OPERATING A MULTIPLE-ACCESS OPTICAL NETWORK

Abstract: A method of operating a multiple-access optical network is described. The network has a number of communication stations having receiving and transmitting portions (1,2; 3,4; 5,6). The stations are physically coupled together by optical waveguides and arranged such that each transmitting portion (1, 3, 5) can transmit signals to one or more receiving portions (2, 4, 6) but cannot transmit signals to the receiving portion of the same station. One method comprises enabling two stations to communicate with each other by transmitting information simultaneously in the same channel. Another method comprises enabling two stations to communicate with each other by causing the transmitting portion of one station to transmit an interfering signal in the same channel as the information transmitted by the transmitting portion of the other station, the interfering signal being such that at other receiving portions the information and interfering signals are received together.

Description: A multiple-access optical network has a plurality of communication stations, the stations being physically coupled together by optical waveguides (usually optical fibres) and arranged such that each station can transmit signals to one or more of the other stations. Such communication networks are referred to as multiple-access (MA) networks since any station can receive signals from a number of the other stations. Recently proposed multiple-access networks have been based on optical fibre links which offer a relatively secure, closed, transmission medium. However, one of the most attractive signal distribution schemes from a network insertion loss and reliability point of view, ie the transmission-star coupler, has a disadvantage of distributing information transmitted from one station to a number of the other stations. This information could be protected using data encryption techniques but this is costly. In accordance with the present invention, a method of operating a multiple-access optical network having a plurality of communication stations, the stations being physically coupled together by optical waveguides and arranged such that each station can transmit signals to one or more of the other stations but cannot receive signals it has itself transmitted, comprises causing one station to transmit information over a selected channel to a receiving station and causing the receiving station to transmit simultaneously a signal over the same channel. This invention finds use, for example, in the type of passive multiple-access network which is described in more detail in published GB patent application number 2172165A, "Optical Signal Power Divider", in the name of British Telecommunications plc. The power dividing network desribed in that patent application sets out to reduce the number of communications channels required in a passive network by using a topology in which a transmitting station is unable to receive its own transmitted signal. The present invention aims to provide a method of operating networks with passive network topologies in which a transmitting station cannot receive its own transmitted signal to provide a natural degree of data security. In some cases, both stations may transmit information to each other over the same channel. Thus, the method according to the present invention of causing one station to transmit information over a selected channel to a receiving station and causing the receiving station to transmit simultaneously a signal over the same channel, may comprise operating the network in full-duplex operation over a single transmission channel. In this case, stations not associated with the two communication stations will receive both transmissions simultaneously and it will be very difficult for those stations to decipher the two simultaneous signals in such a busy channel, giving a degree of natural data security. The two communication stations, on the other hand, can use the same transmission channel without causing mutual interference. In other cases, the method comprises causing the station which is to receive information to transmit an interfering signal in the same channel as the information transmitted by the other station, the interfering signal being received at each of the other stations which also receives the information. This second arrangement provides a more secure transmission system than in the first cases mentioned above allowing each receiving station to protect its own incoming information by effectively blocking all other stations from receiving information in that channel. Conveniently, the station transmitting information may monitor its own receiving channel to confirm that the interfering signal is present before information is transmitted. The channels may be used on any multiple-accessing technique such as wavelength, frequency, time, or code division. The physical connections between stations may be provided by opical fibres, typically monomode optical fibres. The multiple-access network may include an optical signal divider in accordance with any of the examples described in the afore-mentioned published GB patent application 2172165A, the divider comprising X input ports and N output ports; and coupling means for optically coupling each input port with N-Z output ports, where N-Z 1 and Z 0, whereby in use the signal power in each input port is divided between the respective N-Z output ports. An example of a method of opeating a multiple-access optical network according to the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a transfer matrix of a multiple-access network suitable for use with the invention; and, Figure 2 is a block diagram of a multiple-access network. Figure 1 illustrates a transfer matrix for a power divider for use in a multiple-access optical network. This illustrates that each input port P₁,P₃,P₅ is connected to a respective group of two output ports P₂,P₄,P₅ with the input power equally divided between each of the five output ports. For example, the input port P₁ is connected to the output ports P₄,P₆. It should be noted that the input port P₁ is not connected with the output port P₂. This feature leads to a number of advantages including a fifty per cent saving in the number of transmission channels if full-duplex operation is required. In addition, collision detect network access procedures may be greatly simplified since a collision would be indicated by the simultaneous reception of signals in a busy station's transmission channel. The looped signal paths which can occur in interconnected transmissive style networks can also be prevented with these new networks if multiple routes between transmissive star couplers are avoided. An example of a multiple-access network using an optical power divider based on the Figure 1 transfer matrix is shown in Figure 2. The network shown in Figure 2 has three transmitting/receiving stations comprising respective pairs of transmitting and receicing portions 1, 2; 3, 4; and 5, 6. For clarity, the different portions 1-6 have been separated in the drawing although in practice the corresponding transmitting and receiving portions will be physically associated and may be widely spaced from other stations. Each transmitting portion 1, 3, 5 feeds signals to an input port of a respective coupler 7, 8, 9. The output ports of each coupler are connectd to respective pairs of Y couplers 10, 11, 12 whose outputs are connected to respective receivers 4, 6, 2. For example, the output ports of the coupler 7 are connected to input ports of the couplers 10, 11. It will thus be seen that the transmitting portion 1 is physically incapable of transmitting signals to the receiving portion 2. Typically, the signals transmitted will be optical signals in which case the connections between couplers may be provided by optical fibres. Consider an example where the station having transmitting/receiving portions 1, 2 wishes to communicate with the station having transmitting/receiving portions 3, 4. Initially, a suitable channel (A) is chosen which may be based on any multiple-access technique, mentioned above, for example, a TDMA system may be used. As previously explained, if the transmitting portion 1 transmits information on channel A this will be received not only by the receiving portion 4 but also by the receiving portion 6. If simultaneous communication between stations is required then the same channel A can be used by both transmitting portions 1, 3 since the respective portions 2, 4 will not receive information from the associated transmitting portion. However, the receiving portion 6 will receive information in the same channel from both transmitting portions 1, 3. This will make it very difficult for the receiving portion 6 to determine the nature of the information. Alternatively, where non-simultaneous communication is required a different method may be used. If the transmitting portion 1 is to transmit information to the receiving portion 4 on channel A, the transmitting portion 3 transmits an interfering signal on the same channel, channel A, as the information from the transmitting portion 1. This interfering signal will be received by the receiving portions 2, 6 but not by the receiving portion 4. Thus the receiving portion 4 is free to receive the information transmitted by the transmitting portion 1. The interfering signal is chosen to have the maximum interference effect. If a TDMA system using pulse-position-modulation is used, the interfering signal could be provided by a random pulse position signal. In some cases, the transmitting portion 1 may delay sending information until its associated receiving portion 2 has detected the existence of the interfering signal on channel A.

Claims: 1. A method of operating a multiple-access optical network having a plurality of communication stations, the stations being physically coupled together by optical waveguides and arranged such that each station can transmit signals to one or more of the other stations but cannot receive signals it has itself transmitted, the method comprising causing one station to transmit information over a selected channel to a receiving station and causing the receiving station to transmit simultaneously a signal over the same channel. 2. A method according to claim 1, comprising enabling two stations to communicate with each other by transmitting information simultaneously in the same channel. 3. A method according to claim 1, comprising causing the station which is to receive information to transmit an interfering signal in the same channel as the information transmitted by the other station, the interfering signal being received at each of the other stations which also receives the information. 4. A method according to any of claims 1 to 3, wherein the channels are based on one of a wavelength, frequency, time, or code division system. 5. A method according to any of the preceding claims, wherein the network includes an optical signal power divider comprising X input ports and N output ports; and coupling means for optically coupling each input port with N-Z output ports, where N-Z 1 and Z 0, whereby in use the signal power in each input port is divided between the respective N-Z output ports. 6. A method according to any of the preceding claims of operating a passive multiple-access optical network 7. A method according to claim 6 of operating a transmissive star network.

Inventors: HEALEY, PETER
Applicants: BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1986
Main classification: H04B10
IPC codes: H04B10, H04Q3