The Ultra160/m SCSI increases performance, reliability and
manageability. This white paper explains the reasons why and
explores the benefits for your firm
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. Ultra160/m SCSI doubles transfer rates from 80
to 160Mbit/s, improves manageability by automatically testing the
interface's performance level and increases reliability by adding
Cyclical Redundancy Checks (CRC). When Ultra160/m 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.
What is Ultra160/m
SCSI? The ANSI standards T10 committee is revising the SCSI
Parallel Interface (SPI-3). This document is the basis of the
Ultra160/m 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/m SCSI are Double
Transition Clocking, Cyclical Redundancy Checks (CRC), and Domain
Validation.
What technology enables Ultra160/m
SCSI?Ultra160/m SCSI doubles transfer rates to 160Mbit/s 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 160Mbit/s using existing
Ultra2 SCSI cable plants. Alternatively, this technology lets
designers maintain Ultra2 SCSI speeds (80Mbit/s) and improve
reliability by lowering clock speed, allowing more margin for ASICs
and cables. Other Ultra160/m 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/m SCSI is used with LVD signalling,
cable lengths of 12m are maintained providing full backward
compatibility.
Double transition clockingDouble 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
40MHz, while data runs at only 20MHz, or 80Mbit/s on a 16-bit wide
bus. By using both edges of the same 40 MHz request/acknowledge
signal, the data rate can be increased to 40MHz, or 160Mbit/s 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 to 160Mbit/s. Interface
bandwidth is an essential ingredient for Windows NT and UNIX
workstations, video and web servers, and storage area networks
(SANs).
Choosing the reliability advantage with double
transition clocking For a given transfer rate, Double
Transition Clocking 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, 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 designers.
Cyclical redundancy checks
(CRC) The Ultra160/m 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
utilised by FDDI, Ethernet, and Fibre Channel interface. The Ultra3
SCSI CRC detects: All single bit errorsAll double bit errorsAll odd
number of errorsAll burst errors up to 32-bits longAnd has a ~2-32
rate of undetected random error patterns
Domain validation
The third component of Ultra160/m 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
centre 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/m SCSI
devices operate smoothly in existing legacy systems. This testing
is done automatically without changing controller settings, setting
BIOS parameters, or fumbling with manuals.
Ultra160/m SCSI: The
need for speed 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 Ultra160/m 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.
Ultra160/m SCSI:
The low risk interface upgrade 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/m SCSI is the next
natural transition to protect your investment. The implementation
of Ultra160/m SCSI should give customers peace of mind because the
changes are mostly digital and straightforward to simulate and
implement 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 optimise a platform for a
particular market. It enables end users to continue capitalising on
the low-cost connectivity advantages of the SCSI interface.
Ultra160/m SCSI maintains complete backward compatibility with all
earlier SCSI systems. Watch for individual companies' announcements
for full details on products and availability. A lot of work is
being done on this technology and good progress is being made.
Systems that incorporate Ultra3 SCSI devices are expected to ship
in 1999.
Copyright (c) 1999 Quantum CorporationCompiled
by Roger Gann