Abstract:
A method and an arrangement in a data communications system. The object of the invention is to achieve a wireless communication between a processing unit and a printer using a safe transmission and an increased transmission range compared to the infrared transmission. The solution is a way of printing a document in a data communications system using a protocol profiled for printing in the Bluetooth protocol architecture.

Description:
This application claims the benefit of U.S. provisional application No. 60/208,098, filed May 31, 2000. 
    
    
     FIELD OF INVENTION 
     The present invention relates to a method and an arrangement in a data communications system according to the preambles of the independent claims. More specifically it relates to a processing unit wirelessly connected to a printer. It further relates to printing a document by means of the printer, the printer being controlled by the processing unit. 
     DESCRIPTION OF RELATED ART 
     Processing units, e.g. PC&#39;s requiring to print documents uses typically a printer. A processing unit and a printer are generally communicating with each other through cables. But communication disruption caused by wire breakage or inadequate securing of the cable ends, added cost of providing a reliable cable and reliable associated connectors, tangling of the cables and requirements of flexibility, etc. leads to a requirement of replacing the cables. 
     A way of communicating, using a infrared link instead of a cable is shown in the American patent U.S. Pat. No. 6,055,062, which discloses an electronic printer having an attached accessory unit. The accessory unit handles e.g. optional media (e.g. paper) supply units and optional media output. To communicate with the accessory unit, the printer uses a two-ways infrared communications connection to the accessory unit to which it is immediately adjacent. 
     However the range of the infrared link is short, so that the distance between processing unit and the printer have to be less than a few meters and there must be a clear line of sight between them. 
     The so-called Bluetooth interface is an example of a modern radio interface, which was originally intended as replacement for cables between units. The term Bluetooth is in this disclosure used as an example of usage of short-range radio communication. By replacing the cables, the short-range radio technology provides a universal bridge to existing data networks, a peripheral interface, and a mechanism to form small private ad hoc groupings of connected devices away from fixed network infrastructures or connected to a fixed network infrastructure via a gateway. Designed to operate in a noisy frequency environment, the Bluetooth radio uses a fast acknowledgement and frequency hopping scheme to make the link robust. Bluetooth radio modules avoid interference from other signals by hopping to a new frequency after transmitting or receiving a data packet, as shown in  FIG. 1  wherein the X-axis represents the frequency f and the Y-axis represents the time t. Compared with other systems operating in the same frequency band, the Bluetooth radio typically hops faster and uses shorter radio packets. This makes Bluetooth radio more robust than other systems. Use of Forward Error Correction (FEC) limits the impact of random noise on long-distance links. 
     Bluetooth radio is a wireless communication technology using a frequency-hopping scheme in the unlicensed Industrial Scientific Medical (ISM) band at 2,4 GHz. A frequency hop transceiver is applied to combat interference and fading. A shaped, binary FM modulation is applied to minimise transceiver complexity. The gross data rate is 1 Mb/s and Time-Division Duplex (TDD) scheme is used for full duplex transmission. 
     The Bluetooth protocol is a combination of circuit and packet switching. In  FIG. 1 , S 1  denotes one time slot, and P 1  denotes a packet covering three time slots. A time slot is 0,625 ms long. Time slots can be reserved for synchronous packets. Each packet is normally transmitted in a different hope frequency. A packet normally covers a single slot, but can be extended to cover up to five slots. Bluetooth can support an asynchronous data channel, up to three simultaneous synchronous voice channels, or a channel with simultaneously supports asynchronous data and synchronous voice. Each voice channel supports 64 kb/s synchronous (voice) link. The asynchronous channel can support an asymmetric link of maximally 721 kb/s in either direction while permitting 57,6 kb/s in the return direction, or a 432,6 kb/s symmetric link. 
     In  FIG. 2 , the different function blocks of a system using short-range radio transceivers such as Bluetooth are shown. A radio unit  201  is connected to a link control unit  202  providing the base band. The link control unit  202  is connected to the Central Processing Unit, called CPU,  203  providing the link management. The CPU is connected to the memory  204  providing software functions and consisting of two memory units: a SRAM  205  and a FLASH  206 . The CPU  203  is connected to a host interface  207 . A SRAM is a fast temporary memory. FLASH is a programmable ROM. 
     Two or more, up to eight Bluetooth units sharing the same channel form a piconet, i.e. a piconet is a collection of devices connected via Bluetooth technology in an ad hoc fashion. Within a piconet a Bluetooth unit can have either of two roles: master or slave. Within each piconet there may be one and only one master, and up to seven active slaves, i.e. a piconet starts with two connected devices, such as a portable PC and a cellular telephone, and may grow to eight connected devices. All Bluetooth devices are peer units and have identical implementations. Any Bluetooth unit can become master in a piconet. A master unit is the device in a piconet whose clock and hopping sequence are used to synchronise all other devices within the piconet. A slave unit is every device in a piconet that is not a master. 
     The communication within a piconet is organised such that the master polls each slave according to some polling scheme. Master-to-slave transmission always starts in an even-numbered time-slot while slave-to-master transmission always starts in an odd-numbered time slot. With one exception the slave is only allowed to transmit after have been polled by the master. The slave then starts its transmission in a slave-to-master time slot immediately following the packet received from the master. The master may or may not include data in the packets used to poll the slave. The only exception to the above principle is that when a slave has an established Synchronous Connection Oriented (SCO) link, the slave is always allowed to transmit in the pre-allocated slave-to-master slot, even if not explicitly polled by the master in the preceding master-to slave slot. The term SCO-link will be disclosed in more details below. In a Bluetooth communications system there is no direct transmission between slaves in a piconet. 
     The Bluetooth protocol stack will be described, according to the specifications of the Bluetooth system. The protocol stack which is depicted in  FIG. 3 , includes two Bluetooth units  301  and  302 . In the figure the physical layer and the data link layer are shown. 
     Baseband BB 
     The base band describes the digital signal processing part of the hardware, i.e. the Bluetooth link controller, which carries the Bluetooth protocols and other low-level link routines. The Baseband resides in the physical layer  301  and the data link layer  304 . The baseband specification defines two link types: Synchronous Connection-Oriented (SCO) links and Asynchronous Connection-Less (ACL) links. SCO links support real-time voice traffic using reserved bandwidth. ACL links support best effort traffic. 
     Link Manager Protocol LMP 
     
         
         
           
             LMP handles messages used for link set-up, security and control. LMP is layered over the Baseband protocol and resides in the data link layer  304 .
 
Logical Link Control and Adaptation Layer Protocol, L2CAP
 
             L2CAP is also layered over the Baseband protocol and resides in the data link layer  304 . L2CAP provides connection oriented and connectionless data services to upper layer protocols with multiplexing capability, segmentation and reassemble operation, and group abstractions. The L2CAP Specification is only defined for ACL links.
 
Network Layer  305 
 
           
         
       
    
     The network layer is currently not specified in the Bluetooth standard. 
     High Level Protocol or Application  306   
     
         
         
           
             Device information, services and the characteristics of the services can be queried using the Service Discovery Protocol SDP. Like SDP, RFCOMM is layered on top of the L2CAP. RFCOMM is the ‘cable replacement’ protocol, which provides transport capabilities for high-level services (e.g. OBEX protocol) that use serial line as the transport mechanism. 
           
         
       
    
     On top of the link and transport protocols, the applications still need some specific protocols to complete the protocol stack. In the Bluetooth architecture, the application-specific protocols are added on top of RFCOMM or directly on the L2CAP. L2CAP can only be accessed via a protocol which is supported by a Bluetooth profile such as RFCOMM. 
     The enumerated application-specific protocols offer the basic functionality in the Bluetooth environment and they provide only the cable-replacement capabilities. Features such as broadcasting, point-to-multipoint topologies, and scatternet possibilities are not really utilised by these current high-level protocols and usage models. Thus, there are numerous possibilities for developers to create more applications, the nature of which can be totally different from the existing ones. 
     The object of the present invention is to achieve a wireless communication between a processing unit and a printer using a safe transmission and an increased transmission range compared to the infrared transmission used in the above mentioned US-patent. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to unravel the above mentioned drawbacks and achieve a way of printing a document in a data communications system using a protocol profiled for printing in the Bluetooth protocol architecture. 
     This is achieved according to the method and arrangement set forth in the characterising parts of the independent claims. 
     Preferred embodiments are set forth in the independent claims. 
     An advantage of the method and arrangement according to the present invention is that it is possible to communicate wirelessly with a printer at a wide range, up to 10 meters and extendable up to 100 meters. 
     Another advantage is that it offers a safe transferring of data. 
     Yet another advantage is that the present invention makes it possible to wirelessly select a printer among available printers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the relationship between timeslots and frequency hops in a system using Bluetooth. 
         FIG. 2  is a diagram illustrating the different function blocks of a Bluetooth system. 
         FIG. 3  is a diagram showing the Bluetooth protocol stack. 
         FIG. 4  is a schematic block diagram showing a communications system according to the present invention. 
         FIG. 5  is a schematic block diagram showing an entity according to the present invention. 
         FIG. 6  is a schematic block diagram showing a printer entity according to the present invention. 
         FIG. 7  shows a flowchart of the method according to the invention. 
         FIG. 8  is a bloc diagram depicting a protocol overview over the Bluetooth protocols according to the invention. 
         FIG. 9  shows a signalling sequence over a typical SDP transaction. 
         FIG. 10  shows a signalling sequence over typical WPP transactions. 
         FIG. 11  shows a signalling sequence over typical WPP transactions. 
         FIG. 12  shows a signalling sequence over typical WPP transactions. 
         FIG. 13  shows a signalling sequence over typical WPP transactions. 
         FIG. 14  shows a signalling sequence over typical WPP transactions. 
         FIG. 15  shows a signalling sequence over typical WPP transactions. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIGS. 1-3  are related to prior art and described above under “Description of related art”. 
     The wording “client” is in this disclosure defined as the entity sending a request, and the wording “server”, is in this disclosure defined as the entity receiving a request. 
       FIG. 4  shows a possible scenario of the present invention. A Bluetooth data communications system  401  includes two nodes whereof one is a processing unit, which in this example is a PC  402  and the other is a printer  403 . A wireless printer protocol according to the invention is implemented in the Bluetooth protocol stack which is included in a entity, e.g. a PC-card  404 , connected to or implemented in the PC  402 , and in a printer entity, e.g. a printer adapter  405 , connected to or implemented in the printer  403 . According to the Bluetooth standard the distance between the processing unit and the printer is up to 10 meters and extendable up to 100 meters The printer adapter  405  might be connected to the printer port on the printer. The PC  402  and the printer  403  are connected to each other via a Bluetooth air interface  406 . Both entities  404  and  405  comprise a respective computer, each computer comprising an internal memory for storing computer program not visible in  FIG. 4 . 
     The entity  404  connected to or implemented in the processing unit  402 , will now be described more in detail. The entity, now referred to as  501  is shown in  FIG. 5 . The entity  501  includes a Bluetooth protocol stack in which protocol stack a wireless printer protocol is implemented. The printer protocol comprises a printer client which communicates with a printer server by means of the wireless printer protocol, the Bluetooth protocol stack and air interface,. The printer server is included in a printer but is not visible in  FIG. 5 . 
     The entity  501  includes an establishing device  502  arranged for establishing a bi-directional wireless ACL connection between the processing unit and the printer by means of the Bluetooth protocol. 
     The entity  501  comprises further a sending device  503  arranged for sending a connection request message to the printer server and a negotiating device  504  arranged for negotiating configuration parameters with the printer server. The negotiating device  504  comprises a sending device  505  arranged for sending, to the printer server, a configuration request message including no new options if the printer client uses default values. The negotiating device  504  comprises also a sending device  506  arranged for sending, to the printer server, a configuration request message including a suggestion of configuration options. The negotiating device  504  comprises further a sending device  507  arranged for sending, to the printer server, a further configuration request message including a suggestion of configuration options which differs from earlier suggestions of configuration options. This latter sending device  507  is to be used if the printer client receives a response message from the printer server that the configuration request was not acceptable due to e.g. unacceptable parameters, unknown option etc. 
     The entity  501  comprises a sending device  508  arranged for sending a set attribute request message to the printer server, the message comprising e.g. a coding table concerning a negotiated coding type and is to be loaded by the printer server. 
     The entity  501  comprises a sending device  509  arranged for sending keep alive messages frequently to the printer server. A keep alive timer  510  is implemented in the entity  501  and comprises a starting device  511  arranged for starting and restarting the keep alive timer  510  each time a valid message is sent to the printer server and each time a valid message is received from the printer server. The keep alive timer  510  further comprises a closing device  512  arranged for closing the connection between to the printer server, when the keep alive timer  510  expires. 
     For starting one or more printjobs the entity  501  comprises a starting device  513  arranged which starting device  513  comprises a sending device  514  arranged for sending a request message to the printer server comprising a request to start a printjob. 
     The print data that is to be printed by the printer is sent by means of a sending device  515  arranged for sending the print data to the printer server. Said device  515  includes a sending device  516  arranged for sending a number of request messages to the printer server, the messages comprising print data. 
     A printing process might be broken, e.g. because the printer runs out of paper or the ACL connection is broken, etc. This is reported by the printer server in a message received by the printer client. The entity  501  comprises a device  527  arranged for interpret the message and give a note to the user of the processing unit, e.g. by presenting the note on the screen of the PC. 
     E.g. a refill of paper or a new creation of a disconnected ACL connection might make, but the entity  501  comprises a continuing device  517  arranged for continuing the printing process by continuing to send print data request messages to the printer server, starting with the print data subsequent to a last received print data acknowledgement message. 
     The entity  501  comprises a stopping device  518  arranged for stopping the keep alive timer  510  when an ACL connection is disconnected during a printing process. 
     The entity  501  further comprises a requesting device  519  arranged for requesting a reconnection of a session defined by the session identifier in a message sent to the printer server to be used when a new ACL connection is created to the printer, after a break. 
     The entity  501  comprises a stopping device  520  arranged for stopping the print job said stopping device  520  comprises a sending device  521  arranged for sending a message to the printer server, the message comprising a request to stop the printjob. The stopping device  520  will be used when all data to be printed in a printjob is sent to the printer. 
     The entity  501  further comprises a closing device  522  arranged for closing the connection between the processing unit and the printer, the closing device comprising a sending device  523  arranged for sending a message to the printer server, the message comprising a request to disconnect a session identified by a session identity. 
     The entity  501  comprises a stopping device  524  arranged for stopping the sending of keep alive messages after closing a connection between the printer client and the printer server. 
     The entity also comprises a receiver  525  for receiving messages sent from a printer and a transmitter  526  for sending messages to the printer. 
     The printer entity  405  connected to or implemented in the printer  403  shown in  FIG. 4 , will now be described more in detail. The printer entity, now referred to as  601  is shown in  FIG. 6 . The printer entity  601 , including a Bluetooth protocol stack in which a wireless printer protocol is implemented, said protocol comprising a printer server which communicates, by means of the wireless printer protocol, the Bluetooth protocol stack and air interface, with a printer client, e.g. the printer client in the entity  501  described above . The printer client is included in a processing unit  402  and is not visible in  FIG. 6 . 
     The printer entity  601  comprises a receiver  602  for receiving messages sent from a processing unit and a transmitter  603  for sending messages to the processing unit. 
     The printer entity  601  further comprises a responding device  604  arranged for responding upon a connection request whether the connection is successful or not, in a response message sent to the printer client. 
     The printer entity  601  comprises a negotiating device  605  arranged for negotiating configuration parameters with the printer client within the processing unit. 
     The negotiating device  605  comprises a responding device  606  arranged for responding upon a configuration request whether the configuration options in the configuration request are supported by the printer server or not. 
     The negotiating device  605  comprises a loading device  607  arranged for loading a coding table or other optional attributes sent from the printer client. 
     The negotiating device  605  further comprises a sending device  608  arranged for sending a response, whether the loading of the coding table was successful or not, to the printer client. 
     The printer entity  601  comprises a sending device  609  arranged for sending keep alive messages frequently to the printer client. 
     A keep alive timer  610  is implemented in the printer server within the printer entity  601 . The printer entity  601  comprises a starting and restarting device  611  arranged for starting the keep alive timer each time a valid message is received from the printer client and each time a valid message is sent to the printer client. 
     The printer entity  601  comprises a starting device  612  arranged for starting a print job. The starting device  612  comprises a confirming device  613  arranged for confirming a start printjob request message sent to the printer client 
     The printer entity  601  comprises a receiving device  614  arranged for receiving print data from the printer client. The receiving device  614  including a sending device  615  arranged for sending an acknowledgement message to the printer client after receiving a previous decided number of print data request messages. 
     The printer entity  601  comprises an indicating device  616  arranged for indicating, in a message sent to the printer client, that the printer has reported an exemption condition, e.g. that the printer is out of paper, if the printer runs out of paper. 
     The printer entity  601  further comprises an indicating device  617  arranged for indicating, in a message sent to the printer client, when the printer clears the exemption, e.g. that the printer is refilled, when the printer is refilled. 
     The printer entity  601  comprises a stopping device  618  arranged for stopping the keep alive timer when an ACL connection to the processing unit is disconnected during a printing process. 
     The printer entity  601  comprises a sending device  619  arranged for sending a response message to the printer client, according to whether a reconnection request is granted or not. 
     The printer entity  601  comprises a stopping device  620  arranged for stopping the print job. 
     The stopping device  620  including a sending device  621  arranged for sending a response message, after the printer server has received a request to stop the printjob, the message comprising a confirmation that this is apprehended and is sent to the printer client. 
     The printer entity  601  comprises a sending device  622  arranged for sending a response message to the printer client, according to whether a disconnection request is granted or not. 
     The printer entity  601  further comprises a stopping device  623  for stopping the sending of keep alive messages after the connection to the printer client is closed. 
       FIG. 7  shows a flowchart of a possible scenario of the printing process according to the present invention. 
     The method includes the following steps:
       701 . A bi-directional wireless Asynchronous Connection-Less (ACL) connection is established between the processing unit  402  and the printer  403  by means of the printer protocol calling the L2CAP requesting the connection and the L2CAP creating the connection.     702 . A connection is established between the printer client and the printer server for one or more printjobs.     703 . The processing unit  402  and the printer  403  negotiate configuration parameters for said connection.     704 . Keep alive messages are sent frequently during the session from the processing unit  402  to the printer  403  and from the printer  403  to the processing unit  402 .     705 . The processing unit  402  starts the printjob and     706 . sends the printer data to the printer  403 .     707 . The print job is stopped and     708 . the connection is closed between the processing unit  402  and the printer  403 .   

     The method is implemented by means of a computer program product comprising the software code portions for performing the steps of the method. The computer program product is run on a computer stored in a digital computer within the process unit  402  and within the printer  403 , e.g. in the printer adapter  405 . 
     The computer program is loaded directly or from a computer usable medium, such as floppy-disc, CD, Internet etc. 
       FIG. 8  is a bloc diagram depicting a protocol overview over the Bluetooth protocols including the wireless printer protocol WPP according to the invention. The left side represents the PC  801  and the right side represents the Printer  802 . The Host Control Interface HCI is marked as a horizontal line. The HCI provides a command interface to the baseband controller, link manager, and access to hardware status and control registers. 
     SDP, L2CAP and LMP are described above, under Related Art. WPP will be described more in detail below. 
     The interface between two entities on the same layer, a so-called horizontal interface, is defined by it&#39;s protocol  803 ,  804 ,  805  and  812 , e.g. L2CAP on PC communicates with L2CAP on printer using the L2CAP protocol. 
     The actual flow of data (Protocol Data Units, PDU:s) is done between entities in different layers  806 ,  807 ,  808 ,  809 ,  810  and  811 , a so-called vertical interface. 
     On the PC side the protocols is implemented by following applications:
         Client L2CA Application implements L2CAP   Client Printer Application implements WPP   Client Discovery Application implements SDP       

     On the printer side the protocols is implemented by following applications:
         Server L2CA Application implements L2CAP.   Server Printer Application implements WPP.   Server Discovery Application implements SDP.       

     The printing method according to the invention will now be described more in detail. 
     A processing unit requires to print a document, i.e. to perform a printjob, by means of a printer. 
     The processing unit wishes to know which printers that are available, and select one of them, therefore the printing process starts with the Device Discovery procedure, which is a procedure known from the art.  FIG. 9  shows a sequence diagram of a typical SDP transaction between the Client 
     Discovery Application  901  and the Server Discovery Application  902 . It is assumed that inquire has been performed. As a result of inquire the class of device is retrieved. Class of device indicates the type of device and which type of services the device supports. It is also assumed that a point to point connection with the server has been established, using L2CAP. The PrinterServiceClassId is represented as a Universally Unique Identifier (UUID) and is known by client discovery application. 
     A message, e.g. a denoted SDP_ServiceSearchReq message  903  is sent, from Client to Server, to ask which services, in this case printers that are available. The server returns service records handles associated with the respective available printers, e.g. in a denoted SDP_ServiceSearchRsp message  904 . 
     The printer service record database serves as a repository of discovery-related information. All of the information about a service that is maintained by an SDP server is contained in a single service record. The service record consists entirely of a list of attributes. A service record handle uniquely identifies each service record within the SDP server, according to Service Discovery Protocol, Bluetooth Specification version 1.0 B concerning SDP and Appendix VIII, Bluetooth Assigned Numbers, Bluetooth Specification version 1.0 B concerning assigned numbers for predefined attributes and their identity. 
     The Client selects one of the available printers and requests for its attributes, e.g. the address of the printer, a in a message, e.g. a denoted SDP_ServiceAttributeReq message  905  using the service record handle. The attributes are returned in one or more messages, e.g. denoted SDP_ServiceAttributeRsp messages  906 . 
     The Client stores the received attributes and terminate the L2CAP connection 
     A bi-directional wireless asynchronous connection-less (ACL) connection is established ( 701 ) between the processing unit and the printer. This is achieved by means of the printer protocol in the processing unit calling the L2CAP in the within the same unit, requesting the connection to the printer. The printer is connected e.g. by means of the printer address being one of the attributes received. The L2CAP creates the connection and notifies the created connection the printer protocol. 
       FIG. 10  shows sequence diagrams of a typical WPP transactions concerning the connection operations between the WPP Client  1001  and the WPP Server  1002 , according to the invention 
     A creation of a session between a client printer application (source) and a server printer application (destination) is to be requested, i.e. for establishing a connection for one or more printjobs. This is performed by sending a message, e.g. a denoted WPP_Connection_Req message  1003 , from the WPP client  1001  to the WPP server  1002 . This is shown in  FIG. 10 . A status indication to the client printer application whether the connection was successful or not and making the session valid if successful is required. This is be performed in a message by the WPP server  1002 , e.g. in a denoted WPP_Connection_Rsp message  1004 , also shown in  FIG. 10 . This message also includes a session identity. 
     The next step of the printing process is the WPP negotiation procedure according to the invention.  FIGS. 11   a - c  and  12  shows sequence diagrams of a typical WPP transactions concerning the negotiation operations between the WPP Client  1001  and the WPP Server  1002 , according to the invention. 
     After creating the session a configuration of the WPP server  1002  is required. Examples of configuration options are e.g. the number of print data request messages to be received by the printer before return a confirmation message, coding type and table size. 
       FIGS. 11   a, b  and  c  shows three different sub-scenarios of a successful negotiation of a coding type for data compression. A message, e.g. a denoted WPP_Configuration_Req message, is sent from the WPP client  1001  to WPP server  1002  to establish an initial logical link transmission contract between the WPP client  1001  and WPP server  1002  and to negotiate configuration parameters, e.g. the coding type. In this example the WPP server  1002  supports the coding types hamming, table size=80 (default) and huffman table size=80. The three respective sub-scenarios may be a continuation of the connection scenario in  FIG. 10 . 
     In the first sub-scenario, shown in  FIG. 11   a,  the WPP client  1001  uses default values, i.e. hamming, table size=80 and accordingly the WPP_Configuration_Req message  1101  sent, from the WPP client  1001  to the WPP server  1002 , includes no new options. Since that is a coding type that the WPP server  1002  supports, it responses success in a message, e.g. a denoted WPP_Configuration_Rsp message  1102 . 
       FIG. 11   b  shows the second sub-scenario in which the WPP client  1001  requests the WPP server  1002 , in message, e.g. a denoted WPP_Configuration_Req message, if hamming, table size=100 can be used  1103 . This is not a coding type that the WPP server  1002  supports and accordingly it responses in a message, e.g. a denoted WPP_Configuration_Rsp message  1104 , failure and suggests that hamming, table size=80 can be used. The WPP client  1001  supports also hamming, table size=80 and responses this to the WPP server  1002  in a message, e.g. a denoted WPP_Configuration_Req message  1105 . The WPP server responses success in a message, e.g. a denoted WPP_Configuration_Rsp message  1106 . 
     In the third scenario, shown in  FIG. 11   c,  the WPP client  1001  suggests an coding type which is unknown for the printer, i.e. a coding type not supported by the printer, and a size=100, in a message, e.g. a denoted WPP_Configuration_Req message, sent  1107  to the WPP server  1002 . Since this coding type is unknown for the WPP server  1002 , it responses in a message, e.g. a denoted WPP_Configuration_Rsp message  1108  failure and that the coding type is unknown. The WPP client  1001  then tries another coding type that it supports, in this example huffman, size=80, in a subsequent message, e.g. a denoted WPP_Configuration_Req message  1109  sent to the WPP server  1002 . The WPP server  1002  supports huffman, size=80 and accordingly it responses success and confirms huffman, size=80 in a message, e.g. a denoted WPP_Configuration_Rsp message that is sent  1110  to the WPP client  1001 . 
     After the configuration negotiation of coding type according to e.g. the scenarios depicted in  FIGS. 11   a - c,  the WPP client  1001  requests to set an attribute which is illustrated in  FIG. 12 . The WPP client  1001  sends a coding table concerning the negotiated coding type in a message, e.g. a denoted WPP_Set_Attribute_Reg message sent  1201  to the WPP server  1002 . The WPP server loads the coding table to be used and confirms whether it was successful or failure in a message, e.g. a denoted WPP_Set Attribute_Rsp message  1202  sent to the WPP client  1001 . 
     The next step of the printing process is the WPP printing procedure.  FIGS. 13   a - d ,  14  and  15  shows sequence diagrams of a typical WPP transactions concerning the printing operations, between the WPP client  1001  and the WPP server  1002 , according to the invention. 
       FIGS. 13   a - d  shows a first sub-scenario of a successful printing of one print job.  FIG. 13   a  shows the procedure for sending keep alive messages. 
     When the connection has been established and negotiation has been performed, keep alive messages are to be sent, by the WPP client  1001 ,  1303  and WPP server  1002 ,  1304 , frequently, e.g. once each 5 second, as an indication that the source is up and running. Such a message is a denoted WPP_Keep_Alive message. If a break occurs when printing, the printer will find out that, since it does not receive any more keep alive messages. The printer then terminates the printjob and can let other users in. A break can also occur on the printer side. There is also occasions when the printer or processing unit are hard loaded, sending keep alive messages just to tell the receiver that it still alive but it goes slowly at the moment. When a connection has been disconnected by WPP client, WPP client  1001  and WPP server  1002  shall stop sending denoted WPP_Keep_Alive messages. 
     A WPP Keep Alive Timer is restarted each time a valid message is received from the remote endpoint. The timer is implemented on both client and server side. If the Keep Alive timer expires the remote endpoint is considered faulty and the connection is closed and higher level applications is notified. The Keep Alive Timer shall be stopped when a link is disconnected and restarted when a new link is established with the remote endpoint. If a new link is established within a reasonable time, e.g. 10 seconds, the printjob continues where broken. Each WPP message will trigger a restart of a WPP timer. 
     In  FIG. 13   b  a start of a printjob and sending of data to be printed is shown. The WPP client  1001  requests the WPP server  1002  to start a printjob in a denoted WPP_Start_Print_Req message  1305  s, which in turn confirmed by the WPP server ( 1002 ) in a denoted WPP_Start_Print_Cfm message  1306 . The WPP client then requests the WPP server  1002  to print data included in a number of denoted WPP_Print_Data_Req messages  1307 ,  1308 . A confirmation is to be sent after the WPP server  1002  has received a number N WPP_Print_Data_Req messages  1307 ,  1308 . The value of N is negotiated during configuration e.g. N=4. The acknowledgement is e.g. sent in a denoted WPP_Print_Data_Ack message  1309 . This procedure goes on until all data to be printed is received by the printer server. I.e. until the last WPP_Print_Data_Req message  1310  is received. 
     When all data to be printed is sent to the printer server the client requests the printer server to stop the printjob. This is shown in  FIG. 13   c  wherein the WPP client  1001  sends a denoted WPP_End_Print_Req message  1311  to the WPP server  1002 . That this is apprehended by the printer server is reported e.g. in a denoted WPP_End_Print_Rsp message  1312  sent to the WPP client  1001 . 
     After performing one or more printjobs or if a break of the printjob is requested, the client requests a disconnection of a session defined by the session identifier. Depicted in  FIG. 13   d,  this request is performed by e.g. sending a denoted WPP_Disconnect_Req message  1313  from the WPP client  1001  to the WPP server  1002  and a response, whether the disconnection is granted or not, is sent in the opposite direction in a denoted WPP_Disconnect_Rsp message  1314 . 
     When the session is disconnected the WPP client  1001  and the WPP server  1002  stops sending WPP_Keep_Alive messages. 
       FIG. 14  shows a second sub-scenario of a successful printing of one printjob when the printer is out of paper. Negotiation has been performed, a connection is established and keep alive messages are sent as described above though not visible in  FIG. 14 . The WPP client  1001  has requested the WPP server  1002  to start the printjob in a message, e.g. a denoted WPP_Start_Print_Req message  1401 , which is responded success to in a message, e.g. a denoted WPP_Start_Print_Rsp message  1402 . When the WPP client  1001  has requested the WPP server  1002  to print data included in a number of messages, e.g. denoted WPP_Print_Data_Req messages  1403 ,  1404 , being acknowledged by the WPP server  1002  in a message, e.g. a denoted WPP_Print_Data_Ack message  1405 , the printer is out of paper. The printer server then has to report this to the client. This can be performed by the WPP server  1002  sending a message, e.g. a denoted WPP_Status_Ind message  1406 , indicating that the printer is out of paper to the WPP client  1001 . The message is interpreted by the wireless printer protocol and reported to the user of the processing unit, e.g. by presenting a note on the PC screen. The message is obtained by a user of the processing unit including the client, who refills the printer. The printer server then reports that the printer is refilled to the WPP client  1001  by sending a message, e.g. a denoted WPP_Status_Ind message  1407 . The last received denoted WPP_Print_Data_Ack message  1405  defines where to continue the printing by sending messages, e.g. denoted WPP_Print_Data_Req messages  1408 ,  1409  from the WPP client  1001  to the WPP server  1002 . The printer will throw data if already printed or if a part of it has been printed. The printing process then continues as described above. 
       FIG. 15  shows a third sub-scenario of a successful printing of one printjob when the ACL connection is disconnected. Negotiation has been performed, a connection is established and keep alive messages are sent as described above though not visible in  FIG. 15 . The WPP client  1001  has requested the WPP server  1002  to start the printjob in a message, e.g. a denoted WPP_Start_Print_Req message  1501 , which is responded success to in a message, e.g. a denoted WPP_Start_Print_Rsp message  1502 . When the WPP client  1001  has requested the WPP server  1002  to print data included in a number of messages, e.g. WPP_Print_Data_Req messages  1503 ,  1504 , the ACL connection is disconnected, indicated by HCI. The Keep Alive Timer is stopped by the WPP client  1001 . 
     A reconnection of the session is required because it is possible for another client to start a printjob during ACL-disconnected. A session identity is used to identify the different WPP entities. If another job is ongoing the server will not accept the reconnection. The time the server will wait for the reconnection has to be handled by a reconnection timer. If the timer times out the ongoing job will be flushed. After creating a new ACL-connection a reconnection of the session is requested. This can be performed by the WPP client  1001  by sending a message, e.g. a denoted WPP_Reconnect_Req message  1506  requesting a reconnection of the session defined by the session identifier. A response according to whether the reconnection is granted or not is sent in a message, e.g. a denoted WPP_Reconnect_Rsp message  1507 . In this example it is granted. The WPP Keep Alive timer is started again. The last received denoted WPP_Print_Data_Ack message  1505  defines where to continue the printing by sending messages, e.g. a WPP_Print_Data_Req messages  1507 ,  1508  from the WPP client  1001  to the WPP server  1002 . The printer server will throw data if already printed or if the packet is detected to be a retransmission. The printing process then continues as described above. 
     The present invention is not limited to the above-described preferred embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of invention, which is defined by the appendant claims.