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Note: The following technical article was current at the time it was published. However, due to changing technologies and standards updates, some of the information contained in this article may no longer be accurate or up to date.

De-Mystifying Category 5, 5e, 6, and 7 Performance Specifications

Even though the IEEE 802.3ab Gigabit Ethernet specification has released and applications group's are turning their attention towards next generation solutions, there is still a great degree of uncertainty as to the capability of today's telecommunications cabling systems to support tomorrow's high bit-rate applications. Fortunately, the Telecommunications Industry Association (TIA) and International Organization for Standardization (ISO) have made great strides in the specification and clarification of the minimum cabling performance criteria necessary to support these next generation applications.

Additional requirements and recommendations for category 5 and class D cabling that are intended to supplement the existing TIA/EIA-568-A and category ISO/IEC 11801 class specifications have recently been published. These specifications address additional transmission performance characterization required by systems developers to support bi-directional and full four-pair transmission schemes (such as those utilized by Gigabit Ethernet). Table 1 identifies these new cabling documents developed by the TIA and ISO technical committees:

Table 1: Additional Category 5/Class D Specifications

Document Name	Title
TIA/EIA/TSB95	"Additional Transmission Performance Guidelines for 4-pair 100 W Category 5 Cabling", September 1999
TIA/EIA-568-A-5	Addendum 5 to TIA/EIA-568-A entitled, "Additional Transmission Performance Specifications for 4-pair 100 W Enhanced Category 5 Cabling", November 1999
ISO/IEC 11801:1995 FDAM 2	Amendment 2 to ISO/IEC 11801 entitled, "Information Technology - Generic Cabling for Customer Premises Amendment 2", September, 1999

TIA and ISO working groups are also actively developing category 6, category 7, class E, and class F requirements capable of supporting greater frequency bandwidths and higher performance than previously achievable. It is anticipated that these specifications will soon be presented to the industry for technical comment and review.

TSB95

TSB95 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 in order to ensure Gigabit Ethernet application support. The TSB95 return loss and ELFEXT loss recommendations were derived from the worst case transmission performance of channels with only two connection points. The two connector channel topology is consistent with the IEEE committee's assumption that cabling used to support Gigabit Ethernet systems will most likely utilize an interconnect instead of a cross-connect field and will not include a consolidation or transition point connection.

Existing installed category 5 cabling should be verified to ensure that performance meets the minimum recommendations of TSB95 prior to attempting to support the Gigabit Ethernet protocol. It is important to note that existing channel configurations with three or four connectors that satisfy the TSB95 ELFEXT loss and return loss requirements will also support Gigabit Ethernet. Because the recommendations of TSB95 are applicable for the qualification of existing, installed cabling only, they are not recommended to be used as the minimum performance criteria for new category 5 cabling.

Originally balloted as an addendum to '568-A, these recommendations have been published in the form of a Telecommunications Systems Bulletin (TSB). TSB95 is informative in nature and does not contain mandatory or "shall" requirements.

TIA/EIA-568-A-5 ('568-A-5)

Addendum 5 to ANSI/TIA/EIA-568-A 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 '568-A-5 will emerge as the new de facto minimum standard for category 5 cabling. '568-A-5 specifies the minimum equal level far-end crosstalk (ELFEXT) loss and return loss requirements necessary to support developments in applications technology and defines the minimum performance that is required for a worst case four-connector channel to support applications that utilize full-duplex transmission schemes (such as Gigabit Ethernet). To ensure additional crosstalk headroom for robust applications support, '568-A-5 also specifies power sum NEXT and ELFEXT loss performance for category 5e cables, links and channels.

Addendum 5 to TIA/EIA-568-A is a normative document and, unlike TSB95, it provides mandatory requirements, not recommendations.

ISO/IEC 11801:1995 FDAM 2

The performance specifications in ISO FDAM 2 provide new requirements for return loss and ELFEXT loss to compliment the existing ISO class D requirements. The FDAM 2 specified return loss and ELFEXT loss requirements are in harmony with the values in '568-A-5, however, FDAM 2 does not specify additional NEXT loss margin over and above the existing class D requirements. FDAM 2 also includes propagation delay and delay skew requirements for channels and permanent links that are in harmony with the requirements of TIA/EIA-568-A-1.

The requirements of amendment 2 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/Class E

The proposed category 6/class E work area connector shall be an 8-position modular jack interface.

The proposed category 6/class E standards that are currently under development by TIA and ISO working groups describe a new performance range for unshielded and screened twisted-pair cabling. The charter of the working group developing category 6/class E requirements is to specify the best performance that UTP and ScTP cabling solutions can be designed to deliver. It is anticipated that the category 6/class E requirements will be specified in the frequency band of at least 1-250 MHz and be capable of supporting a positive power sum attenuation to crosstalk ratio (ACR) up to 200 MHz.

For category 6/class E cabling topologies, it has been agreed that the 8-position modular jack interface shall be the mandatory work area interface to be consistent with existing category/class requirements. Category 6/class E specifications will be backward compatible meaning that applications running on lower categories/classes will be supported by the category 6/class E infrastructure. If different category/class components are to be mixed with category 6/class E components, then the combination shall meet the transmission requirements of the lowest performing category/class component.

TIA, ISO, CENELEC, and others 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/class E requirements will soon become available for industry review. If TIA and ISO do not encounter unexpected technical issues, it is possible that the industry could have access to published category 6/class E requirements within twelve months.

Category 7/Class F

Proposed category 7/class F cabling will likely be supported by an entirely new connecting hardware design.

Proposed category 7/class F requirements are being developed for fully shielded (i.e., overall shield and individually shielded pairs) twisted-pair cabling. Category 7/class F will most likely 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/class F will also be backward compatible with lower performing categories and classes. It is anticipated that the category 7/class F requirements will be specified in the frequency band of at least 1-600 MHz. At this time, there are no applications, either pending or proposed, that are under development for operation over category 7/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 work area interface design, it is conceivable that the class F requirements will be available in the same time frame as the category 6/class E specifications.

Performance Comparison Chart:

Table 2 provides comparative channel performance data at 100 MHz and other frequency values of interest for the TIA category 5, 5e, and 6 and ISO class D, E, and F performance standards.

Table 2:Industry Standards Performance Comparison
Worst-case channel performance at 100 MHz

Parameter	Category 5 and Class D with additional requirements TSB95 and FDAM 2	Category 5e ('568-A-5)	Proposed Category 6 Class E (Performance at 250 MHz shown in parentheses)	Proposed Category 7 Class F (Performance at 600 MHz shown in parentheses)
Specified frequency range	1-100 MHz	1-100 MHz	1-250 MHz	1-600 MHz
Attenuation	24 dB	24 dB	21.7 dB (36 dB)	20.8 dB (54.1 dB)
NEXT	27.1 dB	30.1 dB	39.9 dB (33.1 dB)	62.1 dB (51 dB)
Power-sum NEXT	N/A*	27.1 dB	37.1 dB (30.2 dB)	59.1 dB (48 dB)
ACR	3.1 dB	6.1 dB	18.2 dB (-2.9 dB)	41.3 dB (-3.1 dB)**
Power-sum ACR	N/A	3.1 dB	15.4 dB (-5.8 dB)	38.3 dB (-6.1 dB)**
ELFEXT	17 dB (new requirement)	17.4 dB	23.2 dB (15.3 dB)	ffs***
Power-sum ELFEXT	14.4 dB (new requirement)	14.4 dB	20.2 dB (12.3 dB)	ffs***
Return loss	8 dB* (new requirement)	10 dB	12 dB (8 dB)	14.1 dB (8.7 dB)
Propagation delay	548 nsec	548 nsec	548 nsec (546 nsec)	504 nsec (501 nsec)
Delay skew	50 nsec	50 nsec	50 nsec	20 nsec

Note: Industry channel-performance requirements for Category 6 and Category 7 are currently under development.

* Class D return-loss requirement at 100 MHz is 10 dB. Class D power-sum NEXT loss is 24.1 dB at 100 MHz.

** Positive ACR at 600 MHz is accomplished with the typical Class F implementation with interconnect environment and without transition point.

*** ffs-The parameters are marked for future study by the ISO standards group, and anticipated performance requirements are under development.

Conclusion:

When designing and installing structured cabling systems, chose the strongest foundation to support your present and future networking needs. To ensure support of emerging technologies that utilize the latest advances in signaling schemes, it is critical to be as informed as possible. Trust the TIA and ISO standards developmental groups to specify complete cabling criteria capable of providing applications assurance for tomorrow's technologies today.

Important Definitions

Attenuation to Crosstalk Ratio (ACR)

A critical consideration in determining the capability of an unshielded twisted-pair (UTP) or screened twisted-pair (ScTP) cabling system is the difference between attenuation and near-end crosstalk (NEXT). This difference is known as the attenuation to crosstalk ratio (ACR). Positive ACR means that transmitted signal strength is stronger than that of near-end crosstalk. ACR helps to define a signal bandwidth (i.e. 200 MHz for category 6) where signal to noise ratios are sufficient to support certain applications. It is interesting to note that digital signal processing (DSP) technology can perform crosstalk cancellation allowing some applications to expand useable bandwidth up to and beyond the point at which ACR equals zero. Even so, the maximum frequency for which positive ACR is assured provides a benchmark to assess the useable bandwidth of twisted-pair (balanced) cabling systems.

Attenuation

Attenuation is a measure of the decrease in signal strength along the length of a transmission line. Ensuring low signal attenuation is critical because digital signal processing technology can not compensate for too much signal attenuation.

Near-End Crosstalk (NEXT) and Equal Level Far-End Crosstalk (ELFEXT)

Pair-to-pair near-end crosstalk (NEXT) requirements quantify undesired signal coupling from adjacent pairs that is received at the same end of the cabling as the transmit end of the disturbing pairs. Standards groups now realize that the sophisticated nature of full-duplex transmission will also require that the crosstalk at the far-end of the cabling be specified. Pair-to-pair far-end crosstalk (FEXT) quantifies undesired signal coupling at the receive end of the disturbing pairs. ELFEXT is calculated by subtracting attenuation from the far-end crosstalk loss. Poor ELFEXT levels can result in increased bit error rates and/or undeliverable signal packets. Note that NEXT margin alone is not sufficient to ensure proper far-end crosstalk performance!

Power Sum

Power sum NEXT and ELFEXT performance provides headroom to ensure cabling channels are significantly robust to handle crosstalk from multiple disturbers. Power summation accounts for the combined performance of all pair combinations. This type of characterization is needed to ensure cabling compatibility with applications that utilize all four pairs for transmitting and receiving signals simultaneously (e.g. Gigabit Ethernet).

Return Loss

Return loss is a measure of the signal reflections occurring along a transmission line and is related to impedance mismatches that are present throughout a cabling channel. Because emerging applications such as Gigabit Ethernet rely on a full duplex transmission encoding scheme (transmit and receive signals are superimposed over the same conductor pair), they are sensitive to errors that may result from marginal return loss performance.

Propagation Delay & Delay Skew

Propagation delay is equivalent to the amount of time that passes between when a signal is transmitted and when it is received at the other end of a cabling channel. The effect is akin to the delay in time between when lightning strikes and thunder is heard - except that electrical signals travel much faster than sound. Delay skew is the difference between the pair with the least delay and the pair with the most delay. Transmission errors that are associated with excessive delay and delay skew include increased jitter and bit error rates.

Bandwidth (fiber)

Bandwidth describes the frequency carrying capabilities of a transmission system and is a function of fiber type, distance, and transmitter characteristics. Bandwidth margin maximizes a system's ability to support advanced applications.

Balance

Twisted-pair transmission relies on signal symmetry or "balance" between the two conductors in a pair. Maintaining proper balance ensures that cabling systems and components do not emit unwanted electromagnetic radiation and are not susceptible to electrical noise. Although these parameters are not industry requirements, it is recommended that the balance performance of cabling components be ensured through measurements of longitudinal conversion loss (LCL) and longitudinal conversion transfer loss (LCTL).

Transfer Impedance

Shield effectiveness directly affects the ability of shielded twisted-pair cable and connecting hardware to maximize immunity from outside noise sources and minimize radiated emissions. Transfer impedance is a measure of shield effectiveness; lower transfer impedance values correlate to better shield effectiveness.

Biography

Valerie Rybinski holds the title of Senior Electrical Engineer at the Siemon Company. Her responsibilities include research and design in the corporate electrical test and development facility. Her standards activities include membership on the TIA working groups responsible for the development of next generation cabling specifications; chairman of the TIA TR42.7.1 Connecting Hardware Working Group; and Secretary of the TIA TR42.7 Copper Cabling Systems Subcommittee.

Valerie holds a Bachelors degree in Electrical Engineering from the University of Connecticut.

Rev. A, 12/99