AU722120B2 - Device and method in telecommunication equipment - Google Patents

Description

WO 98/06224 PCT/SE97/01319 1 DEVICE AND METHOD IN TELECOMMUNICATION EQUIPMENT TECHNICAL FIELD The present invention relates to a device and a method within telecommunication equipment and specifically to a method for connecting two mutually associated exchange units via a public telecommunication network, a device comprising at least two such exchange units and such an exchange unit.

STATE OF THE ART A problem exists within the part of the telecommunication field relating to private exchange units such as corporate exchanges, and this is when there are several different exchange units connected to a public telecommunication network and these exchange units are meant to operate as one single exchange to the user, i.e. additional information which is accessible to users in one exchange unit being possible to transfer over the public telecommunication network to other exchange units in the system. It is known to provide this by a leased line between two such exchange units. This is, however, a high cost solution since such a line may often be unused.

212 691 discloses two PBX:s (Private Branch Exchanges) interconnected by an ISDN network (Integrated Services Digital Network). Between these PBX:s a number of virtual tie trunks can be connected through the ISDN net. One of these lines constitutes a signalling channel between the two PBX:s and the others constitute speech channel lines. The virtual lines are set up depending on calendar data such as which day it is and which time of the day it is.

US-A-5 422 943 discloses a network comprising several PBX:s interconnected by an ISDN network. Both speech and data can be transferred over the network.

EP-A2-584 537 discloses interconnection of private exchanges with the use of an ISDN network. Signalling information is transmitted between the exchanges over a packet switching connection and speech traffic over a basic rate type connection.

EP-A2-641 140 discloses a virtual corporate network where signalling information which is specific to a phone call is transferred between private exchanges in data messages transferred in the same channel as the telephone call.

DESCRIPTION OF THE INVENTION The present invention addresses a problem of how to interconnect at least two exchange units in a corporate network via a public telecommunication network in order to transfer corporate network specific information without using leased lines between the exchange units and without using coding of the corporate network specific information on any speech channel.

:'".Another problem that the invention seeks to address is how to further 15 interconnect at least two exchange units for transfer of corporate network specific information while keeping the set up time and the number of connections between the exchange units down.

An object of the present invention is thus to interconnect two exchange units in a corporate network via a public telecommunication network in order to transfer corporate network specific information without using leased lines.

These problems are solved by the fact that when a call is to be set up between two exchange units, a special signalling channel is established through the public network, if such a signalling channel was not already established for a previous ongoing call, such that corporate network specific 25 information can be transferred over the signalling channel.

In one aspect, the present invention provides a method including the steps: 3 detecting, in a first exchange unit, a demand for set up by a first subscriber connected to the first exchange unit to a second subscriber connected to a second exchange unit, establishing at said detection a certain signalling channel through the network for both exchange units, if no such signalling channel already exists, and establishing a speech channel between the two exchange units for set up of a call between the first and the second subscriber.

In another aspect, the present invention provides an exchange unit to which subscriber terminals can be connected and which includes a control means, which detects a request for set up from a first subscriber terminal connected to the exchange unit to a second subscriber terminal connected to a second exchange unit and in dependence of said detection establishes, by signalling, a special signalling channel through the public telecommunication network between its exchange unit and the second exchange unit, if no such 15 signalling channel already exists, and establishes a speech channel between the first and the second subscriber channel for set up of a call between them.

In a further aspect, the present invention provides a device including at least two such exchange units.

An advantage of the invention is that it only interconnects two exchange 20 units when information is to be transferred and that no connection exists between the two exchanges at other times. A further advantage of the invention is that apart from the speech connections only one more connection, established through the public network between the exchange units, is used.

to This provides a very low cost system since a connection via a public network is only built for the time the network is actually used and there is no need to use expensive leased lines or expensive stationary connections if the exchange units are located at a great distance from each other.

WO 98/06224 PCT/SE97/01319 4 Furthermore, the resources of an exchange unit are better used since no connections are stationarily set up unnecessarily, which makes the exchange unit more flexible.

The invention will now be further described using preferred embodiments and with reference to the annexed drawings.

DESCRIPTION OF DRAWINGS Fig. 1 shows a schematic block diagram of an exchange unit according to the invention, Fig. 2 shows a schematic block diagram of a small corporate network which is connected to a public telecommunication network, Fig. 3 shows a flowchart of a method according to the invention, and Fig. 4 shows schematically how the signalling is made according to the invention.

PREFERRED EMBODIMENTS Fig. 1 shows a simplified kind of exchange unit according to a preferred embodiment of the invention. The preferred exchange unit PINX1 is a corporate exchange, e.g. of MD 110 type from Ericsson. This corporate exchange comprises a switching core 10 to which a control means 12 is connected. The control means 12 is arranged to control the through-connection of calls through the exchange. A number of subscriber terminals 14 are also connected to the switching core 10, as are also ten outgoing lines T1-T10. Two so-called signal terminals ST are also connected to the switching core 10. A DID-memory 16, where DID stands for "Direct-In-Dialling", is also connected to the control means 12. The DID-memory 16 comprises particular phone numbers to be used for initiating and setting up of a call between the exchange unit PINX1 and one or some other exchange units in a corporate network in which the corporate exchange is comprised. As is wellknown to the man skilled in the art, a subscriber 14 is connected to an outgoing line TI-T10 by the switching means 10 controlled by the control means 12, for set up of a call to another sub- WO 98/06224 PCT/SE97/01319 scriber connected to another node such as another corporate exchange or to another subscriber connected directly to a public telephone network. A signal terminal ST can also be connected to an outgoing line within the control of the control means 12.

Each line and connection shown in the figure is bidirectional so that traffic can go in both directions.

A signal terminal ST exists so that an exchange unit is able to communicate with another exchange unit according to a predetermined protocol so that corporate network specific information can be transferred. This, of course, requires that the second exchange unit also comprises a signal terminal. Corporate network specific information means information concerning e.g. who is calling and his telephone number, if a called phone number is engaged, if a called phone number is forwarded and, if so, where to, if the called person is in a meeting etc. All this information is of importance to transfer in a corporate network. The corporate exchange shown in the figure only comprises two signal terminals ST and the corporate exchange can therefore only be connected to two other exchange units at the same time. The corporate exchange could naturally comprise only one or a considerably greater number of signal terminals. The number of signal terminals in an exchange unit is determined by the number of other exchange units to which one wishes to be able to transfer corporate network specific information at the same time. In the same way, the number of connected subscribers and outgoing lines can, of course, be varied. An exchange unit according to the invention is neither limited to corporate exchanges but also comprises, for instance, so-called centrex units. A centrix unit is a part of an exchange in the public network, such as a part of an AXE-exchange, and which is used as a corporate exchange. The control means and.the signal terminals are then always located in the centrex part of the public exchange.

WO 98/06224 PCT/SE97/01319 6 Fig. 2 shows a simplified corporate network with two corporate exchanges PINX1 and PINX2, and a centrex unit CENTREX AXE. The figure is simplified by omitting control means, DID-memory and switching core. The corporate exchanges PINX1 and PINX2 are, however, essentially of the same type as described in connection with Fig. 1.

The two corporate exchanges are connected to a public network IVN each by a first, second, third, fourth, fifth, sixth, seventh, eighth and ninth line T1-T9 and T 11-T19, which public network IVN in this preferred embodiment is an ISDN network (Integrated Services Digital Network). This could, however, equally be a PSTN network (Public Switched Telephone Network). The first corporate exchange PINX1 also has a tenth outgoing line T10 which is in connection with the public network IVN. Also the centrex unit is connected to the public network by a first, second, third and fourth line T20-T23. All lines in this figure are bidirectional too.

The second, third and fourth lines T2-T4 leaving the first corporate exchange PINX1 is in connection with corresponding second, third and fourth lines T12, T13, T14 leaving the second corporate exchange PINX2, i.e. they have been interconnected via the public network IVN. The fifth lines T5 and T15, leaving their respective corporate exchanges, are used for signalling to the public telephone network, i.e.

they work as a D-channel in relation to the ISDN network. If the corporate exchanges were to be connected to an ordinary PSTN network, these lines would obviously be used for ordinary traffic and signalling occur via the normal B-channel connection. The seventh, eighth and ninth lines T7, T8 and T10 leaving the first corporate exchange PINX1 are in connection with the first, second and third lines (T20-T22) leaving the centrex unit CENTREX AXE. The centrex unit CENTREX AXE further uses its fourth line T23 for ISDN signalling, said line obviously also being used for ordinary traffic in case the public network is a PSTN network.

WO 98/06224 PCT/SE97/01319 7 A first signal terminal ST in the first corporate exchange PINX1 is in connection with a first signal terminal ST in the second corporate exchange PINX2 via the fourth lines T4 and T14, leaving the first and the second corporate exchanges PINX1 and PINX2, respectively, and the public network IVN, and a second signal terminal ST in the first corporate exchange PINX1 is in connection with a signal terminal in the centrex unit CENTREX AXE via the ninth line T9 leaving the first corporate exchange PINXI and the third line T22 leaving the centrex unit CENTR- EX AXE and the public network IVN. All lines leaving an exchange unit work as a connection for communication with the public network IVN over a traffic interface using a protocol called "Private Integrated Services Network (PISN); Interexchange Signalling Protocol; Circuit-mode Basic Services" with No. ETS 300 172, and the signal terminals communicate with each others via a signalling interface (Q) each using a protocol called "Integrated Services Digital Network (ISDN) Digital Subscriber Signalling System No. one (DSSI); Data Link Layer" with No. ETS 300 402. Where these interfaces are drawn as dashed lines crossing, as in the case of the traffic interface through all lines connecting the respective exchange units PINX1, PINX2 and CENTREX AXE to the public network and crossing, in the case of the signalling interface through the connections within an exchange unit connecting each signal terminal ST with a line which is connected to the public network. All the lines that are used for any kind of signalling, either to the public network or other exchange units, are shown in bold lines. The exchange units are not constantly connected to each other in the way shown in the figure but are only connected in the manner shown in the figure when needed, i.e. when a call is to be set up. When no traffic occurs between the two exchange units, neither the signalling channels between the signal terminals or any speech channels are connected.

How set-up occurs will be described below with reference to Figs. 3 and 4.

As shown in Fig. 2, two signal terminals communicate with each other across two interfaces, both the traffic interface and the signalling interface whereas the WO 98/06224 PCT/SE97/01319 8 subscriber terminals (not shown) which are connected to the exchange units only communicate with each other across the traffic interface i.e. the protocol used for the signalling channel is on a higher level than the protocol used for the speech channel.

In Fig. 2 it is shown that two calls are going on between subscribers connected to the first corporate exchange PINX1 and subscribers connected to the second corporate exchange PINX2. The subscriber terminals between which the calls are going on are for the sake of clarity omitted in the figure. A first call B I is going on via the second lines T2 and T12 leaving the first and the second corporate exchanges, and a second call B2 is going on via the third lines T3 and T13 leaving the first and second corporate exchanges. Corporate network specific information for both these calls BI and B2 is transferred via a signalling channel (D1-2) which is established via the fourth lines T4 and T14 leaving the first and second corporate exchanges.

In the same way two calls B 1 and B2 are going on between the first corporate exchange PINX1 and the centrex unit CENTREX AXE via the seventh line T7 leaving the first corporate exchange and the first line T20 leaving the centrex unit, and via the eighth line T8 leaving the first corporate exchange and the second line T21 leaving the centrex unit. A signalling channel (D 1-2) is also established for corporate network specific information concerning these calls via the ninth line T9 leaving the first corporate exchange and the third line T22 leaving the centrex unit.

The signalling channels (D1-2) are established by D-over-B signalling, i.e. they are seen by the public network IVN as ordinary speech channels, B-channels, but by the exchange units as D-channels. Corporate network specific information for many more calls than two can of course be transferred via only one such signalling channel.

WO 98/06224 PCT/SE97/01319 9 Fig. 3 shows a flow chart of a method for setting up calls according to the invention and Fig. 4 a schematic showing with arrows how the set up of a signalling channel and a speech channel between two specific exchange units is made. In Fig. 4 eight vertical unbroken lines are shown, from left the to the right denoted TC, DRC, Q, C, C, Q, DRC, TC. A space between the lines denoted C is denoted IVN. TC (Terminal Connection) farthest left here denotes a subscriber connected to the first corporate exchange, DRC (Dynamic Route Controller) thereafter denotes the control means in the first corporate exchange, Q denotes the signalling interface in the first corporate exchange, C the traffic interface between the first corporate exchange and the public network and IVN the public network. Thereafter, C denotes the traffic interface between the second corporate exchange and the public network, Q the signalling interface within the second corporate exchange, DRC the control means in the second corporate exchange and finally TC a subscriber connected to the second corporate exchange.

Fig. 4 shows how information is transferred between two corporate exchanges.

How a signalling channel and a speech channel is set up between two corporate exchanges will now be described with reference to Figs. 3 and 4. Figures and methods are shown and described only for the case with two corporate exchanges, but the method can obviously also be applied to a corporate exchange and a centrex unit or to two centrex units.

With reference to Figs. 3 and 4 the method according to the invention operates in the following manner. When a first subscriber, denoted TC in Fig. 4 and connected to a first corporate exchange, wants to reach a second subscriber, also denoted TC in Fig. 4, where both subscribers belong to the same corporate network but different corporate exchanges, the first subscriber dials a phone number corresponding to the second subscriber's subscription. This is shown in Fig. 4 by an arrow between the lines TC and DRC in Fig. 4, denoted "outg.call" and which states that the first sub- WO 98/06224 PCT/SE97/01319 scriber calls the control means in the first corporate exchange. The control means in the first corporate exchange detects this call, method step 20, and executes an analysis of the number of the second subscriber, usually called B-subscriber. If the control means after this analysis discovers that the B-number is for a subscriber not belonging to the corporate network, method step 22, it will see to it that the call is set up as an ordinary telephone call via the public network, method step 24. If, however, the control means discovers that the number belongs to a subscriber connected to another corporate exchange in the same corporate network, method step 22, this second corporate exchange is identified using the number dialled by the first subscriber and a new analysis is executed. In the new analysis the control means investigates if a signalling channel is set up between the two corporate exchanges, method step 26. If there is a signalling channel set up, the control means selects a particular speech connection or DID-number for use as a call to the second corporate exchange via the public network IVN. Thereafter a speech channel is set up for speech between the two corporate exchanges and corporate network specific information is transferred to the second corporate exchange over the already established signalling channel, method step 28.

If, however, no signalling channel is established, method step 26, the control means selects a particular signalling channel request number and a signalling channel response number from the DID-memory, which numbers are stored in the DIDmemory in advance, and uses these numbers for calling the other corporate exchange, method step 30. This is done in such a way that the control means requires set up of a call sent from the control means to the C-reference point, denoted setup- Bcha (set up B-channel) and signals to the other corporate exchange via the public network IVN, this is illustrated by the signal seizure in Fig.4. The signal seizure, which is a known call signal according to the protocol the public network is using, comprises the signalling channel request number and the signalling channel response number. If the public network were a PSTN-network, only one number would, WO 98/06224 PCT/SE97/01319 11 however, be used, the signalling channel request number. The two numbers are transferred from the public network to the control means in the second corporate exchange, denoted estdrind (establish dynamic route individual). The second control means executes a check of the signalling channel response number, connects its signal terminal to the signalling interface and answers the call with the obtained signalling channel response number if this number was correct, method step 32. The connection of a signal terminal in the second corporate exchange is shown by two arrows going in different directions between the line Q and DRC to the right in Fig.

4, and the answer of the call, made with the signalling channel response number is denoted estdrindack (establish dynamic route individual acknowledged). Subsequently, this B-channel which is to be used as the signalling channel through the public network, is connected, shown by "connected" which is also a known call signal according to the protocol used by the public network, and the control means in the first corporate exchange obtains a signal Bchacon (B-channel connected), stating that the B-channel is set up. Thereafter the first control means connects its signal terminal to the signalling interface, stated by two arrows going in different directions between the lines Q and DRC to the left of Fig. 4. Thereafter the two signal terminals contact each other under control of the first signal terminal, which is shown by the signal estL2 (establish L2) being sent from the first control means to the signalling interface.

Thereafter a stationary connection is established between the two signal terminals between the two signalling interfaces, which is shown by the dashed bidirectional line denoted Layer 2. This connection is established using handshaking between the two signal terminals using an HDLC-protocol. The handshaking is completed by respective signal terminals being told that the signalling channel is established with signals denoted L2est (L2 established). If the public network had been a network of PSTN-type and no signalling channel response number had been transferred during signalling, identification of the two signalling channels would subsequently occur WO 98/06224 PCT/SE97/01319 12 over the established signalling channel, i.e. a check would have been executed to determine if the second corporate exchange was the one to be connected to the first corporate exchange. After this, the signalling channel is established and corporate network specific information concerning the call which is to be set up is transferred over the signalling channel, method step 33.

When the signalling channel in this way has been established between the two corporate exchanges, a speech connection or a DID-number is selected, and the second exchange is called using this DID-number to establish the speech channel, method step 34. This is performed such that a signal estdrreq (establish dynamic route request) is sent from the control means in the first corporate exchange to the public network IVN to call the second corporate exchange, which occurs through the public network using the signal seizure. This signal also comprises DID-number and an answer number. The call is received by the second corporate exchange as the signal estdrind (establish dynamic route individual). The second corporate exchange checks the answer number and replies using the answer number and in dependence of the check by the signal estdrindack (establish dynamic route individual acknowledged), which is transferred through the public network as the signal "connected" and received by the first corporate exchange as estdrreqack (establish dynamic route request acknowledged). The B-channel is thereby established and a call can then continue on this speech channel at the same time as corporate network specific information, such as e.g. A- and B-numbers, is transferred via the signalling channel.

If the public network was a PSTN-network, the check of call numbers for the speech channel would occur via the signalling channel before set up of the speech channel.

The signalling channel is then connected for as long as any call goes on between the two corporate exchanges and is used for transfer of corporate network specific information for all calls going on between the two corporate exchanges. When the last WO 98/06224 PCT/SE97/01319 13 call is disconnected, the signalling channel is also disconnected. The disconnection of channels is well-known to the man skilled in the art and will therefore not be further described here.

With the invention as described above a number of advantages are obtained. Firstly, there is no need for any leased lines between the two exchange units when no corporate network specific information is to be transferred between them, which avoids using expensive leased lines or other stationary connections. Secondly, an exchange unit is used more efficiently, i.e. the capacity needed to keep such a connection connected, when no corporate network specific information is to be transferred between two exchange units, can be used for other purposes.

A number of different embodiments of the invention are possible. The mentioned checks of request numbers need not be executed and may also be partly executed.

Furthermore, if a call is to be set up between a subscriber connected to a first corporate exchange and a second subscriber connected to a second corporate exchange, and it is the first subscriber who is calling the second subscriber, one can according to a different embodiment set up the actual speech channel from the second subscriber such that the public network sees it as if the second subscriber calls the first subscriber. This would in such cases be achieved in that the first control means sees to it that when a signalling channel has been established, a request is transferred for set up of a speech channel via the signalling channel from the first corporate exchange to the second corporate exchange, and the second corporate exchange then sets up the speech channel. This can be of interest if it is cheaper to call from the second corporate exchange than from the first corporate exchange, e.g. depending on different fees in different countries.

The choice of signalling channel response number and DID-number can also be executed in the second corporate exchange. The numbers used for request and response WO 98106224 PCT/SE97/01319 14 to request at set up of speech channels are usually chosen in a known manner according to any local numbering plan within the first corporate exchange, but it would also be possible to use both the A- and B-numbers, i.e. the ones directly belonging to the two subscribers.

A further embodiment of the invention is that speech compression can be used. The corporate exchanges then comprise a voice compression unit which preferably also comprises the signal terminal. A voice compression unit compresses speech according to a compression algorithm so that a speech channel can fill a much smaller part of a speech connection between two corporate exchanges (normally a quarter). The signal terminal receiving a compressed channel expands (in its voice compression unit) the compressed speech to speech of ordinary size. In this way a number of connections between the corporate exchanges can be further reduced, and one can even make sure that the connection or the channel established as the signalling channel is also used for speech channels. This connection normally includes three speech channels and the signalling channel, and more connections are only used when the entire signalling channel connection space has been used.

The control unit in an exchange unit is often also equipped with error check equipment for watching channels set up via the public network regarding error and for reconnecting a by mistake disconnected channel, e.g. due to a fault in the public network.

In the description it has been mentioned that speech channels are established for calls between two subscribers. Such a speech channel can of course also be established for transfer of data or video.

The present invention is particularly suitable when exchange units in a corporate network are located far away from each other. They may even be located in different countries.

The invention may also be combined with establishing signalling and speech channels in dependence of calendar data.

The invention is naturally not limited to the above described and in the drawings shown embodiments but can be modified within the scope of the annexed claims.

The terms "comprise", "comprises", "comprised" and "comprising" when used in this specification are taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

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Claims (15)

1. Method for connection of two mutually associated exchange units (PINX1, PINX2, CENTREX AXE) via a public telecommunication network (IVN) the, method including detection, in a first exchange unit (PINX1), of a request for connection of a first subscriber (14) connected to the first exchange unit to a second subscriber connected to the second exchange unit (PINX2)(20), establishing at said detection, a specific signalling channel (D 1- 2) through the network for the two exchange units if no such signalling channel already exists (26,33), and establishing a speech channel (B1,B2) between the two exchange units (28,34) for connection of a call between the first and the second subscriber, characterized in that the establishing of the speech channel (B1,B2) includes transfering a request for set up of the speech channel over the signalling channel (D1-2) from the first exchange unit (PINX1) to the second exchange unit (PINX2), selecting a particular speech connection number in the second exchange unit and setting up the speech channel (B1,B2) from the second exchange unit to the first exchange unit through the public telecommunication network (IVN) using the particular speech connection number.

2. Method according Claim 1, characterized by transfering corporate network specific information via the signalling channel (D1-2) for the call between the first and the second subscriber, said transfer occurring even if a previously set up speech channel (B1,B2), for which the signalling channel first was established, has been disconnected (28).

3. Method according to Claim 1 or 2, characterized by disconnecting the signalling channel (D1-2) if all the speech channels (B1,B2) have been disconnected, which were connected between the two exchange units (PINX1,PINX2) and for which corporate network specific information has been transferred using the signalling channel (D1-2).

4. Method according to any of the previous claims, characterized in that the establishing of the signalling channel includes selecting, in the first exchange unit (PINX1), of a particular signalling channel request number and transfer of a request using the particular signalling channel request number from the first exchange unit to the second exchange unit (PINX2) for establishing the signalling channel Method according to Claim 4, characterized in that the establishing of the signalling channel (D1-2) also includes selecting, in the first exchange unit (PINX1), of a signalling channel response number, which is transferred together with the signalling channel request number in the transferred request, and answering the request from the second exchange unit (PTNX2) using the signalling channel response number in dependence of a signalling channel response number check for establishing the signalling channel (32).

6. Method according to Claim 4, characterized in that the establishing of the signalling channel includes checking the signalling channel using I: handshaking on the signalling channel before corporate network specific information is transferred, and disconnecting the signalling channel if the handshaking is not correctly executed. *000 :0 0

7. Method according to any of the previous claims, characterized by compressing each speech channel and establishing several speech channels via one and the same connection through the public telecommunication network.

8. Method according to Claim 7, characterized in setting up speech channels via the connection established through the public telecommunication network for the signalling channel in such a way that space corresponding to a compressed speech channel is used for the signalling channel, and the rest of the space is used for compressed speech channels. 18

9. Exchange unit (PINX1) for connection to a public telecommunication network (IVN) and to which subscriber terminals can be connected, the exchange unit including a control means (12) arranged to detect a request originating from a first subscriber terminal (14) connected to the exchange unit for set up to a second subscriber terminal connected to a second exchange unit (PINX2) and, in dependence of said detection, arranged to establish by signalling a particular signalling channel (D1-2) through the public telecommunication network (IVN) between the own exchange unit (PINX1) and the second exchange unit (PINX2), if no such signalling channel already exists, and to set up a speech channel (B1,B2) between the first and the second subscriber terminals for set up of a call between them. characterized in that the control means (12) at the establishing of the speech channel (B1,B2), is also arranged to transfer a request for set up of the speech channel over the signalling channel (D1-2) to the second exchange unit (PINX2), such that the second exchange unit can select a particular speech connection number and set up the speech channel (B1,B2) from the second exchange unit to the first exchange unit through the public telecommunication network (IVN) using the particular speech connection number. Exchange unit according to Claim 9, characterized by a switching core and a number of outgoing lines (T1-T10), wherein said control means (12) being connected to the switching core and each connection point for a subscriber terminal (14) can be switched to an outgoing line via the switching core.

11. Exchange unit according to Claim 10, characterized by a DID-memory (16) connected to the control means (12).

12. Exchange unit according to any of Claims 10 or 11, characterized by a signal terminal (ST) for each additional exchange unit to which the exchange unit is to be able to be connected, each signal terminal being connected to the switching core (10) so that it can be connected to any of the outgoing lines. 19

13. Exchange unit according to any of Claims 9-12, characterized in that, if a signalling channel (D1-2) is already established through the network between the exchange unit and a second exchange unit, the control means is arranged to provide that this signalling channel (D1-2) is used for transfer of corporate network specific information for subsequent connections of subscribers between these two exchange units even if the first requested, set up speech channel has been disconnected or is to be disconnected.

14. Exchange unit according to Claim 9 or 10, characterized in that the control means (12) is arranged to provide disconnecting of the signalling channel if all the speech channels which have been set up between the two exchange units (PINX1, PINX2) have been disconnected for which exchange units corporate network specific information has been transferred using a signalling channel. Device for connection to a public telecommunication network (IVN) and including at least a first and second exchange unit (PINX1, PINX2, CENTREX AXE) to which subscriber terminals (14) can be connected, each exchange unit (PINX1, PINX2, CENTREX AXE) includes a control means (12) or has access to a control means arranged to be able to detect a request, coming from a first subscriber terminal connected to the associated exchange unit (PINX1), for set up to a second subscriber terminal connected to a second exchange unit (PINX2, CENTREX AXE) and, in dependence of said detection, arranged to establish by signalling a particular signalling channel through the public telecommunication network (IVN) between the own exchange unit and the other exchange unit, if no such signalling channel already exists, and to establish a speech channel (B1,B2) between the first and the second subscriber terminal for set up of a call between them, characterized in that the control means.(12) in the first exchange unit (PINX1) at the establishing of the speech channel (B1,B2) is also arranged to send a request for set up of a speech channel over the signalling channel (D1-2) to the second exchange unit (PINX2) and the second exchange unit is arranged to select a particular speech connection number and set up the speech channel from the second exchange unit (PINX2) to the first exchange unit (PINX1) through the public telecommunication network (IVN) using the particular speech connection number.

16. A method for connecting two mutually associated exchange units via a public telecommunication network, substantially as herein described with reference to the accompanying drawings.

17. Exchange unit for connection to a public telecommunications network and to which subscriber terminals can be connected substantially as herein described with reference to the accompanying drawings.

18. Device for connection to a public telecommunications network and including at least two exchange units to which subscriber terminals can be connected, substantially as herein described with reference to the accompanying drawings. e* DATED this 16 t h day of February, 2000 TELEFONAKTIEBOLAGET LM ERICSSON WATERMARK PATENT TRADEMARK ATTORNEYS °o 290 BURWOOD ROAD HAWTHORN VICTORIA 3122 AUSTRALIA RCS:SMM:PCP P6340AU00 20000216_AMENDEDSPECI