Ultra160 SCSI: the next generation

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Ultra160 SCSI: the next generation

The new Ultra160 SCSI interface provides better bandwidth while maintaining full compatibility with previous standards

The first Ultra160 hard drives and SCSI adapters are beginning to hit the market. This is a good time to look at the new standard and where it originated from.

Ultra 160 is fully backwards compatible with Ultra2 SCSI LVD. Ultra2 SCSI (LVD), the previous implementation of the storage industry's dominant, proven interoperability technology, provides the capacity and performance demanded by today's data-intensive applications, and is easily integrated with current hardware and firmware. Compared with other alternatives available today, Ultra2 SCSI (LVD) prevents system performance from becoming I/O limited as drive throughputs increase, performing faster drive buffer-to-host transfers and freeing the bus to accommodate more devices. The enhanced Ultra2 SCSI (LVD) is fully backward compatible, and uses the same physical environment as its predecessor. More importantly, it can be integrated without having to modify or change operating systems, thereby protecting the user's investment in operating system software and drivers.

The literally millions of lines of SCSI software code already written can be used immediately with the Ultra2 SCSI (LVD) interface. Ultra2 SCSI's other benefits include minimal SCSI software or protocol changes, with the support of 15 years of backward compatibility experience with SCSI. Currently, Ultra2 SCSI (LVD) is capable of achieving transfer rates of up to 80Mb/second. It is low cost, easy to integrate, and an optimal solution for improved performance and increased device connectivity. It is available and fully implementable now.

The goal of any peripheral interface is to provide reliable connectivity and avoid being a bottleneck. Just as the Internet has seen dramatic increases in traffic, systems that serve the Internet must deliver unprecedented volumes of data. Internet as well as other new applications impact hard disk drives in two ways. First, the drives must be faster, which in turn puts strain on the interface to keep up. Second, the sheer volume of data being stored in some systems requires a much larger number of drives than ever before. These drives were all being connected with the SCSI interface using a large number of SCSI buses.

Also, since new generations of higher-speed microprocessors are emerging and disk densities are increasing, hard disk drive manufacturers are rolling out drives with faster read channels and higher sustained data transfer rates. Improving internal data rates, however, creates a need for faster external (buffer-to-host) data transfer rates.

Serial interfaces were proposed; however, the transition to a new serial drive interface won't be fast, smooth, or inexpensive. Adopting a serial interface translates into considerable work and costs for systems manufacturers.

Fortunately, such a solution arrived in 1999 in the form of the Ultra2 SCSI (LVD) interface. Ultra2 SCSI (LVD) represented an enhancement of the current parallel Ultra SCSI interface standard that doubles SCSI drive data transfer rates while maintaining backward compatibility.

LVD offered better reliability, backward compatibility through automatic mode sensing of single-ended or differential bus configuration, common-mode noise immunity, reduced EMI, low power requirements, insensitivity to ground shift, extended cable lengths, and greater system margins.

At present, the SCSI definition document does not require nor prohibit hot pluggability for Ultra2 SCSI (LVD) drives. Hot pluggability is the ability to insert or remove a drive from a system with little or no side effects. There are various levels of hot pluggability. The higher the hot pluggability level a drive achieves, the less disruption the drive causes in a RAID system when it is inserted or removed. If the drive can be inserted or removed only when the RAID system is turned off, the drive is not considered hot pluggable.

Preliminary testing indicates that some drives can be "hot inserted" into a SCSI bus (drive data cable) connected with drive power off, followed by turning on drive power without disrupting the system. Furthermore, the system responds with the appropriate SCSI processes to recognise the new drive and re-negotiate modes (from single-ended to Ultra2 SCSI [LVD] mode or from Ultra2 SCSI [LVD] to single-ended mode).

The new successor to Ultra2 SCSI, Ultra160 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/second using existing Ultra2 SCSI cable plants.

Alternatively, this technology lets designers maintain Ultra2 SCSI speeds (80Mb/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 metres 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 80MHz, while data runs at only 40MHz, or 80Mb/second on a 16-bit wide bus. By using both edges of the same 80MHz request/acknowledge signal, the data rate can be increased to 80MHz, or 160Mb/second on a 16-bit wide bus.

DTC doubles the Ultra2 SCSI data transfer rates from 80Mb/second to 160Mb/second Interface bandwidth is an essential ingredient for Windows NT and UNIX workstations, video and web servers, and storage area networks.

For a given transfer rate, 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 without slowing the data rate. Slower clocks should also reduce EMI issues for system designers.

The Ultra160 SCSI reliability enhancements include the addition of a CRC 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, double bit errors, odd number of errors, 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 and such. 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 automatically without changing controller settings, setting BIOS parameters, or fumbling with manuals.

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. The implementation of Ultra160 SCSI should give customers peace of mind because the changes are mostly digital and straightforward to simulate and Implement.

Compiled by Ajith Ram


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This was first published in February 2000

 

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