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A review of 10GBase-T technology

Posted: 21 Oct 2011 ?? ?Print Version ?Bookmark and Share

Keywords:IEEE 802.3an? 10-Gigabit Ethernet? electromagnetic interference? optical fiber? coaxial copper?

The 2006 ratification of IEEE 802.3an, 10-Gigabit Ethernet over twisted pair standard, also known as 10GBase-T, has not led to an immediate proliferation of compliant switches and servers in data centers. However, steady advances in semiconductor lithography, and sophisticated algorithms intended to increase electromagnetic interference (EMI) immunity and lower operating power, allow networking equipment manufacturers to bring to market a slew of new switches and servers promising to make 10GBase-T the connectivity technology of choice in progressive data centers designed to reduce overall costs and improve flexibility. This article will explore the basic operation of a 10GBase-T transceiver and the inherent advantages of 10GBase-T technology as compared to alternatives, such as optical fiber and coaxial copper. Power-reducing algorithms such as Energy Efficient Ethernet (EEE) and Wake-on-LAN (WoL) will be investigated and their impact in real networks explained.

The growing importance of cloud computing C highlighted by the level of attention given it at seminal industry events such as this month's Interop C along with the increasing utilization of unified data/storage connectivity and the advent of server virtualization have elevated the popularity of 10Gbps Ethernet. While several connectivity options are available for 10Gbps Ethernet, both over optical fiber and copper cables, 10GBase-T, is arguably the most flexible, economical, backward-compatible, and user-friendly connectivity option available. It was designed to operate with the familiar unshielded twisted pair cabling technology, which is already pervasive for 1G Ethernet and can interoperate directly with it. It is capable of covering, with a single cable type, any distance up to 100m and thereby reaches 99 percent of the distances required in data centers and enterprise environments.

10GBase-T is the fourth generation of IEEE standardized Base-T technologies which all use RJ45 connectors and unshielded twisted pair cabling to provide 10Mbps, 100Mbps, 1Gbps, and 10Gbps data transmission, while being backward-compatible with prior generations. Because Base-T devices have used an auto-negotiation protocol defined by IEEE to determine the capabilities supported by the other end of the link, this backward-compatibility has meant that upgrades could be performed one end at a time, allowing quick and easy incremental improvement of network speed without either changing the wiring or forklift upgrades of equipment.

10GBase-T transceiver

Figure: Here's a block diagram of a 10GBase-T transceiver showing the major DSP blocks responsible for line equalization, LDPC forward error correction, and analog line code data transformation.

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. Figure 1 depicts an example 10GBase-T transceiver.

Handling EMI
The 128-DSQ line code, used by 10GBase-T systems and mentioned earlier, increases the number of bits per symbol when compared to prior Base-T standards. This is important since an external signal with EMI that couples to the cable's common mode and gets converted to a differential signal may cause errors on a 100-meter 10GBase-T link.

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