GB2171274A - Telephone subset - Google Patents

Abstract

The situation sometimes arises where two telephone systems have to interwork, one example being those of British Telecom (BT) and Mercury Communications Limited (MCL). In the present case when a call is to be made from a BT (or dual access) line to Mercury, a second proceed-to-send (dial tone) signal is used when access to Mercury is needed. To use this, a subscriber's set includes a detector (22) for the second proceed-to-send, but that signal is not heard by the subscriber. The set also has a "last number keyed" store (19), and when the second proceed-to-send is detected, a Mercury authorisation code is sent from a generator (23) thereof to cause call setting in the Mercury system. The set also enables follow-on calls to be made into the Mercury network. Note that the techniques described are applicable to other networks where two telephone systems interwork. <IMAGE>

Description

SPECIFICATION Telephone subset This invention relates to telephone networks and telephone subscribers' apparatus for use where two separate telephone systems have to interwork.

There are now two switched network carriers in the UK; the long established British Telecom (BT) and the newer Mercury Communications Limited (MCL). The licensing arrangements for the two carriers include the provision that interfaces must be provided between the networks to enable BT subscribers to call MCL subscribers, and vice versa, and to enable joint BT-MCL subscribers to use the MCL network for trunk bypass should they so wish. Additionally it is necessary to allow MCL subscribers to "escape" to BT.

The interfaces between the two networks have now been defined. Traffic is passed from the BT network to the MCL network by a PBX group on a local exchange and/or a junction group from a Group Switching Centre (GSC). The PBX group is known as a 3L interface, and the junction group interface is known as a 3J interface. It has been agreed that signalling from the BT network into the MCL network will take the form of DTMF (MF4) and signalling in the reverse direction will take the form of conventional loop interrupt DC signalling or other forms where appropriate.

Objects of the invention are to provide a telephone subscriber's set suitable for use where two networks have to interwork; one example of an application of such a subset being the BT-MCL case referred to above, and to enable follow on calls to the second network.

According to the invention, there is provided an automatic telecommunications network, which includes two separate switching systems so interconnected that calls can be made from a subscriber's apparatus over either or both of said systems, in which when access is needed from one of said systems to the other of said systems a second proceed to send signal is received if that other system can accept the call, in which the wanted number on "dialling" is placed in a storage device at the caller's instrument, in which when during the call setting operations a said second proceed to send signal is received at the subscriber's apparatus that signal is not perceived by the caller, and in which the reception of the second proceed to send signal automatically causes the transmission of an access code appropriate to the other switching system, whereafter the wanted number and any other information required are sent without any further action by the caller and without him hearing the second proceed to send signal.

An embodiment of the invention will now be described with reference to the drawings, in which Figure 1 shows the disposition of the long established BT network, and of the proposed MCL network, and Figure 2 is a schematic representation of a telephone subscriber's set for use in such a system.

System background Referringfirstto Figure 1, in the BTnetwork, both individual direct exchange lines such as 1 and PBX group subscribers such as 2 are shown. For the MCL network, as it is oriented towards the business user, there are MCL only PBX group such as 3 and dual access PBX groups such as 4 where the choice of outgoing route may be determined in the PBX and a choice between BT or MCL routing exists. Such a choice can also be made at the local BT exchange, such as 5. The BT network also includes Group Switching Centres (GSC) and Trunk Switching Centres (TSC).

The basic MCL network consists of three trunk switches 6, 7, 8 to start with. Note that it is currently fashionable to call a telephone exchange or switching centre a switch. The subscriber groups and interfaces from BT route first to an MCL node such as 9 at which signalling and transmission systems are rationalised before being applied to the combined digital trunk group (2Mbit/s) to the MCL trunk switch such as 6.

Subscribers may be: (a) BTsubscribers only, in which case they are constrained to use the BT network as far as possible. When calling another BT number there is no change from the present. However, calling an MCL subscriber they dial the MCL area code [(1-9)xx] and the local MCL number code, this is part of BT national numbering. BT only subscribers are barred from using the MCL network as a trunk bypass (see MCL subscribers).

(b) BTand lndirect DualAccess. Such subscribers have the option of "escaping" to MCL or using the BT network to access the local exchange by a common group of exchange lines. Discrimination between BT and MCL calls may take place at the originating local BT exchange or at the originating GSC. If the former, access to MCL is allocated a number in the local range-number length undefined. If the latter, the dual access subscriber keys an access code, in the present case 0710, which routes to the appropriate MCL switch.

When connected to MCL, a second dial tone, also called a proceed-to send signal, is given by the MCL switch and in response the subscriber must send his authority code and full wanted subscriber number in the multi-frequencyform (DTMF).

(c) MCL only subscribers. Such a subscriber has a code as follows: Prefix Area Code Number Code 071 [(1-9)xx] [4 or perhaps 5 digits] The subscriber's authority and personal identity could also be sent to the MCL exchange.

(d) BTand MCL direct dual access, Here separate groups of circuits to BT and MCL are used. The discrimination for originating calls is then at the PABX, in which case a single discriminating digit may be used at the PABX. The subsequent procedure is as for BT Only or MCL Only subscribers.

(e) MCL direct to BT. This is the reciprocal operation of an indirect dual access subscriber, in that the MCL subscriber can choose whether or not to use the BT network.

It will be appreciated that the need to carry out these manoevres may cause complexities.

To enable the above needs to be met, we provide a new telephone set directed at enabling a dual-access exchange line subscriber connected to a BT local exchange to access the MCL network, without consciously being aware of the second dial tone situation, or the need to change from loop interrupt to DTMF signalling.

Further it can simplify the MCL only operation, and can go some way to preparation for Integrated Services Digital Network (ISDN) working. The problem to be solved is thus the provision of a new telephone set, wherein the subscriber dials, or rather keys, once, only the wanted number, whether it be a BT or MCL number, or whether it is a BT number using the trunk bypass facility of MCL. All the ancilliary functions are taken care of automatically.

In preparation for ISDN with common (or separate) channel signalling the operation of the telephone set can remain the same, changes to its electronic function adjusting to the new conditions.

The new Telephone Set Arran gem en t. This is shown schematically in Figure 2. Essentially, the new set involves modifications to an existing push button loop-interrupt dialling telephone set with "last number called" memory 19. The additional equipment includes a circuit 20 generating the DTMF (MF4) tones, which can form part of the electronics backing on the keypad which is normally a DC only device 21 for the loop interrupt signalling, an "0710" code sender, (0710 being the MCL access code now proposed for use), an MCL route detector 31, a second dial tone detector 22 (for detecting the MCL dial tone), an authorisation code generator 23 (which may have wired "plug" inserts 24, 25 carrying the code), and a means 26 to inhibit the receiver of the telephone set (between the receipt of the second dial tone and the completion of DTMF signalling into MCL).The control circuit 27 ensures the sequence of operations. The various items referred to can be seen in Figure 2.

The operation is, in outline, as follows: (a) In normal use, in calling into the BT Public Switched Telephone Network (PSTN), the subscriber off-hooks and dials by loop-interrupt key pulses sent from the contacts 21 (or any electronic switch) to set up the required call in the normal manner. In this mode, the telephone set operates in the conventional manner with DC supervision. Electronic sets in which a number keyed in by push-button operation is automatically converted into loop interrupt pulses are well known.

(b) When a dual access (indirect) subscriber requires access to the MCL network, the subscriber off-hooks in the normal way and receives BT dial tone. He then presses the * key on the keypad 30 to indicate MCL access required. This key operation is detected by a detector 31 which "tells" the sequence control 27 that a call to the MCL network is needed. The sequence control now pulses the access code 0710 followed by the originating subscriber's number, an authorisation code, and either the full BT STD number (assuming trunk bypass utilisation} or the remainder of the full MCL directory number (for calling an MCL subscriber). Since the keypad-to-loop interrupt conversion mechanism stores the last dialled number in the unit 20, the whole string of numbers is now held in the memory of this circuit until overwritten by further operation of the keypad.The MCL route detector 31 enables the connection of the second dial tone detector 22 across the line, with the assistance of the sequence control 27. Ifforsome reason the caller fails to connect to the MCL network, or receives a busy tone in place of a second dial tone, it is assumed that the subscriber eventually hangs up. However, assuming the receipt of the second dial tone this causes two actions. The receiver in the telephone set is inhibited by the inhibit circuit 26so that the second dial tone is not heard, and at the same time the authorisation code (AC) and originating subscriber number from the generator 23 is activated. This causes the DTMF electronics 20 to send the subscriber authorisation code through the BT network to the MCL switch.After this the directory number stored in the last number keyed store 19 causes the DTMF electronics to send all this directory number information in DTMF form to the MCL network via the BT switches. After this, the receiver inhibit signal is negated, and the telephone set now appears as a conventional telephone set but connected through to the MCL network which has received the called directory number information.

Upon completion of a call the caller normally hangs up and this causes the cradle switch to open the line supervisory loop causing the BT local exchange and the subsequent BT and/or MCL network connections to release (normal calling party release system).

However, at the end of the first call the telephone set arrangement provides the caller with the option of a follow-on call to the second network without releasing the connection to the first network. Instead of hanging up in the normal manner the caller presses the 9 key a second time. This causes a DTMF signal representing # to be sent through the BT network into the MCL network which on receipt of the # signal releases the first connection and returns the MCL dial tone to the caller via the held connection in the BT network. The receiver in the telephone set is again inhibited by the inhibit circuit 26 so that the second dial tone is not heard, and at the same time the authorisation code (AC) and the originating subscriber number from the generator 23 is activated and causes the DTMF electronics 21 to send the subscriber authorisation code through the BT network to the MCL switch. After this identity and authorisation information has been sent, the receiver inhibit circuit 26 is negated, and the caller hears the second dial tone in the receiver of the telephone set.The caller then keys in the directory number required for the follow-on call and the DTMF electronics send all this directory number information in DTMFform to the MCL network via the BTswitches. After this, the telephone set appears as a conventional telephone set but connected through the MCL network for a follow-on call. Alternatively, if the caller wishes to recall the call that he has just cleared then on receipt of the MCL dial tone in the receiver of this telephone set, he can then press the recall key 34, which causes automatic re-sending in DTMF form of the last "dialled" number after the telephone set has automatically sent the caller's directory number and authorisation code in DTMF form.

The process of generating follow-on calls may be repeated indefinitely.

When an MCL subscriber wishes to use his option of routing through the BT network the operation is similar to that for BT-MCL access, but he presses the * key to choose BT. This is also detected in the Mercury route detector 31, with the result that the initial transmission is in multi-frequency pulses but the BT selection information is dialled out.

When an MCL subscriber wishes to make a follow-on call through the BT network the operation is similar to that described above but he presses the * key to maintain the MCL connection and to cause clearance and re-seizure of the BT network. In this case, pressing the * key causes a DTMF signal to be sent through the Mercury network to the BT interface whereupon it cauises release and re-seizure of the BT network.

Re-seizure of the BT netowk may result in either a dial tone indication from the BT network or no indication from the BT network, depending upon the point of interfacing between MCL and BT for calls from MCL to BT.

Because the BT network is unable to act upon any authorisation code or calling line identity information transmitted from the calling subset, this information is not transmitted in this case. Thus, dial tone from BT, if present, is presented directly to the receiver of the telephone set via the Mercury network. If dial tone from BT is not present, and no other tone from BT is received by the telephone subset via the MCL connection within a defined time-out as determined by the second dial tone detector 22, and the sequence controller 27, then a local dial tone generator 35 in the telephone set is enabled to indicate via the receiver of the telephone set that the caller should now key in the directory number of the subscriber required for the follow-on call.

This directory number may be the full number or alternatively a recall of the previous number by pressing the recall key 34. As soon as any of the selection keys or recall key on the keypad is pressed, the local dial tone generator 35 is inhibited. Operation of the selection keys or the recall key causes the DTMF electronics to send the directory number for the follow-on call in DTMF form across the Mercury network to the interface with the BT network whereupon it is converted back into conventional dial pulses which are then received by the BT network and used for setting up the required call.

When ISDN is introduced the addition of an ISDN unit 32, shown dotted, permits the system to have one or two 64 kbit/s channels and one 1 6kbit/s channel. This last is shared by signalling and slower speed data, and provides for separate channel signalling; the whole is carried on the subscriber's pair. There are alternative arrangements in which signalling has a discrete 8 kbit/s channel while slow speed data is handled on another discrete 8 kbit/s channel.

The advantage of the system described is that the originating subscriber is not conscious of the second dial tone from MCL, and does not have to re-key the called subscriber directory number in a DTMF form; this is automatically done for him. This means, that in a trunk bypass situation the subscriber keys # + 10 digits only, whereas if manual keying in response to a second dial tone was required, some 23 digits in total may have to be keyed. This is because in the absence of the above-described arrangements, a whole number may have to be redialled. For calls to MCL only subscribers in the present arrangement, only 10 digits would need to be keyed, avoiding the need to manually add authority code and possibly personal identity code (PIN).

Also note that operation in MCL direct connection demands no second dial tone when completed via the MCL trunk network; operations then remain the same for all uses except choice of MCL or BT networks.

The advantage of providing facilities for a follow-on call is that this minimises the call costs incurred in the first of the two networks.

It will be appreciated that a set such as described above is suitable for any application where a second dial tone must be accepted. One such application would be for cellular radio.

Claims (8)

1. An automatic telecommunications network, which includes two separate switching systems so interconnected that calls can be made from a subscriber's apparatus over either or both of said systems, in which when access is needed from one of said systems to the other of said systems a second proceed to send signal is received if that other system can accept the call, in which the wanted number of "dialling" is placed in a storage device at the caller's instrument, in which when during the call setting operations a said second proceed to send signal is received at the subscriber's apparatus that signal is not perceived by the caller, and in which the reception of the second proceed to send signal automatically causes the transmission of an access code appropriate to the other switching system, whereafter the wanted number and any other information required are sent without any further action by the caller and without him hearing the second proceed to send signal.

2. A network as claimed in claim 1, in which a subscriber with access to said other switching system is allotted an authorisation code which gives him access to said other network, and in which said authorisation code is sent from the subscriber's apparatus as said other information.

3. A network as claimed in claim 1 or 2, in which the wanted number digits sent from a telephone subscribers' apparatus connected to said first system for a call to that first system are sent as loop-disconnect impulses, and in which the wanted number digits sent from such apparatus for a call to the other system are sent using multi-frequency code signalling.

4. A network as claimed in claim 1,2 or 3, in which to ensure that the calling subscriber does not hear the second proceed to send signal his receiver circuitry is disabled in response to the operation by the caller of a key or its equivalent appropriate to a call to the other network.

5. A network as claimed in claim 1, 2, 3 or 4, in which the subscribers served by the second of said systems consist of PBX groups of lines, some of which groups have access to both said systems while the other of said groups have access to only the second of said systems.

6. A network as claimed in claim 1, in which after a call from said one network to said other network another call to said other network (i.e. a follow-on call) may be sent in response to the transmission by the caller of a special signal identifying a follow-on-call, and in which the reception of a said follow-on signal in the other network causes said second proceed to send signal to be sent from the other network to the caller's instrument, which second proceed to send signal is heard by the caller, whereafter he can "dial" the wanted number for that follow-on call.

7. A telephone subscriber's set for use in conjunction with two separate switching systems so interconnected that when a call is to be made from the first of said systems to the second of said systems a special proceed to send signal is received at that set, wherein the set includes a last number keyed store into which a number keyed from the set is inserted, a detection circuit responsive to a said special proceed to send signal to initiate the transmission of an access code appropriate to said second system, which special proceed to send signal is not perceived by the subscriber using the set, and means for sending the wanted subscriber's numberto said second system in response to the reception therefrom of an indication that that system can accept the call.

8. A telephone subscriber's set, substantially as described with reference to Figure 2 of the accompanying drawings.