SAS and Serial ATA Tunneling Protocol (STP)

Author: Michael Micheletti, Product Manager, LeCroy Corp.

Attached SCSI (SAS) is a storage interface developed to meet
the needs of enterprise-class storage applications. Leading
solution vendors are rapidly adopting this new standard because
it offers superior performance while preserving compatibility
with legacy SCSI applications. Central to the SAS value proposition
is its ability to scale storage systems beyond the limits
of today’s parallel SCSI by using a switching technology
called expanders. Designed to provide a fan-out architecture
for SAS, expanders allow SAS initiators to connect up to 16,000
physical links per domain. Yet this is only one aspect of
SAS’ scalability as an enterprise-class storage interface.

important advantage of SAS is interoperability with Serial
ATA (SATA) disk drives. The SAS specification defines an electrical
and physical connection interface within the backplane that
is form-factor compatible with SATA. This feature is ushering
in a new approach to storage system design, enabling manufacturers,
integrators and users to populate a single storage enclosure
with either SAS or SATA disks. Enterprise-class
storage systems can be equipped with high performance SAS
drives for mission-critical applications. The same storage
enclosure can also be introduced with lower cost SATA
drives to address segments of the market that value capacity
over performance.


  • Product segmentation–a single storage platform
    can deliver performance, capacity or a combination
  • Lower design costs–a single backplane design
    can be leveraged across multiple market segments
  • Supply-chain
    management–reduced inventory cost throughout
    the distribution channel
  • Lower
    procurement costs–higher volumes drive lower
    prices for cabling and connectors
  • Flexible
    upgrade options–users can easily reconfigure
    existing storage resources for performance or capacity




Tunneling Protocol (STP) is the foundation for supporting
SATA devices within SAS storage enclosure subsystems. Host
Bus Adapters (HBAs) and expanders that support STP/SATA bridging
exhibit several unique capabilities, including the ability

  • Establish
    connections between SATA target devices and the SAS domain
  • Respond
    to SATA reset sequences
  • Send
    and receive SATA Flow Control primitives including R_RDY
    & HOLD
  • Accept
    SATA Frame Information Sequences (FIS)

attach directly to SATA drives and tunnel SATA frames over
SAS for transport to the SAS initiator. Once an STP connection
is open, the SATA host capability (within the SAS HBA) and
the SATA device communicate as if they were directly attached
on a physical link. A bilingual analyzer has already been
designed to support SAS, STP and pure SATA protocol traffic.
In addition to showing SSP frames, it clearly communicates
STP transactions including OPEN address frames initiated by
either the host or STP target port.


OPEN is transmitted by the initiator port attached to
channel 1. An expander port (acting as an STP target
) responds with Arbitration in Progress (AIP). An STP
connection is established when Open Accept is transmitted
and the Initiator port sends a Register Host-to-Device
FIS (READ DMA EXT) to the STP target port.


Another key differentiator for enterprise-class storage systems
has been fault tolerance within the storage architecture via
support of redundancy for all components in the array. The
SAS protocol specification, like Fibre Channel, has native
support for dual-port drives. In addition, SAS also supports
multi-initiator access. These capabilities provide redundant
pathways within the SAS subsystem, and are an important element
in making SAS robust enough to span a broad
range of applications.

By comparison,
the ATA protocol is not capable of multi-initiator access
or true dual-port capability. SATA devices have no notion
of multiple SATA hosts. SATA devices maintain only a single
ATA task file register image. Within a SAS domain, it’s
possible that more than one STP initiator port might be vying
for access to the same SATA device. This can impact performance
or create potential deadlock conditions.

SAS initiators
rely on SL_CC (connection control) link layer state machine
as the primary mechanism for managing STP connections. In
the example above, an STP target port establishes a connection
with an STP initiator port by responding to STP_Open with
Open_Accept. The SL_CC1 arb select state machine transitions
to SL_CC3 connected by transmitting connection open confirmation.
Once the connection is open, the STP target port (in the expander)
rejects all subsequent connection requests from STP initiator
ports by sending an OPEN_REJECT (STP Resources Busy) message
to the SL transmitter.

are used as an added safeguard to prevent device contention,
in particular when systems (hosts) are added to a configuration
(ie: during PHY resets). Affiliations use register settings
to provide exclusive access to a SATA or STP device for the
duration of a command to ensure coherent access to the SATA
device. The AFFILIATION VALID bit is set to “1”
when a valid STP INITIATOR SAS ADDRESS initiates a transfer.
This serves to lock communication to the STP target until
the connection is closed with CLOSE (CLEAR AFFILIATION) or
PHY reset is transmitted.

This graceful
handling of multiple-initiator environments ensures SAS and
SATA devices will interoperate seamlessly within an STP-enabled
storage enclosure. Together, SAS and SATA technologies will
provide better flexibility and lower overall solution cost
for high volume servers and storage subsystems.

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