Editing Serial Storage Architecture

{{short description|Disk drive protocol}}

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'''Serial Storage Architecture''' (SSA) was a serial transport protocol used to attach [[disk drive]]s to [[Server (computing)|server computer]]s. It was developed by [[IBM]] employee [[Ian Judd]] in 1990 to provide [[data redundancy]] for critical applications. SSA was deployed in server RAID environments, where it was capable of providing for up to 80&nbsp;[[Megabyte|MB]]/s of data throughput, with sustained data rates as high as 60&nbsp;MB/s in non-RAID mode and 35&nbsp;MB/s in RAID mode.

SSA was promoted as an open standard by the SSA Industry Association, unlike its predecessor, the first generation Serial Disk Subsystem.<ref>IBM 7133 Serial Disk Systems Presentation Guide, pp. 9–10</ref> A number of vendors including IBM, [[Pathlight Technology]] and Vicom Systems produced products based on SSA. It was also adopted as an [[American National Standards Institute]] (ANSI) X3T10.1 standard. SSA devices are logically [[SCSI]] devices and conform to all of the [[SCSI command]] protocols.

==History==
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SSA was invented by [[Ian Judd]] of [[IBM]] in 1990. IBM produced a number of successful products based upon this standard before it was overtaken by the more widely adopted [[Fibre Channel]] protocol.

==Link characteristics==
All the components in a typical SSA subsystem are connected by bi-directional cabling. Data sent from the adaptor can travel in either direction around the loop to its destination. SSA detects interruptions in the loop and automatically reconfigures the system to help maintain connection while a link is restored.

Up to 192 [[hot swap]]pable hard disk drives can be supported per system. Drives can be designated for use by an array in the event of hardware failure. Up to 32 separate [[redundant array of independent disks|RAID]] arrays can be supported per adaptor, and arrays can be mirrored across servers to provide cost-effective protection for critical applications. Arrays are connected by thin and inexpensive copper cables situated up to 25&nbsp;metres apart, allowing subsystems to be located in secure, convenient locations, far from the server itself.

The copper cables used in SSA configurations are round bundles of two or four twisted pairs, up to 25 metres long and terminated with 9-pin [[D-subminiature|micro-D]] connectors. Impedances are 75&nbsp;[[Ohm (unit)|ohm]] single-ended, and 150&nbsp;ohm differential. For longer-distance connections, it is possible to use [[fiber-optic]] cables up to 10&nbsp;km (6&nbsp;mi) in length. Signals are [[differential TTL]]. The transmission capacity is 20&nbsp;[[megabyte]]s per second in each direction per channel, with up to two channels per cable. The transport layer protocol is [[non-return-to-zero]], with [[8B/10B encoding]] (10&nbsp;bits per character). Higher protocol layers were based on the [[SCSI]]-3 standard.

==Products==
*[[IBM 7133]] Disk expansion enclosures
*[[IBM Versatile Storage Server|IBM 2105 Versatile Storage Server]] (VSS)
*[[IBM Enterprise Storage Server|IBM 2105 Enterprise Storage Server]] (ESS)
*[[IBM 7190 SBUS SSA Adapter]]<ref>{{cite web |url=http://www-01.ibm.com/support/docview.wss?uid=pub1gc26727401 |title=IBM notice: The page you requested cannot be displayed - United States |access-date=2009-07-16 |url-status=dead |archive-url=https://archive.today/20120717055724/http://www-01.ibm.com/support/docview.wss?uid=pub1gc26727401 |archive-date=2012-07-17 }}</ref>
*[[Pathlight Technology]] Streamline PCI Host Bus Adapter, SSA Data Pump, [[storage area network]] gateway

==See also==
*[[List of device bandwidths]]

==References==
{{Reflist}}

{{Computer-bus}}

[[Category:Serial buses]]
[[Category:SCSI]]
[[Category:IBM storage devices]]
[[Category:American National Standards Institute standards]]
[[Category:Computer storage buses]]