Guide to Alien Crosstalk in Network Cabling Systems
Achieving 10 Gb/s transmission rates over balanced twisted-pair cabling requires advanced planning, proper system specification, and conscientious installation and maintenance practices. Alien crosstalk is the most significant transmission parameter impacting 10GBASE-T performance and should be carefully evaluated by end-users and installers during the cabling specification process.
Alien crosstalk is defined as:
- Unwanted signal coupling from one balanced twisted-pair component, channel, or permanent link to another.
Since alien crosstalk is only caused by differential (or balanced) signal coupling, alien crosstalk is not adversely impacted by common mode noise (e.g. noise from motors, transformers, or florescent lights) that is present in the environment.
Alien crosstalk is specified by 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 alien crosstalk levels can compromise the operation of the 10GBASE-T application by significantly reducing expected signal-to-noise (SNR) margins, thus potentially causing re-transmissions or even auto-negotiation in the network equipment to a lower Ethernet speed. Power sum alien crosstalk measured at the near-end of the transmitter is called power sum alien near-end crosstalk loss (PSANEXT loss). Power sum alien crosstalk measured at the far-end of the transmitter is called power sum alien attenuation to crosstalk ratio, far-end (PSAACRF).
All of the Category 6A/class EA and class FA cabling transmission requirements are significantly more stringent than category 6/class E requirements to specifically to support the high performance 10GBASE-T application environment. For example, the Category 6A/class EA alien crosstalk limits support almost 80% less alien crosstalk voltage than exhibited by a typical installed category 6/class E channel and class FA systems deliver even greater alien crosstalk performance headroom. Category 6A/class EA and class FA systems are also specified to have at least 27% less insertion loss in order to support positive signal-to-alien crosstalk margins over the frequency band required by the 10GBASE-T application.
Alien Crosstalk in 10 GBPS-Ready Cabling Systems
Category 6A/class EA and class FA cabling is specified to support the 10GBASE-T application over worst-case 100 meter, 4-connector channel topologies. Compliant cabling products are carefully designed to satisfy alien crosstalk requirements using the following strategies.
- UTP (Category 6A/class EA)
- Cables have increased diameters, up to 9.0 mm (0.354 in), and connectors are mechanically isolated by increasing the distance between them or via other means in patch panel and faceplates to reduce alien crosstalk
- F/UTP (Category 6A/class EA)
- The foil surrounding the cable and the shielded construction of the connecting hardware virtually eliminates alien crosstalk
- S/FTP (class FA)
- The braid and individual foils surrounding the cable and cable pairs and the shielded construction of the connecting hardware eliminates alien crosstalk
Alien Crosstalk in Category 6A/class EA UTP Cabling Systems
Category 6A/class EA UTP cables rely on thicker and/or specially designed jackets to physically separate internal twisted-pairs from external twisted-pairs and ensure compliant alien crosstalk performance. As a result, care should be taken to avoid installation practices that may deform the jacket (e.g. excessive pathway fill, over-cinched tie wraps, etc.) and compromise alien crosstalk performance.
Siemon's Z-MAX 6AŽ UTP Category 6A/class EA cabling solution combines 10 Gb/s-ready performance with compliance to all TIA and ISO/IEC Category 6A/class EA cabling and component requirements, including alien crosstalk.
Alien Crosstalk in the Category 6A/class EA F/UTP and class FA S/FTP Systems
Shielded (F/UTP) and fully-shielded (S/FTP) cable designs reduce alien crosstalk to virtually zero levels, while offering the added benefit of substantially improved noise immunity at all frequencies. This immunity is especially critical at frequencies above 30 MHz, where the inherent balance of the cable starts to significantly degrade. Shielded and fully-shielded cabling has the added benefit of greatly increased Shannon capacity for future applications.
Siemon's Z-MAX 6A shielded Category 6A/class EA cabling solution combines 10 Gb/s-ready performance with compliance to all TIA and ISO/IEC Category 6A/class EA cabling and component requirements and features significantly improved immunity to alien crosstalk compared to Category 6A/class EA 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 shielded cabling system, Siemon's Z-MAX 6A shielded end-to-end solution represents the cutting edge of Category 6A/class EA cabling. Specifically designed to handle tomorrow's most advanced and performance critical business network applications, Z-MAX 6A shielded cabling performs as well in secure or high EMI environments as it does in standard office spaces.
Siemon's TERA® class F/class FA cabling solution utilizes individual and overall pair shielding to virtually eliminate alien crosstalk and pair-to-pair crosstalk. The result is a 10 Gb/s-ready cabling solution that supports a 15-year lifecycle, provides maximum return-on-investment (ROI), ensures secure data transmission in sensitive environments, and supports cable sharing (i.e. running multiple low-speed, low pair-count applications, such as voice or 10/100BASE T, over one cable). TERA class FA cabling offers performance that far exceeds all performance requirements for 10GBASE-T and, with a bandwidth of 1.2 GHz per pair, Siemon's TERA connector is the highest performing connector on the market today. The need for field testing of alien crosstalk is also eliminated with TERA class FA cabling solutions.
Proper installation practices must be closely followed to help reduce alien crosstalk. Siemon trains its Certified Installers on proper installation techniques.
Siemon Z-MAX 6A UTP cabling solutions comply with Category 6A/class EA alien crosstalk requirements, but, like all 10 Gb/s-ready UTP cabling solutions, may be sensitive to installation practices that deform the outer jacket such as:
- Over-cinched tie wraps
- Excessive conduit/pathway fill
- Exceeding bend radius
Because both Siemon shielded Z-MAX 6A and TERA S/FTP cable designs resist deformation and their shields are significantly less susceptible to damage, their alien crosstalk performance is less likely to be adversely affected by poor installation practices. F/UTP cable offers resistance to deformation due to the foil reinforcement and fewer air spaces in the design. S/FTP cabling offers even more resistance to deformation due to the cable's increased foil and braid content and the connector's robust 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 and internal noise impairments, requirements for field testers capable of assessing the performance of installed Category 6A/class EA cabling systems are specified within the ANSI/TIA-1152 and IEC 61935-1 Edition 3.0 standards. These Standards describe both the measurement procedures and accuracy requirements for level IIIe field testers for all legacy transmission parameters as well as the new PSANEXT loss and PSAACRF alien crosstalk parameters. It's important to keep in mind that the field verification of alien crosstalk or any other transmission parameter is not required by these Standards, although many cabling system vendors require testing of installed cabling prior to providing warranty coverage.
Typically, field tests for alien crosstalk are not performed on F/UTP and S/FTP cabling systems. If installers or end-users are interested in performing alien crosstalk testing at their discretion on Category 6A/class EA UTP cabling systems, 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 and product selection, please Ask Siemon or Contact Sales.
Alien Crosstalk in the Category 6/Class E Cabling Systems
The characterization of alien crosstalk in the installed category 6/class E cabling plant is the main focus of the TIA TSB-155-A and ISO/IEC TR 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 on an assumed "typical" worst case installation environment. This analysis led to the conclusion that 10GBASE-T should operate over category 6/class E UTP channel lengths of up to 37 meters and may operate over category 6/class E UTP channel lengths of 37 to 55 meters depending upon the actual alien crosstalk levels present. Since shielded cabling designs significantly reduce alien crosstalk, these length limitations are not applicable to F/UTP or S/FTP installations.
TIA TSB-155-A and ISO/IEC TR 24750 also specify recommended mitigation practices in the event that an installed category 6/class E UTP 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, unbundling cables, reconfiguring cross-connects as interconnects, and replacing category 6 components with Category 6A components.
It should be noted that category 6/class E cabling is not recommended for new installations targeted for support of the 10GBASE-T application. The reason for this is that, while field test devices for determining compliance to the PSANEXT loss and PSAACRF parameters are commercially available, the test methodology remains extremely time-consuming, overly onerous to implement, and may not be fully conclusive. Furthermore, in a majority of installations, alien crosstalk mitigation will be required. Often, the recognized mitigation methods cannot be easily implemented due to existing pathway fill restrictions and the potential need to replace components. In addition, there is no guidance on qualification procedures for large installations or future Move, Add, and Change (MAC) work.
- Siemon Standards Informant
- Siemon TIA TSB-155-A Overview
- How to Certify or Re-Certify Twisted-Pair Cabling for 10 Gb/s Ethernet (Fluke Networks)