TIA/EIA-568 was developed through the efforts of more than 60 contributing organizations including manufacturers, end-users, and consultants. Work on the standard began with 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 work continues to be maintained by TR-42 within the Telecommunications Industry Association.
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and the popularization of fiber optic cables also drove significant change in the standards, which were eventually superseded by the current TIA/EIA-568-C set.
TIA/EIA-568-C attempts to define structured cabling 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-C.1 defines general requirements, while -568-C.2 focuses on components of balanced twisted-pair cable systems and -568-C.3 addresses components of fiber optic cable systems, -568-C.4, which addressed coaxial cabling components.
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 and future services. Developers hope the standards will 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 a longer one.
All these documents accompany related standards that define commercial pathways and spaces (TIA-569-A), residential cabling (TIA-570-A), administration standards (606), grounding and bonding (TIA-607), and outside plant cabling (TIA-758).
The standard defines categories of unshielded twisted pair cable systems, with different levels of performance in signal bandwidth, attenuation, and cross-talk. Generally increasing category numbers correspond with a cable system suitable for higher rates of data transmission. Category 3 cable was suitable for telephone circuits and data rates up to 16 million bits per second. Category 5 cable, with more restrictions on attenuation and cross talk, has a bandwidth of 100 MHz. The 1995 edition of the standard defined categories 3, 4, and 5. Categories 1 and 2 were excluded from the standard since these categories were only used for voice circuits, not for data.
Structured cable system topologies
TIA/EIA-568-C 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 MDFs, IDFs 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 fibre 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 is 90m of installed cabling, whether fibre or twisted-pair, with 100m of maximum total length including patch cords. No patch cord should be longer than 5m. Optional consolidation points are allowable in horizontal cables, often appropriate for open-plan office layouts where consolidation points or media converters may connect cables to several desks or via partitions.
At the work area, equipment is connected by patch cords to horizontal cabling terminated at jackpoints.
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 8P8C (often incorrectly referred to as RJ45) eight-pin modular connector plugs and sockets. 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 (T568B is equivalent to AT&T 258A).
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 which is white with a stripe of the same color. The specific assignments of pairs to connector pins varies between the T568A and T568B standards.
Note that the only difference between T568A and T568B is that pairs 2 and 3 (orange and green) are swapped. Both configurations 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 connect to the opposite ends that are paired in both configurations: pins 1 and 2 form a pair, as do 3 and 6, 4 and 5, and 7 and 8. One can use cables wired according to either configuration in the same installation without significant problem; problems involving crosstalk can occur (which is normally minimized by correctly twisting a pair together), but are usually insignificant in all but the most stringent specifications such as Category 6 cable. The primary thing one has to be careful of is not to accidentally wire the ends of the same cable according to different configurations (unless one intends to create an ethernet crossover cable).
Use for T1 connectivity
In Digital Signal 1 (T1) service, the pairs 1 and 3 (T568A) are used, and the USOC-8 jack is wired as per spec RJ-48C. The Telco termination jack is often wired to spec RJ-48X, which provides for a Transmit-to-Receive loopback when the plug is withdrawn.
Vendor cables are often wired with Tip and Ring reversed—i.e. pins 1 and 2 reversed, or pins 4 and 5 reversed. This has no effect on the signal quality of the T1 signal, which is fully differential, and uses the Alternate Mark Inversion (AMI) signaling scheme.
Because pair 1 connects to the center pins (4 and 5) of the 8P8C connector in both T568A and T568B, both standards are compatible with the first line of RJ11, RJ14, RJ25, and RJ61 connectors that all have the first pair in the center pins of these connectors.
If the second line of an RJ14, RJ25 or RJ61 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 RJ25 and RJ61 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.
The original idea in wiring modular connectors, as seen in the registered jacks, was that the first pair would go in the center positions, the next pair on the next outermost ones, and so on. Also, signal shielding would be optimized by alternating the "live" and "earthy" pins of each pair. The TIA/EIA-568-B terminations diverge slightly from this concept because on the 8 position connector, the resulting pinout would separate the outermost pair too far to meet the electrical echo requirements of high-speed LAN protocols.
- "TIA-568 Set : Commercial Building Telecommunications Cabling Standards Set".
- William Stallings Knowing UTP wiring basics can boost local net performance, Network World 9 July 1996, page 29
- Charles E. Spurgeon, Ethernet:The Definitive Guide, (O'Reilly Media, Inc., 2000) ISBN 1565926609 page 212
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