Data Backup Glossary (Letter N)
Near-line storage is used by corporations, including data warehouses, as an inexpensive, scalable way to store large volumes of data. Near-line storage devices include DAT and DLT tapes (sequential access); optical storage such as CD-ROM, DVD, and Blu-ray; magneto-optical which utilize magnetic heads with an optical reader; and also slower P-ATA and SATA hard disk drives. Retrieval of data is slower than SCSI hard disk which is usually connected directly to servers or in a SAN environment. Near-Line implies that whatever media the information is stored on, it can be accessed via a tape library or some other method electronically as opposed to off-line which signified some human intervention is required, such as retrieving and mounting a tape. Near-line can be slower, but the type of data (historical archives, backup data, video, and others) dictates that the information will not require instant access and high throughput that SAN and SCSI can provide and is less expensive per byte.
A network-attached storage (NAS) device is a server that is dedicated to nothing more than file sharing. NAS does not provide any of the activities that a server in a server-centric system typically provides, such as e-mail, authentication, or file management. NAS allows more hard disk storage space to be added to a network that already utilizes servers without shutting them down for maintenance and upgrades. With a NAS device, storage is not an integral part of the server. Instead, in this storage-centric design, the server still handles all of the processing of data but a NAS device delivers the data to the user. A NAS device does not need to be located within the server but can exist anywhere in a LAN and can be made up of multiple networked NAS devices.
Also called Nyquist’s Theorem. Before sound as acoustic energy can be manipulated on a computer, it must first be converted to electrical energy (using a transducer such as a microphone) and then transformed through an analog-to-digital converter into a digital representation. This is all accomplished by sampling the continuous input waveform a certain number of times per second. The more often a wave is sampled the more accurate the digital representation. Nyquist’s Law, named in 1933 after scientist Harry Nyquist, states that a sound must be sampled at least twice its highest analog frequency in order to extract all of the information from the bandwidth and accurately represent the original acoustic energy. Sampling at slightly more than twice the frequency will make up for imprecisions in filters and other components used for the conversion.