Electronic Design

DSP Equalization Extends 10GE Serial Transceiver's Optical Fiber Range

For several years now, 10-Gbit Ethernet (10GE) local-area networks (LANs) have been available. But while the price has been high and adoption has been slow, 10GE finally may be on a roll(out). There's a greater demand for video on the Internet, which means a need for higher bandwidth switching capacity, virtualization, more sophisticated encryption, and increased storage capacity.

Market data from Broadcom via Del Oro indicates that the number of 10GE ports will grow from a few hundred thousand in 2006 to 2 million by 2010. Speeding that adoption, Broadcom's BCM8706 10GE serial transceiver chip can be used with standard multisource agreement (MSA) XENPAK and X2 optical modules and the new SFP+ line card standard.

The BCM8706 is a XAUI to serial 10GBase-LRM transceiver. It translates between the 10-Gbit/s serial optical input and output signals and the widely used XAUI Ethernet interface (see the figure). Also, it permits the use of legacy multi-mode fiber (MMF) like OM1, OM2, or OM3 for 10-Gbit/s links in the data center.

Currently, 10GBase-LX4 optical modules with their four wavelengths ( ) of IR are being used, but at a greater cost. In the high-performance mode, the chip can deliver 10 Gbits/s at up to 300 m. In the low-power mode, it can support up to 220 m of MMF as specified in the IEEE 802.3aq standard.

The XAUI interface conforms to the IEEE 802.3ae standards and uses four 3.125-Gbyte/s paths for connections to the media access controller (MAC). It uses the standard 8B/10B error detection and a PCS 64B/65B scrambler/descrambler. A provided I2C serial interface supports external E2 (EEPROM) and XFP circuitry.

The key to the BCM8706 is its DSP electronic dispersion compensation. 10Gbit/s data does not go far on MMF simply because of its high attenuation and especially its various dispersion problems. But according to Broadcom product line manager Mike Furlong, the company's DSP is the "secret sauce" that makes it possible to provide dispersion compensation, adaptive equalization, and other digital control mechanisms at the optical receiver input to greatly extend the reach of MMF at 10 Gbits/s.

The related integrated automatic gain control has a dynamic range of 60 to 700 mV. The BCM8706 also operates from 3.3- and 1.0-V supplies and dissipates less than 1.5 W in its low-power mode. An external 25-MHz crystal or 156.25-MHz clock is required. The serial data rate is 10.3125 Gbits/s, which includes forward error correction and other overhead.

The device also will work with 1-Gbit Ethernet data. It comes in a 13- by 13mm 256-pin fine-pitch ball-grid array (FBGA) package. The core runs at 1 V, and the I/O is at 3.3 V. Power consumption is less than 1.5 W in the low-power mode. Production quantities will be available in the second quarter. Price is $60 in low volume.

Broadcom Corp.

TAGS: Digital ICs
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