SUBSCRIBER MULTIPLEXING DEVICE

Abstract: A subscriber multiplexing device comprising a time-division user data bus (33) and a message-based signalling channel (34) between subscriber interface units (32) and a multiplexing unit (31), the signalling channel being common to all the subscriber interface units. A "message based" signalling channel means that signalling is transferred in discrete protocol data units (PDU), or messages, comprising at least a destination address and the actual signalling data. Each unit connected to the signalling channel receives the message and compares the destination address of the message to its own address. When the addresses match, the unit concerned identifies the message as being addressed to it. If there is no match, the unit rejects the message. Each unit sends a signalling message only when needed, on event basis, in response to an internal change in state or to a message received from another unit, for example. In other words, information about changes is only signalled further through the signalling channel (34).

Description: SUBSCRIBER MULTIPLEXING DEVICE The invention relates to subscriber multiplexing devices used in subscriber interface networks in telephone systems. Although digital exchanges are nowadays widely used in telephone networks, subscriber lines between an exchange and a subscriber are typically analog two-wire lines. Digital exchanges therefore need to be provided with a specific analog subscriber interface which interfaces a time-divisional signal from a digital exchange to an analog subscriber line, as shown in Figure 1 , where a subscriber interface 11 of an exchange 10 is connected to a subscriber line 13 leading to a subscriber terminal 12. Figure 1 also shows an analog subscriber interface 15 which can be located in a multiplexing unit 14 arranged separately from the exchange 10, the multiplexing unit being connected to a digital subscriber interface 19 of the exchange via a digital link 18 (of e.g. 2 Mbit/s). This kind of a remote multiplexing unit 16 can also be connected via a digital link 20 (of e.g. 2 Mbit/s) to another multiplexing unit 21 located close to the exchange. An analog subscriber interface 221 of the other multiplexing unit is connected, in turn, to an analog subscriber interface 23 in a digital or an analog exchange. Multiplexers and digital connections used as described above support longer distances between subscribers and an exchange than a purely analog subscriber line. In addition, a multiplexer can be placed close to the subscribers, which allows a subscriber network to be implemented in a star configuration of short subscriber lines and, thereby, at relatively low cabling costs. The principle of a subscriber line interface of a digital exchange is shown in Figure 2. Wires a and b of an analog subscriber line are connected to a subscriber line interface circuit 30 (SLIC). Audio frequencies appearing on the wires a and b are routed to the SLIC 30 which sets a termination impedance of the subscriber line and separates transmit and receive direction signals from one another. A receive direction signal is supplied via a low-pass filter 31 to a codec 33 which converts the analog signal of the subscriber line into a digital PCM coded form and transfers the PCM signal further to the exchange. Correspondingly, a PCM coded signal arriving from the exchange is converted in the codec 33 into an analog transmit direction signal, which is supplied to the subscriber line via a low-pass filter 32 and the SLIC 30. The most important functions of a subscriber line interface circuit SLIC comprise power supply to the subscriber line, ring voltage supply, detection of subscriber loop activation (off-hook), etc. An active subscriber loop, or off-hook, is a situation in which the subscriber has lifted the handset from its holder and an associated switch connects the a and b wires of the subscriber line together, a direct current loop being thereby formed from the SLIC via the subscriber line to the subscriber terminal, and back. When the subscriber loop activates, the SLIC detects the loop current and informs the exchange about the off-hook state. When the handset is replaced (on-hook), the subscriber loop opens and the loop current disappears. The SLIC detects that there is no loop current and informs the on-hook state to the exchange. A single multiplexing device, such as a multiplexing device 14, 16 or 21 shown in Figure 1 , usually comprises a large number of subscriber interfaces. The reason for this is that an exchange and a multiplexing device are connected by a digital connection which is usually a PCM (Pulse code modulation) link of at least 2 Mbit/s, a capacity of at least 30 subscriber channels and two signalling channels. A high-speed PCM link can be provided by means of a relatively simple multiplexing unit shared by all the subscriber interfaces. A problem with this kind of a subscriber multiplexing device is how to arrange transfer of payload information (speech or data) within the device and, in particular, the signalling between the multiplexing unit and each individual subscriber interface. A prior art solution is a time-division bus interconnecting subscriber interface units and the multiplexing unit. Speech information and signalling can also be transferred on separate time-division buses, one time slot in each transmission direction being allocated, for the duration of a call, for the transfer of speech on a bus. In addition, the multiplexing unit conducts continuous polling of the subscriber interface units connected to the bus, in order to be able to transfer signalling data (such as on-hook, off-hook) from a subscriber interface unit to a multiplexing unit. This requires a particularly large number of time slots and a very high speed bus, the timing of the bus thereby being extremely critical. For this reason, it is also difficult to concentrate a large number of subscriber interfaces in one and the same multiplexing unit. An object of the present invention is to provide a subscriber multiplexing device offering simpler and less time-critical signalling between subscriber interface units and a multiplexing unit. This is achieved with a subscriber multiplexing device of a telecommunications network, the device comprising a multiplexing unit connected to a multi-channel, time-division digital link for bi-directional transfer of user information and signalling information with another multiplexing device and/or an exchange; a plurality of subscriber interface units, each comprising at least one analog subscriber interface which is to be connected to a bidirectional, two-wire subscriber line; a time-division data bus for user information transfer between the multiplexing unit and the subscriber interface units; and a signalling channel for signalling between the multiplexing unit and the subscriber interface units. The subscriber multiplexing unit of the invention is characterized in that the signalling channel is a message-based channel, and that each subscriber interface unit is arranged to transmit signalling messages to the multiplexing unit on event basis. In addition to the time-division user data bus, the invention comprises a message-based signalling channel arranged between the subscriber interface units and the multiplexing unit and common to all the subscriber interface units. 'A message-based' signalling channel means that signalling is transferred in discrete protocol data units (PDU), or messages, comprising at least a destination address and the actual signalling data. Each unit connected to the signalling channel receives the message and compares the destination address of the message with its own address. When the addresses match, the unit concerned identifies the message as being addressed to it. Otherwise it rejects the message. Several transfer protocols are available, which are suitable for use on a signalling channel of the invention. In a preferred embodiment of the invention the signalling channel protocol is a HDLC (High-Level Data Link Protocol), the protocol data unit or message then being an HDLC frame. Another significant characteristic of the invention is that each unit transmits a signalling message only when needed, i.e. on event basis. Transmission on event basis means, for example, that the message is transmitted in response to a change of state within a unit that has to be reported to another unit, or in response to a message received from another unit. In other words, only information about changes is signalled further through the signalling channel. By means of a message-based signalling channel and an event- based message transmission, the requirements for high speed and the timing problems involved in internal signalling in prior art subscriber multiplexing devices are avoided. In the multiplexing unit in particular, a slower speed and/or less efficient signal processor can be used. In addition, the invention allows an extremely large number of subscriber interface units to be concentrated in a single multiplexing unit. In the following, preferred embodiments of the invention will be described with reference to the accompanying drawings, in which Figure 1 illustrates different ways of implementing a subscriber network in an analog or a digital local exchange; Figure 2 is a schematic block diagram illustrating an analog subscriber interface; Figure 3 is a block diagram illustrating a subscriber multiplexing device of the invention; Figure 4 is a block diagram illustrating a subscriber line interface of the invention; Figure 5 is a block diagram illustrating a subscriber multiplexing unit of the invention; Figures 6 and 7 illustrate HDLC frames transferred on a signalling bus; and Figure 8 is a state diagram illustrating the operation of a signalling interface circuit. The present invention can be applied to different subscriber multiplexing devices in telecommunications systems. Remote multiplexing devices, such as devices 14, 16 and 21 shown in Figure 1 , provide a typical application. In the following, a preferred embodiment of the invention implemented in a remote multiplexing device is described. Figure 3 shows a subscriber multiplexing device according to the preferred embodiment of the invention. It comprises a multiplexing unit 31 and an N number of subscriber interface units (SUB) 32 1 ...32 N . Each subscriber interface unit SUB 32 can establish an interface to one or more subscriber lines 13.,...13 N . N and M can be any positive integers. The multiplexing unit MCU31 is connected to a high-speed digital transmission link 18 or 20, such as a PCM link of 2 Mbit/s, or more. The other end of the PCM link 18 or 20 can be connected for example to a digital exchange (such as an exchange 10 shown in Figure 1) or to another multiplexing device (such as the multiplexing device 21 shown in Figure 1). The multiplexing device comprises at least one internal data bus 33 and a signalling bus 34 to which the subscriber interface units 32,...32 N and the multiplexing unit MCU31 are connected. According to the principles of the invention, the data bus 33 is a time-division bus from which a time slot is fixedly allocated, or can be dynamically allocated, to each subscriber interface unit SUB32 for the transfer of user information between the MCU31 and a SUB32. 'User information' refers to any speech or data transferred over the subscriber line. On the data bus 33, user information usually appears in the form of PCM samples. In the preferred embodiment of the invention, the timing and management of the data bus 33 is carried out by the MCU31. In some cases it may be advantageous to utilize a plural number of parallel data buses 33 to which the SUBs 32 ..32 N and the MCU31 can selectively connect. A plural number of data buses may be advantageous to use for instance when the number of subscriber interface units is very large and a high bus capacity is needed. The implementation of the time-division data bus 33 is not, however, essential for the invention, so it will not be described in any further detail here. The signalling bus 34 of the invention is a message-based bus through which the units 32 and 31 transmit messages only on event basis. In the preferred embodiment of the invention, the message-based signalling channel is implemented as a separate bus, but it can also be implemented in one time slot of a time-division data bus. In the preferred embodiment of the invention, the signalling bus 34 uses an HDLC protocol (High Level Data Link Protocol), which is a commonly known data link control protocol. Other similar protocols include ADCCP (Advance Data Communication Control Procedures), LAP-B (Link Access Procedure, Balanced) and SDLC (Synchronous Data Link Control). These protocols are discussed for instance in 'Data and Computer Communications', William Stal lings, Macmillan Publishing Company, 1994, particularly on pages 187 to 197. The invention is not, however, limited to these protocols; any message-based data transfer can be applied to the signalling bus 34. Similarly, 'message' in this context refers to any protocol data unit, or message, packet or frame comprising, when used as defined in the invention, at least a destination address and the actual signalling data. In the preferred embodiment of the invention in which the signalling channel uses the HDLC protocol, the protocol data unit or message is an HDLC frame. Figures 6 and 7 illustrate two HDLC frames that can be used on the signalling bus 34. Figure 6 illustrates a frame which the MCU31 sends to the SUB32. Figure 7 illustrates a frame which the SUB32 sends to the MCU31. Each frame comprises a start flag and a stop flag, the form of which is 01111110 in the HDLC protocol. Both frames also comprise a destination address field DEST ADDR, a source address field SOURCE ADDR, a device type field DEVICE TYPE, a data field GEN. DATA, and error check fields CRC1 and CRC2 (Cyclic Redundancy Check). The frame shown in Figure 6 further comprises a field BYTE COUNT indicating the length of the data field GEN. DATA, which can be from 0 to 8 bytes. The frame shown in Figure 6 also comprises a command field COMMAND in which different commands can be issued to the SUB32. The destination address DEST ADDR and the source address SOURCE ADDR are bus addresses assigned to the interface units 32 T ...32 N and to the multiplexing unit 31 on the signalling bus 34. The field DEVICE TYPE, in turn, can be used for indicating a particular device within the SUB32, such as a subscriber interface circuit or a bus buffer. In a frame sent by the SUB32, the DEVICE TYPE field indicates the subscriber interface the signalling information given in the data field GEN. DATA refers to. In a frame sent by the MCU, the DEVICE TYPE field indicates the interface or the circuit which is to be the subject of the operation given in the COMMAND field or the signalling information given in the data field GEN. DATA. In the preferred embodiment of the invention the most significant bit MSB in the BYTE COUNT field is a read/write bit R/W which indicates whether the MCU wishes to write into a device (R/W = 0), or to read from a device (R/W = 1). In the preferred embodiment of the invention, one of the bits in the SOURCE ADDRESS field is an IRD/NORM bit. When this bit is , the message is generated due to an interrupt from a subscriber line interface. Otherwise (i.e. IRD/NORM = '0') the message is either an acknowledgement of a previous write message or a reply to a read request message. The MCU thus knows the reason for the message directly from the message. It is to be noted that Figures 6 and 7 only show one possible HDLC frame structure, the invention being in no way restricted to it. Figure 4 is a more detailed block diagram of the subscriber line interface unit SUB32 of the preferred embodiment of the invention. The SUB32 comprises an M number of subscriber interfaces, each of which is connected to a respective subscriber line 13.,...13 M . Each subscriber interface comprises a subscriber line interface circuit SLIC41 1 ...41 M and a codec and control circuit 42 1 ...42 M (CODEC & CONTROL). The SLIC41 separates an analog signal transferred over the subscriber line 13 and supplies it as an IN signal to the codec and control circuit 42 which converts the signal into a PCM signal and supplies it to a local PCM bus 45. Correspondingly, the codec and control circuit 42 converts the PCM signal it has received from the bus 45 into an analog OUT signal which is supplied to the subscriber line 13 through the SLIC41. In addition, different control and state signals CONTROL are transferred between the codec and control circuit 42 and SLIC41. Control signals typically include ON/OFF state data, ground key state data, control of polarity reversal, control of ring voltage switching, etc. The local time-division PCM bus 45 is connected to each codec and control circuit 42 1 ...42 N , and to a bus select circuit 43. The bus select circuit connects a particular time slot from the local PCM bus 45 to the time-division data bus 33. The bus select circuit 43 is controlled via a control bus 47 by a signalling interface circuit SIGNIF 44. The signalling interface circuit SIGNIF uses HDLC frames to exchange signalling with the multiplexing unit MCU31 through the message- based signalling bus 34. The SIGNIF 44 is connected to the codec and control circuits 42 ..42 N through a local serial bus 46, which allows the SIGNIF to read the state data of the circuits, to set their states and to perform different operations on them according to the information sent by the MCU31. At its simplest, the signalling interface circuit 44 can convert information received over the local serial bus 46 to HDLC frames for transfer on the data bus 34 to the MCU31. In another transmission direction the SIGNIF 44 converts received HDLC frames into information to be transferred on the serial bus 46. In addition, an interrupt signal INT .. INT M is connected from each codec and control circuit 42 1 ...42 M to the interface circuit SIGNIF 44. When the state of a line interface circuit changes - for example, an ON-HOOK state is activated - a corresponding codec and control circuit 42 sends an interrupt INT1...M to the interface circuit SIGNIF 44. The SIGNIF 44 then reads the state of the line interface circuit in question through the serial bus 46 and informs the MCU31 about the changed state of the HDLC frame via the signalling bus 34. In the HDLC frame, a DECIVE TYPE field indicates the codec and control circuit 42 the signalling message relates to. The SIGNIF focuses the operation to the circuit indicated by the DEVICE TYPE field. Some of the received HDLC frames may be addressed to the bus select circuit 43. In connection with equipment configuration, for example, the MCU31 may use the messages transmitted on the signalling bus 34 to determine which time slot of the data bus 33 each subscriber line signal is to be connected to. The MCU31 can carry out the same control operation also dynamically in connection with each call. In the preferred embodiment of the invention, the SIGNIF 44 can mainly carry out one principal operation at a time, the only exception to this being simultaneous writing through the serial bus and sending of an acknowledgement message to the MCU31. The states of the interface circuit SIGNIF 44 can thus be clearly shown in one state transition diagram, and in theory all SIGNIF 44 state machines could be integrated into one state machine. Such an integrated state machine would, however, be too complicated to be economically feasible. States of the SIGNIF 44 are illustrated in Figure 8. A state of the SIGNIF 44 can be changed by three principal external events: equipment reset, activation of frame reception and signalling interrupt from the codec and control circuit 42. These events can trigger a process that is carried out until the SIGNIF 44 again reaches an idle state. As shown in Figure 8, a hardware reset HW-RESET sets the SIGNIF 44 to a LOST state where it waits for a silent line, i.e. a break on the signalling bus 34. On the HDLC bus, 'silent line' can mean for example a sequence of eight or more consecutive bits '1 '. When the silent line is reached, the SIGNIF 44 enters an IDLE state where it waits for two other external events that are normally possible. When in the IDLE state, the SIGNIF 44 naturally again enters a LOST state if HW-RESET takes place. Normally, however, there are only two other events, i.e. the reception of an HDLC flag from the bus 34 (received FLAG) or the reception of an interrupt from any of the interrupt lines INT1...INT N (INTERRUPT), which wake up the SIGNIF 44 to an active operational state. If the SIGNIF 44 receives an HDLC flag (flag received), the SIGNIF enters a receiving state and starts to receive serial data from the bus 34. The SIGNIF 44 first checks the destination address DEST ADDR of the HDLC frame and compares it with its own address. If the addresses match, reception continues. If the addresses do not match, the message is identified as not being addressed to the subscriber interface unit SUB32 concerned, and the SIGNIF 44 enters the LOST state where it stays until the incoming HDLC frame is completed. If the addresses matched, the CRC fields are checked at the end of the reception of the frame. If a CRC error is detected, the SIGNIF 44 again enters the LOST state. Otherwise the decision concerning the continuation of the process is made on the basis of the status of the received R/W bit. If R/W . = 0, which indicates a write message, the SIGNIF enters an acknowledgement transmission state (TRANSM.ACK state) where a message acknowledging the write command is sent to the MCU31. At the same time, a write operation is carried out on the circuit 42 V ..42 M or 43 denoted by the device type field, or possibly into an internal operations register of the signalling interface circuit SIGNIF 44. The acknowledgement and write operations having been completed, the SIGNIF 44 enters the LOST state. If R/W = 1 , the received HDLC frame is a read request and the process enters a read state (READ). In the READ state, the process reads, according to the frame of the received DEVICE TYPE field, either the internal register of the SIGNIF 44 or the state of one of the codec and control circuits 42. When the reading is completed (READ DONE), the process enters a transmit state (TRANSMIT) which transmits the HDLC frame comprising the data that has been read. After the TRANSMIT state, the process again returns to the IDLE state. If the transmission fails (ARBITRATION LOST), a re- transmission takes place. Figure 5 shows a schematic diagram of the multiplexing unit MCU31. One side of a multiplexing portion 53 is connected to a digital transmission link 18 or 20, such as a PCM link of 2 Mbit/s. The other side of the multiplexing portion 53, which comprises for example PCM signals of 64 kbit/s, is connected to a bus interface circuit 52 (BUS IF). The bus interface circuit 52 connects the 64 kbit/s PCM signals to particular time slots on the data bus 33. The operation of the bus interface circuit 52 and the multiplexing portion 53 is controlled by a control circuit 51. The control circuit 51 is also connected to the signalling bus 34 through which it exchanges signalling messages with the subscriber interface units 32 on event basis, as defined in the invention. The above specification concerns preferred embodiments of the invention. It is to be noted that those skilled in the art will find apparent alternative solutions and variations which can be implemented without departing from the scope and spirit of the accompanying claims.

Claims: CLAIMS 1. A subscriber multiplexing device (16, 14) of a telecommunications network, the device comprising a multiplexing unit (3) connected to a multi-channel, time-division digital link (18, 20) for bi-directional transfer of user information and signalling information with another multiplexing device (21) and/or an exchange (10); a plurality of subscriber interface units (32), each comprising at least one analog subscriber interface which is to be connected to a bidirectional, two-wire subscriber line; a time-division data bus (33) for user information transfer between the multiplexing unit and the subscriber interface units; a signalling channel (34) for signalling between the multiplexing unit and the subscriber interface units, characterized in that the signalling channel (34) is a message-based channel, and that each subscriber interface unit (32) is arranged to transmit signalling messages to the multiplexing unit on event basis. 2. A subscriber multiplexing device according to claim 1, characterized in that the signalling channel is a separate signalling bus (34). 3. A subscriber multiplexing device according to claim 1, characterized in that the signalling channel is one time slot of a time- division data bus. 4. A subscriber multiplexing device according to claim 1, 2 or 3 characterized in that each subscriber interface unit (32) is arranged to send signalling messages to the multiplexing unit (MCU) only in response to internal state changes or to a message from a multiplexing unit. 5. A subscriber multiplexing device according to claim 4, characterized in that internal state changes comprise at least transition from an on-hook state to an off-hook state and transition from the off-hook state to the on-hook state. 6. A subscriber multiplexing device according to any one of the preceding claims, characterized in that at least one subscriber interface unit (32) comprises a plurality of subscriber line interfaces, each of which comprises a subscriber line circuit (41) and a codec (42); a telecommunications means (44) connected to said signalling channel (34) for the transmission and reception of signalling messages; an internal signalling bus (46) for signalling between the subscriber line interfaces (41, 42) and the telecommunications means (44); a bus interface means (43) connected to said time-division data bus (33); an internal time-division data bus (45) for data transfer between the subscriber line interfaces (41, 42) and the bus interface means (43); an interrupt line (INT1...INTM) from each subscriber line interface (41, 42) to the telecommunications means (44), the telecommunications means being arranged to read the state of a corresponding subscriber line interface and to send the state data in a signalling message to the multiplexing unit, in response to an interrupt signal on the interrupt line. 7. A subscriber multiplexing device according to claim 6, characterized in that the telecommunications means (44) is arranged to control the bus interface means (43) in response to the signalling messages sent by the multiplexing unit (MCU). 8. A subscriber multiplexing device according to claim 8 or 7, characterized in that the telecommunications means (44) is arranged to read the state of the subscriber line interface (41, 42) in response to a read message sent by the multiplexing unit (MCU) and to send the state data that has been read in a signalling message to the multiplexing unit (MCU). 9. A subscriber multiplexing device according to any one of the preceding claims, characterized in that the multiplexing unit (MCU) and each subscriber interface unit (32) have a separate signalling channel (34) address, a signalling message being a protocol data unit comprising at least a destination address and signalling information. 10. A subscriber multiplexing device according to any one of the preceding claims, characterized in that the protocol used on the signalling channel (34) is one of the following: HDLC (High Level Data Link Protocol), which is a commonly known data link control protocol, ADCCP (Advance Data Communication Control Procedures), LAP-B (Link Access Procedure, Balanced) and SDLC (Synchronous Data Link Control).

Inventors: NORDSTROEM CAJ, NORDSTROEM, CAJ
Applicants: NOKIA TELECOMMUNICATIONS OY, NORDSTROEM CAJ, NORDSTROEM, CAJ
Application year: 1998
Main classification: H04Q11
IPC codes: H04J3, H04L29, H04Q11