Ultra160 SCSI - Page 1
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 increase performance, reliability, and manageability.
Ultra160 SCSI doubles transfer rates from 80 to 160 Megabytes per second, improves
manageability by automatically testing the interface?s performance level and
increases reliability by adding Cyclical Redundancy Checks (CRC). When Ultra160
SCSI is used with low-voltage differential (LVD) signaling, cable lengths of
12 meters 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.
- What is Ultra160 SCSI?
The ANSI standards T10 committee is revising the SCSI Parallel Interface (SPI-3).
This document is the basis of the Ultra160 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 Ultra160 SCSI are Double Transition Clocking, Cyclical Redundancy Checks
(CRC), and Domain Validation.
- What technology Enables Ultra160 SCSI?
Ultra160 SCSI doubles transfer rates to 160 Megabytes 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 160 MB/second using
existing Ultra2 SCSI cable plants. Alternatively, this technology lets designers
maintain Ultra2 SCSI speeds (80 MB/second) and improve reliability by lowering
clock speed, allowing more margin for ASICs and cables.
Other Ultra160 SCSI improvements include automatic tests of the interface?s
performance level for increased manageability and the addition of CRC for reliable
data transmission. When Ultra160 SCSI is used with LVD signaling, cable lengths
of 12 meters are maintained providing full backward compatibility.
Ultra160 SCSI - Page 2
- Double Transition Clocking
Double transition clocking 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 80
MHz, while data runs at only 40 MHz, or 80 MB/second on a 16-bit wide bus. By
using both edges of the same 80 MHz request/acknowledge signal, the data rate
can be increased to 80 MHz, or 160 MB/second on a 16-bit wide bus.
- Choosing the Speed Advantage with Double Transition Clocking
Double Transition Clocking doubles the Ultra2 SCSI data transfer
rates from 80 MB/second to 160 MB/second. Interface bandwidth is an essential
ingredient for Windows NT and UNIX workstations, video and web servers, and
storage area networks (SANs).
Figure 1. Double Edge Clocking ? Increasing Speed
- Choosing the Reliability Advantage with Double Transition
For a given transfer rate Double Transition Clocking keeps
the maximum clock rate at half the rate of single edge clocking (see Figure
2). This provides more timing margin for ASICs, cables, motherboard traces,
high capacitance devices, extra connectors, etc. Longer pulses reduce the likelihood
of problems by increasing timing margins and tolerance to noise. Double Transition
Clocking reduces the maximum frequency of the clock lines (REQ/ACK) without
slowing the data rate. Slower clocks should also reduce EMI issues for system
Figure 2. Double Edge Clocking - Increasing
Ultra160 SCSI - Page 3
- Cyclical Redundancy Checks (CRC)
The Ultra160 SCSI reliability enhancements include the addition
of a Cyclical Redundancy Check on customer data. CRC provides extra data protection
for marginal cable plants, 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 utilized
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 a ~2-32 rate of undetected random error patterns
The third component of Ultra160 SCSI is Domain Validation.
This technology intelligently tests storage networks including cables, backplanes,
terminators, expanders, bridges etc. 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 HBAs (host bus adapters) and
HDDs (hard disk drives) did not always work smoothly with legacy configurations.
Domain Validation helps assure that Ultra160 SCSI devices operate smoothly in
existing legacy systems. This testing is done auto-matically without changing
controller settings, setting BIOS parameters, or fumbling with manuals.
- Ultra160 SCSI: The Need for Speed
The rule of thumb for the past 15 years has been that bus
bandwidth should be at least 4 times the maximum throughput of a drive. The
Ultra160 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 Ultra160 SCSI is the next natural transition to protect
your investment. The implementation of Ultra160 SCSI should give customers peace
of mind because the changes are mostly digital and straightforward to simulate
These new interface improvements address the most critical
requirements in data storage environments. Ultra3 SCSI provides fast data delivery
and is both a simple and cost effective solution for system OEMs to implement.
From a system designer?s perspective, the big benefit of the new interface is
the flexibility it offers to readily optimize a platform for a particular market.
It enables end users to continue capitalizing on the low-cost connectivity advantages
of the SCSI interface. Ultra160 SCSI maintains complete backward compatibility
with all earlier SCSI systems.