| sampling rate |
| In digitizing operations,
the frequency with which samples are taken and converted into
digital form. The sampling frequency must be at least twice that of
the analog frequency being captured. For example, the sampling rate
for hi-fi playback is 44.1 kHz, slightly more than double the 20 kHz
frequency a person can hear. The sampling rate for digitizing voice
for a toll-quality conversation is 8,000 times per second, or 8 kHz,
twice the 4 kHz required for the full spectrum of the human voice.
The higher the sampling rate, the closer real-world objects are
represented in digital form. Another attribute of sampling is quantizing, which creates a number for the sample. The larger the size of the sample, which is also known as resolution or precision, or just sample size, the more granular the scale and the more accurate the digital sampling. See oversampling and quantization.   |
| SCSI |
| (Small Computer
System Interface) Pronounced "scuzzy." SCSI
is a hardware interface that allows for the connection of up to 15
peripheral devices to a single PCI board called a "SCSI host
adapter" that plugs into the motherboard. SCSI uses a bus
structure and functions like a mini-LAN connecting 16 devices, but
the host adapter counts as one device. SCSI allows any two devices
to communicate at one time (host to peripheral, peripheral to
peripheral). Host adapters are also available with two controllers that support up to 30 peripherals. Introduced in 1986 and originally developed by Shugart Associates, SCSI is widely used in servers, mainframes and storage area networks (SANs). The Daisy Chain SCSI peripherals are daisy chained together. Each device has a second port used to connect the next device in line. An earlier advantage of using SCSI in a desktop PC was that a scanner and several drives (CD-Rs, Zip, hard disks, etc.) could be added to one SCSI cable chain. However, this became less important since the advent of the USB interface. RAID Was Originally SCSI Until the late 1990s, SCSI hard disks were the only ones used in RAID configurations, which provide improved performance and/or fault tolerance. Since the advent of IDE RAID controllers, SCSI and IDE have become more equalized, although SCSI continues to be the drive interface of choice in the server market.  Single Ended Single-ended SCSI allows devices to be attached to a total cable length up to six meters or only three meters, depending on type (see below). Single-ended signaling uses data and ground lines. High Voltage Differential Signaling (HVDS) Also called just "differential SCSI" because it was available before there was a second differential option, HVDS supports cable lengths up to 25 meters. In order to increase distance, differential signaling uses data low and data high lines and costs more than single-ended. Low Voltage Differential Signaling (LVDS) Ultra2 SCSI introduced LVDS, which supports cable lengths up to only 12 meters. LVDS requires less power and is less costly than HVDS, because the transceivers are built into the controller chips.
SCSI SPECIFICATIONS
Tfr Bus Length In
Bus Rate Meters For
Width Max MB Device Types:
Type (bits) Dev Sec SE LVD HVD Pins
SCSI-1 8 8 5 6 12* 25 25
SCSI-2 8 8 5 6 12* 25 50
Fast SCSI 8 8 10 3 12* 25 50
Wide SCSI, aka
Fast Wide SCSI 8 16 20 3 12* 25 68
Ultra SCSI 8 8 20 3 - - 50
Wide Ultra SCSI 16 16 40 - 12* 25 68
Wide Ultra SCSI 16 8 40 1.5 - - 68
Wide Ultra SCSI 16 4 40 3 - - 68
Ultra2 SCSI 8 8 40 - 12 25 50
Wide Ultra2 SCSI 16 16 80 - 12 25 68
Ultra3 SCSI, aka
Ultra160 16 16 160 - 12 - 68
Ultra4 SCSI, aka
Ultra320 16 16 320 - 12 - 68
12* - LVD was not part of these specs; however,
if all devices are LVD, 12 meters applies.
If any device is single ended, then length
in SE column applies.
Information for this chart was obtained from the
SCSI Trade Association (STA), San Francisco, CA
(www.scsita.org).
SCAM Sets IDs Automatically A subset of Plug and Play, called "SCSI Configured Automatically" (SCAM), allows IDs to be set by software rather than manually. Both the host adapter and peripheral must support this. Logical Unit Numbers (LUNs) Each SCSI device can be further broken up into eight logical units, identified by logical unit numbers (LUNs) 0 to 7. Although most SCSI disks contain only one disk inside and are addressed as LUN 0, CD-ROM and optical disk jukeboxes contain multiple units. Each disk in these devices can be addressed independently via LUN numbers; for example, a four-disk jukebox could be assigned LUN 0 to 3. Termination The device at the end of a SCSI chain must be terminated by setting a switch or plugging a resistor module into the open port. Usually, host adapters default to terminated. If both internal and external devices are used, the host adapter termination must be removed, and termination must be applied to the ends of both chains.    |
| sector |
| The smallest unit of storage read or written on a disk. |
| seek time |
| The average of the time it takes to move the read/write from its current location to a particular track on a disk. In 1998, Seagate introduced its line of Cheetah hard disks with 5ms seek time. See access time. |
| serial ATA |
| A serial version of the
ATA (IDE) interface, which has been parallel since its inception in
1986. Ratified by ANSI in 2002 as the next-generation ATA
technology, Serial ATA (SATA) provides a point-to-point channel
between motherboard and drive rather than the Parallel ATA (PATA)
master-slave architecture that supports two drives on the same
cable. Serial ATA (SATA) transfers data in a half-duplex channel at 1.5 Gbps in one direction. With SATA II, introduced in 2003, speed was increased to 3 Gbps. Smaller Cables and Connectors SATA uses a four-wire cable up to one meter in length compared to the 18" wide, flat cable used with PATA drives. The cables and connectors are considerably smaller than their PATA counterpart and take up a lot less space in the case. See IDE and SAS.   |
| serial port |
| A socket on a computer
used to connect a modem, data acquisition terminal or other serial
device via a slow-speed serial interface. Earlier PCs used the
serial port for the mouse, and earlier Macintoshes used the serial
port to attach a printer. Today, the serial port is a legacy
interface, having been superseded by the USB bus. DB (D-Sub) Connectors and COM Ports If present, the serial port on the back of a PC is a male 9-pin connector (DE-9 D-sub connector). Earlier PCs may have had two 9-pin connectors or one 9-pin and one 25-pin (DB-25). On a PC, serial ports are called "COM ports," identified as COM1, COM2, etc. See COM1 and D-sub connectors. Serial, Parallel and Game In earlier PCs, one or two serial ports, one parallel port and one game port were included on the motherboard. On the first PCs, these ports were contained on a stand-alone expansion board plugged into the ISA bus. Contrast with parallel port. See serial interface and RS-232. Faster, But Still Serial The USB and FireWire (IEEE 1394) interfaces were added to PCs in 1998, offering a quantum jump in transfer rate, plus the ability to daisy chain large numbers of devices on the same bus. Like the legacy serial port, USB and FireWire are also serial interfaces. See USB and FireWire.  |
| SIMM |
| (Single In-line Memory Module) An earlier printed circuit board that holds memory chips and plugs into a SIMM socket on the motherboard. See memory module. |
| space/time |
| The following units of measure are used to define storage and transmission capacities. See binary values. |
| SPACE - Bits/bytes - Power of 10 Kilo (K) Thousand 3 Mega (M) Million 6 Giga (G) Billion 9 Tera (T) Trillion 12 Peta (P) Quadrillion 15 Exa (E) Quintillion 18 Zetta (Z) Sextillion 21 Yotta (Y) Septillion 24 TIME - Fraction of second - Power of 10 Millisecond (ms) Thousandth -3 Microsecond (µs) Millionth -6 Nanosecond (ns) Billionth -9 Picosecond (ps) Trillionth -12 Femtosecond (fs) Quadrillionth -15 Attosecond (as) Quintillionth -18 Zeposecond (zs) Sextillionth -21 Yoctosecond (ys) Septillionth -24 Storage/Channel - Capacity Measured In: CPU word size bits Bus size bits Disk, tape bytes Overall memory capacity bytes SIMM and DIMM modules bytes Individual memory chip bits Transmission Speed - Measured In: Network line/channel bits/sec Disk transfer rate bytes/sec Disk access time ms Memory access time ns Machine cycle µs, ns Instruction execution µs, ns Transistor switching ns, ps, fs |
| spam |
| E-mail that is not
requested. Also known as "unsolicited commercial e-mail" (UCE),
"unsolicited bulk e-mail" (UBE), "gray mail" and
just plain "junk mail," the term is both a noun (the
e-mail message) and a verb (to send it). Spam is used to advertise
products or to broadcast some political or social commentary. A Social Plague Like viruses, spam has become a scourge on the Internet as hundreds of millions of unwanted messages are transmitted daily to almost every e-mail recipient as well as to newsgroups. Unfortunately for users and fortunately for spammers, as an advertising medium, spam does produce results. Even if only an infinitesimal number of users reply, it is still cost effective since e-mail is a very inexpensive way to reach people. ISPs Work Overtime In order to alleviate some spam, ISPs have added an enormous number of servers doing nothing more than spam filtering (see spam filter). The CAN-SPAM act in the U.S. became law on January 1, 2004, which provides severe penalties for spammers, if they can be located (see CAN-SPAM). Spam Filters Create More Spam As spam filtering becomes more sophisticated, spammers have to send even more spam to make the same money, but e-mail lists can be purchased for very little or hijacked. There is a thriving business selling e-mail lists to spammers as well as lists of compromised computers (see zombie). There are even third-party spam service providers that will do all the work for you. Easy to Rationalize Spammers justify their existence by citing the enormous amount of unsolicited ads we get via the postal system, which wastes trees and other resources. They claim advertisers pollute the environment every day with obnoxious ads on TV, radio, buses and billboards. Of course, they have a point, especially regarding the tons of paper thrown in the "real" trash can every day. However, none of these other approaches threaten to close down the system they live in.  |
| spam filter |
| Software that diverts
incoming spam. Spam filters can be installed in the user's machine
or in the mail server, in which case, the user never receives the
spam in the first place. Spam filtering can be configured to trap
messages based on a variety of criteria, including sender's e-mail
address, specific words in the subject or message body or by the
type of attachment that accompanies the message. Address lists of
habitual spammers (blacklists) are maintained by various
organizations, ISPs and individuals as well as lists of acceptable
addresses (whitelists) that might be misconstrued as spam. Spam
filters reject blacklisted messages and accept whitelisted ones. Sophisticated spam filters use AI techniques that look for key words and attempt to decipher their meaning in sentences in order to more effectively analyze the content and not trash a real message. Spam filters can also divert mail that comes to you as "Undisclosed Recipients," instead of having your e-mail address spelled out in the "to" or "cc" field. See ad blocker, spam trap and Bayesian filtering. |
| spyware |
| Software that sends
information about your Web surfing habits to its Web site. Often
quickly installed in your computer in combination with a free
download you selected from the Web, spyware transmits information in
the background as you move around the Web. Also known as
"parasite software," "scumware," "junkware"
and "thiefware, spyware is occasionally installed just by
visiting a Web site (see drive-by
download). It Might Even Tell You It's Spyware The license agreement that everyone accepts without reading may actually state that you are installing spyware and explain what it does. For example, it might say that the program performs anonymous profiling, which means that your habits are being recorded, not you individually. Such software is used to create marketing profiles; for example, people who go to Web site "A" often go to site "B" and so on. Spyware may deliver competing products in realtime. For example, if you go to a Web page and look for a minivan, an ad for a competitor's vehicle might pop up (see adware). Spyware Is Focused Merchants place ads with spyware advertisers because they feel their promotions are focused. In fact, many feel that the Internet has opened up the most intelligent marketing system the world has ever seen. Merchants say they are targeting prospects who are really interested in their products, and spyware vendors argue that as long as they treat users anonymously, they are not violating privacy. There are also spyware programs that keep changing the home page in the browser to a particular Web site or just keep popping up ads all the time (see adware). Nevertheless, once you detect spyware, it can be eliminated, albeit with difficulty sometimes. Spyware blockers can detect an invasion of spyware into your computer and have become as popular as antivirus programs. See spyware blocker, adware, snoopware, parasite and spam. Perhaps, it evolved in different camps because the intent of the software is different. Virus writers want to be exposed to the world at large so they can one-up their peers, the "xyz virus contaminated 100 million computers" type of glory. On the other hand, spyware writers want their software to remain hidden and perform their tasks for months to come. However, trojans are viruses that are designed to remain hidden in the computer as well, so the two philosophies do overlap. Perhaps, in time, a new category of "anti-insanity" software will take care of all of it. |
| SRAM |
| See static RAM. |
| static |
| Refers to something that is fixed and unchanging. Contrast with dynamic. |
| static RAM |
| A memory chip that requires power to hold its
content, also known as S-RAM or SRAM. Static RAM chips have access times in the 10 to
30-nanosecond range. Dynamic RAMs (DRAM) are usually above 30,
and Bipolar and ECL memories are under 10. A static RAM bit is made up of a pretzel-like flip-flop circuit that lets current flow through one side or the other based on which one of two transistors is activated. Static RAMs do not require refresh circuitry as do dynamic RAMs, but they take up more space and use more power. |
| switched Ethernet |
An Ethernet network that is
controlled by a switch. The switch cross connects two stations and
gives each sender-receiver pair the full bandwidth of the network. The
total bandwidth is 20 Mbps or 200 Mbps between nodes for full-duplex
operation or 10 and 100 for half-duplex. A major advantage in
migrating from shared to switched Ethernet is that only the hub is
replaced with an Ethernet switch. The existing network adapters (NICs)
in every computer connected to the switch are still valid. See N-way
and LAN switch. |