Putting Network Cabling and Connections to the Test

Last month, we took a look at some of the key tips, products and techniques for terminating network connectors. We became familiar with the EIA/TIA connectivity standards such as T568A/B. This month, we are going to dig a little deeper into some of the testing and performance characteristics of network cabling and connections.

This is a very important and often misunderstood area for technicians, especially those who are still converting from an analog to digital world. As we all know, almost every area of security now has products that connect digitally to computer networks.

In the past, using basic DC theory such as finding an open or shorted connection could solve many alarm circuit problems. At worst, a technician might have to calculate out the resistance and load of an alarm circuit to decide if a sensing device could be properly powered.

I remember recently hearing the common story of how a technician applied these same DC troubleshooting skills in order to find out why a new RJ-45 plug-in connector was not working. The Cat-5 cables were connected pin for pin and all the wires tested for continuity. The devices had been tested in the shop before installation and worked. All the RJ-45 connector pins and cable wires checked out OK for continuity. But when everything was connected in the field, the equipment did not perform properly.

What was the problem? Stay tuned.

See if Connectors Are Wired Right
I thought the best way for us to look at some of these important features of local area network (LAN) testing would be through the eyes of a popular, yet economical, testing device called the LanScaper™ from Test-Um Inc. This Camarillo, Calif.-based company has been around for some time and is a popular designer and manufacturer of Test, Talk and Trace products. I specifically picked this device because of the features it had for the price.

Upon powering up, the tester will do a “Jack ID” test to identify the characteristics of any equipment that might be connected to the jack and cable plugged into it. In this case, an RJ-11 connector and cable are connected and indicate “POTS – Pin 4 & 5, 52V, 49mA.” What does that tell us? If you guessed a phone connection, you are right. Nothing directly to do with networks, but I thought it was a pretty interesting feature. The tester will also report power over Ethernet (PoE) voltage.

If no voltages are present on the connector, the device will start testing with pulses looking for an Ethernet link connection. At this point, the tester will decide to go to the cable or network testing modes.

What are the typical tests for the cable test mode? First, were the connectors wired properly? Tests will be made to make sure there are no opens, shorts, miswires or split-pairs. Open or shorted connections will be indicated by an “X” next to the jack pin number on the LanScaper’s LCD backlight display. Miswires are wires not being connected to the correct pin and will have a flashing “X” next to them as well.

Split-Pairs Can Create Cross Talk
The last, and least understood, cable fault category is split-pairs. First, we have to go back and review last month’s “Tech Talk.” We discussed the network wiring standard EIA/TIA 568A/B. This standard had a specific way to wire each pair of twisted wires to specific pin numbers in an RJ-45 connector. The 568A pairs are 1 & 2, 3 & 6, 4 & 5 and 7 & 8.

Unlike a miswire where the pin numbers do not match on each end of the cable, in a split-pair error you have continuity with pin numbers, but wire pairs have one wire mixed up with another pair. See the diagram above and the mix-up of pins 2 & 3. The tester comes with a remote module that can be connected to the other end of the cable to accurately pinpoint these cabling faults.

Wiring errors such as a split-pair can create conditions like cross talk, also known as near end cross talk (NEXT). (Note: Don’t forget our previous discussions about keeping the twisted pair very tight and close to the pins on the RJ-45 connectors.) Noise is coupled from one unshielded twisted pair (UTP) to another.

Remember from our earlier “Tech Talk” discussion, UTP is designed so that noise on one wire in the pair cancels out the noise on the other pair due to the tight uniform twists. As networks progress into the gigahertz range, coupling and noise issues become an even greater challenge.

Testing equipment needs to check for things like powersum NEXT when the signals on one pair are compared to signals on the other three pairs; and ELFEXT, FEXT (far end cross talk), when one pair is coupled with another pair and measured at the other end of the cable run.

Capacitance Key to Cable Length

Distance is another test for cabling. The length of the cable is measured by calculating its overall capacitance and dividing by a known capacitanceper- foot value. This value can be changed depending on the cable being measured.

It is important to know the total cable length between jacks, which should not exceed 90 meters (295 feet). Service loops and coils in cable runs can sometimes make this distance longer. Yes, you may have heard 100 meters as the rule of thumb, but you have to save 10 meters for connecting equipment to the wall jacks.

The capacitance type of cable length measurement is often confused and even misrepresented by some testing equipment manufacturers with a more sophisticated time domain reflectometer (TDR) process. The TDR measurement process actually sends a signal down a cable and measures the time it takes to reflect the signal back from a point of termination or damage. A graph of lengths to termination points can be displayed.

Testing Network Communications

The next important tester mode is the network mode. When the tester is turned on, it will search for a network connection to a hub, switch or network interface card (NIC). When found, a flashing link LED will be noticed along with an LCD indication such as “100 Base Tx Enet, Auto Rev Full d/x.” This tells us we have a 100Mbps data rate with auto reverse connectivity capability (we do not need a crossover cable per last month’s discussion of T568B).

A ping test can be performed in the dynamic host configuration protocol

(DHCP) IP address request mode. This is similar to your computer’s ping command. Once the MyIP address is provided, it will allow for other tests. The tester will start sending Internet control message protocol (ICMP) echo request (ping) packets to the router. The technician can read, transmit and receive performance of these packets by their transmit/receive consistency and times.

Finally, a validation link test can be performed with the use of two Lan- Scaper testers. One tester, called the Validator, will be locked in NIC mode, while the other tester on the cable end will be put in the ping generator mode with the DHCP off. Then, address resolution protocol (ARP) packets are sent by the ping mode. These packets are slightly modified, so when they are received by the Validator tester a “Val” indication confirms the cable is properly handling network communications.

Another nice feature about this tester is audio tone generation programmability over any selected pairs or pins. It provides a nominal 10V peak-to-peak tone across the pair in up to four tones. This can be traced with a regular tone tester.

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