13 Feb Detecting a Split Pair
Telephone and data circuits all use multi-core cables. ADSL lines usually use four cores that are split into two pairs, while higher specification Cat 5, 5e and 6 cables use eight cores made up into four pairs. Each pair comprises of two wires twisted together. The reason is that twisting wires together provides resistance to crosstalk from adjacent wires as well from electromagnetic interference (EMI). This means that multiple cores can be laid close to each other. Each pair in a data cable performs a unique function.
Although each wire is color coded, it’s relatively easy to get wires mixed up and this is known as a split pair. Split pairs cause numerous problems in circuits, including interference, loss of signal and poor data transmission. Detection of split pairs is not easy using conventional multimeters and is best performed with specialized digital cable testers.
Impact of a Split Pair
Each pair in a data or telephone connection cable carries a different signal and has a different function. In order to work correctly, each wire must be connected in the right position in the 8P8C RJ-45 plug used to connect Ethernet cables. The same applies to four-pin telephone connectors.
If the connection is incorrect at one end of the cable, the signal will be split and, depending upon the actual configuration, this may create an open circuit, disrupt Power over Ethernet (POE) or disrupt the signal. If the error is at both ends of the cable, then the circuit may work but will be affected by crosstalk from one of the adjacent pairs. The impact of this will be an increase in signal errors as a result of crosstalk and a significant reduction in the speed of the connection.
Detecting Split Pairs
One of the difficulties with detecting a split pair using a multimeter or an analog Butt Set is that measurements are made in direct current and, provided the circuit is complete, no error will be identified. If the circuit is open, a tone generator can be used to find the correct pair, but again this will not identify a split pair unless it is visually obvious. What can also cause confusion is if the RJ-45 connector has been connected up using the wrong wiring standards. These are the T-568A and T-568B standards.
The problem with visual inspection is that it’s time consuming and also difficult, because it’s not that easy to read the colors of the wires in most RJ-45 connectors. Apart from this, the split may be caused by another cable that’s connected to a hub or joined further down the line. For this reason, the easiest method to identify a split is to use a digital cable tester like the T3 Innovation Net Prowler™ that can identify splits and other cable faults and, additionally, measure the distance to the fault. Related issues such as excessive skew, poor signal-to-noise ratio and high bit error rates can also be identified using the T3 Innovation Net Chaser™.
Split Pairs: Often Overlooked
In the early days of the internet, when data speeds were low and POE was uncommon, the consequence of a split pair was not as serious as it is today. In fact, splits were sometimes deliberately created when a technician replaced a faulty core with a wire from an unused pair.
Although some circuits may still operate with a split pair, the truth is that the crosstalk created by a spit pair will almost certainly degrade performance, and some functions, like POE, may not operate at all. So when a data network is not performing, the possibility of a split pair should always be considered.