Ultra3 SCSI promises data throughput of up to 160Mb/s along with
full backward compatibility
On September 14, 1998, seven vendors, representing a broad
cross-section of the computer system and storage industry,
announced support for evolutionary changes to the Small Computer
System Interface (SCSI) that increases performance, reliability and
manageability. Ultra3 SCSI doubles transfer rates from 80 to 160Mb
per second, improves manageability by automatically testing the
interfaces performance level and increases reliability by adding
Cyclical Redundancy Checks (CRC). When Ultra3 SCSI is used with
low-voltage differential (LVD) signalling, cable lengths of 12m are
maintained providing full backward compatibility.
The SCSI technology continues to evolve and its evolution is
successfully meeting the increasing demand for I/O bandwidth. The
SCSI interface has the stability, ease of connectivity, large
installed base and a 15-year heritage offering full backward
compatibility. The new implementation of SCSI boosts performance,
reliability and manageability even more.The ANSI standards T10
committee is revising the SCSI Parallel Interface (SPI-3). This
document is the basis of the Ultra3 SCSI technology. Evolutionary
changes have been made to the existing SCSI protocol (SPI-2) to
increase performance, manageability and reliability. All changes
are incremental and existing SCSI protocols are maintained for
backward compatibility. Three new underlying components of Ultra3
SCSI are Double Transition Clocking, Cyclical Redundancy Checks
(CRC) and Domain Validation.Ultra3 SCSI doubles transfer rates to
160Mb per second by using both edges of the request/acknowledge
signal to clock data. This creative solution provides designers
with the choice of improving speed, reliability or connectivity. It
allows system designers to choose bus bandwidths up to 160Mb/s
using existing Ultra2 SCSI cable plants. Alternatively, this
technology lets designers maintain Ultra2 SCSI speeds (80Mb/s) and
improve reliability by lowering clock speed, allowing more margin
for ASICs and cables. Other Ultra SCSI improvements include
automatic tests of the interfaces performance level for increased
manageability and the addition of CRC for reliable data
transmission. When Ultra3 SCSI is used with LVD signalling, cable
lengths of 12m are maintained providing full backward
compatibility.Double Transition Clocking (DTC) changes the digital
protocol to use both edges of the SCSI request/acknowledge signal
to clock data. Data transfer rates can be doubled simply by
increasing the speed of only the data lines. For example, request /
acknowledge signal on Ultra2 SCSI runs at 40MHz, while data runs at
only 20 MHz or 80Mb/s on a 16-bit wide bus. By using both edges of
the same 40MHz request / acknowledge signal, the data rate can be
increased to 40MHz, or 160Mb/s on a 16-bit wide bus.DTC doubles the
Ultra2 SCSI data transfer rates from 80Mb/s to 160Mb/s. Interface
bandwidth is an essential ingredient for Windows NT and UNIX
workstations, video and web servers, and Storage Area Networks
(SANs).DTC keeps the maximum clock rate at half the rate of single
edge clocking. This provides more timing margin for ASICs, cables,
motherboard traces, high capacitance devices, extra connectors and
so forth. Longer pulses reduce the likelihood of problems by
increasing timing margins and tolerance to noise. DTC reduces the
maximum frequency of the clock lines (REQ/ACK) without slowing the
data rate. Slower clocks should also reduce EMI issues for system
designers.The Ultra SCSI reliability enhancements include the
addition of a Cyclical Redundancy Check (CRC) on customer data. CRC
provides extra data protection for marginal cable plants and
external devices, and is one of the best ways to assure data
protection during hot plugging. CRC offers higher levels of data
reliability by ensuring complete integrity of transferred data. It
dramatically reduces undetected error rates by using the same
proven CRC that is utilised by FDDI, Ethernet and Fibre Channel
interface. The Ultra3 SCSI CRC detects all single bit errors, all
double bit errors, all odd number of errors, all burst errors up to
32-bits long and has an approximately 2-32 rate of undetected
random error patterns. The third component of Ultra3 SCSI is Domain
Validation. This technology intelligently tests storage networks
including cables, backplanes, terminators, expanders, bridges and
so forth. Domain Validation ensures that the network is operating
at the required specifications. If reliability is at risk, the
transfer proceeds without a hitch at a lower speed much the way
today's modem and fax transmissions connect despite variations in
equipment.Domain Validation should increase end-user satisfaction
and decreases total cost of ownership by reducing service calls for
under performing systems. In addition, these tests could save on
call center support resources and help alleviate end-user
frustration In the past, new devices such as host bus adaptors
(HBAs) and hard disk drives (HDDs) did not always work smoothly
with legacy configurations. Domain Validation helps assure that
Ultra3 SCSI devices operate smoothly in existing legacy systems.
This testing is done automatically without changing controller
settings, setting BIOS parameters or fumbling with manuals.The rule
of thumb for the past 15 years has been that bus bandwidth should
be at least four times the maximum throughput of a drive. The
Ultra3 SCSI bus bandwidth should stay comfortably ahead of the
internal transfer rates of the next generation 10K HDDs expected in
1999. These HDDs can saturate the Ultra2 SCSI bus with as few as
three drives.SCSI has a 15-year heritage of maintaining full
backward compatibility and excellent forward compatibility. About
every two years small evolutionary changes are made to this
standard to improve speed, reliability and manageability. The low
risk upgrade to Ultra3 SCSI is the next natural transition.
Compiled by Ajith Ram(c) 1999 Quantum