For over 20 years, the parallel bus interface has been the mainstream storage interconnect for most storage systems. But increasing bandwidth and flexibility demands have exposed inefficiencies in the two main parallel interface technologies: SCSI and ATA. The lack of compatibility between parallel ATA and SCSI - including different connectors, cables and software - increases costs for inventory management, R&D, training and product qualification.

While parallel technologies meet most performance requirements of today's enterprise solutions, continued demands for higher speeds, more robust data integrity, smaller designs and wider standardization cast doubt on the ability of parallel technology to economically keep pace with increasing CPU processing power and disk drive speeds. In addition, shrinking budgets are making it increasingly difficult to sustain the costs of developing and managing multiple backplane types, validating multiple interfaces and stocking multiple I/O connections.

Parallel technology poses still other challenges. Parallel transmissions are susceptible to crosstalk across wide ribbon cable paths. This crosstalk adds line noise and can cause signal errors, a pitfall that has been remedied by slowing the signal, limiting cable length or both. Terminating parallel signals is also difficult, requiring individual lines to be terminated, usually by the last drive, to avoid signal reflection at the end of a cable. Finally, parallel's large cable and connector size make it unsuitable for increasingly dense computing environments.

Introducing SAS and SATA

Serial technology, specifically Serial ATA (SATA) and Serial Attached SCSI (SAS), addresses the architectural limitations of its parallel counterparts. The technology draws its name from the way it transmits signals, that is, in a single stream (serially) compared with the multiple streams found in parallel technology. The main advantage of serial technology is that while it moves data in a single stream, it does so much faster than parallel technology because it is not tied to a particular clock speed. Serial technology wraps many bits of data into packets and then transfers the packets up to 30 times faster than parallel down the wire to or from the host.

SATA extends the ATA technology roadmap by delivering disk interconnect speeds starting at 1.5G bps. Due to its lower cost per gigabyte, SATA will continue as the prevalent disk interface technology in desktop PCs, sub-entry servers and networked storage systems where cost is a primary concern.

SAS, the successor technology to the parallel SCSI interface, leverages proven SCSI functionality and promises to greatly build on the existing capabilities of the enterprise storage connection. SAS offers many features not found in today's mainstream storage solutions. These include drive addressability of up to 16,256 devices per port and reliable point-to-point serial connections at speeds of up to 3G bps.

In addition, due to its small connector, SAS offers full dual-ported connections on 3.5-in. and smaller 2.5-in. hard disk drives, a feature previously found only on larger 3.5-in. Fibre Channel disk drives. This is an essential feature in applications requiring redundant drive spindles in a dense server form factor such as blade servers.

SAS improves drive addressability and connectivity using an expander that enables one or more SAS host controllers to connect to a large number of drives. Each expander allows connectivity to 128 physical links, which may include other host connections, other SAS expanders or hard disks. This highly scalable connection scheme enables enterprise-level topologies that easily support multi-node clustering for automatic failover availability or load balancing.

In one of its most significant advances, the SAS interface will also be compatible with lower-cost-per-gigabyte SATA drives, giving system builders the flexibility to integrate either SAS or SATA devices while slashing the costs associated with supporting two separate interfaces. As the next generation of SCSI, SAS bridges the parallel technology gap in performance, scalability and affordability.

Multiple layers of compatibility

• Physical layers
  The SAS connector is a universal interconnection that is form-factor compatible with SATA. It allows SAS or SATA drives to plug directly into a SAS environment for mission-critical applications with high-availability and high-performance requirements or lower-cost-per-gigabyte applications such as near-box storage.
  
  SATA connector signals are a subset of SAS signals that enable the compatibility of SATA devices and SAS controllers. SAS drives will not operate on a SATA controller and are keyed to prevent any chance of plugging them in incorrectly.
  
  In addition, the similar SAS and SATA physical interfaces enable a new universal SAS backplane that provides connectivity to both SAS drives and SATA drives. This eliminates the need for separate SCSI and ATA drive backplanes. This consolidation of designs greatly benefits both backplane manufacturers and end users by reducing inventory and design costs.
  
  
• Protocol layer
  SAS consists of three types of protocols, each of which is used to transfer different types of data over the serial interface, depending on which device is being accessed. Serial SCSI Protocol (SSP) transfers SCSI commands, and SCSI Management Protocol (SMP) sends management information to expanders. Meanwhile, SATA Tunneled Protocol (STP) creates a connection that allows transmission of the SATA commands. By including all three of these protocols, SAS provides seamless compatibility with today's existing SCSI applications, management software and SATA devices.
  
  This multi-protocol architecture support, coupled with the compatibility of SAS and SATA's physical connection, allows SAS to operate as the universal interconnection for both SATA and SAS devices.

Benefits of compatibility

SAS and SATA compatibility offer a number of benefits to system builders, integrators and end users.

System builders can now leverage the universal SAS/SATA connection to deploy common backplanes and common connector and cabling devices. Upgrading from SATA to SAS is as simple as replacing the disk drives. With parallel technologies, upgrading from ATA to SCSI means replacing backplanes, connectors, cables and drives. Other cost-saving benefits of compatibility include simpler validation and inventory management.

VARs and system integrators will be able to easily configure custom systems by simply installing the appropriate disk drive. This is because working with dissimilar technologies and using specialized connectors and different cabling will no longer be necessary. Moreover, the added price/performance flexibility will better enable VARs and system integrators to differentiate their products.

For end users, SAS and SATA compatibility offers a new level of price/performance flexibility. SATA drives will suit those requiring the best price advantage for servers and storage deployments, while SAS drives will deliver the highest performance, reliability and software management compatibility. The ability to upgrade from SATA to SAS drives without having to buy a new system will greatly simplify the purchasing decision, future-proof system investment and reduce the total cost of ownership.

Committee collaboration

On January 20, 2003, the SCSI Trade Association (STA) and the Serial ATA (SATA) II Working Group announced a partnership to enable SAS system-level compatibility with SATA hard disk drives.

This collaboration, as well as cooperation among storage vendors and standards committees, will further define compatibility guidelines - a move that will help system builders, IT professionals and end users more finely tune their systems to optimize application performance and reliability and reduce total cost of ownership.

The SATA specification reached revision 1.0 in 2001, and SATA products are available today from a variety of manufactures. The SAS specification revision 1.0 is targeted for release in early 2003, with product availability in the first half of 2004.

About the author

Paul Griffith is strategic marketing manager for Future I/O Technologies at Adaptec, Inc. For more information, please visit: http://www.scsita.org, or http://www.adaptec.com.