- 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.
Modular Patch Cord Guidelines
How To Ensure Optimium Network Performance
In the competitive world of cabling system design, the price of an installation can be a significant factor when securing new cabling business. When submitting bids for category 5, 5e and 6 jobs, it is often tempting to substitute less expensive components to reduce costs. There are significant risks associated with using lower quality patch cords that are tested only for electrical continuity and not transmission performance. Modular patch cords with poor transmission performance can bring a single user or an entire network down, resulting in significant losses in productivity. Unfortunately, many cabling system designs today still overlook the importance of specifying modular patch cords with factory controlled and tested performance. Considering the wide performance variances due to the combined electrical interactions of two plugs and a short length of cable, the modular patch cord assembly can easily be considered the most significant hurdle to achieving consistent overall transmission performance. To ensure proper system support, modular patch cords must be carefully constructed to ensure component backward compatibility, interoperability, and resistance to damage from moves, adds and changes, in addition to exhibiting tightly specified crosstalk and return loss transmission performance. Understanding the challenges associated with manufacturing high quality patch cords and the risks associated with using inferior patch cords will allow customers to make an informed purchasing decision.
Modular Plug with contact blades
An appreciation of the difficulties associated with producing high quality patch cords begins with an understanding of the performance limitations of the weakest link in a modular cable assembly - the modular plug. The physical geometry of modular plugs used in patch cords makes them more susceptible to crosstalk problems than other cabling components. The large contact blades positioned in parallel at the "nose" of the plug electrically form capacitive plates, which are significant sources of signal coupling. At the entrance of the plug, the cable pairs are untwisted and split as a result of the termination process. This disturbance to the natural cable lay (the geometry of the cable conductor pairs) interferes with the cable's ability to remain immune to crosstalk interference at this location. To complicate matters, the "split pair", (i.e., two conductors of a pair are separated by another pair) construction causes conductors to cross over one another, contributing significantly to signal coupling (See Figure 2). These factors make modular plugs the greatest single crosstalk contributor in a structured cabling system.
Split pair illustration.
A.) Primary Strain Relief
B.) Split Pair
Compounding the inherent design limitations of the modular plug are the wide range of manufacturing variances associated with termination of cable pairs to the modular plug. To maintain optimum manufacturing control and transmission performance, it is critical that the length of wire untwisted is minimized and the conductor pairs are positioned consistently with respect to each other on every single plug termination. Longer untwisted pairs will hinder a cable's ability to reduce crosstalk, resulting in NEXT performance outside of the acceptable margins described below. Only a quality controlled manufacturing environment can ensure that plug termination practices are maintained on a consistent basis.
Mechanical crimp disturbs normal cable geometry.
The process of securing the cable to the plug also plays a critical role in contributing to the final transmission performance of a terminated modular plug. Typical of field-termination practices, a mechanical crimp is commonly used to provide cable strain relief. Very often when a modular plug is crimped, the primary strain relief mechanically crushes the conductor pairs, disturbing the normal geometry of the cable. The crushing action results in deformed conductors and conductor pair invasion, which contributes to both crosstalk and signal reflections (return loss).
Alternately, a factory-controlled plug restraint technique typically involves enveloping the cable jacket over 360 degrees using insert molding. This method eliminates the crushing action of the typical mechanical strain relief and ensures consistent pressure at the strain-relief point from plug to plug. Only a controlled factory environment and advanced manufacturing techniques can ensure consistent and non-damaging strain relief. Another common problem associated with field or improperly terminated patch cords is the amount of movement in the untwisted pairs and through the length of the stranded cable itself. A manufacturer must not only produce cords that meet very narrow transmission performance targets, but they must hold up under a great deal of abuse in the field. Cords used with modular furniture are often pulled through modular furniture or other tight areas, placing significant strain on the cable-plug interface. In other patching situations even normal movement of cords can result in performance variances. Engineers who are experienced with link and component testing know that movement in the test leads can easily vary return loss test results by 3 dB, as well as adversely affect crosstalk performance. It is interesting to note that most field-terminated and improperly manufactured modular cords do not stand up under such adverse conditions. In particular, the mechanical crimp is not sufficient to properly restrain the cable and prevent movement of the pairs. Producing patch cords that not only meet performance requirements but are durable enough to withstand the rigors of day-to-day use is challenging even in a factory environment. As a minimum, experienced operators, precisely controlled materials and processing, and advanced transmission testing are required.
Compounding the manufacturing challenges described above are the precise performance targets that patch cords must meet to achieve proper system performance. To ensure expected channel performance, as well as backward compatible and interoperable mated component performance, modular patch cords must be constructed to fall within a carefully specified crosstalk performance range. For category 6 patch cords to be backward compatible and interoperable with category 5e components, the NEXT loss performance of the terminated modular plugs must consistently fall between 34-38 dB at 100 MHz. The requirements for assured category 6 interoperability are even more difficult to satisfy. To ensure category 6 patch cord interoperability with category 6 outlets, modular plug NEXT loss performance must fall within a range of 36-38 dB - a mere 2dB margin. Achieving performance within this range is challenging considering the many electrical interactions that make up the crosstalk contribution of one terminated modular plug. While modular outlets can be designed to compensate for crosstalk, a single outlet design can not compensate for the wide range of crosstalk levels associated with poorly controlled modular plug terminations.
There are definite risks for both installers and customers who choose to subject inferior or field-terminated modular patch cords to the stress and strain of daily use. Since special test cords are usedfor performance verification of cabling links, the effects of cords used in cabling channels are not evident until after the network is operational. Channel performance is the sum of the performance of all the connecting hardware and cable. A defective patch cord's poor performance at a component level can be masked when tested as part of a channel that has significant headroom. Later, when the same cord is used in a different channel with less headroom, problems are likely to surface. Also, if the cable is not properly secured and pairs move or shift within the plug, the cord's return loss performance can vary greatly, as much as 3db as mentioned previously. Such a cord may perform well initially, yet fail later when moved or unplugged and reinserted. The expense associated with downtime and troubleshooting these types of problems can add up quickly.
There is also the risk that marginal patch cords may work well today yet will fail when used with the next generation of high speed data applications coming in the near future. The performance of category 5e and 6 structured cabling systems far exceeds the bandwidth requirements of currently available applications. Today, most customers are running 10BASE-T (10 Mbps) and 100BASE-T (100Mbps) applications. A marginal category 5e or 6 patch cord may perform adequately for these lower speed applications. However in the next 2-3 years gigabit Ethernet (1,000Mbps) applications and equipment are expected to become available and will begin to test the limits of category 5e and 6 systems. When this occurs, the poor performance of defective modular patch cords will become apparent and will result in channel and application failures. At best, a marginal patch cord failure can bring down a single user. More than likely, the LAN equipment of an individual user experiencing high error rates will persistently attempt to retransmit packets, causing network congestion for all users. At worst, if the patch cord is used in the backbone, it can cause an entire system to crash. The resulting losses in productivity and technician labor to troubleshoot these problems can far exceed the savings resulting from using inferior products.
Avoid Network Downtime with Quality Modular Patch Cords
To ensure proper performance today and tomorrow, The Siemon Company utilizes the highest quality components to manufacture modular cords. It is recommended that raw patch cable satisfy the more stringent impedance requirements of ISO/IEC 11801, in addition to the ANSI/TIA/EIA-568-A cordage requirements. All Siemon modular cords are constructed using specifically designed high performance stranded cable that satisfies both the TIA and the more stringent ISO impedance and return loss requirements. Additionally, Siemon modular plugs have 50 microinches minimum of gold plating over 100 microinches of nickel. This high quality plating results in superior transmission performance and long-term resistance to corrosion from humidity, extreme temperatures and airborne contaminants. Using the highest quality components helps combat the inherent performance limitations of modular plugs and the cord's ability to withstand the rigors of daily use.
Siemon MC6™ Patch Cord with KeyBar™ Technology.
The Siemon Company also employs advanced, patented manufacturing techniques to produce premium quality patch cords with precise performance characteristics. Siemon MC6™ modular patch cords utilize a KeyBar™ (or load bar) which precisely arranges and maintains the position of conductors to the point of termination, ensuring optimum and consistent pair balance. This technique ensures proper positioning of conductors, especially the critical "split pairs". In addition to KeyBar™ Technology, all Siemon patch cords utilize an insert-molded strain relief boot. The boot provides superior cable to plug retention and maximizes performance by preventing the pair deformation commonly caused by a mechanical strain relief. The patented boot design also accepts colored icon tabs for circuit identification and has integral snag protection and bend relief. These design features allow Siemon to produce cord assemblies with consistently high performance as well as enhanced durability and reliability. While it is possible to manufacture patch cords with lower cost materials and production techniques, the end result an inferior product. The combination of high quality and superior manufacturing techniques is the cornerstone of Siemon modular patch cord performance and reliability.
Siemon Modular Patch Cord with insert-molded strain relief boot.
To ensure Siemon modular patch cords deliver guaranteed category 5e or 6 channel performance, The Siemon Company transmission tests 100% of its modular patch cords on a laboratory grade network analyzer and a copy of the test results is included with each shipment. Siemon is the only manufacturer to have consistently applied this level of quality assurance to the cable assembly process since 1994 and to our knowledge is the only manufacturer to do so today. Most manufacturers only test patch cords for electrical continuity. Other manufacturers may randomly test a sample cord, however this approach does not guarantee the performance of all cords. Some important questions to ask when purchasing modular patch cords are whether the manufacturer 100% transmission tests all cords, what type of test equipment was used and what was the test methodology. The Siemon Company is confident that each modular cord we produce will meet its respective category, backward compatibility and interoperability requirements.
Siemon 100% Transmission Testing.
As with all Siemon products, The Siemon Company stands behind its modular patch cords with a written warranty covering both product performance and applications support when installed by a Siemon Certified Installer as part of a complete Siemon cabling solution.
Compromising performance or quality when specifying modular patch cords risks the integrity of the entire network as well as the ability to support future high- speed networking applications.