US 7876743 B2
A method of setting up a session between peer user terminals of a communication system, wherein at least one of the user terminals is a dual-mode terminal having access to both an IP-based packet-switched access network and a circuit-switched access network, and the session extends at least in part across the circuit-switched access network. One of the user terminals initiates the session by sending Session Initiation Protocol (SIP) signaling to a SIP server over the IP-based network. The SIP server sends a notification to a gateway server to establish one or more conversational bearers. The gateway server then sends a callback telephone number to the dual-mode terminal. The dual-mode terminal then calls the callback number to establish a circuit-switched session with the gateway server.
1. A method of setting up a session between first and second peer user terminals of a communication system, wherein at least one of the peer user terminals is a dual-mode mobile terminal having access to an Internet Protocol (IP) based packet switched network and a circuit switched access network, said session extending at least in part across the circuit switched access network, the method comprising the steps of:
transporting signaling to initiate the session via the IP based packet switched network using a call control protocol which is also used for setting up end-to-end packet switched sessions, the signaling being Session Initiation Protocol (SIP) signaling exchanged between the dual mode mobile terminal and a SIP server of an IP Multimedia Core Network Subsystem (IMS);
sending a notification from the SIP server to a gateway server that one or more conversational bearers must be established, the gateway server terminating the circuit switched session within the system;
after notification from the SIP server, sending from the gateway server to the dual-mode mobile terminal, a callback telephone number; and
calling the callback telephone number by the dual-mode mobile terminal to establish a circuit switched call with the gateway server.
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properties of the system known to the SIP server;
prior notification by at least one of the peer user terminals;
information contained in the SIP signaling initiating the packet switched session;
properties defined for at least one of the peer user terminals;
prior notification from a visited network if one of the peer user terminals is roaming; and
prior notification from the packet switched access network used by one of the peer user terminals.
10. A Session Initiation Protocol (SIP) server for use in an Internet Protocol (IP) Multimedia Core Network Subsystem, the server comprising:
means for receiving an INVITE request from a dual-mode user terminal, over an IP based packet switched domain, initiating a packet switched session;
means for determining that the packet switched session requires setting up of at least one circuit switched conversational bearer over a circuit switched access network serving the dual-mode user terminal;
means for sending a notification from the SIP server to a gateway server that one or more conversational bearers must be established, the gateway server terminating the circuit switched session within the system; and
means for receiving a callback telephone number from the gateway server and for forwarding the callback number to the dual-mode terminal.
11. A gateway server for providing an interface between a circuit switched access network and a packet switched network, the gateway server having an interface towards a Session Initiation Protocol (SIP) server of an Internet Protocol (IP) Multimedia Core Network Subsystem, said gateway server comprising:
means for receiving from the SIP server, signaling instructing the setting up of a circuit switched call over the circuit switched access network with a dual-mode user terminal;
means for sending a callback number to the dual-mode user terminal in response to receipt of the signaling; and
means for terminating a call made to the callback number by the dual-mode user terminal.
This application claims the benefit of the filing date as provided in 35 U.S.C. 119 of Great Britain patent application number GB 0303516.9 filed on Feb. 15, 2003, the disclosures of which are incorporated herein by reference.
The present invention relates to negotiating the setting up of conversational bearers in communication networks, which bearers can be used, for example, to carry real time voice and video information.
Digital cellular telephone networks have traditionally relied upon circuit switched channels to carry user traffic such as voice communications. A circuit switched channel is formed by the allocation of one slot per frame in a given TDMA channel. Whilst circuit switched sessions have proved adequate for voice calls, they do not provide an efficient mechanism for transferring large amounts of data which is “bursty” in nature. For example, the setting up of a circuit switched session to download a web page from a web server is likely to result in the connection remaining idle for significant amounts of time, and being overloaded when there is data to transmit.
To facilitate fast data transfers to mobile terminals, packet switched data services are being introduced to digital cellular telephone networks. For example, the General Packet Radio Service (GPRS) is currently being introduced to many GSM networks. Unlike circuit switched sessions, a GPRS session (referred to as a PDP context) for a given user does not necessarily occupy one slot per frame on a given TDMA channel. Rather, slots are only used when the user has data to send or receive. When there is no traffic to transmit, no slots are allocated to the user. When there is a large volume of data to transmit, the user may be allocated one or more slots per frame.
GPRS will be available in future third generation networks such as 3G networks which will rely upon CDMA rather than TDMA. 3G networks will however continue to provide circuit switched services at least for the foreseeable future, although these sessions will not necessarily be end to end. Rather, the links between mobile terminals and the networks will be circuit switched, with data being routed within and between networks via high capacity packet switched networks (which have sufficient bandwidth to handle real time traffic).
It is envisaged that in the future, the packet switched (access) domain will be able to carry real time information streams, for example relating to voice and video telephony.
However, at present the transmission reliability of GPRS is not sufficient to provide users with telephony services of the quality which they will expect, hence the continued provision of circuit switched services (the provision of circuit switched services is also likely to be necessary by the need to continue to service older mobile terminal equipment which is not GPRS enabled).
To facilitate the provision of multimedia services via the packet switched “domain”, the 3rd Generation Partnership project (3GPP) responsible for the 3G standards has been developing a so-called IP Multimedia Core Network Subsystem (IMS). IMS communicates with the GPRS core network and contains all elements that are used to provide IP based multimedia services. For a mobile to mobile call, and assuming the mobiles belong to different networks, an IMS will be provided in each mobile's home network. Each IMS is connected to the GPRS core network of its home network. The base protocol for multimedia services is the IETF Session Initiation Protocol (SIP). SIP makes it possible for a calling party to establish a packet switched session to a called party (using so-called SIP User Agents, UAs, installed in the user terminals) even though the calling party does not know the current IP address of the called party prior to initiating the call. SIP provides other functionality including the negotiation of session parameters (e.g. Quality of Service and codecs).
Translation between circuit switched and packet switched data is performed by an interworking gateway (GW), with the GW establishing the packet switched session to the PC using SIP signalling. Assuming that the packet switched network used by the PC has sufficient bandwidth (e.g. the network is a broadband network), the call will provide the users with a sufficient level of quality for voice and video. In this scenario, the IMS of the home operator's network is not used.
In addition to the need for the destination terminal to have allocated to it a telephone number, a further disadvantage of the architecture of
It is likely that users will prefer to initiate and receive circuit switched and packet switched calls using a common signalling interface. However, under the current proposals, a user would initiate and receive a packet switched call using SIP, e.g. to initiate a packet switched call the user would enter the SIP address for the called party (e.g. email@example.com), whilst he/she would initiate and receive a circuit switched call using the DTAP protocol, e.g. to initiate such a call the user would dial the called party's telephone number (e.g. 012345 . . . ). Network operators would also prefer to use a common signalling interface as this will ease the migration of circuit switched services to the packet switched domain, when that domain has evolved sufficiently to provide the required services.
According to a first aspect of the present invention there is provided a method of setting up a session between peer user terminals of a communication system, said session extending at least in part across a circuit switched access network, the method comprising transporting signalling to initiate said session between at least one of the peer user terminals and said communication system via an IP based packet switched access network using a call control protocol which is also used for setting up end-to-end packet switched sessions, and subsequently establishing said session based upon said signalling.
The term “communication system” is used here to indicate either a single communication network or a collection of networks such as the worldwide telecommunications system made up of various national and international networks.
The signalling which initiates said session in the circuit switched domain may do so indirectly. That is to say that this signalling is interpreted by an appropriate node of the communication system as requiring a circuit switched session. The session is then initiated directly by signalling in the circuit switched domain. Said at least one of the peer user terminals may be unaware in the first instance that a circuit switched session is required.
Embodiments of the present invention have the advantage that session initiation signalling relating to both the circuit switched domain and the packet switched domain can be carried exclusively over the packet switched access network.
It is anticipated that the invention will be particularly applicable where the session being initiated requires one or more conversational bearers, although this need not be the case. Said session may be enhanced by one or more media streams not requiring conversational bearers and establish through the packet access network, e.g. where the session requires one or more conversational bearers, but additionally, is complemented with one or more non-conversational bearers (instant messaging, gaming, etc.). These bearers are established through the packet access network/domain.
Preferably, at least one of the peer user terminals is a dual mode mobile terminal capable of using both said packet switched and circuit switched access networks.
Preferably, said call control protocol is Session Initiation Protocol, SIP, with SIP signalling being exchanged between said at least one of the peer user terminals and a SIP server of an IP Multimedia Core Network Subsystem (IMS). More preferably, said SIP server notifies a gateway server when it receives a session initiation request which requires the establishment of one or more conversational bearers, the gateway terminating the circuit switched session within the system. The gateway provides interworking between the circuit switched session on the one side, and the packet switched session on the other side. The SIP server and the gateway server may be physically co-located.
Typically, following notification from the server, the gateway notifies said at least one of the peer user terminals, via the SIP server, of a callback telephone number, and the peer user terminal calls that number to establish a circuit switched session with the gateway. The gateway maps the established circuit switched session to the SIP signalling session on the basis of the used callback number. Preferably, the gateway selects the callback number from a pool of available callback numbers.
Preferably, the SIP server determines that said session requires the establishment of a circuit switched session as a result of one or more of the following:
Preferably, at least one of the peer user terminals maps the established circuit switched session to the signalling session over the packet switched domain, such that both session can be terminated together.
In an embodiment of the present invention, said peer user terminals are mobile wireless terminals, said at least one of the peer user terminals being a dual mode mobile terminal capable of using both said packet switched and circuit switched access networks. The other mobile terminal may similarly be a dual mode terminal, or may be a single mode terminal capable of accessing only a circuit switched access network (or indeed only a packet switched access network). In other embodiments of the invention, said at least one of the peer user terminals may be a fixed line terminal, e.g. with a broadband link to said communication system providing both packet switched and circuit switched access. Of course other configurations are possible, e.g. said at least one of the peer user terminals is a dual mode wireless terminal and the other user terminal is a fixed line terminal.
SIP is one example of a call control protocol for setting up sessions over the packet switched and circuit switched networks and which is transported over the packet switched network. Other call control protocols such as H.323 may be used.
According to a second aspect of the present invention there is provided a user terminal comprising means for using a circuit switched access network and means for using an IP based packet switched access network, and means for transferring signalling information, using a call control protocol which is also used for setting up end-to-end packet switched sessions, over the packet switched network to initiate a session over the circuit switched network.
According to a third aspect of the present invention there is provided a Session Initiation Protocol server for use in an IP Multimedia Core Network Subsystem, the server comprising:
According to a fourth aspect of the present invention there is provided a gateway server for providing an interface between a circuit switched access network and a packet switched network, the gateway having an interface towards a Session Initiation Protocol server of an IP Multimedia Core Network Subsystem, and means for receiving from the SIP server signalling instructing the establishment of a session over the circuit switched access network with a user terminal.
Typical call session scenarios in existing and proposed telecommunication networks have been described above with reference to
Within the GPRS network 3, two nodes relevant to the UE 1 can be identified. These are the Serving GPRS Support node (SGSN) 6 and the Gateway GPRS Support Node (GGSN) 7. The role of the SGSN 6 is to maintain subscription data—identities and addresses—and to track the location of the UE 1 within the network. The role of the GGSN 7 is to maintain subscription information and allocated IP addresses and to track the SGSN to which the UE 1 is attached. The GGSN 7 is coupled to an IP backbone network 8 (the SGSN is also coupled to the IP network 8, although this session is not shown in the Figure—communication between nodes of the GPRS network, including the GGSN and the SGSN, and between gateway nodes of the UTRAN and the GPRS network, will take place via the IP network 8). Typically, when the UE 1 is turned on it “attaches” itself to the GGSN and a PDP context is established between the UE 1 and the GGSN 7. This context provides a “pipe” for transporting data from the UE 1 to the GGSN 7. This process involves the allocation of an IP address to the UE 1. Typically, the routing prefix part of the address is a routing prefix allocated to the GGSN 7.
Also illustrated in
The S-CSCF 11 performs the session control services for the UE, and maintains a session state as needed by the home network operator for support of services. The main function performed by the S-CSCF 11 during a session is the routing of incoming and outgoing call set-up requests. The main function performed by the P-CSCF 10 is to route SIP messages between the UE 1 and the IMS 9 of the home network 2.
Following GPRS attach by the UE 1, the UE must “discover” the identity (i.e. IP address) of the P-CSCF which it should use. This is done using one of the following mechanisms:
The UE 1 will then notify the S-CSCF 11 of its current location, i.e. the IP address allocated by the GGSN, via the P-CSCF 10 (this process requires authentication of the UE 1 to the S-CSCF and vice versa and makes use of the unique subscriber identity). The S-CSCF 11 makes this information available to a Home Subscriber Server 12 which is used to route subsequent incoming calls to the UE 1.
The S-CSCF 11 verifies the rights of the UE 1 (or rather the subscriber using the UE 1) to use the requested service which is identified in the INVITE message. The S-CSCF 11 must then identify the IP address of the UE 13. It does this by using a look-up table mapping SIP addresses to IP addresses. For a given SIP address, the table provides the IP address of the “home” network of the corresponding subscriber. The identified IP address is used to forward the INVITE message to the S-CSCF 11 b in the B-subscriber's home IMS network 9 b. Using the SIP address contained in the INVITE message, the S-CSCF 11 b identifies the current IP address of the UE 13, and forwards the INVITE message to that address. Upon receipt of the INVITE message, and assuming that the UE 13 answers the call, an OK message is returned to the UE 1. Typically this message is sent via the two S-CSCFs 11,11 b. In order to confirm that the OK message is correctly received by the UE 1, that UE will upon receipt of the message return an ACK message to the peer UE 13. If UE 13 does not receive an ACK message within some predefined time period, it will retransmit the OK message.
As well as its use in establishing PS sessions between mobile terminals, SIP may also be used to establish PS sessions between mobile and fixed terminals and between only fixed terminals. For example, SIP may be used to establish a PS session between a mobile subscriber and a fixed terminal which has a broadband session to the Internet.
As mentioned above, the quality of the packet switched links between the UEs 1,13 and the respective UTRANs may be such that these links are not suitable for transporting real time conversational data, such as voice and video data associated with a call, between the two peer UEs or between one of the UEs and a fixed terminal. Thus, it may be necessary to establish a circuit switched session between the or each UE 1,13 and its circuit switched core network 4,4 b. The following mechanism is used to establish these circuit switched sessions.
A UE is assumed to have a PS domain session to the IMS of its home network, and the UE is registered with the IMS domain. The SIP UA of the UE has already informed its IMS SIP server (which will typically be the S-CSCF of the home network, but could be a P-CSCF of a visited network), e.g. during registration, that conversational bearers should not be established over the PS domain, and that the SIP UA will use the CS domain for such bearers (this requirement may be a default setting for the UE). However, the PS domain and the SIP server are used to convey signalling to set up the conversational bearers over the CS domain. It will be understood that, rather than the UE signalling to the SIP server that conversational bearers should be set up over the CS domain, this requirement may already be know to the SIP server (e.g. it could be a “property” defined for the subscriber), or the SIP server may be informed of the requirement by a visited network used by the UE as its access network.
This procedure is illustrated in
(The SIP server and the gateway may, in some implementations, be physically co-located, either in the home network of a subscriber or in a visited network.)
Following receipt by the SIP server 26 of a SIP INVITE from the UE 20 requesting the setting up of a conversational bearer, the gateway 25 is notified and allocates a “call-back” number to the session. The UE 20 is informed of this number and calls it to established a CS session to the gateway 25. SIP is used to establish the session between the gateway 25 and the called party 24. This procedure is defined in more detail as follows, from the point of view of the mobile terminal as initiating terminal, with reference to the signalling diagram of
The gateway is able to associate the incoming call with the previous SIP dialogue based on the used callback number.
The sequence described above is exemplary only, and may modified as required. In addition, whilst it has been assumed that no pre-existing session has been established between the originating UE and the called party, the procedure is also applicable when such a session exists prior to the establishment of a conversational bearer.
It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention. For example, in an alternative solution the SIP server itself determines the call back number and sends this to the UE. When the UE calls to the number, the call is routed to the gateway, which then establishes a SIP call to the SIP server (notice that this is an additional SIP session from SIP server point of view). The SIP server collects the IP address and port number allocated by the gateway from that SIP signalling and modifies the SDP in the INVITE. However, a disadvantage of this approach is that the INVITE cannot be sent to the terminating side before the CS call is received from the originating UE as it will not know the allocated IP address and port number until the call is received. Also the logic in the SIP server may be complex.