Abstract:
The invention relates to a method for transmitting compression parameters in a mobile communication system, comprising a mobile node, a first and a second packet switching node. In the method the second packet switching node is informed of the entry of the mobile node to an area controlled by the second packet switching node. At least one compression parameter is received from the first packet switching node to the second packet switching node. The second packet switching node informs at least one of the at least one compression parameter to the mobile node by way of layer-3 parameter renegotiation for a logical link connection. The benefits of the invention are related to improved reliability of packet data transmission to and from a mobile node.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to mobile communication systems. Particularly, the invention relates to the transfer of packet header and data compression parameters in a mobile communication system. 
     2. Description of the Related Art 
     A disadvantage associated with wireless data transmission is that the data transfer rate is limited compared to wire-line data transmission. Especially, the data transfer rate is limited in cellular mobile communication systems where a given frequency band must be shared by a number of simultaneous users. When transferring packet data in a cellular mobile communication system, the data transfer rate made available for application layer data transmission is further reduced due to frame and packet headers. For example, when the Transmission Control Protocol (TCP) is carried over the Internet Protocol (IP) version 4, the total header size is 40 bytes of which TCP layer headers take 20 bytes and IP layer headers 20 bytes. In IPv6 the IP layer header size is 40 bytes. The problem is made worse in the cases where IP-in-IP tunneling is used, for example, in association with the Mobile IP. 
     In order to alleviate the aforementioned problems associated with wireless and in general slow links payload data and header compression schemes have been introduced. Examples of such header compression schemes are Van Jacobson header compression defined in the Internet Engineering Task Force (IETF) Request For Comments (RFC  1144 ) and Degermark header compression defined in the IETF RFC  2507 . An example of a payload data compression scheme is V.42bis by International Telecommunications Union (ITU-T). 
     Generally, header compressions rely on the fact that a large part of the headers contain information, which remains constant during the course of a typical TCP/IP connection. Similarly, when Universal Datagram Protocol (UDP) is carried over IP, a significant part of the packet headers remain constant in any given flow, which carries, for example, streaming multimedia. An IP header comprises a number of fields. As a sequence of IP packets is transmitted, there is no need to repeatedly transmit the fields that are not changed between subsequent packets. Those fields that usually change with small or predictable values, for example, TCP sequence numbers in TCP headers, may be encoded incrementally so that the number of bits needed for transmitting the value of those fields decreases significantly. Only those fields that change often and randomly, for example, checksums or authentication data, need to be transmitted in entirety in each header. In header compression is sent occasionally a packet with a full header, which establishes a context. Thereupon, in subsequent packets compressed headers refer to the established context and may contain incremental changes to the context. In order for the header compression to work, it is necessary that a sender and a receive maintain the same context. A number of different contexts may be maintained in the sender and the receiver side to support different packet streams carried over same link, that is, for example, TCP/IP connections or UDP packet flows. 
     Payload data compression relies, on the other hand, upon the redundancy of data to be carried in packet payload. For example, there may be repeating strings in packets. The compression is achieved by maintaining by at the sender and at the receiver side code dictionaries than comprise information on strings comprised in the uncompressed data. The code dictionaries enable the sender to refer to codes instead of entire strings while the transmission occurs in the compressed mode. The V.42bis compression relies on the Lempel-Ziv-Welch algorithm. More information on the Lempel-Ziv-Welch algorithm can be found, for example, in U.S. Pat. Nos. 4,955,066, 4,701,745 and 5,016,009. 
     In association with mobile communication systems the use of header compression and payload data compression has been standardized, for example, in the General Packet Radio System (GPRS). Header compression and payload data compression is used between a mobile station and a Serving GPRS Support Node (SGSN) in order to decrease the amount of data to be transmitted over the air interface between the mobile station and a base transceiver station. 
     Reference is now made to  FIG. 1 , which is a block diagram illustrating the architecture and the protocol stacks in a GPRS system in association with the GSM Edge Radio Access Network (GERAN). The GPRS system is specified, for example, in the 3G Partnership Project (3GPP) specification 23.060. The protocol stacks are illustrated from the user plane point of view. In  FIG. 1  there is a Gateway GPRS Support Node (GGSN)  106 . GGSN  106  is connected to an external network (not shown) via a Gi-interface. The external network may be an arbitrary IP network, for example, the Internet or an intranet. In  FIG. 1  there is also a Serving GPRS Support Node (SGSN)  104 . GGSN  106  communicates with SGSN  104 , which routes packets to and from Mobile Station (MS)  100  via a Base Station Subsystem (BSS). SGSN  104  takes care of the mobility related tasks such as the maintaining of mobile station  100  location information, network registrations, routing area and location updating, Packet Data Context (PDP) activation and deactivation, handovers and the paging of mobile station  100 . Part of the above mentioned tasks are naturally done in other network elements with which SGSN  104  is communicating. The GGSN is responsible for routing and tunneling packets to and from a number of SGSN  104  and other SGSNs. The routing is based on SGSN address information maintained in a Packet Data Protocol (PDP) context held by GGSN  106 . There is at least one PDP context for each network address activated for MS  100 , for example, an IP address or an X.25 address or a PPP link. 
     In  FIG. 1 , the uppermost protocol layer in MS  100  is the application layer (APPL). The application layer may be any protocol, for example, a protocol from the Wireless Application Protocol (WAP) standard or the Transmission Control Protocol (TCP) or the Universal Datagram Protocol (UDP). Over the TCP/IP may be carried, for example, Hypertext Transfer Protocol (HTTP). The application layer communication is exchanged with a peer host, which may be located behind the Gi-interface, for example, in the Internet. Below the application layer there is the IP layer or alternatively X.25 layer, which in GPRS is supported by both MS  100  and GGSN  106 . The IP address for packets addressed to MS  100  points to GGSN  106 . An IP packet  114  is conveyed to MS  100  using GPRS user plane protocols below the IP layer. Between GGSN  106  and SGSN  104  IP packet  114  is conveyed using the GPRS Tunneling Protocol (GTP). A GTP packet carried further over UDP/IP. 
     In SGSN  104  IP packet  114  data is routed based on MS  100  location information and passed to Sub-Network Dependent Convergence Protocol (SNDCP) layer. SNDCP is specified in the 3GPP specification 44.065. SNDCP layer maps network-level characteristics onto the characteristics of the underlying network. For example, SNDCP takes care of the transmission and reception of Network layer Protocol Data Units (N-PDU) carrying IP packets. For example, IP packet  114  is carried in N-PDU  112 . SNDCP multiplexes several packet data protocol packets for the same MS. It segments IP packet  114  to LLC frames, for example, LLC frame  110 . It also reassembles packets from LLC frames. Header compression and data compression is also performed at SNDCP layer. The header compression scheme mentioned in the 3GPP specification 23.060 includes TCP/IP header compression. V.42bis is mentioned as a method for data compression. At the SNDCP layer there are a number of compression entities (not shown) each of which have associated with them an algorithm and the entity specific parameters. Each SNDCP entity, which supports protocol control information compression, is able to negotiate at least one protocol control information compression entity using XID parameter negotiation as specified in the 3GPP specification 44.064. The initiating SNDCP entity defines a set of requested compression entities, together with the algorithm and parameters for each compression entity. The set of compression entities and their algorithms and parameters are transmitted to the peer SNDCP entity. The peer SNDCP entity responds with the set of negotiated compression entities and their algorithms and parameters. The peer SNDCP entity selects the proposed parameter values or other appropriate values for the negotiated compression entities. The information on the set of compression entities used and their algorithms and parameters is referred to hereinafter as the SNDCP compression parameters. Other compression information held at the SNDCP layer comprise, for example, at least one header compression context and at least one code dictionary. SNDCP performs parameter negotiation between MS  100  and SGSN  104 . SNDCP also buffers N-PDUs in the case of acknowledged mode services. 
     The Logical Link Control (LLC) layer provides a highly reliable link between MS  100  and SGSN  104 . The LLC is specified in 3GPP specifications 44.064 and 04.64. The LLC is independent of the underlying radio protocols and hides the BSS and radio interface related tasks from the LLC layer users. LLC supports variable-length information frames. LLC supports both acknowledged and unacknowledged data transfers, that is, acknowledged and unacknowledged modes of operation. LLC provides services typical to a link layer comprising parameter negotiation, flow control in the Asynchronous Balanced Mode (ABM), sequence control to maintain the ordering of LLC-frames, expedited delivery for high-priority data, error detection, error recovery and indication. LLC performs data confidentiality by means of the ciphering of LLC-frame contents. LLC also supports user identity confidentiality by means of the use of Temporary Logical Link Identity (TLLI) instead of International Mobile Subscriber Identity (IMSI). A Service Access Point Identifier (SAPI) identifies a point, at which LLC services are provided by a Logical Link Entity (LLE), in other words an LLC entity, to a layer- 3  entity. Consequently, SAPI identifies an LLE that should process an LLC frame and also a layer- 3  entity that is to receive information carried by the LLC frame. The layer- 3  entity is, for example, an SNDCP entity. 
     The relay layer relays LLC PDUs between the Um and Gb interfaces in the BSS. The Base Station System GPRS Protocol (BSSGP) layer specified in 3GPP specification 08.18 conveys routing and QoS-related information between the BSS and the SGSN. For example, it carries radio resource related requests from the SGSN to the BSS  102 . It also carries LLC frames between the BSS and the SGSN. In addition to LLC frames it also carries signaling PDUs associated with GRPS mobility management. The Network Service (NS) layer transports BSSGP PDUs between BSS and SGSN. NS may be based on Frame Relay (FR). The RLC sub-layer within the RLC/MAC layer provides a radio technology dependent reliable link between MS  100  and BSS  102 . The MAC sub-layer performs the requesting and reservation of radio resources and maps LLC frames onto the GSM physical channels. The task of the MAC layer is to ensure efficient sharing of common radio resources by several mobile stations. The RLC/MAC layer is defined in the 3GPP specification GSM 04.60. 
     Reference is now made to  FIG. 2 , which is a block diagram illustrating packet transmission before and after a routing area update in a prior art GPRS network. In  FIG. 2  there is an MS  100 , Base Transceiver Stations (BTS)  224 - 228  and Base Controller Stations (BSC)  210 - 214  in BSS  216 . There is a GGSN  200 , which is connected to IP network  201 . From IP network  201  is received a downlink packet stream  240 . Initially, downlink packet flow  240  is tunneled from GGSN  200  to SGSN  202  as packet stream  241 . Initially, SGSN  202  routes packets from packet stream  241  to MS  100  via BSC  212  and BTS  222  as packet stream  242 . Packet stream  242  is carried from an SNDCP entity  252  in SGSN  202  to SNDCP entity  230  in MS  100  using an LLC connection serving both SNDCP entities. BSC  212  and BTS  222  are referred to as source BSS  262 . MS  100  communicates with BSC  212  via BTS  222 . In routing area update related signaling SGSN  202  communicates with MS  100  via BSC  212  and a BTS  222  within BSS  262 . 
     When MS  100  detects that a new cell has better radio quality, it must start camping on the new cell, which is served by BTS  224 . The new cell is in the area of a new SGSN  204 . After the handover, packet stream  240  should be routed to MS  100  from GGSN  200  via SGSN  204 , BSC  214  and BTS  224 . BSC  214  and BTS  224  are also referred to as a target BSS  264 . While the PDP contexts have not yet been updated in GGSN  200 , SGSN  202  must forward any unacknowledged packets to SGSN  204 . The packets are sent from SGSN  202  as packet stream  243 . After GGSN  200  has received a PDP context update request and processed it, it may start sending packets from packet stream  240  directly to SGSN  204  as packet stream  244 . SGSN  204  sends packets from packet streams  243  and  244  to MS  100  as packet stream  245 . Packet stream  245  is carried from an SNDCP entity  254  in SGSN  204  to SNDCP entity  230  in MS  100  using an LLC connection serving both SNDCP entities. Before packets may be sent from SGSN  204  to MS  100  after the detection of a routing area update, an XID reset procedure must be performed between SNDCP entities  254  and  230 . Otherwise, the LLC parameters will not match in the MS  100  and SGSN  254  and all LLC frames will be rejected. 
     Reference is now made to  FIG. 3 , which is a signaling diagram depicting routing area update signaling in a prior art GPRS network such as illustrated in  FIG. 2 . At time t 1  an MS  100  detects an event that indicates that it must start using a new cell (not shown). The new cell is within a new routing area, which is controlled by a new SGSN  204 . MS  100  sends a Routing Area Update Request message to SGSN  204  as illustrated with arrow  301 . The message is sent via a target BSS  264 , which controls the new cell. SGSN  204  sends an SGSN Context Request message illustrated with arrow  302  to an old SGSN  202  to get the mobility management and PDP contexts for MS  100 . The SGSN Context Request message comprises, for example, an old routing area identifier, a Temporary Logical Link Identity (TLLI) and the new SGSN address. SGSN  202  responds with message SGSN Context Response illustrated with arrow  303 . The SGSN Context Response message provides the mobility management context and at least one PDP context associated with MS  100 . After receiving the SGSN Content response message SGSN  204  may initiate an XID reset procedure. XID-reset procedure comprises that an SNDCP entity  254  in SGSN  204  requests an LLC-entity in SGSN  204  to send an XID command message to MS  100  as illustrated with arrow  304 . In the case of XID reset, the XID Command message includes a Reset parameter, which tells the peer LLC entity to reset the LLC parameters. MS  100  acknowledges the XID Command with the XID Response message as illustrated with arrow  305 . 
     As defined in 3GPP specification  44 . 064  several actions take place at the XID procedure if a Reset parameter is detected in an XIID Command message. All requests pending from layer  3  to the LLC entities in SGSN  204  and MS  100  are discarded without any further action. Any ongoing Asynchronous Balanced Mode (ABM) establishment, release and XID negotiation procedures are aborted, except the XID negotiation pertaining to a Reset parameter. The LLC layer parameters are set to the default values. Any Logical Layer Entities (LLE) in ABM state is changed to Asynchronous Disconnected Mode (ADM) state. Unconfirmed state variables V(U) and V(UR) are set to 0. Further, all Overflow Counters (OC) for unacknowledged information transfer are set to 0. 
     After having received the SGSN  204  address, SGSN  202  may start forwarding packets to it as illustrated with arrow  306 . SGSN  204  must update PDP contexts in the GGSNs that have PDP contexts active for MS  100 . The updating of PDP contexts specifies, for example, the new SGSN address for the GGSNs. SGSN  204  sends the Update PDP Context Request message to GGSN  200  and receives Update PDP Context Response message as acknowledgement, as illustrated with arrows  307  and  308 , respectively. The routing area update is acknowledged with Routing Area Update Accept message sent by SGSN  204  to MS  100 , which responds by sending Routing Area Update Complete message to SGSN  204 , as illustrated with arrows  309  and  310 . 
     The ABM release that is performed in association with the processing of XID Reset parameter specified in the XID Command message causes significant problems in prior art. After the XID reset the ABM mode must be re-established with Set Asynchronous Balanced Model (SABM) negotiation sent to MS  100 . This is illustrated using arrow  310 . The Unnumbered Acknowledgement (UA) response to the ABM mode re-establishment from MS  100  is illustrated with arrow  311 . 
     However, without knowledge of the previously used SNDCP parameters there is no possibility to re-establish the ABM mode with the SNDCP parameters that were used by the old SGSN, namely SGSN  202 , to communicate with MS  100 . There are three problems related to this. Firstly, if SGSN  204  does not propose any compression entities, in other words, header or data compression entities, in the SNDCP compression parameters negotiated after XID reset procedure, there is no guarantee that the MS  100  will stop using the previously used SNDCP compression parameters. Secondly, if SGSN  204  proposes any compression entities there is no guarantee that MS  100  is capable of changing them. Thirdly, if SGSN  204  in fact proposes a header compression entity, there remains the question as to whether RFC  1144  or RFC  2507  header compression should be used by default. Some mobile stations do not work properly in the first case and some do not work properly in the second and third cases. 
     SUMMARY OF THE INVENTION 
     The invention relates to a method of transmitting compression parameters in a mobile communication network, comprising a mobile node, a first and a second packet switching node. In the method the second packet switching node is informed of the entry of the mobile node to an area controlled by the second packet switching node; at least one compression parameter is received from the first packet switching node to the second packet switching node; the second packet switching node informs at least one of the at least one compression parameter to the mobile node by way of layer- 3  parameter renegotiation for a logical link connection. 
     The invention relates also to a system, which comprises: a mobile node configured to inform a second packet switching node of the entry of the mobile node to an area controlled by the second packet switching node; a first packet switching node; and wherein the second packet switching node is configured to receive at least one compression parameter from the first packet switching node, to inform at least one of the at least one compression parameter to the mobile node by way of layer- 3  parameter renegotiation for a logical link connection. 
     The invention relates also to a packet switching node, which comprises: a user plane protocol entity configured to negotiate compression parameters with a mobile node, to provide at least one compression parameter to a signaling plane protocol entity; and a signaling plane protocol entity configured to receive an indication of a the mobile node entering a new area, to request compression information from the user plane protocol entity, to receive in response the at least one compression parameter from the user plane protocol entity, and to transmit the at least one compression parameter to a second packet switching node. 
     The invention relates also to a packet switching node, which comprises: a user plane protocol entity configured to inform at least one of at least one compression parameter to a mobile node by way of layer- 3  parameter renegotiation for a logical link connection; and a signaling plane protocol entity configured to receive an indication of the mobile node entering an area controlled by the packet switching node, to request information on the mobile node from a second packet switching node, and to receive in response at least the at least one compression parameter. 
     The invention relates also to a computer program comprising code adapted to perform the following steps when executed on a data-processing system: receiving information of the entry of a mobile node to an area controlled by the second packet switching node; receiving at least one compression parameter from a first packet switching node; and informing at least one of the at least one compression parameter to the mobile node by way of layer- 3  parameter renegotiation for a logical link connection. 
     In one embodiment of the invention the layer- 3  comprises Sub-Network Dependent Convergence Protocol (SNDCP) layer and the layer- 3  parameters comprise the Sub-Network Dependent Convergence Protocol (SNDCP) parameters. 
     In one embodiment of the invention, the mobile node comprises a mobile terminal, for example, a UMTS terminal, a GSM terminal, a GPRS terminal, a WLAN terminal or a terminal within an arbitrary cellular radio system. In other words, the mobile node may be a Mobile Station (MS) in a mobile communication system. 
     In one embodiment of the invention, the mobile node comprises a mobile computer, for example, a laptop computer, palmtop computer or a personal digital assistant (PDA). The mobile computer may be equipped with a data card, which provides the mobile station functionality required in order to operate the mobile computer in a mobile communication system. 
     In one embodiment of the invention, the mobile communication system comprises a General Packet Radio Service (GPRS), the first and second packet switching nodes comprises Serving GPRS Support Nodes (SGSN) and the logical link connection is GPRS Logical Link Control (LLC) connection. In one embodiment of the invention, the layer- 3  parameter renegotiation comprises eXchange Identification (XID) negotiation, in other words, the layer- 3  parameter renegotiation is performed using XID negotiation. In one embodiment of the invention second packet switching node informs at least one of the at least one compression parameter to the mobile node during eXchange Identification (XID) negotiation procedure or Asynchronous Balanced Mode (ABM) establishment procedure. XID negotiation is performed, for example, in GPRS LLC protocol. 
     In one embodiment of the invention, the area controlled by the second packet switching node is a Routing Area (RA), which belongs to the second packet switching node. 
     In one embodiment of the invention, the mobile node detects its entry to a cell belonging to a new area. The area is, for example, a routing area. Thereupon, the mobile node informs a base station subsystem of its entry to the new routing area. The base station subsystem informs the second packet switching node of the fact that the mobile node has entered a routing area controlled by the second packet switching node. When receiving a routing area update message from the base station subsystem, the second packet switching node requests information on the mobile node from the first packet switching node. 
     In one embodiment of the invention, sending a routing area update message from the mobile node to the second packet switching node performs the informing of the second packet switching node of the entry of the mobile node to an area controlled by the second packet switching node. In other words, as the mobile node enters an area, for example, a routing area controlled by the second packet switching node, it sends a routing area update message to the second packet switching node. Thereupon, the second packet switching node is aware that the mobile node is within an area controlled by it. 
     In one embodiment of the invention, the at least one compression parameter comprise in-formation on compression entities, algorithms used by the compression entities and state information associated with the compression entities. 
     In one embodiment of the invention, sending a handover preparation request message from the first packet switching node to the second packet switching node performs the informing of the second packet switching node of the entry of the mobile node to an area controlled by the second packet switching node. 
     In one embodiment of the invention, the informing of at least one of the at least one compression parameter to the mobile node is performed during exchange identification negotiation. 
     In one embodiment of the invention, the user plane protocol entity comprises a Sub-Network Dependent Convergence Protocol (SNDCP) layer entity. The signaling plane protocol entity comprises a GPRS Tunneling Protocol (GTP) entity or an application entity communicating with a GTP protocol entity. The protocol entities may be implemented, for example, as a software module, program block or thread. The exchange of information between the protocol entities is configured to be performed, for example, using inter process communication mechanisms such as message passing or shared memory buffers. 
     In one embodiment of the invention, the second packet switching node is a Base Station Subsystem (BSS) node, for example, a base station controller or a base station. In one embodiment of the invention, the first or the second packet switching node is a node, which performs the forwarding and switching of data packets at link layer. The invention is not restricted to packet switching nodes that switch packets at network layer level in the manner of e.g. IP routers. By packets are meant herein throughout this disclosure data packets pertaining to any protocol layer, for example, network layer packets, link layer frames, Asynchronous Transfer Mode (ATM) cells. 
     In one embodiment of the invention, the at least one compression parameter is received from the first packet switching node to the second packet switching node when the first packet switching node requests handover preparation from the second packet switching node. This means that the at least one compression parameter is sent from the first packet switching to the second packet switching node in the message that requests handover preparation. 
     In one embodiment of the invention, the sending of logical link layer frames or any other messages between the mobile node and the packet switching nodes is performed via a radio access network so that the frames and messages are forwarded by one or many intermediate network elements such as base station controllers, radio network controllers and base transceiver stations. In one embodiment of the invention, the first and the second packet switching nodes are directly connected to base transceiver stations and manage the radio network control procedures directly. 
     In one embodiment of the invention, the computer program is stored on a computer readable medium. The computer readable medium may be a removable memory card, magnetic disk, optical disk or magnetic tape. 
     The benefits of the invention are associated with improved reliability of data transmission to and from a mobile node. With the invention it is now possible to provide a uninterrupted packet data connection to a mobile node even as the mobile node changes from an area controlled by a first packet switching node to an area controlled by a second packet switching node. The packet data connection is no longer terminated, if the second packet switching node proposes other compression parameters than previously used. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings: 
         FIG. 1  is a block diagram illustrating the architecture and the protocol stacks in General Packet Radio System (GPRS) in association with the GSM/Edge Radio Network (GERAN); 
         FIG. 2  is a block diagram illustrating packet transmission before and after a routing area update in a prior art General Packet Radio Service (GPRS) network; 
         FIG. 3  is a signaling diagram depicting routing area update signaling in a prior art GPRS network; 
         FIG. 4  is a signaling diagram illustrating routing area update signaling, according to the invention; 
         FIG. 5  is a flow chart depicting one embodiment of a method for compression parameter transfer, according to the invention; and 
         FIG. 6  is a block diagram depicting the architecture of a Serving GPRS Support Node in one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 5  is a flow chart depicting one embodiment of a method for compression parameter transfer, which utilizes a routing area update signaling as illustrated in  FIG. 4 . The signaling is performed in the GPRS system architecture, which is illustrated in  FIGS. 1 and 2 . At step  500  a MS  100  waits for Routing Area (RA) update condition. RA update condition is fulfilled as MS  100  detects an event that indicates that it must start using a new cell (not shown). The new cell is within a new routing area, which is controlled by a new SGSN  204 . MS  100  sends a Routing Area Update Request message to SGSN  204  as illustrated with arrow  401 . Thereupon, the method continues at step  502 . 
     At step  502  SGSN  204  sends an SGSN Context Request message illustrated with arrow  402  to SGSN  202  to get the mobility management and PDP contexts for MS  100 . SGSN  202  obtains the SNDCP parameters associated with an SNDCP entity  252 . The SNDCP parameters comprise at least the SNDCP compression parameters. The SGSN  202  packs the SNDCP parameters  450  to the SGSN Context Response message. In one embodiment of the invention, the SNDCP parameters are packet to the private extension information element, which may be included in any GPRS Tunneling Protocol (GTP) signaling message. The GTP and the private extension information element are defined in the 3GPP specification  29 . 060 . The format of the SNDCP parameters carried in the private extension information element is the same as is used in XID negotiation procedure. The format is defined in the 3GPP specification 44.065, which defines the SNDCP protocol layer in GPRS. Thereupon, SGSN  202  responds with the SGSN Context Response message illustrated with arrow  403 . SGSN  204  takes the SNDCP parameters and provides them to SNDCP entity  254 . The SGSN Context Response message provides also the mobility management context and at least one PDP context associated with MS  100 . 
     At step  504  SGSN  204  starts performing the XID Reset procedure. SNDCP entity  254  requests the XID Reset procedure from the LLC entity associated with Service Access Point Identifier  1  using a primitive LLGMM-RESET. Thereupon, the LLC entity forms an XID Command frame, provides it with the XID Reset parameter and IOV-UI parameter. The LLC entity sends the XID Command to the peer LLC entity as illustrated with arrow  404 . The peer LLC entity resets all other SAPIs and default values are taken into use. 
     The peer LLC entity in MS  100  prepares and sends an XID Response message to SGSN  204  as illustrated with arrow  405 . LLC entity in SGSN  204  responds with LLGMM-RESET-CNF primitive to SNDCP entity  254 , which verifies that a successful XID negotiation of Reset and IOV-UI has been made. 
     After having received the SGSN  204  address from SGSN Context Request message, SGSN  202  may start forwarding packets to it as illustrated with arrow  406 . SGSN  204  must update PDP contexts in the GGSNs that have PDP contexts active for MS  100 . The updating of PDP contexts specifies, for example, the new SGSN address for the GGSNs. SGSN  204  sends the Update PDP Context Request message to GGSN  200  and receives Update PDP Context Response message as acknowledgement, as illustrated with arrows  407  and  408 , respectively. The routing area update is acknowledged with Routing Area Update Accept message sent by SGSN  204  to MS  100  as illustrated with arrow  409 . 
     At step  506  SGSN  204  waits for Routing Area Update Complete message to SGSN  204 , as illustrated with arrow  410 . 
     At step  508  the layer- 3  parameters associated with at least each logical link connection carrying user plane data are re-negotiated. The re-negotiation of connections comprises, for example, the re-establishment of ABM modes for connections between peer LLC entities that were in ABM mode before the XID-reset, and XID-negotiation between peer LLC entities that were in Asynchronous Disconnected Mode (ADM) before the XID-reset. For example, at step  508  SNDCP entity  254  of  FIG. 4  in SGSN  204  requests all LLC entities that were in ABM mode before the XID-reset to re-establish ABM mode. In the case illustrated in  FIG. 4  in SGSN  204  there is one LLC entity for which ABM mode must be re-established. The SNDCP entity requests ABM mode establishment using LL-ESTABLISH-REQ primitive, which provides the SNDCP parameters in turn comprising the compression parameters. In  FIG. 4  the LLC entity in SGSN  204  sends an SABM message as illustrated with arrow  411 . The SABM message comprises XID parameters. The XID parameters comprise the SNDCP parameters  450  as layer- 3  parameters. The XID negotiation is performed as part of the ABM mode re-establishment. The received layer- 3  parameters are sent by the LLC entity in MS  100  to the SNDCP entity  230  in MS  100 . SNDCP entity  230  may change some of the SNDCP parameters to better suit its requirements while MS  100  uses the new cell. For example, MS  100  may decide to change at least one compression algorithm and its compression parameters to suit better the data transfer rates available in the new cell. SNDCP entity  230  provides the changed SNDCP parameters to LLC entity in MS  100 , which responds with an Unnumbered Acknowledgement (UA) message that comprises SNDCP parameters  451  changed by SNDCP entity  230  as XID parameters. The UA message sent from MS  100  to SGSN  204  is illustrated with arrow  412 . Received SNDCP parameters  451  are provided by the LLC entity in SGSN  204  to SNDCP entity  254 . 
     XID-negotiation between peer LLC entities that were in Asynchronous Disconnected Mode (ADM) before the XID-reset involves the exchange of XID Command and XID Response messages, which carry the XID-parameters, for example, the SNDCP parameters. 
     After the negotiation is successful, at step  510  the SNDCP entities in MS  100  and SGSN  204  may start transmitting packets over the logical link connections that have re-negotiated there SNDCP parameters. 
     In one embodiment of the invention, the SNDCP parameters are also sent during handover processing, when a real-time packet stream must be transmitted to a mobile station via a new SGSN, which serves a cell to which the mobile station is performing handover from an old cell. In that case the SNDCP parameters are sent, for example, in association with handover request message that is sent from the old SGSN to the new SGSN. Thereupon, the SNDCP parameters are provided to the mobile station. 
       FIG. 6  is a block diagram depicting the architecture of a Serving GPRS Support Node (SGSN) in one embodiment of the invention. In  FIG. 6  there is an SGSN  600 . SGSN  600  comprises a user plane protocol stack pair  610 . Protocol stack pair  610  comprises a protocol stack for communicating over the Gb-interface, a protocol stack for communicating with a GGSN, and a relay function for relaying packets between the two protocol stacks. SGSN  600  comprises also a protocol stack  612  for communicating with another SGSN. Protocol stack  612  is a GPRS Tunneling Protocol (GTP) stack for control plane. When receiving an SGSN Context Request message from another SGSN, a GTP-C protocol entity  604  is configured to request the SNDCP parameters from an SNDCP entity  602 . Upon receiving the request from GTP-C protocol entity  604 , SNDCP entity  602  is configured to collect information held at SNDCP entity  602  that is associated with the mobile station pointed to using the TLLI that was obtained in the SGSN Context Request message. The SNDCP parameters comprise at least the SNDCP compression parameters pertaining to compression entities, together with the algorithm and parameters for each compression entity. SNDCP entity  602  is configured to respond to the request and to provide the SNDCP parameters to GTP-C protocol entity  604  In one embodiment of the invention, GTP-C protocol entity  604  is configured to return SNDCP compression parameter default values that are always used at the SGSN without separately consulting SNDCP entity  602 . The providing of information from SNDCP entity  602  to GTP-C protocol entity  604  is illustrated in  FIG. 6  with arrow  606 . 
     It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.