Method and apparatus for maintaining a predefined transmission quality in a wireless MAN network

p00001Method and apparatus for maintaining a predefined transmission quality in a wireless MAN network

p00002Field of the invention:

p00003The invention relates to a wireless network for a metropolitan area. More particularly, the invention relates to a technique for maintaining a predefined quality of the transmission link in a wireless network for a metropolitan area.

Background of the invention:

p00004Computers used in modern offices are usually coupled to a local area network (LAN). The LAN allows computer users to share common resources, such as a common printer included in the network, and allows users to share information files, such as by including one or more file servers in the network. In addition, users can usually communicate information with each other through electronic messaging. One type of LAN commonly used is Ethernet. Currently, there are several products that support commercially available Ethernet from various sources.

p00005Business organizations and their subsidiaries are frequently spread over several locations in a metropolitan or geographical area. For example, a business organization may have a central, one or more branches and other facilities. For such business organizations, the LANs located in the various locations will generally need to communicate information between them. Wireless communication links to connect local area networks are known. For example, each of US Patent No. 4,876,742, entitled "Apparatus and Method for Providing a Wireless Link Between Two Area Network Systems" and US Patent No. 5,436,902, entitled "Ethernet Extender", disclose a wireless communication link to connect LAN.

p00006The availability of a wireless link is usually expressed as the percentage of time during which the bit error rate representing the operation of the link is below a given threshold level. The meteorological precipitations cause a severe attenuation of the transmitted signal, especially for links that work in millimeter frequency bands. For example, to maintain 99.99% availability in the presence of meteorological precipitation, the signal must be transmitted at a level of up to 24 dB / km above normal. Therefore, to ensure an acceptable data error rate under all expected conditions, the data is usually communicated over a wireless communication link at a relatively high power and a relatively low speed. However, the amount of data to be communicated on the wireless link may vary widely over time and may vary regardless of environmental conditions. In addition, wireless links, especially those that operate at high power levels, may cause interference with other wireless links that work in the same geographical area.

p00007Therefore, what is needed is a technique to maintain a predefined transmission quality while transmitting data over a wireless communication link in a metropolitan area network (MAN). What is needed is also a technique to transmit data on a wireless communication link in a MAN at a speed sufficient to meet the demand. What is needed is also a technique to transmit data on a wireless communication link in a MAN at a relatively low power. What is additionally needed is a technique to reduce interference between wireless communication links operating in the same geographical area.

p00008WO9427382 describes a communication system in which multiple local area networks that have different characteristics are used to connect portable or mobile computing devices. Radio base stations connected to a backbone LAN and at least one mobile computing device form a high power LAN, which uses a frequency hopping protocol. A low power LAN allows radio communication between a mobile computing device and peripheral devices, using a single frequency spread spectrum protocol. A vehicular LAN provides short-range communication between vehicle terminals and portable terminals. The radio units participate in multiple LANs using the appropriate protocol, frequency and power level to communicate through the communication system.

p00009WO 93/00751 discloses a modem to modem communication system in which the power level can be modified depending on the quality of the communication channel.

p00010Compendium of the invention:

p00011According to the present invention, a method and an apparatus for communicating data between local area networks in a metropolitan area network is disclosed, the method comprising establishing a communication path between pairs of local area networks for data transmission , in which the communication path includes one or more wireless links; transmit data along the communication path; monitor the transmission quality of said one or more wireless links in both directions to obtain a measured performance indicator; periodically exchange performance indicators between nodes at each end of the communication path; keep a history of the performance indicators in each node, so that each node has a complete knowledge of the operating parameters in both directions; and carry out an adequate control procedure to maintain the required transmission operation based on a knowledge of the operation indicators in both directions.

p00012The examples relate to a method and apparatus for maintaining a predefined transmission quality for data transmission in a wireless metropolitan area network (MAN). Each of a series of local area networks (LANs) are coupled to a corresponding router. Each router is coupled to one or more transceivers to interconnect the routers via wireless communication links, thereby forming the MAN. The data to be communicated on the MAN are generated by a user or an application on a source node of a source LAN, and are communicated to a source router coupled to the source LAN. The data is used to modulate a carrier signal. A transceiver for the originating router transmits the modulated signal along a path of one or more wireless communication links to a receiver of a destination router for the data.

p00013If the path includes more than one wireless link, one or more intermediary routers will receive and retransmit the modulated signal along the path. The destination router demodulates the signal and communicates the data to a destination node within the destination LAN.

p00014Each router is coupled to one or more of the wireless links on which the router can selectively transmit data. In addition, each router stores a representative table of the topology of the entire MAN. Each router receives detected indexes representative of the transmission quality of the wireless links in the MAN, and detected indexes representative of the network demand. Based on the stored topology and the indexes detected, each router determines a way in which data on a selected one of the corresponding wireless links is to be transmitted.

p00015Unlike conventional wireless communications, the present invention maintains a global availability of 99.99% or greater, by adapting the transmission mode and transmission path to environmental and other conditions (by detecting transmission quality) and to the demands imposed on the network (by detecting traffic loads) and by relying on the nature tolerant to the delay of packet switching communications. As a result, a MAN according to the present invention can be constructed which is extremely efficient, in terms of cost, power and bandwidth utilization in order to achieve greater coverage and higher network density.

p00016An example of adapting the transmission mode to the detected transmission quality is disclosed. The strength of the received signal over each wireless communication link is monitored periodically to determine if the strength of the received signal is within a predefined range. If the signal strength is outside the range, the power at which the modulated signal is transmitted is regulated to return the signal strength to that interval. In addition, the error rate for the data received on each link is monitored.

p00017If the intensity of the signal received for a communication link remains below the predefined interval despite having adjusted the transmission power to the maximum permissible level, and if the error rate of the data received on the wireless link is close to a predefined limit or exceeds it, one or several techniques are used to reduce the error rate while maintaining a sufficiently high overall network flow. A first technique to reduce the error rate is to reduce the speed at which data is communicated along a path. A second technique to reduce the error rate is to modify a modulation level of the signal transmitted along the path. A third technique to reduce the error rate is to modify a coding scheme with error correction used for the data communicated along the path. A fourth technique is to use spectrum broadening to communicate data along the path.

p00018Each of these techniques to reduce the error rate for the data communicated along a communication path is used dynamically, individually or in combination, in order to maintain an error rate for the data below the predefined limit while maintaining at the same time a bandwidth of communication of the data sufficient to face the demand imposed on the MAN. By transmitting data on each link at a relatively low power, and by using coding with error correction and spread of the spectrum, the interference between links is kept to a minimum, allowing a high network density in the MAN.

p00019In addition, the amount of data reported on each link is also monitored. The communication path is preferably selected according to an algorithm to open the shortest path first (OSPF). If the shortest path does not have enough available bandwidth or results in an excessive error rate, a technique to reduce the error rate and increase the amount of data reported is to select one or more alternative paths to communicate at least one part of the data Preferably, the alternative paths are selected so that they are the next shortest paths available.

Brief description of the drawings:

p00020Figure 1 shows a schematic representation of a metropolitan area in which the present invention forms a metropolitan area network (MAN).

p00021Figures 2A-F show representative MAN topologies, according to the present invention.

p00022Figure 3 shows a functional block diagram of a router, according to the present invention, coupled to a local area network (LAN).

p00023Figure 4 shows a functional block diagram of an outdoor system, according to the present invention, which includes a radio frequency transceiver.

p00024Figure 5 shows a flow chart, according to the present invention, to maintain transmission quality on a wireless link.

p00025Figure 6 shows the timing relationship between the monitored RSL and the transmitted power, of the operation corresponding to the flow chart of Figure 5, during a period of fading due to heavy rain.

p00026Detailed description of a preferred embodiment:

p00027Figure 1 shows an example of a metropolitan area 100 in which the present invention forms a metropolitan area network (MAN). Several locations 102 are located within a geographical or metropolitan area 100

p00028118. Each location 102-118 may be an office building, an industrial estate, a factory, a corporate headquarters, a branch, a warehouse or other facility. Each location 102-118 has one or more local area networks (LAN) located within location 102-118. A router 300 (figure 3) according to the present invention is coupled to each LAN, while one or more external systems 400 (figure 4) according to the present invention are coupled to each router 300. The routers 300 are preferably located in wiring rooms of locations 102-118, in order to facilitate the connection of routers 300 to LANs. Each of the outdoor systems 400 includes a wireless transceiver to communicate data between locations 102-118. Each exterior system 400 is preferably located within a housing 102A118A on the roofs of sites 102-118.

p00029Together, each router 300 and its associated unit or outdoor units 400 form a node of the MAN. The nodes are interconnected via wireless communication links. Multiple simultaneous communication links can be achieved for a node by providing multiple exterior systems 400 for a site. Although nine locations 102-118 are shown in Figure 1, it will be understood that the MAN may include more or less sites, depending on the geography of the metropolitan area 100 and the number of LANs to be interconnected. Therefore, data originating from a node of a LAN located in one of the locations 102-118 can communicate over the MAN to other nodes in other locations 102

p00030118. While the present invention is preferably configured for a metropolitan area, it will be appreciated that under certain circumstances, the network may scale to a greater or lesser region. For example, the present invention could be applied between two or more metropolitan areas, or it could be applied to smaller geographical areas, such as a university or business campus. In addition, one or more of the sites can be configured to communicate data with a geosynchronous satellite. In such a case, the satellite may constitute a repeater for interconnecting MAN located remotely through satellite connections. In such a case, satellite connections can operate according to the present invention or according to known techniques.

p00031Figures 2A-F show representative MAN topologies, according to the present invention, for interconnecting AE nodes. Figure 2A shows the simplest system with a point-to-point topology for interconnecting nodes A and

p00032B. Figure 2B shows a linear topology for interconnecting AD nodes. Figure 2C shows a ring topology for interconnecting AD nodes. Figure 2D shows a ring topology for interconnecting AD nodes with a cross link between nodes B and D. Figure 2E shows a star topology for interconnecting nodes A-

p00033E. Figure 2F shows a star topology for interconnecting AE nodes with BE nodes, also interconnected in a ring topology. It will be apparent that additional topologies according to the invention can be constructed to interconnect any number of nodes based on the topologies shown in Figures 2A-F, and that other topologies according to the present invention can be constructed in addition to those shown.

p00034In general, by providing a large number of links with respect to the number of nodes, the capacity and reliability of the MAN is increased although its cost is also increased. For example, the topology shown in Figure 2B requires a minimum number of links to connect the AD nodes, so that the topology of Figure 2B can be constructed at a minimum cost. However, it should be noted that in figure 2B the data communicated between nodes A and D must pass through nodes B and C. Therefore, data communication will tend to concentrate on the link between nodes B and C, limiting the total network capacity. In addition, there are no alternative communication paths that can be used to avoid a communication link that is degraded due to weather precipitation, or to avoid a link that is inoperative due to the failure of a component. Instead, according to the topology shown in Figure 2C, data can be communicated directly between nodes A and D. The additional link between nodes A and D provides an alternative path for the data, thus expanding the total capacity and reliability of the network. However, it should be noted that in the MAN shown in Figure 2C, the data communicated between nodes B and D must pass through node A or node C. The cross link shown in Figure 2D provides an additional path to communicate data directly between nodes B and D, further expanding the capacity and reliability of the network. If desired, another link can be added to Figure 2D to communicate data directly between nodes A and

p00035C. In addition, redundant links can be arranged between pairs of nodes in the MAN. By having alternative data communication paths, the data communicated in the network can be routed to avoid communication links that are temporarily degraded due to environmental conditions, temporarily congested due to large amounts of demands on the network, or inoperative due to a breakdown. of a component.

p00036It should be noted that the network topology can be configured to take advantage of the knowledge about the demand that the network is expected to receive. For example, assume that it is known that the exchange must communicate large amounts of data with each of the various branches, but that the branches communicate less data between them. A star topology, such as the one shown in Figure 2E, is very suitable for said MAN if the exchange is located at node A and the branches are located at nodes BE. However, it should be noted that other network topologies, such as a ring topology (shown in Figures 2C or 2D), can be used for a MAN with a central and a branch. In the star topology, the data is communicated directly between each branch and the central, although the data communicated between branches must be communicated indirectly through the central. However, if each of the branches must also communicate large amounts of data with the others, additional links can be used, as shown in Figure 2F. The network shown in Figure 2F has great capacity and reliability due to the large number of alternative communication paths. For example, the data communicated between nodes B and D can pass through node E, node A or node C.

p00037Therefore, a network topology can be designed to interconnect a given number of nodes with a selected number of links to provide sufficient alternative communication paths so that the network has sufficient capacity and reliability, while minimizing the associated costs. It should be noted that additional links can be added to the existing MAN, in order to cope with increases in demand. In addition, nodes not connected to the LAN can function exclusively as repeater stations to increase the number of alternative communication paths and to transmit data over long distances.

p00038Figure 3 shows a functional block diagram of a router 300 according to the present invention, coupled to a LAN 302. Router 300 includes several functional blocks 304-318 interconnected by an interconnection 320. The functions of blocks 304-318 can be implemented. by means of physical equipment circuits, in which case, interconnection 320 represents a communication bus. Alternatively, the functions performed by blocks 304-318 can be implemented by a processor that operates according to a stored software program. The router 300 can be manufactured as a separate dedicated unit, or the router 300 can be manufactured by adding one or more circuit boards and software to an existing personal computer system.

p00039LAN 302 can operate according to any LAN protocol, although Ethernet is preferred due to its widespread use in existing locations, partly because Ethernet LANs generally use pre-existing telephone wiring in many buildings. At the level of medium access control (MAC), Ethernet transmits data in packets according to a CSMA / CD protocol. Each Ethernet data packet generally includes a preamble, a destination node address, a source node address, a data field and a data verification field.

p00040Router 300 interconnects with LAN 302 as if router 300 were an additional node in the LAN

p00041302 Thus, the router 300 can send and receive data packets to and from the other nodes of the LAN 302, according to the communication protocol of the LAN 302. The LAN 302 is interconnected with the router 300 through the interface 308 of the LAN. LAN interface 308 monitors data packets from LAN 302; If the address of the destination node of a packet indicates that the packet is destined for a node contained in a LAN other than LAN 302, the LAN interface 308 accepts the data packet. If the destination node address indicates that the data packet is destined for a node within the LAN 302, the LAN interface 308 ignores the packet.

p00042Data packets received from LAN 302 are stored in buffer 318 and conditioned for transmission over the MAN under the control of system control 306. This conditioning may include transforming the data format into a format suitable for transmission over the MAN. Such transformation may be necessary if LAN 302 includes a different type of LAN than Ethernet.

p00043The routing table 304 stores data representative of the MAN topology, which includes a table that identifies which router of the MAN is coupled to each node of each LAN interconnected by the MAN. Router 300 reads the destination address contained in the packet received from LAN 302 and uses routing table 304 to determine which router (destination router) of the MAN is coupled to the LAN that includes the destination node. Next, a header is added to the data packet, which identifies the destination router that the packet should receive. An additional data verification field can also be added to each package, encapsulating the original data package.

p00044Next, the package is communicated on an appropriate one of the links 322A-C to a suitable external system 400 (Figure 4). If there is more than one exterior system 400, the appropriate exterior system 400 is selected according to routing algorithm 312 and according to routing table 304. The routing algorithm 312 preferably includes an algorithm of first opening the shortest path (OSPF), although alternative paths can, however, be selected based on detected transmission quality conditions and traffic levels in the MAN, as explained In the present memory.

p00045When the suitable external system 400 (Figure 4) is available to transmit the data, these are provided to the suitable external system 400 via the interface 316 of external systems through one selected from the links 322A-C. Although the interface 316 of external systems is shown in Figure 3 being able to interconnect router 300 with up to three exterior systems 400 through links 322A-C, it will be apparent that more or less links 322A-C can be arranged and, by therefore, more or less outdoor systems 400 can interact with a single router 300. Preferably, the 322A-C links operate according to high-speed Ethernet standards, such as 100 BASE-TX, 100 BASE-FX or 100 BASE-T4, although other standards can be used, such as 10 BASE-T or 10 BASE- F. Because the router 300 is preferably located in a wiring room and said one or more exterior systems 400 are preferably located on a roof, the links 322A-C will generally extend from the wiring room to the roof. Data packets for communication on the MAN; orders for communication between router 300 and outside system 400 (node ​​orders); and the orders for communication between routers 300 (network orders) are all communicated on links 322A-C at the appropriate times.

p00046Figure 4 shows a functional block diagram of an exterior system 400, according to the present invention. The data packets from the external system interface 316 (Figure 3) of the router 300 are received from a respective one of the links 322A-C via a router interface 402 and transferred to a baseband processor 404. The baseband processor 404 selectively modifies the bit rate of the data packets, selectively performs coding with error correction on the data packets and selectively broadens the spectrum over the data packets, as appropriate. Preferably, the baseband processor 404 can also selectively encrypt the data in accordance with known encryption techniques, for data security reasons. Next, the data packets are transferred to the demodulator section 408A of a modulator / demodulator 408. The modulator section 408A suitably modulates the data in a radio frequency carrier signal, according to a selected modulation scheme, forming a modulated signal.

p00047The modulated signal is then subjected to upstream conversion, amplified and transmitted over a wireless communication link of the MAN via a transmitter section 410A of a radio frequency (RF) transceiver 410. The transceiver 410 is coupled to an antenna 412 for this purpose. Preferably, a high gain, directional antenna is used to reach distances of up to 10 kilometers. This range is related to the preferred transmission frequency of 38 GHz, although a lower transmission frequency may be used, which would result in a greater maximum transmission distance. Similarly, a transmission frequency greater than 38 GHz can also be used for short range. The transceiver 410 is configured to regulate the power at which the modulated carrier signal is transmitted. The power is regulated under the control of the system control 306 (figure 3) of the router 300, by sending node commands to the external system 400 over that appropriate from the links 322A-C.

p00048The data packets received from the wireless link are picked up by the antenna 412 and received by a receiving station 410B of the radio frequency transceiver 410. The received signal is adequately demodulated by a demodulator section 408B of the modulator / demodulator block 408, inverting the modulation that was performed on the data before they were transmitted over the wireless link. Next, the demodulated data is transferred to the baseband processor 404 that adequately decodes the data, applies error correction to the data, and reverses any spread of the spectrum or encryption applied to the data before they were transmitted over the link. Next, the data packet is communicated to the router 300 via the router interface 402.

p00049A transmission quality monitor 406 is coupled to the router interface 402, the baseband processor 404, the modulator / demodulator 408 and the RF transceiver 410. The transmission quality monitor 406 monitors the quality in various ways of the wireless transmission link from which the data was received. For example, the transmission quality monitor 406 calculates the bit error rate (BER) based on data errors reported to the transmission quality monitor 406 by the baseband processor 404. In addition, The transmission quality monitor 406 can monitor the received signal strength (RSL) from the receiver 410B and the signal-to-noise ratio of the signal received from the demodulator 408B . Other operating parameters, such as group variance, eye opening, computation of uncorrected errors, and so on, can also be monitored.

p00050It is hereby considered that the operating parameters, such as the BER, the RSL and the SNR, constitute an indicator of the operation of the wireless link in the direction of reception.

p00051The transmission quality monitor 406 periodically notifies the router 300 of the monitored operation indicator. The transmission quality monitor 406 also sends the measured performance indicator to its counterpart in the other node of the wireless link, so that both nodes have a complete knowledge of the operating parameters in both directions of the communication. Next, the transmission quality monitor 406 performs a suitable control procedure described herein, in order to maintain the required transmission operation. For example, referring to the wireless link that connects nodes B and C in Figure 2C, monitor 406 of the transmission quality of node B periodically sends its monitored operation to monitor 406 of the transmission quality of node C, and vice versa . The monitors 406 of the transmission quality of both nodes maintain a history of the performance indicators, for additional analyzes of the operation and faults, performed by themselves or by the processor 314 of the transmission quality. When the values ​​of the performance indicators are exchanged periodically, there is an integrated form of repetitive transmission and transmission with diversity. As used herein, repetitive transmission refers to the transmission of data redundantly, while transmission with diversity refers to the sending of data according to multiple different formats. As a result, the present invention quickly detects changes in the performance indicator and responds appropriately to such changes. Therefore, any sudden change in the newly received data is filtered immediately.

p00052Each transmission quality processor 314 may be coupled to more than one external system 400, and therefore, each router 300 may receive transmissions of performance indicators from these more than one wireless link. In addition, each transmission quality processor 314 can receive through network commands indicators of the operation of other links in the MAN. Accordingly, each transmission quality processor 314 can analyze the overall transmission quality (of the entire MAN) of the wireless links in the MAN and can determine an appropriate response based on current quality conditions for each of the links wireless coupled to router 300, at previous quality conditions for each of the wireless links coupled to router 300 and based on current or prior quality conditions for other wireless links, notified by another or other routers of the MAN.

p00053The data packets communicated to the router 300 from the external system 400 are received by the external system interface 316 of the router 300. By examining the header attached to the packet, the external system interface 316 determines whether the router 300 is an intermediate router in the communication path for the packet, and if LAN 302 is the final destination for the data packet. If LAN 302 is the final destination for the data packet, the packet is conditioned for communication to LAN 302. This conditioning includes the removal of the additional header that was added to the data packet before transmission. The data packet is stored in buffer 318 until LAN 302 is available to receive the packet.

p00054If LAN 302 is not the final destination for the packet, router 300 is an intermediary router in the communication path for the data packet. In such a case, the exterior systems interface 316 stores the packet in the buffer memory 318. When more than one outside system 400 is coupled to the router 300, the routing algorithm 312 then determines which of these outside systems 400 to communicate the packet for transmission over the MAN. This determination is based on an OSPF algorithm that uses routing table 304 and routing algorithm 312, and on detected conditions of transmission quality and traffic levels in the MAN. When the appropriate outdoor system 400 is available to retransmit the packet to the next router in the communication path for the packet, the packet is retrieved from buffer 318 via the exterior system interface 316 and provided to the appropriate outdoor system 400, which retransmits the data on the MAN.

p00055The amount of data traffic through router 300 (data transmitted and received by router 300) is monitored by monitoring and traffic control 310 based on the amount of data stored in buffer 318 and deleted therefrom, and to the frequency with which the data is stored in buffer 318 and deleted from it. The amount of data traffic is used by routing algorithm 312 to properly route the data in the MAN. The amount of data is the network demand on the node. The system control 306 can periodically communicate performance indicators and network demand detected to the other routers of the MAN through network order packets. While the transmission quality processor 314 and the system control 306 are shown as parts of the router 300, they can also be implemented as software functions located in a workstation or in a network computer connected to the network. Preferably, the overhead associated with the communication of the network order packets is less than two percent of the total data throughput capacity of the MAN.

p00056For normal operation, the BER of the wireless links throughout the MAN is preferably kept below an acceptable threshold. Link availability is defined as the percentage of normal operating time. For example, a 99.99% link availability and a required BER threshold of 10-9 means that for 99.99% of the time, the BER of wireless links throughout the MAN remains below 10-9 . In MAN, high link availability and a reduced BER threshold are preferred. In clear sky conditions, the attenuation of the signal is mainly due to a loss of propagation in the free space. The power of the transmitted signal must be high enough to keep the BER below an acceptable threshold. However, in an anomalous situation, additional losses (rain fade) can be introduced by atmospheric changes such as rain, fog, atmospheric boundary layer and so on. Therefore, in order to maintain normal operation in an abnormal condition, the transmitter must operate with a transmitted power exceeding that required in a clear sky situation. The difference between the power of the transmitted signal and that required in a clear sky situation is known as the fade margin. A higher required link availability or a longer reach requires a greater fading margin. Since the signal attenuation due to rain is very high over high radio frequency bands, such as 38 GHz, a traditional wireless link design based on the fading range requires excessively high transmitted power or provides an impractically short range to achieve the high availability required. The present invention presents a novel method and apparatus for achieving both long range and high link availability with low transmitted power, by using a combination of adaptive modulation techniques, coding with low speed error correction and reduction of the data rate, to compensate for the rain fade. In addition, by including an intelligent routing strategy in which data communicates between nodes through one or more appropriately selected paths (link diversity), the present invention increases the capacity and reliability of the MAN.

p00057By using a combination of adaptive power control, data rate reduction, modulation level reduction and coding with low speed error correction, a range greater than a low transmitted power can be achieved, as described herein. memory. Consider, by way of example, that a quadrature amplitude modulation scheme (QAM) is used M-th programmable, in modulator / demodulator 408 of Figure 3. For example, it is assumed that 16QAM is used to transport high-speed 100 Mbps duplex Ethernet over a wireless link in the MAN. To achieve a BER of more than 10-4, the RSL must be maintained above a predetermined threshold level, referred to herein as RSL 16. In addition, to avoid degradation of operation due to non-linear distortions AM-to-AM and AM-to-PM, the transmitted power is preferably maintained at 6 dB or more below the 1 dB compression point of the power amplifier for 16QAM. The 1 dB compression point refers to an input signal applied to the power amplifier that results in an output signal that is 1 dB below what would be expected if the power amplifier had a linear gain.

p00058In a given time interval during rain fades, if the monitored BER and RSL of a particular link are approaching their predefined threshold levels (for example, 10-4 and RSL16), appropriate countermeasures are invoked in order to improve the functioning of the link and maintain the availability of the necessary link. Countermeasures can be one or more of the following: momentarily reduce the transmission speed of the link, reduce the level of modulation and / or introduce a coding with low speed error correction. For example, if in response to a rain fade, the link operates at a transmission rate that has been reduced to 50 Mb / s, and with a modulation level that has been reduced by 4QAM, its RSL threshold for 10-4 , referred to herein as RSL4, is 8 dB below the RSL16 threshold. In addition, since 4QAM can tolerate more non-linear distortions AM-a-AM and AM-a-PM, the transmitted power can be increased up to 2 dB below the 1 dB compression point of the power amplifier. In other words, with the same power amplifier, the usable transmitted power for 4QAM is 4 dB higher than in the case of 16QAM. Therefore, by switching from 100 Mb / s, 16QAM to 50 Mb / s, 4QAM, the network gain in RSL is 12 dB. Therefore, the link can maintain the required availability regardless of a 12 dB increase in signal attenuation due to rain fade.

p00059By further reducing the transmission speed to 6.25 Mbps and using 4QAM, the gain in speed reduction is 9 dB compared to the case of 50 Mbps and 4QAM. In addition, for the same busy bandwidth, a coding scheme with low speed error correction, such as the super-orthogonal convolutive code, can be used in order to obtain an additional coding gain of 5 dB. In addition, spread of the spectrum can be used in conjunction with reduced data rate in order to reduce interference between communication links. In other words, operation at 6.25 Mb / s and 4QAM provides an overall gain of 14 dB compared to 50 Mb / s, 4QAM, or 26 dB compared to 100 Mb / s, 16QAM.

p00060The gain obtained is used to compensate for the increase in signal attenuation due to rain for a relatively short period of time. For example, consider 99.99% target availability for a wireless link that is required to provide 100 Mb / s duplex high-speed Ethernet. According to conventional design techniques, to maintain the required availability, said link would have a link power balance that includes a large fading margin for operation set at 100 Mb / s. By contrast, according to the present invention, 99.9% link availability can be achieved while providing a comparatively greater range for operation at 100 Mb / s, 16QAM. Furthermore, by means of the present invention, greater 99.99% link availability can be provided for operation.

p00061or at a lower data rate and a lower modulation level.

p00062The nature of Ethernet packet switching is advantageously used by the present invention in the MAN. For example, packet switched communications tend to be bursty because network demand can be characterized by relatively high demand periods and relatively low demand periods. In addition, packet switched communications also tend to be somewhat tolerant of the delay because packets can be stored temporarily before being retransmitted (storage and retransmission). Accordingly, by means of the present invention a global availability of 99.99% can be achieved in a MAN that provides the maximum transmission rate (for example, 100 Mb / s) 99.9% of the time and a reduced transmission rate ( for example, 50 Mb / s or 6.25 Mb / s) the remaining 0.09% of the time, to maintain acceptable operation during heavy rain fades. If during such fades due to heavy rain, the actual traffic flow over said link is below the reduced transmission rate, no further action is required. However, if the demand on said link exceeds the reduced transmission rate, the router 300 redirects the excess traffic to other, alternative, links of the MAN. Data packets are preferably transmitted according to an algorithm of first opening the shortest path (OSPF). Therefore, if the shortest communication path between a source router and a destination router has sufficient bandwidth available, all data will be transmitted along this shorter path. However, if the shortest path momentarily lacks sufficient transmission capacity to accommodate the current traffic flow due to said reduced data rate, all data packets or a selection thereof may be routed along a Alternative communication path, longer but less busy. Preferably, alternative communication paths are selected as the following shorter paths.

p00063It should be noted that in a conventional wireless communication link if, in a given area, a fading margin of 9 dB / km is required to maintain 99.9% availability, then it must be increased to 24 dB / km to maintain 99.99% availability. Therefore, unlike conventional wireless communications, the present invention maintains a global availability of 99.99%

p00064or greater, by adapting the transmission mode and the transmission path to the environmental conditions (by detecting the transmission quality) and to the demand experienced by the network (by detecting traffic loads) and by relying on the nature tolerant to the delay of packet switching communications. As a result, a MAN according to the present invention can be constructed, which is much more efficient in terms of cost, power and bandwidth utilization, than in the prior art, to cover a substantially longer distance. For example, to achieve 99.99% target link availability with a transmission power of 50mW in a given area, the conventional link balance design based on a fixed fade margin of 100 Mb / s, 16QAM can provide an expected range of 3.5 km. For the same transmitted power and environmental conditions, the present invention with adaptive rain fade countermeasure techniques can provide an expected range of 5 km or 7 km with a reduced speed during short periods of fading due to heavy rain of 50 Mb / s or 6.25 Mb / s, respectively.

p00065As an illustrative example of the implementation of a monitoring and control procedure according to the present invention, Figures 5 and 6 respectively show a flow chart and timing relationships between the monitored RSL and the transmitted power during a fading period due to heavy rain. . For example, consider the wireless link between nodes B and C in Figure 2C. Nodes B and C periodically exchange their monitored performance indicators. The flowchart begins in block 500 and is followed by node B, although it will be apparent that node C follows a corresponding flowchart. Node B keeps track of its own transmitted power and the monitored RSL and BER of node C in block 501 (Figure 5), and vice versa. Under normal conditions, node B sets its transmitted power 602 (Figure 6) at a nominal level 606 for a data rate of 100 Mb / s and 16QAM. This nominal level 606 is selected to be low enough to avoid non-linear effects on the operation of the link, the transmit power amplifier of node B as well as the receiver of node C. For 16QAM, this nominal level 606 is preferably 6 dB or more, below the 1 dB compression point of the power amplifier. The nominal level 606 is also selected to ensure that the transmission link is of sufficient quality so that the bit error rate (BER) is generally below the predetermined maximum level, while avoiding transmitting the signal at an excessive power that may interfere with other wireless links. A similar provision applies to node C.

p00066When rain fades over the link between nodes B and C, the RSL of both node B and node C tends to be reduced. When the RSL of node C 601 (Figure 6) is approaching the threshold value RSL16 604, the monitored BER also tends to increase. In block 502 (figure 5), node B continuously compares the monitored RSL of node C 601 with a predetermined alert level RSLw 603 (figure 6), where RSLw is selected to be greater than RSL16 604. When the monitored RSL of node C 601 is below the predetermined alert level RSLw 603, node B sends a control message to node C ordering node C to prepare to switch to a lower data rate and a lower modulation level ( for example, 50 Mb / s, 4QAM) in block 503 (figure 5). The router of node B is also conditioned to prepare for the redirection of excess traffic, if required. Upon receipt of the control message from B, node C prepares for the change and sends a acknowledgment message to B. Node B receives the acknowledgment message from node C, as checked in block 504. Next, node B applies to its own transmitted signal the reduced data rate and the lower modulation level, so coordinated with node C, in block 505. Node B also increases its transmitted power 602 (figure 6) to its maximum level for 4QAM 607. The selection of the RSLw 603 alert level (figure 6) takes into account the delay time 608 for nodes B and C to process the exchange orders, so that switching to operation with the reduced data rate and with the reduced modulation level takes place before the RSL falls below RSL16. Since statistical measurements indicate that the maximum rate of rain fade is approximately 0.5 dB / s, the RSLw alert level is normally relatively close to RSL16.

p00067Assuming that rain fade is transient, low speed data switching occurs at high data speed. Node B continuously receives monitored RSL and BER from node C, in block 506 (Figure 5). When the rain fade is reduced, the RSL increases. In block 507 (figure 5), node B continuously compares the monitored RSL of node C with a predetermined level RSLs 605 (figure 6). When the RSL of node C 601 (figure 6) exceeds that of the predetermined level RSLs 605, node B sends a message to node C requesting a change at a higher data rate and an increased modulation level, in block 508 (figure 5). In response to the reception of this message, node C conditions itself to prepare for the higher data rate and higher modulation level, and sends an acknowledgment message to the node

p00068B. Node B receives the acknowledgment from node C, as verified in block 509 (Figure 5). Next, node B applies the higher data rate and higher modulation level to its transmitted signal, in coordination with node C, in block 510. Node B also reduces its transmitted power back to the normal level, and inform your router of this situation.

p00069In the default level RSLs 605 (figure 6) is selected to be greater than RSLw 603, introducing hysteresis to prevent changing the data format with unnecessary frequency.

p00070The above procedure is executed by the transmission quality processor 314 under the supervision of the system control 306 in Figure 3. In the above descriptions, examples with two operating sets are used to show the prior art: 100 Mb / s with 16QAM; and 50 Mb / s with 4QAM. However, it will be apparent that the prior art is directly applicable to other reduced data rates and to more than two operating sets.

p00071The present invention has been described in terms of specific embodiments that incorporate details to facilitate understanding of the principles of construction and operation of the invention. Said reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended thereto. It will be apparent to those skilled in the art that modifications may be made in the chosen embodiment for illustrative purposes, without departing from the scope of the invention as defined in the claims. Specifically, it will be apparent to one skilled in the art that the device of the present invention could be implemented in several different ways and the apparatus disclosed above is only illustrative of the preferred embodiment of the invention and is not limiting in any way.