Frequently Asked Questions

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Suggestion & Usage

The basic Cat 5 system used to be the only real choice, but developments in Ethernet technology led to the introduction of ‘Enhanced Category 5’ or Cat 5E. Both systems are capable of transmission rates up to 100MHz, but the test parameters for Cat 5 assumed that data signals would only use two of the four pairs (one pair for transmitting and one pair for receiving) and crosstalk measurements were only taken between each pair combination. With Gigabit Ethernet however, all four pairs can be used to transmit simultaneously, and so the cross talk on each pair has to be measured for the combined effects of the other three pairs.

At last! the standard for Cat 6 has been approved for publication by the EIA (TIA/EIA-568-B.2-1). Category 6 is capable of transmission frequencies up to 250Mhz and has a positive power sum attenuation to crosstalk ratio upto 200MHz using improved cables and RJ45 connectors. The problem that manufacturers have, is that to meet the Cat 6 specification, requires the use of cables and connectors which are designed to work together as a ‘tuned’ system. This means that if you install a Cat 6 system the manufacturer will only guarantee performance if all of the components including the patch leads are from their Cat 6 product range. In fact, by mixing Cat 6 components from different manufacturers you could end up with a system with worse performance characteristics than a conventional Cat 5e system. That said, it is worth noting that Cat 6 systems are backwards compatible with Cat 5/5e cabling and when mixed with these lower bandwidth systems the performance criteria of the lower specification will still be met.

Testing Cat 6 cables can be a frustrating process, apart from taking longer because the tester has to scan frequency steps up to 200MHz instead of 100MHz, the fine line between pass and fail is accentuated it seems by the slightest kink and twist. The most significant factor when testing a Cat 6 system can be return loss failures due to the test leads themselves. All connectors have a life cycle and with the average RJ45 connector this is around one or two thousand insertions, so test leads should be replaced after every 1000 tests or so. OK, not a problem but at around $200 per set this cost will have to be considered when pricing jobs.

Fluke seem to have a solution to this problem with their DSP-LIA101S Permanent Link Adapters. The connector at the end of the leads are interchangeable and replaceable with connectors from different manufacturers to ensure compatibility with the system under test. Although a good idea, the adapters are over $500 and a new pair of “Personality Modules” cost over $100. Surely the test plugs should now be considered as ‘consumables’ and the price lowered to reflect this.

This is proposed to be a 600MHz system using a shielded cable with individually screened pairs and a new type of connector. The cable and connectors are slightly bigger than Cat 5e and installation time can be increased because of the complexity of the termination. There are two main draw backs with installing this type of cabling, the first is the additional cost involved, and the second is that almost all networking hardware uses RJ45 jacks. To connect to the cabling system, you have to use Cat 7 to Cat 5e patch leads, and because any system is only as good as its weakest link, your speed is back down to 100MHz. Ratification of the Cat 7 standard could be two years away by which time fibre might be a cheaper alternative.

Cross Over

If you hold both of the RJ45 connectors side by side and look at them from the bottom (ie. the clip is away from you) you can make out which colour is connected to which pin. The pins you are interested in are (from left to right) 1 & 2 and 3 & 6. If they are the same either end then it is a straight through cable, if pin 1 (usually orange/white, but not necessarily) is connected to pin 3 at the other end and 2 is connected to pin 6 (and vice versa) then you have a crossover cable. The ‘crossed pair’ image is actually the correct wiring for a crossover cable and above it is the correct wiring for a straight through cable.
If you have a straight through cable then it won’t work without a hub, if pins 1 & 2 and 3 & 6 are crossed over then the problem lies with the way the network is setup.

If you are using it for 10BaseT or 100BaseT Ethernet then the wiring should be 1 to 3, & 2 to 6
Pins 4 & 5 aren’t used on 10/100BaseT Ethernet systems so it should be OK. If you are planning to use Gigabit Ethernet or Token Ring then it won’t work.

The length limits are not for the particular cables as such, they are for the type of data signal that they carry.

Let me try and explain!

Thinnet (RG58 coax) was used for 10Base2 Ethernet, at 10Mbps on RG58 coax Ethernet can reliably operate upto a distance of 185m. The native cabling environment of the AS400 is Twinax and the standard operating speed is only 1Mbps. At this speed it has a maximum distance of 1800m, however, if Cat 5 forms part or all of the link the distance can drop to between 36m and 364m.

So for a proprietary network such as Thinnet, the distance is set at the maximum length that the signal will work reliably at a given speed over a given type of cable. So far so good!

Now, when we talk about Cat 5, 5e, 6 etc. these are cabling ‘Standards’ which define a method of connecting all types of networking protocols, over a cabling system that uses a common media, common connectors and a common topology. So the length limit was arbitrarily set for the worst case scenario. 10BaseT may well work on Cat 5 for 150m but ATM, AS400, Token Ring etc. may not, and because a structured cabling system has to work for all networking methods, a limit had to be set.

I don’t think I have explained this very well but I hope you get idea. Incidentally, I have heard talk of the length limit being dropped from the standards as it is the overall Attenuation to Crosstalk Ratio which determines a cables ability to transmit a signal successfully, and not the length of the cable. If anyone can elaborate on this point please let me know.

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