Electromagnetic interference and 10GBase-T in the data center

This vendor-written tech primer has been edited by Network World to eliminate product promotion, but readers should note it will likely favor the submitter's approach.

The majority of data center LAN links use 1000Base-T (Gigabit Ethernet) running on unshielded twisted pair structured cabling (Cat5e, Cat6, Cat6a). While increasing demand for bandwidth is driving a shift to 10G Ethernet, that migration is being slowed by the fact that the 10G links being deployed today are based on optical transceivers or SFP+ Direct Attach copper, neither of which is backward-compatible to Gigabit Ethernet.

Rapid technology advances are reducing the price and power of twisted pair-based 10GBase-T and inevitably there will be a rapid shift from Gigabit Ethernet to 10GBase-T.

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This vendor-written tech primer has been edited by Network World to eliminate product promotion, but readers should note it will likely favor the submitter's approach.

The majority of data center LAN links use 1000Base-T (Gigabit Ethernet) running on unshielded twisted pair structured cabling (Cat5e, Cat6, Cat6a). While increasing demand for bandwidth is driving a shift to 10G Ethernet, that migration is being slowed by the fact that the 10G links being deployed today are based on optical transceivers or SFP+ Direct Attach copper, neither of which is backward-compatible to Gigabit Ethernet.

Rapid technology advances are reducing the price and power of twisted pair-based 10GBase-T and inevitably there will be a rapid shift from Gigabit Ethernet to 10GBase-T.

ANALYSIS: What's beyond 10G Ethernet?

While 10GBase-T is similar to Gigabit Ethernet -- both use RJ45 connectors, a structured cabling deployment model with twisted-pair wiring and 100-meter maximum spans, and 802.3 clause 14-based auto-negotiation for backward compatibility -- it also comes with some unique challenges due to the 10x higher data rate.

Providing the 10x rate required higher symbol rates, more bits per symbol and the use of a higher-performance and higher complexity forward error correction scheme in the PHY chips. These requirements translated to increased complexity, which led to high power consumption in the first generation of 10GBase-T chips.

This has been a barrier to adoption, but a combination of design innovation and semiconductor process advances have resulted in 10GBase-T chips that now operate at lower power per bit than the most efficient Gigabit Ethernet chips.

However, the higher symbol rate and the increased number of signaling levels required by 10GBase-T also increase potential vulnerability to electromagnetic (EM) interference, or EMI, from external fields. This article explains why and also discusses techniques to handle the increased vulnerability to EMI.

Key differences

10GBase-T uses four-pair balanced cabling so the 10x higher rate increase comes from two reasons: an increase in the symbol rate to 800MSymbols/sec (from 125MSymbols/sec in Gigabit Ethernet, a factor of 6.4) and an increase in the number of bits per symbol to 3.25 bits/symbol (from 2 bits/symbol in Gigabit Ethernet, a factor of 1.6).

The 6.4x increase in symbol rate causes the 10GBase-T transmit signal spectrum to extend up beyond 400MHz, so the receiver has to allow an input bandwidth close to 500MHz. The bandwidth for a Gigabit Ethernet receiver was 75MHz.

This increase in the required receiver bandwidth is an increase in the window of vulnerability to external interference. A Gigabit Ethernet receiver could easily filter out interfering signals at frequencies above 75MHz but a 10GBase-T receiver cannot do this as a filter cutting off above 75MHz would kill a significant percentage of the desired 10GBase-T.

In a Base-T link, external fields create a common mode signal on the twisted-pair wire. Most of that signal gets attenuated by the choke and the transformer that is built into every Base-T Ethernet port. However, some of this gets converted to a differential signal due to imbalances between the two paths of a differential pair in the cabling, as well as connectors and magnetic components in the signal path.