GB2182227A - Multi-drop digital subscriber loop - Google Patents

Abstract

In a multi-drop digital subscriber loop which allows multiple users to share the same physical loop, each user can be assigned bandwidth according to his requirements. Bandwidth is assigned by the bit configuration in the control information block of each communication frame, thus allowing various voice and data requirements. Each single communication frame addresses all of the devices on the loop. Frequency of addressing is determined by the length of the frame. A small relative bandwidth will be assigned to devices having relatively less frequency of need. <IMAGE>

Description

SPECIFICATION Multi-drop digital subscriber loop This invention relates to a digital subscriber loop which allows multiple users to share the same physical loop.

Digital subscriber loops (DSLs) allowthe exchange of voice between the usersubset and the central office (CO), or private branch exchange (PBX), in the form of digital signals as opposed to the conventional analog techniques. The digital technique allows for better control over the speech quality and is more robust than its analog counterpart.

Existing DSLs are single subscriber loops with fixed voice, data, and control bandwidth. The multidrop digital subscriber loop (MDSL) of the invention allows multiple users to share the same physical loop and, further, each user can be assigned bandwidth according to its requirements. For example, a user can be assigned voice plus 8 kilobits per second (8 Kbps) data, or alternatively 16 Kbps data without voice.

The MDSL is advantageous overthe DSL byfor example, reducing the cost of wiring and simplifying the interface to the PABX. An exemplary MDSL has a total useful bandwidth of 576 Kbps and, therefore, can supportfour 2B+D type terminals, eight B+l/aDtype terminals, orseventytwo 8 Kbps (that is, 9.6K baud) terminals, or a logical combination thereof.

According to one aspect of the invention there is provided a communication system comprising a subscriber loop for providing a serial communication path forsubscribers ofthe loop, and means, coupled tothe loop, to assign the bandwidth of the loop to the subscribers according to the requirements of the subscribers.

According to another aspect ofthe invention there is provided a communication system, comprising a private branch exchange, a plurality of userdevices, a subscriber loop coupling the user devices seriallytothe exchange to provide a serial communication path for the devices to the exchange, and means, coupled to the loop, for dynamically allocating the bandwidth ofthe loop among said devices.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 7 shows a PABX system embodying the invention.

Figure2shows a PABX system embodying the invention employing existing wiring to form the subscriber loop.

Figure 3shows a block diagram of a PABX system embodying the invention.

Figure 4shows the format of a frame of the MDSLC bit stream.

Figure 5shows wiring arrangements of a known PABX system in an analog subscriber loop configuration, in a digital subscriber loop configuration, and in an MDSLconfiguration according to the invention.

Figure 6shows a block diagram of a known PABX system employing an MDSL according to the invention.

Figure 7shows a block diagram of an MDSL line card according to the invention.

Figure 8shows the MDSL logic front end of Figure 7.

Physically, the MDSL may consist ofa pairoftwisted wires intheform ofa ring, as shown in Figure 1.The line card/PABX is the master and the terminals are the slaves.

The MDSLcan supportfrom four2B+D digital subsets orterminals (DDS/Ts) to seventy-two 9.6Kbaud DSS/Ts with just two pairs of twisted wire from the line card/PABX. This greatly reduces the total number of system components and, therefore, the size ofthe system, thus reducing total system cost. It can also usethe existing two pair per subset wiring to form the required ring. This arrangement is shown in Figure 2. This second advantage does not place a penalty on applications that have conventional wiring.

Aspecially designed wall connector is used so that in case a DSS/Tis disconnected, it can close the loopto maintain signal flow. Other techniques can be used to make the system immune to single wire breaks.

The MDSL system consists of an MDSL Controller (MDSLC) and from fourto seventy-two DSS/Ts. Typic- ally, in a PABX,the MDSLCfunction will be a partofthe conventional line card orgroup controller. The MDSLC transmits a bit stream containing data and control for all the slave DSS/Ts. Each DSS/T receives this bit stream, extracts the part addressed to it, inserts its own data and control for the MDSLC, and re-transmits the bit stream. Eventually, after going around the loop, this bit stream is received bythe MDSLC,whereall data and control information is extracted and replaced appropriately. In this manner, the same time slot on the ring is used for both reception and transmission, thus conserving bandwidth. Figure 3 shows, in some detail, the system structure.

The communication format consists offrames of bits. Each frame consists of 104 bits and is 0.125 milliseconds (ms) long in time and carries all the data, control, control address, and frame sync information. This format is shown in Figure 4. Theframe sync Fis an 8-bit unique sequencethat signalsthe start of a frame.The nextfield is the control address CA consisting of 8 bits. The CA indicates the device address for which the control information is contained in the nextfield,the control Cfield. The Cfield is 16 bits wide and carries all the control information for the addressed DSS/T. The last72-bitfield is the data D for all of the devices. The data boundary for each DSS/T is programmed at system installation time.For example, if there are 4 DSS/Ts ofthe 2B+Dtype connected to the MDSL, then each DSS/Tis assigned 18 ofthe 72 bits.

Each MDSLframe takes 0.125 mS and contains 104 bits, therefore, the bit rate on the MDSL is 104 bits divided by 0.125 ms, or 832 Kbps. Thus the twisted pair wire must be capable of transmitting 832 Kbps. Since the stream of bits is continuous, the slaves can recover the clock from the data stream and, therefore, a synchronous data transmission method is used.

Each frame carries a 16-bit control field. The control is associated with each of the 72 data bits bythe address in the CAfield. Thus each data bit is given a sequential address, from Oto 71, and this address determines its position in the Afield. This scheme gives complete flexibility for assigning control bandwidth and/or frequency to any data bit or bits within the maximum total. Below is a table showing the control bandwidth and 16-bit control time period for different types of DSS/Ts. The latter figure is determined bythe product of the frame period of 0.1 25ms with the number of terminals. The control bandwidth is determined by dividing the control field of 16 bits with the 16-bit control time period.It is assumed that for any one configuration all the DSS/Tsareofthetype.

No. of 16-Bit Control Terminal Type Terminals Control Time Period 2B+D 4 32 Kbps 0.5 ms B+1/2D 8 16Kbps 1.0ms 8 Kbps (9.6 Kbaud) 72 1.78 Kbps 9.0 ms The requirementfor a multi-drop digital subscriber loop as a part of a PABX arises from various needs.

Further, the MDSLtechnology has definite advantages over a single subscriber digital loop. An advantageof an MDSL isthe provision of a simplified and cost-reduced installation and wiring scheme for integrated voice/data workstations or subsets. The MDSL allows multiple data and voice to be transported overthesame medium.

For computer/communication systems which employ local area networks (LANs) for networking theircom- puters,the MDSLwill provide specialised LAN capabilities for both data and voice with the added advantages of being part of a PABX and, therefore, a part of other LANs (that is, other MDSLs on the PABX). As an example, each MDSL may be designed to be capable of supporting four2B+D Digital Subsets/Terminals (DSS/T). Thus the total bandwidth on the MDSLwill be 576 Kbps. By the use of dynamic bandwidth allocation, this bandwidth can be split up in any manner. Thus,the MDSLwill be abieto support72 9.6 Kbaudterminals, or it can support any mix of DSS/Ts for a total bandwidth of 576 Kbps.

The MDSL is inherently more robust and less sensitive to interference than the conventional subscriber loop. Furthermore, its capabilities can be changed by new software in the subset, the MDSL controller, and the system, without changing the hardware.

Aspecific application for the multi-drop digital subscriber loop is to employ it in the ITT System 3100 product line of PABXs, manufactured and sold by ITT Corporation. The subscriber loop in the present ITT System 31 00D consists of two wires for analog voice, two wires for di-phase (lights and buttons), and two wires for RS 422 data communications from the auxiliary interface device (AID) card to the integrated data equipment adapter (IDEA) card. Of these sixwires, fourterminate on a voice and di-phase line card,which supports four subsets, and two terminate on the IDEA card, which also supports fours terminals. Each ITT System 31 00D subscriber has one voice channel and one 8 Kbps data channel.

The invention allows the six-wire per subscriber loop to be changed to a four-wire multi-drop digital subscriber loop (MDSL) thatwill carry voice, data, and signalling information for eight subscribers. The existing separate voice and data line cards may be integrated into a new line card, the MDSL line card, which will support eightvoice plus data stations.

This arrangement simplifies the PABX wiring scheme by substituting two pairs per eight subscribers for three pairs per subscriber in the existing PABX configuration. Figure 5 shows the ITT System 31 00D and wiring migration plan from the present analog subscriberlooptothe DSL, and finally to the MDSL. As onecan see, the MDSLwiring is greatly simplified, which results in lower system cost. Currently it takes two cardsto supportfourvoice/data stations. In the proposed configuration, it will take just one card to supporteight stations. Thus can be realised an almost 4to 1 reduction in physical size to achieve the same port capacity, and, therefore, a definite saving in cost.

Figure 6 shows a block diagram of the ITT System 31 00D with a multi-drop digital subscriber loop line card, a digital subscriber line card, and the existing line, trunk, and IDEA cards.

The MDSL line card may be designed to be functionally and physically compatible with the existing ITT System 31000. As an example, the required functional hardware from the IDEA and the line card may be incorporated in the MDSL line card. Thus, the MDSL line card may be comprised of the MDSLfront end and existing IDEA and line card functional hardware. This is shown as a block diagram ofthe MDSL line card in Figure 7.

Figure8showsthe MDSL logic front end of Figure 7. Incoming serial data 1 is decoded into Rx Data Bit Stream 7 and Recovered Clock5 by the decoder ci rcu it 2. Master/Slave select switch 3 selects between Master-mode and Slave-mode. For Master-mode the clock is generated by the Master clock 4 and for Slave-mode the Recovered clock 5 is used by the Master Timing Logic 6. The Rx Data Stream 7 is fed into the Demux8 where the frame is recognised and the control and data are extracted. The Rx Control 9 and the Rx Data 10 are fed into the appropriate buffers 11 and 12. Control Logic 13 controls all the gating and data transfers and gets its stimuli from the Rx Control Buffer 11 and DSS/Tor PABX Control 14. All data transfers between the Rx and Tx Data Buffers 12 and 15 and the Uart 1 7/Codec 18 (slave-mode) orthe Group Controller 25 (master-mode) is under control of the Control Logic 13. Tx Control 19 and Tx Data 20 are fed to the Mux 21 where the Tx Bit Stream 22 is generated and fed to the Encoder 23 fortransmission as the Serial Data Out 24. Inject/Pass-thru switch 25 controls the injection ofthe local control and data intothetransmissionframeattheappropriate time.

The communication scheme is a modification of the SDLC/Token ring type. This scheme has manyadvan- tages over other methods, one of which is dynamic bandwidth allocation, which permits more efficient use of the available bandwidth.

By treating the MDSL line card and the new digital subsets as one subsystem, one can optimise itsfunctionality bytransferring some software modules from the MDSL line card to the digital subsets. As an ex ample, the functional module that performs baud rate conversion could betransferred to allowthe pro- cessors in the digital subsets and the MDSL line card to share the total processing load relatively equally.

The MDSLon the ITT System 3100 forms the basisformany ofthe so called 4th generation features. For example, one can enrich the user/system interaction to provide enhanced messaging. A building security and energy management package can also be provided by a PBX employing the invention, by having the MDSL provide the sensing and control transport mechanism.

Claims (5)

1. A communication system comprising a subscriber loop for providing a serial communication path for subscribers of the loop, and means, coupled to the loop, to assign the bandwidth of the looptothesub- scribers according to the requirements of the subscribers.

2. Acommunication system as claimed in claim 1,wherein communication on the loop is temporarily divided into frames of binarydigits.

3. Acommunication system as claimed in claim 2, wherein each oftheframescomprises a framesyn- chronisation field for sig nal I i ng the start of each of the frames, a control field for providing control information of an addressed one ofthe subscribers, a control address field, for indicating the address of the addressed subscriber, and a data field for providing data to the subscribers.

4. A communication system, comprising a private branch exchange, a plurality of user devices, a sub scriber loop coupling the user devices serially to the exchangeto provide serial communication path forthe devices to the exchange, and means, coupled to the loop, for dynamically allocating the bandwidth ofthe loop among said devices.

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