Serial Storage Architecture (SSA) was a serial transport protocol used to attach

disk drive Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by various electronic, magnetic, optical, or mechanical changes to a surface layer of one or more rotating disks. A disk drive is ...

s to

server computer In computing, a server is a piece of computer hardware or software ( computer program) that provides functionality for other programs or devices, called " clients". This architecture is called the client–server model. Servers can provide var ...

History

SSA was invented by

Ian Judd Ian or Iain is a name of Scottish Gaelic origin, derived from the Hebrew given name (Yohanan, ') and corresponding to the English name John. The spelling Ian is an Anglicization of the Scottish Gaelic forename ''Iain''. It is a popular name in Sc ...

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 Fibre Channel (FC) is a high-speed data transfer protocol providing in-order, lossless delivery of raw block data. Fibre Channel is primarily used to connect computer data storage to servers in storage area networks (SAN) in commercial data cen ...

protocol. SSA was promoted as an open standard by the SSA Industry Association, unlike its predecessor the first generation Serial Disk Subsystem. 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 The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organi ...

(ANSI) X3T10.1 standard. SSA devices are logically

SCSI Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical, optical and logical interface ...

devices and conform to all of the

SCSI command In SCSI computer storage, computers and storage devices use a client-server model of communication. The computer is a client which requests the storage device to perform a service, e.g., to read or write data. The SCSI command architecture was or ...

protocols. SSA provides data protection for critical applications by helping to ensure that a single cable failure will not prevent access to data. 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 Hot swapping is the replacement or addition of components to a computer system without stopping, shutting down, or rebooting the system; hot plugging describes the addition of components only. Components which have such functionality are said ...

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

RAID Raid, RAID or Raids may refer to: Attack * Raid (military), a sudden attack behind the enemy's lines without the intention of holding ground * Corporate raid, a type of hostile takeover in business * Panty raid, a prankish raid by male college ...

arrays can be supported per adaptor, and arrays can be mirrored across servers to provide cost-effective protection for critical applications. Furthermore, arrays can be sited up to 25 metres apart - connected by thin, low-cost copper cables - allowing subsystems to be located in secure, convenient locations, far from the server itself. SSA was deployed in server RAID environments, where it was capable of providing for up to 80

Mbyte The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix ''mega'' is a multiplier of (106) in the International System of Units (SI). Therefore, one megabyte is one million bytes o ...

/s of data throughput, with sustained data rates as high as 60 Mbytes/s in non-RAID mode and 35 Mbytes/s in RAID mode.

Link characteristics

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

micro-D The D-subminiature or D-sub is a common type of electrical connector. They are named for their characteristic D-shaped metal shield. When they were introduced, D-subs were among the smallest connectors used on computer systems. Description, n ...

connectors. Impedances are 75

ohm Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm. Ohm or OHM may also refer to: People * Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm'' * Germán Ohm (born 1936), Mexican boxer * Jörg Ohm (b ...

single-ended, and 150 ohm differential. For longer-distance connections, it is possible to use

fiber-optic An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means t ...

cables up to 10 km (6 mi) in length. Signals are

differential TTL Differential TTL is a type of binary electrical signaling based on the transistor-transistor logic (TTL) concept. Standards implementing differential TTL include RS-422. It enables electronic systems to be relatively immune to noise. Normal TTL ...

. The transmission capacity is 20

megabyte The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix ''mega'' is a multiplier of (106) in the International System of Units (SI). Therefore, one megabyte is one million bytes o ...

s per second in each direction per channel, with up to two channels per cable. The transport layer protocol is

non-return-to-zero In telecommunication, a non-return-to-zero (NRZ) line code is a binary code in which ones are represented by one significant condition, usually a positive voltage, while zeros are represented by some other significant condition, usually a negativ ...

, with

8B/10B encoding In telecommunications, 8b/10b is a line code that maps 8-bit words to 10-bit symbols to achieve DC balance and bounded disparity, and at the same time provide enough state changes to allow reasonable clock recovery. This means that the diff ...

(10 bits per character). Higher protocol layers were based on the

-3 standard.

Products

* IBM 7133 Disk expansion enclosures * IBM 2105 Versatile Storage Server (VSS) * IBM 2105 Enterprise Storage Server (ESS) * IBM 7190 SBUS SSA Adapter * Pathlight Technology Streamline PCI Host Bus Adapter, SSA Data Pump,

storage area network A storage area network (SAN) or storage network is a computer network which provides access to consolidated, block-level data storage. SANs are primarily used to access data storage devices, such as disk arrays and tape libraries from serve ...

gateway

References

{{Computer-bus Serial buses SCSI IBM storage devices American National Standards Institute standards Computer storage buses

History

Link characteristics

Products

See also

References