Going the Distance with PCIe

TORONTO--One of the challenges flash-based storage has faced is that systems designers have been using hard disk architectures and applying them to SSDs, including PCI Express (PCIe).

PCIe, however, was not specifically designed for storage. Although it has a great deal of theoretical bandwidth, it has inherent limitations because it is not a native storage interface. It requires an onboard controller to manage resources between the flash memory and server I/O. And if I/O requests scale beyond controller thresholds, it can dramatically increase latency.
While that has led some vendors to develop alternative technologies, PCIe is still a widely-used interface that has advantages. And despite efforts to build more compact systems that keep everything close together to reduce the distance a signal must travel, sometimes it's not always possible.
Parade Technologies in Santa Clara, Calif., has opted to address this reality with something called a redriver. The high-speed interface IC supplier just introduced a two-lane 8Gb/s PCI Express and SATA redriver targeted at M.2 SSDs and other high speed peripheral applications. The PS8559 features four redriver channels enabling the support of two bi-directional lanes, and is able to handle PCIe up to 8Gb/sec and SATA up to 6Gb/sec.
Alan Yuen, senior director of marketing for Parade's high speed products, said sometimes the design of a system means a signal has to travel farther, which means in can degrade. Although the 8Gb/sec transfer rate of PCIe brings a welcomed performance increase, it also presents challenges to systems designers. At this speed, the allowable distance through PCB traces and other interconnect becomes limited because of signal distortion.
This is where a redriver comes in, Yuen said. “Redrivers help a transaction travel without errors," he said.
The PS8559, placed along the signal path or near the end of the path by the peripheral connector, cleans up and re-transmits the PCIe or SATA signal, both increasing data transmission reliability and enabling system design flexibility, Yuen said. For each of the four PCI Express or SATA channels, the PS8559 integrates a receiver, an adaptive filter that removes distortion and a transmitter, he added.

Parade's PS8559 is a bidirectional, two-lane signal redriver that supports PCIe Gen 3, with data rates up to 8.0 GT/s, and SATA, with rates up to 6 Gb/s.

Because it supports PCIe and SATA, the PS8559 is ideal for the SATA Express interface, which utilizes both of these interface standards and is used for PC storage devices, Yuen said. The support of two PCIe lanes also makes the PS8559 well suited for the newer M.2 interface used in PCs and laptops. Yuen noted that the PS8559 is optimized for low power and includes an automatic power saving mode, making it well suited for portable systems, while other uses include server-class PCIe SSDs as well as PCIe cabling.
Redrivers are not the only option for boosting signals, said Yuen. Retimers are also common and can handle very high speeds, but are common in larger systems. Redrivers are used more often in notebooks and tablets, he said, and to some extent, servers. They are also the most common and generally cheaper. “Redrivers have the benefit being easy to use and cost effective," Yuen said.
PCIe has the advantage of being very scalable, Yuen said. “Between number of lanes and speed per lane, you can get lot of performance headroom," he said. “From our perspective, PCIE has been very useful and widely adopted for high I/O.
The roadmap for PCIe sees products with gen 3.x available roughly from now until 2017, with gen 4 expected in 2018. Yuen said Intel is one the primary drivers of PCIe, and thus the market for redrivers as well.

http://www.eetimes.com/document.asp?doc_id=1329744