WO2001006718A1

WO2001006718A1 - Variable bit rate in circuit switch

Info

Publication number: WO2001006718A1
Application number: PCT/SE2000/001434
Authority: WO
Inventors: Erik Mikael Walles; Bengt Gunnar Larsson; Carl Staffan ÅLUND; Lars-Göran Petersen
Original assignee: Telefonaktiebolaget Lm Ericsson
Priority date: 1999-07-19
Filing date: 2000-07-06
Publication date: 2001-01-25
Also published as: AU6192700A; SE9902738D0; GB2367450A; DE10084803T1; GB0200398D0; DE10084803B4; GB2367450B; GB2367450A8; SE9902738L

Abstract

The present invention present a telecommunication system comprising a circuit switch (1), through which variable bit rate traffic is sent utilising a circuit-switch connection (9). The connection (9) is established between an interworking unit (6) and a pooled functionality device (8) of the circuit switch (1), and may be of a narrow band, wideband or broadband type. Variable bit rate packets received in the interworking unit (6) are mapped to an internal format and sent through the connection (9). Any signalling messages are terminated and used for handling the connections (9) and, if the connection (9) is of a wideband or broadband type, reserving channels within the connection (9) for different calls or sessions. The variable bit rate traffic received at the pooled functionality device (8) is typically demultiplexed according to the channel number and synchronized, which transfers them into a constant bit rate fashion. Similarly, constant bit rate traffic may be packetized and sent over the connection (9) to the interworking unit (6) according to the internal variable bit rate format. ATM networks and Internet networks are two particularly suitable variable bit rate networks for such solutions.

Description

VARIABLE BIT RATE IN CIRCUIT SWITCH

TECHNICAL FIELD

The present invention relates to methods and devices for telecommunication switching, and in particular to telecommunication systems handling both variable bit rate traffic and constant bit rate traffic.

BACKGROUND

Information is transmitted between different actors in a telecommunication system in different transfer modes. In traditional telephony, a fixed connection is arranged for each call. A "circuit" is thus established between the subscribers, and the transfer mode is subsequently named circuit switching. In traditional telephony, a 64 kbps connection is maintained as long as the call lasts. The communication is thus of a constant bit rate. On the contrary, e.g. data communication and mobile telephone communication use different types of packet, frame relay or cell switching modes. Common for these transfer modes is that the information is transported in separate units. Normally, there is no reservation of switched connections involved.

During periods of no activity, no information is sent. The transmission is performed with a variable bit rate, based on the momentary demand.

A tendency of the development is that variable bit rate solutions become more common, especially when employing Internet technology or when introducing the third generation mobile telephony. However, the present circuit switched networks are well established and represent large investments in both technology and money. When using variable bit rate traffic, new networks have to be developed, or gateways between variable and constant bit rate networks have to be developed.

According to prior art, there are solutions where packet switches or routers are used for switching the information. When the packets are directed to circuit switched networks, an interworking unit is used to transfer the variable bit rate traffic into constant bit rate traffic, which is directed out on a circuit switched network.

In the patent application GB 2 315 190 A, an Internet telephony gateway for providing an interface between a circuit switched and packet switched network is disclosed. A circuit switch is placed between the PSTN network and the Internet network.

In the published international patent application WO 94/ 11975, a method for establishing telecommunication call paths in broadband communication networks is disclosed. This illustrates a general position of the prior art for an interface between circuit switched traffic with constant transmission rate and packet/ cell- mediated traffic with variable transmission rate.

Such solutions, however, present difficulties with scaling capability and modularity. Furthermore, different kind of functionality, such as different telephony services, multiplexing, speech coding etc. has to be re-developed for the new devices. Furthermore, when the variable bit rate traffic increases on expense of the constant bit rate traffic, there might be unused capacity in the established constant bit rate networks.

SUMMARY

A general object of the present invention is to provide a gateway between variable bit rate and constant bit rate networks, which requires a minimum of new development. Another object of the present invention is to utilize existing capacity of telecommunication systems. A further object of the present invention is to provide a method of handling variable bit rate traffic in a circuit switch. Yet another object of the present invention is to provide a gateway solution, which is flexible when designing or expanding telecommunication systems. The above objects are achieved by the attached claims.

In general terms, the present invention presents a telecommunication system comprising a circuit switch, through which variable bit rate traffic is sent utilising a circuit- switch connection. The connection is established between an interworking unit and a pooled functionality device of the circuit switch, and may be of a narrow band, wideband or broadband type. Variable bit rate packets received in the interworking unit are mapped to an internal format and sent through the connection. Any signalling messages are terminated in the interworking unit and used for handling the connections and, if the connection is of a wideband or broadband type, the interworking unit reserves channels within the connection for different calls or sessions. The variable bit rate traffic received at the pooled functionality device is typically demultiplexed according to the channel number and synchronized, which transfers them into a constant bit rate fashion. Similarly, constant bit rate traffic may be "packetized", i.e. packed into packets, and sent over the connection to the interworking unit according to the internal variable bit rate format. ATM networks and Internet networks are two particularly suitable variable bit rate networks for such solutions.

In this manner, the resources of a circuit switch and pooled functionality devices attached thereto, are possible to use for variable bit rate applications. The reuse of available well tested systems may, at least in a first phase, increase the development of integrated variable bit rate and constant bit rate networks.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by making references to the following description taken together with the accompanying drawings, in which:

FIG. 1 is block diagram of a telecommunication system according to the present invention; FIG. 2 is a block diagram of an embodiment of an interworking unit according to the present invention;

FIG. 3 is a block diagram of an embodiment of a pooled functionality device according to the present invention; FIG. 4a-d are schematic drawing of different embodiments of the present invention;

FIG. 5 is a schematic drawing of a mobile telephony network;

FIG. 6 is a level diagram of an embodiment according to the present invention with an ATM network; FIG. 7 illustrates schematically a mapping process in an embodiment of the present invention;

FIG. 8 illustrates schematically a synchronising process in an embodiment of the present invention;

FIG. 9 is a schematic drawing of an embodiment of the present invention connected to an Internet network;

FIG. 10 is a level diagram of an embodiment according to the present invention with an Internet network;

FIG. 11 is a flow diagram illustrating a process according to the present invention; and FIG. 12 is a flow diagram illustrating another process according to the present invention.

DETAILED DESCRIPTION

In the following detailed description, the basic general ideas of the invention are first discussed on a general level. These general ideas are applicable to a number of more specific telecommunication systems. Therefore, descriptions of a couple of embodiments, concerning specified systems, where the general ideas are particularly well suited, follow.

In fig. 1, a general telecommunication system according to the present invention is illustrated. A circuit switch 1, normally used for switching constant bit rate traffic in a constant bit rate network 2 is equipped with an exchange terminal 4, which handles the traffic to and from the constant bit rate network 2. The traffic in the trunk connection 5 to the exchange terminal 4 is of a constant bit rate type of voice, video or circuit switched data packets. To the circuit switch 1 , a number of additional functionalities are also normally supplied, such as speech transcoding, speech recognition, rate adaptation, multi-party functionalities, mailbox, speech announcement, forwarding, DTMF signalling etc. Such functionalities are often implemented as pooled functionality devices 8 publicly available for all connections switched through the circuit switch 1. According to the invention, a variable bit rate network 3 is connected to the circuit switch 1 through an interworking unit 6. Variable bit rate traffic in the format of packets, frames or cells is transferred to and from the interworking unit 6. The traffic in the trunk connection 7 to the interworking unit 6 is thus of a variable bit rate type of processed voice or data packets.

"Variable bit rate traffic" refers in this description to different kinds of packet, frame or cell distributing modes.- The most common examples are e.g. X.21, X.25, frame relay, IP and ATM, including AAL1, AAL2 and AAL5. Other less common techniques such as DQDB (distributed queue dual bus) as well as combinations such as IP/ATM, IP/PPP, X.25/ATM, IP/SDH,

IP/WDM (Wavelength Division Multiplexing) are also possible to use together with the present invention. Some adjustments have to be performed for running different variable bit rate traffic according to the present invention, but the basic ideas apply to all different variable bit rate networks.

Variable bit rate traffic of different kinds has certain features in common, distinguishing it from constant bit rate traffic. One obvious feature is the asynchronous information flow or variable bit rate. The information may be transported in packets of constant or varying sizes, and may be sent at a predicted or varying pace. The traffic information rate corresponding to a certain telephone call or data connection is, however, basically varying. In constant bit rate traffic, the information may be contained in packets, e.g. in time slots, but the slot size and duration between subsequent time slots corresponding to the same call is constant. This means that although the instant bit rate may vary, the average bit rate, for times longer than the time between two time slots of one and the same channel, is constant.

Variable bit rate traffic is also normally provided with addressing information, typically provided as a header of the packets or cells. Such addressing information is not present in normal circuit switched constant bit rate traffic, since connections are established for as long the call or data session lasts, and e.g. assigned time slots are reserved for a certain connection. The assignment of the time slot is typically the only "addressing" feature for the bulk traffic information of constant bit rate traffic.

In variable bit rate traffic, signalling information is typically contained in data packets of a similar type as for the data traffic itself. Data packets with signalling information are provided to parties concerned, but are treated elsewhere as any other data packets. In constant bit rate traffic, the signalling is often separately performed e.g. in the beginning and the end of a call or session to establish the connection. The signalling may also be performed via separate channels. The signalling activities are thus easily distinguishable from normal data traffic.

In the case of incoming traffic of variable bit rate to the system illustrated in fig. 1, the interworking unit 6 receives the variable bit rate traffic and transforms the packets into e.g. frames of an internal system variable bit rate (VBR) format. Signalling data, if present, is extracted and used for controlling further signalling. The addresses of the variable bit rate traffic are interpreted in the view of the signalling traffic, to be used for assigning e.g. channel numbers through the circuit switch 1. A connection 9 in the circuit switch 1 from the interworking unit 6 to a pooled functionality device 8, is established according to certain signalling information and/ or address information extracted from the data packets or achieved in other ways, described below. This circuit- switch connection 9 is preferably a semipermanent connection, which in a typical case is established during periods far exceeding a typical single call. However, the circuit-switched connection 9 may also be of a dynamic type controlled by the present need. The control of the connection is performed by call control means 11 , preferably connected to the interworking unit 6, the circuit switch 1 and the pooled functionality device 8. Over this circuit- switch connection 9, the frames of the internal system VBR format are transferred. The capacity of the connection 9 is therefore constant, while the actual traffic in the form of frames still is of a variable bit rate type. The variable bit rate traffic over the circuit- switch connection 9 involves, however, no signalling or call control data, and the address data is in a typical case reduced to a single channel number.

The pooled functionality device 8 receives the frames of the internal system VBR format and extracts the traffic information belonging to a certain telephone call or data session and brings it to a functionality unit assigned to that call or session. The pooled functionality device 8 processes the extracted traffic information basically according to its normal function. Since the received traffic information is of a variable bit rate type, also a synchronisation process has to be performed, at least if the traffic is intended to be forwarded to a constant bit rate network. Such a process may, depending on the actual pooled functionality device 8, be performed before, during or after the normal function of the pooled functionality device 8. By this synchronisation, the last remainder of the variable bit rate character disappears, and the information may now be considered as constant bit rate information.

Anyone skilled in the art easily understands that the conversion from variable bit rate to constant bit rate traffic is performed in a distributed manner. A first part is performed in the interworking unit 6, and a second part is performed in the pooled functionality device 8. The traffic through the circuit- switch connection 9 has a variable bit rate but lacks the typical elements of addressing and signalling. If the processed traffic information of the pooled functionality device 8, which now is of a constant bit rate character, is supposed to be sent further, the processed traffic information is formatted in a constant bit rate manner and sent through the circuit switch 1 once more. The information is sent via a normal connection 10 in the circuit switch 1 to an exchange terminal 4 for access to the constant bit rate network 2. The connection 10 is thus used for traditional constant bit rate traffic. Such switching is assumed to be well known in prior art and is thus not further discussed here.

In some cases the processed traffic information is intended to be sent on to a variable bit rate network. In such a case, the continuation will essentially be a reversal of the incoming procedure. The final variable bit rate network 3, may be of a similar of different kind, compared with the original one. Such special cases are described more in detail further below.

In the case of outgoing traffic to the variable bit rate network 3, the pooled functionality device 8 contains traffic information ready to be sent to the variable bit rate network 3. Such information may in a typical case be information originating from an exchange terminal 4 and processed in the pooled functionality device 8. A connection 9 from the pooled functionality device 8 to the interworking unit 6 is established and according to signalling information associated with the information channel numbers are assigned. The traffic information is packaged and formatted as a frame according to the internal system VBR format and then transferred over the circuit- switch connection 9 to the interworking unit 6. The traffic transferred over the circuit- switch connection 9 has generally a variable bit rate, even if the bit rate in some special occasions may be constant.

The interworking unit 6 transfers the information from the internal system VBR format to the variable bit rate format handled by the variable bit rate network 3. Thereby, addressing information is added, based on the circuit switch connection information and signalling information from the circuit switch 1. Furthermore, in a typical case, signalling messages are added as packets, if necessary. The packets are subsequently transmitted out on the trunk connection to the variable bit rate network 3.

One of the most important constituents of the basic idea of the present invention is to reuse already existing components associated with circuit switches of today to be used in a VBR situation. This opportunity is given by the new idea of transferring variable bit rate traffic through a connection in the circuit switch. Parts of the conversion from variable bit rate to constant bit rate, or to other variable bit rate formats, which are difficult to add to already existing devices , are preferably performed before the transmission through the circuit switch. Parts of the conversion, which are easy to add by e.g. software to existing devices, may advantageously be performed at pooled functionality devices.

The present invention thus utilizes functionality already present in the pooled functionality devices 8, in the conversion between variable and constant bit rate traffic. Since many circuit switches 1 presently available are equipped with functionalities which often forms a part of the conversion, this functionality may readily be reused. However, despite the fact that the functionality and the devices containing it are known in prior art, the use for variable bit rate applications are completely unknown. Using a connection in a circuit switch for transmission of information, appearing for the circuit switch as variable bit rate traffic, is basically unknown prior to the present invention. Since a suitable internal variable bit rate format is selected, the pooled functionality devices 8 only have to be equipped with minor additional functionality in order to be able to handle variable bit rate traffic. It is obvious that a circuit sw tch in a constant bit rate today easily but surprisingly may be updated to accommodate the features of the present invention.

In most circuit switches 1, 64 kbps connections are available, adjusted to the normal PSTN standards. However, using such 64 kbps connection to send variable bit rate frames according to the present invention, will result in that the actual transfer speed is significantly lower than 64 kbps. This is a speed limitation for the interworking unit 6. The limit may, however, be overruled, by establishing several 64 kbps connections, where frames at each connection are associated with one and the same call.

An alternative and preferred solution is to establish a wideband or broadband connection through the circuit switch 1. Many of the circuit switches today have the possibility of establishing wideband or broadband connections of a multiple of 64 kbps. For instance, video conferences over ISDN normally occupy 384 kbps, and many circuit switches may handle such traffic on one and the same connection. In certain circuit switches even braodband connections of 2 Mbps or more are allowed. By establishing such wideband or broadband connections, it is possible to transfer several channels of speech over one and the same connection, from the interworking unit 6 to the pooled functionality device 8. The transmission becomes more efficient, and delays are minimised. In wideband or broadband connection cases, each channel is assigned a channel number, which is used by the interworking unit 6 and the pooled functionality devices 8 for identifying the voice or data packets of a particular call.

Even if the present invention preferably operates with wideband or broadband connections, it is possible to use connections which has a bandwidth of sub rate magnitude, e.g. 8, 16, 32 kbps, as well. Channel numbers may also be used in a general case, i.e. in some cases even for 64 kbps or sub rate connections.

An embodiment of an interworking unit 6 according to the present invention is illustrated in the block scheme in fig. 2. The interworking unit 6 is connected to a variable bit rate network 3 and comprises line termination means 20 for receiving and transmitting variable bit rate packets over a trunk line. A multiplexing unit 21 identifies signalling packets, out-band signalling, and separates them to a processing unit 24. The remaining media stream is forwarded to a mapping unit 22. This stream may also comprise different in-band signalling. In cases, when signalling packets are separated and processed at an earlier stage, the multiplexor unit may be omitted.

The mapping unit 22 takes care of the voice and data content of the packets and transforms them into the internal system VBR format. The processor 24 typically controls the actual conversion, whereby it uses information from signalling packets and/ or information from call control means in other parts of the system for assigning a suitable channel number. The converted data is packed into frames in a framing unit 23. The processor 24 provides the call control means 11 with the information of signalling packets. If a wideband or broadband connection is established, several channels are available and each channel or final destination is assigned to one internal channel number. A circuit switch terminal unit 25 sends the packets of the internal system VBR format on the connection. The information in the internal frame format is sent via a user plane 27 of the circuit switch 1. Packets concerning signalling information are, however, not sent forward to the user plane 27 of the circuit switch. Such packets are, as mentioned above, analysed by the processor 24 and sent to the call control means 11 and suitable control signals are transferred to a control plane 28 of the circuit switch 1.

In a similar manner, the processor 24 of the interworking unit 6 may receive control signals from the circuit switch control plane 28. If such control signals are to be forwarded, they are transformed by the processor 24 to packets of signalling information according to the protocols for the variable bit rate network 3. The control signals are then included in the main stream by the multiplexing unit 21. Frames of voice, video or data packets in the internal system VBR format are received by the circuit switch terminal unit 25 over the circuit- switch connection. The packets are extracted from the frames in the framing unit 23 and the mapping unit 22 transforms the packets into the format accepted by the variable bit rate network. The mapping unit 22 uses information from the processor 24 to add correct addressing information to the packets. The internal channel numbers are e.g. associated with certain final addresses. The packets are eventually multiplexed with any signalling packets and transmitted to the variable bit rate network from the line terminating means 20.

In the above description, the call control means 1 1 is described as a logically separate unit. As anyone skilled in the art understands, the call control means 11 typically has a distributed design, where parts of the call control means 11 or the entire means 1 1 may be comprised in e.g. the processor 24, in the circuit switch control plane 28 and/ or the pooled functionality device 8. The essential feature is, however, a functionality, which is responsible for assigning channel numbers and communicating such numbers between different units in the system.

The call control means 1 1 is also responsible for establishing the connections through the circuit switch 1. This may be performed in different manners. In a system, where few inactivity periods are expected, the connections between the interworking unit and the pooled functionality device may then be established when hardware for the respective means are added to the system. The connection is then kept until the system is closed or the hardware removed.

Another possibility is to establish and disconnect the connection, depending on the traffic situation. The first call requesting a connection according to the present invention will initiate the establishment of such a circuit- switch connection. The circuit- switch connection may then be used for parallel calls, if a wideband or broadband solution is used. The circuit- switch connection is then available until all calls using it are ended and the circuit- switch connection is no longer used. The connection may then be disconnected in order to allow other traffic to use the circuit switch capacity.

An embodiment of a pooled functionality device 8 according to the present invention is illustrated in the block scheme of fig. 3. A connection from an interworking unit was established through the circuit switch 1. A circuit switch terminal unit 30 receives frames of the internal system VBR format. The frame is removed by a framing unit 36, and the variable bit rate traffic is split to different functionality units 35. This is performed by a multiplexing unit 33, which demultiplexes the traffic according to the channel numbers. This demultiplexing is controlled by a processor 34 connected to or comprised in the call control means 11. The functionality units comprises typically synchronisation means 31, and a functionality core 32. The variable bit rate traffic is accordingly synchronized in the synchronisation means 31 , and the data is then handed over to the normal constant bit rate functionality of the pooled functionality device in the core 32 of the pooled functionality device 8.

The pooled functionality device 8 may also be a source of information data. The core 32 of the pooled functionality device then produces data, either as a response of incoming data from elsewhere, or produced mainly within the device itself. The data is synchronized, but this does not mean any problem to the variable bit rate parts, why the synchronisation becomes trivial. In the multiplexor 33, the data is divided into suitable packages, and provided with channel number information, according to information of the processor 34. The data is framed to the internal system VBR format in the framing unit 36 sends the frames out on the established connection by the circuit switch terminal unit 30. The processor 34 also handles the signalling over the circuit switch and is responsible for establishing connections through the circuit switch 1 to the interworking unit, when requested.

The interworking units 6 and the pooled functionality devices 8 according to the present invention enable a flexible connection of various types of traffic and functionalities. Above, the description has been concentrated on one interworking unit 6 and one pooled functionality device, as also shown in fig. 4a. Here one broadband circuit- switch connection 40 is established, through which a number of channels 41 are available. The traffic through the channels are treated in the pooled functionality device 8 and sent as ordinary circuit-switched traffic on connections 10 through the circuit switch once more to exchange terminals 4. The exchange terminals 4 are the interfaces towards e.g. the ordinary PSTN network. In the illustrated case, four channels 41 are used in the broadband connection 40, and four narrow band circuit switched connections 10 are connected to two exchange terminals 4.

In fig. 4b, the system is equipped with two interworking units 6. The may in turn be connected to the same type of VBR network or different types. One could e.g. handle ATM traffic while the other is adjusted for Internet traffic. However, both interworking units 6 operate with the same internal system VBR format. From each one of the interworking units 6, a broadband connection 40 is established to a pooled functionality device 8. In the illustrated case, the first broadband connection has four active channels 41, bringing VBR data to the pooled functionality device 8. The information in two of these channels are treated by the pooled functionality device 8 and is then sent through the second broadband connection 40 in two channels 41 to the second interworking unit 6. The system here acts as a switch and a format interface between two different types of variable bit rate networks. At the same time, traffic from the other two channels in the first broadband connection 40 is sent as ordinary circuit- switched traffic as described earlier. The data treatment in the pooled functionality device 8 could be of any kind, and in the simplest case, the data is just reflected without any real treatment. The pooled functionality device operates in such a case just as a intermediate device for the format conversion from one variable bit rate network format to another.

Fig. 4c illustrates two more alternative embodiments of the systems according to the present invention. In the upper part of the figure, an interworking unit 6 is connected by a broadband connection to a pooled functionality device 8. In this case, the pooled functionality device 8 is of a type, which do not immediately forwards the received information or which by itself creates information to be sent. Such functionality devices are e.g. voice mailboxes or speech announcement. It is thus not necessary, that the information through the channels 41 are forwarded to another device connected to the circuit switch, but the information may simply have its source or end destination within the functionality device 8.

In the bottom part of fig. 4c, another situation, where the data only passes the circuit switch 1 once, is illustrated. In this case, the traffic between the networks connected to two interworking units 6 is intense, and it is beneficial to establish a direct connection 40 between the two interworking units 6. In the illustrated case six channels are presently occupied. This case resembles the case of a pooled functionality device 8 with a pure reflecting operation, but here the information passes the circuit switch only once, which reduces the required capacity of the circuit switch 1. This case is also parallel with the case of a pooled functionality device 8 with a source or end destination for the information, as shown in the upper part of the same figure.

Fig. 4d illustrates another case, where more than one pooled functionality device 8 is present. A connection 40 is established between the interworking unit 6 and each one of the pooled functionality devices 8. The pooled functionality devices 8 could, for instance, provide different types of data treatment, which are of interest for the traffic to and from the interworking unit 6. The pooled functionality devices 8 could of course also provide the same actions, and merely be a resource doubling in the system. Traffic on different channels 41 in the broadband connections 40 are brought to different pooled functionality devices 8, but may very well end up at the same exchange terminals 4.

The situations in figures 4a-d only serves as simple examples of how the system could be composed and developed, by providing more than one of the interworking unit 6, the pooled fucntionality device 8 and the circuit- switch connections 9. This modularity may be extended until the capacity of the circuit switch 1 is fully occupied. Anyone skilled in the art easily understands that the combinations are possible to vary in any configuration, according to the requirements for each system. It is thus probable that a typical system comprises a number of interworking units 6 and a number of pooled functionality devices 8, which are connected by a number of connections 9.

The details of an interworking unit 6 are also influenced by the total system.

If an interworking unit 6 is connected by more than one circuit- switch connection through the circuit switch 1, the mapping unit 22 also has to consider not only the channel numbers for each packet, but also in which connection it is going to be sent. This means that a simpler type of switching mechanism has to be comprised in the mapping unit 22, supported by the processor 24. In such a system, it is preferred to have one set of framing unit 23 and circuit switch terminal unit 25 for each possible circuit-switch connection.

The interworking unit may also be connected to more than one trunk line to variable bit rate networks. The traffic in each one of these then has to be received and treated according to the ^"above description. The mapping procedure may then become even more complex and the mapping unit 22 will act as a smaller type of switch.

In the same manner, the pooled functionality devices 8 have to be adapted to the actual system situation. The multiplexor unit 33 described above is of course not necessary if only one set of synchronisation means 31 and functionality core 32 is available, but in a typical case the board of a pooled functionality device comprises several units, adapted to e.g. the requested number of connections.

Mobile telephony is one of the fastest growing and most requested telecommunication services that has existed. In the future it is possible that more or less all telephony subscribers will have any kind of radio access to the telecommunication networks. One of the features that has caused this enormous success is that the mobile telephone networks are not limited to communication within its own network, but is easily available also from the traditional telephony networks. The development of efficient gateways between e.g. PSTN/ISDN and mobile telephony networks is therefore of crucial importance.

In fig. 5, a mobile telephony network according to GSM is schematically illustrated. A mobile station 101 is freely movable, and communicates with a base transceiver station (BTS) 102, via radio access 106. The mobile station 102 can be some kind of mobile telephone, a facsimile device with radioaccess or a portable computer with radio modem. The BTS 102 comprises equipment for sending and receiving, antennas for one or several cells, and e.g. equipment for ciphering. The BTS 102 is connected to a base station controller (BSC) 103, also referred to as a radio switch, which switches radio channel connections for speech and signalling to a mobile switching centre (MSC) 104. The BSC 103 typically also performs concentration of the information. The MSC 104 is a switching node with special functions, which are required in a mobile network, mainly base changes between MSC's 104 and between mobile networks. One MSC acts as a gateway to other networks, e.g. a PSTN network 107, and is normally referred to as a gateway mobile switching centre (GMSC) 105. All connections to and from a mobile network have to pass a GMSC 105.

In the third generation of GSM, communication within the access and core network will in some parts be based on variable bit rate traffic. The communication between BSC 103 and MSC 104 will probably be performed in a cell transfer mode according to asynchronous transfer mode (ATM) standards. This means that variable bit rate traffic will be received and transmitted in the MSC 104 and the GMSC 105. There will subsequently be a need for the GMSC 105 to convert variable bit rate traffic into constant bit rate traffic, and the opposite, in order to communicate with external constant bit rate networks, e.g. PSTN 107. A telecommunication system according to the present invention is here advantageously used. A circuit switch of today is easily upgraded according to the present invention to be used as a GMSC 105. In fig. 6, a protocol level diagram of an interworking unit 6 according to the present invention, adapted for use with an ATM network 110, is illustrated. A signal from the ATM network 110, is transported on the transport network according to e.g. SDH (synchronous digital hierarchy). The SDH protocol is terminated in the physical layer 1 1 1 of the interworking unit 6 and the ATM cells are made available from the SDH frame. The header of the ATM cells comprises an address field in the form of a logical channel number, which identifies a unique link address between two nodes in the ATM network. This information is used by the ATM level 112 to identify the end destination of the cell content. The ATM channel number is thus used by the control plane of higher levels to control the final destination in the circuit switch.

A mapping facility 1 13 extracts the payload data of the AAL 2 cells into new data packets, which are provided with channel numbers for the circuit- switch connection through the circuit switch. In a channel handling layer 114, these channel numbers are derived from the ATM/AAL2 channel numbers and signalling information available in the processor. The channel number provided packets are then packaged at a HDLC level 1 15 according to the HDLC format, adding a header and in some cases a trailer for indicating the start and end of the packet. A standard format level, e.g. an El level 116 is then used in this embodiment as the physical layer.

In Fig. 7 the handling process of the ATM cells is described more in detail. An ATM cell 130 is received. The payload of the ATM cell comprises in this embodiment a segmentation and reassembly protocol data unit of 48 octets according to the AAL 2 (ATM adaption layer). The header 135 of 5 octets comprises flow control data, virtual path identifiers, virtual channel identifiers, type of payload and error control information. Each AAL 2 cell is also provided with a header 136 and a trailer 137 comprising the channel identifier within the ATM cell, whether the AAL 2 payload is to combine with other packets, cyclic redundancy check etc. The CID (channel identifier) may distinguish up to 248 different channels within one ATM connection. The ATM cell is divided into the individual AAL 2 packets 131. The CID information is used as an entry in a look-up table 132 arranged by the processor, to convert the AAL 2 CID number to an internal channel number (and may be also a connection identifier) for the circuit- switch connection to the pooled functionality device. The header and trailer is thereby removed, and a new header 134 comprising the channel number is added. Finally, the payload and the channel number 134 are provided with a start flag 138 and an end flag 139 to accomplish the HDLC format.

In fig. 6, the pooled functionality device attached to the circuit switch 1 is a transcoder 109. The HDLC packets are thus sent over a broadband connection 40 of 2 Mbps through the circuit switch 1 to the transcoder 109. The transcoder 109 is in its main parts already available in most modern circuit switches 1, but is in the present invention necessary to be provided by some extra software. Accordingly, an El level 117 terminates the packet transport over the 2 Mbps connection 40. A HDLC level 118 terminates the HDLC format, removing header and trailer. In a channel number handling level 119, the internal channel number of the connection is received and used in a control plane for assigning the correct end destination. The pure data is now available for synchronisation. In a synchronisation level 121 the data is collected in a first-in-first-out buffer transferring the asynchronous data traffic from to synchronous traffic. The read-out from the buffer is made in even periods, why the resulting data stream is synchronized. A standard coding-decoding level 120 performs a decoding step and the resulting decoded data is ready for the constant bit rate network. The synchronisation process may also take place after the decoding, if the codec 120 may operate with asynchronous traffic. The data is in a time slot level 122 assigned a time slot, depending on the final destination, which was extracted as the internal channel number. The data is then transferred by an E0 layer out into the circuit switch 1 once more to be switched to the correct final destination, as ordinary PSTN data. Fig. 8, illustrates a first-in-first-out buffer acting as a synchronisation means. This synchronisation is normally referred to as jitter buffer handling. Data packets 141 arrives from the interworking unit in a variable rate. The time periods 142 between two subsequent data packets 141 vary in length around a mean value. The data packets 141 are stored in a first- in-first out buffer 140 for a short period as they arrive. The data packets 141 are read out from the first- in-first-out buffer at regular time intervals 143, i.e. a synchronized data stream is accomplished. The delay time in the buffer 140 should correspond to the difference between the maximum time interval 142 between two subsequent packets at the incoming side and the regular time interval 143 at the outgoing side. This ensures that there always will be a packet waiting to be read at the buffer 140, when the regular time interval 143 is ended. This introduces a time delay, but this delay is often negligible compared with the total time interval 143.

The distributed conversion according to the present invention from variable to constant bit rate is easily distinguishable in fig. 6. The signalling part is terminated in the interworking unit 6 and passed on to the circuit switch 1 at a control plane. The addressing part is used to identify the established connections and for assigning an internal channel number. The data itself is, however, unchanged both as regarding content and the bit rate character. The bit rate character and the content are instead processed in the transcoder 109. Here a decoding of the speech is performed along with a synchronisation process, adapting the variable bit rate and coded speech to the 64 kbps traffic of a common PSTN communication.

Fig. 6 also illustrates the opposite process. Ordinary PSTN data is transferred by an E0 layer through the circuit switch 1 from an original source. The data is assigned a time slot, depending on the original source. The control plane of the circuit switch carries information about the original source and the requested final destination, associated with a certain time slot. A time slot level 122 extracts the data and prepares it for the coding level 120. The output from the coding is arranged in suitable packets and is in the channel number handling level 119 provided with an internal channel number, which indicates the identity of the final destination, according to the control plane information. The HDLC level 118 provides the HDLC format and adds the header and trailer. An El level 117 is then used in this embodiment as the physical layer, sending the HDLC packets over the broadband connection 40 of 2 Mbps through the circuit switch 1 to the interworking unit 6.

In the interworking unit 6, the El level 116 terminates the packet transport over the 2 Mbps connection 40. A HDLC level 115 terminates the HDLC format, removing header and trailer. In a channel number handling level 114, the internal channel number of the connection is received and used in a control plane for assigning the correct end destination. The pure data is now available for mapping and in some cases switching into an AAL 2 format. The mapping facility 113 receives the payload data for the AAL 2 cells and packs it into new data packets. The payload of the ATM cell comprises in this embodiment a segmentation and reassembly protocol data unit of 48 octets according to the AAL 2 (ATM adaption layer). The header and trailer comprises information about the information type, whether the AAL 2 payload is to combine with other packets, cyclic redundancy check etc. This process occurs at the ATM level 112, which also provides ATM/AAL2 channel numbers. These channel numbers are derived from the signalling information available in the processor. The SDH protocol is applied in the physical layer 111 of the interworking unit 6 and the ATM cells are made available for the ATM network 110.

The distributed conversion from variable to constant bit rate is also here easily distinguishable. The signalling part is added in the interworking unit 6, originating from the circuit switch 1 at the control plane. The addressing part is derived from the established connections and from the assignment of an internal channel number. The data itself is in the transcoder 109 allowed to adopt a variable bit rate character. This is done after a suitable coding of the speech has been performed. A 64 kbps traffic is thereby ready to be accepted by an ATM network.

The signalling will here below be described in a schematic manner. When a call request appears from one of the PSTN subscribers to a mobile telephone, the control plane of the circuit switch will start to connect the necessary connections through the circuit switch 1. A free suitable pooled functionality device unit is reserved and an ordinary 64 kbps connection is established between the exchange terminal and the pooled functionality device. Furthermore, a free channel in a connection to the interworking unit connected to the mobile telephony network is searched. If there are no free channels, a new connection may be possible to establish, depending on the system configuration. When a suitable channel number is found, it is reserved for the requested call. Information about the final destination in connection with the selected internal channel number is sent over the control plane to the interworking unit, to be used in connection with the mapping procedure. The processor of the interworking unit further provides necessary signalling messages out on the mobile telephony network. These messages may be multiplexed together with the mainstream of messages or provided on separate connections. When the whole connection is established, speech data may be sent on the connection. The exchange terminal switches the time slot of interest over the established 64 kbps connection to the pooled functionality device, where the speech is coded. The coded speech is packed into HDLC frames and provided with the internal channel number of the broadband connection to the interworking unit.

When reaching the interworking unit, the channel number is used for achieving an appropriate addressing to the final destination.

When a call request appears in the opposite direction, the processor of the interworking unit will receive a message comprising a request for establishing a contact with a certain PSTN subscriber. A free suitable pooled functionality device unit is reserved. Furthermore, a free channel in a connection from the interworking unit to the pooled functionality device is searched. If there are no free channels, a new connection may be possible to establish, depending on the system configuration. When a suitable channel number is found, it is reserved for the requested call. Information about the final destination in connection with the selected internal channel number is sent over the control plane to the pooled functionality device, to be used in connection with the mapping procedure. Finally an ordinary 64 kbps connection is established between a suitable exchange terminal and the pooled functionality device. The exchange terminal further provides necessary signalling messages out on the PSTN network. When the whole connection is established, speech data may be sent on the connection. The interworking unit uses the addresses of the incoming data packets to provide channel number to HDLC frames. The frames are sent to the pooled device , where the channel number decides which unit it is going to end up in. In the pooled functionality device, the speech is decoded and sent in the reserved time slot of interest over the established 64 kbps connection to the exchange terminal, and further on to the final destination.

Another area in telecommunication, which is growing fast is the Internet traffic. The Internet protocol (IP) has the property of being able to be used as an overlay network on top of other transmission techniques. At the same time IP may also manage without connecting bearer networks, since IP involves both multiplexing and switching functions. Internet ha more and more become a universal tool for communication.

Fig. 9 illustrates schematically a telecommunication system comprising an internet network 201 and a PSTN network 107. A computer 200 connected to the Internet network is provided with a telephony application 203, which operates as an ordinary telephone. Calls from the computer application 203 is connected via the Internet network to an interworking unit 6. According to the present invention, the variable bit rate speech is transferred through a circuit switch 1 on a connection to a pooled functionality device 8. The speech is transferred into a normal PSTN fashion and switched through the circuit switch once more to an exchange terminal 4, which eventually connects a receiver telephone 202 via the PSTN network 107. The basic configuration is thus analogue with earlier discussions.

In fig. 10, a protocol level diagram of an interworking unit 6 according to the present invention, adapted for use with an Internet network 201, is illustrated. A signal from the Internet network 201, is transported on the transport network according to IP (Internet Protocol). The IP is terminated in the physical layer 210 of the interworking unit 6 and the content of the IP packets are made available. The header of the IP packets comprises an address fields of source and destination, length information, check sums and a lot more information. This information is used by the UDP level 21 1 to identify the end destination of the packet content. The destination address is thus used by the control plane of higher levels to control the final destination in the circuit switch.

A mapping facility 1 13 extracts the payload data of the Internet packets into new data packets, which are provided with channel numbers for the circuit- switch connection through the circuit switch. In a channel handling layer 1 14, these channel numbers are derived from the IP/UDP addresses and signalling information available in the processor. The channel number provided packets are then packaged at a HDLC level 115 according to the HDLC format, adding a header and in some cases a trailer for indicating the start and end of the packet. An El level 1 16 is then used in this embodiment as the physical layer.

In fig. 10, the pooled functionality device attached to the circuit switch 1 is a transcoder 109. The HDLC packets are thus sent over a broadband connection 40 of 2 Mbps through the circuit switch 1 to the transcoder 109. The transcoder 109 is identical to the one presented in fig. 6, and is thus not further discussed.

In fig. 11, a schematic flow diagram for a receiving process according to the present invention is illustrated. The process starts in step 300. In step 301, a message is received from a variable bit rate network in an interworking unit connected to a circuit switch. A connection between the interworking unit and a pooled functionality device connected to the circuit switch is established in step 302. Any signalling from the variable bit rate network is terminated in step 303 and may be used for reserving of channels in the established connection. The information from the signalling messages and/ or information from the circuit switch control plane is used to map the incoming packets to channel numbers of the established connection and frame it into the internal variable bit rate format in step 304. The frames are sent over the connection 305 and received in the pooled functionality device in 306, where the channel numbers are used to distinguish different calls. In step 307, the information rate of packets of each call, or channel number, is synchronized. The process ends in step 308.

In fig. 12, a schematic flow diagram for an opposite process according to the present invention is illustrated, when information is sent out on a variable bit rate network. The process starts in step 310. A connection between a pooled functionality device and an interworking unit connected to the circuit switch, is established in step 311. In step 312, a message of a constant bit rate character is packetized, provided with channel number of the established connection and framed according to an internal variable bit rate format. The frames are sent over the connection 313 and received in the interworking unit in 314. The channel number information is used in 315 to provide the packets with an address of a connected variable bit rate network. Any signalling from the call control plane of the circuit switch is multiplexed to the packet stream in 316, and the packets are sent out on the variable bit rate network in step 317. The process ends in step 318.

In the particular examples above, the internal format is described as an HDLC format. This is presently regarded as the best available alternative, in the view of available processors for mapping procedures etc. However, as anyone skilled in the art understands, the internal format can be selected in many other ways. Since e.g. transcoders normally operates using a frame format, often referred to as TRAU frames, such frames would also be possible to use for the transmission over the circuit- switch connections. Further alternatives are also easily imaginable. The important feature is, however, that the internal format is of a variable bit rate type, since any synchronisation equipment normally is situated in the pooled functionality device. The internal format permits advantageously also channel number handling, which facilitates the handling of broadband or wideband applications.

It will be understood by those skilled in the art that various further modifications and changes may be made to the present invention without departure from t e scope thereof, which is defined by the appended claims.

Claims

1. A telecommunication system comprising a circuit switch (1); at least one pooled functionality device (8) connected to said circuit switch (1), and characterised by at least one interworking unit (6), connected to said circuit switch (1) and having variable bit rate network line terminals (20); means for establishing at least one circuit- switch connection (9, 40) through said circuit switch (1) between said interworking unit (6) and said pooled functionality means (8) said interworking unit (6) comprises means (22, 23) for transformation of variable bit rate information to and from an internal variable bit rate format for transmission over said circuit- switch connection (9, 40); said pooled functionality device (8) comprises packet handling means (30, 33, 36) for handling of said internal format.

2. The telecommunication system according to claim 1, characterised in that said pooled functionality device(s) (8) is selected from the list of: speech transcoder, speech recognition means, rate adaption means, multi party functionality means, mailbox means, speech announcement means,

TDMF signalling, and forwarding means.

3. The telecommunication system according to claim 1 or 2, characterised in that said transformation means (22, 23) handles at least one of the following traffic types: asynchronous transfer mode, internet protocol,

X.25,

X.21, and frame relay.

4. The telecommunication system according to any of the preceding claims, characterised in that said interworking unit (6) comprises means for handling variable bit rate control signalling (24).

5. The telecommunication system according to claim 4, characterised in that said interworking unit (6) comprises multiplexing means (21) for multiplexing and demultiplexing of control signalling packets and main stream packets.

6. The telecommunication system according to any of the preceding claims, characterised in that said pooled functionality device (8) comprises means for handling control signalling (34).

7. The telecommunication system according to any of the preceding claims, characterised in that said pooled functionality device (8) comprises means for synchronising (31).

8. The telecommunication system according to any of the preceding claims, characterised in that said circuit- switch connection (9) has a bandwidth of 64 kbit/s or a fraction thereof.

9. The telecommunication system according to any of the claims 1 to 7, characterised in that said circuit-switch connection (9, 40) has a bandwidth of more than 64 kbit/s.

10. The telecommunication system according to claim 9, characterised in that said circuit-switch connection (9, 40) has a bandwidth of at least 384 kbit/s.

11. The telecommunication system according to claim 8 or 10, characterised in that said internal variable bit rate format comprises payload and a channel number.

12. The telecommunication system according to claim 11, characterised in that said pooled functionality device (8) comprises multiplexing means (33) for handling packets according to said channel numbers.

13. A telecommunication system comprising a circuit switch (1), and characterised by at least one first interworking unit (6), connected to said circuit switch (1) and having variable bit rate network line terminals (20); at least one second interworking unit (6), connected to said circuit switch (1) and having variable bit rate network line terminals (20); means for establishing at least one circuit- switch connection (9, 40) through said circuit switch (1) between said first and second interworking units (6); said interworking units (6) comprises means (22, 23) for transformation of variable bit rate information to and from an internal variable bit rate format for transmission over said circuit- switch connection (9, 40).

14. The telecommunication system according to claim 13, characterised in that said transformation means (22, 23) handles at least one of the following traffic types: asynchronous transfer mode, internet protocol, X.25, X.21, and frame relay.

15. The telecommunication system according to claim 13 or 14, characterised in that said first and second interworking unit (6) comprises means for handling variable bit rate control signalling (24).

16. The telecommunication system according to claim 15, characterised in that said first and second interworking unit (6) comprises multiplexing means (21) for multiplexing and demultiplexing of control signalling packets and main stream packets.

17. The telecommunication system according to any of claims 13 to 16, characterised in that said circuit-switch connection (6) has a bandwidth of 64 kbit/s or a fraction thereof.

18. The telecommunication system according to any of the claims 13 to 16, characterised in that said circuit-switch connection (6, 40) has a bandwidth of more than 64 kbit/s.

19. The telecommunication system according to claim 18, characterised in that said circuit-switch connection (6, 40) has a bandwidth of at least 384 kbit/s.

20. The telecommunication system according to claim 18 or 19, characterised in that said internal variable bit rate format comprises payload and a channel number.

21. An interworking unit (6), comprising variable bit rate network line terminals (20); circuit switch terminals (25) adapted for handling a circuit-switch connection (9, 40); means for transformation (22, 23) of variable bit rate information into an internal variable bit rate format, for transmission on said circuit- switch connection (9, 40).

22. The interworking unit (6) according to claim 21, characterised in that said transformation means (22, 23) handles at least one of the following traffic types: asynchronous transfer mode, internet protocol, X.25, X.21, and frame relay,

23. The interworking unit (6) according to claim 21 or 22, characterised by means for establishing said circuit switch connection (9, 40).

24. The interworking unit (6) according to any of the claims 21 to 23, characterised by means for handling (21, 24) variable bit rate control signalling.

25. The interworking unit (6) according to claim 24, characterised in that said means for handling variable bit rate control signalling comprises multiplexing means (21) for multiplexing and demultiplexing of control signalling packets and main stream packets.

26. A pooled functionality device (8) for use with a circuit switch (1), comprising circuit switch terminals (30), adapted for handling a circuit switch connection (9, 40); packet handling means (36) for handling of an internal variable bit rate format, for transmission on said circuit switch connection (9, 40).

27. The pooled functionality device (8) according to claim 26, characterised in that said pooled functionality device (8) is selected from the list of: speech transcoder, speech recognition means, rate adaption means, multi party functionality means, mailbox means, speech announcement means, DTMF signalling, and forwarding means.

28. The pooled functionality device (8) according to claim 26 or 27, characterised in that said pooled functionality device (8) comprises means for handling control signalling (34).

29. The pooled functionality device (8) according to any of the claims 26 to 28, characterised in that said pooled functionality device (8) comprises means for synchronising (31).

30. The pooled functionality device (8) according to claim 29, characterised in that said pooled functionality device (8) comprises multiplexing means (33) for handling packets according to said channel numbers.

31. A telecommunication system handling asynchronous transfer mode (ATM) traffic, comprising a circuit switch (1); at least one pooled functionality device (8) connected to said circuit switch (1), and characterised by at least one interworking unit (6), connected to said circuit switch (1) and having ATM line terminals (20); means for establishing at least one circuit- switch connection (9, 40) through said circuit switch (1) between said interworking unit (6) and said pooled functionality means (8) said interworking unit (6) comprises means (22, 23) for transformation of ATM information to and from an internal variable bit rate format for transmission over said circuit-switch connection; said pooled functionality device (8) comprises packet handling means (30, 33, 36) for handling of said internal format.

32. The telecommunication system according to claim 31, characterised in that said interworking unit (8) comprises means for handling ATM control signalling.

33. The telecommunication system according to claim 32, characterised in that said interworking unit (8) comprises multiplexing means (33) for multiplexing and demultiplexing of control signalling packets and main stream ATM cells.

34. The telecommunication system according to claim 31, 32 or 33, characterised in that said internal variable bit rate format is an HDLC format.

35. The telecommunication system according to any of the claims 31 to 34, characterised in that said circuit- switch connection (9, 40) has a bandwidth of at least 2 Mbit/s.

36. The telecommunication system according to any of the claims 31 to 34, characterised in that said interworking unit (8) comprises: mapping means, framing means, and a processor

37. A telecommunication system handling Internet traffic, comprising a circuit switch (1); at least one pooled functionality device (8) connected to said circuit switch (1), and characterised by at least one interworking unit (6), connected to said circuit switch (1) and having ATM line terminals (20); means for establishing at least one circuit- switch connection (9, 40) through said circuit switch (1) between said interworking unit (6) and said pooled functionality means (8) said interworking unit (6) comprises means (22, 23) for transformation of ATM information to and from an internal variable bit rate format for transmission over said circuit-switch connection; said pooled functionality device (8) comprises packet handling means (30, 33, 36) for handling of said internal format.

38. The telecommunication system according to claim 37, characterised in that said interworking unit (6) comprises means for handling Internet telephony control signalling.

39. The telecommunication system according to claim 38, characterised in that said interworking unit (6) comprises multiplexing means (21) for multiplexing and demultiplexing of control signalling packets and main stream Internet packets.

40. The telecommunication system according to claim 37, 38 or 39, characterised in that said internal variable bit rate format is an HDLC format.

41. The telecommunication system according to any of the claims 37 to 40, characterised in that said circuit- switch connection (9, 40) has a bandwidth of at least 2 Mbit/s.

42. The telecommunication system according to any of the claims 37 to 41, characterised in that said interworking unit (6) comprises: mapping means (22), framing means (23), and a processor (24)

43. A method for handling variable bit rate traffic in a circuit switch (1), comprising the steps of: receiving variable bit rate traffic in an interworking unit (6); establishing a circuit- switch connection (9, 40) through said circuit switch (1) between said interworking unit (6) and a pooled functionality device (8) connected to said circuit switch (1); transforming said variable bit rate traffic into an internal variable bit rate format; transmitting said internal variable bit rate format data from said interworking unit (6) over said established circuit- switch connection (9, 40) to said pooled functionality device (8); and handling said internal variable bit rate format data in said pooled functionality device (8).

44. The method according to claim 43, characterised in that said transforming step comprises the steps of : mapping of the incoming variable bit rate packets address to a channel number of said circuit- switch connection (9, 40); and framing of the payload of said incoming variable bit rate packets and said channel number into said internal variable bit rate format.

45. The method according to claim 44, characterised by the further step of terminating variable bit rate signalling messages in said interworking unit (6).

46. The method according to claim 45, characterised by the further step of reserving channel numbers of said circuit- switch connection (9, 40) based on said signalling messages.

47. The method according to any of the claims 44-46, characterised in that said handling step comprises the steps of : receiving said internal variable bit rate format traffic; separating data having different channel numbers; synchronising data with the same channel number.

48. A method for handling variable bit rate traffic in a circuit switch (1), comprising the steps of: establishing a circuit- switch connection (9, 40) through said circuit switch (1) between an interworking unit (6) and a pooled functionality device (8) connected to said circuit switch (1); packetize, in said pooled functionality device (8), constant bit rate traffic into an internal variable bit rate format; transmitting said internal variable bit rate format data from said pooled functionality device (8) over said established circuit- switch connection (9, 40) to said interworking unit (6); and transforming said internal variable bit rate format data in said interworking unit (6) into variable bit rate traffic; and transmitting said variable bit rate traffic.

49. The method according to claim 48, characterised by the further step of reserving channel numbers of said circuit- switch connection (9, 40) based on call control messages in said circuit switch (1).

50. The method according to claim 48 or 49, characterised in that said packetising step comprises the steps of : mapping of the constant bit rate traffic destination to a channel number of said circuit- switch connection (9, 40); and framing of the payload of said constant bit rate traffic and said channel number into said internal variable bit rate format.

51. The method according to any of the claims 48 to 50, characterised by the further step of producing and multiplexing variable bit rate signalling messages in said interworking unit (6).

52. The method according to any of the claims 48 to 51, characterised in that said transforming step comprises the step of mapping of said channel number to a variable bit rate network address.