SSA

Today's huge databases and data intensive applications demand incredible amounts of storage, and transferring massive blocks of information requires technology that is robust, reliable and scaleable. Serial Storage Architecture (SSA) is an IBM-developed interface for connecting storage devices, storage subsystems, servers and workstations in mission-critical PC server applications. However, by the start of 1999 it had failed to win major support, and appeared likely to lose out to the rival Fibre Channel 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 adapter 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 are available in 2.25 and 4.51GB capacities, and particular 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 adapter, 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 MBps of data throughput, with sustained data rates as high as 60 MBps in non-RAID mode and 35 MBps in RAID mode.

Input/output standards

Nearly two decades on, many peripheral devices are still connected into the same serial ports and parallel ports that were present on the very first commercial, and with the exception of the Plug-and-Play standards created as part of Windows 95, the PC's "I/O technology" has changed very little since its invention in 1981. Whilst they may have been adequate for the throughputs required by the peripherals of the day, by the late 1990s the PC's serial and parallel ports fell short of users' needs in a number of important areas:

• Throughput: Serial ports max out at 115.2 Kbit/s, parallel ports (depending on type) at around 500 Kbit/s, but devices such as digital video cameras require vastly more bandwidth
• Ease of use: Connecting devices to legacy ports can be fiddly and messy, especially daisy-chaining parallel port devices through pass-through ports. And the ports are always inconveniently located at the rear of the PC
• Hardware resources: Each port requires its own interrupt request line (IRQ). A PC has a total of 16 IRQ lines, most of which are already spoken for. Some PCs have as few as five free IRQs before peripherals are installed
• Limited number of ports: Most PCs have a pair of COM ports and one parallel port. More COM ports and parallel ports can be added, but at the cost of precious IRQs.

In recent years the field of input/output technology has become one of the most exciting and dynamic areas of innovation in desktop computing today and two emerging serial data standards are about to revolutionise the way that peripheral devices are connected and take the concept of Plug-and-Play to new heights.

They also promise to eliminate much of the fuss and bother involved in connecting devices to computers, including all the spare parts and tangled wires that were so common in the PCs of the past. With these new standards, it will be possible for any user to connect a nearly limitless set of devices to the computer in just a few seconds without the requirement of technical knowledge.