Structured Copper Cabling in the Data Center:
Structured copper cabling standards specify generic installation and design topologies for twisted-pair media that are characterized by a “category” or “class” of transmission performance and deployed in the data center. These cabling standards are subsequently referenced in applications standards as the minimum level of performance to ensure application operation. There are many advantages to specifying standards-compliant cabling. These include the assurance of applications operation, the flexibility of backward compatibility and interoperability, and a structured topology universally recognized by professionals responsible for managing cabling.
Two organizations, the Telecommunications Industry Association (TIA) and the International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) develop structured cabling standards. Committee members work alongside applications development groups to ensure that new grades of cabling will support the latest signal transmission technologies. TIA standards are typically specified by North American end-users, while ISO/IEC standards are commonly specified in international markets.
While the technical requirements of TIA and ISO/IEC are very similar for various grades of cabling, differences in terminology can cause confusion. TIA standards for cabling components (e.g., cables, connecting hardware, and patch cords) are characterized by a performance category and are mated to form a permanent link or channel that is also described by a performance category.
ISO/IEC standards characterize components by performance category, and permanent links and channels are described by a performance class. TIA and ISO/IEC equivalent grades of performance are characterized by bandwidth as shown in table 1.
Category 5e/class D, category 6/class E, and category 6A/class EA media are available in unshielded twisted-pair (UTP) and shielded twisted-pair (F/UTP) constructions. Class F and class FA media are supported by fully-shielded twisted-pair (S/FTP) constructions. Siemon recommends the use of shielded and fully shielded systems in the data center to ensure the highest levels of signal immunity and performance margin.
Whether upgrading an existing or developing a new data center, it is critical to look to the standards for guidance on performance and lifecycle. Both TIA and ISO/IEC state that cabling systems are intended to have a useful life in excess of 10 years. Since applications, such as ethernet, typically have a useful life of five years, cabling should support two generations of network applications. For most data centers, this means specifying a cabling plant capable of supporting 1000 BASE-T (gigabit ethernet) today and an upgrade to 10G BASE-T in five years.
TIA categories and ISO/IEC classes of structured cabling recognized for support of applications in the data center are specified in table 2.
CATEGORY 5E/CLASS D
Category 5e/class D cabling requirements were first published in 2000 in order to address the additional transmission performance required by applications such as 1000 BASE-T that utilize bi-directional and full four-pair transmission schemes. The specification added headroom to category five performance limits and characterized several new transmission criteria required for support of gigabit ethernet over a worst-case four-connector channel. To ensure that additional performance margins were satisfied, category 5e/class D specifications added headroom to the parameters of NEXT loss, ELFEXT loss, and return loss and introduced the characterization of crosstalk using power summation, which approximates the total crosstalk present when all pairs are energized as in a four-pair transmission scheme.
Although category 5e/class D cabling may be found in legacy data centers, it does not support the high data-transfer speeds required in new data centers.
CATEGORY 6/CLASS E
Category 6/class E cabling delivers double the signal-to noise margin of category 5e/class D cabling and provides the performance headroom desired by end-users to ensure that their data centers can withstand the rigors of the cabling environment and still support 1000 BASE-T. Category 6/class E cabling specification development also highlighted the need to limit the conversion of differential mode signals to common mode signals and vice versa through the characterization of component balance, resulting in improved electromagnetic compatibility (EMC) performance.
Although category 6/class E cabling was primarily targeted to support 100 BASE-T and 1000 BASE-T applications, a small portion of the installed base of category 6/class E cabling can support the 10G BASE-T application. However, the use of category 6 cabling to support 10G BASE-T is limited to reduced-length channels in legacy installations and is subject to additional alien crosstalk mitigation and testing requirements that may prove challenging and time-consuming.
To ensure 10G BASE-T capability, Siemon recommends standards-compliant category 6A or higher cabling for data center environments. This position is supported by TIA and ISO/IEC data center standards, which explicitly specify that the minimum grade of cabling to be deployed in the data center is category 6A.
The standards message is clear: category 6 is not recommended for new 10 Gb/s data center deployments.
CATEGORY 6A/CLASS EA
Category 6A/class EA cabling requirements address the extended frequency bandwidth and alien crosstalk margin required to support 10G BASE-T over 100 meters of cabling containing up to four connectors. Category 6A/class EA cabling delivers positive signal-to-alien crosstalk margin up to 500 megahertz (MHz) and is recommended as the minimum grade of cabling capable of withstanding the rigors of the data center environment and supporting 10G BASE-T. Balance requirements for channels and permanent links are also specified for the first time, thereby ensuring better EMC performance than any previous generation of cabling. Category 6A/class EA cabling provides the maximum return-on-investment when calculations are performed using a 10-year lifecycle.
Category 6A/class EA cabling standards are especially critical in light of recent marketing messages advocating the use of non-compliant cable in 10G BASE-T channels as a cost-saving option. Constructed with 26 American wire gauge (AWG) (0.4 mm) conductors and deployed over short-length channels, these cables can be a risky proposition for data centers. Twenty-six AWG conductors do not comply with any TIA or ISO/IEC Standard for horizontal cable requirements, which mandate the use of 24 AWG (0.5 mm) or larger conductors. Even in environments where reduced-length channels can be accommodated by design, 26 AWG cabling has performance drawbacks; including the inability to take advantage of the power savings associated with operating 10G BASE-T in short-reach mode, excessive heat rise when deploying PoE Plus and compromised ability to support expansions, equipment additions, and system upgrades.
ISO/IEC Class F requirements were published in 2002 and describe performance criteria for a fully shielded media type (e.g., cabling with an overall shield and individually shielded pairs). Category F cabling delivers positive attenuation-to-crosstalk margin up to 600 MHz.
Due to its ease of use, performance headroom, ability to support multiple applications under one sheath, and its specification as the recommended category 7 interface in the ISO 15018 Standard, the non-RJ style plug and socket interface specified in IEC 61076-3-104:2002 (e.g., Siemon’s TERA) is the most commonly specified category 7 connector. This interface is commercially available from multiple manufacturers whose products are interoperable.
The significant enhancement in class FA specifications is the extension of the frequency band from 600 MHz to 1,000 MHz. This enhancement allows class FA cabling to be uniquely capable of supporting all channels of broadband video (e.g., CATV) that operate up to 862 MHz. As a result of this extremely high bandwidth performance and unsurpassed EMC performance, class FA specifications have become the de facto requirements for fully-shielded cabling.
It is interesting to note that, although TIA is still investigating category 7A at this time, it is acceptable to specify class FA cabling in North American markets. The rationale for this is that, in addition to being recognized by BICSI, NEMA, IEEE, and other standards organizations, class FA is simply a superset of TIA category 6A requirements.
The advantage that class FA has over other grades of cabling is that it is most capable of being able to support the next Ethernet application beyond 10GBASE-T. However, Robert Carlson, Siemon’s vice president of global marketing, cautions, “In the absence of a call-for-interest by (Institute of Electrical and Electronics Engineers) IEEE, support claims for beyond 10 Gb/s applications cannot be verified. Any such claims are misleading.”
Table 4 provides comparative channel performance data at 100 MHz for category 5e/class D, category 6/class E, category 6A/class EA, class F, and class FA channels. Where there is a slight difference between TIA and ISO/IEC performance limits, ISO/IEC performance limits are indicated in parenthesis.
When designing and installing data center cabling systems, choose the strongest foundation to support your present and future network applications needs. To ensure support of emerging technologies that utilize the latest advances in signaling schemes, it is critical to be as informed as possible. Trust the TIA and ISO/IEC standards developmental groups to specify complete cabling criteria capable of providing applications assurance for tomorrow's technologies today.