Network Cables Online Blog    

Balanced line operation is a transmission method which helps to eliminate the effects of noise on the cable. In the first diagram a coaxial cable is transmitting a 4V signal, this is unbalanced as all of the 4V signal is carried by the centre core of the coax with respect to the grounded screen. If 1V of noise is introduced, it adds to the signal being transmitted making 5V, this could interfere with our data.

With a balanced line transmission our 4V signal is split into +2V and -2V on one twisted pair, so we still have 4V between the two. Now when we introduce the 1V of noise, the +2V becomes +3V, and the -2V becomes -1V, but the potential difference between the two is still 4V. The devices we put on the ends of the cable to make the line balanced are called baluns, this name is derived from the function of the devices of converting between balanced and unbalanced transmission modes.

These days, more and more equipment is being designed to operate on balanced lines without the need for baluns, but there are still a lot of older systems out there that still use these converters.

How can I tell if a cable is a crossover or straight through?
Q. I currently have 2 PCs, one running windows xp and one running windows 98 which I would like to network together using the XP machine as the host. I have 2 rj45 ethernet cards and a 10 metre cat 5e twisted pair cable.
Is there any way to tell if I have the right cable as my computers aren’t communicating and I have tried every other troubleshooting resource, and the only conclusion I can come to is that my cable is not a crossover type.
Any advice would be GREATLY appreciated!

A. 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. If you look at the testing page of the Network Cabling Help website, about half way down there are some pictures of the pin outs for RJ45 connectors. 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.

Cat 5 crossover cable?
Q. I have been looking at your site and it was very helpful.
I’m trying to connect two computers with a cat 5 crossover cable. The cable that I received in the mail was crossed correctly, but it was also crossed at 4 &5. My question is will this cable work by just joining PC to PC without any hubs or anything else. I bought the cable from an individual.

A. 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.

Cable length limits?
Q. I know that cables have restrictions on distance, but I really would like to know why those limits are for each cable. E.g. Why is Cat X cable limited to 100m, and thinnet 185M etc. I have just accepted these values but have been asked why and I do not know a technical enough answer as I have never been a cabler. I could not find this on your site and was hoping it might be added or if you could email me back with why. Thanks.

A. 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.