The present invention relates to the transmission of data over a network and in particular networks of electrically conducting media not originally designed to carry such data. These networks could be coaxial or single conductors or some other form of connection between two or more points.

In any particular building or complex many such conductors exist each installed for a particular purpose. Examples of such installations are audio background music, television or video distribution networks, security or fire alarm systems, intercoms and telephones and of course the mains power and lighting distribution network. Some of these networks extend out from their respective buildings and connect up with other buildings making extremely large networks available for data communication.

The purpose of this invention is to transmit data reliably over such conductors or networks. Such conductors are usually unpredictable as to the amplitude and frequency of signals which may be present on them either as a result of their original purpose or as a result of induced signals from other sources. For our purposes such signals whether intended or otherwise can be considered as interference.

To transmit data reliably over media supporting high levels of spurious interference special techniques need to be adopted in order to maintain the integrity of the said data. This invention provides a multiple point communication system which overcomes the problems of data corruption due to interference and noise.

One of the first problems to recognise is that interfering signals can be introduced onto the network at any point along its length, this in conjunction with the fact that the data signal will become progressively weaker as it travels further away from its point of introduction means that there will eventually be a point where the data signal will become so low in amplitude that even modest amounts of interference will render it irrecoverable.

The attenuation of the data signal is unavoidable as it is caused by the natural properties of the media such as resistance, inductance, and capacitance- Increasing series resistance and reducing parallel resistance as well as series inductance and parallel capacitance will cause a reduction in the amplitude of the data signal. A further problem is that variations in such impedance characteristics are beyond our control and therefore any data system connected to such a network should desirably be capable of adapting to the changing conditions.

It is not possible to overcome the problems caused by attenuation of the signal simply by increasing the launch amplitude at the transmitter as such a solution would inevitably cause interference to other systems in the vicinity. There are both British Standards and Home Office Regulations which limit the maximum output level of any electromagnetic signals which might be used as a carrier for the data signal.

Therefore as we are limited to a maximum launch level at the transmitter, inevitably our data signal will only travel a limited distance before it can no longer be reliably recovered.

The solution to this problem is to boost the data signal back up to a reasonable level at regular points throughout the network. Several such booster stages may be required in order to maintain the data integrity, particularly over long distances or where high levels of interference exist. Boosting or amplifying the data signal is usually achieved by inserting an amplifier unit having a gain over the range of frequencies concerned in "series" with the cable itself.

One problem with such amplifier units or "repeaters" as they are more commonly known is that there is usually a delay between the reception of a signal and its retransmission at higher amplitude, so that the boosted signal may interfere with the original "unboosted" signal. This problem is particularly serious in complex networks in which several repeaters are' receiving and retransmitting the same signal at different times, depending on their distance from the original source of the signals.

This problem has been approached in various ways in the prior art. Some known systems use repeaters with fixed addresses with data being directed from one repeater to the next in a fixed sequence. In some cases each repeater receives data on one channel and transmits it to the next repeater on another channel so that the retransmitted data does not interfere with the incoming data. This approach has several disadvantages. Firstly, in an "existing" network such as the mains power distribution network, the level of interference from the signals normally carried by the network and the impedance of the network may vary from one day to the next, in which case amplification at every repeater -stage may not be necessary. Also, these systems typically rely on an acknowledgement coming back from every repeater so that a re-try can be made if either the original data was not received or the acknowledgement was not sent back. The result is a large amount of network

activity simply validating data. Also, the need for an address location for every repeater is undesirable.

The present invention provides a communication system including at least one transmitter arranged to transmit signals to at least one receiver and at least one repeater for receiving and retransmitting said signals, wherein the signals comprise data words or parts of data words each represented by a particular frequency or combination of frequencies, and the transmitter is arranged to transmit said signals in successive spaced data bursts, in which each data burst represents the same data word or part of a data word throughout its period.

The advantage of transmitting data in "bursts" in this way, with a space between each burst, is that all of the repeaters in the system, as well as the transmitter itself, may be generating the same data burst of the same time without the respective transmissions interfering with each other. If any one section of a particular data burst period is decoded, whether it is the start, the middle, or the end, the resultant data obtained will be the same, thus allowing all of the repeaters to operate simultaneously. Each repeater will transmit a newly generated replica of the incoming signal as soon as possible even though the original transmission is still being broadcast on the network.

It will be appreciated that by operating in this way, repeaters may exchange and retransmit signals sent to each other. In the preferred embodiment of the invention the repeaters are enabled for a predetermined time following reception of a valid data burst, following which they are disabled until a new burst is received. The data bursts are preferably spaced so that one burst is not transmitted until the preceding burst is no longer being transmitted by any of the repeaters in the system.

A particular advantage of the present invention is that the repeaters themselves need not have addresses allocated to them since they simply receive and retransmit an identical signal. Also, the system is particularly adaptable to changing impedance and interference conditions in the network. In the preferred embodiment of the invention the or each repeater has a signal level detector and only boosts the incoming signal if it is below a certain amplitude, thus avoiding unnecessary repetition of signals of a significant amplitude.

A preferred embodiment of the invention comprises several units arranged to receive incoming signals and transmit replies. Each of these units may incorporate a repeater of the type described above possibly including a level detector. In a system of this type the units will each have an allocated address so that they can identify and reply to signals intended for them.

It will be appreciated that the invention is far simpler than prior art systems since it does not require complex software protocols or large addressing overheads. It is particularly applicable to power line networks which may join together at several different nodes. The signal does not follow a predetermined route but will simply find its destination without any intelligent help. For example, if transmission along one path is not possible the signal will automatically find a different route. The same advantage is obtained in internal (e.g. domestic) networks. On a ring main circuit an alternative route between two points always exists.

A system according to the present invention is particularly simple to monitor and control whereas a system with packets of data being directed around on many different channels under the software control of separate repeaters is not.

In the preferred embodiment of the invention the repeaters are connected in parallel with the network so as to require only the minimum of modification of the original network.

The invention will now be described in more detail with reference to the exemplary embodiment thereof illustrated in the accompanying drawings, in which:

FIGURE 1 is a layout of a network of cables connecting several locations together;

FIGURE 2 is a block diagram of a data transmitter;

FIGURE 3 is a block diagram of a data receiver; and

FIGURE 4 is a block diagram of a data repeater.

Figure 1 shows a typical cable distribution network, 1 represents the input for which the network was originally intended, this could be an audio signal in the case of a background music system or the line voltage in the case of a power distribution network. Numeral 2 represents the network, this normally consists of two or more conductors. Numerals 3 to 14, represent twelve output terminals associated with the network's primary purpose, these could be loudspeaker units in the case of a background music system or electrical power points in the case of the power distribution network.

In this example items 3 to 14 can simply be considered as points at which varying impedances and/or interfering signals could be introduced onto the network. Numerals 15 and 16 represent a data transmitter and an auxiliary data transmitter respectively while 18 represents the data receiver. Numeral 17 represents the data output. 19 is a data repeater with an auxiliary data input at 20 and an auxiliary data output at 21. 22 is a data repeater with an auxiliary data input at 23 and an auxiliary data output at 24. The transmitter 15 has a data input indicated as 25 and the transmitter 16 has a data input indicated as 26.

Figure 2 shows a block diagram of the transmitter 15. The carrier oscillator 30 of the transmitter 15 is frequency or phase modulated with the signals from the data encoder 27.

The frequency and amplitude of the signals presented to the network 2 from either a transmitter or repeater is governed both by the characteristics of the network as well as the prevailing regulations regarding electromagnetic radiation levels. The system to be described uses a carrier which complies with the British Standards 6839 Part 1 Recommendations but is not confined to this.

In Figure 2 the data encoder 27 is presented with a digital code at its data input 28. It is this code which is to be recovered at a remote location. The carrier oscillator 29 is enabled at enable control 30 and the modulated output signal appears at the output 31. The circuitry for generating the enable control forms part of the transmitter but is not illustrated in Figure 2. The use of an enable control is well understood in the art and will not be described in detail herein.In Figure 3 the modulated signal is applied to input 32 and amplified and demodulated by amplifier and demodulator 33. The data decoder 34 provides an output signal on output 35 and a data valid signal on a further output 36. The circuitry for receiving the data valid signal forms part of the receiver but is not illustrated in Figure 3 for simplicity. The use of a data valid signal is well understood in the art and not described in detail herein.

In this example a four bit code is used giving a possible sixteen combinations. The modulation technique is an important aspect of this communication system, most data communication systems transmit their data using a serial data bit stream necessitating the correct reception of several bits in succession in order to form a valid data word. With this system it is desirable but not essential to

depart from this serial convention and adopt instead a system where a complete data word is represented by a particular frequency or tone.

An example of this would be a coding system that uses sixteen different tones with values of 0 to 15 assigned to each of the sixteen tone frequencies. On subsequent reception of a tone its value can be deduced from its frequency or period and this value would not be affected by the previous or subsequent tones received. If only a few cycles of the transmitted tone are received they would still represent the same value as a full burst period of several hundred cycles.

A variation of this method of encoding and one which this particular example describes uses eight different tone frequencies, of which four belong to a low group and four belong to a high group. One of the tones from the low group is combined with one from the high group and together form the modulating signal. This is commonly known as dual tone multi-frequency encoding. This "two of eight" system allows for any one of sixteen combinations to be transmitted, providing a 0 to 15 range of coded signals.

With reference to Figure 1 the transmitter 15 is supplied with data at its input 25 which is to be transmitted over the network 2. Referring to Figure 2, this data is encoded by the data encoder 27 and the relevant tones are output to a modulator stage which forms part of carrier oscillator 30, the output of which is coupled onto the network 2. With reference to Figure 1, we will generate a frequency modulated carrier for a duration of 400 milliseconds by enabling the transmitter at 15. The purpose of this transmission is to send data to the receiver 18.

As previously described the signal will become attenuated as it travels along the network and depending on the prevailing conditions may or may not reach receiver 18.

Let us assume in the first instance that the network conditions are such that the signal cannot reach as far as receiver 18. Figure 3 shows a block diagram of a

receiver consisting of a carrier frequency receiver 33 which receives and amplifies the signal, and also comprises a demodulator stage which recovers the modulating tones. The recovered tones are decoded by decoder 34 and will be confirmed as a valid output signal at 35 by an additional signal appearing on 36. Whilst the signal was not able to reach receiver 18 it was able to reach repeater 19.

Figure 4 shows the block diagram of a repeater unit, such as repeater 19. Signals are output to and received from the network 2 via input/output 46. The carrier oscillator 37 and data encoder 38 form a data transmitter capable of producing the modulated tones in much the same way as the transmitter 15 of Figure 2 does.

Figure 4 shows also a data receiver 39 and decoder 40 capable of receiving and decoding the transmitted signal in much the same way as the data receiver of Figure 3.

In Figure 4 the decoded output of decoder 40 is latched into a data latch 41 on receipt of a data valid signal, output from decoder 40, on line 42. If switch 45 (to be described in more detail below) is in the position 49 as is shown in Figure 4 the incoming data is directed into the data encoder 38 and generates the exact tone pair which has just been received.

This new clean tone pair modulates the carrier oscillator 37 and is introduced onto the network cable at approximately the same level as was the original transmitted signal. A timer 43 initiated by the data valid signal on line 42 produces an output enable signal on line 44 which enables the carrier oscillator 37. The repeater may include a data output line 47 and a data input line 48 for connection to additional circuitry to be described below.

In this example this new transmission is enabled for 400 milliseconds. The timer signal 44 is also used to disable the data decoder in order to prevent a positive feedback loop from permanently enabling the output of this transmission.

The overall time taken from data acquisition to data transmission should be kept to a minimum, however, for the system in this example it is approximately 100 milliseconds. Let us assume once again that due to unfavourable network conditions the re-established signal is still not capable of reaching receiver 18- However it will reach repeater 22 which in exactly the same way as repeater 19 will introduce the signal onto the network approximately 100 milliseconds later and therefore 200 milliseconds after the original signal was transmitted from 15. The signal is now sufficiently strong to reach receiver 18 and the data will be successfully recovered.

This might seem to imply that there will be a 100 millisecond delay for every repeater on the network, this is not the case because the original transmission may well activate several repeaters which are within its range, however only the most distant repeaters activated are relevant, as it is from these that the re-established signal will be passed on further down the network.

If for example the maximum distance a valid signal can travel on a particular network under the prevailing conditions is 100 meters, then the data signal will expand outwards from its originating source in approximately 100 meter jumps every 100 milliseconds or so regardless of the fact that there may be repeaters installed at ten meter intervals throughout the network. The network size and worst case conditions are used to determine the duration of, and timing between bursts of tone data, the calculated durations are such that only one burst of data can be in transit on the network at any one time.

It may seem undesirable to have a multitude of transmitters all operating at the same time on the same frequency, but this is of no consequence to the operation of this system because of a phenomenon related to the type of modulation employed known as "capture effect". "Capture

effect" causes a receiver's modulator stage to lock onto the strongest signal available and reject all weaker ones. However, in order to reduce the number of coincident transmissions, a repeater may be arranged not to transmit the regenerated data burst if the network signal at its receiver input is already above a certain threshold level. This may be achieved by incorporating a signal level detector in the repeater, indicated at 51. This level detector generates a "transmit inhibit" signal on line 52 when the amplitude of the received signal is above a certain level. In this example the repeater system operates with its carrier wave, data burst tones and timing intervals derived from a locally generated time base. However in the case of the mains power cable being used as the data network, this aforementioned time base may be phase locked to the power line frequency thus reducing the possibility of intermodulation products being produced by adjacent repeaters.

Eventually the original signal emanating from transmitter 15 terminates. Repeaters in transmit mode will also time out and cease transmitting their duplicate signals, there is now a period of time when no transmissions are generated on the network, after which all repeaters revert back to receive mode ready to receive and transmit the next tone burst data transmission. The duration and timing of tone burst data transmissions followed by the silent period prevents the same data from being passed back and forth from one repeater to another.

In this example each transmission burst communicates the equivalent of a four bit word from its source through the network to the data receiver. We assumed in the foregoing that the original transmission emanating from 15 could not directly reach the receiver 18. However if conditions on the network were favourable and this original transmission did reach as far as 18 it would be

decoded by this receiver and a valid data signal would be output. However shortly after this, repeater 19 would reinforce the signal, as would repeater 22, again this is of no consequence as the data presented to the network is the same no matter which transmitter or repeater is active, the receiver 18 would simply maintain the same decoded four bits at its output for the full tone burst transmission period.

The receiver would after recognising a valid pause in transmission revert back to being ready to receive and decode the next new tone burst transmission which would represent the next four bits of the message.

By this method of transmission any amount of data can be down loaded in tone bursts representing four bits at a time. A number of data bursts received in this fashion are combined to form a complete data transmission message. It is interesting to note that repeaters 19 and 22 because of their parallel connection to the network are in fact bi¬ directional in operation, that is to say it doesn't matter from which end of the network the transmitted signal originates, it will effectively be amplified and passed on down to the other end of the network. This makes communication possible between any points on the network.

In one application many microprocessor systems are connected to the network each with its own built-in address code. Each microprocessor unit acts as a local monitor and controller as well as a network repeater unit. When a request for data is made by the master system controller its transmitted signal will be passed along as described in the above example by all the microprocessor/repeater units on the network. However, in this case each repeater will store each consecutive burst of data transmitted over the network and at the end of these transmissions each will check the accumulated data to see if it needs to reply to the master.

If a particular repeater recognises that a message is addressed to it, then a reply will be made, the bursts of

data transmitted will in turn be passed back along the network by all of the microprocessor/repeater units until finally the master unit having accumulated the data from each successive tone burst can store it for later analyses after which it can make another request for data from a different address location.

It will therefore be appreciated that by this multiple repeater method of communication an almost constant amplitude of signal can be maintained over the entire network.

The repeater of Figure 4 may be connected to a microprocessor via the data output 47 and data input 48. Incoming data received by the repeater may be transmitted to the microprocessor via output 47 and signals from the microprocessor are passed back to the repeater circuitry via data input 48. If the microprocessor recognises its own address in the incoming data, the switch 45 will be moved to position 50 shown in Figure 4 so that reply data from the remote unit can be sent back along the network via data encoder 38 and carrier oscillator 37. It will be appreciated that Figure 4 represents a hardware example of what can also be accomplished by suitable software.

A further improvement in data integrity can be obtained by distributing the output transmission thus described simultaneously over a band of frequencies. The data valid signal 42 can be used as a criterion in deciding which frequency the signal can most reliably be received on. A tone burst at the beginning of a message can be used by the network repeater receivers to ascertain the most favourable frequencies to receive on.

This reduces to a minimum the effect that interference has on the data signal and enables transmissions to be made over much longer distances than would otherwise be possible.