In LTE, the user entity can be in three states: DETACHED, ECM-IDLE and ECM-CONNECTED. In ECM-IDLE state the position of the user entity is known to be within a so called traffic area corresponding to a plurality of cells. In ECM-CONNECTED state the position of the user entity is known to be within a given cell of an eNodeB in a given traffic area.
FIG. 1 shows key nodes of the evolved packet core (EPC) system. There is shown a packet data network (PDN) gateway node PGW, a serving gateway node, SGW, a mobility management entity, MME, a base station, eNodeB, and a user entity UE. The S11 interface between the MME and the SGW is shown. Moreover, the S5/S8 interface between PGW and SGW, the S1-U interface between SGW and eNodeB; and the S1-MME interface between the MME and eNodeB are shown. Key functionalities in the system involves control plane tunnels GTP-C between the PGW, the SGW and the MME and user plane tunnels between the PGW, SGW and the eNodeB. Payload data is transmitted via the user plane tunnel for a given user entity and control information for the user entity is transmitted via the control plane tunnels. There is moreover a S4 interface leading to a S4 interface capable SGSN.
ECM states are described in TS 23.401, section 4.6.3, which specifies that the UE and the MME shall enter the ECM-CONNECTED state when the signalling connection is established between the UE and the MME. Initial NAS messages that initiate a transition from ECM-IDLE to ECM-CONNECTED state are Attach Request and Service Request.
When the UE is in ECM-IDLE state, the UE and the network may be unsynchronized, i.e. the UE and the network may have different sets of established EPS bearers. When the UE and the MME enter the ECM-CONNECTED state, the set of EPS Bearers is synchronized between the UE and network. For a UE in the ECM-CONNECTED state, there exists a signalling connection between the UE and the MME. The signalling connection is made up of two parts: an RRC connection and an S1-MME connection.
The UE shall enter the ECM-IDLE state when its signalling connection to the MME has been released or broken. This release or failure is explicitly indicated by the eNodeB to the UE or detected by the UE. The S1 release procedure changes the state at both UE and MME from ECM-CONNECTED to ECM-IDLE. The SGW is always aware of the ECM state.
In ECM-CONNECTED, there is:                One GTP-C tunnel (per UE) on S11        One GTP-C tunnel per PDN connection on S5/S8        One GTP-U tunnel per bearer on S5/S8        One GTP-U tunnel per bearer on S1-U        
In ECM-IDLE there is:                One GTP-C tunnel (per UE) on S11        One GTP-C tunnel per PDN connection on S5/S8        One GTP-U tunnel per bearer on S5/S8        No GTP-U tunnel(s) on S1-U. In FIG. 1, this tunnel is marked with dashed lines.        
3GPP TS 23.401 describes procedures for a UE to attach to the network.                Section 5.3.2.1 E-UTRAN Initial Attach        Section 5.3.2.2 UTRAN/GERAN Initial Attach        Section 5.5.1.2 S1-based handover (with SGW change)        Section 5.5.2 Inter RAT handover (several cases)        Section 5.10.2 UE requested PDN connectivity        
All these scenarios are based on the same message sequence initiated by the MME: The MME first sends a Create Session Request message 102 to the SGW, then a Modify Bearer Request 107, as shown in the message sequence in FIG. 2.
For the Initial Attach procedure, the Create Session Request message 102 and the subsequent create session response message 115 are used for setting up a control-plane tunnel GTP-C on S11/S4 between the MME and the SGW, a control plane tunnel GTP-C on S5/S8 between the SGW and the PGW, and one user plane tunnel GTP-U on S5/S8 (between the SGW and the PGW).
The Modify Bearer Request message 107 and the subsequent Modify Bearer Response message 123 are then used for putting the UE in state ECM-CONNECTED, whereby a user plane GTP-U tunnel is set up between the SGN and the eNodeB. The message contains the S1-U eNodeB F-TEID that the SGW needs to forward downlink data to the eNodeB.
3GPP TS 23.401 version 9.3.0 deals among others with the so-called attach procedure which is shown in section 5.3.2 and most notably in FIG. 5.3.2.1-1. Key elements of this procedure, namely the create session request message 102, create session response 115, the modify bearer request message 107, the modify bearer response 123 and the ECM connected state 110 are shown in FIG. 2 of the present document.
The standard says nothing about the ECM state in the time between Create Session Request and Modify Bearer Request. Moreover, the standard gives no information on how the network nodes should handle the case in which the Modify Bearer Request never reaches the SGW.
One problem is that if no modify bearer request is received from the mobility management entity, resources are bound in the packet gateway node unnecessarily
One way of dealing with this problem as envisioned by the inventors could be as follows: If no Modify Bearer Request is received, there will be a “hanging” PDN connection in SGW and PGW. Next time the same UE attempts to attach, the MME will (probably) use the same EPS Bearer ID for the default bearer. When the SGW/PGW receives a new Create Session Request it checks (by IMSI) if it already has a PDN connection for that UE. The SGW/PGW checks if the EPS Bearer ID in the Create Session Request is already in use. If it is already in use, the node deletes the existing PDN connection locally (without signalling to other nodes) and handles the new Create Session Request message as usual.
A technical problem with such a solution is that the serving gateway node may still be prone to take up resources unnecessarily for the user entity connection.