The following is a commented excerpt from http://www.epanorama.net/documents/wiring/twistedpair.html
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 on 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:
Campus cabling: Where a site contains more than one building, Campus cable is used to connect and integrate the network within the overall area. EIA/TIA refers to this as Inter-Building Backbone Cable. Backbone Campus Cabling is usually optical fibre based upon 62.5/125um Multimode fibre or single mode fibre. Campus cabling connects the main wiring closets on different buildings.
Riser/Backbone cables: Riser or Backbone has been traditionally installed using screened or unscreened pair cables. 6 x 4 pair or 25 pair cables are available as standard. Those are used for data and telephones. New data applications are increasingly satisfied by Fibre Optic Cables (typically multimode cables). Riser/Backbone cables are run from the house central wiring closet to wiring closets on different building floors.
Horizonal cables: Horizonal twisted pair cables provide the communications link between and into specific work areas. The cables are typically wired from wiring closet to the outlet on the working locations. 4 Pair 24 AWG UTP and FTP cables for high speed networks operating at up to 100 Mbps (even faster speeds up to 1Gbit/s is possible with best cables and newest network techniques). In line with EIA/TIA 568A these cables can be used in lengths of up to 90 metres. 24 AWG is the most commonly used thickness for fixed wiring, but in some cases thicker 23 AWG could be used.
Work area cabling: High performance flexible work area/patch cables are used for localised linking from a wall connection to networked equipment. Patch cables cables are generally available Unscreened or Foil Screened and come in two sizes 0.14mm� (26 AWG) and 0.22mm� (24 AWG).
Patch cables: Short patch cables are used to do the interconnection in the wiring closet from the connectors on the telecom patch panel to active equipment. Patch cables are also used in datacom/telecom racks to interconnect different commmunication equipment. Patch cables cables are generally available Unscreened or Foil Screened and come in two sizes 0.14mm� (26 AWG) and 0.22mm� (24 AWG).
The standard connector for the 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:
Unshielded Twisted-Pair (UTP) Structured Premise Cabling permits many communication protocols to reside in the same wire bundle. Examples include voice, data, and CCTV video and control.
UTP system is Color-coded cabling
A good UTP system gives better interference rejection than coax [WOW]
UTP is less expensive than dedicated coax or fiber
UTP is physically smaller than coax and many other cable types
UTP is a very easy media to install and reconfigure
UTP is extremely easy to terminate
In many cases, the wire is already installed
The disadvantages of structured UTP cabling:
Because there can be different kind signals wired to different outlets and all outlets look the same, the user needs to be careful not to wire a device to an outlet where a wrong type of signal is wired. Wring connection can lead to situation that the system does not work correctly or even to equipment damages.
The application originally designed for some other type of cabling need usually special adaters which cost money
The current UTP systems (CAT5, CAT5e, CAT6) have quite limited high frequency performance, so they do not suit well for transmission of very high frequency signals like cable TV, TV antenna and radio antenna signals. There are product for this available, but the performance which can be got with those is generally quite limited.
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:
100 ohm unshielded twisted pair: modern structured cabling systems (CAT 3, CAT 4, CAT 5, CAT 6) used to carry telephone signal and networking signals within buildings
120 ohm unshielded twisted pair: Some older telecommunication cables on the field (quite typical value for line from cental office to house)
150 ohm shielded twisted pair: Those are generally used for some older netwrking systems like IBM cabling system used for Token Ring network. 150 ohm dhielded twisted pair cable is also sometimes used to carry balanced audio signals and automation systems signals (generally applications which need shielded twisted pair wiring.
75 ohm coaxial cable: video interconnection, CCTV, common antenna wiring, cable TV, some telecommunication signals
50 ohm coaxial cable: antenna wiring for radio transmitters, WLAN cards, cellular phone base stations etc.
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.
The "quality" of the cabling systems to carry high frequency signals is expressed with the folloging marking (those are in use in USA):
Cat 1: Cabling that meets the minimum requirements for analog voice or Plain Old Telephone Service (POTS). Also known with name Grade 1. Commonly called inside wire by the Telco community. (Informal designation)
Cat 2: This is a 100 ohm UTP system capable of operating 1 Mbps Token Ring and similar networks. This is also known as IBM Type 3 cabling system. Also known with name Grade 2. (Informal designation)
Cat 3: This cable type is characterized to 16 MHz and supports applications up to 10 Mbps. Applications may range from voice to 10BASE-T. This is a low performance cable rating which is dissapearing. This is nowadays the minimal requirement for good quality structured telephone cabling system. This is also known as ISO/IEC 11801 Class C cabling. This was the standard for UTP performance as late as 1988. The FCC recently changed the requirement for telephone inside wiring to minimum of Cat 3 due to crosstalk problems with nontwisted quad-four. CAT 3 is no longer recognized by TIA.
Cat 4: This cable type is characterized to 20 MHz and supports applications up to 16 Mbps. Applications may range from voice to 10BASE-T and 16 Mbps Token Ring. This cable type is not much used nowadays.
Cat 5: The traditional rating of cables for high speed data installation. Rated frequency is 100 MHz. This cable works well from voice to 100BASE-T Ethetnet and 155Mbps ATM. This cable type is also known as ISO/IEC 11801 Class D cabling. Today Cat 5 copper communications wiring is the recognized minimum for broadband services. The standard for thie wiring are ISO/IEC-11801 and TIA/EIA-568-A-5. CAT5 performance is only possible when cable, connector modules, patch cords, and all electronics carry the same CAT5 rating.
Cat 5e: New rating developed in USA. Rated frequency is 100 MHz. Cat 5E is becoming the new standard for premises wiring, because it is recommended as the minimum for all future installations by TIA/EIA, IEEE and many equipment manufacturers. Enhanced Category 5, was ratified in 1999.
Cat 6: A new rating just developed in US, ISO/IEC and CENELEC. Rated frequency is 200 MHz with some requirements specified for 250 MHz. Category 6 is being specified concurrently by both ISO in the 11801-2001 document and the TIA in its Category 6 addendum to TIA 568B (ANSI/TIA/EIA-568-B.2-1 ratified by the TIA/EIA in June 2002). This presents the best performance possible with the current T568A and T568B wiring configurations on an 8 position 8 conductor modular connector (RJ-45). In Europe this is known as ISO/IEC 11801 Class E cabing.
Cat 7: A rating for individual pair screened cables derived from the german DIN 44312-2 standard requirements. Rated frequency is 600 MHz. The work is on progress. This is also known as ISO/IEC 11801 Class E. This cable is fully shielded and uses non-standard RJ-45 interface (Alcatel hybrid RJ-45 connector).This cabling is primarily for European market place. Other alternative connector style is IBM Mini-C connector. In Europe this is known as Class F cabling.
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:
UTP = Unshielded Twisted (Balanced) 4-Pair Cable, 100 Ohms
STP = Overall foil/braid Shielded 2-pair Cable w/ Individually shielded, 150 Ohm
FTP = Overall foil shielded 4-pair Cable, 100 Ohm
ScTP = Overall foil/braid Shielded Cable, 100 or 120 Ohm
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 should 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, remember 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).