Friday, December 26, 2008

Hard Disk Drive (HDD)

A hard disk drive (HDD), commonly referred to as a hard drive, hard disk, or fixed disk drive, is a non-volatile storage device which stores digitally encoded data on rapidly rotating platters with magnetic surfaces. Strictly speaking, "drive" refers to a device distinct from its medium, such as a tape drive and its tape, or a floppy disk drive and its floppy disk. Early HDDs had removable media; however, an HDD today is typically a sealed unit (except for a filtered vent hole to equalize air pressure) with fixed media.

HDDs (introduced in 1956 as data storage for an IBM accounting computerwere originally developed for use with general purpose computers. In the 21st century, applications for HDDs have expanded to include digital video recorders, digital audio players, personal digital assistants, digital cameras and video game consoles. In 2005 the first mobile phones to include HDDs were introduced by Samsung and Nokia. The need for large-scale, reliable storage, independent of a particular device, led to the introduction of embedded systems such as RAID arrays, network attached storage (NAS) systems and storage area network (SAN) systems that provide efficient and reliable access to large volumes of data.

Form factor Width Largest capacity Platters (Max)
5.25″ FH 146 mm 47 GB[13] (1998) 14
5.25″ HH 146 mm 19.3 GB[14] (1998) 4
3.5″ 102 mm 1.5 TB[6] (2008) 5
2.5″ 69.9 mm 500 GB[16] (2008) 3
1.8″ (CE-ATA/ZIF) 54 mm 250 GB[17] (2008) 3
1.3″ 43 mm 40 GB[18] (2007) 1
1″ (CFII/ZIF/IDE-Flex) 42 mm 20 GB (2006) 1
0.85″ 24 mm 8 GB[19] (2004) 1

IDE interface (PATA)

SATA Interface

Sunday, December 21, 2008

VOIP (Voice Over Internet Protocol )


Voice over Internet Protocol (VoIP) is a general term for a family of transmission technologies for delivery of voice communications over the Internet or other packet-switched networks. Other terms frequently encountered and synonymous with VoIP are IP telephony and Internet telephony, as well as voice over broadband, broadband telephony, and broadband phone, when the network connectivity is available over broadband Internet access.

VoIP systems usually interface with the traditional public switched telephone network (PSTN) to allow for transparent phone communications worldwide.

VoIP can be a benefit for reducing communication and infrastructure costs by routing phone calls over existing data networks and avoiding duplicate network systems. Skype and Vonage are notable service provider examples that have achieved widespread user and customer acceptance and market penetration.

Voice-over-IP systems carry telephony speech as digital audio, typically reduced in data rate using speech data compression techniques, packetized in small units of typically tens of milliseconds of speech, and encapsulated in a packet stream over IP.