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Testing the limits of copper

As a rule of thumb, it is projected that 10GBASE-T will provide 10 times the speed at three times the price of 1000BASE-T ports.

May 1, 2005  

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For this month’s article, I want to provide an overview of the 10GBASE-T technology that is under development in IEEE.

I have been at-tending the IEEE 802.3an task force for about two years now and have listened to many technical contributions.

I am amazed at the expertise and sophistication that is being built into the networking electronics to counteract the effect of noise and to be able to provide a reliable transmission over twisted pair copper.

The first essential feature is that it will be backward compatible with all earlier technologies and will be capable of auto negotiation from 10 Gb/s down to 1000 Mb/s, 100 Mb/s and 10 Mb/s.

This is an important feature in order to interface with legacy equipment or to accommodate lower performance cabling or excessive noise in the environment.

The second is that 10GBASE-T uses the same MAC (Media Access Control) and Logical Link Control (LLC) layers as other 10 Gigabit technologies such as 10GBASE-SR (10 Gb/s Ethernet over multi-mode fiber) and 10GBASE-LR (10 Gb/s Ethernet over singlemode fiber).

10GBASE-T is a Physical Layer specification (PHY) that takes the data at the 10 Gigabit Media Independent Interface (XGMII) and arranges, fortifies and conditions this data into a suitable format for transmission over the twisted pair link segment, 4-pairs at the same time.

At the opposite end of the link segment, the data is recovered at the receiver, after equalization and removal of all known noise sources. In fact this is the key that unlocks the 10 GbE technology and sets the wheels into motion.

10GBASE-T utilizes a startup sequence to train the receiver to detect known sources of noise as the transmitters are turned on, one at a time, at the near end and the far end of a twisted pair link segment.

Feedback loop

A feedback loop in the receiver is programmed to remove the detected amount of noise caused by each transmitter over a prescribed interval of time.

Four echo cancellers, twelve Near End Crosstalk (NEXT) cancellers and twelve Far End Crosstalk (FEXT) cancellers are used to effectively remove most of the internal noise in the cabling.

What is left over is the external noise due to alien crosstalk and due to the environment.

Another important feature is the signal processing that is used to fortify and condition the data before it is sent over the line.

The 10GBASE-T PHY uses 16-level Pulse Amplitude Modulation (PAM) with a modulation rate of 800 million symbols per second on each of the wire pairs.

Ethernet data and control characters are encoded at a rate of 3.125 information bits per PAM16 symbol to achieve an information rate of 2500 Mb/s for each wire pair.

Two consecutively transmitted PAM16 symbols are mapped into a Double Square 128 (DSQ128) constellation with 128 possible values.

Each DSQ128 symbol carries 7 bits of information. Five hundred and twelve of these DSQ128 symbols are placed in a low-density parity check (LDPC) code block that includes redundant bits for error checking and correction. What is the net result of all this coding? The coding scheme is able to achieve a coding gain of 10 dB.

This means that it can tolerate 10 dB higher noise levels compared to an uncoded PAM signal for the same bit error rate objective of 1 bit error in 1,000,000,000,000 bits of information.

Another notable feature is that it senses the transmission characteristics of the cabling and adjusts the power and the spectrum of the transmit signal accordingly to minimize the effect of inter-symbol interference and alien far end crosstalk.

A Tomlinson Harashima Precoder (THP) is used to map the DSQ128 input into a quasi-continuous discrete time value in the range (-16, 16). The coefficients of this precoder are determined during the start-up sequence.

This THP processed symbol stream may be further processed by a digital transmit filter. It is then passed on to four digital to analog converters (DACs).

Finally, in order to minimize the alien far end crosstalk noise on short links, the power of the transmit signal is backed off in eight increments depending on the level of the receive signal.

What we have here is an application that truly tests the limits of copper. And like the processing power of personal computers all this power is available in a small package and at an affordable price. As a rule of thumb, it is projected that 10GBASE-T will provide 10 times the speed at three times the price of 1000BASE-T ports.

That is not bad at all.

Paul Kish is Director, IBDN Systems & Standards at Belden CDT Networking Division (NORDX). He is also vice chair of the TR-42 engineering committee.

Disclaimer: The information presented is the author’s view and is not official TIA correspondence.