Clarifying category and performance specifications

Even as the 802.3ab committee of the Institute of Electrical and Electronics Engineers (ieee--New York City) nears completion of its work on the Gigabit Ethernet specification, there is still great uncertainty about the capability of today`s telecommunications cabling systems to support tomorrow`s high-bit-rate applications. Fortunately, the Telecommunications Industry Association and Electronic Industries Alliance (TIA/EIA--Arlington, VA) and the International Organiza-tion for Standardization

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Now more than ever, a knowledge of standards is vital.

Valerie Rybinski,

The Siemon Co.

Even as the 802.3ab committee of the Institute of Electrical and Electronics Engineers (ieee--New York City) nears completion of its work on the Gigabit Ethernet specification, there is still great uncertainty about the capability of today`s telecommunications cabling systems to support tomorrow`s high-bit-rate applications. Fortunately, the Telecommunications Industry Association and Electronic Industries Alliance (TIA/EIA--Arlington, VA) and the International Organiza-tion for Standardization and International Electrotechnical Commission (ISO/IEC--Geneva) have made great strides in clarifying the minimum cabling-performance criteria necessary to support next-generation applications.

Additional requirements and recommendations for Category 5 and Class D cabling are expected to be published within the next few months and will supplement the existing TIA/EIA-568a category and ISO/IEC-11801 class specifications. These updated specifications address additional transmission performance characteristics required by systems to support bidirectional and full 4-pair transmission schemes, such as those used by Gigabit Ethernet. The table lists the new cabling documents that are under review by the TIA and ISO technical committees.

High-frequency bandwidth specs

The TIA and ISO working groups are also developing Category 6, Category 7, Class E, and Class F requirements for cabling capable of supporting higher-frequency bandwidths and higher performance than was previously achievable. These specifications will likely be presented to the industry for technical comment and review in early 1999.

TIA document PN-4292, proposed telecommunications systems bulletin tsb-95, provides recommendations for the new Category 5 channel parameters of return loss and equal-level far-end crosstalk (elfext) loss. These recommendations are specified to verify the performance of installed, or "legacy," Category 5 cabling to ensure that it can support Gigabit Ethernet applications. tsb-95`s return-loss and elfext-loss recommendations were derived from the worst-case transmission performance of channels with only two connection points. This two-connector channel topology is consistent with the ieee committee`s assumption that cabling used to support Gigabit Ethernet systems will most likely use an interconnect instead of a crossconnect field and will not include a consolidation or transition point connection.

Before attempting to run Gigabit Ethernet over existing installed Category 5 cabling, you should verify that its performance meets the minimum recommendations of tsb-95. Existing channel configurations with three or four connectors that satisfy the tsb-95 elfext-loss and return-loss requirements will also support Gigabit Ethernet. Because the recommendations of tsb-95 are applicable for the qualification of existing installed cabling only, they should not be used as the minimum performance criteria for new Category 5 cabling.

Originally balloted as an addendum, tsb-95 is informative in nature and does not contain mandatory or "shall" requirements.

Category 5E requirements

Formally known as TIA SP-4195-A, proposed Addendum 5 to TIA/EIA-568a specifies Enhanced Category 5 (Category 5E) performance requirements. It is strongly recommended that new Category 5 cabling installations be specified to satisfy the minimum requirements of this document, and it is expected that TIA/EIA-568a-5 will emerge as the new de facto minimum standard for Category 5 cabling. The standard specifies the minimum elfext-loss and return-loss requirements necessary to support developments in applications technology and defines the minimum performance required for a worst-case four- connector channel to support applications that use full-duplex transmission schemes. To ensure additional crosstalk headroom for robust applications support, TIA/EIA-568a-5 also specifies power-sum near-end crosstalk (next)-loss and elfext-loss performance for Category 5E cables, links, and channels.

Proposed Addendum 5 to TIA/EIA-568a is a normative document and, unlike tsb-95, it provides mandatory requirements, not recommendations.

Amendment to ISO/IEC-11801

The performance specifications in ISO-proposed draft amendment pdam 3 provide new requirements for return loss and elfext loss to complement existing ISO Class D requirements. pdam 3 specifies return-loss and elfext-loss requirements that are in harmony with the values proposed in TIA/EIA-568a-5; but, the amendment does not specify additional next-loss margin above existing Class D requirements. pdam 3 also includes propagation-delay and delay-skew requirements for channels and permanent links that are in harmony with the requirements of TIA/EIA-568a.

The requirements of proposed Amendment 3 to ISO/IEC-11801 are normative, and this document is expected to become the de facto standard for new Class D cabling installations.

Category 6 and Class E

The proposed Category 6 and Class E standards under development by TIA and ISO working groups describe a new performance range for unshielded twisted-pair (utp) and screened twisted-pair (sctp) cabling. The charter of these working groups is to specify the best performance that utp and sctp cabling can be designed to deliver. It is anticipated that these requirements will be specified in the frequency band of at least 1 to 250 megahertz and will be capable of supporting a positive power-sum attenuation-to-crosstalk ratio (acr) to 200 MHz. At this time, there are no applications, either pending or proposed, under development for operation over Category 6 and Class E cabling.

For Category 6 and Class E cabling topologies to be consistent with existing category and class requirements, the standards groups have agreed upon the 8-position modular-jack interface as the mandatory work-area interface. Category 6 and Class E specifications thus will be backward-compatible, which means that applications running on lower categories or classes of cabling will be supported by the Category 6 and Class E infrastructure. If components of a different category or class are mixed with Category 6 and Class E components, the combination shall meet the transmission requirements of the lowest-performing component.

TIA/EIA, ISO/IEC, CENELEC (the European Committee for Electrotechnical Standardization--Brussels), and other groups are collaborating closely on the development of Category 6 and Class E standards, and their proposed requirements are very much in harmony. It is expected that Category 6 and Class E requirements will be available for industry review within the next six months. If TIA and ISO do not encounter unexpected technical issues, the industry could have access to published Category 6 and Class E requirements within 12 months.

Category 7 and Class F

Proposed Category 7 and Class F requirements are being developed for fully shielded (i.e., overall shield and individually shielded pairs) twisted-pair cabling. Category 7 and Class F will be supported by an entirely new interface design (i.e., plug and socket). Even though these requirements will be supported by a new connecting hardware interface, Category 7 and Class F will also be backward-compatible with lower-performing categories and classes. It is anticipated that Category 7 and Class F requirements will be specified in the frequency band of at least

1 to 600 MHz. At this time, there are no applications, either pending or proposed, under development for operation over Category 7 and Class F cabling.

It is interesting to note that TIA is not actively developing a standard for Category 7 and will most likely harmonize with the Class F requirements put forth by ISO. If industry consensus is achieved on the selection of a Category 7 interface design, Class F requirements might be available at approximately the same time as the Category 6 and Class E specifications.

The table compares channel-performance data at 100 MHz and other frequency values of interest for the proposed TIA Category 5, 5E, 6, and 7 and ISO Class D, E, and F performance standards.

Think ahead

When designing and installing structured-cabling systems, choose the strongest foundation to support your present and future networking needs. To ensure support of emerging technologies that use the latest advances in signaling schemes, it is critical to be as informed as possible. Trust the TIA and ISO standards groups to specify complete cabling criteria capable of supporting tomorrow`s technologies today.

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The 8-position modular-jack interface is the proposed Category 6 and Class E work-area connector.

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Proposed Category 7 and Class F cabling will be supported by an entirely new connecting hardware design.

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Valerie Rybinski is senior electrical engineer at The Siemon Co. (Watertown, CT). She is a member of TIA working groups responsible for the development of next-generation cabling specifications, chairman of the TIA PN-2948 connecting hardware task group, and secretary of the TIA PN-3727 utp systems task group.

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