Negotiation of used communication mode in a telecommunications system

1. A method for negotiating a used communication mode between system elements in a telecommunications system, the method comprising:

performing message exchange between the system elements,

checking based on the exchanged messages if the system elements support a first in-band signalling protocol,

selecting the communication mode according to said first signalling protocol based on supported communication modes if the system elements support said first signalling protocol,

if there is at least one system element not supporting said first signalling protocol, checking based on the exchanged messages if the system elements support a tandem free operation (TFO) in-band signalling protocol, and

continuing communication between the system elements according to the procedures of the TFO protocol if the system elements support the TFO protocol.

2. A method according to claim 1 , further comprising: selecting the communication mode between the system elements according to a second in-band signalling protocol if there is at least one system element not supporting said first signalling protocol when checking the support for said first signalling protocol and there is at least one system element not supporting the TFO protocol when checking the support for the TFO protocol.

3. A method according to claim 1 , wherein the communication modes supported by the system elements are indicated if the system elements support said first signalling protocol,

the supported communication modes in the exchanged messages of said first signalling protocol are checked, and

a communication mode that the system elements support is selected.

4. A method according to claim 3 , wherein

the communication modes are prioritized, and the communication mode is selected based on the prioritization.

5. A method according to claim 2 , wherein messages according to said first signalling protocol are sent from a system element to another system element,

messages of both said first signalling protocol and said second signalling protocol are sent to the other system element if no messages according to said first signalling protocol are received from the other system element within a first pre-defined time limit, and

messages to the other system element are sent according to said second signalling protocol if no messages according to said first or said second signalling protocol are received from the other system element within a second pre-defined time limit.

6. A method according to claim 2 , wherein said second signalling protocol is based on the V.140 protocol.

7. A method according to claim 5 or 6 , wherein messages comprising V.140 signatures are sent by the system elements according to the second in-band signalling protocol,

the connection between the system elements is checked by V.140 phase 2 procedures if the system elements receive at least one V.140 signature, and

the used communication mode is negotiated by V.140 phase 3 procedures if the phase 2 procedures are successful.

8. A method according to claim 1 , wherein said first signalling protocol is based on the tandem-free operation (TFO) protocol.

9. A method according to claim 1 , wherein the messages according to said first signalling protocol are sent by the least significant bits of the connection between the system elements.

10. A method according to claim 1 , wherein the communication mode is a multimedia call.

11. An apparatus in a telecommunications system in which a connection is arranged between said apparatus and one or more other parties,

said apparatus is configured to check if the other parties of a connection support a first in-band signalling protocol,

said apparatus is configured to negotiate the communication mode with the other parties based on supported communication modes according to said first signalling protocol if the other parties support said first signalling protocol,

said apparatus is configured to, if there is at least one other party not supporting said first signalling protocol, check if the other parties support a tandem free operation (TFO) in-band signalling protocol, and

said apparatus is configured to continue communication with the other parties according to the TFO protocol if the other parties support the TFO protocol.

12. An apparatus according to claim 11 , wherein said apparatus is configured to negotiate the communication mode with the other parties according to a second in-band signalling protocol if there is at least one other party not supporting said first signalling protocol when checking the support for said first signalling protocol and there is at least one other party not supporting the TFO protocol when checking the support for the TFO protocol.

13. An apparatus according to claim 11 , wherein said apparatus is configured to search at least for messages according to said first signalling protocol from the other parties and for messages according to the TFO protocol from the other parties.

14. An apparatus according to claim 12 or 13 , wherein said apparatus is configured to send messages to the other parties according to said first signalling protocol to check if the other parties support said first signalling protocol or the TFO protocol at least if said apparatus has initiated the connection with the other end or if said apparatus does not support the messages received from the other parties, and

said apparatus is configured to send messages to the other parties according to said first signalling protocol and said second signalling protocol if no messages according to said first signalling protocol are received from the other parties within a first pre-defined time limit, and

said apparatus is configured to send messages to the other parties according to said second signalling protocol if no messages according to said first or said second signalling protocol are received from the other parties within a second pre-defined time limit.

15. An apparatus according to claim 11 , wherein

said apparatus is configured to send an indication on the supported communication modes to the other parties if the other parties support said first signalling protocol,

said apparatus is configured to check the communication modes supported by the other parties, and

said apparatus is configured to select a communication mode that the other parties also support.

16. An apparatus according to claim 15 , wherein said apparatus is configured to select the communication mode if said apparatus has initiated the connection with the other parties or if the other parties have permitted said apparatus to select the communication mode.

17. An apparatus according to claim 12 , wherein

said apparatus is configured to send messages to the other parties comprising V.140 signatures according to said second signalling protocol,

said apparatus is configured to execute V.140 phase 2 procedures if it receives at least one V.140 signature from the other parties, and

said apparatus is configured to negotiate the used communication mode with the other parties by V.140 phase 3 procedures if the phase 2 procedures are successful.

18. An apparatus according to claim 11 , wherein

said first signalling protocol is based on the tandem-free operation (TFO) protocol.

19. An apparatus according to claim 11 , wherein

said apparatus is a network element such as a mobile switching centre comprising an interworking function.

20. An apparatus according to claim 11 , wherein said apparatus is a terminal comprising a terminal adaptation function.

21. An apparatus according to claim 11 , wherein said apparatus comprises

transmission and reception means for sending and receiving in-band signalling messages to and from the other parties,

checking means for checking the signalling protocols and communication modes supported by the other parties based on the received signalling messages, and

selecting means for selecting the communication mode based on the signalling protocols and communication modes supported by the other parties.