DATA PACKET REPEATER

Abstract: A packet-based communications system, such as a power line communications system, uses a repeater to reach subscriber stations that cannot be directly reached by a base station. The repeater is arranged to receive a packet in a first time period and to retransmit it in a subsequent time period, thus avoiding the need to reserve part of the frequency band for use by a frequency-translating repeater. The system can be a polled system and the base station allows for the time taken to reach a subscriber station via the repeater before polling another subscriber station.

Description: DATA PACKET REPEATER FIELD OF THE INVENTION This invention relates to the use of a repeater in a packet-based communications system. BACKGROUND OF THE INVENTION Communications signals are attenuated by the medium over which they are carried. In both wireline and wireless communications systems, it is common to use repeaters to extend the reach of a transmitted signal. In most wireline systems this can be achieved by inserting a repeater in the transmission line, the repeater receiving a weak signal, amplifying it, and retransmitting it. There has been considerable interest in transporting telecommunications signals over a power line. Existing power lines are used to carry the telecommunications signals to subscribers, thus avoiding the need to install new cabling to each subscriber. However, the power line poses a harsh medium for carrying communications signals, with high attenuation, high noise levels and regulatory limits on the level of transmitted power that can be used on the line. Consequently, it has been found that the achievable reach between a transmitter and a receiver on the powerline is less than the length of many of the distribution cables fed by an electricity substation. In many cases less than half of the electricity customers could receive a direct communications connection from a base station sited at the electricity substation, and the level of transmitted power cannot simply be increased because of regulatory limits. The reach of power line communications systems can be extended by siting repeater units along distribution cables leading from the electricity substation. However, unlike a dedicated communications cable, it is not easily possible to insert conventional repeaters in a power line. This would involve cutting the cable to insert an RF blocking device in series with the power cable to prevent source communications signals on one side of the repeater from interfering with the repeated signals on the other side of the repeater. US Pat. No. 5,726,980 describes a way in which repeaters can be provided on a power line. A received signal is repeated into a different frequency band. This avoids the need for RF blocking devices since the original and repeated signals are separated in frequency. However, repeating the signal into another frequency band has a disadvantage that part of the available spectral band has to be allocated for repeater use. SUMMARY OF THE INVENTION The present invention seeks to provide an alternative way of using a repeater in a communications system. Accordingly, a first aspect of the invention provides a power line communications system comprising: - a power line; - a plurality of communications subscriber stations coupled to the line; - a base station, coupled to the line, for serving the plurality of subscriber stations; and, - a repeater, coupled to the line, for use in reaching subscriber stations that cannot be directly reached by the base station, the system being a packet-based system, and wherein the repeater is arranged to receive a packet in a first time period and to retransmit it in a subsequent time period. Another aspect of the invention provides a repeater for use in the system. The use of a repeater which repeats in a different time period rather than a different frequency band offers particular advantages in a power line communications system where there are considerable constraints on the available spectrum. An advantage of using this type of repeater is that the full spectral band that is allowed for powerline communications can be used at once, rather than having to reserve part of the band for use by a frequency- translating repeater. By using the full spectral band, data throughput can be increased. A further aspect of the invention provides a communications system comprising: - a plurality of subscriber stations; - a base station for serving the plurality of subscriber stations; and, - a repeater for use in reaching subscriber stations that cannot be directly reached by the base station, the system operating in a polled manner whereby the base station sends a polling packet in a downstream direction to a subscriber station and receives, in reply, a reply packet in an upstream direction and wherein, to serve a subscriber station via the repeater, the repeater is arranged to receive a packet in a first time period and to retransmit it in a subsequent time period in both the downstream direction and the upstream direction, and the base station is arranged to allow for the time taken to reach a subscriber station via the repeater before polling another subscriber station. Thus, the reach of the polled system is extended without the need for a frequency- translating repeater and its associated disadvantages of permanently allocating part of the frequency band for use by the repeater. The polling mechanism is adapted to allow for the additional time needed to reach a subscriber via a repeater. A further aspect of the invention provides a repeater for use in a communications system which comprises a plurality of subscriber stations and a base station for serving the plurality of subscriber stations, the repeater being for use in serving subscriber stations that cannot be directly reached by the base station, the repeater being arranged to receive a packet in a first time period, to inspect the received packet and, if the inspection indicates that the packet should be repeated, to retransmit the packet in a subsequent time period. A similar problem is encountered when operating a polled packet data system on other cabled systems, where time division duplexing and multiplexing is employed within a certain band on cables primarily used for another purpose, where it is also not practical to insert isolation into the cable at the location of the repeater. A wireless packet data system employing omnidirectional antennas which do not afford isolation between receive and transmit sections also requires this solution, if efficient use is to be made of the bandwidth available. The system may be used to extend reach in low power systems, or for in-fill of fades or obstructions to line-of-sight paths. Preferred features may be combined as appropriate and may be combined with any of the aspects of the invention, as would be apparent to a person skilled in the art. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, and to show by way of example how it may be carried into effect, embodiments will now be described with reference to the accompanying drawings, in which:- FIGURE 1 shows a power line communications system; FIGURE 2 shows the communications network in more detail; FIGURE 3 shows equipment for use at a subscriber premises; FIGURE 4 shows operation of the direct polling method; FIGURE 5 shows operation of the polling method via a repeater; FIGURE 6 shows timing detail of the polling method via a repeater; FIGURE 7 shows a packet structure; FIGURE 8 shows a repeater; FIGURE 9 shows functional blocks of a repeater; FIGURE 10 shows a base station; and, FIGURE 11 shows a flow chart for a polling method by the base station. DESCRIPTION OF PREFERRED EMBODIMENTS A preferred embodiment of the invention is a powerline communications system, which will now be described in more detail. Figure 1 shows an electricity distribution network which is adapted to carry telecommunications signals. Mains electricity enters the network from an 11 kV or 6.6kV transmission line 105 and is transformed by substation 100 into a 400V supply which is delivered over distribution cable 120 to customer premises S1 to S6. A substation 100 typically has between 4 and 8 distribution cables of the kind shown as 120, 121 leading from it, each distribution cable serving a number of premises. A distribution cable can extend for several hundreds of metres. Distribution cable 120 comprises blue, red and yellow phase lines and a neutral line. Subscriber communications stations CPE are typically located at houses or businesses. A full system will usually include more than the six premises shown here and will typically include a more elaborate tree-and-branch distribution network. Subscriber premises may receive a single phase electricity supply (230V) or a three-phase electricity supply (400V). Domestic subscriber premises usually receive a single phase supply and neighbouring subscriber premises are usually coupled to different phase lines. In figure 1 subscriber S1 is shown coupled to the red phase line, and subscriber S2 is coupled to the yellow phase line. This helps to distribute the load of the network evenly across the three phases. A base station BS couples data communications signals onto distribution cable 120. The base station can be coupled to one or more distribution cables 120 at a point near to substation 100, as shown in figure 1 , or it may be coupled to the bus bars at substation 100, the bus bars acting as a star point for serving all of the distribution cables. The communications signals propagate over the electricity distribution cable to transceiver stations at subscriber premises S1 to S6, with a coupling unit CU 151 coupling the communications signals to/from the power line. Subscriber premises couple to a phase line of distribution cable 120 by a branch line 150. In the upstream direction, communications signals are transmitted from the subscriber transceiver stations CPE over cable 120 towards the base station. Communications signals can be transmitted between a phase line and neutral or earth, or in a differential manner. A repeater 160 is shown connected to cable 120. The purpose of the repeater is to extend the reach of the base station BS to those subscriber stations that cannot be directly reached by BS. The positioning of the repeater is a matter of choice, determined by measurements made on the particular network. Such measurements determine those parts of the network which will receive an inadequate service and which need a repeater. A repeater may serve one phase line or a number of phase lines. A dedicated repeater can be coupled to the distribution line 120 at a convenient point, such as a lamp post, or a subscriber station CPE can be used as a repeater. Figure 2 shows a top level diagram of an example power line communications (PL) system configuration. The end-user computer connects to the customer premises equipment (CPE), within the customer premises. The CPE is connected to the power line base station using a power line protocol across the low-voltage LV power line. Communications traffic from multiple base stations accesses the core network through the main station via concentration in a hub, FDDI ring or Ethernet daisy chain. Within the core network a manager station allows remote management of the base stations and CPEs. Intranet and Internet access is available directly or indirectly via the core network, via gateways where appropriate. An example customer premises installation is shown in figure 3. The CPE is a freestanding unit, powered from the customer premises mains supply. The CPE provides two data connections: • a connection to the subscriber's computer, e.g. 10BaseT Ethernet connection • a connection to the power line coupling unit CU. The coupling unit provides the link between the CPE and the external power line. Preferably this is at the street side of the electricity meter. The CPE need not be located close to the customer's computer. For ease of installation within the customer premises, the Ethernet link is preferably via unshielded twisted-pair (UTP) cable. A number of data terminals can connect to the CPE in the form of a local area network, as shown in figure 3. The CPE is intended to be left powered up continuously, i.e. it does not need to be turned on and off at the same time as the computer. Frequency shift keying (FSK) is preferred as a modulation scheme. The special case of minimum shift keying (MSK) and more spectrally-efficient modulation schemes such as QAM can also be used, although these require a higher carrier-to-noise ratio (CNR) and will hence reduce the achievable range for a given transmitter power limit. As described below, subscribers located closer to a base station may use the more spectrally-efficient schemes as they will experience a higher SNR. The PL system delivers around 1 Mbps peak bit rate, shared between all CPEs connected to a given Base station. This peak shared bit rate may be reduced, such as to 250kbps or 500kbps, or raised depending on available bandwidth and modulation schemes that are used. Reduced bit rates will also be supported to provide service to end-users at the extremes of the system range and end-users with adverse link transmission characteristics. To compensate for difficult CPE-Base station links (e.g. CPEs at the extremes of transmission range) one or more reduced bit rates can be used. The bit rate and frequency band for the CPE can be set via control signals from a network management unit. The range over which a PL link can be reliably maintained is dependent on the loss characteristics of the electricity distribution cable and the noise levels experienced at the receiver. For reliable operation, it is expected that a carrier to (background) noise ratio (CNR) of greater than 10 dB is required. The use of multiple frequency bands and multiple data rates allows individual CPEs to be configured to best match the transmission characteristics to the customer premises. Where appropriate, such parameters will be capable of being configured across the network from the management system. The base station and repeater units support the same frequency bands and data rates as the CPEs connected to it. When communicating with any CPE, the Base station and repeater automatically switch to the frequency and data rate used by that CPE. The communications network comprises a primary station (base station) and a number of secondary stations (subscriber stations) communicating over a power line system. The primary station controls access to the network by the secondary stations through the use of polling. The primary station allocates physical station addresses to each secondary station. As the Power line protocol emulates an Ethernet LAN service each station is also uniquely identified by the use of an Ethernet address which would be fixed at manufacture. Each secondary station on the network is registered with the primary station through the management interface, using its unique Ethernet address, before service can be provided to it. The primary station is responsible for determining if registered secondary stations are operating in an active, degraded or inactive state. Changes in the operational state of a secondary station do not affect the normal operation of other stations. The primary station continuously polls all active secondary stations in sequence using either information or polling frames, selecting the appropriate modem frequency and data rate as required. Secondary stations can only transfer information when they have been polled. The primary station waits for a response to its poll or moves on to the next active secondary station if a response is not received within the expected time. Additionally, a repeater is inserted between a base station and an outstation in this polled packet data system and it performs a store and forward function. The repeater has both its receive and transmit stages connected to the power line with little or no isolation between the two. If communication is required between a base station and an outstation, and if the two are beyond communications range by a direct path, then a packet is first sent by the base station and received by the repeater unit. The packet is demodulated and stored, and then retransmitted in the next timeslot. The polling sequence is under control of the base station, which ensures that only the repeater is transmitting in its allocated timeslot. The repeated packet is then received by the outstation. The reverse procedure is employed in the return path, also under the control of the base station. Polling is now described with reference to figures 4 and 5. In fig. 4 base station BS polls subscriber S1. S1 is located close to BS and can be reached directly. BS transmits a polling packet POLL 201 and receives, in reply, a reply packet REPLY 202 in the next time period. In fig. 5 base station BS polls subscriber S4. S4 cannot be reached directly by the BS and requires the help of repeater 160. BS transmits a polling packet POLL 210. This is received by repeater 160, amplified and retransmitted as POLL' 211. S4 receives the polling packet and replies with a reply packet REPLY 212 which is again received, amplified and retransmitted by the repeater as REPLY' 213. The base station is aware that S4 cannot be reached directly and waits a suitable time before polling another subscriber station. Figure 6 shows timing of the signalling sequence of fig. 5. The activities of the three elements: base station BS, repeater and subscriber station are shown on respective lines. Firstly, BS transmits a polling packet 301 to the repeater. This is received 302 by the repeater. The repeater waits until BS has finished transmitting before retransmitting the polling packet as POLL' 303. This is received 304 by the subscriber station. The subscriber station then transmits a reply packet 305 which is received 306 by the repeater. The repeater waits until the subscriber station has finished transmitting before retransmitting the reply packet 307 which is received 308 by BS. Once BS has transmitted the polling packet 301 to the repeater, it waits a suitable time for the reply 308 from the repeater. In the example shown, the polling packet POLL and the reply packet REPLY are of equal length. In this case, BS waits for a reply for a period of at least three times the length of a packet before polling another station. The polling packet and reply packet may be of different lengths, and doing so can improve throughput. Where a station is polled and has nothing to send in return both the polling and reply packets will be short. Where data is primarily being delivered to a station e.g. downloading a web page to a terminal at the subscriber, the polling packet will contain a long payload and the reply packet will be short. Where a subscriber is sending data, this will result in a short polling packet and a long reply packet. The base station, upon polling a station that it knows is being served by a repeater, waits until either: (a) it receives a reply, or (b) a time-out period has expired, the time-out period being based on the expected packet length. Because there is no isolation between the transmit and receive parts of the stations, the reception of a packet and the subsequent transmission of a packet occur in separate time periods. However, due to the unpredictable length of packets, these separate time periods are not rigidly defined timeslots such as would exist in a TDM system. Figure 7 shows an example format for a polling packet. This comprises a synchronising field 320, an address field 321 , repeater information field 322 and a payload portion 323. The repeater information field can be used in a number of ways. In one embodiment, the repeater field can simply indicate whether or not the packet requires repeating. A repeater, upon receiving the packet can use this field to determine whether or not it needs to repeat the packet. For greater control over how a packet is handled during transmission, it is preferable to assign a unique address to a repeater or to a group of repeaters. A repeater, upon receiving a packet, examines the address and compares it with its own address to determine whether or not it needs to repeat the packet. Each repeater or group of repeaters on a particular phase line (red, yellow, blue), distribution line (120, 121 fig. 1) or combination of these can be assigned a unique address. It may even be desirable to allocate a unique address to each repeater. Considering the previous example where a base station wishes to reach a subscriber S4 on red phase, then BS sends a polling packet that identifies a repeater on red phase. This prevents repeaters on other phase lines from attempting to repeat the packet, since mutual coupling between phase lines will often cause repeaters on blue and yellow phase lines to receive the packet as well as those repeaters on the red phase line. A base station has knowledge of the repeater or group of repeaters that will be needed to serve a given station and inserts the appropriate address into the packet. While a single repeater should be sufficient to serve most subscribers, it is possible to concatenate several repeaters. By allocating a common address to a group of repeaters, it is possible to "daisy-chain" a packet along the group of repeaters to reach the desired station. Where several repeaters are to be used, without modifying the repeater address field, it is important to ensure that each repeater is suitably spaced apart along the power line so that two repeaters are not able to receive and retransmit the same packet simultaneously, which would severely degrade performance. Figure 8 shows a repeater unit. This can be based on a subscriber station CPE. Indeed, one alternative to providing dedicated repeater units on a distribution line is to designate certain home units as repeaters. The repeater includes a transformer 400, a receive chain comprising an amplifier 410 and a demodulator/decoding block 420 and a transmit chain comprising a coding/modulator block 440 and an amplifier 430. The modulator/coding block converts raw digital data signal into a coded form before modulating it to RF in a suitable form for transmission over the power line. The modulation may be Frequency Shift Keying (FSK), Quadrature Amplitude Modulation, Orthogonal Frequency Division Multiplexing (OFDM) or some other appropriate modulation scheme. Demodulator/decoding block 440 performs a complementary operation. Processor 450 processes the digital data stream supplied by the demod/decode block 420. As described previously, a repeater can examine an address field in a received packet to determine whether the packet needs to be repeated. Figure 9 shows how this can be achieved. A header extraction block 460 extracts the repeater address field from a received packet. The packet is stored in a buffer 470 and also supplied to a control function 465 which compares the address in the received packet with a stored address from address store 467. If the stored address matches the address in the received packet, a control signal causes a gate 480 to release the packet from buffer 470 to the output, for retransmission. The functions shown in figure 9 can be performed by processor 450 under the control of software, by hardware or by a combination of these. The address store 467 can be implemented as a code stored in a non-volatile memory, by a code set by a DIL switch or some other appropriate manner. Figure 10 shows an example of equipment provided at base station BS. Communications signals at RF are received from power line coupling units at the power line drivers 500. These signals are fed to RF modems 510 which convert the signals between RF and base band. Backhaul card 530 converts the data into a format for transmission over the backhaul network. The digital interface card 520 manages the polling protocol on the distribution cables. This card comprises a processor under software control, with a memory that stores a table of details for each subscriber station in the system. This table includes the following details for each subscriber station: - whether the station is reachable directly - address of a repeater for use in reaching the station. Figure 11 shows a flow chart of a method that the base station processor can perform in polling the subscriber stations. Firstly, at step 600, the address of the next station is retrieved, together with the connection details for that station. At step 605 it is determined whether the station is reachable directly, by inspecting the retrieved connection data. If the CPE is reachable directly, it is polled at step 610 and two tests are performed. Firstly, at step 615, it is determined whether a reply has been received. If no response has been received, the processor waits a 'normal' time out period at step 620. This is the normal period expected for sending a polling packet and receiving a reply packet without the use of a repeater. Should the CPE not be reachable directly, then at step 630 the CPE is polled via a suitable repeater, the repeater being determined by the stored details. The same two tests are performed at steps 635 and 640 but there is a difference in that the time out period is extended to cover the time that it would normally take to send a polling packet and receive a reply via a repeater.

Claims: I CLAIM: 1. A power line communications system comprising: - a power line; - a plurality of communications subscriber stations coupled to the line; - a base station, coupled to the line, for serving the plurality of subscriber stations; and, - a repeater, coupled to the line, for use in reaching subscriber stations that cannot be directly reached by the base station, the system being a packet-based system, and wherein the repeater is arranged to receive a packet in a first time period and to retransmit it in a subsequent time period. 2. A repeater for use in a power line communications system, the system comprising a base station for serving a plurality of communications subscriber stations in a packet-based manner, the repeater being for use in permitting the base station to reach stations that cannot be directly reached by the base station, the repeater being operable to receive a packet in a first time period and to retransmit it in a subsequent time period. 3. A communications system comprising: - a plurality of subscriber stations; - a base station for serving the plurality of subscriber stations; and, - a repeater for use in reaching subscriber stations that cannot be directly reached by the base station, the system operating in a polled manner whereby the base station sends a polling packet in a downstream direction to a subscriber station and receives, in reply, a reply packet in an upstream direction and wherein, to serve a subscriber station via the repeater, the repeater is arranged to receive a packet in a first time period and to retransmit it in a subsequent time period in both the downstream direction and the upstream direction, and the base station is arranged to allow for the time taken to reach a subscriber station via the repeater before polling another subscriber station. 4. A base station for use in a communications system comprising a plurality of subscriber stations served by the base station and a repeater for use in serving subscriber stations that cannot be directly reached by the base station, the base station being arranged to operate in a polled manner whereby the base station sends a polling packet in a downstream direction to a subscriber station and receives, in reply, a reply packet in an upstream direction and wherein, in serving a subscriber station via the repeater, the base station is arranged to allow for the time taken to reach a subscriber station via the repeater before polling another subscriber station, the repeater being arranged to receive a packet in a first time period and to retransmit it in a subsequent time period in both the downstream direction and the upstream direction. 5. A base station according to claim 3 further comprising a store of connection details for each subscriber station that it serves, the connection details including an indication of the need to use a repeater to serve the station. 6. A base station according to claim 3 further comprising a store of connection details for each subscriber station that it serves, the connection details including an indication of the repeater that is needed to serve the station. 7. A method of operating a base station in a communications system comprising a plurality of subscriber stations served by the base station and a repeater for use in serving subscriber stations that cannot be directly reached by the base station, the method comprising operating the base station in a polled manner whereby the base station sends a polling packet in a downstream direction to a subscriber station and receives, in reply, a reply packet in an upstream direction and wherein, in serving a subscriber station via the repeater, the base station allows for the time taken to reach a subscriber station via the repeater before polling another subscriber station, the repeater being arranged to receive a packet in a first time period and to retransmit it in a subsequent time period in both the downstream direction and the upstream direction. 8. A method according to claim 7 further comprising the base station maintaining a store of connection details for each subscriber station that it serves, the connection details including an indication of the need to use a repeater to serve the station. 9. A repeater for use in a communications system which comprises a plurality of subscriber stations and a base station for serving the plurality of subscriber stations, the repeater being for use in serving subscriber stations that cannot be directly reached by the base station, the repeater being arranged to receive a packet in a first time period, to inspect the received packet and, if the inspection indicates that the packet should be repeated, to retransmit the packet in a subsequent time period. 10. A repeater according to claim 9 wherein the repeater stores an address and a packet contains an address field, the repeater being arranged to compare the address in the packet with its own address to determine whether to repeat the packet. 1 1. A repeater according to claim 9 wherein a packet contains a field indicative of whether repeating is required and the repeater is arranged to inspect this field to determine whether to repeat the packet.

Inventors: RICKARD ROBIN PAUL, RICKARD, ROBIN, PAUL
Applicants: NORTEL NETWORKS LTD, RICKARD ROBIN PAUL, NORTEL NETWORKS LIMITED, RICKARD, ROBIN, PAUL
Application year: 2000
Main classification: H04B3
IPC codes: H04B3