Digital subscriber line
Digital subscriber line (DSL) is a high-speed method of transferring data over existing twisted-pair telephone lines. DSL is commonly available through your local telephone provider or a third-party provider, if the service is available in your area.

DSL encompasses various protocols, including asymmetric DSL (ADSL), rate adaptive DSL (RADSL), high bit-rate DSL (HDSL), and symmetric DSL (SDSL). It also includes different subvariations, such as CAP-encoding ADSL versus discrete multitone (DMT)- encoding ADSL. The acronym xDSL is used to refer to the set of DSL protocols, rather than to a specific protocol. (more…)

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In this coming topic, I am going to give some basic introduces on what is Integrated Services Digital Network (ISDN), digital subscriber line (DSL), and universal serial bus (USB) standards. It explains how each standard enables device communication, as well as its advantages and disadvantages.

ISDN overview
Integrated Services Digital Network (ISDN) refers to a set of communication protocols proposed by telephone companies to permit telephone networks to carry data, voice, graphics, music, and video. ISDN was developed to permit faster access over existing telephone systems without the additional call setup time. It enables the simultaneous transmission of voice and data signals using end-to-end digital connectivity. (more…)

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Last month was mention about the Linux Processes on PIDs , parent and child processes and using the ps command. In this part 2, let see how the background and foreground background process work.

Background and foreground processes
An interactive process can run in the foreground or background. To place a process in the background, you use the command bg or the ampersand symbol (&). This lets you do other jobs while waiting for the command to finish. Let’s say you use the syntax

$ find / filename 2 > /dev/null | grep filename | tee result

A command using this syntax could take a long time to execute, especially if you have a lot of files and are listing several network drives. Only one job at a time can run in the foreground, so no further commands can be issued until the prompt returns. To continue working on the command line while it processes, you can execute the same command in the background, using the ampersand. For example, you could enter the command using the following syntax:

$ find / filename 2 > /dev/null | grep filename | >result &

After the command has been issued, the shell displays a message, which looks similar to the following: (more…)

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After read some linux article, and having some basic knowledge, now can have basic understanding on linux processes, a process is a single program that is executed within its own virtual address space. For example, a shell is a process that is created each time a user logs in to the system. It creates a new process every time it implements a program such as a system command. A system command is an example of a collection of related commands called a job. Jobs create a series of separate processes from a single command line. So, in fact, every program running under Linux is a process.

The following table shows the three main categories of processes.

Linux can share its processing power, storage capabilities, and input and output mechanisms with several users, or with several processes created by one user. Processes such as user jobs, operating system tasks, mail, and background jobs like printing need to be monitored simultaneously by Linux. (more…)

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Text processing is different than word processing. In word processing, text is edited and manipulated in a “What-You-See-Is-What-You-Get” (WYSIWYG) environment. This enables you to produce printed copies of the text, complete with features such as graphics and tables.

Linux stores files in plain-text format. This allows you to use different processing, or filtering, utilities on the text. These utilities let you format, analyze, and manipulate text in many different ways. For example, you can format pages and paragraphs, check spelling, add page numbers and headers, and count the lines, words, and characters that a file contains.

Text-processing utilities

The general command syntax used for text-processing commands is

command [option ] filename(s)

Reading files
To work with files, you need to know what information they contain. One way of doing this is to use the cat command to display the contents of a file on your terminal screen. This command provides a number of options.

Options for the cat command

Viewer commands
A quicker way of viewing the contents of a file is by using viewer commands, which display the information at the start or end of a file. Examples of viewer commands include head and tail.

By default, the head command displays the first 10 lines of a file, and the tail command displays the last 10 lines of a file. Additional options can be used with these commands to view more specific areas of a file. For example, you can use the -n option with the commands to specify the number of lines they must display.

Pager commands
If a particular file is longer than one screen, it may scroll past too quickly for you to read it. You can scroll through documents at your own pace by using pager commands, which display documents one screen at a time. Examples of pager commands include less and more.

The more command is the original pager command and derives from the Berkeley version of UNIX. It enables you to move through a document in a forward direction only.

The less command was developed to replace the more command and provides a wider range of options for viewing files.

Pager command options

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