Monday, March 31, 2008

Handheld Cable Checkers

A cable-checker device is usually a small battery-operated unit that is used to check STP or UTP cables. This simple test is usually done when cables are first installed as a quick check to be sure that the process of pulling the cables through the ceiling or walls has not damaged them.

If the cable is already attached to a network device, you have to disconnect it and attach it to the unit. A cable checker operates by placing a voltage on a wire and determining whether it can be detected at the opposite end. This can be used to determine whether the cable has a break anywhere along its path and whether you are looking at the same cable on both ends when several cables are traversing a single path. Most cable checkers consist of two components, which you attach to opposite ends of the cable.

Cable Testers

A cable tester is a small step up from the basic checker. This device can be used to measure NEXT, attenuation, impedance, and noise on a line. Some cable testers even perform length measurements, of both the total cable and the distance to a fault on the cable, such as a kink in the wire that is causing reflections of the signal to radiate back to the transmitting side of the cable. Another function you might see is wire-mapping, which checks to be sure that the correct wire-pairs in a cable have been mapped to the correct pins on the connector attached to the end of the cable. In cables used for 10BASE-T networks, for example, the standard specifies specific pairs of wires in the cable that must be used for transmitting and receiving data. The actual decisions about which pins are chosen for a particular connector are not made arbitrarily. If the wires are not correctly mapped to the pin-out on the connector specified by the standard, the cable might generate errors due to noise or cross-talk.

Internet 2010

Small handheld instruments like these usually have LED lights that indicate a pass or fail condition for the test you are performing. They do not require a keyboard or monitor to display data. Some have a small screen that displays limited text, sometimes showing the suspected type of error that has caused a fail condition. Most are battery powered and can use an AC adapter, which makes them useful portable instruments for installing or troubleshooting cabling.

When you begin to go up the price ladder for these types of instruments, you will find some that can perform more advanced monitoring functions, such as showing network use and Ethernet collisions. Another useful feature to look for if you can afford it is the capability to log data to a memory buffer for later review. Some cable testers are even capable of connecting to a PC or printer to produce awritten report. This allows you to leave the device connected for a while to monitor a line.

Depending on the capabilities of the particular device, you can expect to pay from several hundred dollars up to a thousand or more for a good cable tester. When evaluating products, be sure to compare features. Price doesn't always reflect the quality of a device. And you should carefully check the literature and documentation that is available for each device when making a purchasing choice. Although some features, such as the capability to produce a written report, might sound great, do you really need that capability? In a large network, probably so; in a small one, probably not.

Bit Error Rate Testers (BERT)

Data travels through the wire (or the fiber) as a series of signals that indicate a single bit, representing either zero or one. The statistic called bit error rate (BER) is calculated as a percentage of bits that have errors when compared to the total number of bits sampled:

BER = number of bit errors during sampling interval / total number of bits transmitted

Whereas LAN analyzers operate on data captured from the wire in units of frames (depending on the LAN protocol, such as Ethernet or Token-Ring), a bit error rate tester (BERT) performs a more basic function to determine whether the line is capable of carrying the network signaling at the bit level with a minimum of errors.

This kind of instrument is normally used when installing a connection to a network service provider, and it might be used to demonstrate the quality of service that the provider establishes for your link.

The instrument used to perform this kind of error detection usually does so by generating a specific bit pattern on the line and then checking it at another location to compare the generated signal with that which is received. A pseudorandom binary sequence (PRBS) of bits is produced by the instrument. It is pseudorandom because it simulates random data. However, because the pattern is also known by the receiving connection so that it can make the comparison, it's not truly random, but instead is a predefined pattern. Other tests include sequences of specific bits, either zeros or ones, for extended periods, or specific user-defined bit patterns.

When you have a line that exhibits a high bit-error rate, using a slower transmission speed usually improves performance. This is because when you lower the number of errors that occur, higher-level protocols do not have to resend packets as often to compensate. Although one bit error in a frame usually is easily recovered by a network protocol using an error correction code (ECC) technique, multiple bit errors might be all that it takes to cause an entire frame of several hundred thousand bits to be re-sent.

No comments:

Internet Blogosphere