doc/printing-overview.shtml

   1 <H1 ALIGN="RIGHT"><A NAME="OVERVIEW">1 - Printing System Overview</A></H1>
   2
   3 <P>This chapter provides an overview of how the Common UNIX Printing System
   4 works.
   5
   6 <H2>The Printing Problem</H2>
   7
   8 <P>For years <I>the printing problem</I> has plagued UNIX. Unlike
   9 Microsoft&reg; Windows&reg; or Mac OS, UNIX has no standard interface or
  10 system in place for supporting printers. Among the solutions currently
  11 available, the Berkeley and System V printing systems are the most
  12 prevalent.
  13
  14 <P>These printing systems support line printers (text only) or
  15 PostScript printers (text and graphics), and with some coaxing they can
  16 be made to support a full range of printers and file formats. However,
  17 because each varient of the UNIX operating system uses a different
  18 printing system than the next developing printer drivers for a wide
  19 range of printers and operating systems is extremely difficult. That
  20 combined with the limited volume of customers for each UNIX varient has
  21 forced most printer vendors to give up supporting UNIX entirely.
  22
  23 <P>CUPS is designed to eliminate <I>the printing problem</I>. One
  24 common printing system can be used by all UNIX varients to support the
  25 printing needs of users. Printer vendors can use its modular filter
  26 interface to develop a single driver program that supports a wide range
  27 of file formats with little or no effort.  Since CUPS provides both the
  28 System V and Berkeley printing commands, users (and applications) can
  29 reap the benefits of this new technology with no changes.
  30
  31 <H2>The Technology</H2>
  32
  33 <P>CUPS is based upon an emerging Internet standard called the Internet
  34 Printing Protocol. IPP has been embraced by dozens of printer and
  35 printer server manufacturers and is supported by Microsoft Windows
  36 2000.
  37
  38 <P>IPP defines a standard protocol for printing as well as managing
  39 print jobs and printer options like media size, resolution, and so
  40 forth. Like all IP-based protocols, IPP can be used locally or over the
  41 Internet to printers hundreds or thousands of miles away. Unlike other
  42 protocols, however, IPP also supports access control, authentication,
  43 and encryption, making it a much more capable and secure printing
  44 solution than older ones.
  45
  46 <P>IPP is layered on top of the Hyper-Text Transport Protocol ("HTTP")
  47 which is the basis of web servers on the Internet. This allows users
  48 to view documentation, check status information on a printer or server,
  49 and manage their printers, classes, and jobs using their web browser.
  50
  51 <P>CUPS provides a complete IPP/1.1 based printing system that provides
  52 Basic, Digest, and local certificate authentication and user, domain,
  53 or IP-based access control. TLS encryption will be available in future
  54 versions of CUPS.
  55
  56 <H2>Jobs</H2>
  57
  58 <P>Each file or set of files that is submitted for printing is called a
  59 <I>job</I>. Jobs are identified by a unique number starting at 1 and
  60 are assigned to a particular destination, usually a printer. Jobs can
  61 also have options associated with them such as media size, number of
  62 copies, and priority.
  63
  64 <H2>Classes</H2>
  65
  66 <P>CUPS supports collections of printers known as <I>classes</I>. Jobs
  67 sent to a class are forwarded to the first available printer in the
  68 class.
  69
  70 <H2>Filters</H2>
  71
  72 <P>Filters allow a user or application to print many types of files
  73 without extra effort. Print jobs sent to a CUPS server are filtered
  74 before sending them to a printer. Some filters convert job files to
  75 different formats that the printer can understand. Others perform page
  76 selection and ordering tasks.
  77
  78 <P>CUPS provides filters for printing many types of image files,
  79 HP-GL/2 files, PDF files, and text files. CUPS also supplies PostScript
  80 and image file Raster Image Processor ("RIP") filters that convert
  81 PostScript or image files into bitmaps that can be sent to a raster
  82 printer.
  83
  84 <H2>Backends</H2>
  85
  86 <P>Backends perform the most important task of all - they send the
  87 filtered print data to the printer.
  88
  89 <P>CUPS provides backends for printing over parallel, serial, and USB
  90 ports, and over the network via the IPP, JetDirect (AppSocket), and
  91 Line Printer Daemon ("LPD") protocols. Additional backends are
  92 available in network service packages such as the SMB backend
  93 included with the popular SAMBA software.
  94
  95 <P>Backends are also used to determine the available devices. On
  96 startup each backend is asked for a list of devices it supports,
  97 and any information that is available. This allows the parallel
  98 backend to tell CUPS that an EPSON Stylus Color 600 printer is
  99 attached to parallel port 1, for example.
 100
 101 <H2>Printer Drivers</H2>
 102
 103 <P>Printer drivers in CUPS consist of one of more filters specific to a
 104 printer. CUPS includes sample printer drivers for Hewlett-Packard
 105 LaserJet and DeskJet printers and EPSON 9-pin, 24-pin, Stylus Color,
 106 and Stylus Photo printers. While these drivers do not generate optimal
 107 output for the different printer models, they do provide basic printing
 108 and demonstrate how you can write your own printer drivers and
 109 incorporate them into CUPS.
 110
 111 <H2>Networking</H2>
 112
 113 <P>Printers and classes on the local system are automatically shared
 114 with other systems on the network. This allows you to setup one system
 115 to print to a printer and use this system as a printer server or spool
 116 host for all of the others. Users may then select a local printer by
 117 name or a remote printer using "name@server".
 118
 119 <P>CUPS also provides <I>implicit classes</I>, which are collections of
 120 printers and/or classes with the same name. This allows you to setup
 121 multiple servers pointing to the same physical network printer, for
 122 example, so that you aren't relying on a single system for printing.
 123 Because this also works with printer classes, you can setup multiple
 124 servers and printers and never worry about a single point of failure
 125 unless all of the printers and servers go down!