ANSI/TIA-568
TIA/EIA-568-B is a set of three standards that address commercial building cabling for telecommunications products and services. The three standards are formally titled ANSI/TIA/EIA-568-B.1-2001, -B.2-2001, and -B.3-2001)
The TIA/EIA-568-B standards were first published in 2001. They supersede the TIA/EIA-568-A standards set, which are now obsolete.
Perhaps the best known feature of TIA/EIA-568-B.1-2001 are the pin/pair assignments for eight-conductor 100-ohm balanced twisted pair cabling. These assignments are named T568A and T568B, and are frequently referred to (erroneously) as TIA/EIA-568A and TIA/EIA-568B.
History
TIA/EIA-568-B was developed through the efforts of more than 60 contributing organizations including manufacturers, end-users, and consultants. Work on the standard began in 1985, when the Computer Communications Industry Association (CCIA) asked the Electronic Industries Alliance (EIA), a standards organization, to define standards for telecommunications cabling systems. EIA agreed to develop a set of standards, and formed the TR-42 committee, with nine subcommittees to perform the work.
The first revision of the standard, TIA/EIA-568-A.1-1991 was released in 1991, and was updated in 1995. The demands placed upon commercial wiring systems increased dramatically over this period due to the adoption of personal computers and data communication networks and advances in those technologies. The development of high-performance twisted-pair cabling and the popularization of fiber optic cables also drove significant change in the standards, which were eventually superceded by the current TIA/EIA-568-B set.
Goals
TIA/EIA-568-B attempts to define standards that will enable the design and implementation of structured cabling systems for commercial buildings, and between buildings in campus environments. The bulk of the standards define cabling types, distances, connectors, cable system architectures, cable termination standards and performance characteristics, cable installation requirements and methods of testing installed cable. The main standard, TIA/EIA-568-B.1 defines general requirements, while -568-B.2 focuses on components of balanced twisted-pair cable systems and -568-B.3 addresses components of fiber optic cable systems.
The intent of these standards is to provide recommended practices for the design and installation of cabling systems that will support a wide variety of existing services, and the likelihood of supporting future services that are designed with standard cabling in mind. The standards hope to provide a lifespan for commercial cabling systems in excess of ten years. This effort has been largely successful, as evidenced by the definition of category 5 cabling in 1991, a cabling standard that (mostly) satisfied cabling requirements for 1000BASE-T, released in 1999. Thus, the standardization process can reasonably be said to have provided at least a nine-year lifespan for premises cabling, and arguably longer.
All these documents accompany related standards that define commercial pathways and spaces (569-A), residential cabling (570-A), administration standards (606), grounding and bonding (607) and outside plant cabling (758).
Structured cable system topologies
TIA/EIA-568-B defines a hierarchical cable system architecture, in which a main cross-connect (MCC) is connected via a star topology across backbone cabling to intermediate cross-connects (ICC) and horizontal cross-connects (HCC). Telecommunications design traditions utilized a similar topology, and many people refer to cross-connects by their older, nonstandard names: "distribution frames" (with the various hierarchies called MDF's, IDF's and wiring closets). Backbone cabling is also used to interconnect entrance facilities (such as telco demarcation points) to the main cross-connect. Maximum allowable backbone cable distances vary between 300m and 3000m, depending upon the cable type and use.
Horizontal cross-connects provide a point for the consolidation of all horizontal cabling, which extends in a star topology to individual work areas such as cubicles and offices. Under TIA/EIA-568-B, maximum allowable horizontal cable distance varies between 70m and 90m for twisted-pair cable types, depending upon patch cord length and gauge. Fiber optic horizontal cabling is limited to 90m. Optional consolidation points or transition points are allowable in horizontal cables, although many industry experts discourage their use.
At the work area, equipment is connected to horizontal cabling by patch cords.
TIA/EIA-568-B also defines characteristics and cabling requirements for entrance facilities, equipment rooms and telecommunications room.
T568A and T568B termination
Perhaps the widest known and most discussed feature of TIA/EIA-568-B.1-2001 is the definition of pin/pair assignments for eight-conductor 100-ohm balanced twisted-pair cabling, such as Category 3, Category 5 and Category 6 unshielded twisted-pair (UTP) cables. These assignments are named T568A and T568B and they define the pinout, or order of connections, for wires in RJ-45 eight-pin modular connector plugs and jacks. Although these definitions consume only one of the 468 pages in the standards documents, a disproportionate amount of attention is paid to them. This is because cables that are terminated with differing standards on each end will not function normally.
TIA/EIA-568-B specifies that horizontal cables should be terminated using the T568A pin/pair assignments, "or, optionally, per [T568B] if necessary to accommodate certain 8-pin cabling systems." Despite this instruction, many organizations continue to implement T568B for various reasons, chiefly associated with tradition. The United States National Communication Systems Federal Telecommunications Recommendations do not recognize T568B.
The primary color of pair one is blue, pair two is orange, pair three is green and pair four is brown. Each pair consists of one conductor of solid color, and a second conductor of the same color with a white stripe. The specific assignments of pairs to connector pins varies between the T568A and T568B standards.
Mixing T568A-terminated patch cords with T568B-terminated horizontal cables (or the reverse) does not produce pinout problems in a facility. Although it may very slightly degrade signal quality, this effect is marginal and certainly no greater than that produced by mixing cable brands in-channel.
Wiring
Regardless of the wiring standard, RJ-45 modular jack pins are numbered 1 through 8, pin 1 being the leftmost pin on the connector side of the plug, 8 the rightmost. Pins on jacks are numbered correspondingly, pin 1 being the rightmost and pin 8 the leftmost connector when the connectors are on the bottom side of the socket.
The assignments of wire pairs to plug and jack pins are as follows:EATME
| Pin | T568A Pair | T568B Pair | Wire | T568A Color | T568B Color | Pins on plug face (jack is reversed) |
|---|---|---|---|---|---|---|
| 1 | 3 | 2 | tip |  white/green stripe |
 white/orange stripe |
|
| 2 | 3 | 2 | ring |  green solid |
 orange solid | |
| 3 | 2 | 3 | tip | white/orange stripe |
white/green stripe | |
| 4 | 1 | 1 | ring |  blue solid |
blue solid | |
| 5 | 1 | 1 | tip |  white/blue stripe |
white/blue stripe | |
| 6 | 2 | 3 | ring | orange solid |
green solid | |
| 7 | 4 | 4 | tip |  white/brown stripe
|
white/brown stripe
| |
| 8 | 4 | 4 | ring |  brown solid
|
brown solid
|
Note that the only difference between T568A and T568B is that pairs 2 and 3 (orange and green) are swapped. Both standards wire the pins "straight through", i.e., pins 1 through 8 on one end are connected to pins 1 through 8 on the other end. Also, the same sets of pins are paired in both standards: pins 1 and 2 form a pair, as do 3 and 6, 4 and 5 and 7 and 8. And although many cables implement some small electrical differences between pairs, these effects are negligible, so cables wired to either standard are interchangeable.
Backwards compatibility
Because pair 1 connects to the center pins (4 and 5) of the RJ-45 jack in both T568A and T568B, both standards are compatible with the first line of RJ-11, RJ-14, RJ-25 and RJ-61 connectors that all have the first pair in the center pins of these connectors.
If the second line of a RJ-14, RJ-25 or RJ-61 plug is used, it connects to pair 2 (orange/white) of jacks wired to T568A but to pair 3 (green/white) in jacks wired to T568B. This makes T568B potentially confusing in telephone applications.
Because of different pin pairings, the RJ-25 and RJ-61 plugs cannot pick up lines 3 or 4 from either T568A or T568B without splitting pairs. This would most likely result in unacceptable levels of hum, crosstalk and noise.
Because 10BASE-T and 100BASE-TX use only pairs 2 and 3, pairs 1 and 4 need not even be present in the cable. It is also common in some networks to use one 4-pair Category 5 cable to provide two separate 10BASE-T or 100BASE-TX links, assigning only two pairs to each link. However, such jacks cannot be used with 1000BASE-T as it requires all four pairs for each link. They are also incompatible with direct use by single-line telephones with standard RJ-11 plugs as nothing is connected to pair 1 in the jack. However, a separate telephone line could be connected to pair 1, thus allowing a single jack to be used for either voice or Ethernet without reconfiguration.
See also
References
National Communications System Federal Telecommunications Recommendation 1090-1997
External links
