Abstract:
The invention relates to a method and system for performing packet switched handover in a mobile communication network. The system comprises a mobile node, a first and a second packet switching node. The method enables the parallel sending of logical link layer frames from the first and the second packet switching node. This is achieved so that the mobile node does not reject incoming frames received from two logical link layer entities having different states. The benefits of the invention are related to improved quality of service and the avoiding of gaps in received data during handover.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to mobile communication systems. Particularly, the invention relates to the performing of packet switched handover in a mobile communication system.  
         [0003]     2. Description of the Related Art  
         [0004]     The introduction of conversational and streaming services in Global System of Mobile Communications (GSM) has created a demand for efficient handovers from user perspective in GSM/Edge Radio Access Network (GERAN). The General Packet Radio Service (GPRS) and the IP Multimedia System (IMS) support the conversational and streaming services on their side and impose requirements on the GERAN side. It is necessary to be able to perform Packet Switched (PS) handovers frequently enough and to be able to minimize interruptions in a constant packet stream to a mobile terminal. The interruptions must preferably be short enough to enable a packet buffering mechanism in the mobile terminal to hide the interruptions. Previously in GPRS it was sufficient to provide a loss-free link layer service for interactive applications such as Wireless Application Protocol (WAP) browsing. In browsing applications moderate extra delays caused by handovers are acceptable. However, in streaming or conversational class services interruptions in the supposedly constant packet stream are immediately noticeable unless, of course, they can be hidden using large enough buffers in the receiving ends. However, such buffering introduces always a delay in the media streams provided to the user. In the case of conversational voice services any significant delays are unallowable, especially considering other factors already introducing a delay in the speech path such as noise filtering and speech coding.  
         [0005]     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 GERAN. The GPRS system is specified, for example, in the 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 PDP context information held by GGSN  106  for each network address activated for MS  100 , for example, an IP address or an X.25 address or a PPP link.  
         [0006]     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 WAP protocol or Transmission Control Protocol (TCP) or Universal Datagram Protocol (UDP). Over 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.  
         [0007]     In SGSN 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 packet payload compression is also performed at SNDCP layer. SNDCP performs parameter negotiation between MS  100  and SGSN  104 . SNDCP buffers N-PDUs in the case of acknowledged mode services.  
         [0008]     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).  
         [0009]     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.  
         [0010]     The standardization organization  3 G Partnership Project (3GPP) is currently specifying the packet switched handover for GERAN A/Gb mode. One of the key aspects of the packet switched handover is duplicated packet forwarding to both a source BSS and a target BSS during handover, which has not yet been thoroughly covered in the specifications.  
         [0011]     Reference is now made to  FIG. 2 , which is a block diagram of GPRS architecture illustrating problems in prior art associated with duplicated packet forwarding. According to current GPRS specifications, an LLC entity in a new SGSN can only be started so that an LLC connection is establishing at the request of an SNDCP entity or the peer LLC entity. An LLC entity can only be created in its initial state where the LLC connection variables have their initial values. 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  246  for which a real-time service is required. Initially, downlink packet stream  246  is tunneled to SGSN  202  as packet stream  240 . Initially, SGSN  202  routes packet stream  240  to MS  100  via BSC  212  and BTS  222  as packet stream  242  using an LLC connection terminating at an LLC entity  230 , which is located in MS  100 . BSC  212  and BTS  222  are referred to as source BSS  262 . MS  100  communicates with BSC  212  via BTS  222 . BSC  212  performs handover related tasks including the handover determination algorithms and decisions. In handover related signaling an SGSN communicates with a BSC within a BSS. Similarly, in handover related signaling an MS communicates with a BSC within a BSS. The signaling between an MS and a BSC goes via a BTS.  
         [0012]     However, when MS  100  receives a report indicating that a cell served by BTS  224  has better radio quality, it must start performing handover to the cell served by BTS  224 . The new cell is under the area of a new SGSN  204 . After the handover, packet stream  246  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 handover is not fully complete, SGSN  202  must forward packets to both BSC  212  and SGSN  204 . In order to be able to process packets from packet stream  240  SGSN  204  must receive them as a GTP tunneled packet stream  241  from SGSN  202 . Packets from GTP tunneled packet stream  241  are forwarded in SGSN  204  to its LLC entity  254 . The LLC entity is started from initial state with initial LLC connection variables. GTP tunneled packet stream  241  is routed from SGSN  204  as packet stream  244  carried over an LLC connection. The problem in the packet duplicated forwarding mechanism described above is that LLC entity  254  in the new SGSN, namely SGSN  204 , has different state compared to LLC entity  252  and LLC entity  230 . This means that LLC entity  230  in MS  100  receives packets from two different independent LLC entities. The corresponding peer LLC entity  230  in MS  100  is not capable of receiving simultaneously packets from two different LLC entities, if the states of the LLC entities comprising the LLC variables are not synchronized. The different states essentially lead to the rejection of LLC frames carrying packet stream  244  or the receiving of duplicate LLC frames in an uncontrolled manner.  
         [0013]     The rejection is due to the fact that LLC entity  252  sends LLC frames with sequence numbers that are overlapping with the sequence numbers sent by LLC entity  254  even though they are different LLC frames. Frames are rejected in LLC entity  230  also due to the fact that LLC entity  254  sends LLC frames using different ciphering parameters. Because the ciphering parameters are different, LLC entity  230  is unable to decipher the LLC frames and discards them due to failing Frame Check Sequence (FCS) verification. A further problem is that SGSN  204  is unaware of the LLC frame sizes negotiated between MS  100  and SGSN  202 . If SGSN  204  uses values that exceed the maximum values supported by MS  100 , it discards all LLC frames. This in turn may lead to the releasing of the PDP context carrying packets streams  240 ,  241 ,  242  and  244 . MS  100  may additionally also perform reset.  
         [0014]     As explained in the 3GPP specification 44.064, the ciphering parameters for LLC frames comprise IOV, LFN, OC and SX. IOV is an Input Offset Value, which is a 32 bit random value generated by the SGSN. LFN is the LLC Frame Number (LFN) in the LLC frame header. OC is an overflow counter that is calculated and maintained independently at the sending and the receiving sides. An OC for acknowledged operation must be set to 0 whenever asynchronous balanced mode operation is re-established for the corresponding Data Link Connection Identifier (DLCI). An LLC layer connection is identified using DLCI, which consists of Service Access Point Identifier (SAPI) and the TLLI associated with MS  100 . OC shall be incremented by 512 every time when the corresponding LFN rolls over. Due to this fact, OC is never sent directly in LLC frames. The aim of OC is to add variation to the ciphering process in order to make it more robust. SX is an XOR mask calculated from the LLC entity identifier. There are two IOV values, one for numbered information frames associated with acknowledged operation and another for unconfirmed information frames associated with unacknowledged operation. There are also two LFN values, one for acknowledged operation and another for unacknowledged operation. There are four OC counters associated with each DLCI. There is one OC counter per operation mode, which is either unacknowledged or acknowledged, and direction of transmission, which is either uplink or downlink.  
         [0015]     Naturally, the session key K c  used in the ciphering algorithm is one of the ciphering parameters.  
         [0016]     Reference is now made to  FIG. 3 , which is a signaling diagram illustrating signaling during a packet switched handover in accordance with the current 3GPP proposals. The current proposals are described in TSG document GP-032710 “Packet Switched Handover for GERAN A/Gb mode, Stage  2 ”, version 0.2.0, 2004-01. The architecture associated with the signaling is as illustrated in  FIG. 2 . MS  100  sends radio quality measurement information pertaining to neighboring cells to source BSS  262  using message  301 . Based on the measurement information source BSS  262  determines that handover is required. At time t 0  source BSS  262  determines that handover is to be performed to a new cell, which is in the area of a new SGSN, which is SGSN  204 . Source BSS  262  sends a PS Handover Required message  302  to old SGSN  202 . The message comprises, for instance, the source cell, the target cell, TLLI, cause and a transparent container. SGSN  202  determines based on the target cell if the handover is an intra- or inter-SGSN handover. SGSN  202  determines the identity of the new SGSN and sends a Prepare PS Handover Request message  303  to SGSN  204 . SGSN  204  sends a PS Handover Required message  304 , which requests target BSS  264  to reserve radio resources for MS  100  in the target cell. When radio resources have been successfully allocated, target BSS  264  sends a PS Handover Request Acknowledge message  305  indicating successful allocation. SGSN  204  sends a Prepare PS Handover Response message  306  to SGSN  202 , which tells, among other things, that SGSN  202  may issue to MS  100  a command to complete handover to the new cell. SGSN  202  receives message  306  at time t 1 .  
         [0017]     However, simultaneously a packet from GTP packet stream  307  is received by SGSN  202 . SGSN  202  forwards packets one by one from GTP packet stream  307  to SGSN  204  as packet stream  308 . SGSN  204  sends packets from packet stream  308  further to target BSS  264  as packet stream  309 . Target BSS forwards packets from packet stream  308  to MS  100  as packet stream  310 . There is a delay before MS  100  is able to receive packets from SGSN  204  via target BSS  264 . SGSN  202  sends PS Handover Command message  311  to source BSS  262 . Source BSS sends further PS Handover Command message to MS  100 . Thereupon, MS  100  tunes to the radio channel and timeslot allocated in the target cell by target BSS  264 . This is illustrated using arrow  312 . Target BSS  264  sends Physical information to MS  100  for MS  100  to synchronize. After MS  100  has synchronized, it sends a PS Handover Complete message  314  to target BSS  264  at time t 2 . Only after time t 2  MS  100  is prepared to receive packets via target BSS  264  normally, which shows that there is an intolerable delay unless MS  100  receives packets via both target BSS  264  and source BSS  262 . Target BSS  264  sends a PS Handover Complete message  315  to SGSN  204 . Thereupon, SGSN  204  performs PDP context update messaging represented using arrows  316  and  317  with GGSN  200 . PDP context update indicates to GGSN  200  the address of current SGSN  204 . After having received PDP context update at time t 3 , GGSN  200  is able to start routing GTP packet stream  318  to right SGSN, which is now SGSN  204 . Thereupon, MS  100  receives packet stream  320  from target BSS  264 , which has received it from SGSN  204  as packet stream  319 .  
         [0018]     Reference is now made to  FIG. 4 , which is signaling diagram illustrating the delay associated with a solution, which merely forwards packets from a source node to a target node during handover processing. The solution is similar to the solution utilized in UMTS in association with Serving Radio Network Server SRNS relocation. SRNS relocation is explained in 3GPP 23.060. In  FIG. 4 a  source node  452  receives a packet stream  401  sent by an upper node  450 , which is connected to an IP network  451 . At time to upper node sends a specific packet  460  in packet stream  401 . Source node forwards packet stream further  402  to MS  100  via an access network  456 . At time t 1  MS  100  decides to start using a target node  454  instead of source node  452  for receiving packet streams. At time t 1  MS  100  acknowledges last frame received via source node  452  using message  403 . Packet  460  has not been completely received, for example the last frame from packet  460  may be pending. MS  100  sends a request message  403  for source node  452  indicating the abandoning of source node  452  for MS  100  traffic. After receiving message  403 , source node  452  starts forwarding all packets addressed to MS  100  via target node  454  as packet stream  405 . Packet stream  405  is forwarded by target node  454  to MS  100  as packet stream  406 . At time t 2  MS  100  receives a first packet since MS  100  received the last frame via source node  452  at time t 1 . The time difference between t 1  and t 2  represents the gap in the receiving of packets at MS  100 , whereas the time difference between t 0  and t 2  represent a delay in receiving packet  460  from upper node  450  to MS  100 . The delays explained above are intolerable for realtime services.  
         [0019]     As has been illustrated in association with  FIGS. 2, 3  and  4 , there are problems in performing packet switched handover using current GPRS architecture and the solutions proposed in prior art. On the one hand, it must be possible for an MS to receive packets simultaneously from a source node and a target node during the handover signaling. On the other hand, this is not possible in current GPRS specifications and leads to the rejection of forwarded frames at the MS side.  
       SUMMARY OF THE INVENTION  
       [0020]     The invention relates to a method of performing handover in a mobile communication system comprising a mobile node, a first and a second packet switching node. In the method a handover condition associated with the mobile node is detected in the first packet switching node; the first packet switching node requests handover preparation from the second packet switching node; logical link layer information is received from the first packet switching node to the second packet switching node; the state in a logical link layer entity is set in the second packet switching node based on the logical link layer state information; and logical link layer frames are sent from the first and second packet switching nodes to the mobile node during handover.  
         [0021]     The invention relates also to a method performing handover in a mobile communication system comprising a mobile node, a first and a second packet switching node. In the method a handover condition associated with the mobile node is detected in the first packet switching node; the first packet switching node requests handover preparation from the second packet switching node; a packet is received at the first packet switching node; a logical link layer Protocol Data Unit (PDU) is formed from data in the packet; a first frame containing the logical link layer Protocol Data Unit (PDU) is sent to the mobile node from the first packet switching node; the logical link Protocol data Unit (PDU) is sent from the first packet switching node to the second packet switching node; and a second frame containing the logical link layer Protocol Data Unit (PDU) is sent to the mobile node from the second packet switching node.  
         [0022]     The invention relates also to a method performing handover in a mobile communication system comprising a mobile node, a first and a second packet switching node. In the method a handover condition associated with the mobile node is detected in the first packet switching node; the first packet switching node requests handover preparation from the second packet switching node; at least one ciphering parameter is received from the first packet switching node to the second packet switching node; a logical link parameter exchange is performed between the mobile node and the first packet switching node; and logical link layer frames are sent from the first and second packet switching nodes to the mobile node during handover.  
         [0023]     The invention relates also to a method performing handover in a mobile communication system comprising a mobile node, a first and a second packet switching node. In the method a first logical link layer entity is formed in the mobile node; a handover condition is detected in the mobile node; a second logical link layer entity is formed in the mobile node; logical link layer frames are sent from the first and second packet switching nodes to the mobile node during handover; handover completion is detected; and logical link layer parameters between the mobile node and the second packet switching node are renegotiated after the detecting of the handover completion if the logical link layer parameters are not suitable.  
         [0024]     The invention relates also to a system, which comprises a mobile node, a first and a second packet switching node. The system further comprises: signaling means in the first packet switching node for detecting a handover condition associated with the mobile node, requesting handover preparation from the second packet switching node and sending logical link layer information to the second packet switching node; signaling means in the second packet switching node for receiving logical link layer information from the first packet switching node; control means in the second packet switching node arranged to set the state in a logical link layer entity based on logical link layer information from the first packet switching node; and control means in the first packet switching node arranged to send logical link layer frames to the mobile node during handover.  
         [0025]     The invention relates also to a system, which comprises a mobile node, a first and a second packet switching node. The system further comprises: signaling means in the first packet switching node for detecting a handover condition associated with the mobile node and requesting handover preparation from the second packet switching node; logical link layer means in said first packet switching node for forming logical link layer Protocol Data Units (PDU) and sending said logical link layer Protocol Data Units (PDU) to said second packet switching node; and logical link layer means in said second packet switching node for sending said logical link layer Protocol Data Units (PDU) transparently to said mobile node.  
         [0026]     The invention relates also to a system, which comprises a mobile node, a first and a second packet switching node. The system further comprises: signaling means in the first packet switching node for detecting a handover condition associated with the mobile node, requesting handover preparation from the second packet switching node and sending at least one ciphering parameter to the second packet switching node; signaling means in the second packet switching node for receiving at least one ciphering parameter from the first packet switching node; logical link layer means in the first packet switching node for performing a logical link parameter exchange with the mobile node.  
         [0027]     The invention relates also to a system, which comprises a mobile node, a first and a second packet switching node. The system further comprises: control means in the mobile node arranged to form a first logical link layer entity in response to connection establishment and a second logical link layer entity in response to a handover condition; signaling means in the mobile node for detecting the handover condition and a handover completion; logical link layer means in the mobile node arranged to renegotiate logical link layer parameters with the second packet switched node after the handover completion if the logical link layer parameters are not suitable.  
         [0028]     In one embodiment of the invention, the mobile node is 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.  
         [0029]     In one embodiment of the invention, the mobile node is a mobile computer, for example, a laptop computer, palmtop computer or a personal digital assistant (PDA).  
         [0030]     In one embodiment of the invention, the mobile communication system is a General Packet Radio Service (GPRS), the first and second packet switching nodes are Serving GPRS Support Nodes (SGSN) and the logical link layer is GPRS Logical Link Control (LLC) and the logical link parameter exchange is Logical Link Control (LLC) exchange Identification (XID) negotiation. 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.  
         [0031]     In one embodiment of the invention, the logical link parameter exchange is performed in response to the detection of handover condition at the first packet switching node.  
         [0032]     In one embodiment of the invention, the first logical link layer entity in the mobile node is removed after the detecting of handover completion.  
         [0033]     In one embodiment of the invention, the at least one ciphering 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 ciphering parameter is sent from the first packet switching to the second packet switching in the message that requests handover preparation.  
         [0034]     In one embodiment of the invention, the logical link layer information 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 logical link layer information is sent from the first packet switching node to the second packet switching in the message that requests handover preparation.  
         [0035]     In one embodiment of the invention, the logical link parameter exchange is performed in response to the condition where the mobile node receives an LLC frame, which has a duplicate flag set. The duplicate flag indicates the duplication of the LLC frame for handover purposes. In one embodiment of the invention, the duplicate flag is only accepted by the mobile node while handover is being performed. Otherwise, the receiving of the flag results in an error indication to the peer LLC-entity.  
         [0036]     In one embodiment of the invention, the logical link layer means in the mobile node and in the first and second packet switching nodes are represented by one or many Logical Link Control (LLC) entities, a Logical Link Management Entity (LLME) and a multiplexing entity associated with them. On transmission the multiplexing entity generates and inserts the FCS, performs a frame ciphering function and provides SAPI-based logical link control layer contention resolution between the various logical link entities. The functions performed by multiplexing entity and LLME are described in 3GPP specification 23.060.  
         [0037]     In one embodiment of the invention, the control means in the first and second packet switching node comprise the higher protocol layer entities above the logical link layer. For example, in a SGSN the control means may comprise the relay layer entities, the SNDCP layer entities and the GTP layer entities.  
         [0038]     In one embodiment of the invention, the control means in the mobile node comprise the higher protocol layer entities pertaining to the GPRS user plane.  
         [0039]     In one embodiment of the invention, the signaling means in the mobile node comprise the signaling protocols used to communicate with the first and the second packet switching nodes. In a GPRS mobile terminal the signaling means comprises the GPRS control plane signaling protocol stack entities. In one embodiment of the invention, the actual mobility management and radio control related application logic are performed in control means or in separate control means in association with signaling means. In this embodiment the exchange of signaling messages is handled by separate means reserved for the task.  
         [0040]     In one embodiment of the invention, the signaling means in the first and the second packet switching nodes comprise the signaling protocols used to communicate with the mobile node. In a SGSN the signaling means comprises the GPRS control plane signaling protocol stack entities.  
         [0041]     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.  
         [0042]     The benefits of the invention are associated with improved quality of service. With the invention it is now possible to provide a continuous packet stream to a mobile station during handover. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0043]     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:  
         [0044]      FIG. 1  is a block diagram illustrating the prior art architecture and the protocol stacks in a General Packet Radio Service (GPRS) system in association with the GSM/EDGE Radio Access Network (GERAN);  
         [0045]      FIG. 2  is a block diagram illustrating General Packet Radio Service (GPRS) network architecture and problems in prior art associated with duplicated packet forwarding;  
         [0046]      FIG. 3  is a signaling diagram illustrating signaling during a packet switched handover in prior art;  
         [0047]      FIG. 4  is a signaling diagram illustrating the delay associated with a solution, which merely forwards packets from a source node to a target node during handover processing;  
         [0048]      FIG. 5  is a signaling diagram depicting one embodiment of packet switched handover method utilizing state transfer, according to the invention;  
         [0049]      FIG. 6   a  is a block diagram depicting one embodiment of packet switched handover method utilizing frame forwarding via Serving GPRS Support Node (SGSN), according to the invention;  
         [0050]      FIG. 6   b  is a block diagram depicting one embodiment of packet switched handover method utilizing frame forwarding directly to target Base Station Subsystem, according to the invention;  
         [0051]      FIG. 7  is a signaling diagram depicting one embodiment of packet switched handover method utilizing logical link parameter reset, according to the invention;  
         [0052]      FIG. 8  is a block diagram depicting one embodiment of packet switched handover method utilizing duplicate logical link control entities, according to the invention;  
         [0053]      FIG. 9  is a signaling diagram depicting one embodiment of packet switched handover method utilizing a duplicate frame indicator, according to the invention;  
         [0054]      FIG. 10  is a flow chart depicting one embodiment of packet switched handover method utilizing context transfer, according to the invention;  
         [0055]      FIG. 11  is a flow chart depicting one embodiment of packet switched handover method utilizing frame forwarding, according to the invention;  
         [0056]      FIG. 12  is a flow chart depicting one embodiment of packet switched handover method utilizing logical link reset, according to the invention;  
         [0057]      FIG. 13  is a flow chart depicting one embodiment of packet switched handover method utilizing duplicate logical link control entities, according to the invention;  
         [0058]      FIG. 14  is a flow chart depicting one embodiment of packet switched handover method utilizing a duplicate frame indicator, according to the invention;  
         [0059]      FIG. 15  illustrates a Serving GPRS Support Node (SGSN) in one embodiment of the invention; and  
         [0060]      FIG. 16  illustrates a mobile node in one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0061]     Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0062]      FIG. 10  is a flow chart depicting one embodiment of packet switched handover, which utilizes state transfer using a signaling illustrated in  FIG. 5 . The signaling is performed in GPRS system architecture, which is illustrated in  FIG. 2 . At step  1000  it is checked if handover occurs. In case there is handover MS  100  sends radio quality measurement information pertaining to neighboring cells to source BSS  262  using message  301 . Based on the measurement information source BSS  262  determines that handover is required. The determination is performed using an algorithm that is executed in a Base Station Controller (BSC) within the source BSS  262 . At time t 0  source BSS  262  determines that handover is to be performed to a new cell, which is in the area of a new SGSN, which is SGSN  204 . Source BSS  262  sends a PS Handover Required message  302  to old SGSN  202 . The message comprises, for instance, the source cell, the target cell, TLLI, cause and a transparent container. SGSN  202  determines based on the target cell if the handover is an intra- or inter-SGSN handover. SGSN  202  determines the identity of the new SGSN and sends a Prepare PS Handover Request message  303  to SGSN  204 .  
         [0063]     At step  1002  the state pertaining to the logical link is obtained by the LLC-entity in SGSN  204 . This is achieved so that Prepare PS Handover Request message carries LLC state information element  500 . LLC state information element  500  comprises information that is used to synchronize LLC-entities in SGSN  202  and SGSN  204 . Information element  500  comprises at least the session key K c , the IOV values for both modes of operation, both LFN values and the four OC values. SGSN  204  stores information element  500  until SGSN  202  forwards packets to it. The handover signaling between network elements continues as explained in association with  FIG. 3 .  
         [0064]     At step  1004  when a first forwarded packet is received from SGSN  202 , an LLC entity is initialized in SGSN  204 . During initialization SGSN  204  uses information element  500 . By having information element  500  and the LLC state information in it, it is possible for SGSN  204  to construct an LLC-entity, which is an exact replica of the LLC-entity in SGSN  202  from MS  100  point of view. Thereupon, MS  100  is able to receive LLC frames from both LLC-entities without noticing a difference. In one embodiment of the invention the LLC-entity in SGSN  204  is initialized and started already after SGSN  204  has received message  303  and no packets to be forwarded have yet been received by SGSN  204 . At step  1006  SGSN  204  starts forwarding packets received via SGSN  202  using the LLC-entity constructed and initialized at step  1006 .  
         [0065]      FIG. 11  is a flow chart depicting one embodiment of packet switched handover, which utilizes frame forwarding in a system as illustrated in  FIG. 6   a  or  6   b . At step  1100  SGSN  202  waits for a message from source BSS  262  indicating that handover is required. In one embodiment of the invention the handover indication may also be received from MS  100 . When the message is received method continues in step  1102 . At step  1102  SGSN  202  waits for an event where SGSN  202  receives a packet  610  from GGSN  200 , which is the first user plane packet after the start of handover. At this event a first LLC frame  614  that carries data from packet  610  is to be sent by SGSN  202 . When the event occurs packet  610  is received by an SNDCP entity  600  in SGSN  202  via the GTP and relay layers as illustrated in  FIG. 1 .  
         [0066]     Packet  610  is received to SGSN  202  via tunnel  240 . The SNDCP entity  600  performs packet segmentation for packet  610  and other SNDCP level tasks and issues a request to an LLC-entity  252  to send first LLC-frame  614 . The request is issued in the form of an LLC Service Data Unit (SDU). At step  1104  LLC-entity  252  prepares an LLC-PDU using the information contained in LLC-SDU and the LLC-entity  252  state variables. At step  1106  LLC-entity  252  sends the prepared LLC-PDU in a first LLC-frame  614  to source BSS  262  and BSC  212  therein.  
         [0067]     At step  1108  LLC-entity  252  passes the LLC-PDU in a second LLC-frame  616  to a frame forwarding entity  604  in association with SNDCP entity  600 . It should be noted that second LLC-frame  616  is a duplicate of LLC-frame  614 . Frame forwarding entity  604  sends the second LLC-frame  616  to SGSN  204  using a connection  241 , which tunnels LLC-frames prepared by LLC-entity  252  to SGSN  204 . Connection  241  is, for example, a GTP tunnel established between SGSN  202  and SGSN  204  for the transparent forwarding of LLC-frames. The second LLC-frame  616  is received by LLC-entity  606  in SGSN  204 . LLC-entity  606  is configured to receive LLC-frames via connection  241  and forward them transparently towards target BSS  264 . The transparent forwarding means in this case that the LLC-entity does not alter the LLC-frame fields indicating LLC-entity  252  state. In one embodiment of the invention, relay LLC PDU formed from LLC-frame  616  is not relayed through SNDCP protocol entity in SGSN  204 . In another embodiment of the invention the LLC PDU from LLC-frame  616  is relayed through protocol entity chain GTP-SNDCP-LLC-BSSGP in order to be sent to target BSS  264 .  
         [0068]     In one embodiment of the invention illustrated in  FIG. 6   b  SGSN  202  passes second LLC-frame  616  directly to target BSS  264 . This is achieved so that a connection  241   b  is formed between SGSN  202  and target BSS  264 . This is achieved so that at step  1108  is omitted from the method. Instead, at step  1110  LLC-entity  252  passes the LLC-PDU in a second LLC-frame  616  to a frame forwarding entity  604   b  in association with SNDCP entity  600 . Frame forwarding entity  604   b  sends the second LLC-frame  616  to target BSS  264  using connection  241   b . Target BSS  264  is configured to receive LLC-frame  616  and other duplicate LLC-frames for handover and to prepare them for transmission to MS  100 .  
         [0069]      FIG. 12  is a flow chart depicting one embodiment of packet switched handover, which utilizes logical link reset achieved using a signaling illustrated in  FIG. 7 . The signaling is performed in GPRS system architecture, which is illustrated in  FIG. 2 . At step  1200  it is checked if handover occurs. In case there is handover MS  100  sends radio quality measurement information pertaining to neighboring cells to source BSS  262  using message  301 . Based on the measurement information source BSS  262  determines that handover is required. The determination is performed using an algorithm that is executed in a Base Station Controller (BSC) within the source BSS  262 . At time t 0  source BSS  262  determines that handover is to be performed to a new cell, which is in the area of a new SGSN, which is SGSN  204 . Source BSS  262  sends a PS Handover Required message  302  to SGSN  202 . The message comprises, for instance, the source cell, the target cell, TLLI, cause and a transparent container. SGSN  202  determines based on the target cell if the handover is an intra- or inter-SGSN handover. SGSN  202  determines the identity of a new SGSN, which in this case is SGSN  204 , and sends a Prepare PS Handover Request message  303  to SGSN  204 .  
         [0070]     At step  1202  cipher parameters pertaining to the logical link are obtained by the LLC-entity in SGSN  204 . This is achieved so that Prepare PS Handover Request message carries cipher parameter information element  700 . Information element  700  comprises, for example, the session key K c  and any other parameters not re-negotiated at during XID-reset procedure. At step  1204  SGSN  202  starts XID-reset procedure so that LLC-entity  252  in SGSN  202  sends an XID command message  701  to MS  100  via source BSS  262 . XID command message  701  includes information on LLC parameters such as, for example, LLC version number, IOV values, retransmission timeout, maximum number of retransmissions, maximum information field lengths in the two acknowledgement modes, frame buffer sizes in uplink and downlink direction, window sizes in uplink and downlink directions and layer- 3  parameters. XID command message  701  proposes LLC parameter values that correspond to initial LLC values set when a new SGSN initializes its LLC-entity. At the receipt of XID command message  701 , MS  100  sets LLC parameters to the values proposed and issues XID response message  702  acknowledging the proposed parameter values. In one embodiment of the invention MS  100  is configured to accept the parameters proposed by SGSN  202  automatically when it is aware that a handover process is pending. In one embodiment of the invention MS  100  accepts a downlink PDU automatically from SGSN  204  if it is flagged accordingly and if it is received during handover.  
         [0071]     At step  1206  SGSN  204  starts receiving packets forwarded from SGSN  202 . In  FIG. 7  such packets are carried in packet stream  308 . SGSN  204  initializes its LLC-entity  254  to have initial LLC parameter values. The initial values correspond to the LLC-parameter values negotiated between SGSN  202  and MS  100  during XID-reset procedure at step  1204 . Thereupon, SGSN  204  starts sending the forwarded packets towards MS  100 . Afterwards, SGSN  204  and MS  100  may negotiate more optimal LLC parameters. Typically the re-negotiation of LLC parameters is performed after routing area update.  
         [0072]      FIG. 13  is a flow chart depicting one embodiment of packet switched handover, which utilizes illustrated in  FIG. 8 . At step  1300  MS  800  has only one LLC-entity, which is first LLC-entity  802 . First LLC-entity  802  is the peer entity for LLC-entity  252  in SGSN  202 . There is an LLC connection  842  between LLC-entities  252  and 802. LLC connection  842  carries a packet stream originating from GGSN  200  to MS  800 . MS  800  waits for a condition where handover is required. This is determined based on, for example, a handover command received from BSS  262 . When the condition is detected the method continues in step  1302 . At step  1302  MS  100  constructs a second LLC-entity  804 , which exists simultaneously with first LLC-entity  802  at least during handover. Second LLC-entity  804  is the peer entity for LLC-entity  254  in SGSN  204 . At step  1304  MS  800  initializes second LLC-entity  804 . The LLC parameters are initialized to values compatible with the values to which SGSN  204  initializes the LLC parameters while it initializes LLC-entity  254  at step  1306 . At step  1306  SGSN  204  receives packets forwarded from SGSN  202  via a tunneling connection  241 . Tunneling connection  241  is, for example, a GTP tunnel. SGSN  204  sends the forwarded packets towards MS  800  using LLC connection  844 , which it sets up between LLC-entities  254  and  804 . At step  1308  MS  800  checks if handover is finished. If handover is not finished method continues at step  1308 .  
         [0073]     When the handover is finished LLC connection  842  between LLC-entities  252  and  802  is no longer used to carry LLC-frames. In one embodiment of the invention at step  1310  MS  800  checks if LLC parameters pertaining to LLC connection  844  are suitable taking into consideration, for example, the radio conditions at the cell served by BTS  224 . MS  800  may also readjust the parameters depending on available memory and the data rate on LLC connection  844 . In one embodiment of the invention LLC parameters at LLC-entity  254  are initialized first to moderate values, which are made suitable for most mobile stations under different radio conditions. Mobile stations may have also varying memory sizes and software versions. For example, information field lengths, frame buffer and window sizes may be first set to values lower than would otherwise be negotiated between peering LLC-entities. If MS  800  determines that LLC parameters are not suitable, it readjusts them to different values at step  1312 . The parameters are to be readjusted, for example, using an XID reset procedure involving the exchanging of XID command and XID response between LLC-entities  804  and  254 . If parameter values are suitable no readjusting is needed.  
         [0074]     In one embodiment of the invention, MS  800  removes the first LLC-entity, which was used prior to handover, after the handover is complete. At step  1314  MS  800  performs the procedures necessary for removing LLC-entity  802 , which is no longer used. MS  800  may also remove LLC-entity  802  directly after step  1308  before checking whether the LLC parameters are suitable. The removing of LLC-entity comprises, for example, the releasing of memory reserved for the use of LLC-entity  802  and LLC connection  842  in MS  800 . Similarly, information pertaining to LLC-entity  802  and LLC connection  842  may be removed from memory tables maintained in MS  800 .  
         [0075]      FIG. 14  is a flow chart depicting one embodiment of packet switched handover method, which utilizes a duplicate frame indicator conveyed and processed using a signaling illustrated in  FIG. 9 . The signaling is performed in GPRS system architecture, which is illustrated in  FIG. 2 . At step  1400  it is checked if handover occurs. In case handover occurs MS  100  sends radio quality measurement information pertaining to neighboring cells to source BSS  262  using message  301 . Based on the measurement information source BSS  262  determines that handover is required. The determination is performed using an algorithm that is executed in a Base Station Controller (BSC) within the source BSS  262 . At time t 0  source BSS  262  determines that handover is to be performed to a new cell, which is in the area of a new SGSN, which is SGSN  204 . Source BSS  262  sends a PS Handover Required message  302  to SGSN  202 . The message comprises, for instance, the source cell, the target cell, TLLI, cause and a transparent container. SGSN  202  determines based on the target cell if the handover is an intra- or inter-SGSN handover. SGSN  202  determines the identity of a new SGSN, which in this case is SGSN  204 , and sends a Prepare PS Handover Request message  303  to SGSN  204 .  
         [0076]     At step  1402  cipher parameters pertaining to the logical link are obtained by the LLC-entity in SGSN  204 . This is achieved so that Prepare PS Handover Request message carries cipher parameter information element  900 . Information element  700  comprises, for example, the session key K c  and any other parameters not re-negotiated at during a XID-reset procedure.  
         [0077]     At step  1404  SGSN  204  waits for packets forwarded from SGSN  202  to it. When such a packet is received in message  308 , the method continues at step  1406 . At step  1406  an SNDCP entity in SGSN  204  indicates to LLC-entity in SGSN  204  while requesting the sending of an LLC-SDU that the LLC-SDU is a first LLC-SDU comprising data from packets forwarded from SGSN  202  to SGSN  204 . The LLC-PDU is therefore a duplicate of another LLC-PDU sent from SGSN  202 . LLC-entity in SGSN  204  sets a duplicate for handover flag in the header of the LLC-PDU to be sent. The flag may be carried in, for example, in one of the reserved bits in LLC address field or in one of the UI control field bits. Therefore, no extra bits are needed in LLC-PDU header. LLC parameters are set to default handover values. The default values may be standardized so that optimization is maximized or normal default values are used. When MS  100  receives the LLC-PDU in an LLC frame, it detects that the duplicate for handover bit is set. At step  1408  MS performs implicit XID-reset for the LLC-entity in it. In implicit XID-reset the MS  100  sets automatically the LLC parameters to values, which are compatible with values set by LLC-entity in SGSN  204  when it is first created and initialized. Implicit XID-reset is required in MS  100  before it is able to process any LLC frames from SGSN  204 . For example, this is due to the differing ciphering parameters, for example overflow count, which have not been received at step  1402 .  
         [0078]      FIG. 15  illustrates a Serving GPRS Support Node (SGSN) in one embodiment of the invention. SGSN  1500  comprises a signaling entity  1504 , which communicates with a logical link layer entity  1506 . Signaling entity  1504  performs GPRS control plane signaling. Logical link layer entity  1506  carries both control plane and user plane messages as specified in 3GPP 23.060 pertaining to LLC. In the embodiment of the invention disclosed in association with the description of  FIGS. 6 and 11  logical link layer entity  1506  is responsible for forming logical link layer Protocol Data Units (PDU) and sending the logical link layer Protocol Data Units (PDU) to new SGSN. In one embodiment of the invention the sending of the logical link layer PDUs to new SGSN is achieved so that logical link layer entity  1506  passes the PDUs to control entity  1502 , which sends them via, for example, a GTP entity  1510  to the new SGSN. In one embodiment of the invention signaling entity  1504  is responsible for detecting handover conditions, requesting handover preparation from other SGSNs, receiving handover preparation requests from other SGSNs, sending logical link layer state information, ciphering parameters and other information to other SGSNs. In one embodiment of the invention, the actual mobility management and radio related application procedures associated with signaling messages received to signaling entity  1504  are performed by control entity  1502  or by a separate control entity within signaling entity  1504 . In one embodiment of the invention control entity  1502  is responsible, for example, for setting the state in logical link layer entity  1506  based on logical link layer information received from another SGSN and sending logical link layer frames to mobile node during handover. The actual sending of logical link layer frames is performed via lower protocol layers  1508 . The arrows in  FIG. 15  illustrate directions of information flows between the entities within SGSN  1500 .  
         [0079]      FIG. 16  illustrates a mobile node in one embodiment of the invention. In  FIG. 16  mobile node is more specifically a GPRS mobile terminal. Mobile node  1600  comprises a signaling entity  1604 , which communicates with a logical link layer entity  1606 . Logical link layer entity  1606  carries both control plane and user plane messages as specified in 3GPP 23.060. In one embodiment of the invention signaling entity  1604  is responsible for receiving signaling messages from the base station subsystem and detects handover conditions and handover completion based on received signaling messages. Logical link layer entity  1606  performs the Logical Link Control (LLC) protocol related tasks. In the embodiment of the invention disclosed in association with the description of  FIG. 12  logical link layer entity  1606  is arranged to renegotiate logical link layer parameters with new SGSN after the handover completion. Mobile station  1600  comprises also a control entity  1602 , which performs higher protocol layer related tasks and overall coordination of communication. In one embodiment of the invention control entity  1602  is arranged to form a first logical link layer entity during connection establishment procedure and a second logical link layer entity in response to a handover condition. The arrows in  FIG. 15  illustrate directions of information flows between the entities within mobile node  1600 .  
         [0080]     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.