consistent throughout the length of the cable. I hate working with the stuff, but I can see why it's rated for higher performance. As Andrew pointed out, all four pairs are used in gigabit, so do be certain you have one of those continuity testers that detects pair reversal or miswiring. I find that when I've miswired an RJ45 plug, it's usually because as I was holding the wires flat and inserting them into the plug, a couple of them transposed. Please be aware that the inexpensive cable testers do not have the capability of detecting if you did the wiring right (pin 1 to pin1, up to pin 8 to pin 8) but you got the pairing wrong. An example of wrong pairing which I've seen in the real world was when a contractor set up pair 1 to pins 1&2, pair 2 to pins 3&4, etc. The wiring continuity was right - and it simply refused to work with data. The high end cable testers detect this faux pas, and that's why they cost thousands of dollars. Use the Mark One optical inspection technique and you'll be fine. But borrow a continuity tester if you have to. I've never seen a Cat5/5e/6 cable that wasn't 24 gauge solid wire. It's not wire gauge that determines the quality of the cable. Yes, you can buy stranded wire for making up patch cables - but it's harder to terminate properly. As others have mentioned, don't have more than 1/2 inch of untwisted cable at a termination - you'll cause an impedance bump in the overall circuit and have nasty little reflections running around with your pristine data. Don't kink the cable - that's a huge impedance bump right there, and don't try for a really sharp radius bend. Some of the best cable I've worked with is Siamese Cat 5e. Two Cat 5e cables with the jackets bonded together, and the resulting cable is stiff enough that it resists kinking and is really nice to work with. Oh, and you always have two circuits, which is great for later expansion. But despair - it's really expensive. So stick with Cat 5e and be really careful not to kink it during installation - you will have great success. Curt --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "NLUG" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/nlug-talk?hl=en -~----------~----~----~----~------~----~------~--~---
