1. Technical Field
The invention relates in general to cellular networks that support transmission of packet data. In particular the invention relates to performing a serving network entity relocation from a certain network entity to a peer network entity.
2. Discussion of Related Art
Traditionally cellular systems, for example the Global System for Mobile telecommunications (GSM), have been used to transmit speech and they have implemented circuit switching. In circuit switching a certain amount of transmission resources is reserved in all the networks through which the connection goes. For new data applications there is usually need to transmit bursts of data every now and then. For this kind of data transmission circuit switching is not an efficient way to transmit data.
In the Universal Mobile Telecommunication System (UMTS) both packet switching and circuit switching are supported. When a mobile station, or a user equipment as the portable terminal communicating with the UMTS system is usually called, is changing its position, the network elements through which its packet switched data or circuit switched connections change. The UMTS system keeps track on the position of the user equipment, and the circuit switched connections are re-routed in a similar manner as in GSM, for example, and the packet switched data is re-routed similarly as in General Packet Radio Service (GPRS).
In the core UMTS network the packet switched data and the circuit switched connections may be transmitted using separate networks. The core network of UMTS transmitting the packet data may be, for example, and Internet Protocol (IP) based network. The user equipment is attached to the core UMTS network through a radio access network (RAN). Both the packet switched data and the circuit switched connections flow through the same radio access network.
FIG. 1 presents a schematic diagram of a UMTS radio access network (RAN) 110 and UMTS core network 120. In FIG. 1 the UMTS core network 120, as one example of realizing the UMTS core network, comprises two separate core networks: the circuit switched core network (CS-CN) 130 and the packet switched core network (PS-CN) 140. The CS-CN 130 is very similar to GSM core network and comprises mobile services switching centers (MSCs), through which circuit-switched connections run. FIG. 1 presents MSCs 131a and 131b. In the common part of the UMTS core network 120 there is a data base, Home Location Register (HLR) 121, where information about the mobile stations is stored. The PS-CN comprises GPRS Supporting Nodes (GSNs) which act as routers with additional mobility functions. The data packets run through the GSNs. The GSNs facing the UMTS RAN 110 are Serving GPRS Supporting Nodes (SGSN). The SGSN is at the same hierarchical level as the MSC; it, for example, keeps track of the individual MSs' location and performs security functions and access control. The GSNs facing other packet data networks are Gateway GPRS Supporting Nodes (GGSN). FIG. 1 presents two SGSNs 141a, 141b, and one GGSN 142 which faces a packet data network 150. The packet data networks to which UMTS core network is connected through a GGSN may be, for example, the Internet or a X.25 data network.
FIG. 1 shows base stations (BTS) 111a, 111b, 111c, 111d of the UMTS RAN 110. A mobile station communicates with these base stations over the radio interface. One or more base stations are connected to a radio network controller (RNC). The RNCs face the UMTS core network and they are connected to MSCs and/or SGSNs. It is possible to connect to each MSC or SGSN many RNCs. The RNC is responsible, for example, for allocation of radio resources and for handling handovers, where a mobile station changes cell. FIG. 1 shows two RNCs 112a, 112c. 
Here, the term cell is used to refer to the area covered either by a base station or, if a base station comprises many transmitters, by one or more transmitters. A group of cells, whose base stations are connected to a part of, one or more RNC form a Location Area (LA), which term is used in connection with circuit switched connections, or a Routing Area (RA), which term is used in connection with packet switched data. FIG. 1 present two Location/Routing areas 113a and 113c. 
A mobile station performs a UMTS PS attach procedure in order to obtain packet data services, in other words to inform to the UMTS system that it can send and/or received packet data. The UMTS PS attach establishes a logical link between the MS and the SGSN, and makes the MS available for, for example, paging via SGSN and notification of incoming packet data. When it actually has packet data to transmit, it performs a Packet Data Protocol (PDP) context activation.
Consider, as an example, a data packet flow between MS 101 and a terminal 151 in FIG. 1. The route of the data is MS 101-BS2111b-RNC1112a-SGSN1141a-GGSN 142-packet data network 150-terminal 151. The PDP context activation has established a tunnel between the SGSN1141a and the GGSN 142. Data between SGSN and GGSN is transmitted using a GPRS tunneling protocol (GTP), which typically runs on IP. Signaling takes place using the same protocol. Between the SGSN and the RNC data is transmitted using the same GTP protocol and there is also a tunnel. The Tunnel Endpoint Identifier (TEID) indicates the data packet belonging to a certain data packet flow related to a certain MS. Each packet transmitted towards a SGSN, GGSN, or RNC carries a SGSN, GGSN or RNC TEID, correspondingly. In the RAN, the data packets are transmitted between the RNC and the MS typically based on the International Mobile Subscriber Identifier (IMSI) of the mobile station or on a Radio Network Temporary Identity (RNTI) and according to the negotiated QoS (Quality of Service) parameters.
A mobile station may be either in a PMM (Packet Mobility Management) connected or PMM idle state. In a PMM connected state the mobile station is known by an RNC and it has a RRC connection. The RNC tracks the location of the mobile station, when the mobile station is in the PMM connected state. When the mobile station is in a PMM idle state, its location is tracked by a SGSN. The SGSN tracks the location of a mobile station only at the Routing Area level, whereas a RNC knows the cell in which a mobile station is.
When a mobile station changes location, it may change cell. Usually the cell is selected based on the strength of the downlink radio transmissions. If the source (original) cell and the target cell do not belong to the same routing area, a PS-attached mobile station performs a routing area update. Consider as an example in FIG. 1 a situation, where the mobile station (which is in a PMM Idle state) changes the cell from the BS2111a to the BS3111c. FIG. 2 presents an example of a message sequence chart of a inter-SGSN routing area update, where the target SGSN is not the source SGSN, e.g. the target SGSN is SGSN2 and the source SGSN is SGSN1 in FIG. 1. The routing area update procedure begins with a Routing Area Update Request 201 sent by the MS to the target SGSN, to which the target RNC is connected. The target SGSN sends a SGSN Context Request 202 to the source SGSN. This message indicates the PTMSI or IMSI of the MS, the target Routing Area identifier (RAI) and some security information. Once the MS has been authenticated, the source SGSN which responds with a SGSN Context Response 203. This messages carries, for example, information about the IMSI of the MS and about the PDP contexts of the MS. After the SGSN Context Response, the target SGSN may trigger the RAN to perform some security functions with the MS. Thereafter, if the MS is authenticated properly, the target SGSN sends a SGSN Context Acknowledgement 204 to the source SGSN.
The target SGSN sends Update PDP Context Request 205 to the proper GGSN or GGSNs. This message indicates, for example, the address of the target SGSN Address, a SGSN Tunnel Endpoint Identifier and a GGSN Tunnel Endpoint Identifier. The GGSN Tunnel Endpoint Identifier is used by the GGSN to identify the PDP context. The GGSNs update their PDP context fields and return an Update PDP Context Response 206 to the target SGSN. Thereafter the target SGSN informs the HLR of the change of SGSN by sending Update Location message 207 indicating the target SGSN and the IMSI of the MS.
Cancel Location and Cancel Location Ack messages 208 and 209 between the HLR and the source SGSN inform the source SGSN that it can delete information about the MS. Insert Subscriber Data message 210 is sent by the HLR to the target SGSN and it carries subscriber information related to the MS. The target SGSN confirms by sending Insert Subscriber Data Ack message 211. Thereafter the HLR sends an Update Location Ack message 212 to the target SGSN. Thereafter the target SGSN sends a Routing Area Update Accept message 213 to the MS, and the MS responds with a Routing Area Update Complete message 214. If the routing area update is an intra-SGSN, then only messages 201, 213 and 214 are sent.
A mobile station may communicate with more than one cell at the same time. If the cells are controlled by more than one RNC, one of the RNCs is the Serving RNC (SRNC). The serving RNC is responsible, for example, for the control of the radio resources and for the control of the uplink and downlink packets. The other RNCs are usually called drift RNCs and they typically just relay data packets between the MS and the Serving RNC. In FIG. 1, for example, the mobile station may be communicating with base stations BS2111b and BS3111c, and the serving RNC may be the RNC1112a. When the mobile station, for example, moves farther away from the base station BS2111b, the relocation of the Serving RNC becomes relevant because the MS may communicate only with the base station BS3111c. The route of packet data between the GGSN and the MS may be, for example, before the SRNC relocation GGSN-SGSN1-RNC1-RNC2-BS2-MS.
When the mobile station is in a PMM connected state, an SRNC relocation procedure is always performed before RAU (routing area update) procedure. FIG. 3 presents a message sequence chart of the steps of the relocation of a Serving RNC before the routing area update (RAU) procedure. The source RNC sends a SRNC Relocation Required message 301 indicating the target RNC and carrying an information field to be passed to the target RNC to the source SGSN. If the SRNC relocation is an inter-SGSN SRNC relocation, the source SRNS sends a Forward SRNC Relocation message 302 to the target SGSN. Thereafter the target SGSN sends a SRNC Relocation Request message 303, which carries the information field originally sent in the SRNC Relocation Required message 301, to the target RNC. The target RNC and the target SGSN establish necessary bearers to the user data between themselves (i.e. over the Iu interface). Thereafter the target RNC sends a SNRC Relocation Ack message 304 to the target SGSN. This message indicates the RNC IP address(es) (possibly one address per PDP context) on which the target RNC is willing to receive packet data and Tunnel End Point Identifier(s) which are the identifiers to be used in GTP level when sending packets to this RNC.
When the traffic resources between the target RNC and the target SGSN are allocated and the target SGSN is ready for the SRNC move, then a Forward SRNC Relocation Response message 305 is sent from the target SGSN to the source SGSN. This message indicates that the target SGSN and RCN are ready to receive from the source RNC the downstream data packets not yet acknowledged by the MS.
When the Forward SRNC Relocation Response message 305 is received in the source SGSN, the source SGSN indicates the completion of a preparation phase at the CN PS domain side for the SRNC relocation by sending the SRNC Relocation Command message 306 to the source RNC. The message comprises IP address(es), and Tunnel End Point Identifier(s) that can be used by the source RNC to send the downstream packets not yet acknowledged by the MS to the target RNC.
When the source RNC has received the SRNC Relocation command message 306, it sends a SRNC Relocation Commit message 307 to the target RNC. This Commit message indicates the GTP sequence number for the next downlink packet (SND) to be received from the GGSN, the GTP sequence number for the next uplink packet (SNU) to be tunneled to the GGSN and UP_RLC_Ack contains the acknowledgements for upstream PDU received by the source RNC on each RLC connection used by the UE. The source RNC stops the exchange of the packets with the MS, and starts tunneling the buffered and incoming downstream packets towards the target RNC using RNC IP address(es) and Tunnel End Point Identifier(s) indicated in the SRNC Relocation Command message 306.
At this point the target RNC, i.e. RNC2, starts acting as the Serving RNC. The downlink and uplink packets are transmitted between the target RNC and the MS, and the target RNC, instead of the source RNC, controls the transmission. The target RNC sends New Mobility Management System Information message 309 to the MS indicating e.g. relevant Routing Area and Location Area. The target RA identifier, which is different from the source RAI, triggers a RA Update procedure which starts by the mobile station sending Routing Area Update message 201. Before this procedure the downlink packets arrive, again referring to the example presented in FIG. 1, the following route: GGSN-SGSN1-RNC1-RNC2-BS53-MS. After the RA Update, the GGSN can send packets intended for the MS directly to the SGSN2 and the route is GGSN-SGSN2-RNC2-BS3-MS.
The uplink traffic is not stopped due to the new Mobility Management System Information or the RA Update procedure, because the target SGSN may send an uplink packet to the GGSN before the update PDP context is performed. This is because the address of the SGSN sending the uplink packet to the GGSN is not checked.
Immediately after a successful switch at the RNC, the target RNC, which is now the serving RNC, sends SRNC Relocation Detect message 308 to the SGSN2. After sending out the New MM System Information message 309, the target RNC sends an SRNC Relocation Complete message 310 to the target SGSN. The target SGSN sends an Update PDP Context Request message 311 (new SGSN Address, QoS Negotiated, SGSN Tunnel Endpoint Identifier, GGSN Tunnel Endpoint Identifier) to the GGSNs concerned. GGSN Tunnel Endpoint Identifier is used by GGSN to identify the PDP context. The GGSNs update their PDP context fields and return an Update PDP Context Response message 311 (GGSN Tunnel Endpoint Identifier). The target SGSN sends Complete Serving RNC relocation message 312 to the source SGSN when all GGSNs have been updated.
If the serving RNC relocation is an inter-SGSN SRNC relocation, there is no need for the RA update procedure which starts with the message 201. Further, the Forward SRNC Relocation messages 302 and 305, Update PDP Context messages 311 and Complete SRNC Relocation message 312 in FIG. 3 are not needed.
As can be seen in FIGS. 2 and 3, the relocation of a serving SRNC including a routing area update involves a large number of messages. If only the phase of SRNC relocation preceding the RA update is studied, there are at least 12 messages involved in the inter-SGSN SRNC relocation. In an error situation, where the source RNC, for example, does not receive the SRNC Relocation Command, the problem can be caused by the source SGSN, the target SGSN or the target RNC. Furthermore, the uplink and downlink packet transmission is usually interrupted during SRNC relocation.