TIA Category 6 and 7 physical media for Gigabit Ethernet



Introduction

Unshielded Twisted Pair is the most common kind of copper telephone wiring. Twisted pair is the ordinary copper wire that connects home and many business computers to the telephone company. To reduce crosstalk or electromagnetic induction between pairs of wires, two insulated copper wires are twisted around each other. Each signal on twisted pair requires both wires. Since some telephone sets or desktop locations require multiple connections, twisted pair is sometimes installed in two or more pairs, all within a single cable. However, the twisted pair wire required for reliable data transmission is of a heavier gauge than the telephone wire; it uses 24 to 26 AWG (AmericanWire Gauge). For some business locations, twisted pair is enclosed in a shield that functions as a ground. This is known as shielded twisted pair (STP).

Twisted pair comes with each pair uniquely color coded when it is packaged in multiple pairs. Different uses such as analog, digital, and Ethernet require different pair multiples. Although twisted pair is often associated with home use, a higher grade of twisted pair is often used for horizontal wiring in LAN installations because it is less expensive than coaxial cable.

The purpose of twisting the copper wires together is� to minimize the interference between the two adjacent wires. Otherwise, there� would be too much line noise to transfer voice or data efficiently. There� are two different types of twisted pairs that are used to transmit data. The STP (Shielded Twisted Pair) and the UTP (Unshielded Twisted Pair). Just like it sounds, the STP is a special kind of copper telephone wiring used in some business installations. An outer covering or a shield is added to the twisted pair acting as a ground. This process shields the twisted pair from EMI (Electro-magnetic Interference), which will disturb the data transmission greatly. The UTP is just a twisted pair without the extra shielding. It just relies on the copper twists to protect it from the EMI. The twisted pairs are grades into five different categories by their data carrying ability.


Maximum Data Rate Usual Application
CAT 1 Less than 1 Mbps Analog voice (plain telephone service)ISDN Basic Rate Interface Doorbell wiring
CAT 2 4 Mbps Mainly used in the IBM Cabling System for Token Ring� networks
CAT 3 16 Mbps Voice and data on 10BASE-T Ethernet
CAT 4 20 Mbps Used in 16Mbps Token Ring Otherwise not used much
CAT 5 100 Mbps 100 Mbps TPDDI155 Mbps ATM

As you can see, the twisted pairs can range greatly in bandwidth by the different�levels of CATs. For a LAN environment, the CAT 2 and above are used because� they can transmit greater amounts of data. These generally are used for Token�Ring LANs except for the CAT 3, which is only supported by the Ethernet LANs.� Most companies who have networks in the offices use twisted pair system to�run their LAN. Using the twisted pair media for LANs is very attractive because� of its low cost; mainly the UTP setup because it costs much less than the� STPs and the STP cabling are far more difficult to put into walls than the� UTPs. On the other hand, the risk of going with the twisted pair setup is� the instability of the data flow because of it?s susceptibility to line interference.

The latest levels of CAT are CAT 5e (category 5 enhanced), CAT6 and CAT7.� This page talks about Category 6 and Category 7 standards for physical media in a network. Most applications today run at 10 or 100 Mbps.� Category 5 products are able to support gigabit applications (on the majority of existing installations) and Category 5e on new installations. In the future, Category 6 products may replace Category 5e products, once the standard is completed and the compatibility and testing issues are resolved. Category 6 and 7 products will have the potential of reducing the cost associated with the network electronics [e.g., network interface cards, (NICs)] and have the potential to take us beyond the gigabit performance level.


Category 6/Class E


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.

�� � � � � � � � � � � � � cat6 interface �� � � � � � � � � � � � � � � cable

� � � � � � � � � � � � � � � � � � � �� � � � � � � � � � � � � � �
�� � � � � � � � � � � The proposed category 6� interface will be
�� � � � � � � � � � � � � an 8-position modular jack interface.�


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



� �� � � � � cat7 connector
� � � � � � � � � � � � � � � � � � � � � � �
� � � � � � Proposed category 7/class F cabling will likely be
� � � � � � supported by an entirely new connecting hardware design.
�� � � � � � � (shown here, a non-RJ45 connector)

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:-

The Table below 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: Industry Standards Performance Comparison, Worst-case channel performance at 100� MHz
Parameter � � � � � � � � � � � � � � � � � Category 5 and Class D
� � �
� � � � with additional
� � � � requirements
� TSB95and 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
� parantheses)
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.





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.


Links to Industry information on latest developments in CAT 6 and 7 standards: -

White papers on standard updates by Siemon Cabling System.
Technology Information by Simon Hollobone at www.findarticles.com.
Technical Information on Gigabit Ethernet at www.icc.com and article from communication news here.
Gigabit Ethernet over copper using category 6 cable, article by David Marsh for EDN Magazine.