Twisted pair cables

Twisted pair wiring

Twisted pair cable consists of a pair of insulated wires twisted together. It is a cable type used in telecommunication for very long time. Cable twisting helps to reduce noise pickup from outside sources and crosstalk on multi-pair cables.

Twisted pair cable is good for transferring balanced differential signals. The practice of transmitting signals differentially dates back to the early days of telegraph and radio. The advantages of improved signal-to-noise ratio, crosstalk, and ground bounce that balanced signal transmission bring are particularly valuable in wide bandwidth and high fidelity systems. By transmitting signals along with a 180 degree out-of-phase complement, emissions and ground currents are theoretically canceled. This eases the requirements on the ground and shield compared to single ended transmission and results in improved EMI performance.

The most commonly used form of twisted pair is unshielded twisted pair (UTP). It is just two insulated wires twisted together. any data communication cables and normal telephone cables are this type. Shielded twisted pair(STP) differs from UTP in that it has a foil jacket that helps prevent crosstalk and noise from outside source. In data communications there is a cable type called FTP (foil shielded pairs) which consists of four twisted pair inside one common shield (made of aluminium foil).

When cable twisted at constant twist rate over the lenght of the cable, a cable with wel defined characteristic impedance is formed. Characteristic impedance of twisted pair is determined by the size and spacing of the conductors and the type of dielectric used between them. Balanced pair, or twin lines, have a Zo which depends on the ratio of the wire spacing to wire diameter and the foregoing remarks still apply. For practical lines, Zo at high frequencies is very nearly, but not exactly, a pure resistance. Because the impedance of a cable is actually a function of the spacing of the conductors, so separating the conductors significantly changes the cable impedance at that point.

When many twisted pairs are put together to form a multi-pair calbe, individual conductors are twisted into pairs with varying twists to minimize crosstalk. Specified color combinations to provide pair identification.

The most commonly used twisted pair cable impedance is 100 ohms. It is widely used for data communications and telecommunications applications in structured cabling systems. In most twisted pair cable applications the cable impedance is between 100 ohms and 150 ohms. When a cable has a long distance between the conductors, higher impedances are possible. Typical wire conductor sizes for cables used in telecommunications 26, 24, 22 or 19 AWG.

Here are some common impedances related to twisted pair lines:

Shielded Twisted Pair Cable is used to eliminate inductive and capacitive coupling. Twisting cancels out inductive coupling, while the shield eliminates capacitive coupling. Most applications for this cable are between equipment, racks and buildings. Shielding adds usually some attenuation to the cable (compared to unshielded), but usually not because in the case of balanced transmission, the complementing signals will effectively cancel out any shield currents, so shield current losses are negligible.

The noise pickup characteritics of twisted pair cable is determined by the following cable characteristics: number of twists per meter (generally more twists per meter gives better performance), uniform cable construction, capacitance balance (less capacitance difference to groud, the better), cable diameter (less are between wires is better) and the amount of shielding (more shielding, the better).

Structured wiring systems

Telecommunication cabling is a wide topic. Most generally when we talk about telecommunication cabling, we are talking about twisted pair cabling used to carry telephone and other telecommunication signals inside building and in the cabling otn the telephone company outside plant. In some applications some other cable types (twisted pair, coaxial cable, fiber) are used, but twisted pair is the most commonly used cable type.

The customer premises cabling can be divided to the following parts:

The standard connector for whe wiring described above is 8 contact RJ-45 modular connector (specified in IEC 60603-7-4/5).

When talking about telecommunion cabling, you can usually see term "structured cabling system". The term structured cabling system refers to all of the cabling and components installed in a logical, hierarchical way. It's designed to be relatively independent of the computer (or telephone) network which uses it, so that either can be updated with a minimum of rework to the cable plant. Until a few years ago, each different data communications technology required its own type of wiring. Now, a single wiring technology (structured cabling system) will support all the major existing data networking technologies and those which are appearing on the horizon.

Benefits of structured UTP cabling include:

The disadvantages of structured UTP cabling:

The T568-A standard published by the Electronic Industry Association and Telecommunications Industry Association defines a system for building a data and voice communications network in an office environment that will have a lifespan of at least ten years and support networking products made by multiple vendors. The most commonly used structured cabling system uses unshielded twisted pair cabling wired according EIA/TIA-568A standard. In this kind of wiring the rooms are wired in star-topology from the central wiring room. The most commonly used cable type nowadays is CAT 5 unsielded twister pair cable terminated to RJ-45 (ISO 8877 / IEC 60603-7 8-position modular connectors) connectors (four twisted pairs per cable). This kind of cabling can be used to carry telephone signals (both analogue and digital) as well as data communication needs (Ethernet and my other networking techniques). EIA/TIA-568A cabling standard is for USA markets. EIA/TIA-568A standard gives two options for cable color coding TIA568A and TIA568B. EIA/TIA 568A and 568B are two wiring methods used to indicate which colors are assigned to which pin of the modular jack. From those color coding the most commonly used one is TIA568B (I recommend using this). The T568-A standard standard also defines the categories used to grade UTP cable. These categories have become the industry standard for UTP cable performance and are widely used by many manufacturers. Europe has it's own (quite similar in main details) cabling standard EN 50173.

Computing and data communications are fast-moving technologies where equipment often has a practical lifetime of a few years, at most, before it is overtaken by something newer and better. In a few year's time, the unified wiring technology that is now recommended may be inadequate. However, modern networking technologies are being developed around the use of twisted-pair wiring so, although none of us can see into the future reliably, the best advice that can be given now is that new building wiring should be category 5 UTP or better (CAT6). Wiring using structured cabling becomes (CAT 5 UTP) relevant in homes.

Here are some common cable types you might encounter in telecommunication installstions:

For twisted pair data applications there are three possible nominal impedance levels: 100 ohms, 120 ohms and 150 ohms. 100 ohm cables are having the predominant market share and therefore the development od hardware and software is mainly focusing on 100 ohm systems. If you do not have any specific reasons to choose otherwise, I recommend 100 to use 100 ohms cable for data installation. The typical cable copper wire thickness used here is 24 AWG. The most common jacketed cable consists of four tightly twisted pairs of #24 gauge insulated copper conductors. It is extensively used in commercial applications and is finding its way into new homes to meet rising consumer demand.

Typical UTP cable has four pairs of wires in each cable. Not all four pairs are used in actual applications. For most LANs, only two pairs are used, one in each direction to allow full duplex, simultaneous bidirectional communications. Due to the limitation on bandwidth and emission of radiation that could potentially affect other electronic devices, the higher speed networks are migrating toward using all four pairs.

Cable categories

The "quality" of the cabling systems to carry high frequency signals is expressed with the folloging marking (those are in use in USA):

Generally the specification for different groups is determined by Attenuation/Cross Talk Ratio, the gap between attenuation and NEXT. Practically a minimum gap of 10 dB is required for a data signal to be readable. CAT-3 UTP cable rated at 16 MHz with 10 dB of headroom at 16 MHz. CAT-4 UTP cable rated at 20 MHz with 10 dB of headroom at 20 MHz. CAT-5 UTP cable rated at 100 MHz with 10 dB of headroom at 100 MHz.

For installation to meet specific Category requirements all components must meet or exceed the designated Category. Using a Cat 3 receptacle (or patch cord) on Cat 6 reduces performance to Cat 3.

Some naming for differetn cable types:

Recommendations: Installing cable with less performance (including Category 5 and 3) than Category 5e cable risks costly re-wiring in the near future. Cat 5 is now considered obsolete except for maybe household use. Cat 5E or Cat 6 is the standard now. Telephone, data, computer network and video cabling should be "home runs" from each phone, workstation, TV, etc. to a central location typically near the incoming service of the telephone company and cable provider. Two Category 5e home run cables (each with 4 pairs) are recommended for every wall opening. One UTP cable is for computer network and the other UTP cable is for telephone, modem and fax.

Generally it is a good idea to install an UTP cable system, unless you have a very good reason why you shoudl use STP cable system. STP cabling systems are more expensive and harder to install and maintain than UTP cabling systems, but are not necessarily any better in normal home / office environment.

When installing cable, remeber that there are different cable types. In-wall wiring is designed to be done with solid core cable (usually CMR cable). This is the rightr cable type to use. Stranded wire patch cables are often specified for cable segments running from a wall jack to a PC and for patch panels. They are more flexible than solid core wire. If you hard used the solid core cable for this, the constant flexing of patch cables may wear-out solid core cable. Another reason is that solid core cable does not terminate reliably to a normal RJ-45 connector used in patch cables (solid core terminates very niceky only to RJ-45 wall plugs and patch panel connectors). Stranded cable does have also it's weaknesses. Stranded cable is susceptible to degradation from moisture infiltration, may use an alternate color code, and should not be used for long cables because of usually poorer specifications than same category solid core cable designed for in-wall wiring. TIA/EIA 568A specification specify a one network link to have up to 90 meters of in-wall wiring (thicker solid core cable) and in addition to this up to 10 meters of patch cable (thinner stranded wire).

European cable classes

European cabling standards use different naming and classes. The permanent links and channels are classified as follows:

Cabling and EMC

Cabling systems performance must also meet EMC standards. Above 30 MHz, the European Norms (EN 50081-1/EN 55022) specify limits for the allowable free space emission. This means that whenever the LAN-system bandwidth exceeds 30 MHz, special care has to be taken to ensure that the EMC requirements are met.

In the data world, a lot of work has been done to keep actual running frequencies low. Below 30 MHz, the FCC has no radiation specifications and European specifications are quite ree here. Therefore, one of the key drivers to technology has been to get systems at or near that number to avoid the requirements of the FCC and European standard..

In order to reduce emission significantly, the use of shielded cabling proves to be far the most straight forward solution. But balanced transmission or balanced transmission plus shielding are two different means to achieve good EMC performance. With a well balanced system, will shielding really be necessary? Will STP always give better EMC performance than UTP? The answer is no clear and depends on application. Both cable types have threir good and bad properties. If the cable link and the transmit/receive circuits of the communications equipment were perfectly balanced, we would need no shielding at all. In differential mode both UTP and STP perform in the same way (STP only usually has somewhat higher attenuation and/or is harder to handle because is usually thicker). In the common mode, the UTP link acts as a long antenna, while the FTP or S-FTP cable behave more or less as a coaxial cable (=is well known that the radiation is low and susceptibility in the immunity case is good). Experiments with STP cable have demonstrated the necessity of maintaining continuity of the shield to get good performance. And well grounded STP can perform very well in very varying enviroments (for example STP is advantageous in cases when cable is surrounded with lots of metal objects).

A well-designed and properly installed FTP cabling network undoubtedly offers improved signal integrity over an unshielded system. However, in order to work effectively, the shielding must be properly earthed. In practice this is not always easy to achieve. A badly earthed, shielded system will actually offer poorer performance levels than one which is not shielded in the first place. In the worst case, the shield itself can become a radiating source, producing current earth loops, or capturing ambient emissions and radiating harmful interference right next to the data cable.

When STP is grounded at the both ends (for normal operation needed for good performance), the cable shield can start carry grounding current which affect the cable performance. If a ground potential differences of less than 1 V r.m.s. cannot be met, this can cause problems for STP installation. In those special cases, the wiring system needs to use non-continuous grounding where continuity (in high frequency terms) can still be achieved by using equipment cables with discontinous, but capacitively coupled shields.

When installing communication cabling, the communication cabling should be installed enough far away from power cabling to avoid interference pickup from power cables to LAN wiring. Power cables and LAN wiring should never be installed inside some electrical wiring pipe for both safety and interference reasons. When installing cables, the communication cables should be kept at least 15 cm away from 230V AC lines and 20 cm away from 400V AC lines (those recommendations are taken from industrial cabling recommendations and are good starting point for residential installations also).

Flexible stranded core cable and solid core cables

No matter if the cable is shielded or not, there are two types of cables: flexible stranded core cable and solid core cable. Solid core cable is usually less expensive and is used for inside wall wiring. Solid core cable is easy to work with on inside-wall wiring (easy to terminate well to sockets) and usually has better performance characteristics than the flixible cables. Solid core cable is a poor choose for patch cords since it is not very flexible and difficult to terminate correctly to RJ-45 plug. You can try to make patch cords from solid core cable, but you will soon find out that they are very unreliable, because standard RJ-45 plugs do not connect to solid core cable reliably. Al the patch cables should be built from flexible stranded cable.

Installing tips

When installing cables, be careful when handling the cable. There are many things that can cause Cat 5 or higher specification cable not to meet spec including bending, stress, and etc.

It is relatively easy to install cables around the house when the house is being built. In a timber frame house the cables are installed before the internal wallboards are installed. In a brick and block house trunking or ducting should be installed/laid in the floor screed or walls. The use of trunking or ducting is also recommend for a timber frame house, because when those are installes, the wiring can be updated quite easily if necessary. When installign cables, it is a good way to label the cables as soon as you pull them to the place. A large number of cables without marking will be a large mess that is hard to work out later which cable is going where. CAT 5 wire is available in reel-in-box packaging. This is very handy for pulling the wire without putting twists in it. Without this kind of package or a cable reel stand, pulling wire is a two-person job.

The fire safety codes needs to be taken into account when installing wiring. Typically the cables must meet the international fire safety standards (for example IEC 332-1) and the local building codes. When installing cable in an office building, fire codes often dictate that cables running through the air spaces in the building's walls (called plenums) must have an outer sheath made from a material that does not emit toxic gases when it burns. The PVC sheathing used on most cables does emit these gases, so there is a separate grade of cable, called plenum cables, that have sheathing made of a safer material. Plenum cables are less flexible and more expensive, but if local codes require it, use them. There are several fire code classifications for the outer insulation of CAT 5 cable (CMR cable, "riser cable", plenum cable etc.). You may be required by local, state or national codes to use the more expensive plenum-jacketed cable if it runs through suspended ceilings, ducts, or other areas, if they are used to circulate air or act as an air passage from one room to another. If in doubt, use plenum. CMR cable is generally acceptable for all applications not requiring plenum cable.

Nowadays you can wire a building with nothing but Category 5 UTP and do almost anything one might want. You can theoretically even run more than one service over one four pair cable in some applications. However, there are no standards within TIA/EIA 568A currently for "sharedsheath" non-data applications. Installers should be aware that in TIA/EIA 568A, while it does not specifically disallow multiple uses for any cable, if such multiple uses are installed, that cable will not be certifiable as being 568A compliant. This does not mean that the cable next to it in the bundle is or isn't compliant. So the best idea is to designate which cables will be 568A compliant and to install them to that standard. The shared-sheath, non-standard cables, can be wired as needed. It is also possible to install all cables to be 568A compliant, and to make the non-standard applications appear outside each wall jack. In that way, all wiring could, at any time, be part of a 568A compliant network and yet, at the change of a connector, be a video feed, telephone, T-1 line, RGB cable, or many other uses.

There are standards and recommendations how ther grounding of telecom wring systems should be grounded. This topic is covered in telecom standards, cabling systems specifications and electrical safety codes. Some standards / proposed standards related to structured cabling systems grounding are TIA/EIA-607, PrEN50303 and PrEN50174-2.

Nowadays by installing twisted pair wiring to all rooms in the home or office, you can virtually provide any type of service required to each room. Those services include 10/100BaseT data feeds for computer networking, Internet access, printer-sharing, fax machines, telephony feeds for phones, conferencing, intercoms, audio feeds and video feeds. This home networking setup is of a simple design. By terminating these different media into a sophisticated multiplexing device which can split audio/video and voice/data input signals without degradation, a home owner will have the ability to branch these signals across the twisted pair wire to any location in the home.

Radio audio (550kHz - 108MHz), TV broadcasts, cable TV and satellite TV signals are traditionally distributed using coaxial wiring. With good enough twisted pair wiring you can even use the same wiring also for most of the services traditionally done with coaxial antenna cabling.

Home networking will allow central location of satellite and cable receivers, AM/FM antennas, computer resources, etc., which plug into these multiplexers. Then by simply making patch cord changes to a small rack, located in a closet, the homeowner can control what piece of information is fed to each room.

Standard telecom jack wirings

There are many wiring standard, some more common than other. There is a difference between telephone wiring and LAN wiring. The most differences are the way pairs are allocated in the cables and the cable type used (data applications generally demand higher spec cable).

Telephone Wiring in USA is defined by USOC (Universal Service Order Code - now handled and maintained by the TIA). Silver satin cable is often used for telephone wiring and does not support the more stringent requirement (higher speeds) used in LAN cables.

LAN Wiring is defined by EIA/TIA. The most important wiring standard in this is EIA/TIA 568 wiring standard. EIA/TIA 568A and 568B are two wiring methods used to indicate which colors are assigned to which pin of the modular jack

Many Rj-45 connectors with wiring markings on them have two wire color possibilities printed to the connector, usually referred as "A" and "B". What is meant by the "A" and "B" markings? The "A" Wiring Scheme is the new wiring for telephone companies and are found in all new residential and commercial wiring applications. The "B" Wiring Scheme was used by AT&T for commercial wiring applications when they were "the only game in town" wiring new buildings. Those both wiring color schemes are listed in the EIA/TIA 568 wiring standard. When doing the wiring it basically does not matter which one of those you use, as long as you do the whole wiring in the same way. There is no performance difference, just different wire colors used for different pins, but the wiring from connector to connector is electrically same.

Pin designations of plugs

Male Modular connectors are numbered LEFT to RIGHT when viewed from the TOP (TOP is when the plastic lever is on the bottom). Female connectors are numbered from LEFT to RIGHT when viewed from the FRONT.

Common wiring practices

There are many common wiring practices in telecom and LAN world. EIA/TIA 568A and 568B are two wiring methods used to indicate which colors are assigned to which pin of the modular jack. Those are the standard of modern "stuctured cabling" and "LAN" world.

In telephone installetion the coloring practice has traditionally been the following: "Ring" is normally assigned to the RED wire, "Tip" is normally assigned to the GREEN wire.

The normal modular phone cord has one end flipped over from the other, so using a four conductor flat cord, black on one end is on the left, and right on the other end. For some other applications cable very likely does not have that flipped end, but uses the straight thru connection (for example datacom application use straight thru connection).

Safety

There are special safety considerations with telephone wiring that may be unknown by workers new to this field. The following hints and guidelines should be followed closely to help avoid safety hazards, and ensure trouble-free installations and high-quality telephone service:

The tips above gnerally apply to any telecommunications network installing.

Installing cable

Nowadays UPT systems are generally installed with CAT5 or better cable. CAT5 wiring will have certain characteristics including:

CAT5 typically comes in 1,000' spools.

Copper wire resistance table for cable thicknesses used in typical telecommunication / LAN wiring: 

AWG   Feet/Ohm  Ohms/100ft  Ampacity*   mm^2   Meters/Ohm  Ohms/100M  diameter/mm
 22     30.3       3.30        2.1     0.644     9.24       10.8        0.64
 24     19.1       5.24        1.3     0.511     5.82       17.2        0.51
 26     12.0       8.32        0.8     0.405     3.66       27.3        0.41
 28     7.55       13.2        0.5     0.321     2.30       43.4        0.33
These Ohms / Distance figures are for a round trip circuit. Specifications are for copper wire at 77 degrees Fahrenheit or 25 degrees Celsius. The most commonly used wire in structured cables inside walls is 24 AWG. Patch cables generally use 26 AWG wire.

Typical transmission characteristics of CAT5 wiring:

Attenuation an crosstalks on CAT5 channel: (A Channel may consist of up to 90 meters of horizontal cable, one or two transition connectors on each end of the horizontal cable, and up to 10 meters of user patch cables for a total maximum length of 100 meters.) 
Frequency   Attenuation     NEXT
 (MHz)        (dB)          (dB)
  1.0         2.5           60.0
  4.0         4.0           50.6
  8.0         6.3           45.6
 10.0         7.0           44.0
 16.0         9.2           40.6
 20.0        10.3           39.0
 25.0        11.4           37.4
 31.25       12.8           35.7
 62.5        18.5           30.6
100.0        24.0           27.1
The limits above are derived from TIA TSB-67 Cat 5 Channel Specifications. (Data from http://www.chem.ox.ac.uk/it/staff/Network%20Documentation/standards.html. The cable iself gives you much better performance, but the performance of of a channel is reduced by crosstalk and attenuation on connectors and patch cables. For 100 meters if a good solid core CAT5 cable (cable only) the specications at 100 MHz frequency is impedance 100+-15 ohms, attenuation 22 dB, NEXT 32.3 dB and return loss 16 dB.

Typically the transmission delays vary somewhat between different pairs (I have seen somewhere figure of easily at least 1% difference on CAT5 cable).

Then installing the wiring keep in mind that CAT5 UTP wire is more fragile than coax cable. Per specifications, CAT5 will withstand about 30 lbs. of pull when the wire is being run. This means that the cable should move fairly freely when being pulled through ceilings and walls. Otherwise, the cable will stretch and distort and no longer meet specification when subjected to too much pull.

Specifications also instruct installers to cut back no more than 2 inches of the outer jacket when attaching the line to a punch-down block. If you cut back much more than 2 inches of the cable's jacket to make it "pretty" in their wiring rack, you risk installing the wire out of spec and may experience unpredictable communication problems. This happens for a simple reason, electromagnetism. Our environment is saturated with it. When the wires are all inside the jacket and twisted around each other in pairs, they are affected in the same way, at the same point by these external forces.

For this same reason, CAT5 should not be laid directly on fluorescent lighting or parallel to power cables.

Patch panels

A patch panel is a mounted hardware unit containing an assembly of port locations in a communications or other electronic or electrical system. In a network, a patch panel serves as a sort of static switchboard, using cables to interconnect computers within the area of a local area network (LAN) to the network equipment (hubs, switches) and cabling going to different locations. Typically all telephone connections and data outlets in a modern office are wired to some sort of patch panel, from there they are connected where-ever needed (networkign equipment end incoming telephone lines). A patch panel uses a sort of jumper cable called a patch cord to create each interconnection.

There are many different types of patch panels in use in telecommunication wiring. Most modern patch panels designed for data communication applications use patch panels which have RJ-45 connectors in them. This is convient in data communication wiring applications, where one four pair cable is used for one application.

Patch panels have been used for long time also in telephone wiring applications. In this kind of applications the patch panels are designed such that there is easy access for separate telephone wire pairs and where plain wires (no connector on the end) can be easily terminated. The most common types of patch panels:

Modern patch panels are generally built as modules that mount in EIA standard 19" relay rack cabinet or wall-mount brackets.

Punch-down blocks come in many sizes. They are made up of grooves/slots the individual wires are punched into and look very much like what you will find in telephone closets. Specifications for CAT5 wiring instruct installers to cut back no more than 2 inches of the outer jacket when attaching the line to a punch-down block.

Suitablility of UTP wiring for different special applications

Besides telecom and datacom wiring applications, like telephone lines and LAN applications, typical structured wiring can be used for many non-traditional uses. This is a short description for what CAT5 is suitable an for what not.

Good ideas:

Video signals can be carried over CAT5 UTP wiring with suitable adapter. There are different kind of adapters to send video signals areas where there is no coax in place. Baluns are available for Composite Video plus Stereo Audio, SDI Serial Digital Video, S-VGA and UHF up to 850MHz.

Line level audio can be carried over CAT5 wiring if suitable audio adapters are connected to both ends of cable to convert the typical unbalanced audio (RCA connectors) to balanced signals that can travel nicely over CAT5 wiring. Directly soldering RCA jacks to CAT5 wire does not give any good results (will pick up lots of humming and noise). Balanced signal source and receiver combinations can be directly wired to CAT5 UTP usually with quite good results (you need to think custom special wiring practice for this, there are no standards for this).

Bad ideas:

CAT5 wiring inside wall is not a good choise for speaker wiring (for normal 4-8 ohm speakers). It is too too thin for this. The resistance of long runs will effect sound quality and if you turn the volume up a lot, you will heat the cable with considerable amount of amplifier power.

CAT5 for low voltage lights is a bad idea. They can draw decent amounts of current at low voltages, overheating the wire. Check the manufacturer to see what gauge wire they recommend sine lights draw all sort of current based on their power/intensity.

Do not use CAT5 wiring for smoke detectors, because smoke detectors should be always wired with a special listed fire resistant cable designed for smoke detector control. Most building codes require this.

Should check:

CAT5 for temp sensors and such? It will work, especially if they are the digital type. If you are using analog temp sensors, this is where the length issue will most affect you since you are working with very low votlage readings anyway and losses in small wire (22-24 gauge) is high at low voltage levels.

Wiring testing

Proper testing of wiring system after installation is essential to guarantee good operation later. The cabling system needs to be measured after installation and the results of those measurements should be documented for later use. Many of the problems encountered in UTP cable plants are a result of miswired patch cables, jacks and crossconnects.

Cable testing verifies you've completed an installation in accordance with all of the terms and conditions of the contract and industry standards. When you're testing copper cabling, it's important to understand and follow three distinct phases: visual inspection; test measurements; and documentation.

Potential cable fault types in typical telecom installation include:

The multimeter is probably the most basic and widely used field tester. Available in analog and digital varieties, multimeters measure voltage, current, and resistance in copper wires. By using a shorting device on one end of the pair, you can also test continuity. The most common cable fault is an open circuit, usually due to problems close to or at the ends of the cables. A simple ohm meter test generally suffices. It is possible to calculate the length of the cable by determining the actual resistance of the loop (you need to be able to short the cable from other or have both cable ends near to perform this measurements).

Induction amplifier/tone generator, also known as a toner or cable tracer, enables installers to identify a specific pair by generating a tone on one end of the pair - with an inductive amplifier identifying it at the opposite end. Most units are now a combination of tone generator and continuity tester, commonly known as a wand and toner set. This solution is ideal for cable identification and troubleshooting in field. Application Hints for induction amplifier/tone generator system: When tracing wires terminated to a terminal block such as a "66 block", attaching both generator leads to the cable or pair tends to contain the signal within the cable. The tracer must nearly touch the end of the cable to detect the signal, which is helpful when the wires are close together, as when terminated. When tracing along cable runs and to maximize radiated signal, connect one lead of the generator to the wire or cable, and the other end to ground (case of an electrical box, electrical conduit, metallic water pipe or ground rod). Do not touch to live AC circuits! If no ground is available, do not connect the other lead to anything, let it dangle as near to the earth as possible. Connect the generator to the ungrounded shield of a coax cable. The shield will do it�s job, if connected to the center lead. Inductive amplifier are generally available with two types of probing tips. Nickel plated brass tips offer maximum performance when probing insulated wires or making direct metallic contact. The other type is partially conducting plastic tip specially formulated with a carbon powder and carbon fiber blended plastic, these tips perform much the same as traditional metal tips while reducing the risk of shorting terminals (typical resistance around 300 ohms). The signal generators generally connect to the wiring through modular connector, "alligator" clips (for use on uninsulated wires and 66 style punch-down blocks) or "bed-of-nails" penertrator (for use on insulated wires).

Wire map testers, also known as pair scanners, are low-cost cable testers that usually test for opens, shorts, crossed pairs, and miswires (such as reversed pairs in a 4- or 25-pair cable). Some testers in this category also test for split pairs. These devices are good for quick, basic tests. Good Wiremap tester a checks a cable for open or short circuits, reversed pairs, crossed pairs and split pairs. This is essential tool for checking stuctured datacom wiring.

Installers use certification field testers to verify a cabling system meets the transmission performance requirements as specified in TIA/EIA TSB-67. All variations of these units test a cabling system up to at least 100 MHz. In the autotest mode, they include length, attenuation, wire map, and near-end crosstalk (NEXT) tests. When operating in autotest mode, the field tester compares the actual measured values with required values for Cats. 3, 4, or 5, and displays pass or fail for the entire battery of tests. It also displays pass or fail and the actual tested values for each parameter. Usially this kind of devices can also perform other measurements, including impedance, capacitance, resistance, delay, delay skew, equal level far-end crosstalk (ELFEXT), and attenuation-to-crosstalk ratio (ACR) calculations. In addition to the TIA/EIA TSB-67 standard, they usually include the classes of ISO/IEC and the pass/fail criteria in their database. Certification testers can store test data and export it to a database on PC or output it to a printer.

Certification test units test a UTP and ScTP cabling system to at least 100 MHz, and measure/record the following parameters: wire map length; attenuation; NEXT, as well as return loss; ELFEXT; ACR; propagation delay; delay skew; power sum NEXT; power sum ACR; and power sum ELFEXT.

The TDR locates and tests all cable defects, splices, and connectors and gives loss values for each occurrence. Originally developed for use on coaxial networks, the TDR can measure the electrical length of a cable and is an excellent troubleshooting tool for UTP, ScTP etc. To measure the cable, you inject a high rise-time pulse into the cable, and then look for the reflections caused by impedance mismatches to return. Checking cable length is usually done using a time domain reflectometer (TDR), which transmits a pulse down the cable, and measures the elapsed time until it receives a reflection from the far end of the cable. Each type of cable transmits signals at something less than the speed of light.

A telephone test set is used to simulate the user's telephone equipment, identify circuits, telephone circuit diagnostics and troubleshoot.

The cabling installer must have proper adapters available to connect the test equipment to the cable under test.

Here are some notes for testing differnet media options:

Optical fibre testing

75 ohm coaxial cable testing

100 ohm UTP cable testing for Category 5e cable

Twisted pair alternatives have replaced coaxial cabling on today�s LANs. At the Category 5 performance level or above, there are a bewildering number of options. All standards require that installed links pass three tests: wire map (end-to-end pin-to-pin connectivity), attenuation and near end crosstalk (NEXT).

Necesary cable mesurements are noise, attenuation and NEXT (near-end crosstalk). Cable and connecting hardware installed using poor practices can have their NEXT performance reduced by as much as a whole Category. Split pairs will cause NEXT performance reduced so much that cable becomes completely unusable for data applications. Knowing the attenuation (and NEXT) of a link allows you to determine whether it will function for a particular access method, and how much margin is available to accommodate increased losses due to temperature changes, aging, etc.

In twisted pair wiring testing you can sometimes see terms "Basic Link" and "Channel". The "Basic Link" configuration is intended for use by cable installers and contractors, testing the fixed wiring in a building. A Basic Link may consist of up to 90 meters of horizontal cable, only one transition connector on each end of the horizontal cable, and two Test Equipment Cords with a length of no more than 2 meters each.

A "Channel" configuration is intended for LAN owners and LAN systems designers to test the end-to-end connections of their LAN cabling. A Channel may consist of up to 90 meters of horizontal cable, one or two transition connectors on each end of the horizontal cable, and up to 10 meters of user patch cables for a total maximum length of 100 meters. The user's patch cables are used to make the connection to the networking hardware, such as the HUB to Patch Panel and Workstation to Wall Outlet connections.

Sometimes there is need to identify cables in unknown cabling systems or physical location of them. There are many different tools available for this.

Cable-end locator kit. Sometimes called an office locator kit, this is a set of numbered 8-pin modular plugs, which the cable test equipment can identify. With this option, you insert the plugs into outlets in the work area, then search with the tester until it finds the plug at the opposite end of the cable.

Induction amplifier/tone generator, also known as a toner or cable tracer, enables installers to identify a specific pair by generating a tone on one end of the pair - with an inductive amplifier identifying it at the opposite end. Most units are now a combination of tone generator and continuity tester, commonly known as a wand and toner set. This solution is ideal for cable identification and troubleshooting. The idea is that the tone generator is connected to a wire line. It sends the indication tone to the cable. The Line Tracer is a hand held inductive tracer that will help to identify wires without piercing the insulation. The idea is the inductive tracer will hear the tone sent to the cable (usually indicated with a small speaker and indication LED) when it is near the cable. It can trace Tone Generator signals through dry wall, wood and many other non metal surfaces. More near the tracer gets to the right cable, the stronger the sound gets. So the audible signal will tell the user of the cirrect wire carrying the tone signal and a LED light provides visual indication.

Cable tracing and identification is made easy with a professional tone generator and inductive probe. Simply connect the tone generator to a modular jack or wires. Trace the transmitted signal at the other end of the wires with the inductive or capacitive probe. The signal emitted by the tone generator can be easily traced by the probe even when cables are in a bundle or hidden in a tangle of punchdown blocks or wall plates.

Tone Generator and Inductive Amplifier are often used to trace cable pairs, follow cables hidden in walls or ceiling. The tone generator will typically put a 1 kHz or 2 kHz audio tone on the cable under test, the inductive amp detects and plays this through a built-in speaker. Typically transmitter sends out a pulsating 2 KHz signal down the cable which is heard in the receiver when you�re close to the cable. There are also systems which use different frequencies for testing. Many listenign devices have two detectors in its tip which are tuned to pick up the electromagnetic signals ("short" mode) or electrostatic signals ("open" mode) generated by the other components. . The inductive amplifier can detect this signal without having to cut or damage the cable being tested. The inductive amplifier usually has a range of about 1"-4" from the cable. Typically used to identify or trace cables.

So the whole testing operation operates by injecting a "warbling tone" on one end and "listening" for a strong tone on each wire on the other end�. This kind of devices typically work at around 800-1100 Hz frequency, most typically the systems us 900/1100 Hz "warble" signal (there are also systems that use different frequencies, for example 2 kHz). The main drawback of this system is that if you have cross-coupling between wires (there is always soem capacitive or inductive coupling between the wires because they co-exist in parallel with other wires in a cable!) then you have to "listen" for the strongest signal! Since the Human ear has a logrythmnic response curve to sound, it can be sometimes hard to accurately identify which is the strongest signal source. Same listening ideas with suitable frequency transmitter and receiver are used to locate the location of cables, pipes etc. inside wall and in land (the signal is in this case usually sent between ground and the cable wires, signal is picked up then capacitively or inductively depending on the application).

There is another way to trace the right pair: Feed the signal to the pair you want to trace between the wires in the pair. Take the probe to the distribution device or telecommunications closet. When the probe is held near the pair carrying tone, the tone will be quite strong. (Some lesser volume may be picked up on adjacent leads or pairs, but this is normal.) The pair carrying tone can be verified by touching the tip of the probe across the leads. The tones will cease if that pair is the one carrying the tone.

Home networking

Modern homes are nowadays filled with lots of electronic gadgetry. Currently there are more than 15 million homes with multiple PCs in USA alone. Computer networking, whole-house audio, home theater, video distribution, home controls and security systems are becoming increasingly commonplace. All of this electronic gadgetry requires proper wiring if the devices are to function in harmony. With homenetworking, their households receive the benefits of simultaneous, shared Internet access, printer/peripheral sharing, file and application sharing, and networked gaming. In addition, consumers can enjoy the use of each of these home entertainment and information services using existing wiring in the home.

It is estimated that about 15-20% of new homes in USA are now (2002) being fitted with what the industry calls "structured" wiring, and perhaps 42% of new homes will be so equipped by 2004. This phenomenon isn't limited to new homes. It is surprisingly affordable to retrofit structured wiring systems into existing structures, thereby increasing their functionality as well as their value. A proper communications wiring system allows the home's electronic devices to work together. Wiring using structured cabling becomes (CAT 5 UTP) relevant in homes. It is extensively used in commercial applications and is finding its way into new homes to meet rising consumer demand.

To build a home network, several conditions must be met. First, a home must be wired properly for a computer network. All cables are home runs back to a central location (star topology). And each room has sufficient outlets to support the desired applications and equipment. Next, each computer or peripheral to be connected to the network must have an Ethernet card, called an adapter, or a Network Interface Card (NIC). These cards connect to a computer or peripherals' bus (circuit board) and convert data into electrical signals for transmission through the network. Many computer products ship with such cards preinstalled. An Ethernet hub or switch is also required.

Ethernet switch or hub is at the center of the network, the interchange point that permits data to flow among devices. The front of the hub or switch contains a number of RJ-45 connections, linked by an internal printed-circuit board electronics. Through the hub and its connected cabling, any computer on the network can talk to any other computer, compatible peripheral, or Internet service provider (typically through cable modem or ADSL router with Ethernet interface). With a hub the data sent by one computer is sent to all other computers in the network and they use it if they need. An Ethernet switch is a more advanced device that sends each part of data to only those devices that they are ment for, thus increasing the network speed and security in networks where there are many users in the network.

There has been standardizing efforts on home wiring. TIA has published a standard EIA/TIA 570 "Residential Telecommunications cabling standard". This standard addresses the wiring for residential premises. It applies to telecommunications premises wiring systems installed within an individual building with residential (single family or multioccupant) end users. The standard was approved September 1, 1999 to standardize requirements for residential telecommunications cabling. The Cabling infrastructure specifications within this Standard are intended to include support for security, audio, television, sensors, alarms and intercom. This Standard is intended to be implemented for new construction, additions, and remodelled single and multi-tenant buildings.

TIA-570-A is a comprehensive standard that provides expertly conceived models, recommendations and considerations, not only for cabling media types, but demarcation, topology, pathways, separation from sources of EMI, space requirements, distribution devices (DD), auxiliary disconnect outlets (ADO), equipment and patch cords, single or multi-tenant property layouts, link and channel performance criteria and field test requirements, among others.

TIA-570-A establishes minimum "Grades" of Residential Cabling to serve today�s requirements and, more importantly, the more stringent demands of the future:

A minimum of one outlet to each: Kitchen, Bedroom, Family/great room and Den/study room. Six or Eight-position modular outlet/connector must comply with ANSI/TIA/EIA-568-A.

Recognised cables for telecommunication in EIA/TIA 570 are:

The whole system is generally wired using "star" wiring approach. Coaxial cables and components are used for CATV distribution (5 -1000 MHz).

Devices such as intercom, security system keypads, sensors, and smoke detectors May be wired in a star, loop, or daisy chain. Per equipment manufacturer�s instructions and Can be hard wired

Video and audio over twisted pair wiring

Normally video transmission is done using 75 ohm coaxial cables, but unshielded twisted pair or UTP cable is a very inexpensive interconnection when compared to a coaxial cable connection. Modern unshielded twisted pair wiring (CAT5 or better) can be used to transport video signals when this is done in the right way.

The secret to sending signals over UTP is to balance them well in order to limit both radiation and noise pick up. This kind of unshielded twisted pair wiring method is used in many CCTV applications nowadays to use existing in-house twisted pair wiring instead of installing new coaxial cable for the CCTV camera. For at least 20 years products have been available capable of transmitting video using UTP wire.

When wiring A/V signals to unshielded twisted pair wiring (RJ-45 comnnectors), two of the eight conductors found in Cat 3 or Cat 5 cables are used for each signal. Line noise, cross talk and attenuation are low in this way. This means that one eight conductor (4 pairs) cable can be used to carry up to four different signals. Extra, unused conductors can even be used for carrying power to the device or other signals.

Unshielded twisted pair or UTP cable is a very inexpensive interconnection when compared to a coaxial cable connection. The attenuation characteristics of the UTP are an order above those of coaxial cable. Here is some data on loss in dB per 100ft (~30m) of common cable types: 

Freq.(MHz)  RG59  RG6  CAT-5 
    1       0.4   0.2   1.8 
   10       1.4   0.6   5.8 
   50       3.3   1.4  11.0 
  100       4.9   2.0  19.3 
  200       7.3   2.8  29.3 
  400      11.2   4.3  42.0
The data presented on table above was taken from http://www.intersil.com/design/elantec/DataTransmissionOverUTPCable.asp.

Video signal can be adapted to UTP wiring using a special balun transformer between BNC video connector and the wisted pair wiring. This converter converts the unbalanced audio signal to balanced signa which nicely travels through the cable. A similar transformer can be used on the other end of the cable to convert the video signal back to unbalanced format which fits to BNC connector. There are both passive solutions (balun transformers) and active converters available on the market for this application. Generally this kind of adapters can be used for both NTSC and PAL video signals. Simple UTP baluns just convert an unbalanced coax signal to a balanced signal for use on the twisted pair. Some companies add gain with active converters to extend the distance. There are difference how well different products perfom. Some products provide inadequate noise immunity, some perform very well. Some products provide also ground loop isolation and surge protection.

You can also find special adapters for S-video and VGA signals. S-video adapter included two baluns in one case to transport both S-video signals (Y and C) through separate wire pairs. VGA baluns generally use four pairs to transmit RGB signals and sync signal.

It is interesting to note that one of the major problems with baluns and analog video is not the high-frequency limit but the low frequency limit. Very low frequencies are difficult to pass through a transformer and other similar devices. Higher frequencies are much easier to pass. If you wish to use UTP for analog video, be sure you get performance data on the baluns that show the entire operating range. Broadcast-quality video requires performance all the way down to DC. Since a traditional transformer cannot pass DC, other methods are used to design these baluns.

Balance is also a critical parameter. The nature of a balanced line means that the two conductors in the twisted pair are identical (identical length, identical size). The more identical they are, and the closer together they are, the easier it is for the balun to reject noise and interference generated outside the pair. The less identical the two conductors, (and standard POTS lines are often very unequal), the more noise will get through. For best perfomance choose a cable that is very well balanced. For example Category 5 is better balanced than Category 3. Balancing is specified by the amount of capacitance difference ('unbalance') built up over a given distance. The standard for Category 5 is 1000 pF/1000 ft. capacitance unbalance. Some of the best cables have less than 150 pF/1000 ft.

To help reduce this problem, a number of companies have special passive and active devices to attempt to balance lines more perfectly. Active adapters contain circuitry to "adjust" the balancing, cost more but can send a video signal much farther than a passive device.

One a characteristic of Category-5 cable is that the pairs of wires are twisted at different rates inside the cable (this is done intentionally to reduce pair to pair crosstalk). Therefore, for a given length of Cat-5 cable the total length of a particular pair could be longer than others. Since the signals travel in the cable at a fixed rate (approximately 90% of light speed), the arrival times of signals can be skewed in a long cable (those that have to travel farther arrive later and the corresponding image shifts to the right). This can be a problem in high resolution video systems where for example RGB signals are transported. This is seen on the monitor as separation, or lack of convergence in colors. For example a vertical white line on the screen may look to have a red tinge on the left edge and blue tinge on the right edge. This effect gets worse at high resolutions, high refresh rates, long cables (in excess of 200-300 feet), and depends on the cable construction itself. To compensate for this skew, many commercial VGA to UTP adapters have "skewcompensation" pots. The pots are ty�pically adjusted by a screwdriver and need to be only set once per installation (adjustment is independent of resolution and refresh rate). The skew effect is very subtle, it can only move the colors for a few pixels.

Bottom line is that CAT5 cable is not great stuff for video signals. Certainly no better than any cable designed for video transmission UNLESS the CAT5 is combined with some sort of box that handles the conversion between what is good for video cable and what is good for CAT5. With such combination of devices you can get good results with video signal transmission over CAT5 cable. The converter will convert the unbalanced video signal to the balanced signal that can nicely go though the twisted pair cable without picking up too much noise or radiating too much interference. The converter will also made the necessary impedance conversion (75 ohms to 100 ohms and back). The converters have generally good common mode rejection characteritics (some prodicts boast over 60 dB common mode rejection). Some converters will pass DC and some other don't.

How about using UTP cable to carry audio ? Unshielded twisted pair is suitable cable to carry balanced signals (balanced audio, 10/100Base-T Ethernet, telephone, etc.), but is far from optimal for unbalanced signals (like home hifi audio interfaces with RCA connductors). To properly transfer unbalanced signal over UTP the signals need to be balanced (there are baluns for this). If you are carrying unbalanced audio signals through some short distances, I recommend you to use a cable with coaxial construction (typical shielded audio cable) or use shielded twisted pair cabling (best cable for balanced audio, works well also with unbalanced signals).

When audio signals are transferred with video, a separate wire pair (or two pairs for stereo) is used for audio signal. The audio signal is also converted between unbalanced (RCA connectors) and balanced formats (in wire pair) using audio signal transformers on both ends of the UTP cable.

When transmitting audio and video signals through twisted pair wiring, use the adapters from the same manufacturer on the both ends of the cable. There are no generic standards how this kind of adapters work, so adapters from different manufacturers are most propably not compatible with each other.

For example CCTV industry is shifting to UTP video transfer. Until recently, closed-circuit television (CCTV) security and surveillance equipment has been predominantly installed using coax cable. Although the technology to support twisted pair in the CCTV environment has been around for many years, today more and more CCTV dealers and installers are specifying twisted pair for the entire cabling system simply because it makes good business sense.

Promoted as a simple, inexpensive way to drive SVGA, XGA and SXGA signals long distances, Cat5 converters (or sometimes referred to as Twisted Pair converters) are catching on. Routing or driving RGBHV signals through high-resolution coax cable is certainly still the defacto-standard for the majority of the ProAV systems market, but more and more integrators are starting to use RGBHV to Cat5 converters for cost and simplicity. Why? Simple. It's a lot less expensive. Cat5 cable is very inexpensive (15 cents per foot price range) and pulling Cat5 cable through a wall and ceiling is a lot easier and faster than pulling Coax cable. Finally, crimpling Cat5 means two crimps per run versus 10 per run with RGBHV cable. With good converters Cat5 cabling will work out just fine. This kind of converter converts a asymmetrical RGB video signal (as used for coaxial video transmission) in a symmetrical one. This symmetricla one will transport nicely through CAT5 UTP cable. On the receiving end the signals are converted back to asymmetical one. One word to the wise: not all Cat5 converters are alike. Not all of them are good and there is a quality difference. They don't all use the same technology to do the signal conversion and you need to compare which one is good enough for your application.

Then user over UTP is used, the system will generally over wire gauges from 26AWG through 12AWG. Category 2, 3, 4 or 5 cable may be generally used. The better the cable, less possibility to interefence and longer range is supported. Individually shielded pairs should be avoided, as they usually drastically reduce the operating range of the systems. Video can generally be operated in the same communication cable coexistent with telephone, computer, control signals, power voltages and other video signals. While video may be routed through telephone punch down block terminals, any bridgetaps, also called T-taps and any resistive, capacitive or inductive devices MUST BE removed from the pair.

There are also some plans for running broadband RF video through CAT5 or better twisted pair wiring. One of the most demanding applications on the market today is broadband video, commonly known as CATV or cable television. It carries a broad range of signals extending from 54 MHz to beyond 600 MHz (usually up to around 900 MHz). Coaxial cable (RG-59 or RG-6) is commonly used for these applications, primarily for home networks and those systems are designed for 75 ohms impedance. The ever-increasing bandwidth available on modern twisted pair wiring systems has created ideas to carry RF video like cable TV signal through twisted pair wiring. The solution is to use a small transformer to convert from an unbalanced to a balanced signal on the sending end and and vice-versa on the receiving end. Baluns also make the necessary impedance transformation between coaxial cable (75 ohms) and twisted pair wiring (100 ohms). The use of CAT 5 for wireband RF video has been pretty limited because of quite high attenuation, especially at higher frequencies. The standard specifies attenuation of 24 dB per 100 meters of CAT5 cable, and when frequencies go higher, the attenuation increases quicly. The recent publication of the Category 6 standard by TIA marks an important milestone in this cabling system evolution. Category 6 at least doubles the bandwidth (usable frequency range) compared with Category 5/5e cabling. How far can you transmit video signals over Category 5e and Category 6 cabling? Most video receivers (e.g. TV sets) are designed to accommodate a wide dynamic range of signals. The minimum signal level at the remote television receiver is around 1 mV (-10 dBmV). For weaker signals, the picture is snowy and also much more susceptible to external noise. The maximum output level from the local amplifier is 50 dBmV, giving a dynamic range of up to 60 dB for the cabling. It should be noted that the signal level may need to be reduced below 50 dBmV because of radiated emission requirements that can further limit the dynamic range for the application. If the maximum allowed attenuation is 60 dB and cable length is 100 meters, we get the following maximum frequencies that stay within the limits expecting that the cables are well-behaving at high frequencies (data based on http://www.nordx.com/public/htmen/pdf/Video_over_Twisted_Pair_Cabling.pdf): 

Cable     Frequency

CAT5      not specificed (propably 250-300 MHz)
CAT5e     400 MHz
CAT6      500 MHz
CAT6+     650 MHz
In real-life you can't usually accept the 60 dB attenuation, thus the maximum distance and/or frquency range is reduced. Distance achievable is a function of signal level, balance and the degree to which the cable used unbalances the signal. Cat5e maintains signal balance at least 5 dB better than Cat5. Level 7 or equivalent cable is another 5 dB better, providing for a signal drive level of not more than 46 dBmV.

Broadband video is definitely one application that can take full advantage of the improved transmission performance offered by Category 6 cabling and beyond. A low Insertion Loss and a high Signal-to-Noise Ratio are the most important cabling parameters for the broadband video application. Although the composite video signal is an analog signal today, future digital television signals will use the same broadband frequency spectrum and channel allocation but a different digital modulation scheme. Category 6 or better cabling is well positioned to meet the bandwidth requirements for these applications.

 
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