Alan Flatman / LAN Technologies
There is no doubt that the early 1990s will be remembered for its prolific developments in cabling technology and standards. The first industry standard for structured cabling was approved by the Electronic Industries Alliance and Telecommunications Industry Association (EIA/TIA- Arlington, VA) in the United States in 1991. The EIA/TIA-568 building standard was quickly embraced by suppliers and end-user organizations worldwide, despite its scope being limited to voice-grade (Category 3) twisted-pairs plus 62.5-micron multimode fiber. Sadly, EIA/TIA-568 did not specify an optical connector and, hence, optical-fiber implementations continued to be application-specific or proprietary.
Improved versions of twisted-pair cables and connector hardware quickly followed and were specified by the TIA's telecommunications systems bulletins TSB-36 and TSB-40. The performance of twisted-pair cables was improved by more controlled twisting and the use of better dielectric materials. Bandwidth capability was increased by a factor of six within a period of less than two years, resulting in the creation of 100-MHz Category 5.
The market for structured cabling experienced strong growth from 1993, and field testers were soon introduced. Unfortunately, early testers were of little use because they could not credibly qualify installed links. Uncontrolled mixtures of Category 3, 4, and 5 components plus the damaging effects of poor installation practices could not be checked. Consequently, the performance of many installations is still in question today.
In 1995, international, European, and U.S. standards were published for 100-MHz cabling systems, and accurate field testers finally emerged, based on TSB-67. In many respects, this development represented a turning point for the industry. But by this time, an estimated 40 million Category 5 outlets had been installed in Europe (100 million worldwide). Due to the low confidence in field testing prior to this date, compliance of pre-1995 installed Category 5 cabling has depended on a number of factors controlled by individual cabling vendors; these factors include the quality of components plus design and installation practices. We are only now establishing consistency in this area through the publication of planning and installation standards.
Amendment of U.S. standards
The U.S. cabling standard TIA/EIA-568A has undergone a series of amendments to correct errors, complete existing specifications, and introduce a number of additional requirements. The following addenda are being published to reflect the recent maintenance activity conducted by TIA committee TR-42:
- TIA/EIA-568A-1 introduces a new requirement for horizontal cable propagation delay and delay skew, based on limits for twisted-pair Ethernet. The channel (end-to-end) limit for propagation delay is 555 nsec at 10 MHz, and delay skew is 50 nsec.
- TIA/EIA-568A-2 introduces a number of corrections and additions, including the specification of common-mode and differential-mode termination for near-end crosstalk (NEXT) measurement in the field (to improve measurement accuracy and repeatability).
- TIA/EIA-568A-3 introduces a requirement to limit multiple-disturber NEXT in bundled cables. It requires power-sum NEXT (PS-NEXT) from multiple pairs in hybrid, pre-loomed, or speed-wrapped cables to be at least 3 dB better than the worst pair-to-pair NEXT for that cable.
- TIA/EIA-568A-4 specifies a nondestructive test methodology and performance limits for modular plug cord NEXT.
- TIA/EIA-568A-5 takes account of recent improvements in design and manufacturing of Category 5 cable and connector hardware and therefore specifies an enhanced version of Category 5 cabling. So-called Enhanced Category 5 (Category 5E) cabling reflects what the supply industry can make today. NEXT and return-loss requirements are more stringent than in TSB-95, and PS-NEXT is also specified. Category 5E requirements are summarized in the table. Addendum 5 specifies additional measurement procedures plus Level II-E accuracy limits for field testers. Early verification of Category 5E systems indicated a high level of noncompliance and, consequently, the return-loss specification for cables and connector hardware has been elevated to guarantee channel limits.
In addition to the preceding addenda, TSB-95 was developed to specify the additional parameters required to support 1000Base-T Gigabit Ethernet over Category 5 cabling. Minimum requirements for return loss, equal-level far-end crosstalk (ELFEXT), and power-sum ELFEXT (PS-ELFEXT) are as specified in IEEE 802.3ab. It is important to note that these characteristics have been based on statistical measurements and therefore do not capture 100% of installed Category 5 links. Additional measurement procedures are also specified for field-testing new parameters. It was decided to publish this specification as a telecommunications systems bulletin, rather than a formal amendment to the U.S. standard, so as not to disenfranchise suppliers and users of existing Category 5 cabling. Additional requirements are summarized in the table above.
Amendment of international and European standards
International and European cabling standards ISO/IEC-11801 and EN 50173 are also being amended to correct errors, complete existing specifications, harmonize with U.S. specifications, and introduce a number of additional requirements. The International Organization for Standardization and International Electrotechnical Commission (ISO/IEC-Geneva) are processing two amendments within committee SC25/WG3, and the European Committee for Electrotechnical Standardization (CENELEC-Brussels) is processing a single amendment within TC215/WG1. Both sets of amendments are technically identical.
The most significant architectural amendment is the introduction of channel and permanent link specifications. End-to-end channel specifications have been adopted from TIA/EIA-568A, without modification. Permanent link specifications define the performance of cabling between an outlet and the horizontal cable side of a patch panel. The permanent link replaces the existing ISO/IEC "link," which represented cabling between an outlet and the equipment side of a patch panel. The "permanent link" was introduced as the installation contractor's reference points for field-testing and verification, which closely corresponds to the "basic link" specified by TIA/EIA-568A and the field-testing specification, TSB-67.
There will no longer be a separate attenuation-to-crosstalk ratio (ACR) specification for ISO/IEC-11801 and EN 50173; the existing specification has been simplified to be the numerical difference between NEXT and attenuation.
Values of propagation delay and skew published in TIA/EIA-568A-1 will be introduced to ISO/IEC-11801 and EN 50173. Also, Class D channel return loss will be aligned with Category 5E, as specified by TIA/EIA-568A-5.
ELFEXT and PS-ELFEXT specifications will be introduced to ISO/IEC-11801 and EN 50173. ELFEXT values are 0.4 dB less stringent than those adopted by TIA for Category 5E, as shown in the table.
Second-generation cabling standards
Next-edition cabling standards are scheduled for publication later this year. These standards are expected to contain several major technical and architectural changes.
Withdrawal of obsolete cable types: It has been proposed to withdraw Category 3, Category 4, and 150-ohm balanced copper cabling (i.e., IBM Type 1 STP cable), including the connector originally specified for Token Ring (IEC 807-8). This proposal reflects market trends and is intended to focus media choice.
Alignment of Class D cabling with TIA Category 5E: It has been proposed to align all parameters for Class D channel and permanent links with values specified by TIA/EIA-568A-5, the outstanding differences being 3-dB NEXT and PS-NEXT (Category 5E being more stringent). This proposal may not be straightforward, due to differences in cordage attenuation premium allowed by TIA/EIA-568A (up to 20% higher than horizontal cable) and ISO/IEC-11801 and EN 50173 (up to 50% higher than horizontal cable).
Centralized optical architecture (COA): As the name suggests, COA provides connections directly from outlets to a centralized equipment room. COA is currently defined by TIA/EIA TSB-72, which limits cable length to 300 meters across a building. Only the concept is being proposed for adoption by ISO/IEC-11801 and EN 50173, thus supporting larger buildings and campus-centralized equipment rooms over distances in excess of 300 meters.
COA has a number of benefits. It may reduce the administration costs associated with distributed equipment and patching facilities (for example, servers may be centralized for this reason). COA may also enhance responsiveness to change and eliminate the need for special rooms for equipment and cabling distributors. Being passive, COA is intrinsically more reliable than distributed architectures.
Open-office, or zone, cabling: This approach to cabling improves distribution in high-churn, open-plan office environments. The key component is the "consolidation point," which facilitates the clustering of outlets. A consolidation point is typically a "break out" box containing a number of modular (RJ-45 style) connectors located behind a false floor or ceiling.
Open-office cabling is specified by TIA/EIA TSB-75 using cords with only a 20% attenuation premium. A more flexible version of this cabling architecture has been developed for EN 50173 to address the use of 4-, 3-, or 2-connector models (i.e., crossconnect and consolidation point [as shown in the figure], interconnect and consolidation point, or interconnect to outlet). The total channel length will be reduced, as more consolidation-point (CP) cable is deployed due to its higher attenuation, and is specified by:
H = 105 - (A + B + C)X - DY,
where X = attenuation premium of cordage and Y = attenuation premium of CP cable.
Next-generation cabling: Two new categories and classes of cabling are being developed. Class E (Category 6) cabling will extend transmission performance to 200 MHz, while Class F (Category 7) cabling is targeting 600 MHz. These performance limits are anticipated for state-of-the-art cabling channels based on unscreened or overall foil-screened cable and for cable with individually screened pairs. The upper frequency limits of 200 and 600 MHz correspond to positive PS-ACR. A total of four mated connectors is assumed for the horizontal channel to facilitate a crossconnect and consolidation point.
The following objectives have been agreed upon for the development of next-generation cabling:
The current status of next-generation cabling development is as follows:
Category 6 (Class E) cabling: The Class E channel and Category 6 cable specifications are technically stable, having accomplished a positive PS-ACR (0.1 dB) at 200 MHz based on four RJ-45 connectors. All parameters now have formula-based specifications, and the upper frequency limit has been extended to 250 MHz. Development of the Category 6 RJ-45-style connector is ongoing, with further work required to verify multivendor operation and backward-compatibility with Category 5 and 5E connectors. A new test method known as "de-embedding" is being used to support connector evaluation, providing greater accuracy and improved repeatability at frequencies above 100 MHz.
Category 7 (Class F) cabling: Class F channel and Category 7 cable specifications are technically stable; these specifications were taken from the published German standard, DIN 44312-5. However, the channel performance falls short of its target of 600 MHz. PS-ACR is zero at 475 MHz based on four connectors. It has been agreed not to extend the upper frequency limit to accommodate the use of DSP electronics to exploit negative ACR performance.
Two Category 7 connector types have been chosen: one based on the RJ-45, the other using an entirely different footprint. The RJ-45-type connector employs two additional pin-pairs for Category 7 operation and the conventional 8 inline pins for Category 5 or 6. IEC has been requested to develop standards for both connector types to support electrical and mechanical compatibility, multivendor interoperation and backward-compatibility with lower-performance connectors. The RJ-45-type connector is being viewed as the preferred choice, but the outcome will depend on technical developments within the IEC standards committee, TC48B.
Optical-fiber cabling: Currently, ISO/IEC-11801 and EN 50173 specify 50- and 62.5-micron multimode fibers in terms of their lowest common values of attenuation and modal bandwidth. There are plans to specify these fiber types separately in the future to distinguish their modal bandwidths. Multimode fiber with higher modal bandwidth has also been proposed to support 10-Gigabit Ethernet up to 300 meters using low-cost 850-nm VCSELs (vertical-cavity surface-emitting lasers). This proposal is based on 50-micron core size and has an incredible modal bandwidth performance of 2200/500 MHz-km.
Challenges and opportunities
There are a number of significant challenges associated with the ongoing development of copper cabling systems. One of the most outstanding challenges relates to backward-compatibility of the RJ-45 connector, which was originally designed to operate at frequencies up to 3 MHz. Different compensation techniques used above and below 100 MHz make backward- compatibility difficult to accomplish. Having said this, there are promising signs that industry experts will resolve this challenge quite soon.
The introduction of new parameters such as ELFEXT is raising the complexity and skill associated with field-testing copper cabling, which has now become more sophisticated than optical-fiber testing. Today, it is possible to enhance existing field-tester technology to 350 MHz to provide accurate testing of Class E (Category 6) cabling. However, operation up to 600 MHz represents a formidable challenge. Consequently, no field testers are yet available to verify Class F (Category 7) cabling performance up to 600 MHz.
The specification and testing of screening continues to be underdeveloped. Cable and connector screens are currrently specified as "surface transfer impedance" at a few spot frequencies. Coupling attenuation will, one day, provide a better mechanism to characterize and test screening effectiveness, but it may take years to complete. Consequently, screening cannot be tested in the field today.
Despite the challenges, there are also some exciting opportunities associated with new developments in cabling standards. The most obvious short-term opportunity is the harmonization of U.S., international, and European standards. Perhaps the most significant long-term opportunity is the delivery of a comprehensive package of design, planning, installation, and field-testing standards this year. Standards-makers have recognized that "drip feeding" the market with a series of uncoordinated specifications generally leads to incompatibility and confusion.
Alan Flatman is founder of LAN Technologies (Congleton, Chesire, UK), an independent consultancy specializing in the introduction and deployment of new network technologies. He has chaired standards committees in Europe and the United States and has also represented the United Kingdom at ISO/IEC and CENELEC throughout the development of their standards for structured cabling systems. Flatman can be contacted at: [email protected]. See www.cable-install.com for an expanded version of this article.