GB2310778A

GB2310778A - Telephone network signalling

Info

Publication number: GB2310778A
Application number: GB9604381A
Authority: GB
Inventors: Christopher William Lewi Hobbs
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1996-02-28
Filing date: 1996-02-28
Publication date: 1997-09-03
Anticipated expiration: 2016-02-28
Also published as: GB9604381D0; GB2310778B

Abstract

A telecommunications network comprises a switch 210 and a terminal 200 joined by a traffic channel. A part of the channel is provided by a digitally multiplexed link 310 which has a restricted bandwidth. Terminal 200 and switch 210 communicate using signaling messages which occupy a frequency band lying outside of that restricted bandwidth, such as an 8kHz carrier tone. Signaling converters 340, 350 receive the 8kHz signaling messages, convert the received messages to a different frequency band, such as FSK tones lying within the speech band, and re-transmit the converted signalling messages. The converters may buffer received messages and re-order them before re-transmission. The converters may also append flags to the beginning and end of speech-band signaling to differentiate signaling from speech or data on the channel.

Description

TELEPHONE NETWORK SIGNALLING This invention relates to signalling within a telephone network.

In the local subscriber loop of a telephone network signalling occurs between a switch and a terminal to perform tasks such as call set-up and disconnection. One known method of signalling is loop disconnect signalling. This method modifies the state of the direct-current loop between the switch and the terminal and is effective for transferring simple information. Another known method of signalling is multifrequency (MF) signalling. This method uses various combinations of two voice-band frequencies and offers a faster transfer of signalling information.

A further type of signalling system is shown in figure 1. This uses a carrier tone 110 lying outside of the frequency band 100 allocated to speech to carry signalling information. In a system manufactured by Nortel known as 'P Phone' speech is limited to a bandwidth of around 3.5kHz and an 8kHz out-of-band tone is modulated to carry signalling information.

A variety of features can be provided with this additional signalling capacity, such as display of calling number, display of time and date and automatic call redirection. Terminals typically have a display and a number of keys to invoke these extra features.

Terminals which use this out-of-band form of signalling are usually connected directly to a switch, as shown in figure 2. A terminal 200 couples to a wall socket 230. A direct connection is made via a line 220, such as a twisted-wire pair, to exchange 210. This arrangement is practical for an office, where the exchange will be close to the terminal, or in a network where individual lines link each terminal to the switch.

Switch 210 couples to other parts of the telecommunications network via trunks 260.

An alternative to individually coupling each terminal directly to the switch is to use a remote unit installed nearby to a group of terminals. The remote unit multiplexes the speech bands for the group of terminals and combines the out-of-band signalling for the group of terminals into a separate channel on the multiplexed line. Remote units are expensive and require substantial local power and space.

It is an object of the present invention to allow terminals which use an out-of-band signalling system to be connected to a switch in a manner other than those just described.

According to the present invention there is provided a telecommunications network comprising a switch and a terminal joined by a traffic channel, a part of the channel having a restricted bandwidth, wherein the terminal and switch communicate using signalling messages occupying a frequency band lying outside of that restricted bandwidth, there being first and second signalling converters associated respectively with the switch and the terminal, the signalling converters being arranged to receive signalling messages, to convert the received signalling to a different frequency band and to re-transmit the converted signalling whereby to convey the signalling within the restricted bandwidth over the restricted bandwidth part of the channel.

Preferably the terminal and switch also communicate using a speech band lying within the restricted bandwidth, the converters being arranged to convert the signalling lying outside the restricted bandwidth to signalling lying within the speech band.

Preferably the converters are arranged to convert the signalling lying outside the restricted bandwidth to frequency-shift-keyed signalling lying within the restricted bandwidth.

Preferably the converters are arranged to append flags to the beginning and end of each signalling message lying within the restricted bandwidth whereby to differentiate between signalling messages and speech or data on the channel.

Preferably the converters are arranged to acknowledge receipt of a signalling message before re-transmitting that message.

Preferably the converters have a buffer to store a group of siynalling messages, and are operable to re-order messages stored in the buffer before re-transmitting them. This feature has the advantage of minimising the number of in-band signalling tones which a subscriber hears.

According to another aspect of the present invention there is provided a method of operating a telecommunications network comprising a switch and a terminal joined by a traffic channel, a part of the channel having a restricted bandwidth, the method comprising signalling between the terminal and switch using signalling messages occupying a frequency band lying outside of the restricted bandwidth, and receiving, at first and second signalling converters associated respectively with the switch and the terminal, signalling messages, converting the received signalling to a different frequency band and re-transmitting the converted signalling whereby to convey signalling within the restricted bandwidth over the restricted bandwidth part of the channel.

In this manner terminals and switches which use an out-of-band signalling system can be connected together using conventional digitally multiplexed lines with a bandwidth of 64kb/s. This allows a significant cost reduction compared with providing individual direct connections for terminals or providing remote units for groups of terminals.

The embodiments described use a modulated 8kHz out-of-band signalling tone, but it will be apparent that other frequencies could be used.

Embodiments of the invention will now be described with reference to the accompanying drawing sheets, in which: Figure 1 shows the allocation of bandwidth in a signalling system which uses an out-of-band tone; Figure 2 shows a terminal directly connected to a switch in a known manner; Figure 3 shows an arrangement in which signalling converters are installed at each end of a multiplexed link; Figure 4 shows an alternative arrangement to that of figure 3; Figure 5 shows one of the signalling converters shown in figures 3 and 4 in more detail.

Figure 3 illustrates an embodiment of the present invention. As in figure 2, a terminal 200 is coupled to a switch 210. However, part of the route between the switch and exchange is provided by a digitally multiplexed network 310, having multiplexers 320, 330 at either end of trunk lines 325. Typically this multiplexing equipment limits the bandwidth of a call to the range 300Hz - 3200Hz before sampling and quantizing the call to generate a 64Kbit/s data stream. A number of such data streams are multiplexed together on a trunk 325. At the other end of the trunk a complementary demultiplexing process reconstructs a set of analogue signals for onward transmission. Attempts to pass an out-of-band signalling tone over the multiplexed network will be unsuccessful as the bandwidth limiting process will remove this tone.

A first signalling converter 350 is inserted into the path between terminal 200 and multiplexer 320. The operation of this converter for signalling flowing from terminal 200 in direction 301 will be considered. Converter 350 receives signalling messages from terminal 200 which are carried by an Amplitude Shift Keyed (ASK) 8kHz carrier tone. The tone is demodulated and the received data are re-transmitted as Frequency Shift Keyed (FSK) tone signalling liying within the speech band. The inband tones can be carried over section 310 of the route where the bandwidth is limited. This first converter 350 also performs a complementary conversion of signalling from in-band tones to an out-ofband 8kHz tone in direction 302. A second signalling converter 340 is inserted into the path between multiplexer 330 and switch 210 to convert in-band signalling tones back to out-of-band 8kHz tone signalling that switch 210 recognises. As with the first converter, converter 340 performs complementary conversions in directions 301 and 302.

A subscriber with a signal terminal 200 has a single converter in the form of a line card 350. This may be housed in a moulded box with an RJ45 connector for plugging into adaptor 360, and a 603A connector for coupling to terminal 200. A mains adaptor 365 couples to a mains supply to generate a 15V supply for powering the display and circuitry of terminal 200. The converter taps this supply to power the conversion circuitry.

At the switch end, a line card 340 is installed which can provide signalling conversion for a number of lines, typically twelve separate lines. A set of these line cards may be installed to provide conversion for a full set of lines leading to switch 210. Conveniently, the line cards may be designed to fit into a rack 345 of the type which houses Programmable Digital Multiplexers (PDMX) 330. The rack provides physical support for the cards and a power supply.

In the alternative arrangement of figure 4, signalling converter line cards are installed in shelf 400 which also houses the multiplexing equipment.

Figure 5 shows converter 350 in more details. The converter performs signalling conversion in two directions. In direction 500 conversion is from an out-of-band 8kHz tone to in-band tones. In direction 510 conversion is from in-band tones to an out-of-band 8kHz tone.

Operation of the converter is controlled by a microcontroller 520, which derives timing from a 14.31818 MHz oscillator 515.

In direction 500, a signal from terminal 200 comprising speech and outof-band signalling enters the converter via socket 501. This is passed through an isolation transformer 511 and to a 2:4 wire hybrid circuit 512.

A portion of the signal is tapped and passes through an 8kHz bandpass filter 513 to isolate the signalling information. This is fed to microcontroller 520 as an interrupt. Baseband data passes from microcontroller 520 to modem 530, where a frequency-shift keyed signal is generated. The V23 modem standard uses a 2100Hz tone to represent a logic '0' and a 1300 Hz tone to represent a logic '1' While signalling is being converted speech passes through amplifier 517 to 4:2 hybrid circuit 516 where the speech and converted in-band signalling are combined. This combined signal is passed through a further isolation transformer 519 and to an output plug 521.

Trailing and leading flags are preferably added to in-band signalling transmissions to allow the next converter placed downstream along the line to detect the in-band tones and differentiate between them and other similar in-band tones occurring as part of speech or data transmissions. These flags are preferably five bit flags which have a characteristic bit pattern.

In direction 510 a complementary conversion process is performed. A signal having speech with in-band signalling enters the converter at plug 521 and passes through isolation transformer 519 to 2:4 hybrid 516. Inband signalling is converted to baseband data by modem 530 and passed to microcontroller 520. Where leading and trailing flags have been added to the in-band transmissions, these will be recognised by comparing the bit sequence of the flags with stored sequences. The microcontroller outputs baseband data to shaper 514 which use the microcontroller output to gate an 8kHz tone. This process results in an amplitude shift keyed 8kHz tone. At hybrid 512 the modulated 8kHz tone and amplified speech is combined. The combined speech and outof-band signalling passes through isolation transformer 511 to socket 501.

The converter receives a 50Vdc supply from the switch. The converter also receives a 15Vac supply from mains adaptor 365 (shown in figure 3). Switch mode converter 540 derives a 50Vdc supply for the telephone interface and +/- 5vdc supply for the circuitry. Mains adaptor 365 may c o house rechargeable 22.5Vdc batteries which provide an alternative to the mains supply in case of a mains failure.

Converter 345 adjacent switch 210 operates in the same manner as the single line converter just described but duplicates that function for each line. The oscillator and other parts of the converter circuitry may be shared by the group of line converters. This converter derives a 48Vdc supply from the back plane of the PDMX rack which it is housed within.

One of the consequences of allowing transmission over a digitally multiplexed link is that long transmission delays can occur. Where there is a direct link between the terminal and switch transmission across the loop generally takes iess than toms, a figure which can be accommodated. With longer delays there may be circumstances where there would be insufficient time for a response to be made. One way of solving this is as follows. When a message is received at a converter an acknowledgement is immediately forwarded to the local device, i.e. the terminal or switch. The entire received message is then converted and forwarded on.

One of the consequences of converting out-of-band signalling to in-band signalling is that the in-band tones may be heard by a subscriber before or during a call. One of the ways of reducing this is to receive a set of commands at the converter, to store the set in a local buffer and re-order them before retransmission. As an example, at call set-up a number of signalling messages are transmitted from a switch to a terminal. One of the first commands is a 'voice on' command to instruct the terminal to demute the voice path, which will cause the following tones to be heard.

By re-ordering the commands at the local buffer, the 'voice on' command can be delayed to prevent the subscriber from hearing most of the tones.

A further consequence of allowing transmission over a digitally multiplexed link is that there is no direct current path between a terminal and a switch. A direct current path can be used to detect breaks in the link. A handshaking protocol can be used to periodically establish connection between converters. In this protocol a converter, during a convenient quiet period, transmits an 'Are You There' signal to the other converter and waits for an acknowledgement. This mechanism can also be used to determine the expected delay of a message transmission and to calculate a suitable time-out period to wait for a response.

Claims

1. A telecommunications network comprising a switch and a terminal joined by a traffic channel, a part of the channel having a restricted bandwidth, wherein the terminal and switch communicate using signalling messages occupying a frequency band lying outside of that restricted bandwidth, there being first and second signalling converters associated respectively with the switch and the terminal, the signalling converters being arranged to receive signalling messages, to convert the received signalling to a different frequency band and to re-transmit the converted signalling whereby to convey the signalling within the restricted bandwidth over the restricted bandwidth part of the channel.

2. A telecommunications network according to claim 1 wherein the restricted bandwidth part of the traffic channel comprises a digitally multiplexed channel having filters which bandwidth limit analogue signals passing along the traffic channel before digitally encoding them.

3. A telecommunications network according to claim 1 or claim 2 wherein the terminal and switch also communicate using a speech band lying within the restricted bandwidth, the converters being arranged to convert the signalling lying outside the restricted bandwidth to signalling lying within the speech band.

4. A telecommunications network according to any preceding claim wherein the converters are arranged to convert the signalling lying outside the restricted bandwidth to frequency-shift-keyed signalling lying within the restricted bandwidth.

5. A telecommunications network according to any preceding claim wherein the converters are arranged to append flags to the beginning and end of each signalling message lying within the restricted bandwidth whereby to differentiate between signalling messages and speech or data on the channel.

6. A telecommunications network according to any preceding claim wherein the converters are arranged to acknowledge receipt of a signalling message before re-transmitting that message.

7. A telecommunications network according to any preceding claim wherein the converters have a buffer to store a group of signalling messages, and are operable to re-order messages stored in the buffer before re-transmitting them.

8. A terminal having a signalling converter which is adapted for use in a telecommunications network according to any one of claims 1 to 7.

9. A switch having a signalling converter which is adapted for use in a telecommunications network according to any one of claims 1 to 7.

10. A method of operating a telecommunications network comprising a switch and a terminal joined by a traffic channel, a part of the channel having a restricted bandwidth, the method comprising signalling between the terminal and switch using signalling messages occupying a frequency band lying outside of the restricted bandwidth, and receiving, at first and second signalling converters associated respectively with the switch and the terminal, signalling messages, converting the received signalling to a different frequency band and re-transmitting the converted signalling whereby to convey signalling within the restricted bandwidth over the restricted bandwidth part of the channel.

11. A telecommunications network substantially as described herein with reference to figures 3 to 5 of the accompanying drawings sheets.

12. A method of operating a telecommunications network substantially as described herein with reference to figures 3 to 5 of the accompanying drawings sheets.