Serial Storage Architecture
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Serial Storage Architecture (SSA) is serial transport protocol used to attach disk drives to servers. It 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.
SSA was promoted as an open standard by the SSA Industry Association. A number of other vendors including Pathlight Technologies produced products based on SSA. It was also adopted as an ANSI X3T10.1 standard.
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 swappable 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 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.
With its inherent resiliency and ease of use, SSA is being increasingly deployed in server/RAID environments, where it is capable of providing for up to 80 Mbit/s of data throughput, with sustained data rates as high as 60 Mbit/s in non-RAID mode and 35 Mbit/s in RAID mode.
Link characteristics
The cables are round bundles of two or four twisted pairs, up to 25 metres long and terminated with 9-pin micro-D connectors. Impedances are 75 ohm single-ended, and 150 ohm differential. Signals are differential TTL. The transmission capacity is 20 Mbyte/second in each direction per channel, with up to two channels per cable. Encoding is non return to zero, with 10 bits per character. Higher protocol layers were based on the SCSI-3 standard.