Understanding the NexSAS™ Serial Attached SCSI Product Family

Author: Sam Barnett, Product Line Manager for Serial Attached SCSI and Serial ATA,
Vitesse Semiconductor

The NexSAS™ family of Serial Attached SCSI (SAS) products from Vitesse offers server and storage enclosure OEMs, ODMs, and contract manufacturers (CMs) the industry’s most comprehensive portfolio of solutions for the emerging SAS market. Comprised of edge expanders, fanout expanders, RAID-on-CHIP offerings, intelligent mux/de-mux devices, discrete enclosure management/SAS backplane controllers, and rate-agile signal conditioners, the NexSAS™ line delivers on the promise of high performance storage solutions for SMB (small/medium business) as well as enterprise systems.

The Vitesse NexSAS™ product family is divided into four functional areas: expander technology, RAID-on-CHIP, SAS/SATA Support devices, and Enclosure Management. This article explores the NexSAS™ expander and RAID-on-CHIP technology in detail.

NexSAS™ Expander Technology

Designed with high-performance interconnect in mind; the NexSAS™ expander family takes advantage of decades of proven switching technology. A single functional design insures scalability from low-port offerings to the high-port-count devices needed for blade servers or storage enclosures. Figure 1 outlines the basic functional block diagram of all expanders in the NexSAS™ family.


Figure 1: Expander Functional Diagram


All Vitesse expanders divide the core systems function into two separate components – the expander function and the management function. Included in the expander functional block are the Expander Connection Manager (ECM), Expander Connection Router (ECR), the Broadcast Primitive Processor (BPP), and the actual physical interfaces (PHYs) to the device (ranging in number from six to 36 depending on application need).

Rounding out the core functions is the management function block, which provides for expander management (SMP) and vendor-specific enclosure management functions (SSP/SES). Also included in this block are various peripheral interfaces needed for interaction in the overall system. These interfaces include two-wire serial, JTAG, UARTs, general purpose I/O (GPIO), and serial peripheral interfaces. The number and types of these interfaces vary with the particular expander and specific operating mode(s) employed.

General Features

All Vitesse expanders share a common set of features:

  • High Performance Switching Architecture – low latency, non-blocking switching matrix that provides up to 54 Gb/s of aggregate switching capacity (in large-port-count expanders with linear scaling for lower-port-count expanders).
  • Vitesse Universal PHYs – self-configuring, rate adaptable PHYs supporting SAS initiators and SAS/SATA targets at both 1.5 and 3.0 Gb/s data rates; multiple per-PHY pre-emphasis/de-emphasis capability (granularity depends on series), and multiple per-PHY output levels (granularity depends on series) build on over 20 years of Vitesse long-haul and ultra-long haul transport technology. They may be combined into SAS “wide-ports” of up to n-PHYs wide.
  • Virtual Initiator and Target Capability – SMP (Serial Management Protocol), SSP (Serial SCSI Protocol), and STP (Serial ATA (SATA) Tunneling Protocol) initiator and target functions enable robust value-added services including self-configuration, self-discovery, SES-mastering, rouge drive identification, etc.
  • Integrated Enclosure Management Subsystem – robust EM capabilities maximize OEM investment in legacy firmware/software/custom features and allow flexibility in system designs.

Since each product in the expander family is equipped with an identical set of base features, differentiation is delivered based on the application space being addressed. Vitesse recognizes two distinct application spaces – Servers and Enclosures.

Server Series Expanders

Vitesse Server Series Expanders address the specific needs of the SMB and Enterprise server OEM and ODM. Each member of the Server Series (the VSC7149 6-PHY expander, VSC7150 12-PHY expander, VSC7151 9-PHY expander, VSC7160 12-PHY expander with table-routing, and the VSC7161 10-PHY expander with table-routing) was designed to optimize package, power, and I/Os for the server market.

With the addition of a 160-entry content-addressable-memory-based routing table, the VSC7160 and VSC7161 are ideal for usage either in expandable enterprise servers or small enclosures (see Figure 2).


Figure 2: Typical Usage Model for Server Series Expanders in an Embedded Server Application


Enclosure Series Expanders

Enclosure Series expanders are identical in base features to the Server Series. In additional to these features, each enclosure series expander (including the VSC7154 36-PHY expander, VSC7153 24-PHY expander, and VSC7155 18-PHY expander) has been enhanced to better address the needs of the storage enclosure market. A summary of the additional features follows:

  • Enhanced High-speed I/O – more density and granularity in output levels, programmable edge rate controls, and improved signal equalization (all on a per-PHY basis) delivers freedom to system designers without compromising signal integrity.
  • Improved Routing Table – increase to 320 entries effectively doubles system address space in support of large volume stores.
  • Larger On-Chip EM Memories – gives value added features more “room to grow and run”

(See Figure 3)


Figure 3: A Storage Subsystem Comprised of Enclosure Series Expanders


Next Generation Expanders (Generation 1.5)

Recent additions to the NexSAS™ family include the VSC7156 24-PHY and VSC7157 36-PHY expanders. Like their predecessors, the VSC7153 and VSC7154 (referenced above), these devices are also targeted at both enterprise server and enclosure applications. These evolutionary expander products offer a multitude of new enterprise-class features to the storage OEMs seeking advanced functionality and product differentiation.

With the feature set offered in the VSC7156 and VSC7157, OEMs are able to build more integrated, higher function storage solutions. These feature additions bridge the gap between the SAS 1.1 and planned SAS-2 specifications and raise the bar on both functional integration and price/performance for today’s SAS market. Features include:

  • End-to-End Address-based zoning – for control, security, and performance (much like Fibre Channel today)
  • Enhanced Multi-affiliation STP/SATA Bridge – providing multi-initiator operation for Serial ATA end-devices
  • Wholesale improvements in table routing capacity – making more dense arrays and subsystems possible
  • Growth in on-chip user-accessible enclosure services memory – giving the OEM more flexibility in enabling differentiating features in firmware/software
  • Broad new statistics functions – enable accountability and control
  • Pin and firmware compatibility with previous generation NexSAS™ expanders – deliver on the promise of re-use for existing designs

NexSAS™ RAID-on-CHIP (ROC) Technology

To address the growing need for end-to-end SAS solutions, Vitesse introduced the VSC7250 and VSC7251 single-chip RAID controllers. Combined with enterprise class RAID software suite, the fully integrated NexSAS™ ROC solutions provide a cost effective, high-performance alternative to traditional RAID products that require a separate IO processor and a SAS/SATA protocol chip.

The NexSAS™ ROC solution offers PCI-Express host connectivity and up to eight independent SAS PHYs, which support direct or expander connections to both SAS and SATA targets as well as other SAS 1.1 compliant devices.

The VSC7250 is ideal for higher performance applications that requires up to eight PCI-Express lanes and eight SAS PHYs. The VSC7251 is the stripped down version of the VSC7250 that provides the same RAID feature set but with four-lane PCI-Express interface and four SAS PHYs.

The NexSAS™ ROC product family integrates a high-speed DDR2 memory controller and XOR hardware acceleration. Each SAS PHY supports 1.5/3.0 Gb/s SAS and SATA connections. SAS PHYs can be combined to create wide ports in any width up to eight. In addition, both the VSC7250 and VSC7251 provide a Flash/NVSRAM/SRAM interface to simplify system design. The integrated MIPS 5Kc processor and dedicated hardware acceleration engines combine to deliver the best price/performance SAS ROC solution in the industry.

Along with commercially available software offerings, the NexSAS™ ROC product family provides enterprise class RAID features for greater data protection and management (offerings include RAID 0, 1, 5, 5E, 5EE, 50, 6, 10, snapshot or copyback).

(See Figure 4)


Figure 4: Typical VSC7250 Application in the ROMB (RAID-on-Motherboard) Space


Features of the VSC7250 and VSC7251 are outlined below:

  • RAID-on-Chip – a fully integrated solution that reduces bill-of-materials costs and minimizes printed circuit board real estate
  • PCI-Express host interface – the VSC7250 supports up to eight host processor connections via x1, x2, x4, or x8 lane PCI-Express links while the VSC7251 supports up to four PCI-express links
  • Up to eight SAS/SATA PHYs – provide direct or expander-based connections to SAS or SATA HDDs or other SAS 1.1 compliant devices
  • Embedded Microprocessor Complex – RAID-optimized processor offloads host CPU from RAID calculations as well as volume rebuilds and enables open source driver support
  • On-Chip RAID 5 and 6 Acceleration – Build-in hardware XOR accelerator offloads RAID parity calculation for greater data protection and performance
  • DDR-II External Memory Interface – High-performance memory interface supports optimized I/O and RAID processing

NexSas™ Summary

The following section provides a “decoder” ring for the various members of the NexSAS Serial Attached SCSI product family.

Server Series Expander Devices

VSC7149 6-PHY Edge Expander
VSC7150 12-PHY Edge Expander
VSC7151 9-PHY Edge Expander

VSC7160 12-PHY Edge/Fan-out Expander with Table Routing and SSP
VSC7161 10-PHY Edge/Fan-out Expander with Table Routing and SSP
VSC7162 12-PHY Generation 1.5 Expander with Advanced Features
VSC7163 10-PHY Generation 1.5 Expander with Advanced Features

Enclosure Series Expander Devices

VSC7153 24-PHY Edge/Fan-out Expander with Table Routing and SSP
VSC7154 36-PHY Edge/Fan-out Expander with Table Routing and SSP
VSC7155 18-PHY Edge/Fan-out Expander with Table Routing and SSP

VSC7156 24-PHY Generation 1.5 Expander with Advanced Features
VSC7157 36-PHY Generation 1.5 Expander with Advanced Features

RAID-on-CHIP Devices

VSC7250 8-PHY PCI-Express to SAS Single Chip RAID Controller
VSC7251 4-PHY PCI-Express to SAS Single Chip RAID Controller

SAS/SATA Support Devices

VSC7170 2:1/1:2 SAS/SATA Active/Active Mux/Port Multiplier
VSC7171 Enhanced 2:1/1:2 SAS/SATA Active/Active Mux/Port Multiplier
VSC7173 Enhanced 2:1/1:2 SAS/SATA Active/Passive Mux/Port Multiplier
VSC7177 2:1/1:2 SAS/SATA Active/Passive Mux/Port Multiplier

SAS Backplane Controller/Enclosure Management Controllers

VSC410 SAS/SATA Backplane Controller
VSC420 SATA Backplane Controller
VSC425 Enhanced SATA Backplane Controller


In all, the future of SAS storage systems based on NexSAS™ technology are dependent on the needs of the end-user. End-to-end solutions like those provided by Vitesse enable unique OEM applications while features of discrete components give way to multiple usage models.

With any new technology, be it SAS or something else, one size will never fit all. Proper solutions will differ by application, connectivity requirements, scalability, performance, and price sensitivity – all functions of innovation.


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