# Patent

## Title
Changing of channel capabilities

## Abstract

In a telecommunications system, in which a connection comprises a part with an interworking function at both ends, a channel is allocated to the connection between the interworking functions. The required channel capabilities may vary during the connection, whereby the channel capabilites should be changed. When the first interworking function detects that a channel capability must be changed, a first message, which indicates the desired capability change, is transmitted (2-1) to the second interworking function; and the channel capability is changed (2-3, 2-5) into the desired one at both ends of the part.

## Background
FIELD OF THE INVENTION
The invention relates to conveying transport service parameter information in a packet-switched network, which conveys service that is conventionally conveyed in a circuit-switched network. In particular, the invention relates to conveying transport service parameter information in an ATM (Asynchronous Transfer Mode) network. By means of the transport service parameter information, it is possible to control capabilities of a channel allocated to a connection.
BACKGROUND OF THE INVENTION
Conventionally, speech and data are transferred in a circuit-switched network by allocating a dedicated channel to each connection. The dedicated channel permits to ensure certain quality and reliability. However, a problem with the circuit-switched network is that the channel is reserved for a connection, even though nothing would be transferred. In the packet-switched network a plurality of connections can share the same channel. However, since the end-users want to use familiar circuit-switched devices, these two different transmission technologies have been combined such that circuit-switched devices are used at the user end, but the actual data transmission is carried out by packet switching, for instance, using an ATM-based network. The packet-switched data transmission should be transparent to end-users and end-user applications, such as telephone traffic, ISDN (Integrated Service Digital Network), voice-band data and fax. The end-users require the same reliability and quality of service as when the circuit-switched network is used alone. To this end, various protocols have been standardized internationally, which support voice transfer over the ATM-based network, for instance. ATM Forum's ‘Loop Emulation Service Using ATM adaptation layer type 2’, or more briefly, ‘LES Using AAL2’ is one example of these standards.
However, there is a problem that current standards do not include all requirements and needs of service aware transport, such as control of codecs and echo cancellation. For instance, ELCP (Emulated Loop Control Protocol) defined in LES specifications allows exchange of channel allocation messages and user port control messages between an interworking function CP-IWF in an access device of the end-user and an interworking function CO-IWF in a gateway at the other end of the ATM network. However, these messages are not able to signal more detailed information on the state of the network or free capacity between CP-IWF and CO-IWF, for instance. In other words, they are not able to convey information on which capabilities the channel should have at any given time. This information is needed, however, in order that the user could be provided with the reliability and quality of the circuit-switched network.
BRIEF DESCRIPTION OF THE INVENTION
The object of the invention is thus to provide a mechanism, by which channel-specific transport service parameter information can be conveyed between interworking functions. This is achieved by a method, a system and nodes, which are characterized by what is set forth in the independent claims. The preferred embodiments of the method, the system and the nodes of the invention are disclosed in the attached dependent claims.
The invention is based on detecting a problem and solving it by extending the existing message-based data exchange to cover also channel-specific conveyance of transport service parameter information in such a manner that interworking functions can adjust channel capabilities. The invention has an advantage that thereby it is possible to take into account transport service requirements, i.e. capabilities, set for the channel and to change the channel capabilities from default values after a connection establishment any time a need for a change is detected. In this manner it is possible to make sure that the subscriber will always have a desired service. For instance, if ADPCM (Adaptive Differential Pulse Code Modulation) is configured to default coding for speech, but for instance the digital signal processing capacity of CP-IWF is limited, CP-IWF can request that PCM (Pulse Code Modulation) be used for speech. Even though the user's other simultaneous applications may lose some of the bandwidth as a result of the change in coding, the user will nevertheless receive the desired speech service.



BRIEF DESCRIPTION OF THE DRAWINGS
In the following the invention will be described in greater detail in connection with reference to the attached drawings, wherein
FIG. 1 shows a system according to a first preferred embodiment of the invention;
FIGS. 2 and 3 show message-based data transmission according to the first preferred embodiment of the invention;
FIG. 4 shows operation in CP-IWF according to the first preferred embodiment of the invention; and
FIG. 5 operation in CO-IWF according to the first preferred embodiment of the invention.



DETAILED DESCRIPTION OF THE INVENTION
The present invention can be applied in connection with any system, where services are transferred over a circuit-switched or packet-switched network using a protocol with no defined mechanism for data exchange relating to channel capabilities. The invention suits particularly well to systems, in which circuit-switched services are transferred over a packet-switched network. A system of this kind is shown in FIG. 1, for instance, where circuit-switched services are transferred over an ATM-based network using the previously mentioned ELPC between the interworking functions. Even though the invention and its preferred embodiments are described in the following in connection with the system of FIG. 1 and the protocol employed therein, the invention is not restricted thereto, however, but it can be applied e.g. to systems utilizing wireless data transmission.
FIG. 1 shows simplified network architecture of a system 1, in which a service is conveyed to circuit-switched service subscribers via an ATM network 2. In the example of FIG. 1, a connection between subscribers thus comprises two circuit-switched parts and one packet-switched part such that at both ends of the packet-switched part there is an interworking function which adapts circuit-switched and packet-switched data transmission to be mutually compatible.
Between a user terminal, i.e. a subscriber device 3, and an integrated access device IAD it is possible to use data transmission according to PSTN (Public Switched Telephone Network) or ISDN, for instance. Between the integrated access device and DSLAM (Digital Subscriber Line Access Multiplexer) device is used e.g. SDSL (Symmetric Digital Subscriber Line), ADSL (Asymmetric Digital Subscriber Line), HDSL (High-Speed Digital Subscriber Line) or VDSL (Very high speed Digital Subscriber Line) technology. Between a gateway GW and a local exchange LE of the PSTN network it is possible to use V5 data transmission. V5 is a standardized interface between the subscriber network and the local exchange defined by ETSI.
Of the interworking functions, the customer premises inter-working function CP-IWF is located in the integrated access device IAD and the central office interworking function CO-IWF is part of the gateway GW. In the first preferred embodiment, the above-mentioned LES protocol of the ATM Forum is used between the interworking functions. Both of the interworking functions listen to traffic passing on the channel allocated to a connection. The allocated channel comprises two sub-channels: a speech channel and a signalling channel. On the basis of the listening the interworking function is able to conclude what kind of a connection is required, and triggers, when needed, a functionality described in greater detail in FIGS. 2, 3, 4 and 5. The same functionality can also be triggered by the fact that the operator may have provided the connection with a level (i.e. the desired capabilities) that differs from the level indicated by the default values of parameters. For instance, the capabilities may have been provided in CO-IWF or CP-IWF. The interworking function may also receive from a network element information that triggers the functionality presented in FIGS. 2, 3, 4 and 5. For instance, CO-IWF may receive from the local exchange LE additional information that triggers the changing of channel capabilities.
The system and its network nodes implementing the functionality of the present invention comprise, in addition to prior art means, means for implementing functionalities described in greater detail in connection with FIGS. 2, 3, 4 and 5. To put it more precisely, they comprise means for changing connection parameters (i.e. channel capabilities). The present network nodes comprise processors and memory, which can be utilized in operations according to the invention. All changes necessary for implementing the invention can be executed as added or updated software routines, with application-specific circuits (ASIC) and/or programmable circuits (EPLD, FPGA).
FIGS. 2, 3, 4 and 5 show data transmission according to the invention in a first preferred embodiment, in which it is assumed that CO-IWF is a so-called master. In the first preferred embodiment, when CP-IWF desires to change a connection parameter/connection parameters, it transmits to CO-IWF a request, to which CO-IWF responds with a command. When CO-IWF desires to change a connection parameter /connection parameters, it transmits a command directly to CP-IWF. This has an advantage that the master-slave position is stable and it is known which one makes a decision on the quality of the connection.
In FIG. 2, the process starts in a situation, where a channel has already been allocated to a connection. In other words, a speech path is available between the user device and the local exchange shown in FIG. 1, and the connection has been established at default values configured for the parameters. CP-IWF desires to change a parameter reserved for the connection, for instance, for the reason that the quality of the connection has changed, because a fax machine was added to the connection. Therefore, CP-IWF transmits a channel request 2-1 to CO-IWF. The channel request preferably comprises the following elements in the following order: protocol discriminator, channel reference number, message type, information transfer capability, cause and channel parameters. These elements will be described in greater detail later on in the text. Either one of the elements, channel parameters or information transfer capability, can be omitted in the first preferred embodiment of the invention.
After receiving the message 2-1, CO-IWF checks whether it is able to provide the requested channel capabilities. In the example of FIG. 2, it is assumed that it is. Consequently, CO-IWF transmits a channel command 2-2. The channel command preferably comprises the following elements in the following order: protocol discriminator, channel reference number, message type, information transfer capability and channel parameters. Also in this message either one of the elements, channel parameters or information transfer capability, can be omitted in the first preferred embodiment of the invention. In the example of FIG. 2, CO-IWF takes the channel parameters and/or the contents of the information transfer capability directly from the channel request 2-1 for the message 2-2. If CO-IWF is not able to provide the requested parameters it may provide other values in the message 2-2. In another embodiment of the invention CO-IWF may provide other values in the message 2-2, for the reason, for instance, that while listening to the connection, it has detected that the requested channel capabilities do not meet with the channel capabilities required by the connection.
After receiving the message 2-2, CP-IWF checks whether it is able to provide the capabilities requested in the message 2-2. In the example of FIG. 2, it is assumed that it is. So, CO-IWF makes the requested changes in settings at step 2-3 and transmits a channel acknowledgment 2-4 to CO-IWF. The acknowledgment message preferably comprises the following elements in the following order: protocol discriminator, channel reference number and message type.
After receiving the acknowledgment 2-4, CO-IWF makes the corresponding changes in its settings, whereafter the connection has new parameters in its use. Thus, the channel capabilities have been changed to meet the need.
The meaning of the elements used in the messages of FIG. 2 is as follows:

    
    
        the protocol discriminator is the first part of all messages, also of prior art messages;
        the channel reference number is used for connection identification after the channel has been allocated and the connection established. In prior art messages the channel is identified by a channel allocation reference. Because connection parameters can be changed at any step of the connection, it is advantageous to identify the connection in a novel manner;
        the message type refers to the name of the message according to the prior art. A new value for each new message needs to be defined for the message type;
        in connection with ISDN calls, the information transfer capability supports detecting content type (e.g. speech, video, audio) of the calls. For instance, the Information Transfer Capability part of the Bearer Capability Information element as defined in ITU-T recommendation Q.931 can be used as the information transfer capability element;
        the cause refers to the source from which the service is selected. the value of the cause element can be e.g. a provided service, a call phase service or no resources available;
        the channel parameter identifies the requested service in a greater detail.
    
    


In FIG. 3, the starting step is a situation, where a channel has already been allocated to the connection. After receiving additional information on the quality of the connection, for instance as a result of monitoring, CO-IWF desires to change a parameter reserved for the connection, having first checked that its resources allow the parameter change. For changing the channel capabilities, CO-IWF transmits a channel command 3-1. The channel command has been described in greater detail in connection with FIG. 2. This time, CO-IWF itself gives the contents of the channel parameters element and/or the information transfer capability element.
After receiving the message 3-1, CP-IWF checks whether it is able to provide the capabilities requested in the message 3-1. In the example of FIG. 3, it is assumed that it is not. So, CP-IWF transmits a channel reject message 3-2 to CO-IWF. The channel reject message preferably comprises the following elements in the following order: protocol discriminator, channel reference number, message type and cause. The meanings of the elements have been explained above in connection with FIG. 2.
After receiving the channel reject message, CO-IWF can transmit a new command to CP-IWF with new parameter values.
Signalling according to FIGS. 2 and 3 can be triggered at any step of the connection. It is sufficient that the interworking function receives additional information on the quality of the established connection or on a change in the quality and detects that a capability or capabilities of the channel must be changed. The channel capability/capabilities can be changed several times, if necessary.
The use of the messages according to the first preferred embodiment presented in FIGS. 2 and 3 requires that the standard ELCP be extended by adding thereto the messages shown in the figures with their new elements. In addition, four new values (i.e. one new value/one new message) indicating the above-mentioned messages must be added to the existing ‘message type’ parameter. An advantage with the first preferred embodiment is that with slight changes in the existing protocol it is possible to provide a message-based transmission mechanism, by which the capabilities of the channel allocated to the connection can be changed.
Between the messages and/or operations shown in FIGS. 2 and 3 it is possible to transmit other messages and/or perform other operations. The messages and the operations are not in an absolute order, but e.g. the message 2-4 can be transmitted at the same time with or prior to the change of settings of step 2-3. The elements included in the messages are not in any absolute temporal order either, but their order can be changed. The messages need not include all the above-described elements either. One element can also convey more information than what is presented above. For instance, the data of the above-mentioned two different elements can be combined into one element. Correspondingly, the data transmitted in the abovementioned one element can be conveyed by a plurality of elements. The above messages are only examples and may comprise a plurality of separate messages for transferring the same information. Other information can also be transferred in the messages. For instance, a reject message may contain information on what kind of a capability change would be possible. The message names can also differ from the above names. The same information can also be transmitted as a part of some old message by adding the necessary element(s) thereto. The messages can also be messages of another protocol.
FIG. 4 is a flow chart of the operation of CP-IWF in the first preferred embodiment of the invention after a channel has already been allocated to the connection. At step 401, CP-IWF listens to the speech and signalling channels of the channel monitoring whether it receives a command (step 402) or detects a need for changing the channel capabilities (step 403). If CP-IWF detects the need, it transmits a request to CO-IWF at step 404 and starts monitoring time (405) and whether it receives a command within the time 1 (step 406). If CP-IWF receives the channel command within the time 1, it checks at step 407 whether it has resources to provide the requested capabilities. If it has the resources, CP-IWF transmits an acknowledgment to CO-IWF at step 408 and changes settings at step 409. Thereafter, CP-IWF returns to step 401 to listen to the channel.
If CP-IWF does not have the necessary resources (step 407), it transmits a rejection to CO-IWF at step 410 and returns to step 401 to listen to the channel.
If the time 1 expires (step 405) prior to the reception of the command, CP-IWF retransmits the request at step 411. Thereafter, CP-IWF starts monitoring the time (step 412) and whether it receives the command within the time 2 (step 413). The time 2 can differ in length from the time 1. If CP-IWF receives the command within the time 2, it proceeds to step 407 to check whether it has resources to provide the requested capabilities.
If the time 2 expires (step 412) prior to the reception of the command, CP-IWF transmits information thereon to a resource management entity or a management device of the integrated access device at step 414 and proceeds thereafter to step 411 to retransmit the request. These two different times are used in the first preferred embodiment both to ensure that the packet containing the request can be delivered and to serve as an accessory of the resource management entity to provide it with information if there is something wrong with the system. Information is not transmitted if one packet is lost.
If the command is received at step 402, CP-IWF proceeds to step 407 to check its resources.
CP-IWF executes the above-described steps until the channel allocated to the connection is released by deallocation message exchange.
FIG. 5 is a flow chart of the operation of CP-IWF in the first preferred embodiment of the invention after a channel has already been allocated to the connection. At step 501 CO-IWF listens to the speech and signalling channels of the channel monitoring whether it detects a need to change channel capabilities (502) or receives a request (step 503). If CO-IWF detects a need (502) or receives a request (step 503), it checks at step 504 whether it has resources to provide the requested capability/capabilities. If it has resources, CO-IWF sets a time counter n to zero at step 505 and transmits a command to CP-IWF at step 506. Thereafter CO-IWF starts monitoring time (step 507) and whether it receives a rejection (step 508) or an acknowledgment (step 509) within the time 3. The time 3 may differ in length from the times 1 and 2 mentioned in connection with FIG. 4. If CO-IWF receives the acknowledgment (509) within the time 3, it changes the settings at step 510. Thereafter CO-IWF reverts to step 501 to listen to the channel.
If CO-IWF receives the rejection (step 508) within the time 3, it proceeds to step 511, where it adapts the connection parameters to meet with its resources. Thereafter CO-IWF continues from step 505 by setting the time counter n to zero. The time counter is used both to ensure that the packet containing the command can be delivered and to serve as an accessory of the resource management entity to provide it with information if there is something wrong with the system. Information is not transmitted if one packet is lost.
If the time 3 expires (step 507) prior to the reception of the acknowledgment or the rejection, CO-IWF checks the value of the time counter n. If n is zero (step 512), CO-IWF sets the time counter n to one at step 513 and proceeds thereafter to step 506 to retransmit the channel request. If the value of the time counter n is not zero (step 512), CO-IWF transmits information thereon to the gateway resource management entity or management device at step 514 and proceeds to step 501 to listen to the channel. If the command transmission was triggered by a detected need for change, the command is retransmitted, because the need for change still exists (step 502). If the command was transmitted in response to a request, steps 501, 502 and 502 will be repeated.
If CO-IWF detects at step 504 that its resources are not sufficient to provide the necessary or requested capabilities, CO-IWF proceeds to step 511 to adapt the parameters to meet with the resources.
CO-IWF executes the above-described steps until the channel allocated to the connection will be released by deallocation message exchange.
The order of the steps shown in FIGS. 4 and 5 may deviate from what is described above, and the steps may take place in parallel. Between the steps it is possible to execute other steps that have not been shown in the figures, and some of the steps appearing in the figures can also be omitted, such as steps 414 and 514. Some of the steps can also be replaced by an alternative solution that produces the same outcome. The time counter of FIG. 5 is an alternative to monitoring two different times as in FIG. 4.
In a second preferred embodiment, neither of the interworking functions is assumed to be the master. In the second preferred embodiment, when the interworking function detects a need for change, it checks for which kind of channel capabilities it has free resources and transmits a channel command to a second interworking function, whereafter it remains waiting for a rejection or an acknowledgment for a given period of time, or possibly retransmits the command. In the second preferred embodiment, the functionalities of both interworking functions are thus similar. The message exchange shown in FIGS. 2 and 3 changes such that the message 2-1 is not transmitted, but both interworking functions can transmit the other messages. In the second preferred embodiment, the functionalities shown in FIGS. 4 and 5 are combined for example such that from step 403 of FIG. 4 the process proceeds to step 504 of FIG. 5 to check the available resources, when a need is detected to change channel capability/capabilities. In the second preferred embodiment the channel command can be replaced by a channel request.
Also in a third preferred embodiment of the invention, the functionalities of both interworking functions are similar. In the third preferred embodiment of the invention, the interworking function which detects a need to change parameters transmits a channel request, i.e. acts in the same way as CP-IWF in the first preferred embodiment of the invention. Correspondingly, the interworking function which receives the request acts in the same way as CO-IWF in the first preferred embodiment of the invention.
Even though the invention is described in the above assuming that one interworking function is a customer premises interworking function and the other is a central office interworking function, the invention can also be applied between two customer premises interworking functions or between two central office interworking functions.
Even though the invention is described in the above assuming that both interworking functions are located in network nodes, all embodiments of the invention can also be applied if one of the interworking functions is located in the network node (being the host according to the first preferred embodiment) and the other is located in an end-user device or in a device to which the end-user device is directly coupled. Both of these devices are referred to as terminal devices in the present document. In that case there is no circuit-switched network between the end-user and the interworking function, but the part between the user interface and the interworking function can be regarded as the circuit-switched part of the connection, because it is based on circuit-switching. The second and the third preferred embodiments of the invention can also be applied when neither of the interworking functions is located in the network node, but instead in an end-user device or in a device to which the end-user device is directly coupled.
Even though the invention is described in the above by means of an example of a system, in which the connection comprises two circuit-switched parts and therebetween a packet-switched part, with an interworking function at both ends, which interworking function adapts circuit-switched data transmission and packet-switched data transmission to one another, the invention is not restricted to these systems. The connection may also comprise one circuit-switched part and one or two packet-switched parts. There may also be more than two or three parts. All parts can be either circuit-switched or packet-switched. Different parts can use different data transmission technologies, for instance, one packet-switched part can use ATM technology and the other packet-switched part can use IP (Internet Protocol) technology. The interworking function that is located between two different parts (e.g. an ATM part and an IP part, or a circuit-switched part and a packet-switched part) adapts the data transmission methods and technologies to one another. The interworking function between two similar parts, in turn, does not adapt two different data transmission methods or technologies to one another but it works together with the interworking function at one end of either part. In the present document, the interworking function covers both the functionality of adapting two different data transmission methods or technologies to one another and the functionality of participating in channel allocation at one end of the part, which supports the same protocol as the interworking function adapting two different functionalities to one another at the other end of the part.
It is apparent to a person skilled in the art that as technology progresses the basic idea of the invention can be implemented in a variety of ways. Thus, the invention and its embodiments are not restricted to the above-described examples, but they may vary within the scope of the claims.

## Claims

1. A method comprising:
establishing a channel to a connection, where the channel established to the connection comprises at least a part with a first interworking function at a first end and a second interworking function at a second end;
detecting in the first interworking function that a channel capability of the channel established to the connection must be changed;
transmitting to the second interworking function a first message which indicates a desired capability change for the channel established to the connection;
checking in the second interworking function whether the desired capability change can be performed in response to the first message; and
if the capability can be changed into the desired one:
transmitting a second message to the first interworking function, which message indicates that the desired capability change of the channel established to the connection can be performed at the second end;
transmitting a third message to the second interworking function, which message indicates that the desired capability change can be performed at the first end;
changing the capability into the desired one at the first end; and
changing the capability into the desired one at the second end only in response to the reception of the third message.

2. The method as claimed in claim 1, wherein, if the capability cannot be changed into the desired one, the method further comprises:
checking which kind of a change can be performed at the second end; and
transmitting to the first interworking function a second message which indicates the change that can be performed at the second end as the desired capability change.

3. The method as claimed in claim 1, wherein, if the capability cannot be changed into the desired one, the method further comprises:
transmitting to the second interworking function a fourth message, which indicates that the desired capability change cannot be performed at the first end;
modifying the desired capability change in response to the reception of the fourth message in the second interworking function; and
transmitting to the first interworking function a new second message, which indicates that the modified capability change is the desired one.

4. The method as claimed in claim 1, further comprising:
checking, in response to the detected need for capability change, what kind of a change can be performed at the first end; and
indicating in the first message the change which can be performed at the first end as the desired capability change.

5. A telecommunications system comprising
at least a first part for transferring data between end-users of the system,
at least a first interworking function at a first end of the first part,
at least a second interworking fUnction at a second end of the first part; and
the first and the second interworking functions being arranged to establish a channel in the first part to a connection between the end-users,
wherein the first interworking function is arranged to detect a need for change in a channel capability of the channel established to a connection in the first part and to transmit to the second interworking fUnction a first message that indicates a desired capability change for the channel established to the connection,
the second interworking fUnction is arranged to check in response to the first message, whether the desired capability change can be performed, and if the capability can be changed into the desired one, to transmit to the first interworking function a second message which indicates that the desired capability change can be performed at the second end,
the first interworking function is arranged to check in response to the reception of the second message, whether the desired capability change can be performed, and if the capability can be changed into the desired one, to change the capability into the desired one and to transmit to the second interworking function a third message which indicates that the desired capability change can be performed at the first end, and
the first and the second interworking functions are arranged to change the channel capability of the channel established to the connection into the desired one.

6. The telecommunications system as claimed in claim 5, wherein, if the second interworking function is not able to change the capability into the desired one, it is arranged to check what kind of a change can be performed, and to indicate in the second message the change which can be performed as the desired capability change.

7. The telecommunications system as claimed in claim 5, wherein the first interworking fUnction is arranged to check in response to detecting a need for capability change, what kind of a change can be performed and to indicate in the first message the change which can be performed at the first end as the desired one.

8. The telecommunications system as claimed in claim 5, wherein the first interworking function is arranged to detect the necessary change on the basis of the information received from another entity of the system.

9. The telecommunications system as claimed in claim 5, wherein the first interworking function is arranged to detect the necessary change from the subscriber information provided for the subscriber using the connection.

10. The telecommunications system as claimed in claim 5, wherein the first interworking function is arranged to detect the necessary change by listening to the channel established to the connection.

11. The telecommunications system as claimed in claim 5, wherein the first part is packet-switched.

12. The telecommunications system as claimed in claim 11, wherein an ELCP protocol is employed between the first interworking function and the second interworking function.

13. The telecommunications system as claimed in claim 5 further comprising at least one circuit-switched part.

14. A telecommunications system node comprising a first interworking function arranged to establish a channel to a user of the telecommunications system between itself and a second interworking function located in a second node of the telecommunications system, to receive a first message which indicates a need for change in a channel capability of a channel established to the connection, to check whether it can perform the change indicated in the first message, and if it ca, to transmit a second message which indicates that the first interworking function can perform a desired capability change for the channel established to the connection, and said first interworking function is arranged to receive a third message, which indicates that the second interworking function can perform the desired change, and in response to the reception of the third message to perform the desired change.

15. The node as claimed in claim 14, wherein the first interworking function is arranged to perform the desired change in response to the transmission of the second message.

16. The node as claimed in claim 14, wherein if the first interworking function cannot perform the desired change indicated in the first message, the first interworking function is arranged to modify the desired capability change and to transmit the modified capability change in the second message as the desired capability change.

17. The node as claimed in claim 14, wherein the first interworking function is arranged to receive a fourth message, which indicates that the second interworking function cannot perform the desired capability change, and in response to the fourth message to modify the desired capability change and to transmit the modified capability change in a new second message as the desired capability change.

18. The node as claimed in claim 14, the node being a network node.

19. The node as claimed in claim 14, the node being a terminal device in the telecommunications system.