GB2058517A - Digital subscriber loop - Google Patents

Abstract

In a fully digital telecommunication system such as a multi-exchange system of the integrated Systems Digital Network type, the subscriber's sets are each connected to the exchange via a full- duplex digital loop. Digitally-encoded speech, data and other digital services are carried on the loop alternately with in-band signalling and supervisory messages and framing data, the messages conveying this being suitably formatted with start and end flag codes. The formatting is effected via a special codec 17 which is connected to the line circuit 10 when such traffic is being handled. Speech from/to a hardset 3 is conveyed to or from the line circuit 10 via a PCM codec 8, and data from a data interface 11 is suitably formatted to go direct to or from the line circuit 10. The line circuit arrangements continuously monitor the loop for such messages so that when one is detected it can be dealt with. <IMAGE>

Description

SPECIFICATION Digital subscriber loop This invention relates to a fully digital intelligence transmission system, i.e. one in which the intelligence to be conveyed, whether it be speech, data or other digital services is conveyed in digital form right up to the subscriber's apparatus.

Such an arrangement becomes an economic possibility as a result of the rapidly falling cost of digital circuitry as compared with other apparatus, etc. used in telecommunication systems. This also has the useful feature that all intelligence, and this includes supervisory and signalling intelligence, can be conveyed as various combinations of 1 and O signal elements. Hence on reception it is merely necessary to be able to distinguish between two message conditions, which minimises the deleterious effect of interference on the intelligence being conveyed.

This invention has as its object the provision of a relatively economical implementation of a subscriber's loop arrangement for use in a system of the type referred to above.

According to the invention there is provided a data transmission system in which communication between a system subscriber's apparatus and the telecommunication exchange over a subscriber's loop is effected in digital manner using pulse code modulation when speech is to be conveyed, in which the messages which convey speech, data and/or other digital services are carried on the loop alternately with inband signalling and supervisory messages, so that said signalling and supervisory messages are conveyed in band, and in which the code groups to be conveyed to respect of the speech, data and/or other digital services are aligned with respect to framing information derived from the in-band signalling frame structure.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows schematically typical message formats for use in a system embodying the invention.

Fig. 2 illustrates in highly simplified form a subscriber loop arrangement to which the invention is applicable.

Fig. 3 shows schematically equipment at the subscriber's end of a loop in a system embodying the invention.

Fig. 4 shows schematically equipment at the exchange end of a loop in a system embodying the invention.

With the development of an Integrated Services Digital Network (ISDN) it becomes necessary to have a suitable terminating equipment for connection to the subscriber loop. Such equipment, known as an Integrated services Circuit terminating Equipment (ICE), has to cater for signalling and supervisory messages, and for framing information which is conveyed in-band on the subscriber loop, which avoids the need for additional digital bandwidth for framing and/or signalling, as hitherto postulated. The ICE has facilities for digital speech and the connection of data and other digital services, including character mode data, packet mode data, frame mode data and facsjmile.The ICE employs Integrated services Data Link Control (IDLC) procedures, which are based on the High Level Data Link Control (HDLC) frame structure in accordance with international standards, for control of the subscriber loop. In addition, the IDLC procedure provides a capability of end-to-end signalling.

IDLC accommodates the following types of digital signal: (a) unformatted digitally-encoded speech, e.g.

A-law PCM (b) data and other digital services already formatted in accordance with the HDLC frame structure (c) data and other digital services not already formatted as in (b) (d) signalling and supervisory messages and framing information formatted in the HDLC frame structure by the ICE or the exchange equipment.

The HDLC frame structure used is one specified by an international standard, and is shown in Fig.

1 a. It consists of start-of-frame and end of frame flags, an address field, a control field, an information field, and a frame checking sequence FCS. "Transparency" is ensured by a process of bit insertion and deletion to prevent the emulation of the flag sequence.

Fig. 1 b shows a sequence of HDLC frames including an idle sequence filled with flags between two such frames.

Signalling and supervisory messages are generated within the ICE, or the exchange equipment, and are frame formatted by an HDLC codec. Such messages when received from the line are de-formatted by the codec. Only frames with a valid FCS and which meet the constraints set by the higher level control procedures are deemed valid. The generation of an FCS group for a message, and the checking performed on a message and its FCS group can follow established procedures in the field of error detection and correction, and are not described herein.

The higher level procedures used permit signalling between the ICE and the local exchange, and between the ICE and a remote ICE, once a connection has been set up. The subscribersubscriber signalling facility permits mode changes to be made during a connection.

Unformatted digitally-encoded speech at 64 K bit/sec. is transmitted directly to the line without frame formatting. PCM octets, i.e. 8 bits, are aligned to the last HDLC flag sequence transmitted to the line during signalling, see Fig.

1 c. The risk that such octets will emulate signalling and supervisory messages is reduced to a negligible level by the constraints imposed by the frame structure including the FCS and the higher level control procedures.

Data and other digital services already formatted as per HDLC frame structure (Fig. la are transmitted to the line without further formatting.

Subscriber loop signalling according to the higher level control procedures may be handled by the terminating equipments, or the ICE may handle such procedures and connect the terminating equipment to the line when the connection has been set up. For terminating equipments operating at speeds below 64 K bit/sec. the ICE absorbs the additional band-width by inserting dummy control frames or flag sequences, which will be deleted by the distant ICE or at an interface with a non-lSDN network.

Data and other digital services not already formatted accqrding to the HDLC frame structure are formatted into frames by an HDLC codec in the ICE (see below), with deformatting at a distant ICE or at an interface with a non-lSDN network. The higher level control procedure enables subscriber loop signalling to be handled by the terminating equipment, or the ICE may handle the higher level signalling protocol and connect the terminating equipment to the line via the HDLC codec when a connection has been set up. Terminating equipments with user rates up to 48 K bit/sec. can be accommodated; speed difference between the terminating equipment and the subscriber loop are accommodated by the frame length and/or the insertion or deletion of additional flag sequences and/or control frames.

Fig. le already mentioned, shows a typical transmission for a call set up from the ICE using IDLC procedure. The call is set up with signalling and supervisory and framing indicated at SSMF between the ICE and the loop. When the connection is set up, formatted data or digitallyencoded speech octets aligned to the last flag sequence may be transferred. Under control of the user, end-to-end signalling may be used, e.g.

change of the transmission mode to allow the use of a different service, the mode changes being controlled by the higher order control procedure.

Fig. 2 shows the arrangement highly schematically, with an ICE 1 coupled to the local ISDN exchange via a full duplex 64 K bit-sec.

digital connection. The ICE is shown with a telephone handset 3 and a data input device 4 to indicate the variety of services available. At the exchange, there is a signalling extraction circuit 5 controlled by a control unit 6, the signalling being extracted by the circuit 5 for use by the unit 6, before the speech or data etc. is applied to the switch 7. In some cases this may involve multiplexing.

Fig. 3 is a functional block diagram of the ICE at a subscriber's end of the loop and it includes a telephone handset 3 connected to a PCM codec 8 which is connectable via switching means 9 to a line circuit 10 to the exchange. There is also a data interface 11 for the transmission and reception of data, connected to the line circuit 10 in the transmit direction via the switching means 9. This interface 11 is connected to the line circuit 10 in the receive direction via further switching means 12. The switching means shown are "ganged" together and although shown schematically as mechanical switches would be electronic switches.

The keyboard 13, with its associated logic circuitry, is connected to the signalling and control unit 14 whose functions include the control of the switching means shown in the drawings. Other items controlled by the unit 14 include a display 1 5 with its associated logic circuitry which displays numbers before sending, or information received from a remote exchange or ICE. Incoming call annunicator uses a loudspeaker 16 by which suitable sounds to attract the attention of a subscriber are emitted.

The signalling and control unit 14 also controls= an HDLC codec via which the signalling and supervisory messages go and which is connected to the line circuit 10 when such messages are being handled.

The handset 3, codec 8, data interface 11, keyboard 13, display 15, annunicator 1 6 and line circuit interface 10 all perform their usual functions.

The signalling and control unit 14 determines whether the PCM codec 8, data interface 11 or HDLC codec 17 should be connected to the line, and whether the data interface 11 or signalling and control unit 14 should be connected to the codec 1 7 in accordance with the procedures referred to above. In the case of outgoing calls these connections occur, dependent on the various controls operated by the caller, and in the case of a terminating call they occur in the main under control of received signalling or supervisory messages.

The codec 17 performs the-formatting and deformatting referred to above, and passes timing information derived from the validated flag sequence to the ICE timing unit 1 8 which controls octet alignment and other timing functions. The codec 1 7 continuously monitors the incoming line for valid frames while the line is active.

The corresponding equipment at the exchange end of the subscriber loop, Fig. 4, is somewhat simpler as it does not have the subscriber controlled items: in the circumstances it is fact that no specific description of it is needed.

At all times during operation the loop is monitored at each of its ends for signalling, supervisory or framing messages, this involving an inspection of incoming data for flags.

Claims (5)

1. A data transmission system in which communication between a system subscriber's apparatus and the telecommunications exchange over a subscriber's loop is effected in digital manner using pulse code modulation when speech is to be conveyed, in which the messages which convey speech, data and/or other digital services are carried on the loop alternately with in . band signaliing and supervisory messages, so that said signalling and supervisory messages are conveyed in band, and in which the code groups to be conveyed in respect of the speech, data and/or other digital services are aligned with respect to framing information derived from the in-band signalling frame structure.

2. A system as claimed in claim 1, in which speech is applied to the line circuit via a PCM codec, the connection including the PCM codec only being effective when speech is being handled, and in which at the exchange the PCM code combinations are conveyed in time division multiplex manner, being multiplexed with PCM code combinations from other lines.

3. A system as claimed in claim 1 or 2, and in which when data is to be conveyed the data interface is connected directly to the line circuit.

4. A system as claimed in claim 1, 2 or 3, and in which when signalling or supervisory messages or framing information is being conveyed the connection to the line circuit is via a further codec which formats outgoing messages and deformats incoming messages.

5. A data transmission system substantially as described with reference to the accompanying drawings.