Abstract:
In compression of packet data headers the compressor side ( 100 ) in a point-to-multipoint connection transmits ( 301 ) a message announcing a particular service. The entities ( 200 ) receiving the announcement message are adapted to respond to the message and based on the response message(s) ( 305 ) the compressor can adapt the compression scheme used. In particular the compressor may compress ( 315 ) or not compress ( 311 ) the header of the packet data header.

Description:
TECHNICAL FIELD 
     The present invention is related to the field of header compression in data packets in a wireless communication network. In particular the present invention relates to a method and device for use in point to multi point services such as Multimedia Broadcast Multicast Services (MBMS). 
     BACKGROUND 
     An important problem for packet transfer over radio interfaces is the overhead caused by the packet headers. If a large number of relatively small packets are transferred, such as in interactive voice conversations, the overhead accounts for a significant part of the available bandwidth. Consequently, header compression protocols are usually supported in most radio interface standards. 
     One header compression technique is specified in the Internet Engineering Task Force (IETF) Request For Comments (RFC) 3095 where the compression scheme is denoted as Robust Header Compression (RoHC), see “RObust Header Compression (RoHC): Framework and four profiles: RTP, UDP, ESP and uncompressed”, IETF RFC3095. This compression scheme is designed to work well when used over links with high error rates and long-round trip times, as opposed to other existing compression schemes. 
     In RoHC the compressor and de-compressor operates using multiple states. The operation always starts in the lowest compression state and transitions are made gradually towards higher compression states. The mode of operation controls the logic of state transitions and what actions to perform in each state. 
     Generally, there are two different types of modes of operation; unidirectional and bidirectional. In unidirectional mode, packets are sent from compressor to de-compressor and thereby transitions between compressor states are performed without any knowledge about the de-compressor state, e.g. periodically. In turn bidirectional modes have feedback mechanisms for error recovery requests and acknowledgements of context updates from de-compressor back to compressor. 
     Bi-directional modes typically do not have as frequent transitions to low compression states as unidirectional mode and therefore they have (in some sense) better compression efficiency. 
     In certain cases however, such as, for example, Multimedia Broadcast Multicast Services (MBMS) of which mobile TV is one example, a fundamental difference compared to conventional mobile communication services is that MBMS is characterized by point-to-multipoint connections instead of point-to-point connections. 
     One consequence of point-to-multipoint communication is that there are multiple de-compressor machines (typically one in each terminal) and typically one compressor machine per service at the network side. Similarly, feedback channels between compressor(s) and de-compressors are undesirable (or infeasible) and therefore unidirectional mode of operation is the preferred choice. 
     In point-to multipoint connections a problem arises with conventional technology, since the number of involved de-compressor machines is constantly changing because some users join and leave the MBMS service every now and then. Hence, in order to create contexts for all de-compressors, the compressor must perform a transition to the lowest compression state whenever a new user has joined the service. In general, the arrival of a new user is unpredictable and therefore the compressor must perform these transitions in a very conservative manner. In reality the compressor may need to always assume worst-case situations with a large user population and short inter-arrival times. Although this type of design creates the necessary context for all de-compressors, the overall header compression efficiency is impaired due to frequent transitions to the lowest compression state. 
     Hence there exists a need for a more efficient transmission of data in point-to-multipoint connections. 
     SUMMARY 
     It is an object of the present invention to provide a more efficient transmission of data in point-to multipoint connections. 
     This object and others are obtained by the method and device as set out in the appended claims. 
     Thus, by providing a compression scheme for a point-to multi point connection where the compressor is provided with information relating to the arrival and/or status of the de-compressors receiving a compressed data stream without a dedicated feedback channel from the de-compressor(s). This is obtained by providing a mechanism at the compressor side in a point-to-multipoint connection that transmits a message related to a particular service. The entities receiving the announcement message are adapted to respond to the message and based on the response message(s) the compressor can adapt the compression scheme used. In particular the compressor may compress or not compress the header of the packet data header. 
     In accordance with one embodiment of the present invention a method for header compression in a data packet is provided. First an announcement message is transmitted announcing a data packet service; and a polling message for counting the number of interested receiving entities. After receiving an interest message indicating the number of entities interested in the announced data packet service; and after receiving information indicative of at least one entity having received the polling message for the first time or having experienced a data packet header decompression error; data packets comprising uncompressed data packet headers related to the announced data packet service are transmitted. After receiving a status message indicative of the state of the entity having received the announcement message data packets comprising compressed data packet headers related to the announced data packet service are transmitted. 
     The invention also extends to a computer program product and to a compression device for compressing data in accordance with the above method. The compression device adapted to compress data can, for example, be a base station transceiver, a gateway, a gateway which is part of a base station transceiver, an access point or some other infrastructure device performing equivalent functions. 
     The invention further extends to a mobile terminal for communication in a wireless communication network where the mobile terminal comprises a receiver for receiving an announcement message announcing a data packet service and for receiving data packets comprising uncompressed and compressed data packet headers related to the announced data packet service. The mobile terminal also comprises a transmitter for transmitting a state of the mobile terminal, and is adapted to transmit an interest message indicating that the mobile terminal is interested in the announced data packet service and also adapted to transmit an information message indicating that the mobile terminal has received the first information message for the first time or having received a data packet header in error. 
     In this fashion the transmitter of the compressed data packet headers will not be forced to unnecessarily often send uncompressed packet headers and therefore the overall compression efficiency will be increased. This in turn will lead to a more efficient compression of data over a point to multipoint connection leading to a higher quality of received data for each receiver in the point-to-multipoint connection. 
     In accordance with one embodiment a method for header compression in a data packet comprising the steps: a) transmitting an announcement message announcing a data packet service; and a polling message for counting the number of interested receiving entities b) receiving an interest message indicative of a number of entities interested in the announced data packet service; c) receiving an information message indicative of at least one entity having received the polling message for the first time or having experienced a data packet header decompression error; d) transmitting data packets comprising uncompressed data packet headers related to the announced data packet service; e) receiving a status message indicative of the state of the entity having received the announcement message; f) transmitting data packets comprising compressed data packet headers related to the announced data packet service is provided. 
     In accordance with one embodiment, the announced data packet service comprises a multicast data packet service. 
     In accordance with one embodiment, the announced data packet service 20 comprises a broadcast data packet service. 
     In accordance with one embodiment, the status message indicative of the state of the entity having received the first information message comprises a de-compressor context. 
     In accordance with one embodiment, the at least one entity comprises a mobile terminal. 
     In accordance with one embodiment, a computer program for header compression in a data packet comprising instruction 30 sets for: a) transmitting an announcement message announcing a data packet service and a polling message for counting the number of interested receivers b) receiving an interest message indicative of a number of entities interested in the announced data packet service; c) receiving an information message indicative of the at least one entity having received the polling message for the first time or having received a data packet header in error; d) transmitting data packets comprising uncompressed data packet headers related to the announced data packet service; e) receiving status message indicative of the state of the entity having received the first information message; f) transmitting data packets comprising compressed data packet headers related to the announced data packet service is provided. 
     In accordance with one embodiment, a device for communication in a wireless communication network, the device comprising a transmitter for transmitting an announcement message announcing a data packet service and for transmitting data packets comprising uncompressed and compressed data packet headers related to the announced data packet service, a receiver for receiving a status message indicative of the state an entity having received the first information message, characterized by that the receiver being further adapted for receiving at least one interest message indicative of at least one entity being interested in the announced data packet service and by a processing unit for registering the number of entities interested in the announced data service, the receiver being further adapted for receiving an information message indicative of at least one entity having received the announcement message for the first time or having received a data packet header in error is provided. 
     In accordance with one embodiment, the processing unit is further adapted for compressing data packet headers related to the service announced. 
     In accordance with one embodiment, the device is a base station transceiver, an access point or some other equivalent entity. 
     In accordance with one embodiment, a mobile terminal for communication in a wireless communication network comprising a receiver for receiving an announcement message announcing a data packet service and for receiving data packets comprising uncompressed and compressed data packet headers related to the announced data packet service, the mobile terminal further comprising a transmitter for transmitting a state of the mobile terminal, wherein the transmitter is further adapted for transmitting an interest message indicative of the mobile terminal being interested in the announced data packet service and for transmitting an information message indicative of the mobile terminal having received the first information message for the first time or having received a data packet header in error is provided. 
     In accordance with one embodiment, the mobile terminal further comprising a processing unit for decompressing data packets related to the data packet service announced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described in more detail by means of non-limiting examples and with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates a compression device according to one embodiment of the present invention. 
         FIG. 2  illustrates a mobile terminal according to one embodiment of the present invention 
         FIG. 3  illustrates steps performed when compressing data in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     At the outset, it should be remarked that the present description describes different embodiments of the present invention which are for illustrative purpose only. 
     In  FIG. 1  a base transceiver station BTS  100  comprising a receiver  110  and transmitter  120  in communication with a processing unit  130  is shown. Also, the base transceiver station  100  may further comprise a memory  140  and, optionally, a gateway  150 . 
     While the functions of the receiver  110  and transmitter  120  are known of to the person having ordinary skills in the art, the processing unit  130  uses them to receive and send control signals and data from and to a mobile terminal, such as the mobile terminal illustrated in  FIG. 2 . The processing unit  130  comprises a compressor  135  for compressing data packet headers and possibly also a de-compressor  137  for decompressing compressed data received via the receiver  110 . The de-compressor  137  is however not used in the described embodiments of the invention. 
     In accordance with one embodiment one compressor  135  per service is present in the processing unit  130  of the base transceiver station  100  and that also one de-compressor per service is present in each receiving mobile terminal. 
     Here, the term mobile terminal is to be interpreted as any mobile entity adapted for communication in a wireless communication network. 
     The base transceiver station  100  may also comprise a gateway  150  in communication with the processing unit  130 , where instead of the processing unit  130 , the gateway  150  may itself perform the header compression. The gateway  150  may be part of the base transceiver station or be located outside of it in some other part of the network. In the case the gateway is located outside of a base transceiver station, the compressed packets may be transmitted by the gateway to the mobile terminal or mobile terminals by one or several base transceiver stations, such as the base transceiver station  100  over the radio interface, i.e. from the network side to the terminal side. 
     Other examples of transceivers besides the Base Transceiver Station may be a Node B, an Access Point AP or some other network entity adapted for transmission of data packets. The transceivers are able to establish two-way communication links with mobile terminals and thereby some control information can be exchanged between the network side and the terminal side. However, no dedicated feedback channels are available from the de-compressors back to the compressor, such as the one located in the gateway or in the processing unit  130 . 
     Also, the base transceiver station  100  may comprise a memory  140  for storing at least the number of mobile terminals using a service provided by the network via the base transceiver station  100 . 
       FIG. 2  illustrates a mobile terminal  200  according to one embodiment of the present invention. The mobile terminal  200  can advantageously be used for communication with a base transceiver station, such as the base transceiver station  100  from  FIG. 1  or any other entity comprising a compressor  135  adapted to compress data for a point-to-multipoint connection. 
     In addition to other features well known to the person having ordinary skill in the art, the mobile terminal  200  according to the present invention comprises a receiver  210  and a transmitter  220 , a processing unit  230  in communication with the receiver  210  and transmitter  220 , a user interface  240  and a memory  250  also in communication with the processing unit  230 . 
     Via the receiver  210  and transmitter  220  the mobile terminal is able, among other functions, to receive and send control signals and data packets from and to a network entity, such as a base transceiver station. 
     The processing unit  230  can in accordance with one embodiment comprise a de-compressor  235  for decompressing data packet headers received via the receiver  210  and to transmit a notification message to for example a base transceiver station or a gateway about the state of the de-compressor  235  or a message indicating an interest in receiving a service provided in a wireless network and announced by the network side, for example via a gateway or a base transceiver station. 
     In  FIG. 3  one embodiment of a procedure used when providing a service including transmission of compressed data in a point-to multipoint connection according to the present invention is shown. 
     First in a message  301  the network side, such as a gateway or a base transceiver station  100  announces a point-to-multipoint service, such as a MBMS service. The actual data packet transmission may or may not have started yet at this time. In principle, the network side can be adapted to announce the service without actual transmission until at least one terminal has shown interest on the service. Next in a step  303  a polling message is transmitted where the terminals are asked to reply if they are interested in the announced services and they need to create or repair the de-compressor context. The message  303  may be transmitted as a separate message but may also be a part in implied by the message  301 . 
     In response to the message  301  and or  303  a mobile terminal  200  can reply with an interest message  305  to the announced service. 
     Next, in a step  307 , the network side, checks if the message  305  indicates interest in the announced service indicated in message  301 . If no message indicating interest is received or if the message shows no interest message the network side continues to send messages  301  and  303 . The messages  301  and  303  can for example be transmitted at certain pre-defined time intervals or when there is no other data to transmit or at other suitable times. 
     However, if a mobile terminal  200  has indicated interest in the announced service in the message  305 , it is checked in a step  309  if the mobile terminal is being polled for the first time, i.e. the terminal has recently joined the service and needs to create de-compressor context or if it has a damaged de-compressor context. 
     In case the mobile terminal is not being polled for the first time or it does not have a damaged de-compressor context, the network side continues to send messages  301  and  303 . The two checks performed at step  307  and  309  may be performed intermittently or at regular time intervals. The network side may perform other operations during the remaining time, such as receiving and transmitting data packets and control signals. 
     However, if the message  305  received form the mobile terminal  200  indicates that the messages  301  and or  303  has been received for the first time or if the message  305  indicates a damaged de-compressor context. In response to receiving a the message  305  indicating that the de-compressor is out of context, for example because the mobile terminal  200  is indicating interest in the service for the first time or the mobile terminal is out of context for some other reason such as by receiving an erroneous data stream, the network side transmits uncompressed headers in a stream  311 . The data stream  311  transmits uncompressed headers to the mobile terminal in question in order to repair the context or to provide the de-compressor in the mobile station with enough information enable it decompress compressed packet data headers. 
     Next in a step  313 , the current state of the de-compressor(s) in the one or more mobile terminals  200  is checked. If the state of the de-compressor(s) indicates that the mobile terminal receiving the service are ready to receive compressed data packet headers, compressed data packet headers related to the announced service announced are transmitted in a data stream  315 . 
     In other cases if one or many mobile terminals are not ready to receive compressed headers, the network side continues to send uncompressed data packet headers and checks regularly whether the one or more mobile terminals are able to receive compressed data packet headers in a data stream  311 . 
     The procedure described above in conjunction with  FIG. 3  can be implemented by a computer program stored on a digital storage medium. The computer program can be loaded into a computer which can be adapted to execute the program to make a suitable entity on the network side function in accordance with the described procedure. For example the computer program may be executed by a computer program stored in a central entity, such as a gateway. Also, the central entity may be part of a transmitting entity, such as a Base Transceiver Station, Node B an access point or some other equivalent entity. 
     The compression procedure of the present invention may take the following form: 
     If ((the terminal is interested in the announced service AND it is being polled for the first time) OR (the terminal is interested in the announced service AND it has a damaged de-compressor context)) then reply with an information message, else do nothing. 
     The individual transmitting entities, such as base transceiver stations then count the number of these feedback messages and propagate the counted values to the central entity, such as the gateway, where the compressor state is changed accordingly if needed. 
     Using the method, device and computer program as described herein enables a transmitter of compressed data packet headers to not unnecessarily often send uncompressed packet headers and therefore the overall compression efficiency will be increased. This in turn will lead to a more efficient compression of data over a point to multipoint connection leading to a higher quality of received data for each receiver in the point-to-multipoint connection.