WO2009003359A1

WO2009003359A1 - Method for establishing a call, signaling controller, network element and system

Info

Publication number: WO2009003359A1
Application number: PCT/CN2008/001085
Authority: WO
Inventors: Xinmin Zhang; Junwen Wang; Fangxin Liu; Minmin Xue
Original assignee: Alcatel Lucent
Priority date: 2007-06-29
Filing date: 2008-06-03
Publication date: 2009-01-08
Also published as: CN101335659A

Abstract

The present invention proposes a method for establishing a call, a signaling controller for controlling a network element to allow a terminal to access to a packet network, a network element for allowing a terminal to access to a packet network and a communication system including at least two terminals, network elements and a media gateway controller, wherein at least a first terminal and a second terminal are connected respectively via at least a first network element and a second network element to a first network which is in turn connected to a second network via a third network element that is address translation enabled, said first network and second network are packet networks, said method for establishing a call between said first terminal and said second terminal comprising: creating context for the call with translated addresses of said first network element and said second network element in said second network; determining if said first network element and said second network element in the context for the call are corresponding to the same translated address in said second network; and establishing a traffic connection for the call with respective addresses of said first network element and second network element in said first network if the addresses of said first network element and second network element in the context for the call are corresponding to the same translated address in said second network.

Description

Method for Establishing a Call, Signaling controller, Network Element and System

Field of the Invention

The present invention relates to the field of communication, and more particularly to establishing a call via packet networks.

Background of the Invention

At present, with evolvement of traditional telecommunication networks towards NGN networks, soft switch has become a key for the network evolvement and the call and control in the next generation packet network. The soft switch is an IP network-based real time communication technology and related applications, simulates the circuit switch with software and provides the next generation network with call and connection controlling function for real time traffic.

As one of the media gateway control protocols (MGCP) between a media gateway controller and a media gateway, the H.248 protocol, also known as MeGaCo protocol, is proposed by the cooperation of the ITU-T and the IETF, which is an improvement based on earlier MGCP protocols. The H.248 supports a larger scale of network applications, facilitates the extension of protocols, and becomes the standard of media gateway control protocols by replacing the MGCP.

Figure 1 is a flowchart illustrating the process of establishing a call between two users according to for example the H.248. In this process, the physical terminal IDs of terminals 1 , 2 are respectively AO, A1 , to which users A, B are respectively connected. The particular flow is as follows.

- When user A hangs off, the gateway informs the MGC of this event with a NTFY REQ message, and the MGC returns a response message after validating reception of said event.

- The MGC indicates the gateway to send a dial tone to user A with a MOD_REQ message after receiving the calling user hanging off event, and terminal AO returns MOD_REPLY message as response, the dial tone thus being sent to user A.

- User A dials, terminal AO collects the dialed numbers and sends it to the MGC with a NTFY_REQ message, and then the MGC sends a NTFY_REPLY message as acknowledgement.

- The MGC creates a new context in the MG and incorporates a TDM terminal and a RTP terminal into that new context, and then the MG returns an ADD_REPLY message as response and assigns a new connection descriptor and a new RTP terminal descriptor.

- The MGC analyses the called number, determines that the called user B is connected to the terminal A1 of the MG, and requests the MG to incorporate terminal A1 and a certain RTP terminal into the new context with an ADD-REQ message, and then the MG returns an ADD_REPLY message as response and assigns a new connection descriptor and a new RTP terminal descriptor.

- The MGC sends a MOD_REQ message to terminal A1 , modifies the attribute of A1 and requests the MG to send a ringing tone to user B, and then the MG returns a MOD REPLY message as response and sends the ringing tone to user B.

- The MGC sends a MOD_REQ message to terminal AO, modifies the attribute of AO and requests the MG to send a ring back tone to user A, and then the MG returns a MOD REPLY message as response and sends the ring back tone to user A.

When the called user B hangs off, the MG informs the MGC of this event with a NTFY REQ message, and then the MGC returns a NTFY REPLY as acknowledgement.

- The MGC sends to the RTP terminal associated with terminal A1 a connection description of the RTP terminal associated with terminal AO with a MOD REQ message and modifies the status of the RTP terminal associated with terminal A1 as receiving/sending, and then the MG returns a MOD_REPLY message as acknowledgement.

- The MGC sends to the RTP terminal associated with terminal AO a connection description of the RTP terminal associated with terminal A1 with a MOD_REQ message and modifies the status of the RTP terminal associated with terminal AO as receiving/sending, and then the MG returns a MOD_REPLY message as acknowledgement.

- User A and user B communicate with each other via the established RTP connection.

- When the calling user A hangs up, the MG informs the MGC by sending a NTFY_REQ message, and then the MGC sends a NTFY_REPLY message as acknowledgement.

- The MGC sends the MG a MOD_REQ message for modifying the attribute of terminal AO, requesting the gateway to further monitor event of terminal AO, such as hanging off etc., and modifying the status of the RTP terminal associated with AO as deactivation, and then the MG sends a MOD_REPLY message as response and executes that command.

- The MGC sends the MG a SUB REQ message for removing the context and canceling the call, and then the MG returns a SUB_REPLY message as response.

- The MGC send a MOD REQ message to the MG for modifying the attribute of terminal A1 and requesting the MG to further monitor event of terminal A1 , such as hanging up etc., and sending terminal A1 a busy tone, and then the MG returns a MOD_REPLY message as response while sending the busy tone to user B.

- The MGC send the MG a MOD REQ message for requesting the MG to monitor event of terminal AO, such as hanging off etc., and then the MG returns a MOD REPLY message as response, here the context being empty.

- When the called user B hangs up, the MG informs the MGC by sending a NTFY-REQ message, and then the MGC returns a NTFY-REPLY message as response.

The MGC sends the MG a SUB_REQ message for removing the context and canceling the call, and then the MG returns a SUB-REPLY message as response.

- The MGC sends the MG a MOD_REQ message for requesting the MG to monitor event of terminal A1 , such as hanging off etc., and then the MG returns a MOD REPLY message as response, here the context being empty. Figure 2 shows an example of a network environment comprising edge routers employing the existing VoIP technology. In this system, telephone terminals 105, 106 access to a local network 100 respectively via access gateway (AGW) 103, 104, and the local network is connected to a public network 200 via a router 101 that is network address port translation (NAPT) function enabled. The establishment of call between the telephone terminals 105 and 106 is managed by the media gateway controller (MGC201) according to the H.248 protocol. Here, assume that the private addresses of two access gateways 103 and 104 are respectively 192.168.2.2 and 192.168.2.10; the local addresses corresponding to terminals 105 and 106 are respectively 192.168.2.2/3001 and 192.168.2.10/3002; the public address of MGC 201 is 200.1.1.2; the public address and private address of router 101 are respectively 200.1.1.1 and 192.168.2.1/24.

In the above system, since the two access gateways are connected to the public network via a NAPT router, when a call is established between the internal telephone terminals 105 and 106 behind the two access gateways, the local SDP (session description protocol) message and the remote SDP message of this call will use same public address as the NAPT router and different user datagram protocol (UDP) ports. In this case, the destination address of real time transport protocol (RTP) traffic will use the public address (and port) of the MAPT router of a remote user. When this traffic arrives the NAPT router, the source address/port will be translated into the same NAPT router address by the NAPT router, and this IP packet will be discarded by the NAPT router and could not be forwarded back to the private address of the remote user. Therefore, the call can not be established normally. This is the "twice address translation problem".

More particularly, for example, when the terminal 105 calls the terminal 106, the MGC 201 commands AGWs 103 and 104 to create two pieces of context for both of the terminals. These pieces of context contain respectively the translated public addresses of the terminals 105 and 106, such as 200.1.1.1/3001 and 200.1.1.1/3002, as the addresses of traffic connection between the terminals 105 and 106. Here, AGWs 103 and 104 are allowed to know their translated public addresses and create context by those translated public addresses, by configuring AGWs 103 and 104. However, when two users could communicate with each other via the established RTP connection, the terminal 105 sends RTP traffic to the terminal 106 with the translated public address 200.1.1.1/3002 of the terminal 106. A traditional router 101 does not support the establishment of RTP connection between same addresses, that is to say, the router 101 can not forward packets for traffic connection between two AGWs 103 and 104 having translated addresses within the local network, thus resulting in disabling the transport of media stream for that traffic.

Figure 3 illustrates another example similar to the system of Figure 2, except that the two media gateways in the system of Figure 3 are controlled by a same signaling controller. In this system, an ISAM access system of ALCATEL is for example employed, which comprises two media gateway nodes 111 and 112 respectively connected to the terminals 105 and 106 as well as a signaling controller (IVPS) 113 controlling the two media gateway nodes 111 and 112. Here, assume that the private addresses of the two media gateway nodes 111 and 112 are respectively 192.168.2.2/3001 and 192.168.2.10/3002; the public address of the MGC 201 is 200.1.1.2; the public address and private address of the router 101 are respectively 200.1.1.1 and 192.168.2.1/24; and the private address of the signaling controller 113 is 192.168.2.100/24.

The above mentioned "twice address translation problem" exists also in the system shown in Figure 3. More particularly, when the terminal 105 calls the terminal 106, the MGC 201 commands the signaling controller 113 to create two pieces of context for these two terminals. This signaling controller informs the MGC 201 of the SDP messages of the terminals 105 and 106 with the translated public addresses 200.1.1.1/3001 and 200.1.1.1/3002 of the terminals 105 and 106. Then, the signaling controller configures the processor for the terminals 105 and 106, such as a digital signal processor (DSP), respectively with remote addresses 200.1.1.1/3002 and 200.1.1.1/3001. Once the two users could begin to communicate, the terminal 105 sends RTP traffic to the terminal 106 with the translated public address 200.1.1.1/3002 of the terminal 106. However, the traditional router 101 does not support the establishment of RTP connection between same addresses, that is to say, the router 101 can not forward packets for traffic connection between two AGWs 103 and 104 having translated addresses within the local network, thus resulting in disabling the transport of media stream for that traffic.

The existing routers can not address this problem in internal communication. The above technical problem exists not only in network environment applying H.248 but also in network environment applying other media gateway control protocol.

Summary of the Invention

To solve the above problem in the prior art, according to an aspect of the present invention, a method for establishing a call is proposed, wherein at least a first terminal and a second terminal are connected respectively via at least a first network element and a second network element to a first network which is in turn connected to a second network via a third network element that is address translation enabled, said first network and second network are packet networks, said method is adapted to establish a call between said first terminal and said second terminal which comprises: creating context for the call with translated addresses of said first network element and said second network element in said second network; determining if said first network element and said second network element in the context for the call are corresponding to the same translated address in said second network; and establishing a traffic connection for the call with respective addresses of said first network element and second network element in said first network if the addresses of said first network element and second network element in the context for the call are corresponding to the same translated address in said second network.

According to another aspect of the present invention, a signaling controller for controlling a network element to allow at least one terminal to access to a packet network is proposed, which comprises: a context creation unit configured to create context for a call when said at least one terminal (caller) establishes the call with another terminal (callee); a determination unit configured to determine if the addresses of the caller and the callee in the context for the call are corresponding to the same translated address; and a traffic connection controlling unit configured to control said network element to establish a traffic connection for the call with local addresses of said caller and callee when said determination unit determines that the addresses of the caller and the callee in the context for the call are correspond to the same translated address.

According to yet another aspect of the present invention, a network element for allowing at least one terminal to access to a packet network is proposed, which comprises the signaling controller according to the present invention.

According to yet another aspect of the present invention, a communication system is proposed, which comprises at least two terminals, at least two network elements according to the present invention respectively connected to said at least two terminals, and a controlling unit (MGC) for managing a call between said at least two terminals.

Brief Description of the Drawings

These and many other features and advantages of the present invention will become apparent from the following description of the embodiments of the present invention with reference to the drawings, wherein:

Fig.1 is a flow chart illustrating the process for establishing a call between two users by employing for example the H.248 protocol;

Fig.2 shows a system, in which telephone terminals access via respective access gateways to the local network which is in turn connected to the public network via the NAPT router;

Fig.3 shows another system, in which telephone terminals access via respective ISAM-V nodes to the local network which is in turn connected to the public network via the NAPT router and the above two ISAM-V nodes share a same signaling controller (IVPS);

Fig.4 is a flow chart illustrating the method for establishing a call according to an embodiment of the present invention;

Fig.5 is a flow chart illustrating the method for establishing a call according to another embodiment of the present invention;

Fig. 6 is a block diagram of the signaling controller according to an embodiment of the present invention;

Fig. 7 is a block diagram of the signaling controller according to another embodiment of the present invention; and

Fig.8 is a block diagram of the network element according to an embodiment of the present invention.

Detailed Description of the Invention

The present invention proposes a method for establishing a call. The method for establishing a call according to an embodiment of the present invention will be explained below with reference to Figure 4. The method of this embodiment is applied for example to the systems shown in Figures 2 and 3, and thus the description of the above systems will not be repeated herein.

As shown in Figure 4, firstly, in step 401 , context for a call is created with the translated addresses of the first and second network elements respectively connected to a caller and a callee. Here, the first network element may be for example the AGW 103 shown in Figure 2 or the ISAM node 111 shown in Figure 3, and the second network element may be for example the AGW 104 shown in Figure 2 or the ISAM node 112 shown in Figure 3. As mentioned above, in this embodiment, the first and second terminals are connected respectively via at least the first and second network elements to the local network (the first network) which is in turn connected to the public network (the second network) via a router (the third network element) that is address translation function enabled, both of the local network and public network being packet networks.

In this embodiment, the process for creating context for the call may adopt any existing or future solutions, standards, specifications etc., for example (but not limited to) the above mentioned H.248. More particularly, as for the system shown in Figure 3, for example, the terminal 105 dials to the terminal 106. After receiving the number dialed by the terminal 105, the MGC 201 commands the signaling controller 113 to create two pieces of context for the call respectively for the terminals 105 and 106.

The signaling controller 113 informs the MGC 201 of the SDP message of the terminal 105 with the translated public address 200.1.1.1/3001 of the media gateway 111 and of the SDP message of the terminal 106 with the translated public address 200.1.1.1/3002 of the media gateway 112. After that, the MGC 201 modifies the context for the terminals 105 and 106 respectively with the remote SDP addresses 200.1.1.1/3002 and 200.1.1.1/3001 within the signaling controller 113. The process for creating context for the call with respect to the system shown in Figure 2 is similar, and thus will not be repeated herein.

It should be noted that, in the step of creating context for a call with public addresses, the first network element and second network element (access gateway or IVPS etc.) could penetrate routers that are NAT (Network Address Translation) function enabled by means of various manners in order to communicate with the MGC with messages containing public addresses, such as (but not limited to) the following manners:

A first manner, as described above as prior art, makes it possible to configure the routers and access gateways to allow the routers to allocate public addresses to the AGWs 103 and 104 in advance, thus enabling the AGWs 103 and 104 to be aware of their translated public addresses and create context with those translated public addresses.

A second manner is to implement the ALG (Application Level Gateway) function on the routers. The ALG function allows for identifying specific application level protocols, such as H.248 or SIP. With this ALG function, the address information encapsulated in the data portion of a message could be read and modified, and the address in a message (or signaling) could be modified in accordance with the public address allocated actually to an access gateway.

Moreover, other NAT router penetration solutions could be adopted, such as MIDCOM (Middlebox Communications), STUN (Simple Traversal of UDP through NAT) or TURN (Traversal Using Relay NAT) etc.. However, these solutions cause change of network topology as well as deployment of extra MIDCOM/STUN/TURN servers.

It should be noted that, the above process for creating context is varied for different media gateway control protocols, and the present invention is not limited to the above described process for creating and modifying context.

In addition, the signaling controller of this embodiment may be implemented in software, hardware or a combination of them. It may be any types of signaling controlling system, such as interactive voice processing system (IVPS). This signaling controller may not only be incorporated in said media gateway, such as access gateway AGW or integrated access device etc., but also be separated from said media gateway.

Next, in step 402, a determination is made if the first and second network elements in the context for the call are corresponding to same translated public network address. More particularly, according to the present embodiment, the addresses of a caller and a callee in the context for the terminals 105 and 106 are compared with each other. If they have different addresses, we proceed to step 404 and continue the call according to a normal call processing flow, for example according to the above mentioned H.248 protocol.

If the addresses of the caller and callee in the context for the call are the same, i.e. the translated public addresses are the same, for example 200.1.1.1 , we determine that the call is one between telephone terminals within the local network, and proceed to step 403.

In step 403, a traffic connection for the call is established with the respective addresses of local network of the first and second network elements. More particularly, according to the present embodiment, for example, the AGWs 103 and 104 or ISAM nodes 111 and 112 establish a RTP connection for the call with their respective addresses of local network. Thereby, the caller and callee transmit media data respectively with the local addresses of their opposite party but not the public addresses informed by the MGC 201 , thus preventing the router 101 from discarding traffic packets.

Thereby, the problem that a call can not be normally handled and a traffic connection can not be established in a local network due to address translation may be solved by using the method for establishing a call according to the present embodiment, without modifying the existing routers, media gateway controller etc. as well as the existing protocols, standards etc..

Figure 5 is a flow chart illustrating the method for establishing a call according another embodiment of the present invention. The same parts of this figure are marked with the same reference numbers as Figure 4, and thus the description for the same parts will be omitted below for simplicity.

In the above embodiment shown in Figure 4, assume that the caller and callee are aware of the local addresses (and ports) of their opposite party. However, in practice, it is possible that the caller and callee are not aware of the local addresses (and ports) of their opposite party in advance. This embodiment aims at this kind of case, i.e. a mechanism for obtaining local address (and port) information is introduced on the basis of the embodiment shown in Figure 4.

As shown in Figure 5, steps 401 , 402, 403 and 404 of the present embodiment are the same as the embodiment of Figure 4. However, a further step 505 is introduced before step 403 of the present embodiment. In step 505, before establishing the traffic connection for the call, the respective addresses (IP addresses and ports) of local network of the first and second network elements are obtained.

More particularly, for example with reference to the system shown in Figure 2, the local addresses may be obtained by means of the following manners: the access gateways 103, 104 inquiring the router 101 for the private address of the terminal 106 with a public address 200.1.1.1/3002, and for the private address of the terminal 105 with a public address 200.1.1.1/3001. After obtaining the private addresses of remote users from the router 101 , the access gateways 103, 104 configure respectively processors, such as digital signal processor (DSP), for the terminals 105, 106 based on those private addresses, and establish a traffic connection for the call with the local addresses. Then, the terminal 105 transmits traffic media to the terminal 106 with the private address of the terminal 106, and vice versa.

Thereby, the problem that a call can not be normally handled and a traffic connection can not be established in a local network due to address translation may be solved by using the method for establishing a call according to the present embodiment, without modifying the existing routers, media gateway controller etc. as well as the existing protocols, standards etc.. Furthermore, the present embodiment introduces the mechanism for obtaining local addresses, which makes it possible to meet the situation where the local address of the opposite party can not be learned in advance.

Although obtaining local addresses is done by inquiringing the router 101 in the present embodiment, the present invention is not limited to this. For example, the respective local addresses may also be exchanged between the caller and the callee by transferring signaling or other communication ways. All the means for obtaining local addresses are applicable, and the present invention does not intend to limit on it.

Based on the same inventive concept, according to another aspect of the present invention, a signaling controller for controlling a network element to allow at least one terminal to access to a packet network is proposed. The signaling controller will be explained below with reference to Figure 6.

Figure 6 shows a signaling controller 600 according to an embodiment of the present invention. The signaling controller 600 comprises a context creation unit 601 , a determination unit 602 and a traffic connection controlling unit 603. The context creation unit 601 is configured to create context for a call when at least one terminal (caller) establishes the call with another terminal (callee), for example creating two pieces of context for a call while the terminals 105 or 106 initiates the call. The determination unit 602 is configured to determine if the addresses of the caller and the callee in the context for the call are corresponding to the same translated address, for example determining if the addresses of the caller and callee in the context for a call between the terminals 105 and 106 are same public address such as 200.1.1.1. If they are the same, it means that the call is one between two terminals within a local network; if they are not the same, the call will proceed according the normal call processing flow. The traffic connection controlling unit 603 is configured to control said network element to establish a traffic connection for the call with local addresses of said caller and callee when said determination unit 602 determines that the addresses of the caller and the callee in the context for the call are correspond to the same translated address (i.e. the call is one between two terminals within a local network). For example, when the determination unit 602 determines that the caller and callee have the same public address 200.1.1.1 in the context, a traffic connection for the call will be established respectively with the private addresses 192.168.2.2/3001 and 192.168.2.1. /3002 of the terminals 106 and 105.

In implementation, the signaling controller 600 of the present embodiment as well as the context creation unit 601, the determination unit 602 and the traffic connection controlling unit 603 it includes, may be implemented in software, hardware or a combination of them. For example, those skilled in the art are familiar with a variety of devices which may be used to implement these components, such as micro-processor, micro-controller, ASIC, PLD and/or FPGA etc.. The signaling controller of the present embodiment may be either implemented as integrated into the network element, or implemented separately, and each of the components of the signaling controller of the present embodiment may also be implemented separately physically but interconnected operatively.

In operation, said signaling controller for controlling a network element to allow at least one terminal to access to a packet network of the embodiment illustrated with reference to Figure 6, may implement the above described method for establishing a call. The problem that a call can not be normally handled and a traffic connection can not be established in a local network due to address translation may be solved by using the signaling controller, without modifying the existing routers, media gateway controller etc. as well as the existing protocols, standards etc..

Figure 7 shows a signaling controller 800 according to another embodiment of the present invention. The same parts of this figure are marked with the same reference numbers as Figure 6, and thus the description for the same parts will be omitted below for simplicity.

The signaling controller 800 of the present embodiment, which is similar to the signaling controller shown in Figure 6, comprises a context creation unit 601 , a determination unit 602 and a traffic connection controlling unit 603. In addition, the signaling controller 800 of the present embodiment comprises still a local address obtaining unit 801. The local addresses obtaining unit 801 is configured to obtain respective local addresses of said caller and said callee, for example inquiringing the router 101 for the local addresses of the terminals 106 or 105 with public addresses 200.1.1.1/3002 or 200.1.1.1/3001 when the determination unit 602 determines that the caller and callee have the same public address 200.1.1.1. The traffic connection controlling unit 602 then establishes a traffic connection for the call with the local addresses of the caller and callee obtained by the unit 801 for obtaining local addresses.

Similarly to the preceding embodiment, the signaling controller 800 of the present embodiment as well as the context creation unit 601, the determination unit 602, the traffic connection controlling unit 603 and the unit 801 for obtaining local addresses it includes, may be implemented in software, hardware or a combination of them. For example, those skilled in the art are familiar with a variety of devices which may be used to implement these components, such as micro-processor, micro-controller, ASIC, PLD and/or FPGA etc.. The signaling controller of the present embodiment may be either implemented as integrated into the network element, or implemented separately, and each of the components of the signaling controller of the present embodiment may also be implemented separately physically but interconnected operatively.

In operation, said signaling controller for controlling a network element to allow at least one terminal to access to a packet network of the present embodiment, may implement the above described method for establishing a call. The problem that a call can not be normally handled and a traffic connection can not be established in a local network due to address translation may be solved by using the signaling controller, without modifying the existing routers, media gateway controller etc. as well as the existing protocols, standards etc.. Moreover, the present embodiment introduces also a mechanism for obtaining local addresses to meet the situation where the local address of the opposite party can not be learned in advance. Based on the same inventive concept, according to yet another aspect of the present invention, a network element for allowing terminals to access to packet networks is proposed.

Figure 8 shows a network element 700 according to an embodiment of the present invention, including the signaling controller 600 as shown in Figure 6 and a traffic connection implementing unit 701. The network element 700 is adapted to allow at least one terminal to access to a packet network.

It should be noted that, although not shown in Figure 8, the shown network element 700 may also include the signaling controller 800 as shown in Figure 7, as well as various variants as described in the preceding embodiments.

In implementation, the network element 700 of the present embodiment as well as the signaling controller 600 and the traffic connection implementing unit 701 it includes, may be implemented in software, hardware or a combination of them. For example, those skilled in the art are familiar with a variety of devices which may be used to implement these components, such as micro-processor, micro-controller, ASIC, PLD and/or FPGA etc.. Each of the components of the network element 700 of the present embodiment may be either implemented as integrated together or implemented separately, and may also be implemented separately physically but interconnected operatively.

In operation, the network element 700 of the present embodiment, may implement the above described method for establishing a call. The problem that a call can not be normally handled and a traffic connection can not be established in a local network due to address translation may be solved by using the network element 700, without modifying the existing routers, media gateway controller etc. as well as the existing protocols, standards etc.. Further, the situation where the local address of the opposite party can not be learned in advance could be met by incorporating the signaling controller 800 as shown in Figure 7 into the network element of the present embodiment.

In practice, a network element for allowing terminals to access to packet networks usually includes a part for controlling signaling and a part for processing media. Although examples are made above with respect to the ISAM node and IVPS, the present invention is not limited to them. The signaling controller 800 and the network element 700 of the above described embodiments of the present invention may be implemented in various ways, include various variants and combine with other functions, which would not obstruct the implementation of technical solution of the present invention.

Based on the same inventive concept, according to yet another aspect of the present invention, a communication system is proposed. This communication system includes at least two terminals, at least two network elements as described in the preceding embodiment respectively connected to said at least two terminals, and a controlling unit for managing a call between said at least two terminals. Furthermore, said communication system may also include other network elements, such as routers etc..

For example, in the present embodiment, a traffic call may be established between a caller and a callee whether the terminals of the caller and callee intervene a packet network through network elements within the local network or not. The particular operation flow could refer to the above description about the method for establishing a call according to the embodiments of the present invention, and will not be repeated herein. Although the exemplary embodiments of the method for establish a call, the signaling controller for controlling a network element to allow terminals to access to packet networks, the network element for allowing terminals to access to packet networks and the communication system including at least two terminals and network elements and the media gateway controller are described above in detail, the above embodiments are not exhaustive, and those skilled in the art can make numerous changes and modifications within the spirit and scope of the present invention. Therefore, the present invention is not limited to those embodiments, the scope of which is defined only by the appended claims.

Claims

1. A method for establishing a call, wherein at least a first terminal and a second terminal are connected respectively via at least a first network element and a second network element to a first network which is in turn connected to a second network via a third network element that is address translation enabled, said first network and second network are packet networks, said method is adapted to establish a call between said first terminal and said second terminal, comprising:

- creating context for the call with translated addresses of said first network element and said second network element in said second network,

- determining if said first network element and said second network element in the context for the call are corresponding to the same translated address in said second network, and

- establishing a traffic connection for the call with respective addresses of said first network element and second network element in said first network if the addresses of said first network element and second network element in the context for the call are corresponding to the same translated address in said second network.

2. The method for establishing a call according to claim 1 , wherein the step of creating context for the call comprises creating context for the call by means of a controlling unit (MGC) for managing call, which is connected to said second network.

3. The method for establishing a call according to claims 1 or 2, wherein said traffic connection for the call conforms to the real time transport protocol.

4. The method for establishing a call according to any of claims 1-3, wherein said first network element and second network element are controlled by a single signaling controller.

5. The method for establishing a call according to any of claims 1-4, wherein said first network element and said second network element are controlled respectively by two signaling controllers.

6. The method for establishing a call according to any of claims 1-5, further comprising: obtaining respective addresses of said first network element and second network element in said first network before establishing the traffic connection for the call.

7. A signaling controllerer for controlling a network element to allow at least one terminal to access to a packet network, characterized by comprising:

- context creation unit configured to create context for a call when said at least one terminal (caller) establishes the call with another terminal (callee),

- determination unit configured to determine if the addresses of the caller and the callee in the context for the call are corresponding to the same translated address, and

- traffic connection controlling unit configured to control said network element to establish a traffic connection for the call with local addresses of said caller and callee when said determination unit determines that the addresses of the caller and the callee in the context for the call are correspond to the same translated address.

8. The signaling controller according to claim 7, wherein said context creation unit is configured to create the context for the call by means of a controlling unit (MGC) for managing call.

9. The signaling controller according to claims 7 or 8, further comprising a local address obtaining unit configured to obtain respective local addresses of said caller and said callee.

10. The signaling controller according to any of claims 7-9, wherein said traffic connection for the call conforms to the real time transport protocol.

11. The signaling controller according to any of claims 7-10, wherein a plurality of said network elements are controlled by a single signaling controller.

12. The signaling controller according to any of claims 7-11 wherein a plurality of said network elements are controlled by their respective signaling controllers.

13. A network element for allowing at least one terminal to access to a packet network, characterized by comprising the signaling controller according to any of claims 7-12.

14. A communication system, comprising: - at least two terminals,

- at least two network elements according to claim 13 respectively connected to saidst two terminals, and

- controlling unit (MGC) for managing a call between said at least two terminals.