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A method for transferring a data packet from a compressor to a decompressor said data packet including a header with header data fields. A number of the header data fields that remain constant during the data transfer are stored in the decompressor. In a compressed data packet, a header data field t

A method for transferring a data packet from a compressor to a decompressor said data packet including a header with header data fields. A number of the header data fields that remain constant during the data transfer are stored in the decompressor. In a compressed data packet, a header data field that varies is replaced by a compressed value identifying a data packet in a compression sequence. In the decompressor context data comprising information for relating the received compressed value to a corresponding compression sequence is maintained and the information is updated according to the received compressed values. The compressed value and the information of the corresponding compression sequence are used for mapping the compressed value into a decompressed header data field. Thus compressed data is unambiguously mapped to a full packet data header field in the decompressor side throughout the session.

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1. A method for transferring a data packet from a compressor (MS) to a decompressor (SGSN), said data packet including a header with header data fields, a number of the header data fields that remain constant during the data transfer being stored in the decompressor, the method comprising: sending

1. A method for transferring a data packet from a compressor (MS) to a decompressor (SGSN), said data packet including a header with header data fields, a number of the header data fields that remain constant during the data transfer being stored in the decompressor, the method comprising: sending from the compressor and receiving at the decompressor a data packet comprising information on one or more header data fields that vary during the data transfer; decompressing the header using the stored header data fields and the received information on the one or more header data fields that vary during the data transfer; comprising sending from the compressor and receiving at the decompressor a compressed value for a header data field, said compressed value identifying the data packet in a compression sequence and the compressed value being formed by taking a first number of least significant bits of the header data field;maintaining in the decompressor context data comprising information for relating the received compressed value to a corresponding compression sequence, the information being updated according to the received compressed values; using the compressed value and the information of the corresponding compression sequence for mapping the compressed value into a decompressed header data field. 2. A method according to claim 1, wherein the compression sequence comprises a set of successive data packets that can be differentiated from each other with the resolution provided by the compression value.3. A method according to claim 1, wherein the header is a RTP/UDP/IP header, the data field is one of the following: IP identification, RTP sequence number, RTP time-stamp, and the compressed value is a first number of least significant bits of the data field.4. A method according to claim 3, wherein the context data comprises at least a second number of the most significant bits of the data field.5. A method according to claim 1, wherein the compressor and the decompressor are network elements of an access network to a IP packet data network.6. A method according to claim 5, wherein the compressor and the decompressor are network elements of a wireless access network to a IP packet data network.7. A method according to claim 6, wherein the compressor and the decompressor are network elements of a mobile communication network supporting GPRS.8. A method according to claim 7, wherein the compression and decompression functionalities are implemented in the SNDCP layer of the GPRS.9. A method for processing a data packet comprising: receiving a data packet comprising an ordinal, said ordinal indicating the order of the packet in a series of transmitted packets; comparing the ordinal of the received packet with an earlier stored comparison ordinal; initiating, as a response to the difference between the ordinal of the received packet and the comparison ordinal exceeding a predefined limit, an error function; initiating, as a response to the difference between the ordinal of the received packet and the comparison ordinal being less than a predefined limit, a header compression algorithm; storing, as a response to the ordinal of the received packet being greater than the comparison ordinal and the difference between the ordinal of the received packet and the comparison ordinal being less than a predefined limit, the ordinal of the received packet as the comparison ordinal. 10. A method according to claim 9, wherein the error function comprises discarding the new packet.11. A method according to claim 9, wherein the ordinal is the time-stamp of a data packet.12. A method according to claim 9, wherein the ordinal is the sequence number of a data packet.13. An access network element (MS) comprising: means for transferring a data packet to a decompressor (SGSN) said data packet including a header with header data fields: means for compressing the header by excluding header data fields that remain constant during th e data transfer from transmission;means for sending to the decompressor a data packet comprising information on one or more header data fields that vary during the data transfer;comprising:means for sending a compressed value for a header data field associated with identifying the data packet in a compression sequence, the compressed value being formed by taking a first number of least significant bits of the header data field.14. An access data network element according to the claim 13, comprising: means for receiving a data packet comprising an ordinal, said ordinal indicating the order of the packet in a series of transmitted packets; means for comparing the ordinal of the received packet with an earlier stored ordinal; means for initiating, as a response to the difference between the ordinal of the received packet and the comparison ordinal exceeding a predefined limit, an error function; means for initiating, as a response to the difference between the ordinal of the received packet and the comparison ordinal being less than a predefined limit, a header compression algorithm; means for storing, as a response to the ordinal of the received packet being greater than the comparison ordinal, the ordinal of the received packet as the comparison ordinal. 15. An access network element (MS) comprising means for receiving data packets said data packets including a header with header data fields; means for storing header data fields that remain constant during the data transfer; means for receiving compressed data packets comprising information on one or more header data fields that vary during the data transfer; means for decompressing compressed data packets using the stored header data fields and the received information on the one or more header data fields that vary during the data transfer; comprising: means for receiving in a data packet a compressed value identifying the data packet in a compression sequence, the compressed value being a first number of least significant bits of the header data field;means for maintaining context data comprising information for relating the received compressed value to a corresponding compression sequence, said information being updated according to the received compressed values;means for using the compressed value and the information of the corresponding compression sequence for mapping the compressed value into a header data field in a decompressed data packet.16. An access data network element according to claim 13, wherein the element is a mobile terminal of a mobile communication network.17. An access data network element according to claim 13, wherein the element is a SGSN of a mobile communication network supporting GPRS.