Modulation Scheme Enables 10GE Transmission over UTP Cable
QAM-encoded multi-bit per baud modulation approach handles the echo cancellation, crosstalk, reach, and cost objectives of full-duplex 10-Gbit transmission over Cat 5e and Cat 6 cables.
By
P.J. Sallaway and Sreen Raghavan, Vativ Technologies
While delivering 10 Gigabit Ethernet (10GE) transport over optical connections is growing in metro and core networks, adoption of optical-based 10GE solutions has been slowed in the enterprise data center environments due to cost drawbacks. To solve this problem, we propose a more viable transmission technique for delivering full-duplex 10-Gbit/s connections over existing Cat 5e and Cat 6 cabling for short reach, rack-to-rack data links in the corporate data center.
Delivering 10GE services over UTP cables is not an easy task. Issues such as price, power dissipation, and user manageability make it difficult for IT managers to roll out 10 Gigabit Ethernet connections using existing technologies. At the same time, by their nature, Cat 5e and Cat 6 cables were not designed to support 10-Gbit data rates and reach necessary to make copper-based 10 Gigabit Ethernet connections an effective option for enterprise and data center deployments.
This article details a quadrature amplitude modulation (QAM)-based multi-bit per baud modulation scheme that makes it possible to deliver 10GE connections over UTP cables by solving the design challenges associated with delivering full-duplex 10Gbps datarate over Cat 5e and Cat 6 cabling.
A Multi-Bit Approach
These are several existing and developing technologies that provide 10Gbps transmission; optical, 10GBASE-CX4 and 10GBASE-T. Each of these technologies has drawbacks that prevent the implementation of cost effective solutions for enterprise data centers. Traditionally, fiber-optic links traditionally more expensive than copper due to the cost of the optics while 10GBASE-CX4 requires an unwieldy, expensive InfiniBand cable and connectors. Likewise, 10GBASE-T will consume too much power and is far too complex to be a cost effective solution.
Figure 1 shows the general architecture for a multi-bit per baud modulation scheme that uses QAM encoding. The scheme shown in Figure 1 incorporates I and Q modulation to make efficient use of the signal spectrum.
Figure 1: Block diagram of the proposed 10 Gigabit Ethernet QAM-encoded modulation scheme.
Quadrature amplitude modulation (QAM) is a modulation scheme whereby multiple bits are encoded into a symbol by mapping two component values or amplitudes onto two orthogonal carriers of the same frequency. By using an in phase (I) and quadrature phase (Q) carrier, the effective bandwidth utilization is doubled. For example, using four discrete amplitudes on I and Q (-3, -1, 1, 3), there are 16 possible combinations per QAM symbol allowing up to four bits to be represented per transmitted QAM symbol.
An I/Q symbol mapping is shown in Figure 2 for a 16-QAM constellation. The resultant baud rate is 1/4th the input bit rate.
Figure 2: QAM space diagram for M = 16.
Reducing the baud rate lowers the complexity of digital signal processing required to implement the receiver functions such as adaptive equalization, echo cancellation, near-end crosstalk (NEXT) cancellation, and far-end crosstalk (FEXT) cancellation. For example, equalization is typically performed by finite impulse response (FIR) filtering consisting of multiple stages or taps. Hence, equalization filter complexity is proportional to the length of the impulse response of the UTP cable channel divided by the transmission baud period. When the baud rate is reduced, the number of taps is reduced as well.
As an example, a 1-Gigabaud equalizer with 10 taps could be reduced to four 500-Megabaud equalizers with 5 taps. This simplification of signal processing circuitry not only makes the implementation of the receiver more feasible, it also results in power savings.
Similar to 1000BASE-T, the QAM-encoded modulation scheme uses full-duplex communication to more efficiently utilize the channel bandwidth. Data is transmitted and received on the same twisted pair. Since there are four twisted pairs, the aggregate bit-rate is 2.5 Gbit/s on each twisted pair in both directions. Additional overhead for error correction and coding is necessary to obtain further signal to noise ratio benefit in the system.
Handling Impairments
A significant impairment in full-duplex transmission is the echo signal generated on each twisted pair. Since the local transmit and remote receive signals are both present at the local receiver, echo due to the transmit signal must be cancelled in the receiver to effectively deliver a 10GE full-duplex connection over these cables. In addition to echo, designers must also cancel NEXT and FEXT at the receiver to enable error-free full-duplex 10Gbps transmission.
QAM provides many advantages over an equivalent PAM solution, which are mainly based on a reduced baud rate. By combining the lower baud rate and cable length that is much less than the 100 m proposes by the 10GBASE-T task force, much less equalization is necessary. A smaller amount of equalization not only directly reduces the equalizer complexity, but allows the equalization and the FEC decoder to be decoupled, thus providing significant additional complexity reduction in the FEC decoder.
Further power advantages are found in the DSP data path, as it is also running at a low baud rate. These advantages are compounded in the NEXT, FEXT and echo cancellers as number of computations necessary is also much lower. This results in significant power and die size advantages when compared to competing solutions.
Wrap Up
The Ethernet world has reached a clear inflection point in its advancement toward ever higher bandwidths. The IEEE standards bodies have left a gap in their 10 Gigabit Ethernet standards efforts that leave data center managers with poorly defined and unmanageable 10GE link solutions. This has put cost and implementation restraints on the deployment of high bandwidth backbones to support Gigabit Ethernet networks.
By using QAM modulation with forward error correction, designers can develop full-duplex 10-Gbit/s Ethernet transceivers capable of providing low-power stacking and uplinking solutions over a single Cat 5e or Cat 6 cable for enterprise data center applications. In addition, this transceiver can be integrated into a 10GE short reach copper module that would allow data center managers to implement easily deployable, pay as you grow, 10 Gigabit Ethernet backbones for their enterprise networks.
Reference
1. "10GBASE-T Channel Criteria, PHY Vendors' Perspective", IEEE 802.3 - 10GBT Study Group, May 2003.
About the Author P. J. Sallaway is the director of DSP development at Vativ Technologies. P. J. has a bachelors degree and a masters degree in electrical engineering from the Massachusetts Institute of Technology. He can be reached at pj@vativ.com.
Sreen Raghavan is the president and CEO of Vativ Technologies. Sreen received a bachelors degree in Electrical Engineering from the Indian Institute of Technology, Madras, and received his M.S. and Ph.D. degrees in Electrical Engineering from the University of California, San Diego.He can be reached at sreen@vativ.com.
