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Maximizing 10GBase-T connectivity in data centers (Part 1)

Posted: 11 Apr 2012 ?? ?Print Version ?Bookmark and Share

Keywords:10GBase-T? RJ45 connectors? PHYs?

The 10GBase-T transceiver uses full duplex transmission with echo cancellation on each of the four twisted pairs available in standard Ethernet cables, thereby transmitting an effective 2.5Gbps on each pair. These bits are transformed into a bandwidth-reducing line code called 128-DSQ (for double square), which limits the analog bandwidth utilization of the 10GBase-T modem to 400MHz. High-performance line equalization countermands the low-pass filter effects of the transmission channel, and additional digital signal processing (DSP) functions cancel the crosstalk and echo impairments present in the cabling. Additionally, powerful low-density parity check, or LDPC, forward error correction coding rounds out some of the DSP functions and allows nearly error-free detection at close to fundamental limits in signal-to-noise ratio.

Since the DSP capabilities of the 10GBase-T transceivers result in full internal pair-to-pair crosstalk cancellation, these transceivers are limited in their performance by alien crosstalk, i.e. the undesired signal coupling between adjacent components and cabling. Bearing this sensitivity in mind, while 10GBase-T transceivers are capable of operation over various types of twisted pair cabling, Cat6A cabling was purpose-built for this type of transmission. Specifically, Cat6A cabling requirements were developed to address the alien crosstalk headroom required to support 10GBase-T over 100m of cabling containing up to four-connectors and to deliver positive signal-to-alien crosstalk margin and the extended frequency bandwidth of up to 500MHz.

While Cat6A cabling is ideal, Cat6 cabling, which was primarily targeted to support 100Base-T and 1000Base-T transceivers, may also be applicable to some 10GBase-T applications. Because the alien crosstalk in Cat6 UTP cabling is extremely dependent upon installation practices (e.g., bundling, the use of tie-wraps, and pathway fill), deployment guidelines were developed based upon a typical worst-case environment. These show that 10GBase-T transceivers should operate over Cat6 UTP channel lengths of up to 37m and may operate over channel lengths of 37 to 55m, depending upon the actual alien crosstalk levels present. The TIA TSB-155-A and ISO/IEC 24750 technical bulletins identify the additional performance headroom, as well as applicable field qualification test requirements and procedures, which must be satisfied by the installed base of Cat6 cabling in order to support 10GBase-T.

The previously mentioned 128-DSQ line code used by 10GBase-T system increases the number of bits per symbol when compared to prior Base-T standards. This is important since an external signal that introduces electromagnetic interference (EMI), couples to the cable common mode and gets converted to a differential signal may cause errors on a 100-meter 10GBase-T link.

Common EMI tests, such as those mandated by the Telcordia GR1089 standard, call for testing with field strengths of 8.5V/m. Measurements using Cat5 and Cat6 unshielded twisted pair cabling in 8.5V/m electromagnetic fields indicate that differential pickup can easily reach 60mV, thereby exceeding the voltage margin at the receiver of a 100-meter 10GBase-T link.

To contend with such EMI events, particularly in an unshielded cabling system, 10GBase-T transceivers support adaptive interference cancellation. Such an algorithm requires three key steps: detecting the interferer, identifying it, and then removing it.

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