This invention relates to electronic communication systems and more particularly to packet-data communication systems.
Electronic communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements, such as General Packet Radio Service (GPRS) and Enhanced Data Rates for GSM Evolution (EDGE), and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards. Electronic communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates specifications for UMTS, WCDMA, and GSM communication systems.
FIG. 1 depicts a cellular radio telephone system 10. A base station controller (BSC) 12 and a radio network controller (RNC) 14 control various radio network functions, including for example radio access bearer setup, diversity handover, etc. More generally, the BSC and RNC direct connections to/from mobile stations (MSs) 16 and user equipments (UEs) 18, which may be mobile telephones or other remote terminals, via the appropriate base transceiver station(s) (BTSs) and Node Bs, which communicate with each MS and UE through downlink (i.e., BTS/Node B to MS/UE) and uplink (i.e., MS/UE to BTS/Node B) channels. BSC 12 is shown coupled to BTSs 20, 22, and RNC 14 is shown coupled to Node Bs 24, 26. Each BTS/Node B serves a geographical area that can be divided into one or more cell(s). The BTSs/Node Bs are coupled to their corresponding BSC/RNC by dedicated telephone lines, optical fiber links, microwave links, etc. The BSC 12 and RNC 14 are connected to external networks such as the public switched telephone network (PSTN), the Internet, etc. through one or more nodes in a core network 28 indicated by dashed lines. As depicted in FIG. 1, the core network 28 includes a mobile switching center (MSC) 30, and packet radio service nodes, such as serving GPRS support nodes (SGSNs) 32, 34, and a gateway GPRS support node 36. It will be appreciated of course that various names can be used for the devices depicted in FIG. 1, and for simplicity, the terminals 16, 18 will be commonly called UEs in this application.
Handover, which may also be called hand-off, is generally a process of maintaining on-going connections as UEs move with respect to the BTSs/Node Bs, and possibly vice versa. For example, as a UE moves from one cell to another, the UE's connection is handed over from Node B 26 to Node B 24. Early cellular systems used hard handovers (HHOs), in which a first BTS (covering the cell that the UE was leaving) would stop communicating with the UE just as a second BTS (covering the cell that the UE was entering) started communication. Modern cellular systems typically use diversity, or soft, handovers (SHOs), in which a UE is connected simultaneously to two or more BTSs/Node Bs. A control communication link between the BSCs/RNCs 12, 14 permits diversity communications to/from the UEs 16, 18 via the BTSs/Node Bs 20-26.
Packet-switched (PS) handover (HO) enables a network to control cell changes during packet-data transfers, e.g., GPRS sessions. It will be understood that the “network” here means generally the BSCs/RNCs 12, 14 and entities in the core network 28. One version of the PS HO procedure is specified in 3GPP Technical Specification (TS) 43.129 V6.8.0, Packet-switched handover for GERAN A/Gb mode, Stage 2 (Release 6) (June 2006).
During the PS HO procedure, the network might either reset certain transmission parameters used in the old cell to their default values, or alternatively the network might keep those parameters unchanged. This is indicated by an old eXchange IDdentifier (XID) indicator included in the non-access stratum (NAS) container for a PS HO information element part of a PS HO command message. If an old XID indicator is set, the Logical Link Control (LLC) layer and Layer-3 transmission parameter values that were applicable before receipt of the PS HO command message have to be kept; otherwise, all LLC-layer and Layer-3 transmission parameters have to be reset to default values. The transmission parameters define transmission attributes for Sub-Network Dependent Convergence Protocol (SNDCP) and LLC entities of a GPRS protocol stack. Such entities are typically included in a UE and an SGSN. The transmission parameters are negotiated by the network and the UE during an active session between the UE and an SGSN, and negotiations may be initiated by either side (i.e., the network or the UE) at any time. The purpose of the negotiations is to agree about the best SNDCP- and LLC-related parameters for packet-data transmission used by the UE and the network.
The SNDCP and LLC and their places in the GPRS protocol stack are described in, for example, 3GPP TS 44.065 V6.6.0 (June 2005), Technical Specification Group Core Network and Terminals; Mobile Station (MS)—Serving GPRS Support Node (SGSN); Subnetwork Dependent Convergence Protocol (SNDCP) (Release 6); and 3GPP TS 44.064 V6.1.0 (September 2005), Technical Specification Group Core Network; Mobile Station—Serving GPRS Support Node (MS-SGSN); Logical Link Control (LLC) layer specification; (Release 6); and other technical specifications. In general, the SNDCP and LLC specifications determine the nature of communications between UEs and packet radio service nodes, with the SNDCP functionality mapping network-level characteristics onto the characteristics of the underlying network and the LLC layer providing a reliable logical link. Logical link management functions involve maintenance of communication channels between UEs and the network across the radio interface, co-ordination of link state information between the UEs and network, and supervision of data transfer activity over the logical links between the UEs and the network.
In general, entities at Layer 3 select service access points, logical control channels, and the mode of operation of layer 2 (acknowledged or unacknowledged) as required for each individual message. Layer 3 is described in, for example, 3GPP TS 44.018 V7.9.0 (June 2007), Mobile Radio Interface Layer 3 Specification, Radio Resource Control (RRC) Specification (Release 7), and other technical specifications.
The current PS HO procedure restricts the UE's ability to negotiate the transmission parameters. If the network has indicated to the UE to reset LLC-layer and Layer 3 parameters to default values (i.e., the old XID indicator was not set in the PS HO command message), directly after performing a PS HO, the UE has to wait for an XID command or message sent by the network that either initiates the negotiation or confirms that the default values should be used. Being able to send an XID command to confirm the use of the default values of the transmission parameters is mandatory for a 3GPP-compliant network, but an XID command starting a negotiation of different values is not.
Such operation can cause trouble because the current PS HO procedure does not set a time limit for the UE to wait before the UE is allowed to start its own transmission-parameter negotiation after a PS HO. If the network does not send an XID message dealing with the transmission parameter values directly after the PS HO, the UE cannot start the negotiation itself. One consequence of this can be non-optimal conditions for packet-data transmission.
In addition, some networks never start a transmission-parameter negotiation by proposing values different from the defaults and let the UE take the initiative to start the negotiation. To comply with the current PS HO procedure, such networks would be forced to change their implementations or else face a deadlock: both the UE and the network wait for the other to start the negotiation.
Moreover, in a case of reset to default values after a PS HO, it is the responsibility of an SGSN to send an empty XID command message if it wishes to use the default parameters. Such operation can cause trouble in various situations, such as the loss of the empty XID command message on the radio link (i.e., the UE does not receive the message) and a failure to send the empty XID command message due to an erroneous (non-3GPP-compliant) SGSN implementation.
Furthermore, the default values specified by 3GPP TS 44.065 for the SNDCP and by 3GPP TS 44.064 for the LLC layer are simply the minimum requirements for the SNDCP and LLC layer to perform their functionalities. Better performance typically requires better values, which can be important for packet-based services requiring low latency, e.g., the conversational quality-of-service (QoS) class of services.