WO2002032166A1

WO2002032166A1 - Connection release in a two-layer communication network

Info

Publication number: WO2002032166A1
Application number: PCT/EP2000/009885
Authority: WO
Inventors: Tuija Hurtta
Original assignee: Nokia Corporation
Priority date: 2000-10-09
Filing date: 2000-10-09
Publication date: 2002-04-18
Also published as: EP1327364A1; EP1622401A1; DE60022913D1; EP1327364B1; CN1454435A; BR0017350A; DK1327364T4; CN1214668C; JP4406204B2; JP2004511982A; DK1327364T3; KR20030043992A; CA2423278A1; EP1327364B2; ATE305706T1; KR100719167B1; DE60022913T2; ATE535126T1; US7672312B1; ES2248137T5

Abstract

The present invention proposes a method for releasing at least a part of a connection in a two-layer communication network comprising a first communication network layer adapted to establish a communication channel to a terminal, and a second communication network layer adapted to establish an application layer connection, said method comprising the steps of: deciding in one of said first and second communication network layers that at least said part of said connection associated to said terminal is to be released, and forwarding said decision from said one of said communication network layers in which said decision has been taken, to said other of said communication network layers via an interface between said first and second communication network layers. Accordingly, each communication network layer has a knowledge of the necessity to release a connection and is enabled to initiate suitable measures in order not to waste network capacities for a connection call which is or can no longer be active. The present invention also relates to a correspondingly operated communication network.

Description

TITLE OF THE INVENTION

CONNECTION RELEASE IN A TWO-LAYER COMMUNICATION NETWORK

FIELD OF THE INVENTION

The present invention relates to a method for releasing a connection in a two-layer communication network.

BACKGROUND OF THE INVENTION

Throughout the subsequent specification, a two-layer communication network is intended to mean a communication network comprising a first communication network layer adapted to establish a communication channel to a terminal communicating in a call and transmitting said call, and a second communication network layer adapted to establish said call. An application layer as used in the present specification is intended to mean a set of security mechanisms, files, data and protocols (excluding transmission layer protocols) , a call is intended to mean a logical association between two or several users, while a communication channel is intended to mean a logical connection in the transmission layer between at least two end-points (e.g. between a mobile terminal and a support node) . Of course, the present invention is not limited to voice calls, but the expression call is intended to represent a subset of sessions.

A terminal may be any communication end-point which is adapted to send and/or receive data via a communication channel. In particular, a terminal can be a wireless terminal, or a terminal communicating via wires. Examples of terminals are radio terminals also referred to as a mobile station MS, or a user equipment UE (terminal conforming to UMTS standards also known as third generation (3G) terminal), or a server device.

Fig. 1 shows an example of a basic network architecture for such a two-layer communication network. The illustrated example is given with reference to a UMTS/GPRS network as a first communication network layer adapted to establish a communication channel to a terminal (UE) communicating in a call and transmitting said call, and an Internet Protocol based IP network as a second communication network layer adapted to establish a call (e.g. Internet Protocol Telephony IPT) . However, networks operated according to other standards may be chosen while the applicability of the present invention will not substantially be influenced thereby.

A user equipment UE as a terminal communicates with and/or via the communication network. This means that the user equipment UE may communicate with another user equipment (not shown) or with a server entity (not shown) within or external to the communication network. An external server entity could in turn be regarded as a terminal. The party initiating the call is referred to as the caller, while the called party is referred to as the callee. The communication network shown in Fig. 1 is a two-layer communication network consisting of a GPRS/UMTS network as an example for a first communication network layer, and of an Internet Protocol based (IP) network as an example for a second communication network layer. Between the first and second communication network layers at least one interface I/F is present. In the illustrated case, a network functional entity known as Policy Control Function (PCF) constitutes the interface. Nevertheless, the interface may be provided directly between the network layers, i.e. between respective nodes thereof. In such a case, and with reference to the illustrated example, the interface would be provided between the Gateway GPRS Support Node GGSN and the Call State Control Function CSCF (sometimes also referred to as proxy-CSCF) . The CSCF in turn is adapted to communicate with other functional entities and elements of the IP network such as for example a home subscriber server HSS, etc. (not shown).

Upon a call, the user equipment communicates via a Packet Data protocol (PDP) context. For the PDP context, a radio bearer (RB) is established between the user equipment UE and a Node_B of a radio access network RAN of the GPRS/UMTS network layer. (A Node_B corresponding to a base station in a GSM network. ) The Node_B is under control of a Radio Network Controller RNC (corresponding to a base station controller BSC in GSM) . Of course, a radio access network generally comprises a plurality of Node_B elements. For simplicity of the drawings, however, only one Node_B has been illustrated. An example for such a radio access network is UTRAN (UMTS Terrestrial RAN) .

The radio access network RAN as an example for the access network (a non-radio access network (i.e. wired/fixed access network is also conceivable) is connected to a core network CN. A core network denotes the part of the

GPRS/UMTS network which is independent of the connection technology of the terminal UE. In the illustrated example, the core network is a GPRS (General Packet Radio Service) core network. The core network comprises a Serving GPRS Support Node (SGSN) and a Gateway GPRS Support Node GGSN, which nodes are known as such and a detailed description thereof is therefore considered to be dispensable. It is assumed that the terminal and/or user equipment UE / MS communicating with/via the above briefly outlined communication network has subscribed to GPRS.

Such a GPRS subscription contains the subscription of one or more PDP addresses (Packet Data Protocol). Each PDP address is described by one or more PDP contexts in the terminal (UE and/or MS), the SGSN and the GGSN. Each PDP context may be associated with a TFT (Traffic Flow Template) . At most one PDP context associated with the same PDP address may exist at any time with no TFT assigned to it. Every PDP context exists independently in one of two PDP states (active / inactive) . The PDP state indicates whether data transfer is enabled for that PDP address (and TFT) or not. In case all PDP contexts associated with the same PDP address are deactivated, data transfer for that PDP address is disabled. All PDP contexts of a subscriber are associated with the same MM context (Mobility Management) for the IMSI (International Mobile Subscriber Identity) of that subscriber.

The inactive state characterizes the data service for a certain PDP address of the subscriber as not activated. The PDP context contains no routing or mapping information to process PDP PDUs (Protocol Data Units) related to said PDP address. No data can be transferred.

The terminal (UE and/or MS) initiates a transition from the inactive to the active state by initiating the PDP context activation procedure.

In the active state, the PDP context for the PDP address in use is activated in the terminal MS, the SGSN and GGSN. The PDP context contains mapping and routing information for transferring PDP PDUs for said particular PDP address between the terminal (MS) and the GGSN. The active PDP state is permitted only when the mobility management state of the subscriber is standby, ready, PMM-idle or PMM- connected. The Iu interface radio access bearer (RAB) may or may not be established for an active PDP context.

An active PDP context for a terminal (MS) is transferred to inactive state when the deactivation procedure is initiated.

At call release of a call of a terminal having subscribed to a GPRS network and communicating with/via the network, there occur situations in which the call is terminated, but the PDP contexts are still active. In such a case, data packets may still be tried to be transmitted to or from the PDP address associated to the still active PDP context.

As the charging of a service for a subscriber may be based on the amount of data transmitted to and from him, or on the time during which the PDP context is active, such a situation is undesirable for a subscriber in that he will be charged for data services he did not actually make use of and/or receive.

Also, as the GPRS network is only enabled to serve a limited number of subscribers, i.e. forward data to a limited number of addresses for which PDP contexts are in an active state, such a situation is undesirable in terms of a waste of communication network resources, as other subscribers could be blocked by active PDP contexts of subscribers no longer actually participating in a call.

SUMMARY OF THE INVENTION Consequently, it is an object of the present invention to provide a method for releasing a connection or part of said connection in a two-layer communication network which method prevents the occurrence of the above mentioned drawbacks.

According to the present invention, this object is achieved by a method for releasing at least a part of a connection in a two-layer communication network comprising a first communication network layer adapted to establish a communication channel to a terminal, and a second communication network layer adapted to establish an application layer connection, said method comprising the steps of: deciding in one of said first and second communication network layers that at least said part of said connection associated to said terminal is to be released, and forwarding said decision from said one of said communication network layers in which said decision has been taken, to said other of said communication network layers via an interface between said first and second communication network layers.

According to further developments of the present invention,

- said first communication network layer is a GPRS/UMTS system;

- said second communication network layer is an IP Multimedia Subsystem;

- said terminal is a mobile station (MS) or a user equipment (UE) ; - said interface between said first and said second communication network layers is constituted by a network functional entity;

- said network functional entity is a Policy Control Function (PCF) ; - said connection is a session; - the method further comprises the steps of releasing an established session to said terminal communicating in said session, modifying a communication channel used for said session; - said modification is a deactivation;

- said modification affects at least one parameter of a set of parameters defining said connection;

- said at least one parameter is the quality of service parameter „guaranteed bitrate,,, and the guaranteed bitrate is modified such that it is set to zero;

- said modification affects charging for said communication channel;

- said communication channel is represented by using a PDP context; - said connection is a session;

- the method further comprises the steps of releasing or modifying said communication channel, and modifying said session carried on said communication channel;

- said modification is a release; - said modification affects charging for said session;

- said communication channel is represented by using a PDP context;

- the method further comprises a step of requesting, by said terminal, said two-layer communication network to release said terminal from said session, and wherein said decision concerning the release is taken by said second communication network layer based on said release request from said terminal, and forwarded from said second to said first communication network layer; - the method further comprises a step of monitoring an inactivity state of said terminal by said first communication network layer, and wherein in case of detecting an inactivity state, said decision concerning the release is taken by said first communication network layer, and forwarded from said first to said second communication network layer;

- said inactivity state is represented by a break in a radio connection.

Also, the present invention concerns a communication system consisting of a terminal (UE) communicating via a connection with a two-layer communication network, the network comprising a first communication network layer adapted to establish a communication channel to said terminal (UE) communicating in said call, and a second communication network layer adapted to establish an application layer connection for said call, wherein said system is adapted to carry out the method steps for releasing the connection as set out above.

Thus, according to the present invention:

1) releasing a call in the application layer, indicating the call release to the transmission layer, performing specific functions in the transmission layer due to the indication (e.g., releasing the communication channel or including the indication to charging information. )

2) releasing a communication channel (i.e. PDP context) or part of the communication channel (i.e. RAB) in the transmission layer, indicating this to the application layer, releasing the call in the application layer, is enabled. (It should be kept in mind that a connection is intended to mean either a call or a PDP context or part of the PDP context (i.e. a RAB.))

Accordingly, the present invention provides the advantage that each of said communication network layers has a knowledge of the necessity to release a connection and is enabled to initiate suitable measures which are related to its network layer in order not to waste network capacities for a connection which is or can no longer be active.

Moreover, the terminal is relieved from the burden to take care of initiating the deactivation of the PDP contexts, as this duty has now been transferred to the network.

Further, due to the network initiating the PDP context deactivation it is ensured that the PDP context is not used for other purposes than the call.

It is thereby enabled that the network may release, e.g. an IP multimedia call irrespective of whether the terminal is in a position to do that, e.g. if the terminal is out of coverage and/or the transmission quality (quality of service) is below a predetermined threshold level.

As a call can be released by the network automatically after a predetermined period of time, the subscriber will not be charged for data packets routed in vain to his PDP address specified by a still active PDP context.

Still further, as a terminal might try to misuse the PDP context for other traffic than the call, by modifying / deactivating the PDP context according to the present invention, such a misuse is prevented or at least detected when the present invention is implemented.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further objects and advantages of the present invention will become more fully apparent upon referring to the accompanying description of preferred embodiments in conjunction with the attached drawings, in which: Fig. 1 shows a block circuit representation of a two-layer communication network with regard to an example of a GPRS/UMTS layer and IPT layer communication network;

Fig. 2 illustrates a signaling scenario of a first embodiment of the present invention; and

Fig. 3 illustrates a signaling scenario of a second embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will now be described in greater detail with reference to the drawings.

Figs. 2 and 3, respectively, illustrate a signaling scenario of signals transmitted between individual network parts and/or network elements. The numbering of the arrows representing the signal transmission direction is representative of the consecutive transmission of the signals and/or execution of processes over time.

The terminal (UE in Fig. 1) is represented as a mobile station MS, the radio access network RAN is represented by using UTRAN as an example radio access network, the GPRS network elements SGSN and GGSN are individually shown, the interface between the (GPRS/UMTS) first layer communication network and the (IPT) second layer communication network is represented by the PCF functional element (Policy Control Function) . The IPT layer (second layer) of the ^""two-layer communication network is represented by a proxy-CSCF as the network functional element thereof mainly involved in connection with the present invention (so that the other network elements of the IPT layer are omitted from the illustration) . [First Embodiment]

The signaling scenario in connection with a first embodiment of the present invention is shown in Fig. 2.

Fig. 2 illustrates a network initiated PDP context deactivation or change in the PDP context.

In brief, the terminal MS initiates the call release. The call release is informed to the proxy-CSCF, which sends a corresponding indication to the PCF. The PCF sends the indication, i.e. a decision, to the GGSN. The GGSN, in response thereto, changes a previous decision, i.e. it initiates a PDP context deactivation or PDP context modification or performs a change in the PDP context upon receipt of the indication from the PCF. The change in the PDP context may affect charging. Afterwards, the GGSN sends a request to the PCF to remove the state information related to the PDP context.

Thus, upon call release, an inter network layer communication is involved, which takes place via the interface between the communication network layers.

The individual steps of the involved signaling are now explained with reference to Fig. 2.

In step #1, the MS as the terminal sends a "Bye" message. The "Bye" message is sent to the proxy-CSCF, which forwards the message towards the callee (not shown) in order to inform the communication partner terminal of the intention of the MS to terminate the call.

In step #2, the proxy-CSCF sends a "Release" message to the PCF. The PCF (Policy Control Function) constitutes the interface between the two layers of the communication network. This function ensures that users do not violate promised traffic characteristics. Thus, the task of the Policy Control Function resides in the division of available network resources in a defined manner over a set of network users competing for those resources and thus serves to enforce a connection admission policy.

In step #3, the PCF may acknowledge the receipt of the release message by sending a "Release Ack" message to the proxy-CSCF.

In step #4, the PCF sends the "Decision" message (including a request ID) to the GGSN in order to change the previous decision, i.e. to indicate the call release. Stated in other words, as a PDP context (s) was (were) in the active state for the ongoing call (corresponding to a previous decision to set the PDP context in the active state) , the decision taken by the PCF (step #4) resides in deciding that the PDP context (s) concerned for the call are to be modified or set inactive (deactivated) .

In step #5, the GGSN may report to the PCF that it has successfully completed performing the decision, i.e. to translate the decision into an appropriate action, by sending the "Report State" message (including the request ID) to the PCF.

In subsequent steps #6 to #9, the GGSN initiated PDP context deactivation or modification may be performed. The former case is presented in Fig. 2. The PDP context deactivation and modification are known as such, so these steps are not described in further detail. As an alternative to the PDP context deactivation or modification, the GGSN may perform a change in the PDP context. The change may affect charging for the PDP context. The functions performed may be network operator specific.

In step #10, the GGSN thereafter sends a "Delete Request State" message (including the request ID) to the PCF in order remove the request state from the PCF.

Following step #10, the radio access bearer release procedure is performed in step #11.

Note that the messages transmitted in steps #4 (decision) , #5 (report state) , and #10 (delete request state) can be COPS messages. COPS is an IETF (Internet Engineering Task Force) specified protocol for a PEP (Policy Enforcement

Point) to request policy information from the PDP (Policy Decision Point) . However, it is not required that these messages are COPS messages.

It is also to be noted that the steps #2 to #11 are the same if the other communication end-point (not shown) initiates the call release.

As an alternative to the above mentioned sequence of signaling steps, it is also possible that the PCF may be adapted to remove the request state information already upon receipt of the call release indication from the proxy- CSCF. In that case, the GGSN would not have to send the "Delete Request State" message (step #10) , thereby leading to a reduced signaling amount.

[Embodiment 2]

According to the second embodiment, a call such as an IP multimedia call is released by the network if the terminal MS is impeded to do so. For example, in case the MS is out of the coverage area of the serving Node_B and no handover was successful.

Generally, the RAB is released upon a break in the radio connection or upon an inactivity state of an MS has been monitored and detected by said first communication network layer.

In brief, according to the second embodiment, the radio network controller RNC initiates the radio access bearer (RAB) release procedure. Then, the RAB is released. The SGSN modifies or deactivates (as a kind of modification) the concerned PDP context. The GGSN informs the PCF about the PDP context modification / deactivation. The PCF sends an indication to the proxy-CSCF to release or modify the concerned call. To release the call, the proxy-CSCF sends the message (e.g. a SIP Bye message (SIP= Session Initiation Protocol) ) to the mobile station MS as the terminal (which is optional) and to the other communication endpoint. The proxy-CSCF may receive acknowledgments for this message.

Thus, upon RAB release, an inter network layer communication is involved, which takes place via the interface between the communication network layers.

The individual steps of the involved signaling are now explained with reference to Fig. 3.

After a certain time of break in a radio connection or after a certain time of inactivity, the network is enabled to release or modify a call, e.g. if the terminal MS has left the coverage area. Such network initiated call release or modification is particularly important if charging for the call is based on time.

Fig. 3 illustrates a call release signaling scenario initiated by the network upon detection of an inactivity of the terminal MS.

In step #1, the radio network controller (RNC) detects and/or notices that the radio connection to the MS has broken or the terminal MS has been inactive. Upon detection of this situation, the RNC sends a "RAB Release Request" message to the SGSN.

In a following step #2, the radio access bearer release is performed.

Thereafter, in step #3, the SGSN modifies and/or deactivates the concerned PDP context for the subscriber by ■sending an "Update PDP Context Request" (in case of a modification, to be explained later) or a "Delete PDP

Context Request (in case of deactivation) to the GGSN. The latter case is represented in Fig. 3.

As shown in step #4, the GGSN sends a "Delete Request State" message (including the request ID) to the PCF to remove the request state from the PCF.

In response thereto, the PCF sends in step #5 the "Release" message to the proxy-CSCF to request a call release or modification.

As show in step #6, the proxy CSCF may acknowledge the receipt of the message by sending a "Release Ack" message to the PCF. In a step #7, the proxy-CSCF (optionally) may send a "Bye" message to the terminal now being e.g. out of coverage (as the terminal, if out of coverage, may not necessarily receive this message, this message may optionally be sent) , and may send the "Bye" message to the communication partner (not shown) of the illustrated terminal MS. As an alternative, the proxy-CSCF may modify the call (not shown). The modification may affect charging for the call.

During steps #8 to #10, the PDP context deactivation procedure (which is known as such) is performed.

In step #11, the terminal MS (optionally, e.g. if not fully out of coverage and some signaling is still possible) and the other endpoint (not shown) may acknowledge the call release by sending an appropriate acknowledgment message (e.g. "200 OK" message) .

In Fig. 3, the messages transmitted in step #4 (Delete Request State) may be a COPS message.

Both embodiments have been described in terms of deactivating a concerned PDP context for a call. However, deactivation of the PDP context represents a boundary case of a modification of a PDP context. Thus, instead of deactivation, a modification of the PDP context is also possible.

Such a PDP context modification is performed after RAB release (with reference to Fig. 3) and before RAB release (with reference to Fig. 2) . The PDP context as such is preserved in the course of a modification, but (as an example) the QoS parameter guaranteed bitrate is set to zero upon a modification as conceivable in connection with the present invention. In such a case, (in the second embodiment) the GGSN should send the "Request" message (including request ID and QoS negotiated as parameters) to the PCF. After receiving the request message with the (requested) guaranteed bitrate of zero, the PCF should send the "Release" message to the proxy-CSCF. In connection with the first embodiment (Fig. 2) , however, the PCF should, in case of modifying PDP contexts instead of deactivation, send the "Decision" message having such a content that GGSN modifies the concerned PDP context such that the guaranteed bitrate is set to zero.

Nevertheless, a deactivation of concerned PDP contexts as illustrated in Figs. 2 and 3, respectively, is optimum for the network resources. That is, since the PDP context is only used for the call, after releasing the call, the PDP context is no longer needed and could therefore be deactivated without causing any trouble.

Further, with regard to Fig. 2, the PDP context deactivation may happen immediately. Alternatively, the GGSN may set a timer when the "Decision" message is received and initiate a PDP context deactivation after the expiry of the timer.

Further, with regard to Fig. 3, call release may happen immediately. Alternatively, The CSCF may set a timer when the release message is received and initiate call release after the expiry of the timer.

In the foregoing, the inter network layer signaling has been described in that it takes place via the PCF as an interface network element. However, in both embodiments, the communication between the GGSN and proxy-CSCF may be effected without involving the PCF network element. The PCF may be located in the CSCF or in the GGSN. In this case, however, a new interface between GGSN and (proxy) CSCF would have to be defined.

Thus, either a call or a PDP context is released by a network layer, with the release being informed to the other network layer. A call release may affect the PDP context used to carry the call. A PDP context release may affect the call carried on the PDP context. The expression "connection" as used herein above was chosen to describe either a call or a PDP context (or even a part of the PDP context, i.e. a radio access bearer), while the term "communication channel" denotes a PDP context.

Accordingly, as has been described herein above, the present invention proposes a method for releasing at least a part of a connection in a two-layer communication network comprising a first communication network layer adapted to establish a communication channel to a terminal, and a second communication network layer adapted to establish an application layer connection, said method comprising the steps of: deciding in one of said first and second communication network layers that at least said part of said connection associated to said terminal is to be released, and forwarding said decision from said one of said communication network layers in which said decision has been taken, to said other of said communication network layers via an interface between said first and second communication network layers. Accordingly, each communication network layer has a knowledge of ^"-the necessity to release a connection and is enabled to initiate suitable measures in order not to waste network capacities for a connection call which is or can no longer be active. The present invention also relates to a correspondingly operated communication network. Although the present invention has been described herein above with reference to its preferred embodiments, it should be understood that numerous modifications may be made thereto without departing from the spirit and scope of the invention. It is intended that all such modifications fall within the scope of the appended claims.

Claims

1. A method for releasing at least a part of a connection in a two-layer communication network comprising a first communication network layer adapted to establish a communication channel to a terminal, and a second communication network layer adapted to establish an application layer connection, said method comprising the steps of: deciding in one of said first and second communication network layers that at least said part of said connection associated to said terminal is to be released, and forwarding said decision from said one of said communication network layers in which said decision has been taken, to said other of said communication network layers via an interface between said first and second communication network layers.

2. A method according to claim 1, wherein said first communication network layer is a GPRS/UMTS system.

3. A method according to claim 1, wherein said second communication network layer is an IP Multimedia Subsystem.

4. A method according to claim 1, wherein said terminal is a mobile station (MS) or a user equipment (UE) .

5. A method according to claim 1, wherein said interface between said first and said second communication network layers is constituted by a network functional entity.

6. A method according to claim 5, wherein said network functional entity is a Policy Control Function (PCF) .

7. A method according to claim 1, wherein said connection is a session.

8. A method according to claim 7, further comprising the steps of releasing an established session to said terminal communicating in said session, modifying a communication channel used for said session.

9. A method according to claim 8, wherein said modification is a deactivation.

10. A method according to claim 8, wherein said modification affects at least one parameter of a set of parameters defining said connection.

11. A method according to claim 10, wherein said at least one parameter is the quality of service parameter _^guaranteed bitrate,,, and the guaranteed bitrate is modified such that it is set to zero.

12. A method according to claim 8, wherein said modification affects charging for said communication channel .

13. A method according to claim 8, wherein said communication channel is represented by using a PDP context.

14. A method according to claim 1, wherein said connection is a session.

15. A method according to claim 14, further comprising the steps of releasing or modifying said communication channel, and modifying said session carried on said communication channel .

16. A method according to claim 15, wherein said modification is a release.

17. A method according to claim 15, wherein said modification affects charging for said session.

18. A method according to claim 15, wherein said communication channel is represented by using a PDP context .

19. A method according to claim 7, further comprising a step of requesting, by said terminal, said two-layer communication network to release said terminal from said session, and wherein said decision concerning the release is taken by said second communication network layer based on said release request from said terminal, and forwarded from said second to said first communication network layer.

20. A method according to claim 1, further comprising a step of monitoring an inactivity state of said terminal by said first communication network layer, and wherein in case of detecting an inactivity state, said decision concerning the release is taken by said first communication network layer, and forwarded from said first to said second communication network layer.

21. A method according to claim 20, wherein said inactivity state is represented by a break in a radio connection.

22. A communication system consisting of a terminal (UE) communicating via a connection with a two-layer communication network, the network comprising a first communication network layer adapted to establish a communication channel to said terminal (UE) communicating in said call, and a second communication network layer adapted to establish an application layer connection for said call, wherein said system is adapted to carry out the method steps for releasing the connection as set out in any of the claims 1 to 21.