printer

                      Dot-matrix printer

A type of printer that produces characters and illustrations by striking pins against an ink ribbon to print closely spaced dots in the appropriate shape. Dot-matrix printers are relatively expensive and do not produce high-quality output. However, they can print to multi-page forms (that is, carbon copies), something laser and ink-jet printers cannot do.

Dot-matrix printers vary in two important characteristics:

·  speed: Given in characters per second (cps), the speed can vary from about 50 to over 500 cps. Most dot-matrix printers offer different speeds depending on the quality of print desired.

·  print quality: Determined by the number of pins (the mechanisms that print the dots), it can vary from 9 to 24. The best dot-matrix printers (24 pins) can produce near letter-quality type, although you can still see a difference if you look closely.

Laser printer

A type of printer that utilizes a laser beam to produce an image on a drum. The light of the laser alters the electrical charge on the drum wherever it hits. The drum is then rolled through a reservoir of toner, which is picked up by the charged portions of the drum. Finally, the toner is transferred to the paper through a combination of heat and pressure. This is also the way copy machines work.

Because an entire page is transmitted to a drum before the toner is applied, laser printers are sometimes called page printers. There are two other types of page printers that fall under the category of laser printers even though they do not use lasers at all. One uses an array of LEDs to expose the drum, and the other uses LCDs. Once the drum is charged, however, they both operate like a real laser printer.

One of the chief characteristics of laser printers is their resolution -- how many dots per inch (dpi) they lay down. The available resolutions range from 300 dpi at the low end to 1,200 dpi at the high end. By comparison, offset printing usually prints at 1,200 or 2,400 dpi. Some laser printers achieve higher resolutions with special techniques known generally as resolution enhancement.

In addition to the standard monochrome laser printer, which uses a single toner, there also exist color laser printers that use four toners to print in full color. Color laser printers tend to be about five to ten times as expensive as their monochrome siblings.

Laser printers produce very high-quality print and are capable of printing an almost unlimited variety of fonts. Most laser printers come with a basic set of fonts, called internal or resident fonts, but you can add additional fonts in one of two ways:

·  font cartridges : Laser printers have slots in which you can insert font cartridges, ROM boards on which fonts have been recorded. The advantage of font cartridges is that they use none of the printer's memory.

·  soft fonts : All laser printers come with a certain amount of RAM memory, and you can usually increase the amount of memory by adding memory boards in the printer's expansion slots. You can then copy fonts from a disk to the printer's RAM. This is called downloading fonts. A font that has been downloaded is often referred to as a soft font, to distinguish it from the hard fonts available on font cartridges. The more RAM a printer has, the more fonts that can be downloaded at one time.

In addition to text, laser printers are very adept at printing graphics. However, you need significant amounts of memory in the printer to print high-resolution graphics. To print a full-page graphic at 300 dpi, for example, you need at least 1 MB (megabyte) of printer RAM. For a 600-dpi graphic, you need at least 4 MB RAM.

Because laser printers are nonimpact printers, they are much quieter than dot-matrix or daisy-wheel printers. They are also relatively fast, although not as fast as some dot-matrix printers. The speed of laser printers ranges from about 4 to 20 pages of text per minute (ppm). A typical rate of 6 ppm is equivalent to about 40 characters per second (cps).

  

Impact printer

printers that work by banging a head or needle against an ink ribbon to make a mark on the paper. This includes dot-matrix printers, daisy-wheel printers, and line printers. In contrast, laser and ink-jet printers are nonimpact printers. The distinction is important because impact printers tend to be considerably noisier than nonimpact printers but are useful for multipart forms such as invoices.

Non-impact printer

type of printer that does not operate by striking a head against a ribbon. Examples of nonimpact printers include laser and ink-jet printers. The term nonimpact is important primarily in that it distinguishes quiet printers from noisy (impact) printers

Port number / Port address

                               Port number / Port address

Every system connect to the internet has a number of ports open on it. Ports are basically virtual doors that allow the inflow and out flow of data packets. Without the opening of ports, no data communication can take place on a particular system . Typically, each time a client established a new connection over the network, a randomly chosen port number gets opened.

Similarly, each time a service is enabled on a server, it automatically opens a predefine port number and listens for any clients who might want to established connection.

Port number are of three types

v Well-known port number

           Range of this port number is 0 to 1023.

It use service likely FTP(21 port number),SMTP,POP etc.

v Registered Port number

           Range of this port number is 1024 to 49151.

It use service likely for check mail.

v Dynamic/private port number

          Range of this port number is 49152 to 65535.

It use service for malicious programs likely Trojans, keyloggers, or spayware tools.

                         

Dynamic Host Configuration Protocol (DHCP)

The Dynamic Host Configuration Protocol (DHCP) is a standardized networking protocol used on IPnetworks that dynamically configures IP addresses and other information that is needed for Internet communication. DHCP allows computers and other devices to receive an IPaddress automatically from a central DHCP server, reducing the need for a network administrator or a user from having to configure these settings manually.

DHCP servers maintain a database of 

 available IP addresses and other kinds 

of addresses, such as a default route, 

and one or more DNS server addresses. 

DHCP may be used to configure some of these settings and the remaining settings may be manually configured.

DHCP servers typically grant IP addresses to devices only for a limited interval. Devices are responsible for renewing their IP address lease before that interval expires, and must stop using the address once the interval has expired, if they have not been able to renew it.

Non-technical

DHCP allows computers (clients) to be assigned settings from a server in a client-server model. DHCP is very common in modern networks and is used in home networks as well as larger campus networks. In home networks, the Internet Service Provider's server may assign a globally unique external IP address to a home router (or modem) and this IP address is used in Internet communications. The home router (or modem) may also use DHCP to supply a usable IP address to devices connected to the home network, allowing these devices to communicate on The Internet. The global unique IP address assigned by the Internet Service Provider (ISP) is different from any of the IP addresses assigned to the devices connected to the home router. This is because of a design consideration of IPv4 to help conserve IPv4 addresses.

Technical

Dynamic Host Configuration Protocol automates network-parameter assignment to network devices from one or more DHCP servers. Even in small networks, DHCP is useful because it makes it easy to add new machines to the network.

When a DHCP-configured client (a computer or any other network-aware device) connects to a network, the DHCP client sends a broadcast query requesting necessary information to a DHCP server. The DHCP server manages a pool of IP addresses and information about client configuration parameters such as default gateway, domain name, the name servers, other servers such as time servers, and so forth. On receiving a valid request, the server assigns the computer an IP address, a lease (length of time the allocation is valid), and other IP configuration parameters, such as the subnet maskand the default gateway. The query is typically initiated immediately after booting, and must complete before the client can initiate IP-based communication with other hosts. Upon disconnecting, the IP address is returned to the pool for use by another computer. This way, many other computers can use the same IP address within minutes of each other.

Because the DHCP protocol must work correctly even before DHCP clients have been configured, the DHCP server and DHCP client usually must be connected to the same network link. In larger networks, this is not practical. On such networks, each network link contains one or more DHCP relay agents. These DHCP relay agents receive messages from DHCP clients and forward them to DHCP servers. DHCP servers send responses back to the relay agent, and the relay agent then sends these responses to the DHCP client on the local network link.

Depending on implementation, the DHCP server may have three methods of allocating IP-addresses:

·         dynamic allocation: A network administrator assigns a range of IP addresses to DHCP, and each client computer on the LAN is configured to request an IP address from the DHCP server during network initialization. The request-and-grant process uses a lease concept with a controllable time period, allowing the DHCP server to reclaim (and then reallocate) IP addresses that are not renewed.

·         automatic allocation: The DHCP server permanently assigns a free IP address to a requesting client from the range defined by the administrator. This is like dynamic allocation, but the DHCP server keeps a table of past IP address assignments, so that it can preferentially assign to a client the same IP address that the client previously had.

·         static allocation: The DHCP server allocates an IP address based on a table with MAC address/IP address pairs, which are manually filled in (perhaps by a network administrator). Only clients with a MAC address listed in this table will be allocated an IP address. This feature, which is not supported by all DHCP servers, is variously called Static DHCP Assignment by DD-WRT, fixed-address by the dhcpd documentation, Address Reservation by Netgear, DHCP reservation or Static DHCP by Cisco and Linksys, and IP reservation or MAC/IP binding by various other router manufacturers.