ANTI VIRUS
"Antivirus" redirects here. For antiviral medication, see antiviral drug.
Antivirus software are computer programs that attempt to identify, neutralize or eliminate malicious software. Antivirus is so named because the earliest examples were designed exclusively to combat computer viruses; however most modern antivirus software is now designed to combat a wide range of threats, including worms, phishing attacks, rootkits, trojan horses and other malware. Antivirus software typically uses two different approaches to accomplish this:
examining (scanning) files to look for known viruses matching definitions in a virus dictionary, and
identifying suspicious behavior from any computer program which might indicate infection.
The second approach is called heuristic analysis. Such analysis may include data captures, port monitoring and other methods.
Most commercial antivirus software uses both of these approaches, with an emphasis on the virus dictionary approach. Some people claim that a Firewall program performs the functions of a Anti-Virus, however they are mistaken.
Contents[hide]
1 Approaches
1.1 Dictionary
1.2 Suspicious behavior
1.3 Other approaches
2 Issues of concern
3 Mobile devices
4 History
4.1 Effectiveness
5 See also
6 Notes
7 External links
//
[edit] Approaches
[edit] Dictionary
In the virus dictionary approach, when the antivirus software looks at a file, it refers to a dictionary of known viruses that the authors of the antivirus software have identified. If a piece of code in the file matches any virus identified in the dictionary, then the antivirus software can take one of the following actions:
attempt to repair the file by removing the virus itself from the file,
quarantine the file (such that the file remains inaccessible to other programs and its virus can no longer spread), or
delete the infected file.
To achieve consistent success in the medium and long term, the virus dictionary approach requires periodic (generally online) downloads of updated virus dictionary entries. As civically-minded and technically-inclined users identify new viruses "in the wild", they can send their infected files to the authors of antivirus software, who then include information about the new viruses in their dictionaries.
Dictionary-based antivirus software typically examines files when the computer's operating system creates, opens, closes, or e-mails them. In this way it can detect a known virus immediately upon receipt. Note too that a System Administrator can typically schedule the antivirus software to examine (scan) all files on the computer's hard disk on a regular basis.
Although the dictionary approach can effectively contain virus outbreaks in the right circumstances, virus authors have tried to stay a step ahead of such software by writing "oligomorphic", "polymorphic" and more recently "metamorphic" viruses, which encrypt parts of themselves or otherwise modify themselves as a method of disguise, so as to not match the virus's signature in the dictionary.
An emerging technique to deal with malware in general is whitelisting. Rather than looking for only known bad software, this technique prevents execution of all computer code except that which has been previously identified as trustworthy by the system administrator. By following this default deny approach, the limitations inherent in keeping virus signatures up to date are avoided. Additionally, computer applications that are unwanted by the system administrator are prevented from executing since they are not on the whitelist. Since modern enterprise organizations have large quantities of trusted applications, the limitations of adopting this technique rest with the system administrators' ability to properly inventory and maintain the whitelist of trusted applications. As such, viable implementations of this technique include tools for automating the inventory and whitelist maintenance processes.
[edit] Suspicious behavior
The suspicious behavior approach, by contrast, doesn't attempt to identify known viruses, but instead monitors the behavior of all programs. If one program tries to write data to an executable program, for example, the antivirus software can flag this suspicious behavior, alert a user, and ask what to do.
Unlike the dictionary approach, the suspicious behavior approach therefore provides protection against brand-new viruses that do not yet exist in any virus dictionaries. However, it can also sound a large number of false positives, and users probably become desensitized to all the warnings. If the user clicks "Accept" on every such warning, then the antivirus software obviously gives no benefit to that user. This problem has worsened since 1997[citation needed], since many more non-malicious program designs came to modify other .exe files without regard to this false positive issue. Therefore, most modern antivirus software uses this technique less and less.
[edit] Other approaches
Some antivirus software use other types of heuristic analysis. For example, it could try to emulate the beginning of the code of each new executable that the system invokes before transferring control to that executable. If the program seems to use self-modifying code or otherwise appears as a virus (if it immediately tries to find other executables, for example), one could assume that a virus has infected the executable. However, this method could result in a lot of false positives.
Yet another detection method involves using a sandbox. A sandbox emulates the operating system and runs the executable in this simulation. After the program has terminated, software analyzes the sandbox for any changes which might indicate a virus. Because of performance issues, this type of detection normally only takes place during on-demand scans. Also this method may fail as viruses can be nondeterministic and result in different actions or no actions at all done when run — so it will be impossible to detect it from one run. [1]
Some virus scanners can also warn a user if a file is likely to contain a virus based on the file type.
[edit] Issues of concern
The regular appearance of new malware is certainly in the financial interest of vendors of commercial antivirus software, but there is no evidence of collusion. [2]
Some antivirus software can considerably reduce performance. Users may disable the antivirus protection to overcome the performance loss, thus increasing the risk of infection. For maximum protection, the antivirus software needs to be enabled all the time — often at the cost of slower performance (see also software bloat).
It is important to note that one should not have more than one memory-resident antivirus software solution installed on a single computer at any given time. Otherwise, the computer may be crippled and further damaged.[3]
It is sometimes necessary to temporarily disable virus protection when installing major updates such as Windows Service Packs or updating graphics card drivers.[citation needed] Active antivirus protection may partially or completely prevent the installation of a major update.
When purchasing antivirus software, the agreement may include a clause that your subscription will be automatically renewed, and your credit card automatically billed at the renewal time without your approval. For example, McAfee requires one to unsubscribe at least 60 days before the expiration of the present subscription.[4] In that case, the subscriber may contest the charges with the credit card issuer, but this recourse is likely to fail if in fact the subscriber had authorised such a "continuous payment authority". Norton Antivirus also has a default setting that includes the automatic renewal of your subscription. [5]
Some antivirus programs are actually spyware masquerading as antivirus software. It is best to double-check that the antivirus software which is being downloaded is actually a real antivirus program.[6]
Some commercial antivirus software programs contain adware. For example, the home/small business version of CA Anti-Virus 2008 displays an advert for CA products whenever the desktop is unlocked after a period of inactivity.
Most widely-accepted antivirus programs often do not detect newly-created viruses. This can be verified by making a program with destructive code in a language like C++.
PRINTERS..
A computer printer, or more commonly a printer, is a computer peripheral which produces a hard copy (permanent human-readable text and/or graphics) of documents stored in electronic form, usually on physical print media such as paper or transparencies. Many printers are primarily used as local peripherals, and are attached by a printer cable to a computer which serves as a document source. Some printers, commonly known as network printers, have built-in network interfaces (typically wireless or Ethernet), and can serve as a hardcopy device for any user on the network. Individual printers are often designed to support both local and network connected users at the same time.
In addition, a few modern printers can directly interface to electronic media such as memory sticks or memory cards, or to image capture devices such as digital cameras, scanners; some printers are combined with a scanners and/or fax machines in a single unit. Printers that include non-printing features are sometimes called Multifunction Printers (MFP) or Multi-Function Devices (MFD).
A printer which is combined with a scanner can function as a kind of photocopier if so designed. Most MFPs include printing, scanning, and copying among their features. Printers are designed for low-volume, short-turnaround print jobs; requiring virtually no setup time to achieve a hard copy of a given document. However, printers are generally slow devices (30 pages per minute is considered fast; and many consumer printers are far slower than that), and the cost-per-page is relatively high. In contrast, the printing press (which serves much the same function), is designed and optimized for high-volume print jobs such as newspaper print runs--printing presses are capable of hundreds of pages per minute or more, and have an incremental cost-per-page which is a fraction of that of printers.
The printing press remains the machine of choice for high-volume, professional publishing. However, as printers have improved in quality and performance, many jobs which used to be done by professional print shops are now done by users on local printers; see desktop publishing. The world's first computer printer was a 19th century mechanically driven apparatus invented by Charles Babbage for his Difference Engine.[1]
Contents[hide]
1 Printing technology
2 Modern print technology
2.1 Toner-based printers
2.2 Liquid inkjet printers
2.3 Solid ink printers
2.4 Dye-sublimation printers
2.5 Inkless printers
3 Obsolete and special-purpose printing technologies
3.1 Typewriter-derived printers
3.2 Teletypewriter-derived printers
3.3 Daisy wheel printers
3.4 Dot-matrix printers
3.5 Line printers
3.6 Pen-based plotters
4 Other printers
5 Printing mode
6 Monochrome, color and photo printers
7 The printer manufacturing business
8 Printing speed
9 References
//
[edit] Printing technology
Printers are routinely classified by the underlying print technology they employ; numerous such technologies have been developed over the years.
The choice of print engine has a substantial effect on what jobs a printer is suitable for, as different technologies are capable of different levels of image/text quality, print speed, low cost, noise; in addition, some technologies are inappropriate for certain types of physical media (such as carbon paper or transparencies).
Another aspect of printer technology that is often forgotten is resistance to alteration: liquid ink such as from an inkjet head or fabric ribbon becomes absorbed by the paper fibers, so documents printed with liquid ink sublimation printerare more difficult to alter than documents printed with toner or solid inks, which do not penetrate below the paper surface.
Checks should either be printed with liquid ink or on special "check paper with toner anchorage".[2] For similar reasons carbon film ribbons for IBM Selectric typewriters bore labels warning against using them to type negotiable instruments such as checks. The machine-readable lower portion of a check, however, must be printed using MICR toner or ink. Banks and other clearing houses employ automation equipment that relies on the magnetic flux from these specially printed characters to function properly.
[edit] Modern print technology
The following printing technologies are routinely found in modern printers, as of April 2006:
[edit] Toner-based printers
Main article: Laser printer
Toner-based printers work using the Xerographic principle that is at work in most photocopiers: by adhering toner to a light-sensitive print drum, then using static electricity to transfer the toner to the printing medium to which it is fused with heat and pressure.
The most common type of toner-based printer is the laser printer, which uses precision lasers to cause adherence. Laser printers are known for high quality prints, good print speed, and a low (Black and White) cost-per-copy; they are the most common printer for many general-purpose office applications. They are far less commonly used as consumer printers due to a high initial cost.
Laser printers are available in both color and monochrome varieties.
Another toner based printer is the LED printer which uses an array of LEDs instead of a laser to cause toner adhesion to the print drum.
Recent research has also indicated that Laser printers emit potentially dangerous ultrafine particles, possibly causing health problems associated with respiration [1] and cause pollution equivalent to cigarettes.[3] The degree of particle emissions varies with age, model and design of each printer but is generally proportional to the amount of toner required. Furthermore, a well ventilated workspace would allow such ultrafine particles to disperse thus reducing the health side effects.
[edit] Liquid inkjet printers
Main article: Inkjet printer
Inkjet printers spray very small, precise amounts (usually a few picolitres) of ink onto the media. These droplets of ink will carry a slight electrical charge. The placement of the ink on the page is then determined by the charge of a cathode and electrode between which the ink moves towards the paper. Inkjet printing (and the related bubble-jet technology) are the most common – quality inkjet printers are inexpensive to produce.
Virtually all modern inkjet printers are color devices; some, known as photo printers, include extra pigments to better reproduce the color gamut needed for high-quality photographic prints (and are additionally capable of printing on photographic card stock, as opposed to plain office paper).
Inkjet printers consist of nozzles that produce very small ink bubbles that turn into tiny droplets of ink. The dots formed are the size of tiny pixels. Ink-jet printers can print high quality text and graphics. They are also almost silent in operation. Inkjet printers have a much lower initial cost than do laser printers, but have a much higher cost-per-copy, as the ink needs to be frequently replaced.
In addition, consumer printer manufacturers have adapted a business model similar to that employed by manufacturers of razors; the printers themselves are frequently sold below cost, and the ink is then sold at a high markup.
Various legal and technological means are employed to try and force users to only purchase ink from the manufacturer (thus leading to vendor lock-in); however there is a thriving aftermarket for such things as third-party ink cartridges (new or refurbished) and refill kits.
Inkjet printers are also far slower than laser printers. Inkjet printers also have the disadvantage that pages must be allowed to dry before being aggressively handled; premature handling can cause the inks (which are adhered to the page in liquid form) to run.
No comments:
Post a Comment