Guide to Alien Crosstalk in Network Cabling Systems
Achieving 10Gbps 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 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 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 of the switch 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).
Alien Crosstalk in 10GBPS-Ready Cabling Systems
Category6A/class EA and class F/FA cabling are 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.
- UTP (Category6A/class EA)
- Has increased cable diameter up to 9.0 mm (0.354 in.) and separation between connectors to reduce alien crosstalk
- F/UTP (Category6A/class EA and class F/FA)
- Foil screen virtually eliminates alien crosstalk
- S/FTP (Class F/FA)
- Full shielding eliminates alien crosstalk
Alien Crosstalk in the Category 6A/Class EA UTP Cabling Systems
The main difference between category 6A/class EA and category 6/class E UTP cables is the greatly increased outside jacket wall thickness. 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 those specified for category 6/class E cabling. For example, category 6A/class EA alien crosstalk limits support almost 80% less alien crosstalk voltage than that exhibited by a typical installed category 6 channel! Furthermore, category 6A/class EA systems are also specified to have 27% 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 Z-MAX 6A UTP category 6A cabling solution combines 10Gbps-ready performance with compliance to all of the pending category 6A/class EA cabling and component requirements, including alien crosstalk.
Alien Crosstalk in the Category 6A/Class EA F/UTP and Class F/FA S/FTP Systems
Shielded (F/UTP) and fully-shielded designs (S/FTP) 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. Screened 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 10Gbps-ready performance with compliance to all of the 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 performs as well in secure or high EMI environments as it does in standard office spaces, by virtue of its shielded construction.
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 10Gbps-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 (running more than one low-speed, high speed application such as voice or 10/100BASE-T over one cable). TERA class F/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 F/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 10Gbps-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 Z-MAX 6A shielded and TERA S/FTP cable designs resist deformation and their shields are significantly less susceptible to damage, their overall performance is less likely to be adversely affected by poor installation practices. F/UTP cable offers resistance to crushing due to the foil reinforcement and fewer air spaces in the design. S/FTP cabling offers even more resistance to crushing 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, the requirements for field testers capable of assessing the performance of installed category 6A/class EA cabling systems are specified within the pending TIA-568-B.2-10 and IEC 61935-1 standards. These Standards specifiy both the measurement procedures and accuracy requirements for level IIIe field testers for all historical parameters as well as the new alien crosstalk parameters PSANEXT loss, PSAFEXT loss and PSAACRF. Keep in mind that the level IIIe field test devices for determining compliance to these new parameters have just recently been introduced to the market and the field verification of alien crosstalk parameters is not required by these Standards.
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 10G 6A 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 was 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.
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 just now being introduced to the market, 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 Guide to Cabling Standards
- TSB-155 on track for imminent publication (Cabling Installation and Maintenance Magazine)
- Testing 10 Gigabit/sec Ethernet Over Copper and Fiber Cabling (Fluke Networks)