Guide to Alien Crosstalk (AXT) in Cabling Systems
Achieving 10 gigabit Ethernet and higher cabling grades requires advanced planning and understanding of performance parameters. Alien crosstalk is an important performance parameter end users and installers should consider during cabling grade selection.
Alien crosstalk (AXT) is defined as:
- the occurrence of unwanted signal coupling from one component, channel, or permanent link to another.
Since alien crosstalk is an indicator of differential (or balanced) signal coupling, alien crosstalk cannot be adversely impacted by common mode noise (e.g. noise from motors or florescent lights) that is present in the environment.
Alien crosstalk is only specified by the current Standards as a power sum parameter for components and cabling to approximate the energy present when all cabling pairs are energized.
High power sum AXT levels can compromise the operation of the 10GBASE-T application causing performance below standard. Power sum alien crosstalk measured at the near-end is called power sum alien near-end crosstalk loss (PSANEXT loss). Power sum alien crosstalk measured at the far-end is called power sum alien attenuation to crosstalk ratio, far-end (PSAACRF).
AXT in the category 6/class E systems
The characterization of alien crosstalk in the installed category 6/class E cabling plant is the main focus of the TIA TSB-155 and ISO/IEC 24750 technical bulletins. Because the alien crosstalk in category 6/class E UTP cabling is extremely dependent upon installation practices (e.g. bundling, the use of tie-wraps, and pathway fill), performance values were developed based upon a "typical" worst case environment meaning that 10GBASE-T should operate over category 6/class E UTP channel lengths of up to 37 meters and may operate over channel lengths of 37 to 55 meters of category 6/class E UTP cabling depending upon the actual alien crosstalk levels present. Since the overall foil in category 6/class E F/UTP cabling designs significantly reduces alien crosstalk, these length limitations are not applicable to F/UTP cabling.
TIA TSB-155 and ISO/IEC 24750 also specify recommended mitigation practices in the event that an installed category 6/class E channel does not satisfy the minimum alien crosstalk levels. Mitigation techniques include using non-adjacent patch panel ports to support the 10GBASE-T application, separating or using improved equipment cords, using F/UTP equipment cords, unbundling cables, reconfiguring cross-connects as interconnects, and replacing category 6/class E components with category 6A/class EA components.
AXT in 10GBASE-T systems
Compliant cabling products are carefully designed to comply with alien crosstalk requirements.
- UTP (Augmented Cat 6)
- Have increased cable diameter up to 9.0mm (0.354") to reduce alien crosstalk
- F/UTP (Augmented Cat 6)
- Foil screen virtually eliminates alien crosstalk
- S/FTP (Cat 7/Class F)
- Full shielding eliminates alien crosstalk
The significant difference between a Category 6A/class EA and category 6 UTP cables is the greatly increased diameter wall jacket. Design strategies use thicker jackets to separate the copper cores from each other and ensure compliant alien crosstalk performance. Installation practices that deform the jacket (e.g. excessive pathway fill, over-cinched tie wraps, etc.) can compromise alien crosstalk performance.
The transmission specifications of Category 6A/class EA cabling are significantly more stringent than that specified for category 6. For example, Category 6A/class EA systems are specified to have almost 30% more stringent insertion loss requirements in order to support the positive signal-to-alien crosstalk margin up to 500 MHz required by the 10GBASE-T application.
Siemon's 10G 6A UTP Category 6A cabling system combines 10Gb/s performance with meeting or exceeding all performance requirements of the pending Category 6A (augmented category 6) standard, including alien crosstalk.
One of the key changes in the 10G 6A UTP products is the increase in cable diameter to ensure proper immunity to alien crosstalk. In lieu of using a screened solution, the primary method for reducing the effects of alien crosstalk along the length of a channel is to create greater separation between cables. As opposed to cabling mitigation techniques such as manual separation of cables, the best method is achieved by design. Increasing space within the overall cable ensures that adjacent cables are properly separated to reduce the effects of alien crosstalk. This increased cable diameter is present on both UTP horizontal and patch cables. 10G 6A MAX patch panels are designed with optimized port spacing to ensure alien crosstalk mitigation.
In contrast, screened and fully-shielded designs reduce alien crosstalk while substantially improving noise immunity at all frequencies and especially above 30 MHz when cable balance starts to significantly degrade. Screened and fully-shielded have the added benefit of significantly increased Shannon capacity for future applications.
Siemon's 10G 6A F/UTP category 6A system features superior immunity to alien crosstalk versus UTP systems. This immunity eliminates the need for field testing of alien crosstalk.
Combining 10 Gb/s performance with the security, noise immunity and pathway space maximization of a screened cabling system, Siemon's 10G 6A F/UTP end-to-end solution represents the cutting edge of Category 6A (augmented category 6) cabling. 10G 6A F/UTP meets or exceeds all pending standard performance requirements for 10GBASE-T, including alien crosstalk parameters. Specifically designed to handle tomorrow's most advanced and performance critical applications, 10G 6A F/UTP performs as well in secure or high EMI environments as it does in standard office spaces, by virtue of its screened construction.
Siemon's TERA Cat 7/Class FA solution implements full shielding to eliminate alien crosstalk.
TERA offers performance to 10Gb/s and beyond, far exceeding all performance requirements for 10GBASE-T. With bandwidth of 1.2 GHz per pair, TERA performs at twice the category 7/class F specifications and the highest of any available copper system.
Installation practices
Proper installation practices must be closely followed to help reduce or eliminate AXT. Siemon trains its Certified Installers on proper installation techniques.
10G 6A UTP increased diameter cables meet AXT parameters, but exhibit increased sensitivity to installation practices that may deform outer jackets:
- Over-cinched tie wraps
- Excessive conduit/pathway fill
- Exceeding bend radius
10G 6A F/UTP and S/FTP cable design resists deformation, allowing less restrictive installation practices. F/UTP cable offers resistance to crushing due to the foil reinforcement and fewer air spaces in the design.
As part of the installation process, field testing for alien crosstalk should be considered.
Field Testing for Alien Crosstalk
Since 10GBASE-T applications are sensitive to alien crosstalk, the requirements for field testers capable of assessing the performance of installed category 6A cabling systems are specified within the pending TIA-568-B.2-10 standard.
This Standard specifies both the measurement procedures (Annex E) and accuracy requirements (Annex I) for level IIIe field testers for all historical parameters as well as the new alien crosstalk parameters PSANEXT loss, PSAFEXT loss and PSAACRF (PSAELFEXT).
Keep in mind that the level IIIe field test devices for determining compliance to the new parameters have just recently been introduced to the market and the field verification of alien crosstalk parameters is not required by the Standard.
If installers or end users are interested in performing alien crosstalk testing at their discretion on 10G 6A UTP, sample testing should be conducted based upon evaluating links that meet all of the following conditions:
- Longest installed lengths
- Cables within the same bundle
- Adjacent ports in the patch panel
Siemon offers Network Cabling Services to ensure proper network cabling installation and design from the work area to the data center. For further assistance in answering your questions about alien crosstalk, please Ask Siemon or Contact Sales.
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