Abstract:
A system for printing a document over a network including a network, a printer and a raster image processor. The network connects at least three computing resources. The printer is coupled to the network and included in the computing resources. The printer includes a printer network connection, a printer virtual connection mechanism and a mechanism for printing the document based on the raster image. The printer network connection transfers data packets to and from the computing resources. The printer virtual connection mechanism receives a raster image representative of a document from a member of the plurality of computer resources to which a unique virtual connection has been established. The raster image processor is coupled to the network and included in the computing resources. The raster image processor includes raster image processor network connection mechanism, a rendering, mechanism, and a raster image processor virtual connection mechanism. The raster image processor network connection mechanism transfers information to and from the computing resources. The mechanism rendering mechanism renders a series of page description language instructions representative of the document into the raster image. The raster image processor virtual connection mechanism transmits a raster image to a printer via the unique virtual connection. The virtual connection maintenance mechanism opens, maintains, and closes unique virtual connections between printer and raster image processor.

Description:
This is a Division of application Ser. No. 08/270,046, originally filed Jul. 1, 1994 and CPA filed Feb. 13, 1998, now U.S. Pat. No. 6,091,507. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is a method and apparatus for printing a document over a network. More specifically, the present invention provides a method and apparatus wherein a host computer generates a raster image from a series of page description language instructions representative of the document, and the raster image is transferred over a network to one or more printers where it is printed. 
     In the prior art, a document is printed over a network by preparing a series of page description language (PDL) instructions at a host computer and transferring those instructions to a printer over a network. The printer includes a raster image processor (RIP) that processes the series of PDL instructions into a raster image that is stored in a frame buffer, and the printer&#39;s print engine prints the raster image from the frame buffer. 
     Processing the series of PDL instructions into a raster image is known in the art as RIPing, which is a computation intensive and memory intensive process that requires a significant amount of time relative to the amount of time it takes for the print engine to print the image. Therefore, prior art printers that process PDL instructions into raster images generally spend significant amounts of time RIPing the PDL image. During much of this time, the print engine must remain idle waiting for the raster image. 
     This problem is addressed in U.S. Pat. No. 5,113,494 to Menendez et al., which discloses a high speed raster image processor that RIPs PDL instructions fast enough to minimize the idle time of a laser printer&#39;s print engine. The RIP disclosed by Menendez et al. resides in a common printer node with the print engine, and specifies a dedicated connection between the RIP and the print engine. This approach dedicates a significant amount of hardware to perform the RIP function, and increases the cost of the printer. 
     Another prior approach to this problem is to provide the host computer with a RIP, which is typically implemented by software on the host computer. While this approach does not generally decrease the time required to RIP a document, multiple hosts can RIP documents in parallel, with each host submitting a raster image of a document to a printer&#39;s print engine when RIPing is complete. The drawback to this approach, however, is that a tremendous amount of data must be sent over the network. An 8½×11 inch page of color text represented by a sequence of PDL instructions and printed at a resolution of 300 dots per inch (dpi) will generally not require more than twenty kilobytes of data to be transferred over a network. However, a raster image of this same page of color text will require about four megabytes of data to be transferred over a network. In addition, for a printer having a large form factor, such as a 36 inch wide color printer, the amount of data to be transferred makes this approach prohibitive. A 300 DPI color image at a size of 54×54 inches requires about 125 megabytes of data to be transferred over the network. Since ripless printers do not contain data storage resources of this magnitude, the raster image must be retransmitted over the network for each printed copy. 
     On a typical Ethernet network adhering to the IEEE 802.3 specification, raster data cannot be transferred to printer fast enough to feed a moderately fast print engine. In a laser printer, the printer engine must pause between pages to wait for the raster image to be received. In an ink jet printer, the printer may have to pause while printing a page to wait for additional raster data. These pauses may result in a banding effect because the pause may cause the ink deposited during the preceding pass of the print head to dry before the next pass can occur, while most passes will occur continuously and the ink will not dry. When the ink is not dry, the ink deposited between successive passes will blend together and minimize the banding associated with successive passes of the print head, while banding will be more pronounced if the ink has dried before the next pass of the print head occurs. 
     Another problem associated with RIPing the series of PDL instructions at the host computer is that host computer&#39;s RIP will generally lack information about the media and inks of the printer that will eventually print the document. When the RIP is integrated in the printer, the RIP is generally provided with this information. 
     SUMMARY OF THE INVENTION 
     The present invention is method and apparatus for printing a document over a network. In the present invention, a host computer based raster image processor processes page description language instructions representative of a document to form a raster image representative of the document. A high-speed virtual connection between the raster image processor and the printer is opened and raster data is transferred over the network to a selected printer where the document is printed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a typical prior art networked computing system. 
     FIG. 2 is a diagram showing the ISO-OSI communication standard. 
     FIG. 3 is a block diagram showing a typical prior art implementation of the physical, data-link, and network layers. 
     FIG. 4 is a block diagram of a computing system in accordance with one presently preferred embodiment of the present invention. 
     FIG. 5 shows a networked computing system in accordance with a second embodiment of the invention. 
     FIG. 6 shows a computer system shown in FIG.  5 . 
     FIG. 7 shows a printer shown in FIG.  4 . 
     FIG. 8 shows a raster network interface that is generic to a network system wherein the raster data network and the conventional data network share the same physical media such as that shown in FIG. 5, and a network system wherein the raster data network and the conventional data network have separate data networks such as that shown in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a block diagram of a typical prior art networked computing system  10 . System  10  is comprised of network bus  12 , printing devices  14  and  16 , and host computers  18  and  20 . 
     When either computer system  18  or  20  desires to print a document, it assembles page description language (PDL) instructions represented in FIG. 1 by PDL generation modules  36  and  40  in computer systems  18  and  20 , respectively. PDL generation modules  36  and  40  may be a word processor, a desktop publishing program, a CAD program, or any other program resident at the host computer that is capable of generating a series of PDL instructions. Additionally, modules  36  and  40  may provide PDL instructions that were previously generated and are now stored on storage media, such a hard disc drive. Examples of PDL&#39;s include Adobe&#39;s PostScript® language, Hewlett Packard&#39;s HP-GL/2 language, and the PCL printer control language. 
     Before a printer can print the document, the series of PDL instructions must be processed by a raster image processor (RIP) to form a raster image. This process is known in the art as raster image processing, or RIPing. The raster image is a bit-mapped representation of the document to be printed, with each bit in the bitmap representing the absence or presence of a dot (or pixel) on the printed page. 
     In the prior art, it was common for the printer to include a RIP. For example, in FIG. 1 host computer  18  generated a series of PDL instructions at PDL generation module  18 , and the instructions were sent through network interface  38  to network  12 . A printer such as printing device  14  received the PDL instruction from network  12  through network interface  22 , processed PDL instructions into a raster image at RIP module  24 , and printed the raster image at print engine module  26 . Print engine module  26  may represent any type of printer known in the art, such a laser printer or an ink jet printer. 
     The process of converting an image from a series of PDL instructions into a raster image requires a large amount of computation and generates a vast quantity of data. For example, to generate a raster image of an 8½×11 inch black and white page at a resolution of 300 dots per inch (dpi) requires approximately a megabyte of data, while a similar four-color document requires approximately four megabytes of data. Because of the computational resources required to produce a raster image, the printer&#39;s RIP is often the bottleneck that determines the throughput of the printer. 
     One solution to this problem is to provide a RIP in the host computer, and send the processed raster image from the host computer to the printer. With the advent of powerful microprocessors such as the 80486, Pentium™, and PowerPC™ microprocessors, RIPing can be performed in the background by a host system without the user perceiving significant system degradation. In addition, this solution lowers the cost of the printer because the printer need not be provided with a RIP. 
     For example, in FIG. 1 RIP module  42  of host system  20  processes a sequence of instructions from PDL generation module  20  to form a raster image. The processed raster image is then sent to a printer via network interface  44  and network  12 . Because the raster image has already been generated, the document can be printed by a printer not having a RIP, such printing device  16  in FIG.  1 . In printing device  16 , the raster image is received from network  12  via network interface  28 , and provided to print engine  34 . Alternately, the raster image can be provided to a printer having a RIP. such as printer  14 , in which case the raster image will be provided directly to the print engine. 
     One complication that arises when the RIP is provided in a host computer is that the RIP may not have information about the printer&#39;s media and ink colors. When the RIP is in the printer, in theory the RIP will know how to properly interpret a color represented in a PDL instruction. For example. to generate a certain shade of red specified in the PDL instructions, the printer will have to deposit onto the print medium a combination of ink dots at certain ratios and positions. For a given color specified in a PDL instruction, the required combination and ratio will vary from printer to printer. When the RIP is in the host computer, a user must preset parameters of the RIP based on media information of the printer that will print the raster image. 
     Since individual host computers can RIP PDL instructions in parallel, and the print engine can print a raster image as soon as it receives it. the only other factor which can prevent a printer from printing at the maximum speed of its print engine is the bandwidth of the network. 
     Many networks conform to the IEEE 802.3 Ethernet specification, which defines the hardware requirements of the network, the size of data packets that are transported by the network, and a data communication standard called ISO-OSI. The ISO-OSI communication standard defines a seven layer stack of primitives that ensures accurate data transfers between the physical Ethernet hardware and applications accessing the network. Protocols that implement the ISO-OSI standard include TCP/IP, IPX/SPX, and AppleTalk™/EtherTalk™. 
     FIG. 2 is a diagram showing the ISO-OSI communication standard. With the exception of the application layer, this diagram represents network interfaces  22 ,  28 ,  38 , and  44  in FIG.  1 . The lowest (or first) layer is the physical layer, which represents network interface hardware  46 . Network interface hardware  46  is responsible for transmitting data packets to and receiving data packets from network bus  12  and will be described in greater detail below with reference to FIG.  3 . 
     The second layer is the data-link layer which represents data-link module  47 . When receiving data from the network, data-link module  47  retrieves a data packet from local memory in the hardware  46  and examines the data packet to determine whether the packet conforms to one of the supported protocols. In FIG. 2, protocol stack  60  represents protocol A, protocol stack  62  represents protocol B, and protocol stack  64  represents protocol C. If the packet conforms to a supported protocol, the packet is provided to the network layer of the protocol stack of the protocol associated with the packet. When sending data to the network, data-link module  47  receives a data packet from the network layer of a protocol stack and provides the packet to the physical layer for transmission to the network. 
     The remaining five layers of the ISO-OSI standard are the network, transport, session, presentation, and application layers. The application layer represents the application that is communicating with the network. The other four layers perform various functions such as encoding and decoding addresses of packets, high level error correction, partitioning data into packets, maintaining packet order and flow control, implementing process-to-process data flow, and formatting data for applications. Generally, as data flows from layer to layer, it is repeatedly transferred and copied from a memory area associated with one layer to a memory area associated with another layer. 
     FIG. 3 is a block diagram showing a typical prior art implementation of the physical, data-link, and network layers. The physical layer comprises network interface hardware  46 , which includes network transmit and receive module  48 , local RAM  50 , and DMA-I/O module  52 . The data-link layer comprises data-link module  47 , which includes host processor RAM  54  and data-link implementation software  56 . Finally, the network layer is comprised of a portion of each of the protocol stacks  60 ,  62 , and  64 . 
     Typically, network transmit and receive module  48  includes digital-to-analog converters, analog-to-digital converters, modulators, demodulators, and other components known in the art and required to convert the signals carried by network  12  into digital data suitable for manipulation by a computer system. When a data packet is received, module  46  stores the data packet in local RAM  50  and signals data-link module  47  by asserting interrupt  58 . Data-link implementation software  56  responds by requesting DMA-I/O module  52  to transfer the contents of local RAM  50  into host processor RAM  54 . 
     In prior systems, a complete data packet is transferred from the local RAM of the physical layer to the host processor RAM of the data link layer, even if the data packet contained data formatted in accordance with an unsupported protocol, and even if only a few bytes of data in the data packet were required by the application layer. In other words, a large number of bytes were transferred from one memory location to another unnecessarily. 
     The layers defined by the ISO-OSI standard ensure accurate and reliable data transfers between computer resources connected by an Ethernet network. The layers also ensure modularity and compatibility because the vendor of a product need only design the product to communicate with an adjacent layer. For example, a word processor (which is represented by the application layer) must only communicate with the presentation layer. Likewise, Ethernet hardware need only communicate with the data-link layer. 
     While the ISO-OSI standard ensures accuracy, reliability, modularity, and compatibility, these attributes are achieved at the expense of speed. The continual copying of data (even unneeded data) from one memory location to another and the frequent handshaking that provides accuracy and reliability detract from potential throughput that could be obtained by a network based on the IEEE 802.3 Ethernet specification. 
     FIG. 4 is a block diagram of a computing system  65  in accordance with one presently preferred embodiment of the present invention. System  65  includes a conventional network bus  66 , as is known in the art. and a raster network bus  68 . Raster network bus  68  is a dedicated network designed to transmit raster data from a RIP to a printer. 
     In FIG. 4, computer systems  70  and  72 , printer  76 , and print server  82  are connected via network bus  66 . Computer system  72 , printers  76  and  78 , raster data front end  80 , and print server  82  are connected via raster network bus  68 . 
     Printer  76  includes network interface  84 , RIP  86 , print engine  88 , raster data buffer  90 , raster connection module  92 , and raster network interface  96 . Since printer  76  is provided with a RIP, printer  76  can accept and process PDL instructions. For example, computer system  70 , which includes PDL generation block  98  and network interface  100 , but does not include a RIP, can send PDL instructions through network interface  100  to network bus  66 . Printer  76  can receive the PDL instructions through network interface  84 , RIP the instructions into a raster image at RIP  86 , and print the raster image at print engine  88 . 
     Printer  76  is also coupled to raster network bus  68  via raster network interface  96 . Raster connection management  92  maintains unique virtual connections between printer  76  and devices providing raster data, and raster data buffer  90  stores raster data in preparation for printing the raster data at print engine  88 . In one embodiment, raster data buffer  90  is large enough to ensure that print engine  88  can print a complete page at its maximum speed, thereby minimizing the banding effect associated with ink from a previous scan drying before the next scan. In another embodiment, raster data buffer  90  is large enough to store at least one complete raster image of a document, thereby allowing multiple copies of the same document to be printed without requiring reRIPing the PDL instructions, and without retransmitting the raster image over the network. In yet another embodiment, raster data buffer  90  is large enough to store two or more raster images, thereby allowing print engine  88  to print one image while raster data buffer  90  receives another. In this embodiment, images may be stored in a FIFO queue and printed on a first-in first-out basis, or prioritized in some other manner. If raster data buffer  90  is sufficiently large, raster images may be permanently stored in buffer  90 , and repeatedly printed at the initiation of the user. Because raster data buffer  90  must hold vast amounts of data, in a preferred embodiment of the present invention, buffer  90  comprises at least one hard disc drive. 
     Printer  76  and  78  are also provided with profile information modules  94  and  108 , respectively. Profile information modules  94  and  108  provide media and ink profile information to computer systems that RIP PDL instructions, as will be explained below. 
     Printer  78  includes print engine  102 , raster data buffer  104 . raster connection management  106 , printer profile information module  108 , and raster network interface I  10 . The elements referenced in printer  78  perform the same functions as the identically named elements in printer  76 . However, printer  78  does not include a RIP nor a conventional network interface. Accordingly, printer  78  does not process PDL instructions such as those from computer system  70 , but can only process RIP data provided via raster network bus  68 . 
     Printers  76  and  78  are designed to utilize the system of the present invention. However, simpler printers may also be provided with raster data front end  80  to take advantage of the features of the present invention. Raster data front end  80  includes printer profile information module  112 , raster network interface  114 , raster connection management  116 , raster data buffer  118 , and printer interface  120 . Printer  122  is coupled to raster data front end  80  and includes printer interface  124  and print engine  126 . Printer interface  124  of printer  122  and printer interface  120  of raster data front end  80  are connected by line  128  and together may form any common interface as in known in the art. such as a parallel interface, a serial interface, a SCSI interface, etc. With the exception of printer interfaces  120  and  124 , the elements referenced in raster data front end  80  and printer  122  perform the same functions as the identically referenced elements of printer  78 . 
     Computer system  72  is comprised of raster network interface  130 , raster connection management module  132 , media/ink correction module  134 , RIP  136 , PDL generation module  138 , and network interface  140 . 
     Network interface  140  provides access to conventional network  66  for typical network operations, such as file access, E-mail, and the like. PDL generation module  138  represents a device that provides PDL instructions. such as a word processor, a CAD program, or a storage device storing previously generated PDL instructions. 
     RIP  136  processes the PDL instructions into a raster image. Thereafter, raster connection management module initiates a connection dialogue with printers coupled to raster network bus  68 . The connection dialogue results in a virtual connection being opened to a selected printer and will be described in greater detail below. 
     After a printer is selected, the selected printer provides media and ink information from the printer&#39;s printer profile information module (or the printer profile information module of the raster data front end attached to the printer). The information includes the print media presently engaged by the printer, including thickness, transparency and reflectivity characteristics, size, and other factors affecting the print media. The ink information includes the ink lot number, color, and chromatic characteristics of the inks. Media/ink correction module  134  uses the media and ink information of the selected printer to adjust for media and ink differences between printers to produce corrected raster image data. Raster connection management module  132  then transmits the corrected raster image data through raster network interface  130  and raster network bus  68  to the selected printer. 
     Print server  82  is a device configured to receive PDL instructions from a computing device coupled to network  66 , RIP the PDL instructions, and provide the resulting raster data to a printer via raster network bus  68 . Print server  82  includes raster network interface  142 , raster connection management module  144 , media/ink correction module  146 , RIP  148 , routing  150 , and network interface  152 . Print server  82  is provided to receive a document represented by a series of PDL instructions from a computer system not coupled to raster network bus  68 , such as computer system  70 , or simply to off-load RIP processing from another computer system. Routing module  150  is provided to route printing jobs to a selected printer, and provide print job status information back to the computer system that originated the print job. The other referenced elements of print server  82  perform the same functions as the identically referenced elements of computer system  72 . 
     FIG. 5 shows a networked computing system  154  in accordance with a second embodiment of the invention. In system  154 , the conventional network and the raster network share the same physical network  156 . In addition to network  156 , system  154  includes printers  158 ,  160 , and  164 , raster data front end  162 , computer systems  166  and  170 , and print server  168 . 
     Printer  158  includes printer profile information module  172 , RIP  174 , print engine  176 , raster data buffer  178 , raster connection management module  180 , and raster network interface  182 . Printer  160  includes print engine  184 , raster data buffer  186 , raster connection management module  188 , printer profile information module  190 , and raster network interface  192 . Raster data front end  162  includes printer profile information module  194 , raster network interface  196 , printer interface  198 , raster data buffer  200 , and raster connection management module  202 . Printer  164  includes printer interface  204  and printer engine  206 . 
     Computer system  166  includes raster network interface  208 , raster connection management module  210 , media/ink correction module  212 , RIP  214 , and PDL generation  216 . Print server  168  includes raster network interface  218 , raster connection management  220 , media/ink correction module  222 , RIP  224 , and routing module  226 . Finally, computer system  170  includes PDL generation  228  and network interface  230 . 
     Generally, the elements referenced in FIG. 5 perform the same functions as similarly labeled elements in FIG.  4 . However, the raster network interfaces of printers  158  and  160 , raster data front end  162 , computer system  166 , and print server  168  communicate via the network using standard ISO-OSI protocols, as well as the unique raster data protocol defined by the present invention. 
     FIG. 6 shows computer system  166  of FIG.  5 . In FIG. 6, network hardware  230  of raster network interface  208  is coupled to network  156 . Raster network intervention module  232  of interface  208  intervenes between network hardware  230  and raster connection management module  210  and conventional protocol layers  234 . FIG. 6 is also representative of print server  168  of FIG.  5 . In the embodiment shown in FIG. 4, FIG. 6 is representative of computer system  72  and print server  82 , however, conventional protocol layers  234  are not present. 
     Protocol layers  234  implement the ISO-OSI standard shown in FIG.  2 . When a data packet is received by network hardware  230 , raster network intervention module  232  examines the packet and determines whether the packet contains raster data. If it does, module  232  sends the relevant portion of the data packet to raster connection management module  210 . 
     In one embodiment of the present invention, module  232  sends any packets that are not raster data packets to conventional protocol layers  234 , thereby providing maximum modularity with an existing implementation of conventional protocol layers  234 . In another embodiment of the present invention, raster network intervention module  232  is aware of the protocols supported by conventional protocol layers  234 , and ignores any packets that are not supported. In this embodiment, the functions of the data link layer and the raster network intervention module may be incorporated into a single module. 
     Raster connection management module coordinates virtual connections between RIPs and printers. In other embodiments, raster connection management module  210  compresses raster data, decompresses raster data, encrypts raster data, and decrypts raster data as is known in the art. Module  210  also requests printer profile information from a selected printer, and adjusts outgoing raster data at media/ink correction module  212  based on media and ink characteristics of the selected printer. 
     In FIG. 6, PDL generation modules  216 A and  216 B provide PDL instructions that are RIPed by RIPs  214 A and  214 B, respectively. Computer system  166  may have any number of RIPs, which are coordinated by raster connection management module  210 . Raster connection module  210  then sends raster data to the selected printer via network bus  156  using the transmission protocol of the present invention, which is described below. 
     FIG. 7 shows printer  78  of FIG.  4 . Printer  78  is only coupled to raster data network bus  68  and includes print engine  102 , raster data buffer  104 , raster connection management  106 , and raster network interface module  110 . Raster network interface module  110  includes raster network data link module  238  and network hardware  236 . If printer  78  were configured to support other protocols, printer  78  would be provided with conventional protocol layers  234  of FIG. 6, and raster network data link module  238  would be replaced with raster network intervention module  232  of FIG.  6 . 
     FIG. 8 shows a raster network interface  240  that is generic to a network system wherein the raster data network and the conventional data network share the same physical media such as that shown in FIG. 5, and a network system wherein the raster data network and the conventional data network have separate data networks such as that shown in FIG.  4 . 
     Network interface  240  comprises network hardware  244  and raster network intervention/data link module  252 . Also shown in FIG. 8 are network bus  242 , raster connection management module  256 , and conventional protocol stack  254 . 
     Network hardware  244  includes network transmit and receive module  246 , local RAM  248 , and DMA-I/O module  250 . Network transmit and receive module  246  is coupled to network bus  242  and includes digital-to-analog converters, analog-to-digital converters, modulators, demodulators, and other elements required to transmit and receive data from network bus  242 . Local RAM  248  is provided to store data that has just been received from the network or is about to be transmitted to the network. DMA-I/O module  250  is provided to transfer the contents of local RAM  248  to protocol layers  254  or raster connection management module  256 , or alternately, to transfer data from layers  254  or module  256  into local RAM  248 . 
     The present invention implements a method of receiving data that greatly reduces overhead processing and data transfer. As discussed above with reference to FIG. 3, in the prior art a received packet is always transferred from local RAM into other RAM that is part of the data link layer. 
     In contrast, the present invention allows raster network intervention/data link module  252  to examine the contents of local RAM  248 . If the contents of RAM  248  are formatted in accordance with a supported protocol, module  252  directs DMA-I/O module  250  to transfer data from local RAM  248  to conventional protocol layers  254  or raster connection management module  256 . However, if the contents of RAM  248  do not conform to a supported protocol, the contents are ignored. 
     In addition, only required data need be transferred from local RAM  248 . For example, some commands that are transmitted over a network require only a few bytes to be transferred from local RAM  248 , while data packets containing raster data will require over a thousand bytes to be transferred from local RAM. Accordingly, network interface  240  minimizes the amount of data that must be transferred from local RAM  248 . 
     When a computer system or print server system desires to print a document, the system enters into a connection dialogue using the protocol of the present invention. The protocol defines three types of addressing modes and two types of packets. The three addressing modes are directed, broadcast, and multicast. A directed packet is sent to a single device on the network having a unique network address. A broadcast packet is sent to all devices on the network, and a multicast is sent to a subset of devices on a network. 
     The two types of packets are command packets and data packets. Data packets are used to transfer raster data to printers, and are “connected packets”, meaning they are only transmitted in association with a previously established virtual connection. Data packets are directed packets, and are further classified based on the type of raster data they carry. Command packets, on the other hand, may be “connected” or “connectionless packets”, and also may be directed, broadcast, or multicast. 
     When the system desires to print a document, the system first transmits a printer ID request. This printer ID request includes a network address for the system and is contained in a connectionless data packet. The printer ID request may either be broadcast to all network entities, or be multicast only to printers. 
     All printers capable of handling raster data respond with a printer ID response. The printer ID response is contained in a directed connectionless packet and includes the printer&#39;s network address, the printer type, and other information. 
     The system that issued the printer ID request then issues an open virtual connection request to a printer based on the received printer ID responses. The open virtual connection request is contained in a directed connectionless packet and includes the network address of the system and the maximum number of data packets the system can transmit in a single burst. 
     The selected printer must respond to the open virtual connection request with an open virtual connection response within a predetermined time limit. The open virtual connection request is contained in a directed connectionless command packet. Receipt of the open virtual connection response by the requesting system establishes the virtual connection. The open connection response includes a connection ID that uniquely establishes the connection in time and the maximum number of data packets that can be received by the printer in a single burst. The maximum number of data packets that the printer can receive in a single burst will be less than or equal to the maximum number of data that the system can transmit in a single burst. This maximum number of data packets will be referred to below as an end sequence number. 
     Once the virtual connection has been established, the system and the selected printer communicate via the virtual connection using connected command packets and connected data packets. The system sends a job information request command to the printer that includes information about the color streams present in the raster data, the size of the job, and the number of copies to print. The selected printer then issues a job information response command through the virtual connection to the system. The response informs the system whether the printer accepts the information contained in the job information command as valid. 
     Until this point, the system has initiated the dialogue and the printer has responded. However, at this point the printer becomes the initiator of raster data transfer operations. The printer issues a request/acknowledge command contained in a connected command packet that simultaneously acknowledges data received so far, and requests additional data. The request/acknowledge command includes a data parcel type field that identifies the color stream of the data required by the printer and a current sequence number that acknowledges receipt of all packets comprising a burst up to the current sequence number. 
     Normally, the system will transmit a complete burst of data packets to the printer without error, in which case the printer responds with a request/acknowledge command having the current sequence number equal to the end sequence number, thereby acknowledging to the system that all the data was received for that partial sequence. However, if a partial or whole burst of data packets is lost or otherwise corrupted, the printer will respond immediately with a request/acknowledge command having the current sequence number, thereby informing the system that a portion of the burst must be retransmitted. In other words, if the printer detects a problem with a burst, the printer requests that the system retransmit the burst, even if the system has not finished transmitting the original burst. By immediately requesting retransmission, the protocol of the present invention recovers from errors faster than prior art techniques. 
     The system transmits data to the printer using data packets that have a size of approximately one kilobyte in one presently preferred embodiment of the present invention. Accordingly, if the printer and the system negotiated a maximum burst size of 256 kilobytes, a burst would comprise 256 data packets. 
     In addition to the connected commands described above, there are several other connected commands. A probe command is contained in a directed, connected command packet sent by the system to the printer to inform the printer that the system is still functioning and the virtual connection is still open. In a preferred embodiment, a probe is sent about every 30 seconds. The printer responds to a probe command with a probe response command, which is also contained in a directed, connected command packet. If the printer fails to respond to a probe command with a probe response command within a specified period of time, then the virtual connection is terminated. Finally, a close command is contained in a directed connected packet and terminates the virtual connection. The close command can be initiated by either the system or the printer. 
     When the transmission of raster data to the printer is complete, either the printer or the system may issue a close connection command, which terminates the virtual connection. 
     The present invention facilitates printing a document over a network by increasing the speed of raster data transfers over the network. This is achieved by several unique features of the present invention. In one embodiment, a separate network is provided to transmit raster data. By providing a separate network, the total bandwidth of the network is available to transmit raster data. 
     In addition, the present invention provides a protocol which minimizes protocol overhead within a printer or computer system. A received packet is examined in the local memory of the network hardware, and is ignored if not needed, and only those bytes of the received packet that are required are transferred from local memory. Further, data is transferred directly from the local memory of the network hardware directly to the memory raster connection management module, as opposed to the multiple transfers that occur as data moves up and down a conventional ISO-OSI stack. Finally, the data may be compressed and decompressed to obtain additional network bandwidth. The present invention also defines a unique raster data transmission format that minimizes the handshaking required by prior art transmission protocols. 
     For any given network, the present invention has the potential to achieve 2-10 times the effective data transfer rate of raster data through the network compared to conventional techniques. Accordingly, the present invention facilitates host based RIPing of documents represented by PDL instructions and anticipates other advances in the art that are dependent on network throughput. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.