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author | Timothy Pearson <kb9vqf@pearsoncomputing.net> | 2011-11-06 15:56:34 -0600 |
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committer | Timothy Pearson <kb9vqf@pearsoncomputing.net> | 2011-11-06 15:56:34 -0600 |
commit | b529f046c9a64ac5fcfa60747af940cf972b3ebc (patch) | |
tree | 83c28cf7fa8fed1960ebd3924b579e7ed8c95cc6 /doc/kdeprint/theory.docbook | |
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diff --git a/doc/kdeprint/theory.docbook b/doc/kdeprint/theory.docbook deleted file mode 100644 index 7b137f1c3..000000000 --- a/doc/kdeprint/theory.docbook +++ /dev/null @@ -1,690 +0,0 @@ -<chapter id="theory"> -<title>Some Theoretical Background: &CUPS;, <acronym>IPP</acronym>, -&PostScript; and <application>Ghostscript</application></title> - -<para>This chapter aims to give a bit of theoretical background to -printing in general, and to &CUPS; especially. If you are not in need -of this, you might like to skip ahead to the <link -linkend="getting-started">next chapter</link>. Chances are you will -come back to this chapter at some point anyway, because sometimes one -needs extra theory to solve a practical problem.</para> - -<sect1 id="basics-of-printing"> -<title>Basics About Printing</title> - -<para>Printing is one of the more complicated chapters in -<acronym>IT</acronym> technology.</para> - - -<para>Earlier on in history, every developer of a program that was -capable of producing printable output had to write his own printer -drivers too. That was quite complicated, because different programs -have different file formats. Even programs with the same purpose, for -example: word processors, often do not understand each other's formats. -There was therefore no common interface to all printers, hence the -programmers often supported only a few selected models.</para> - -<para>A new device appearing on the market required the program authors -to write a new driver if they wanted their program to support it. Also -for manufacturers, it was impossible to make sure their device was -supported by any program known to the world (although there were far -fewer than today).</para> - -<para>Having to support ten application programs and a dozen printers, -meant a system administrator had to deal with 120 drivers. So the -development of unified interfaces between programs and printers became -an urgent need.</para> - -<para>The appearance of <quote>Page Description Languages</quote>, -describing the graphical representation of ink and toner on sheets of -paper (or other output devices, like monitors, photo typesetters, &etc;) -in a common way, was a move that filled a big gap. <!-- FIXME --></para> - -<para>One such development was &PostScript; by Adobe. It meant that an -application programmer could concentrate on making his program generate -a &PostScript; language description of his printable page, while -printing device developers could focus on making their devices -&PostScript; literate.</para> - -<para>Of course, over time, there came the development of other description -methods. The most important competitors to &PostScript; were -<acronym>PCL</acronym> (<quote>Print Control Language</quote>, from -&Hewlett-Packard;), <quote>ESC/P</quote> (from Epson) and -<acronym>GDI</acronym> (<quote>Graphical Device Interface</quote> from -&Microsoft;).</para> - -<para>The appearance of these page description languages made life easier, -and facilitated further development for everybody. Yet the fact that there -still remained different, incompatible, and competing page description -languages keeps life for users, administrators, developers and -manufacturers difficult enough.</para> - -<sect2> -<title>&PostScript; in memory - Bitmaps on Paper</title> - -<para>&PostScript; is most heavily used in professional printing -environments such as PrePress and printing service industries. In the -&UNIX; and &Linux; domains, &PostScript; is the predominant standard -as a <acronym>PDL</acronym>. Here, nearly every program generates a -&PostScript; representation of its pages once you push the -<quote>Print</quote> button. Let us look at a simple example of -(hand-made) &PostScript; code. The following listing describes two -simple drawings:</para> - -<example id="coded-postscript"> -<title>&PostScript; Code</title> -<screen>%!PS -100 100 moveto -0 50 rlineto -50 0 rlineto -0 -50 rlineto -closepath -.7 setgray fill -% first box over; next -160 100 moveto -0 60 rlineto -45 10 rlineto -0 -40 rlineto -closepath -.2 setgray fill</screen> -</example> - -<para>This tells the imaginary &PostScript; <quote>pen</quote> to draw -a path of a certain shape, and then fill it with different shades of -gray. The first part translates into more comprehensive English as -<quote>Go to coordinate (100,100), draw a line with length 50 upward; -then one from there to the right, then down again, and finally close -this part. Now fill the drawn shape with 70% darkness gray.</quote></para> - -<example id="rendered-postscript"> -<title>Rendered &PostScript;</title> -<mediaobject> -<imageobject> -<imagedata fileref="ps-boxes.png" format="PNG"/> -</imageobject> -<textobject> -<phrase><xref linkend="coded-postscript"/> example rendered as an -image.</phrase> -</textobject> -</mediaobject> -</example> - -<para>Of course, &PostScript; can be much more complicated than this -simplistic example. It is a fully fledged programming language with -many different operators and functions. You may even write -&PostScript; programs to compute the value of Pi, format a hard disk or -write to a file. The main value and strength of &PostScript; however -lies in the field to describe the layout of graphical objects on a -page: it also can scale, mirror, translate, transform, rotate and -distort everything you can imagine on a piece of paper -- such as -letters in different font representations, figures, shapes, shades, -colors, lines, dots, raster...</para> - -<para>A &PostScript; file is a representation of one or more pages -to be printed, in a relatively abstract way. Ideally, it is meant -to describe the pages in a device-independent way. &PostScript; is -not directly <quote>visible</quote>; it only lives on hard disks -and in <acronym>RAM</acronym> as a coded representation of -future printouts.</para> - -</sect2> - -<sect2> -<title>Raster Images on Paper Sheets</title> - -<para>What you see on a piece of paper is nearly always a -<quote>raster image</quote>. Even if your brain suggests to you that -your eyes see a line: take a good magnifying glass and you will -discover lots of small dots... (One example to the contrary are -lines that have been drawn by <quote>pen plotters</quote>). And that -is the only thing that the <quote>marking engines</quote> of today's -printers can put on paper: simple dots of different colors, size and -resolution, to make up a complete <quote>page image</quote> composed of -different bitmap patterns.</para> - -<para>Different printers need the raster image prepared in different -ways. Thinking about an inkjet device: depending on its resolution, -the number of inks used (the very good ones need 7 different inks, while -cheaper ones might only use 3), the number of available jets (some print -heads have more than 100!) dispensing ink simultaneously, the -<quote>dithering algorithm</quote> used, and many other things, the -final raster format and transfer order to the marking engine is heavily -dependent on the exact model used.</para> - -<para>Back in the early life of the <quote>Line Printer Daemon</quote>, -printers were machines that hammered rows of <acronym>ASCII</acronym> -text mechanically on to long media, folded as a zig-zag paper -<acronym>snake</acronym>, drawn from a cardboard box beneath the -table... What a difference from today!</para> - -</sect2> - - -<sect2> -<title><acronym>RIP</acronym>: From &PostScript; to Raster</title> - -<para>Before the final raster images are put on paper cut-sheets, they -have to be calculated somehow out of their abstract &PostScript; -representation. This is a very computing-intensive process. It is called -the <quote>Raster Imaging Process</quote>, more commonly -<quote><acronym>RIP</acronym></quote>).</para> - -<para>With &PostScript; printers the <acronym>RIP</acronym>-ping is -taken care of by the device itself. You just send the &PostScript; -file to it. The <quote>Raster Imaging Processor</quote> (also called the -<acronym>RIP</acronym>) inside the printer is responsible (and -specialized) to fulfill quite well this task of interpreting the -&PostScript;-page descriptions and put the raster image on paper.</para> - -<para>Smaller &PostScript; devices have a -hardware-<acronym>RIP</acronym> built in; it is etched in silicon, on a -special chip. Big professional printers often have their -<acronym>RIP</acronym> implemented as a software-<acronym>RIP</acronym> -inside a dedicated fast &UNIX; run computer, often a Sun SPARC Solaris -or a &SGI; &IRIX; machine.</para> - -</sect2> - -<sect2> -<title><application>Ghostscript</application> as a Software -<acronym>RIP</acronym></title> - -<para>But what happens, if you are not lucky enough to have a -&PostScript; printer available?</para> - -<para>You need to do the <acronym>RIP</acronym>-ing before you send -the print data to the marking engine. You need to digest the &PostScript; -generated by your application on the host machine (the print client) -itself. You need to know how the exact raster format of the target -printer's marking engine must be composed.</para> - -<para>In other words, as you can't rely on the printer to understand -and interpret the &PostScript; itself, the issue becomes quite a bit -more complicated. You need software that tries to solve for you the -issues involved.</para> - -<para>This is exactly what the omnipresent &ghostscript; package is -doing for many &Linux;, *BSD and other &UNIX; boxes that need to print -to non-&PostScript; printers: &ghostscript; is a &PostScript; -interpreter, a software <acronym>RIP</acronym> capable of running many -different devices.</para> - -</sect2> - -<sect2> -<title><quote>Drivers</quote> and <quote>Filters</quote> in General</title> - -<para>To produce rasterized bitmaps from &PostScript; input, the -concept of <quote>filters</quote> is used by &ghostscript;. There are -many different filters in &ghostscript;, some of them specialized for -a certain model of printer. &ghostscript; filterspecializedin devices -have often been developed without the consent or support of the -manufacturer concerned. Without access to the specifications and -documentation, it was a very painstaking process to reverse engineer -protocols and data formats.</para> - -<para>Not all &ghostscript; filters work equally well for their -printers. Yet, some of the newer ones, like the -<application>stp</application> Filter of the -<application>Gimp</application> Print project, produce excellent -results leading to photographic quality on a par or even superior to -their &Microsoft; &Windows; driver counterparts.</para> - -<para>&PostScript; is what most application programs produce for -printing in &UNIX; and &Linux;. Filters are the true workhorses of -any printing system there. Essentially they produce the right bitmaps -from any &PostScript; input for non-&PostScript; target -engines.</para> - -</sect2> - -<sect2> -<title>Drivers and Filters and Backends in CUPS</title> - -<para>&CUPS; uses its own filters, though the filtering system is -based on Ghostscript. Namely the pstoraster and the imagetoraster -filters are directly derived from Ghostscript code. &CUPS; has -reorganized and streamlined the whole mechanics of this legacy code -and organized it into a few clear and distinct modules.</para> - -<para>This next drawing (done with the help of &kivio;) gives an -overview of the filters and backends inside &CUPS; and how they fit -together. The <quote>flow</quote> is from top to bottom. Backends -are special filters: they don't convert date to a different format, -but they send the ready files to the printer. There are different -backends for different transfer protocols.</para> - -<screenshot id="architecture-diagram"> -<screeninfo>&kprinter; dialog started (&kivio; draft drawing) -</screeninfo> -<mediaobject> -<imageobject> -<imagedata fileref="cups-filterarchitecture-kivio-70Percent-scaled.png" -format="PNG"/></imageobject> -<textobject> -<phrase>&kprinter; dialog started (&kivio; draft -drawing)</phrase></textobject> -</mediaobject> -</screenshot> - -</sect2> -<sect2> -<title>Spoolers and Printing Daemons</title> - -<para>Besides the heavy part of the filtering task to generate a -print-ready bitmap, any printing software needs to use a SPOOLing -mechanism: this is to line up different jobs from different users for -different printers and different filters and send them accordingly to -the destinations. The printing daemon takes care of all this.</para> - -<para>This daemon is keeping the house in order: it is also -responsible for the job control: users should be allowed to cancel, -stop, restart, &etc; their jobs (but not other peoples's jobs) and so -on.</para> - -</sect2> - -</sect1> - - - -<sect1 id="cups-and-ppd"> -<title>Excursion: How <quote>CUPS</quote> uses the power of -&PPD;s</title> - -<para>Now that you know how a &PostScript; language file (which -describes the page layout in a largely device independent way) is -transformed into a Raster Image, you might ask: -<quote>Well, there are different kinds of raster output devices: first -they differ in their resolution; then there are the different paper -sizes; it goes on with many finishing options (duplex prints, -pamphlets, punched and stapled output with different sheets of colored -paper being drawn from different trays, &etc;). How does this fit into -our model of device-independent &PostScript;?</quote></para> - -<para>The answer comes with so called &PostScript; Printer Description -(&PPD; files. A &PPD; describes all the device dependent features -which can be utilized by a certain printer model. It also contains -the coded commands that must be used to call certain features of the -device. But &PPD;s are not a closed book, they are simple -<acronym>ASCII</acronym> text files.</para> - -<para>&PPD;s were <quote>invented</quote> by Adobe to make it easy for -manufacturers to implement their own features into &PostScript; -printers, and at the same time retain a standard way of doing so. -&PPD;s are well documented and described by Adobe. Their -specification is a de-facto open standard.</para> - -<sect2 id="ppd-files"> -<title>Device Dependent Print Options</title> - -<para>Remember, advanced &PostScript; printing was originally only -developed for use on &Microsoft; &Windows; and Apple &Mac; systems. -For a long time, all of the feature rich printing on modern devices -was simply unavailable for &Linux; and &UNIX;. &CUPS; changes this -decisively. &CUPS; is closely tied with &PPD;s, and therefore existing -&PPD;s can be utilized to the full by all systems powered by -&CUPS;.</para> - -<para>Using &PPD;s, printer manufacturers were able to insert -device-specific hardware features into their products, for features such -as duplexing, stapling, punching, finishing, &etc;. The printer drivers -load this &PPD; just like an additional configuration file. Thus the -printer driver learns about the available device options and how to -call them; the driver also presents them in a &GUI; to the user. Through -this mechanism you are still able to print -<quote>device-independent</quote> &PostScript; page description -language files and specify device-dependent finishing options on top, -which are added to the application-generated &PostScript;.</para> - -</sect2> - -<sect2> -<title>Where to get the &PPD;s for &PostScript; Printers</title> - -<para>&PPD;s originally were not used routinely in &UNIX; and &Linux; -systems. The vendors providing those &PPD;s never intended them for -anything other than the originally supported &OS;s: &Microsoft; &Windows; and -&MacOS;. Through its brilliant move to fully support and utilize -the existing &PPD; specification, &CUPS; now gives the power to use -all features of modern printers to users of &Linux; and &Linux;-like -systems. &tdeprint; makes its usage even more comfortable than the -&CUPS; developers ever dreamed of.</para> - -<para>&CUPS; can use original &Windows; &PPD;s, distributed by the -vendors in the case of &PostScript; printers. Those normally don't -cost any money, and they can be grabbed from any &Windows; computer -with an installed &PostScript; driver for the model concerned, or from -the disks provided with the printer. There are also several places on -the web to download them.</para> - -</sect2> - -<sect2> -<title>How Special &PPD;s are Now Useful Even For Non-&PostScript; -Printers.</title> - -<para>Now you know how &PostScript;-Printers can use &PPD;s. But what -about non-&PostScript; printers? &CUPS; has done a very good trick: by -using the same format and data structure as the &PostScript; Printer -Descriptions (&PPD;s) in the &PostScript; world, it can describe the -available print job options for non-&PostScript; printers just the -same. For its own special purposes &CUPS; just added a few special -options (namely the line which defines the filter to be used for -further processing of the &PostScript; file).</para> - -<para>So, the developers could use the same software engine to parse -the Printer Description Files for available options for all sorts of -printers. Of course the &CUPS; developers could not rely on the -non-&PostScript; hardware manufacturers to suddenly develop &PPD;s. -They had to do the difficult start themselves and write them from -scratch. More than 1000 of these are available through the commercial -version of &CUPS;, called <application>ESP -PrintPro</application>.</para> - -<para>Meanwhile there are a lot of &CUPS;-specific &PPD;s available. -Even now those are in most cases not originating from the printer -manufacturers, but from Free software developers. The &CUPS; folks -proofed it, and others followed suit: where &Linux; and &UNIX; -printing one or two years ago still was a kludge, it is now able to -support a big range of printers, including 7-color inkjets capable of -pushing them to Photo Quality output.</para> - -</sect2> - -<sect2> -<title>Different Ways to get &PPD;s for non-&PostScript; -Printers</title> - -<para>You can get &PPD;s to be used with &CUPS; and non-&PostScript; -printers from different areas in the Web:</para> - -<itemizedlist> -<listitem> -<para> first, there is the repository at <ulink -url="http://www.linuxprinting.org">www.linuxprinting.org</ulink>, -which lets you generate a <quote>CUPS-O-Matic</quote>-&PPD; online for -any printer that had been supported by traditional &ghostscript; -printing already. This helps you to switch over to &CUPS; with little -effort, if you wish so. If your printer was doing well with the -traditional way of &ghostscript; printing, take CUPS-O-Matic to plug -your driver into th e &CUPS; system and you'll have the best of both -worlds.</para> -</listitem> - -<listitem> -<para>second, there are &CUPS;-&PPD;s for the more than 120 printer -models, which are driven by the new universal -<application>stp</application> driver. <application>stp</application> -(stood originally for Stylus Photo) is now developed by the gimp-print -project; it was started by Mike Sweet, the leading &CUPS; developer -and is now available through <ulink -url="http://gimp-print.sourceforge.net">gimp-print.sourceforge.net</ulink>. -This driver prints real Photo quality on many modern inkjets and can -be configured to make 120 &CUPS;-&PPD;s along its own -compilation. &HP; Laser- and DeskJet, <trademark -class="registered">Epson</trademark> Stylus and Photo Color models as -well as some <trademark class="registered">Canon</trademark> and -<trademark class="registered">Lexmark</trademark> are covered.</para> -</listitem> - -<listitem> -<para>third, there is the commercial extension to &CUPS; from the -&CUPS; developers themselves: it is called <application>ESP -PrintPro</application> and comes with more than 2.300 printer -drivers. There are even improved imagetoraster and pstoraster filters -included.</para> -</listitem> -</itemizedlist> - -<para>&CUPS; makes it really easy for manufacturers to start -supporting &Linux; and &UNIX; printing for their models at reasonably -low cost. The modular framework of &CUPS; facilitates to plug in any -filter (=driver) with minimal effort and to access and utilize the -whole printing framework that &CUPS; is creating.</para> - -<para>Read more about the exciting &CUPS; features in the available -&CUPS; documentation at <ulink -url="http://www.cups.org/documentation.html">http://www.cups.org/documentation.html</ulink> -and <ulink -url="http://wwww.danka.de/printpro/faq.html">http://www.danka.de/printpro/faq.html</ulink>. -Also at <ulink -url="http://www.linuxprinting.org">http://www.linuxprinting.org/</ulink> -is a universal repository for all issues related to &Linux; and &UNIX; -printing.</para> - -</sect2> - -</sect1> - -<sect1 id="cups-ipp-support"> -<title>How &IPP; Support Makes &CUPS; the Best Choice Around</title> - -<sect2> -<title><quote><acronym>LPD</acronym> Must Die!</quote></title> - -<para>For a long time many developers were deeply dissatisfied with good -old <acronym>LPD</acronym>. Quite a few new projects were started to -improve printing: <application>LPRng</application> is the best known -example. Others are <acronym>PDQ</acronym>, <acronym>PPR</acronym>, -<acronym>PLP</acronym>, <acronym>GNUlpr</acronym> and -<acronym>RLPR</acronym>. But none of the new programs were seen as a -<quote>big shot</quote>; most of them are just implementing the same old -<acronym>LPD</acronym> specification with a few (or many) new -extensions, which again make them incompatible with each other.</para> - -<para>Having seen the development of not just one, but different -viable alternatives to venerable <acronym>BSD</acronym>-style -<acronym>LPD</acronym>, Grant Taylor, author of the <citetitle>Linux -Printing HOWTO</citetitle>, finally rallied the call <citetitle>LPD -Must Die!</citetitle> in his <quote>Campaign To Abolish The Line -Printer Daemon</quote>.</para> - -<!-- FIXME: look up URLs for the above --> - -</sect2> - -<sect2> -<title>How the &IPP; Came to Be</title> - -<para>Along with the above, on the industry side of things, there were -efforts to overcome the well-known weaknesses of -<acronym>LPD</acronym>. It started with proprietary extensions to -plain old <acronym>LPD</acronym>, and stretched as far as -&Hewlett-Packard;'s attempt to establish &HP; JetDirect as a new -standard for a network printing protocol. The result were even more -incompatibilities.</para> - -<para>In the end, an initiative to define a new common industry and -<acronym>IETF</acronym> standard took shape. The <quote>Printer -Working Group</quote> or <acronym>PWG</acronym>, a loose aggregation -of vendors in hardware, software, and operating systems, drafted the -new <quote>Internet Printing Protocol</quote>, &IPP;. &IPP; v1.1 has -now been approved by the <acronym>IETF</acronym> (Internet Engineering -Task Force) as a proposed standard, and now enjoys the unanimous -support throughout the industry in Europe, USA and Japan. Most -current network printer models have now built in &IPP; support on top -of traditional <acronym>LPR</acronym>/<acronym>LPD</acronym> or -JetDirect Printing.</para> - -</sect2> - -<sect2> -<title>Why &IPP; is Solving Many Problems</title> - -<para>&IPP; promises to solve a lot of problems network administrators -face. This trade normally deals with heterogeneous network -environments and spends more than half of its working hours dealing -with printing problems.</para> - -<para>By creating a unified set of query functions for &IPP; enabled -printers and servers, for transferring files and setting job-control -attributes &etc;, &IPP; is destined to work across all &OS; platforms. -It's rollout however, will not happen overnight, as many legacy print -devices will still be in use for many years to come. Therefore, in -&IPP; there is a provision made for backwards compatibility of all -&IPP; implementations. &CUPS; is proving the viability of &IPP; -printing in all environments.</para> - -<para>The most striking advantage will be it's integration into the -existing set of other robust <acronym>IP</acronym> protocols. Being -an extension of the proven and robust <acronym>HTTP</acronym> 1.1 -protocol, for the special task of handling print file and related -data, it is also very easy to plug in other standards as they are -being developed and deployed:</para> - -<itemizedlist> -<listitem> -<para>Basic, Digest, and Certificate Authentication for users seeking -access to print services.</para> -</listitem> -<listitem> -<para>SSL3 and <acronym>TLS</acronym> encryption for transferring -data.</para> -</listitem> -<listitem> -<para>Bi directional communication of clients with print devices, using -the <acronym>HTTP</acronym>/&IPP; <command>GET</command> and -<command>POST</command> mechanism.</para> -</listitem> -<listitem> -<para>LDAP directory service integration to keep a consistent database -of available printers, their capabilities and page-costs, &etc;, as well -as user passwords, <acronym>ACL</acronym>s &etc;.</para> -</listitem> -<listitem> -<para><quote>Pull</quote> (as opposed to the usual <quote>Push</quote> -model) printing, where a server or printer just needs to be told the -&URL; of a document, whereupon it is retrieved from the resource on the -internet and printed.</para> -</listitem> -</itemizedlist> - -</sect2> - -<!-- -<sect2> -<title>&CUPS;, &IPP; and &kde;</title> - -<para>&CUPS; is the most advanced implementation of &IPP; on all &OS; -platforms. That makes &CUPS; a crucial ally to help "conquer the -desktop" for projects like &kde;. &tdeprint; is the best utility to -make &CUPS; core functionality available to &kde; Desktop -users.</para> - -</sect2> --> - -<sect2> -<title>Printer <quote>Plug'n'Play</quote> for Clients</title> - -<para>Have you ever seen a demonstration about &CUPS; capabilities in -the network? You must have been quite impressed if you didn't know in -advance what to expect.</para> - -<para>Imagine you as the administrator of a <quote>LAN</quote>. For -testing purposes you fully installed one &kde;/&CUPS; box on your net, -complete with a dozen printers configured and functional: -&PostScript;, LaserJets, InkJets and BubbleJets, and so on. Your -&kde; users on that box are very happy, they can print like never -before, <quote>ringing all the bells and whistles</quote> of every -printer. It took you 2 hours to make everything run perfectly... and -now all the other 100 users on the network want the same. Two hours -again for every box? No way you could do that before next year, you -think?</para> - -<para>Wrong. Just change one setting in the original &CUPS; box to -make it a <quote>server</quote>. Install &CUPS; on five other boxes, -as <quote>clients</quote>. By the time you turn back to your first -client, you find the users happily playing with the settings for the -dozen printers you had defined earlier on the <quote>server</quote>. -Somehow magically the printers had appeared on all the -<quote>Print</quote> dialogs of the five new &CUPS; client -boxes.</para> - -<para>Your users print, but not a single driver had been installed on -the clients, nor a printer queue defined.</para> - -<para>So, how does this magic work?</para> - -</sect2> - -<sect2> -<title><quote>Seeing</quote> Printers Not Installed Locally?</title> - -<para>The answer is not complicated at all.</para> - -<para>If a &CUPS; server is on the <acronym>LAN</acronym>, it -broadcasts the names of all available printers to the -<acronym>LAN</acronym>, using the <acronym>UDP</acronym> protocol and -port 631. Port 631 is reserved as a <quote>well-known port</quote> by -<acronym>IANA</acronym> (the <quote>Internet Assigning Numbers -Authority</quote>) for &IPP; purposes. All &CUPS; clients listen to -&CUPS; server info sent to their port 631. That's how they know about -available printers, and that's how they learn about the -<quote>path</quote> to the printers as well.</para> - -<para>Using &IPP;, which is really a clever extension to -<acronym>HTTP</acronym> v1.1, &CUPS; is able to address all objects -related to the printing system via <quote>Universal Resource -Locators</quote> or <acronym>URL</acronym>s. Print jobs to be deleted -or restarted, printers to be queried or modified, admin tasks to be -performed on the server, with &IPP; and &CUPS;, everything is -addressable by a certain <acronym>URL</acronym>. Many important -things can be done through the web interface to &CUPS;, accessible for -example with &konqueror;.</para> - -</sect2> - -<sect2> -<title>Printing Without Installing a Driver</title> - -<para>And more, the clients basically can <quote>administer</quote> -and <quote>use</quote> any printer they see, just as if it was a -locally installed one. Of course, you can set restrictions on it with -access control lists &etc;, so that not <emphasis>any</emphasis> -clients may use <emphasis>any</emphasis> printer as it likes.</para> - -<para>The clients even are able to print without the appropriate filter -(or driver) installed locally.</para> - -<para>So how does this work? If a client wants to know about and -select printer-specific options, it sends a request (called -<command>CUPS-get-ppd</command>) to the server. The server tells the -client all about all printer-specific options, as read from the server -side &PPD;. The user on the client side can see the options and -select the required ones. He then sends the print file, usually -unfiltered <quote>raw</quote> &PostScript;, spiced up with the -printer-options to the printer server, using &IPP; as the transport -protocol. All further processing, especially the filtering to -generate the final format for the target printer, is then done by the -server. The server has the necessary programs (<quote>drivers</quote> -or <quote>filters</quote>) to do this.</para> - -<para>This way a client prints without needing to install a driver -locally.</para> - -<para>Any change on the server, such as adding or modifying a printer, -is instantly <quote>known</quote> to the clients with no further -configuration.</para> - -</sect2> - -<sect2> -<title><quote>Zero Administration</quote>, Load Balancing, and -<quote>Failover Switching</quote></title> - -<para>Some other advanced features built into &CUPS; are the capacity to -do <quote>load balancing</quote>.</para> - -<para>If you define the same printer queues on two or more different -servers, the clients will send their jobs to the first responding or -available server. This implies an automatic load balancing amongst -servers. If you have to take one server off the network for -maintenance, the others will just take over its tasks without the users -even noticing the difference.</para> - -</sect2> - -</sect1> - -</chapter> |