Print log
Printer protocols are sets of rules for communication that define the rules, syntax and semantics of communication between a computer or printer server and a printer.
There are very simple protocols in which only the pure print data are transmitted. More extensive protocols optionally also include encryption, user / access rights, order prioritization, order cost recording, order routing, order time control, error messages and their handling as well as queries of status and equipment features, as well as control commands for, for example, the selection of the paper tray, stapler, etc., or control commands for default settings .
All protocols functionally consist of 2 parts: page content and control data and are to be assigned to the application layer (7) in the OSI model. The content is always transmitted with a defined page description PDL (PS, XPS, PDF, JPEG, TIFF ...) and accompanied by control instructions in a Printer Command Language (PCL, PostScript). This generally applies to both locally connected printers and network printers , with the difference that the transport channel is layer 4 in the OSI model instead of on a USB stack : TCP or UDP on a specified TCP or UDP port.
Some manufacturers have extended existing printer protocols in a proprietary manner, or supplemented them with other communication protocols in order to map certain functions, for example the print data is delivered via port 9100 and the status queries are made with the Simple Network Management Protocol on port 161. LPR was initially used on Unix / Linux , which has practically disappeared today because too many functions are missing. Today the IPP is used here in the form of the Common Unix Printing System , which is very extensive today. Under Windows, printing is usually done with Server Message Block releases, these are more general protocols for providing any network resources that also contain a print protocol. In server-based computing, print jobs are also transmitted to printers that are locally connected to a workstation computer via the RDP ( Microsoft ), ICA ( Citrix ) or Red Hat RHEV SPICE session protocols . These are then integrations of print protocols into the respective session protocols.
Since the actual print output is always a time bottleneck, print jobs are always temporarily stored in a queue. This queue then serves the actual output. This queue is absolutely necessary for network printers n, since a printer in the network is a shared resource which, unlike locally connected printers, can be used by several network participants. Without the queue and its management via the print protocol, the delivering client would be partially blocked for the duration of the print and the printer would not be ready for all other participants.