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Title:
CONTROLLER AND METHOD FOR CONTROLLING THE SETUP OF CALLS FROM A FIRST TERMINAL TO A SECOND TERMINAL IN A COMMUNICATION SYSTEM AND COMPUTER PROGRAM MEDIUM
Document Type and Number:
WIPO Patent Application WO/2018/224143
Kind Code:
A1
Abstract:
The invention relates to a controller in a communication system in which a first control unit and a second control unit of the controller independently carry out a first call setup procedure and a second call setup procedure for setting up a call between a first and a second terminal. Since the second call setup procedure running independently from the first call setup procedure may result in unnecessary and undesired alerting (Ghost or Phantom ringing) at the second terminal, the invention provides for sending a cancel message to the second terminal to stop the continuously running second call setup procedure under certain conditions. Such conditions can be the failure of the first call setup procedure, or the receiving of a cancel message actively sent by the first terminal. The invention is particularly useful in TCP based IMS systems.

Inventors:
PATIL SIDDANAGOWDA (IN)
Application Number:
PCT/EP2017/063849
Publication Date:
December 13, 2018
Filing Date:
June 07, 2017
Export Citation:
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Assignee:
NOKIA SOLUTIONS & NETWORKS OY (FI)
International Classes:
H04L29/06
Foreign References:
US20030231753A12003-12-18
US20170150468A12017-05-25
US20160029228A12016-01-28
Other References:
None
Download PDF:
Claims:
CLAIMS

1. A controller for controlling a setup of a call from a first terminal to a second terminal in a communication system, the controller comprising:

a first control unit including a first call setup unit adapted to carry out a first call setup procedure to setup the call between the first terminal and the first control unit in response to receiving a call-setup request message from the first terminal; and

a second control unit including a second call setup unit adapted to carry out a second call setup procedure to set up the call between the second control unit and the second terminal in response to said first call setup unit receiving the call setup request message from the first terminal, the second control unit carrying out the second call setup procedure independently from the first control unit carrying out the first call setup procedure; wherein

the second control unit further comprises a call setup procedure cancellation unit adapted to send a control message to the second call setup unit to cancel the second call setup procedure carried out by the second call setup unit.

2. The control according to claim 1, wherein the second call setup unit, in response to receiving the control message, sends a cancel message to the second terminal.

3. The controller according to claim 1, wherein the call setup cancellation unit sends the control message in response to said first call setup unit receiving a cancel message from said first terminal .

4. The controller according to claim 1, wherein the first control unit comprises a first timer unit and a first resource allocation unit adapted to allocate resources in said first control unit during a time interval provided by the first timer unit .

5. The controller according to claim 1, wherein the second control unit comprises a second timer unit and a second resource allocation unit adapted to provide resources in said second control unit during a time interval provided by said second timer unit.

6. The controller according to claim 5, wherein the second timer unit provides a first time interval during which the second resource allocation unit allocates first transmission resources and a second time interval started at the expiry of the first time interval during which the second resource unit allocates second transmission resources.

7. The controller according to claim 6 and 2, wherein the second call setup unit sends the cancel message to the second terminal at the expiry of the first time internal or the second timer interval .

8. The controller according to claim 7, wherein the communication system is a TCP based communication system, wherein the second call setup unit sends the cancel message to the second terminal at the expiry of the time interval without 18x response from the second terminal. 9. A method for controlling the setup of a call from a first terminal to a second terminal in a communication system, the method, comprising: sending a call setup message from the first terminal to a controller of the communication system;

carrying out, by a first call setup unit of a first control unit of the controller, a first call setup procedure to setup the call between the first terminal and the first control unit; carrying out, by a second call setup unit of a second control unit of the controller, a second call setup procedure to set up the call between the second control unit and the second terminal in response to said first call setup unit receiving the call setup request message from the first terminal, the second call setup procedure being carried out independently from the first call setup procedure; and

sending, by a call setup cancellation unit of the second control unit, a control message to the second call setup unit to cancel the second call setup procedure carried out by the second call setup unit .

10. The method according to claim 9, further comprising the second call setup unit, in response to receiving the control message, sending a cancel message to the second terminal.

11. The method according to claim 9, further comprising the step of sending a cancel message from the first terminal to the controller, wherein the control message is sent in response to receiving the cancel message from the first terminal.

12. The method according to claim 9, further comprising allocating, by a first resource allocation unit in the first control unit, resources in the first control unit during a time interval provided by a first timer unit of the first control unit .

13. The method according to claim 9, further comprising providing, by a second resource allocation unit in the second control unit, resources in the second control unit during a time interval provided by a second timer unit of the first control unit .

14. The method according to claim 13, further comprising allocating first resources by the second resource allocation during a first time interval provided by said second timer unit, and allocating second resources by the second resource allocation during a second time interval started by the second timer unit at the expiry of the first time interval.

15. The method according to claim 10 and 14, wherein the cancel message is sent to the second terminal at the expiry of the first time internal or at the expiry of the second time internal.

16. The method according to claim 16, wherein the communication system is a TCP based communication system, comprising the step of sending the cancel message to the second terminal at the expiry of the time interval without 18x response from the second terminal .

17. A computer program medium comprising instructions to carry out the steps of one or more of the method claims 10 to 16.

Description:
CONTROLLER AND METHOD FOR CONTROLLING THE SETUP OF CALLS FROM A FIRST TERMINAL TO A SECOND TERMINAL IN A COMMUNICATION SYSTEM AND COMPUTER PROGRAM MEDIUM

FIELD OF THE INVENTION

The present invention generally relates to setting up of calls between a first terminal and a second terminal in a

communication system. The call setup is centrally controlled by a controller to handle the call setup from the first terminal to the second terminal in response to receiving a call set-up request message from the first terminal. In particular, the present invention relates to a controller and a method for setting up calls between IMS (IP Multimedia Subsystem) users connected over a TCP transport system.

Moreover, the present invention relates to a computer program medium comprising instructions to carry out the suggested method according to at least one embodiment of the present invention .

BACKGROUND OF THE INVENTION As illustrated in FIG. 1, in a communication system 3, a first terminal 1 and a second terminal 2 are connected to a central controller 10 which sets up calls initiated by the first terminal 1 to the second terminal 2. In particular, as also shown in FIG. 1, the controller 10 comprises a first control unit 11-1 with a first call setup unit 11-11 which sets up the call between the first terminal 1 and the first control unit 11-1 by running a predetermined first call-setup procedure. This is for example done in response to the first terminal 1 sending a call setup request message to the central controller 10.

The first control unit 11-1 forwards the received call set up request to the second control unit 11-2 and the second control unit 11-2 starts a call between the second control unit and the second terminal 2. That is, the second control unit 11-2 starts a second call setup procedure between the second control unit 11-2 and the second terminal 2 by a second call setup request unit 11-22 to set up the call. As indicated with the bi-directional arrows, in the first and second setup procedure, many messages are exchanged until the call is setup, in particular request, acknowledgement and timing (time out) messages as explained below with more details.

One example of such a call setup from an originating first terminal 1 to a second terminal 2 is in an IP multimedia subsystem (IMS) environment where the call setup procedures follow the transmission of request (INVITE) and acknowledge messages (ACK) , as will be described below with particular reference to an IMS environment using TCP messages.

It may be noted that in many communication systems the call setup unit 11-11 runs a first call setup procedure (including the providing of call setup messages and acknowledgement messages) independently from the second setup unit 11-22 running the second call setup procedure. The fact that they run independently means that once they have been started they will continue to be executed even if no communication takes place between the first control unit 11-1 and the second control unit 11-2 through an interface unit IU shown in FIG. 1. For example, when the first control unit 11-1 decides, due to a missing acknowledgement from the second terminal side, to cancel the first call setup procedure, the second call setup procedure (including re-transmissions of second call setup request messages between the second control unit 11-2 and the second terminal 2) will continue even though the first terminal 1 has already hung up.

However, this unnecessary re-transmission of second call setup procedures in the absence of any acknowledgement from the second terminal 2 can cause false alert or false ringing on the terminating second terminal 2. For example, during the call setup originating from the first terminal, the second terminal may still be busy with another call from a third terminal 3, such that the second call setup procedure

continues to send call setup messages, for example, INVITE messages in TCP, to the second terminal 2 and the second terminal 2 did not acknowledge the INVITE message received of failed to respond which will cause a false ringing or alert when the second terminal 2 terminates its call with the third terminal 3.

FIG. 2 shows an example of a call flow diagram describing this situation of false alert within the framework of an IP

(Internet Protocol) multimedia subsystem using TCP. FIG. 2 is in particular a flow diagram describing a scenario where the endpoint B starts ringing even after the IMS terminating P-

CSCF cleared the call leg towards the A-side. As is well known to the skilled person, in such a system, a controller might be formed by the combination of the IMS core/ P-CSCF whilst the combination of the pgw, sgw and eNode units essentially constitute the packet routing network. The controller may therefore be referred to as a routing engine and the packet routing network may be referred to a packet core. As is also well known to the skilled person, within the framework of a TCP communication system, the controller and the terminals not only use re-transmission sequences of call setup request messages and acknowledgement messages, but they also use various timers (they are called Timer A and Timer B) in connection with the transactions on the server side and the client side. The timers are used for timing various time-outs of signaling between the first terminal, the controller, the packet core and the second terminal. As indicated in FIG. 2, when in step (at timing) A the Timer B expires and a 408 time-out message is received from the SIP stack, this leads to a clearance of transmission resources on the application layer of the first control unit (IMS core/P- CSCF and to the termination of the call setup between the first terminal UE-A and the controller whilst the second call setup procedure, i.e. the re-transmission of TCP INVITE messages, between the controller and the second terminal UE-B continues because no acknowledgement is received due to the second terminal UE-B being busy with a call from a third terminal (indicated with the "CS Call-in-Progress" at step B in FIG. 2) or delay at intermediate nodes or other issues at the second terminal UE-B (e.g. a malfunction or similar) .

As indicated with the step B in FIG. 2, after the ongoing call of the second terminal with the third terminal call is terminated (or due to other reasons as explained below) as indicated with "CS Call got over", the independently running second call setup procedure (the re-transmissions) will continuously send the TCP INVITE messages and eventually the second terminal UE-B will enter a ringing state ("UE Starts Ringing") . That is, with the Timer B expired and no client side transaction present at the P-CSCF, the second terminal UE-B starts ringing and then stops ringing ("US Stops Ringing") when the user answers the call. This is a particular situation in which the user of the second terminal UE-B hears or notices a ringing phenomenon although in the first terminal UE-A the first call setup procedure has already been cancelled due to the 408 timeout signal, that is the first terminal UE-A does remain in the call setup anymore.

With more detail, in FIG. 2, the second terminal UE-B goes into a ringing state much later after the originating first terminal UE-A or any intermediate NE in the IMS environment has terminated the call during the call setup phase and an indication that the call has terminated has not reached the terminating second terminal UE-B. For the GM interface over TCP, the terminating P-CSCF on reception of the SIP INVITE triggers a TCP connection

establishment procedure if there is no connection or forward SIP INVITE over an already existing TCP connection. The SIP Timer B is also started when the SIP INVITE is received and it is set to typically 64*Tl (Tl = 0.5 seconds as prescribed by rfc-3261) = 32 seconds.

In the absence of a TCP ACK (acknowledgement) for the SIP INVITE TCP message due to various reasons (listed below) , the TCP stack at the P-CSCF will keep re-transmitting the buffered SIP INVITE message even after the Timer B expired and the client-side transaction has been removed from the P-CSCF.

Therefore, in FIG. 2, the TCP INVITE re-transmission will continue until the TCP re-transmitting timer expires. In the absence of any response for SIP INVITE from the second terminating terminal UE-B, the originating first terminal UE-A or any intermediate NE shall trigger SIP CANCEL. However, an indication to terminate the session cannot reach the second terminating terminal UE-B due to one of the reasons listed below and SIP CANCEL will be buffered and dropped on expiry of the SIP Timer B by the P-CSCF if a lxx response is not received. The false alerting (also called Ghost or Phantom ringing) could have been caused because SIP INVITE did not reach the terminating second UE-B at the end of SIP Timer B expiry and the P-CSCF received the 18x response after the expiry of SIP Timer B. Since the SIP CANCEL was dropped there is no way for the terminating second terminal UE-B to know that the call has been cancelled and consequently it enters a false alerting/ringing state. This issue of Ghost or Phantom ringing is commonly found in the IMS field and reported by many operators, however, it is not limited to IMS. As

explained with reference to FIG. 1, it may be commonly significant whenever two call setup procedures operate independently in any communication system. In IMS the false ringing may in particular occur in the second terminal UE-B which is connected to P-CSCF over TCP transport in the following situations: a) Delay in LTE (Long Term Evolution) paging and causing SIP INVITE to reach the second terminating terminal UE-B after Timer B expired at P-CSCF; and/or b) Dual SIM Dual Standby (DSDS) handsets (mobile terminals) where only one SIM shall have data connectivity at any time.

Apart from the fact that the ringing is unnecessary and undesirable for the user of the second terminating terminal, the continuation of the second call setup procedure (the re ¬ transmissions of SIP INVITE messages) independently from the possibly already cancelled first call setup procedure

unnecessarily occupies transmission resources in the kept open socket between the controller and the terminating second terminal .

SUMMARY OF THE INVENTION

Therefore, the present invention aims at providing a

controller for a communication system, a method and a

communication system for controlling the setup of calls between a first terminal and a second terminal which can avoid Ghost or Phantom ringing at the second terminal when a call is setup between the originating first terminal and the

terminating second terminal.

The aim of the invention is also to overcome the false alerting in IP Multimedia Subsystem (IMS) when UEs are connected over TCP transport .

In one aspect of the invention, a controller in accordance with the invention for controlling the setup of a call from a first terminal to a second terminal in a communication system, the controller comprises a first control unit including a first call setup unit adapted to carry out a first call setup procedure to setup the call between the first terminal and the first control unit in response to receiving a call-setup request message from the first terminal, and a second control unit including a second call setup unit adapted to carry out a second call setup procedure to set up the call between the second control unit and the second terminal in response to said first call setup unit receiving the call setup request message from the first terminal, the second control unit carrying out the second call setup procedure independently from the first control unit carrying out the first call setup procedure wherein the second control unit further comprises a call setup procedure cancellation unit adapted to send a control message to the second call setup unit to cancel the second call setup procedure carried out by the second call setup unit. In another aspect, a method in accordance with the invention for controlling the setup of a call from a first terminal to a second terminal in a communication system comprises the sending a call setup message from the first terminal to a controller of the communication system, carrying out, by a first call setup unit of a first control unit of the controller, a first call setup procedure to setup the call between the first terminal and the first control unit, carrying out, by a second call setup unit of a second control unit of the controller, a second call setup procedure to set up the call between the second control unit and the second terminal in response to said first call setup unit receiving the call setup request message from the first terminal, the second call setup procedure being carried out independently from the first call setup procedure, and sending, by a call setup cancellation unit of the second control unit, a control message to the second call setup unit to cancel the second call setup procedure carried out by the second call setup unit.

Hereinafter, first some general aspects of the invention shall be summarized for convenience.

As will be appreciated, one of the aspects of the present invention lies in the realization that the re-transmission process in the second call setup procedure is unnecessary under the conditions that the transaction with the first originating terminal has already been terminated or due to other reasons, for example an intentional cancellation by the first originating terminal. According to the present

invention, in the advent of the first call setup procedure being stopped or an intentional cancel message being sent by the first originating terminal, the second call setup

procedure will be stopped by sending a control message to the second call setup unit of the second control unit . Eventually, this sending of the control message will result in the cancellation of the second call setup procedure by sending a cancel message (CANCEL) to the second terminating terminal.

Another aspect of the present invention is that for allowing the sending of the cancel message to the second terminating terminal, a timer unit which times the assignment of resources to the application layer in the second control unit extends its timer value from a first predetermined time interval to a second longer time internal such that the cancel control message can actually be sent from the second control unit to the second terminal.

That is, for allowing the sending of the CANCEL message to the second terminating terminal, the application layer in the second control unit extends its resources allocated for this call for a certain configurable or constant time after an initial time period by the timer unit (in TCP: Timer B) expires so as to gracefully cleanup the session with the second terminal without false alerting.

In this manner, the "independency" of the second call setup procedure is stopped - it becomes controllable by the second control unit being able to influence (to stop) the second call setup procedure even if the first terminal side transaction has already been cleared.

In IMS systems using TCP transport, such a timer which is being extended over its original defined value could be an extension of the Timer B such that during an extended Timer B period transmission resources for the application layer of the second control unit are available allowing the cleaning up of the signaling on the second terminal side in a controlled manner.

Essentially, by extending the Timer B and by allocating resources to the application layer, the client side

transaction at the terminating P-CSCF is kept for a certain additional time (which can be a configurable parameter) . In particular, in IMS systems using TCP transport, the

terminating P-CSCF can send a SIP CANCEL message when it receives an 18X response from the second terminating terminal or on the Timer B expiry. When the SIP CANCEL message is delivered to the terminating second terminal UE-B as soon as it receives the SIP INVITE, it will stop ringing.

The present application contains various embodiments under which conditions and in which way a cancellation of the ongoing second call setup procedure is initiated. Essentially, there are three different scenarios which will finally stop the execution of the second call setup procedure.

Therefore, according to another aspect of the controller it is further advantageous that the second call setup unit, in response to receiving the control message, sends a cancel message to the second terminal.

According to yet another aspect of the controller, it is advantageous that the call setup cancellation unit sends the control message in response to said first call setup unit receiving a cancel message from said first terminal. According to yet another aspect of the controller, it is advantageous that the first control unit comprises a first timer unit and a first resource allocation unit adapted to allocate resources in said first control unit during a time interval provided by the first timer unit.

According to yet another aspect of the controller, it is advantageous that the second control unit comprises a second timer unit and a second resource allocation unit adapted to provide resources in said second control unit during a time interval provided by said second timer unit .

According to yet another aspect of the controller, it is advantageous that the second timer unit provides a first time interval during which the second resource allocation unit allocates first transmission resources and a second time interval started at the expiry of the first time interval during which the second resource unit allocates second transmission resources .

According to yet another aspect of the controller, it is advantageous that the second call setup unit sends the cancel message to the second terminal at the expiry of the first time internal or the second timer interval.

According to yet another aspect of the controller, it is advantageous that the communication system is a TCP based communication system, wherein the second call setup unit sends the cancel message to the second terminal at the expiry of the time interval without 18x response from the second terminal. According to another aspect the method further comprises the second call setup unit, in response to receiving the control message, sending a cancel message to the second terminal. According to another aspect the method further comprises the step of sending a cancel message from the first terminal to the controller, wherein the control message is sent in response to receiving the cancel message from the first terminal. According to another aspect the method further comprises allocating, by a first resource allocation unit in the first control unit, resources in the first control unit during a time interval provided by a first timer unit of the first control unit .

According to another aspect the method further comprises providing, by a second resource allocation unit in the second control unit, resources in the second control unit during a time interval provided by a second timer unit of the first control unit.

According to another aspect the method further comprises allocating first resources by the second resource allocation during a first time interval provided by said second timer unit, and allocating second resources by the second resource allocation during a second time interval started by the second timer unit at the expiry of the first time interval.

According to another aspect of the method the cancel message is sent to the second terminal at the expiry of the first time internal or at the expiry of the second time internal. According to another aspect of the method the communication system is a TCP based communication system and the method comprises the step of sending the cancel message to the second terminal at the expiry of the time interval without 18x response from the second terminal.

According to another aspect there is provided a computer program medium comprising instructions to carry out the above listed method steps .

Hereinafter, the invention will be described with reference to its advantageous embodiments with reference to the drawings. These drawings, where like reference numerals refer to

identical or functionally similar elements throughout the separate views, together with the detailed description below, are incorporated in and form part of the specification, and serve to further illustrate embodiments of concepts that include the claimed invention, and explain various principles and advantages of those embodiments

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings :

FIG. 1 shows a principal overview of the communication system considered by the present invention including a controller and a first and second terminal for setting up a call;

FIG. 2 is a call flow diagram in a prior art communication system relating to an IMS system using TCP, describing a situation in which the terminating terminal UE-B due to the expiration of a Timer B clearing the transaction to the second terminating terminal UE-B causes the second terminal UE-B to enter a ringing state during successive TCP INVITE retransmissions in the second call setup procedure;

FIG. 3 is a principal block diagram of a communication system with a controller using an application layer AL and a network layer NL called a routing engine, and a packet core also containing an application layer AL and a network layer NL communicating with the routing engine;

FIG. 4-1 is a block diagram similar to the one in FIG. 1, however including a controller of the invention and

demonstrating the cancellation of the second call setup procedure in the communication system of FIG. 1 with a control message CM and a cancellation message CNCL;

FIG. 4-2 is a principle flow chart relating to FIG. 4-2 and showing the steps of sending the control message CM and the cancellation message CNCL for cancelling the second call setup procedure with the second terminating terminal;

FIG. 5-1 a general block diagram in accordance with the first embodiment of the invention in which the second call setup procedure in the case of an IMS TCP system is cancelled by extending the Timer B value to keep call setup resources in the application layer of the second call control unit for an extended time to allow a graceful cleanup of the second call setup procedure;

FIG. 5-2 is a messaging flowchart which illustrates the extension of the Timer B of step C and the cancellation of the second call setup procedure in step D of FIG. 5-1 with more details of the TCP messages; Fig. 6 shows a messaging flowchart for the case of an IMS TCP system in which the cancellation of the second call setup procedure in step D is initiated by a CANCEL message in step C originated by the first originating terminal UE-A during the first call setup procedure;

FIG. 7 a signalling flowchart similar to the one in FIG. 6 also initiating a cancellation message CANCEL from the first originating terminal, however, with a different timing than in FIG. 6;

FIG. 8 a signalling flowchart similar to the ones in FIG. 5-7 in which the Timer B is not extended in step C ' but the cancellation message is sent shortly before the Timer B expires; and

FIG. 9 a detailed flow diagram of the best mode of the

invention showing a terminating P-CSCF with an application layer and a network layer on the first control unit (client side transaction) and the second control unit (the server side transaction) , with the indication of the blocks respectively shown in FIG. 4-1.

Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and locations of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. The arrangement and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the invention will be described with reference to its detailed embodiments with reference to the drawings.

However, first of all, some general overview of the invention is given with reference to FIG. 4-1. The controller 10 in FIG. 4-1, viewed in a layer OSI sense, operates in an application layer AL and a network layer NL as shown in FIG. 3. Since also the routing network RN operates on an application level and a network level, it should be understood that also in FIG. 4-1 the controller 10 is structured in layers to communicate with the application layer/network layer of the packet core.

Furthermore, it is to be understood that also the originating terminal UE-A and the terminating terminal UE-B are layer structured such that the respective layers cooperate through an interface unit as principally indicated with the

bidirectional arrows in FIG. 3. However, hereinafter, the more detailed block diagram of FIG. 4-1 according to the principle of the invention is described. The layered operation will be described with more details below regarding the best mode of the invention in FIG. 9.

As is shown in FIG. 4-1, on a general level, the present invention comprises a controller 10 for controlling the setup of a call from a first terminal 1 to the second terminal 2 in the communication system. The controller 10 can comprise a first control unit 11-1, for example, formed by the client side transaction in FIG. 9, including a first call setup unit 11-11 adapted to carry out a first call setup procedure to set up a call between the first terminal 1 and the first control unit 11-1 when a call setup request message CSM is received from the first (originating) terminal 1. The controller 10 in FIG. 4-1 also comprises a second control unit 11 2 including a second call setup unit 11-22 adapted to carry out a second call setup procedure to set up the call between the second control unit 11-2 and the second terminal 2 in response to said first call setup unit 11-1 receiving the call setup request message CSM from the first terminal 1. As already explained with reference to FIG. 1 and FIG. 2, the second control unit 11-2 carries out the second call setup procedure independently from the first control unit 11-1 carrying out the first call setup procedure. The term

"independently" may be understood, as explained above, by the specific examples of message flows in the IMS TCP system in FIG. 2 and also in the flowcharts of FIG. 5-8 described below. That is, once the call setup message CSM has been handed over from the first control unit 11-1 to the second control unit 11-2, the second call setup procedure will continue re ¬ transmissions of the call setup message or messages based on this call setup message CSM to set up the call essentially independently to what happens on the client side transaction. For example, as explained, even if there is a timeout for the first call setup procedure or an active cancellation message from the first originating terminal, this has no influence on the still ongoing re-transmissions in the second call setup procedure to set up the call to the second terminal 2. This may, however, cause the situation of a ghost or phantom ringing on the side of the terminating terminal 2.

In accordance with the present invention and as shown in FIG. 4-1 (and FIG. 4-2), the second control unit 11-2 further comprises a call setup cancellation unit 11-25 which is adapted to send a control message CM (which can also be regarded as a cancellation message to indicate the stoppage of the second call setup procedure) to the second call setup unit 11-22 to cancel the second call setup procedure carried out by the second call setup unit 11-22. That is, the second control unit 11-22 comprises an own cancellation unit 11-25 which controls the second call setup unit 11-22 to cancel the call setup procedure by sending a corresponding cancel message CNCL to the second terminal 2. Of course, the first control unit 11-1 and the second control unit 11-2 respectively contain resource allocation units 11-14, 11-24 for respectively allocating resources for the client side transaction and the server-side transaction controlled by respective timer units 11-13 and 11-23 which provide respective time intervals to the resource allocation units 11-14 and 11-24 during which resources are allocated in the first and second control units 11-1, 11-2. That is, as well known to the skilled person in the technology of IMS systems using TCP, the control units 11- 1, 11-2 each comprise a timer unit 11-13, 11-23, which are denoted as the Timer B in IMS technology. Between the first control unit 11-1 and the second control unit 11-2 there is provided an interface unit IU which is used for coupling the messages therebetween. In particular, considering the layer model shown in FIG. 3, the interface unit IU transfers messages between the application layers of the first and second network units 11-1, 11-2 as is also shown in the best mode of the invention in FIG. 9. The cancellation procedure is also illustrated in the

flowchart of FIG. 4-2 relating to the block diagram in FIG. 4- 1. That is, a first call setup procedure is carried out with steps SI, S2 between the first terminal UE-A and the first control unit 11-1. For example, the step SI might be a call setup (INVITE) message and the step S2 might be an

acknowledgement message AK or failure message as will be seen with more concrete details in FIG. 5-2. In response to the call setup message CSM having been rooted to the second control unit 11-2, also a second call setup procedure is independently carried out between the second control unit 11-2 and the second terminal UE-B. Steps S3 and S4 may also comprise INVITE and ACK messages. As shown with the message CM, the second control unit 11-2 and in particular the call setup cancellation unit 11-25 causes the stoppage of the second call setup procedure under specific conditions.

Therefore, the sending of the control message CM (which is in fact a cancellation message) , in the method in accordance with the invention stops the second call setup procedure to the second terminal to avoid the ghost or phantom ringing on the second terminal side UE-B because the sending of the control message to the second call setup unit 11-22 causes the sending of the cancel message CNCL to the second terminal UE-B. It may be noted that in the figures the first, second and third terminal 1, 2, 3 are equally denoted with reference numerals UE-A, UE-B and UC-C in most flow charts.

Whilst FIG. 4-1 and Fig. 4-2 describe generally that the second call setup procedure is stopped by the second control unit 11-22 and that this can be done, in accordance with one embodiment, by sending the CANCEL message to the second terminal UE-B after sending the control message CM to the second call setup unit 11-22, it may be noted that the CANCEL message can also be directly sent from the second control unit 11-2 without going through the second call setup control unit 11-22. This is in particular the case when a cancel message is sent from the first terminal UE-A or from other upstream network elements which cancel message is then routed from the first control unit 11-1 to the second control unit 11-2 and from the second control unit 11-2 to the second terminal UE-B. Thus, whenever the second call setup procedure is to be stopped, this can be done directly by sending the cancel message to the second terminal UE-B and/or by controlling first the second call setup unit 11-22, e.g. by cancelling stopping the re-transmissions in the second call setup unit in the second control unit 11-2 or by sending the cancel message to the second terminal UE-B from the second call setup unit 11-22. Thus, the principle idea of the present invention to stop the second call setup procedure can be achieved in various different manners, as will be seen below in the specific embodiments and examples.

Extension of the Timer B

FIG. 5-1 illustrates more details of the flowchart in FIG. 4-2 for the case in which the sending of the cancel message (CNCL in FIG. 4-1 and 4-2 and TCP (cancel) in FIG. 5-1) is made possible by an extension of the Timer B contained in the timer unit 11-23 of the second control unit 11-2 in FIG. 4-1. As understood with reference to FIG. 3, the controller 10 is divided in a first control unit 11-1, formed as a routing engine, and a second control unit 11-2 coupled to by the packet core which contains the switching network for routing packets/messages. Each of them has, as explained, a structure of an application layer and the network layer. Also in FIG. 5-1 a first call setup procedure is run between the first terminal UE-A and the first control unit and a second call setup procedure is initiated call setup messages. In step CO the initial call setup in the first call setup procedure fails, for example because the second terminal UE-B is still busy (call in progress) with another call from the third terminal UE-C and does not send an acknowledgement to the call setup message sent from the second control unit, and thereafter in step C the call setup timer (Timer B) in the first timer unit 11-13 of the first control unit 11-1 and the second timer unit 11-23 of the second control unit 11-2 expires independently. However, as shown in FIG. 5-1, when step C occurs, the second call setup procedure continues running by executing the TCP re-transmissions. These re-transmission, as explained, are the result of the second terminal being unable to acknowledge the TCP INVITE messages, e.g. because of the second terminal UE-B conducting a call (call in-progress) with a certain third terminal UE-C (because of delays in LTE paging or for other reasons as explained above) .

In FIG. 5-1, a first embodiment of the invention is shown in which the initial timer value of for example 32 seconds is extended to extend the call setup at the routing engine for a configurable (extended) time period. As shown in FIG. 5-1, with the extension of the timer period in step C ' ' , the existence of the call setup resources are prolonged or extended at the routing engine for an extended period. The resources concerned are in particular those at the application layer of second control unit 11-2 of the routing engine, such that a cancel call setup message TCP (cancel) is initiated in step D immediately after the alerting signal is transmitted from the second terminal UE-B. The TCP cancel message is followed by the transmission of an acknowledgement message indicating the cancel success. As will be understood from FIG. 5-1, due to the fact that the Timer B value is extended over the original value, it is possible that a cancel message is sent to the second terminal to cancel the second call setup procedure. As will be understood with more details below from the specific embodiments in FIG. 5-8, the sending of the TCP cancel message in step D is possible because due to the extension of the timer period resources are available on the application layer such that in response to the control message CM a cancel message can be sent from the application layer of the second control unit 11-2 to the second terminating terminal UE-B. Since the resources between the second control unit 11-2 and the second terminal UE-B are still existing (there is still an open socket used for the TCP-transmissions) the cancel message which is sent from the application layer of the second control unit 11-2 can be inserted in the existing message flow (open socket) . For this purpose, the second timer unit 11-23 provides not only a first time interval ("original period in FIG. 5-1) during which the second resource

allocation unit 11-24 allocates first transmission resources but also a second time interval (extended time in FIG. 5-1) started at the expiry of the first time interval during which the second resource unit 11-24 allocates second transmission resources .

TCP IMS Signaling with Extension of Timer B

In terms of an IMS TCP system FIG. 5-2 illustrates the flowchart in FIG. 5-1 with particular emphasis on the

particular units used in the TCP system. In particular, FIG. 5-2 is a call flow diagram describing how the phantom ringing is resolved and how gracefully the transaction at the

terminating P-CSCF to endpoint B is cleaned up. Therefore, FIG. 5-2 shows the more detailed TCP INVITE and TCP INVITE re ¬ transmissions sessions as well as the respective states into which the respective units enter. However, also in FIG. 5-2, similarly as in FIG. 5-1, in step C the first timer period is extended (a second time interval is added) during which the second resource allocation unit 11-24 is to maintain the resources (or allocate second resources) which triggers the sending of the control message CM and of the cancel message from the second control 11-2 to the second terminal UE_B in step D. Therefore, similarly as in FIG. 5-1, also FIG. 5-2 illustrates that the adding of the second time interval (the "extension" time interval) in which the second resource allocation unit 11-24 allocates second resources following the allocation of first resources in the first (unextended) time interval allows to send the CANCEL message to clean up the re ¬ transmissions on the server side transaction.

In connection with the FIG. 5-2, the present invention contains the following further technical characteristics, advantages and further advantageous features/steps which a skilled person in the TCP art will immediately understand from the respectively indicated messages in the embodiments in FIG. 5-2 and in FIGs . 6-8.

1. Send CANCEL as soon as Timer B expires (FIG. 8) This makes SIP CANCEL buffered at the P-CSCF and/or various intermediate nodes between terminating P-CSCF and UE-B. SIP CANCEL shall be delivered to UE-B after SIP INVITE as soon as UE-B is able to send TCP Ack to SIP INVITE TCP message. 2. P-CSCF will keep client side transaction for a certain duration post Timer B expiry.

As explained, this is to allow graceful clean-up of transaction at UE-B and P-CSCF if 18X is sent/received after Timer B expired with the following scenarios: a. P-CSCF receives 18X response post Timer B expired, P-CSCF will send SIP CANCEL towards terminating UE and generate SIP ACK for "487 Request Terminated" received from the second terminal UE-B for graceful SIP transaction clean-up regarding the second call setup procedure. b. P-CSCF not receiving 18X response while Client side transaction is kept alive for certain duration (can be a configurable timer) . P-CSCF shall send SIP CANCEL on expiry of this timer and clear client side (UAC) transaction. Any subsequent SIP messages received by P-CSCF for associated transaction from terminating UE-B shall be dropped. This is shown in the embodiment in FIG. 6. In particular, FIG. 6 is a call flow diagram describing the resolving of the phantom ringing when the terminating P-CSCF has not received an 18X response neither during the initial Timer B first time interval nor during the extended Timer B second time interval.

In the embodiment of FIG. 6, there is also an active sending of a cancellation message from the first originating terminal UE-A in step C and shortly before the end of the extended Timer B time period (keeping the transmission resource for the application layer of the second control unit 11-1), a cancel message CANCEL is sent in step D' (and repeated in step D) in response to the cancellation unit 11-25 of the second control unit 11-2 sending the control message CM to the second call setup unit 11-22. As will be understood, this cancellation message in steps D, D' finally results in a TCP cancel acknowledgement message TCP Ack (CANCEL) and a cancellation confirmation message (200 OK (CANCEL) ) which confirms the stopping of the second call setup procedure, and thereby the stopping of the ringing at the second terminating terminal UE- B . c. P-CSCF receiving SIP CANCEL from upstream intermediate node or originating UE-A to terminate INVITE transaction while Timer B in the second timer unit 11-23 in the second control unit 11-2 is running. For such cases, P-CSCF shall respond with 200 OK (for CANCEL) towards upstream nodes and send "487 Transaction Terminated" upon clearing server side (UAS) transaction (towards Mw interface) . On client side (UAC) transaction, P-CSCF shall send SIP CANCEL after 18X response from UE-B is received or Timer B has expired. This shall make SIP CANCEL delivered to UE-B as soon as it is again available to receive TCP packets and shall stop alerting at UE-B. This is shown in the embodiment in FIG. 7. In particular, FIG. 7 is a call flow diagram describing the resolving of the false alerting when the terminating P-CSCF receives a CANCEL request from upstream nodes (such as UE-A) before the Timer B in the second control unit 11-2 expires.

In the embodiment in FIG. 7 there (as in FIG. 6) is an active sending of a CANCEL message from the originating terminal UE-A in the step C followed by a cancellation message being sent in step D) within the extended timer period.

Fig. 7 differs from FIG. 6 as follows. In Fig 6, there is still no response received from UE-B during the extended timer period and hence the CANCEL message is triggered as soon as the extended timer expires. All responses received at P-CSCF after the extended time period shall be dropped as there no transaction exists any more at the P-CSCF. In Fig 7, while running extended timer by the second timer unit 11-23 at the P-CSCF, it receives TCP Ack and SIP 18X response and gracefully clean up the session resource at P- CSCF and UE-B. Hence, the cases in FIG. 6 and FIG. 7 differ that basically in FIG. 6 the cleanup occurs when the extended Timer B has expired (no resources are allocated by the second resource allocation unit 11-24) whilst in FIG. 7 the cleanup occurs with resources still available in the extended Timer B period .

Hence, one advantageous feature of this invention concerns the sending of the SIP CANCEL message on Timer B expiry without 18x response for SIP INVITE request received from terminating UE-B and keeping client side transaction at terminating P-CSCF for certain extended duration (fixed as in FIG. 8 or

configurable) post extended Timer B expiry to provide a mechanism for graceful clean-up of transaction at P-CSCF and UE-B . However, a similar concept may also be used, as shown in FIG. 8, even without a Timer B extension. In particular, FIG. 8 is a call flow diagram describing the resolving of the phantom ringing when the terminating P-CSCF has not received the 18X response during the initial Timer B first timer interval and the extended Timer B second timer interval has been set to 0 (i.e. has not been extended) . In FIG. 8 the cancel message CANCEL in step D is sent shortly before the expiration of the original basic timer period (first time interval) set by the Timer B (of the second timer unit 11-24) . That is, once the control message CM is sent from the cancellation unit 11-25 of the second control unit 11-2, the cancel message is sent to the terminating UE-B by the second call setup unit 11-22. That is, once Timer B expires at second control unit 11-2, the cancel message is sent to the terminating UE-B by second control unit 11-2.

As explained, FIG. 8 shows a flowchart of the invention which does not use a Timer B extension. Here, even during the initial original Timer B time period (first time period) resources will be available/held on the application layer of the routing engine. Therefore, as soon as the first call setup procedure is stopped (failed) or is about to be stopped, either through a timeout as in FIG. 8, or due to an active cancellation message as in FIG. 7 and FIG. 6, a CANCEL message may be sent even during the initial timer period (e.g. shortly before it expires) . Thereafter, the transaction resources are cleared on the client side (application layer) . On the other hand, if the second terminal is still busy with another call even after the Timer B expiry, re-transmissions take place (the skilled person experienced in the TCP procedures knows that TCP messages are always re-transmitted through the open socket until they are confirmed or are timed-out or fail) such that during a re-transmission process of the cancellation message in step D*, even if no resources are assigned any more on the application layer of the second control unit, a cancellation of the second call setup procedure is achieved in step D* .

Further implementations

As already briefly mentioned above, P-CSCF shall send SIP CANCEL towards terminating side UE-B when transaction timeout (Timer B) occurs at Client side transaction (UAC) of

terminating P-CSCSF and "408 Request Timeout" response shall be sent towards originating side UE-A. This shall ensure when UE-B is available to acknowledge to TCP INVITE packet, UE-B shall start ringing and send 18X response. Immediately, it shall receive SIP CANCEL so that it shall stop ringing and cancel INVITE transaction at UE-B. It may be noted that extending Timer B at terminating P-CSCF post initial Timer B expiry is considered to be the best implementation to keep the Client side transaction alive for graceful clean-up of transaction at P-CSCF and UE . Further variations and embodiments

1. Terminating P-CSCF forwards SIP INVITE towards terminating UE-B but due to delay in LTE paging UE-B failed to respond within Timer B. In this scenario, the following possibilities 1., 2. and 3. for graceful cleanup may be considered further: a. Terminating P-CSCF forwards SIP INVITE towards UE-B and start Timer B

b. Due to delay in LTE paging or due to other reasons, SIP INIVTE has not been delivered to UE-B or UE-B is unable to respond .

c. SIP INVITE is buffered at TCP stack at P-CSCF or

intermediate nodes between P-CSCF & UE . TCP stack can then continue re-transmitting SIP INVITE in the absence of TCP ACK for TCP SIP INVITE.

d. Timer B expires on P-CSCF, clears server and client transaction

e. LTE paging successful or UE-B is able to respond after Timer B at P-CSCF has expired. SIP INVITE is delivered to UE-B which shall send TCP ACK.

f. UE responds SIP INVITE with 18X and goes into ringing state . g. P-CSCF drops 18X messages without respective transaction existing .

2. Terminating UE-B is delayed responding to the call destined for one SIM as the UE-B was handling active call in the other

SIM in a Dual SIM Dual Standby (DSDS) handset. This situation can happen when terminating UE-B goes into a ringing state as delayed session initiation message received at the terminating UE-B and session cancelled indication has not reached the terminating UE : a. SIM 1 is in Active call, SIM 2 gets incoming call.

b. P-CSCF send TCP INVITE but no TCP ACK is received, such that TCP on the second control unit re-transmits SIP INVITE. c. Timer B expires at P-CSCF, this clears Client and Server transaction.

d. Call on SIM 1 ends, UE-B (SIM 2) responds TCP ACK for TCP INVITE, sends 18X and starts ringing

e. P-CSCF drops incoming 18X, UE-B keeps ringing until subscriber answers or Timer at UE-B expires .

3. Another possibility to prevent false alerting in UE-B ' s connected over TCP transport is to enable lOOrel/PRACK on UE- B's but this has side effect that UE-B's would keep re ¬ transmitting 18x in the absence of PRACK from originating endpoint and might block UE using the application. The present invention as described in this disclosure stops such re ¬ transmission of 18X even when endpoints are enabled with lOOrel/PRACK and will help cleaning up SIP INVITE transactions gracefully . BEST MODE OF THE INVENTION

FIG. 9 show the allocation of resources and the usage of the Timer B and the application layer/network layer message flow for the first originating terminal UE-A, the second

terminating terminal UE-B and the first control unit 11-1 and second control unit 11-2 on the client side transaction and the server side transaction. As is well known to the skilled person in the IMS and TCP technology, the application

layer/network layer model is used in FIG. 9 (see also FIG. 3) . FIG. 9 also show details of the message signaling in the respective layers. The components within the bolded rectangles denoted with NL are at the network layer and components outside the rectangle are at the application layer.

Therefore, it should be understood that FIG. 9 does not show the application layer and the network layer arranged in a vertical arrangement as in FIG. 3. FIG. 9 is to illustrate how the resources are allocated for the timer extension and in particular the forwarding of the cancel messages from the application layer of the second control unit to the network layer of the second control unit. However, in connection with FIG. 3, the skilled person clearly realizes that the

interaction of the message flow between the application layer and the network layer on the client side and the server side transaction are within the P-CSCF network element of IMS. The reference numerals indicate the blocks shown in FIG. 4-1 for the first control unit 11-1 and second control unit 11-2. The client side and server side transactions are within the P-CSCF of the IMS. The dashed lines show the features which are specifically claimed in the claims and constitute the

invention . As shown in FIG. 9 there is an autonomous first call setup procedure, running between the first terminal UE-A and the first control unit 11-1 where the Timer B expiration in the first timer unit 11-13 is monitored. In this case, a socket exists for the reception of the INVITE message from UE_A.

Likewise, on the server side transaction side (second control unit) an independent second call setup procedure is run by the application layer and the network layer with sockets to send and receive messages to/from the terminating terminal UE-B. If the Timer B value in the second timer unit 11-23 is extended or the timer expires, the client side transaction resource is deleted and the Timer B is stopped. However, when the

maintaining of the resources is achieved by extending the Timer B time interval, the control message CM can be sent from the cancellation unit 11-25 to the second call setup unit 11- 11 and the CANCEL message CNCL is sent to the open socket by the cancellation unit 11-25. However, this is due to the case that the server side transaction resource is extended for a certain duration to allow handling of the SIP messages.

Hence, FIG. 9 shows the resource allocation indicated within the rectangles. The allocation of resources in connection with the timer values allow the sending of a CANCEL message in the block 11-25.

As explained above, generally, and with specific reference to the TCP layered system, the present invention provides the major advantage that an unwanted alerting of the second terminating terminal is avoided by sending a cancel control message from the second control unit to the second terminal when the Timer B expires. However, once the re-transmission of the cancellation message to UE-B is initiated this will finally lead to the cancellation of the second call setup procedure. Alternatively, the Timer B may be extended and the resources at the application layer be held such that a control message for cancelling the second call setup procedure can be sent during the extended timer period. Therefore, since resources are used for actively cancelling the second call setup procedure, false alerting (Ghost or Phantom ringing) can be prevented.

INDUSTRIAL APPLICABILITY

Summarizing, the described invention can prevent false alert (also called ghost or phantom ringing) at the second

terminating terminal UE-B and thus avoid inconvenience to users. Whilst the invention has been described with particular emphasis on terminals connected over TCP transports, the invention is also applicable to TLS encryption methods and endpoints behind NAT over TCP. The invention may also be generally used for any communication system in which a server side transaction carries out a first call setup procedure and a client side transaction carries out an independent second call setup procedure when a call is initiated from the

originating first terminal to the second terminating terminal.

It is not expected that every UE available in the market shall support lOOrel/PRACK and in these circumstances the present invention is particularly useful for stopping false alerting independently from the lOOrel/PRACK features.

In the foregoing specification, specific embodiments have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present teachings . The benefits, advantages, solutions to problems, and any element (s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.

Moreover, in this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms "comprises," "comprising," "has," "having," "includes," "including," "contains," "containing," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises, has, includes, contains a list of elements does not include only those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by "comprises ... a," "has ... a," "includes ... a," or "contains ... a," does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises, has, includes, or contains the element. The terms "a" and "an" are defined as one or more unless explicitly stated otherwise herein. The terms "substantially," "essentially," "approximately," "about," or any other version thereof, are defined as being close to as understood by one of ordinary skill in the art, and in one non-limiting embodiment the term is defined to be within 10%, in another embodiment within 5%, in another embodiment within 1%, and in another embodiment within 0.5%. The term "coupled" as used herein is defined as connected, although not necessarily directly and not necessarily mechanically. A device or structure that is "configured" in a certain way is configured in at least that way, but may also be configured in ways that are not listed. It will be appreciated that some embodiments may be comprised of one or more generic or specialized processors (or "processing devices") such as microprocessors, digital signal processors, customized processors, and field programmable gate arrays (FPGAs), and unique stored program instructions (including both software and firmware) that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of the method and/or apparatus described herein. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used.

Software programs containing software instructions for carrying out the functionalities and method steps in the described units may be used. Therefore, one or more embodiments can be implemented as a computer-readable storage medium having computer readable code stored thereon for programming a computer (e.g., comprising a processor) to perform a method as described and claimed herein. Examples of such computer-readable storage mediums include, but are not limited to, a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a ROM (Read Only Memory) , a PROM (Programmable Read Only Memory) , an EPROM (Erasable Programmable Read Only Memory) , an EEPROM (Electrically Erasable Programmable Read Only Memory) and a Flash memory. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein, will be readily capable of generating such software instructions and programs and ICs with minimal experimentation.

The Abstract of the Disclosure is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims .

In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in various embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter. LIST OF ABRIVIATIONS

DSDS Dual SIM Dual Standby

IMS IP Multimedia Subsystem

IP Internet Protocol

LTE Long Term Evolution

NAT Network Address Translation

NE Network Element

P-CSCF Proxy Call Session Control Function

PRACK Provisional Acknowledgement

SIM Subscriber Identification Module

SIP Session Initiation Protocol

TCP Transport Control Protocol

TLS Transport Level Security

UAC User Agent Client

UE User Endpoint

OSI Open Source Interconnection