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Structured Data Cabling
In recent years, structured cabling has emerged as a basic building utility, comparable to HVAC, electricity or plumbing. It forms the essential communications infrastructure of any IT system, and is every bit as important as the equipment connected to each end of cable. Cabling is often overlooked or taken for granted, but a little tie and effort in the choice of cabling system will save tim, money and frustration in the near term, and, in the longer term, minimise the need for disruptive upgrades.
The beginning
When office computers were first introduced, each manufacturer selected a medium to connect the device where all the information was stored (the Mainframe) to the dumb terminals sitting on user's desks. Each computer manufacturer chose the medium best suited to their equipment, with many opting for some form of coaxial cable.
In general, one mainframe was not enough to host the different applications required by the users, which in turn lead to the use of two or more mainframes - each with its own incompatible cabling system.
The resulting multiplicity of cable types lead to high costs and congestion on cabling routes. Clearly, a simpler system was required, with one type of cable able to handle all applications.
The answer came in the form of twisted-pair cabling, which had originally been introduced for telephony systems, and subsequently developed to support the transmission of higher-frequency signals. The availability of a single medium made life much easier for computer manufacturers and building owners alike.
Owners of earlier generation computers could use special adapters that would allow the signals to be transmitted over twisted-pair cable in place of the original medium. New, non-proprietary cabling systems, along with the rise of networking protocols, especially Ethernet and Token Ring, made it possible to transmit relatively large amounts of data, and did much to stimulate the development and take-up of the personal computer.
Technical overview
Three elements define the concept of structured cabling: a generic cable type (usually based on four-pair twisted-pairwiring); flood wiring (whereby cabling is pre-installed at every usable location within a building); and patch panels (allowing users to be connected to any service). The cabling is generally terminated in an eight-position modular jack, known as an RJ45.
By the early 1990s, standards writing bodies began to take an interest in the rapidly developing world of structured cabling, leading to a system of Categories and Classes to classify different cabling standards. A key parameter of any cabling system is the bandwidth of tyhe cable. Early structured cabling systems had a bandwidth of 16MHz, which, with a simple coding scheme and conservative approach to bandwidth utilisation, supported data transmission of 10 Mbit/s.
As cable systems and network protocols have developed, there has been a trend toward higher bandwidths and more sophisticated signal encoding schemes. For example, Category 4 cabling was introduced in response to IBMs need for a 20MHz bandwidth to support Token Ring LAN with its 16 MBit/s data rate.
The table below summarises bandwidths and bit rates for the various categories and classes, along with a timescale indicating their year of introduction. While higher bit rates could be achieved in any category by the development of more compicated protocols (encoding schemes) and electronics, this increased complexity results in higher costs. The choice of what category system to specify is often a vital decision.
| Category/Class | Frequency | Typical Bit-rate | Year Introduced |
| 1 / A | 100 kHZ | 'voice grade' | n/a |
| 2 / B | 1 MHz | 1 MBit/s | n/a |
| 3 / C | 16 MHz | 10 MBit/s | 1985 |
| 4 | 20 MHz | 16 MBit/s | 1991 |
| 5 / D | 100 MHz | 100 MBit/s | 1992 |
| 5 / D | 100 MHz | 1000 MBit/s | 1995 |
| 6 / E | 200 MHz | 1000 MBit/s + | 1998 |
| 7 / F | 600 MHz | 1000 MBit/s ++ | 1998 |
Notes:
- In European standards, Class refers to the end-to-end performance of a cabling link and Category refers to the individual components;
- In American standards, Category applies to both component and link performance;
- Category 5e (enhanced) was introduced in the late 1990s to beter support 1000 MBit/s services;
- Category 6 is 'charcterised' and tested to 250MHz;
- The connector for Category 7 systems has yet to be defined.
Performance
In the UK, Category 5e and 6 systems both commonly use an unshielded twisted-pair (UTP) cable that has four pairs. There are other options, such as shielded cables (STP), but they are still referred to as catagory 5e or 6. Category 7 systems are currently only available using cables that have a shield around each pair and an overall shield around all four pairs. The connectors at either end are also shielded. It is not expected that category 7 can be made in an unshielded version.
If data requirements continue to grow beyond the cost effective abilities of Category 6, the market will opt for either Category 7 shielded systems, or fibre optics.
Fibre and copper
Category 5e/6 and 7 all refer to the use of copper cable. However, optical fibre also plays a key role in modern data networks. IN general, optical fibre is preferable for data transmission in LAN backbone networks. In general, however, copper is the most economic media type for backbone cabling and voice applications.
With a copper backbone, the cabling itself tends to be application specific, and may not be suitable for use with alternate protocols. Fibre, on the other hand, is much more adaptable, and can be sed for many different applications. Other important benefits of fibre include higher bandwidth, lower losses, immunity to ecternal interference, and the fact that it occupies much less space than its copper equivalent.
See also:
- Go back
- Network hardware information
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