Data communications network

Abstract: Methods and apparatus are set forth which enable a data communications network (1) to perform decentralised uplink network control functions (decentralised away from the centralised general communications controller (GCC) level of the network hierarchy); (2) to minimise, or in some cases eliminate, the overhead expended in making power lever (signal strength) determinations and assessments at both the GCC and base station levels of the network hierarchy (for the purpose of managing uplink communications); and (3) to allow individual subscriber radios (terminals) to dynamically determine the most desirable base station to communicate with when exchanging information a host computer via a shared RF communication channel (the link between a terminal and the base station). Additionally, an illustrative radio frequency data communication network, built in accordance with the teachings of the invention as a modified version of an existing network, is described where the modified network incorporates the aforementioned methods and apparatus.

Description: The present invention relates generally to cellular networks used for performing digital communications over a large geographical area via radio frequency ("RF") links sharing a single assigned channel or frequency. Cellular radiotelephone networks are well known that provide communications services over a predefined geographical area divided into zones, sometimes to as "cells". Each cell typically includes a plurality of base stations and associated antennas located at each base station, for transmitting and receiving message signals between a given base station and subscriber radios, sometimes referred to as "terminals", "cellular phones", "radiotelephones", "data radios" and the like, located in or passing through a given cell. Each base station in a given cell typically communicates with a general communications controller (a "GCC"), which functions as a centralised control mechanism for coordinating communications between the subscriber radios in a given cell and a host computer, often coupled to or forming part of a telephone switching network. An example of such a system in commercial use is the Advanced Radio Digital Information System ("ARDIS"), developed jointly by Motorola, Inc. and International Business Machines Corporation. A specific example of such a system is described in U.S. Patent Numbers 4,550,443 and 4,850,032, to Freeburg, and in many other technical publications and patents. In the ARDIS, RF links between the various terminals in a given cell and the base stations within that cell, are dynamically established as the need for communications services and resources arise. Many of the aforesaid technical publications and patents have addressed the problem of how to support a multiplicity of simultaneous digital communications randomly attemping to make use of the single frequency (also referred to herein as the shared channel) that is usually assigned for a predefined geographical area. This is the case, for example, in U.S. Patent No. 4,866,788, to Mouly et al, which describes a process for controlling the retransmission of messages from transmitting stations belonging to a cellular system. The process taught by Mouly et al is based on the recognition that the probability of having to retransmit a request message to use a shared channel depends on (is a function of) the state of the shared channel and the power received by the base station from the transmitting subscriber radios. Subscriber radio transmitting power is measured at the base station and controlled from the base station in order to impact the amount of traffic attemping to access the shared channel at any given point in time. Mouly et al is one example of how assessing the power level of a signal transmitted by a subscriber radio can be used in performing network control functions. Mouly et al also exemplifies a system in which power level assessment overhead is expended at the base station level of a network hierarchy in order to implement the process described in the reference. Other examples of how power level assessment overhead is expended to support dynamic power level adjustement techniques used in radio telecommunications networks to perform network control functions, to minimise "collisions" with respect to the use of a shared channel, etc., are described in U.S. Patent No. 4,512,033 to Schrock; in U.S. Patent No. 4,613,990 to Halpern; and in the aforementioned U.S. Patent No. 4,550,443 to Freeburg. Schrock describes circuitry used in a generalised bidirectional communication system, with the circuitry being resident at each of a plurality of remote terminals, for responding to externally generated power level adjustement signals. The externally generated power level adjustement signals are used to control a plurality remote terminals connected to a master terminal. Halpern is an example of prior art that teaches performing power level measurement at the fixed base station level of a network hierarchy to dynamically control the power of radiotelephone transmitters. Neither the Schrock or Halpern references deal with the problem of managing and/or reducing "uplink" message traffic overhead, i.e., overhead associated with message traffic flowing from the remote terminals towards a host computer, particularly in situations where a message transmitted by a single terminal can result in a plurality of uplink messages. This phenomenon occurs whenever a transmitting terminals is within range of two or more of the plurality of base stations typically located within a given cell site. U.S. Patent No. 4,550,443, to Freeburg, does present a system that is capable of handling the aforementioned uplink message traffic; however, as will be explained hereinafter, the uplink traffic message overhead expended is considerable and has the potential for having an adverse affect on uplink message throughput. The Freeburg reference (U.S. Patent No. 4,550,443) describes a data communications system that covers a geographic area divided into a plurality of cells and includes a general communications controller (GCC), a plurality of channel communications modules (GCMs, or base stations), a set of transmitter and receiver pairs each associated with a given base station, and a plurality of portable terminals. Data signals, included in packets of information that also include control signals, are communicated between the GCC and the portable terminals by way of a radio channel (the data signals are also referred to herein as the "data portion" of a packet). Each base station takes a signal strength measurement every time it receives a packet from a portable terminal. In systems similar to the one taught in the Freeburg references cited hereinabove, the GCC gathers the signals strength measurements from the base station receivers and all of the packets received by the various base stations (even duplicate messages received from a given terminal) are passed to the GCC. The GCC then computes an adjusted signal strength for each input and selects the input having the largest adjusted signal strength for determining the location of the portable radio that transmitted the packet. The GCC sends the packet associated with the strongest signal to the host, after determining that the packet contains valid data as will be explained hereinafter. Additionally, the GCC takes note of the identification (ID) of the terminal/base station pair that provided the strongest signal. Whenever the GCC thereafter transmits a message to a portable terminal, the base station associated with the recorded terminal/base station pair ID, is the base station of choice to set up the RF link to the terminal. In other words, the base station that covers the area having the targest adjusted signal strength for the last transmission from the target portable terminal is the first choice when trying to communicate "downlink" (i.e., from the host or GCC toward the terminal). Since the GCC can be simultaneously transmitting message signals to portable radios in other portions of the cell using non-interfering base station transmitters, downlink information throughput is greatly enhanced in systems similar to the one taught by Freeburg. The above described Freeburg reference (U.S. Patent No. 4,550,443) is an example of a power level adjustement technique which affects the size of the transmitting "spheres" of base station transmitters. This technique for adjusting the power level of these transmitters facilitates the ability to communicate with more than one terminal in a given cell (in a downlink sense) at any one point in time. Altough the size of the transmitting spheres can be made non-overlapping for downlink communications purposes, uplink communications in a given cell having a plurality of base stations still suffer from the aforementioned centralised overhead problems associated with two or more antennas within a cell picking up signals being transmitted by any given terminal at any point in time. As a result, not only is equipment required at the base station level of the network hierarchy to determine signal strength for each and every message received from a terminal, the GCC in systems similar to the one described by Freeburg are also forced to expend overhead to sort out which of the received packets generated by a single terminal should be passed on to the host computer. This means that the GCC must determine which of the received packets resulted in the strongest received signal; record the ID of the base station (and the transmitting terminal) that received the signal to provide an indication of the best path to use for downlink communications back to the transmitting terminal; and determine the validity of the packets received from the various base stations before choosing a packet to send on to the host computer. In order to determine the validity of received packets, cyclic redundancy checking (CRC) techniques, well known to those skilled in the art, are often employed at the base station of a network hierarchy. A validity indication, like signal strength indication, is placed in the packets that are passed to the GCC which again must expend overhead in making the aforementioned packet validity determinations. It should be noted that the strongest signal received by a base station and passed to the GCC may contain questionable data. In this case it is not desirable to pass the strongest signal on to the host computer. The GCC would typically throw out the questionable data in favour of a weaker received signal so long as the weaker signal includes a packet that passes, for example, a CRC test, or some other validity test established to verify proper data transfer within the network. All of the presently known radio frequency data communications networks utilise a GCC or its equivalent to assure that a high quality message is passed on to the host computer, and to record downlink control information to be used in transmitting information from the host or GCC back to a specific terminal. It can be readily appreciated by those skilled in the art that where even a single message sent by a given terminal results in the GCC (1) having to interpret the signal strength of a set of received packets; and (2) having to determine the highest quality signal from among the set of packets, etc., the aforestated GCC overhead problem becomes quickly compounded when many messages are being transmitted by a given terminal, and/or messages are being simultaneously transmitted by a plurality of terminals within a given cell. Accordingly, it would be desirable to minimise the amount of GCC overhead expended in interpreting signal strength measurements, controlling uplink communications, and determining the quality of a set of packets received from a plurality of base stations, where the set of packets relate to the transmission of a single message by a given terminal. As indicated hereinbefore, the desirability of being able to minimise GCC overhead for these purposes becomes even more acute in practice where multiple copies of a plurality of messages are all being input to a GCC by the base stations located in a given cell. In the article "Cellular Packet Communications", IEEE Transactions on Communications, vol. 38, no. 8, August 1990; by D.J.Goodman the "third generation" of wireless communication is introduced which realizes a sort of decentralization of the operations from the GCC to the single terminals and reduces the GCC overhead by letting the single terminals determine the base station best able to serve the terminal in its current location. It would be desirable to determine the optimal terminal/base station pair, at the terminal level, based on signal strength. This is particularly true since, as those skilled in the art will readily appreciate, the terminal side of the terminal/base station link is the weakest link in the network hierarchy due to factors such as the terminals often being mobile, terminal power supplies being typically less reliable than the power supplies used by the base stations etc. EP-A-528522, discloses a system with a plurality of remote terminals communicating with a plurality of base stations. Each base station monitors the number of remote terminals it can hear and decides, on the basis of signal strength, number of users attached and base station coverage, which base station should be attached to a particular remote terminal and then notifies a communication controller. However it would be desirable to assure that only a single valid packet is sent to GCC by only one of the base stations in a given cell (for uplink transactions), even where a plurality of base stations each receives a packet being transmitted by a given terminal. This would significantly reduce GCC overhead since the GCC, whenever a packet is received from a base station, would simply pass the already validated packet to the host computer. It would also be desirable to reduce the need for power level (signal strength) measurement equipment at the base station level in the network hierarchy. As indicated hereinbefore, such equipment is presently being used for the purpose of achieving reliable uplink communications and enabling the GCC to be able to select an appropriate terminal/base station pair for downlink communications. It is an object of the present invention to provide a technique which alleviates the above drawbacks. According to one aspect of the invention there is provided a data communications network (100) of the type in which packets of digital information signals are exchanged between a general communications controller (GCC) (101) and at least one terminal located within a predefined geographical area, the data communication network including: a plurality of base stations (102), including a target and non-target base station, each coupled to said general communications controller (GCC) (101), for transmitting and receiving signals over said predefined geographical area; a single channel, shared by the at least one terminal and said plurality of base stations, over which communications between the at least one terminal and said plurality of base stations (102) may be performed; the at least one terminal including: means for dynamically selecting an optimal one of said plurality of base stations (102) to communicate with over said channel; the data communication network further including means for forwarding a packet received by said non-target base station to said general communications controller (GCC) (101) only if the packet is valid and has not been forwarded to the general communications controller (GCC) (101) by said target base station or by any other of said plurality of base stations (102). In preferred embodiments the terminal further comprises means for including a terminal/base station pair ID signal in packets transmitted to the GCC by the terminal once it has selected a base station with which to communicate. The GCC then records the terminal/base station pair ID for each packet it receives from the terminal side of the network and sends at least the data portion of each packet so received to a host computer. Hence the GCC does not have to interpret or determine packet signal strength or data validity information. The recorded pair ID may then be used to identify the downlink terminal/base station routing of choice (until, for example, the subscriber radio moves out of range of the base station, and the radio dynamically determines that another base station is a more suitable path for exchanging information with the rest of the network etc). In preferred embodiments of the invention, validity checks on packets being sent uplink are performed at the base station level of the network hierarchy. The base stations resolve (among themselves) which of a set of duplicate packets to send to the GCC based on the terminal/base station pair ID contained in the packets (as the primary source for determining which base station is to forward the packet to the GCC); backed up by a predetermined priority ordering of which of the other base stations in the network is to attempt to send a valid packet to the GCC if the primary designated base station determines it has received invalid or otherwise questionable data. The aforementioned technique for resolving which packet is to be sent, by only one of the networks plurality of base stations, to the GCC, does not depend on the strength of the signal received by any base station over an RF link to the transmitting terminal. Viewed from a second aspect, the present invention provides a method for exchanging packets of digital information signals in a data communication network between a general communications controller (GCC) (101) and at least one terminal located within a predefined geographical area, comprising the steps of: coupling a plurality of base stations (102), including a target and a non-target base station to said general communications controller (GCC) (101), wherein each of said base stations (102) is capable of transmitting and receiving signals over said predefined geographical area; sharing a single channel over which links between said at least one terminal and said plurality of base stations (102) may be established; dynamically selecting a terminal/base station pair, for each active terminal, between which a link may be established to form an optimal communications path, wherein selection is performed locally by each active terminal; and wherein a packet received by said non-target base station is forwarded to said general communications controller (GCC) (101) only if the packet is valid and has not been forwarded to the general communications controller (GCC) (101) by said target base station or by any other of said plurality of base stations (102). This approach relieves the GCC from having to determine and/or interpret received packet validity and signal strength data. In fact, in accordance with the preferred embodiments, signal strength measurement at the base station level of the network hierarchy (for uplink data control purposes) can be eliminated altogether because, by definition, the contents of each packet specify the terminal/base station routing of choice. In preferred embodiments the network can determine when to issue a retransmit request to a subscriber radio in situations where none of the plurality of base stations is able to pass valid data uplink. The present invention will be described further, by way of example only, with reference to an embodiment thereof as illustrated in the accompanying drawings, in which: FIG. 1 depicts a radio frequency data communications network for exchanging packets of information between at least one host computer and at least one subsrciber radio located within a predefined geographical area serviced by the network in accordance with the preferred embodiment of the present invention; FIG. 2 depicts the format of a subscriber radio initiated echo message which, in accordance with the preferred embodiment of the invention, may be used in a radio frequency data communications network to dynamically identify and select a suitable base station to communicate with from the terminal level of the network hierarchy depicted in FIG. 1; FIG. 3 depicts of a base station echo message which, along with other echo messages from the base stations within range of a given terminal, may be used in the preferred embodiment to determine the optimum base station for the terminal to pair up with to establish RF communications over a shared channel at a given point in time. FIG. 4 depicts a packet format that is used in the preferred embodiment to assess packet validity after transmission over an RF link, and to provide information to the GCC and the base stations within range of a given terminal of the terminal/base station pair selected by the terminal; and FIG. 5 depicts, in the form of a flowchart, a method in accordance with the preferred embodiment for resolving which of a set of duplicate valid packets (where the method is also used to determine packet validity) is to be sent uplink to the GCC by a specific one of the plurality of base stations receiving the packets. The network in FIG. 1 is similar to the one described in the aforementioned Freeburg Patent, No. 4,550,443, at least to the extent that large contiguous geographic areas can be covered with logically overlapping radio broadcast spheres (broadcast from the base station level of the network hierarchy), which may, with a high degree of probability, be dynamically optimised into non-overlapping physical spheres in order to increase network efficiency. As indicated hereinbefore, this optimisation relates to the broadcast spheres established by the base station transmitters, allowing simultaneous communications to be conducted by two or more base stations in the network on a single assigned radio frequency. The preferred embodiment of the invention goes beyond the type of optimisation achieved by Freeburg. In particular, it addresses uplink communications management problems, such as GCC level overhead, etc. and the decentralisation of network management functions in a network in which a single preassigned RF channel carries signals transmitted by the terminals in or passing through a given cell. Accordingly overall network efficiency is increased by the decentralisation and redistribution of heretofore centralised network computer from the GCC level of a network hierarchy to the network base stations and terminal nodes. More particularly, with reference to FIG. 1, a prior art network 100, which may be modified so as to be accordance with the preferred embodiment of the invention, is comprised of: a GCC 101, also referred to herein as the "GCC level" of the depicted network hierarchy; a plurality of base stations, 102-1, 102-2, 102-3,..., 102-n (n base stations being represented in FIG. 1), also referred to herein as the "base station level" of the depicted network hierachy; and at least one terminal, 103-1, shown together with other terminals 103-2, 103-3, ..., 103-m (m terminals in total being represented in FIG. 1), passing through (or permanently located within) the predefined geographic area (cell) covered by the network. The terminals constitute the "terminal level" of the network hierarchy. The geographic area is literally "covered" by the overlapping base station broadcast spheres shown, for example, as broadcasts spheres 104-1, 104-2, 104-3,..., 104-n. Each "sphere" represents the broadcast range of a transmitter associated with a given base station, examples of which are designated as transmitters 105-1, 105-2, 105-3,..., 105-n in FIG. 1. For the sake of illustration only, it is assumed that a receiver associated with each base station is located in proximity to the aforementioned transmitters and that each receiver is capable of receiving the signals transmitted by any terminal within a depicted sphere. The sphere shown in dashed line form in FIG.1, sphere 199, is meant to illustrate the contiguous overlapping coverage over the predefined geographic area by the set of remaining base stations and associated transmitting and receiving antennas not specifically shown in FIG. 1. FIG. 1 also shows host computer 110 which the GCC 101 may be coupled. Messages flowing "downlink" are those flowing from, for example, host computer 110 to the terminal level of the network; whereas "uplink" messages are, for example, those flowing from the terminal level of the network towards GCC 101 and/or host computer 110. As indicated hereinbefore, the centralised GCC level of network 100 has heretofore been heavily relied upon to manage uplink communications. The techniques used required GCC computing overhead to be expended in the interpretation of signal strength measurements and validity information inserted into packets passed to GCC 101 by the plurality of base stations. Signal strength determination equipment located at the base station level is also required (by the known techniques for managing uplink communications) to measure the signal strength of duplicate packets transmitted by a single terminal within range of the receivers associated with a plurality of base stations. In the preferred embodiment of the invention, in order to simplify the management of uplink communications and achieve the desired decentralising (offloading) of GCC overhead, an optimised terminal sign-on process is used which involves only the terminal signing on and the base stations within range of the effective physical broadcast sphere of the terminal. The terminal is used in the novel network to establish the optimal communication link between the terminal itself and a base station selected by the terminal. Once this link is established the terminal, when transmitting an uplink message, inserts a terminal/base station pair ID signal into the packet being transmitted. As indicated hereinbefore, the packet is defined to contain at least the data being transmitted, the terminal/base station pair ID signal, and control information from which to determine the validity of the packet once received on the base station side of the RF link with the terminal. A new technique is then implemented at the base station level of the network hierarchy for determining which (if any) among any two or more duplicate packets received by the base stations from a given terminal is to be passed on the GCC 101. Any packet ultimately passed to GCC 101 will contain at least a data portion and the terminal/base station pair ID signal, with the data being presumed to be valid since validity determination and interpretation will have been performed at the base station level of the network. Finally, GCC 101, is modified so that (for uplink control purposes) it only needs to record the terminal/base station pair ID signal from each packet received from the terminal side of the network, and passes a presumably valid packet to host computer 110. Each of the above aspects of the preferred embodiment will now be described in greater detail with reference to FIGs. 2-5 First, the optimal terminal sign-on process used in the preferred embodiment requires an echo message to be transmitted by the terminal to all base stations within range of the terminal. An exemplary echo message packet is shown in FIG. 2, where the packet includes a signal synchronisation (or start message) indication, in slot 201; as end message indicator, in slot 204; a slot 202 for a base station ID (initially shown to contain the value "0"); and a slot 203 for the terminal ID number. At the start of the sign-on process, the only information conveyed uplink by the packet shown in FIG. 2 is (a) the terminal ID number; and (b) a predefined delimiter for the sign-on (or periodic reoptimisation message). According to the preferred embodiment of the invention, the sign-on delimiter is represented by the base station ID number being set equal to "0" by the transmitting terminal. Any base station successfully receiving the echo message returns an echo packet to the identified terminal with its base station ID embedded in the returned packet. An exemplary return echo message packet is shown in FIG. 3. In the example, base station ID "n" is inserted in slot 301 by base station n which is identifying itself to the transmitting terminal that is trying to locate the optimal base station to communicate with. It should be noted that the echo type messages referred to hereinabove, and shown by way of example in FIGS. 2 and 3, need pass no further uplink than the base station level of the network hierarchy. Resolving the establishment of optimised communications at the terminal and base station levels will, as indicated hereinbefore, simplify network operation and efficiency. According to the preferred embodiment of the invention, once a terminal has dispatched an echo message (of the type depicted in FIG. 2), it goes into a listening mode for any echo message (of the type depicted in FIG. 3) returning from a base station. In addition the terminal also listens for echo message "crashes", based on the well known use of standard field strength discrimination characteristics. Many commercially available terminals, such as those presently in use in the ARDIS and the recently available IBM PCradio type terminals ("IBM" and "PCradio" are trademarks of International Business Machines Corporation), are programmable and may be set up so that echo message received from the base stations in response to sign-on or reoptimisation procedure, message crash information, etc., may be analysed at the terminal level of the network and stored in a terminal resident record. In addition, such "smart" terminals may be programmed by those having ordinary skill in the art to select the base station with which the terminal desires to engage in communications with. Two examples of how to make a terminal/base station pair selection so that an active RF link can be established will now be discussed. First, a given terminal may be set on and remain in an "always listening" mode, listening for a response (an echo message of the type shown in FIG. 3) to an echo message generated by the terminal. Standard radio characteristics of an FM discriminator can lock onto a signal stronger than other signals concurrently presented from other echoing base stations. From this lock onto the FIG. 3 type packet sent by the "strongest" base station transmitter, the terminal can identify the base station to pair up with. As a second example, the terminal may be set on and be directly addressed (called) by a specific base station as a result of direction from host computer 110 or GCC 101. The terminal will then try to respond to that base station, and if successful the terminal/base station pairing is once again known and inserted into packets transmitted uplink by the terminal. If unsuccessful, the terminal could initiate an echo sequence to try and identify a base station to pair up with. Once the terminal determines the optimal connection to a base station, the terminal will begin sending (or receiving) messages that include the data being exchanged. Theses messages may, for example, be of the format type shown in FIG. 4. In the preferred embodiment the message packets must include at least the data being exchanged, the terminal/base station pair ID signal, and information from which to determine the validity of the packet after transmission over an RF link. The exemplary packet shown in FIG. 4 may be used to (1) assess packet validity after transmission over an RF link (the check sum information could, for example, be contained in slot 401 as shown in FIG. 4); (2) provide information to the GCC and the base stations within range of a given terminal of the terminal/base station pair selected by the terminal (the terminal/base station pair ID signal may be sent, for example, in slots 402 and 403 of the packet shown in FIG. 4); and (3) contain (besides the data count and actual data as shown in slots 404 and 405 respectively) other information, such as an assessment of what the base station broadcast power will need to be to create an effective physical sphere of range for downlink communications (shown in slot 406 of FIG. 4). According to a novel technique performed at the base station level of the network hierarchy, to be more particularly described hereinafter, the selected base station (once it determines that it is indeed the selected base station) will forward any message it receives up the system hierarchy to GCC level, only if the message is determined by the base station to be valid. As indicated hereinbefore, the selected base station could perform any one of many well known validity tests, such as a cyclic redundancy test, to make the required packet validity determination. The check sum error information shown in slot 401 of the exemplary packet depicted in FIG. 4 may be used for validity test purposes. The information contained in the aforementioned terminal resident record (echo analysis, message crash information, etc.), may be conveyed uplink as part of the data or in a separate slot not shown in FIG. 4. Such information may be used at the GCC or host computer level to further optimise system performance, for record keeping and performance analysis purposes, etc. Once GCC 101 has recorded the terminal/base station pair ID information conveyed with a given valid packet to the GCC level of the network hierarchy, the GCC, according to a preferred embodiment of the invention, will dynamically maintain (with each subsequent communication iteration) the current terminal broadcast power requirement, for a given terminal/base station pair, according to the information contained in slot 406 of the exemplary packet depicted in FIG. 4. Power requirement information will of course be subject to change if the terminal establishes itself with a different base station, if a terminal reoptimisation sequence is performed, even where the same base station is selected, assuming any changes take place in radio conditions, geography, etc. The new power requirement information (when a change occurs) will be passed uplink in a revised of the type depicted in FIG. 4, in slot 406. Utilising this technique, base station physical broadcast spheres can be kept to a minimum, system inefficiency cause by having to manage physically overlapping broadcast spheres can be kept to a minimum, and downlink system throughput can remain optimised. Uplink communication can be further optimised in the network by not only utilising the terminals to dynamically select an optimal base station, but also by eliminating the aforementioned GCC overhead presently expended whenever duplicate messages sent by the same terminal are received by a plurality of base stations, and by eliminating signal strength determination equipment at the base station level of the network hierarchy. According to the preferred embodiment of the invention, whenever a packet transmitted by a terminal is received by any base station, the base station first checks to see whether its ID (base station ID) is included in the packet. Any base station receiving the message, but not specified as the target base station in the message, temporarily stores the message at the base station level of the system hierarchy and does not pass the message to GCC 101. Hence, GCC 101 no longer receives duplicate messages from two or more base stations. If a packet received by the targest base station (i.e., the base station identified as part of the terminal/base station pair ID information contained in the received packet) has been corrupted, the target base station "ask" the other base stations in the network if they received the message properly. The target base station would not forward the corrupted packet to GCC 101. A key distinction therefore between the prior art and the network in the preferred embodiment is that the base stations now resolve (among themselves) which of a set of duplicate packets to send to the GCC based on the terminal/base station pair ID contained in the packets (as the primary source for determining which base station is to forward the packet to the GCC), backed up by a predetermined priority ordering of which of the other base stations in the network is to attempt to send a valid packet to the GCC if the primary designated base station determines it has received invalid or otherwise questionable data. If one or more base stations, other than the base station, did receive a duplicate (but valide) packet, the target base station can "ask" one of the other base stations to send the valid received packet to GCC 101. In practice, to implement the concepts set forth hereinabove, the preferred embodiment of the invention relieves GCC 101 from having to determine and/or interpret received packet validity and signal strength, by (a) arranging the base stations into a local network (LAN); (b) assigning different time delays to, for example, the base stations surrounding the target base station; and (c) determining, at each of said plurality of base stations, if any of the base stations having a smaller time delay has already sent (to GCC 101) a copy of the packet being sent uplink. In fact, signal strength measurement at the base station level of the network hierarchy (for uplink data control purposes) is eliminated altogeteher because, by definition, the contents of each packet specify the terminal/base station routing of choice. One embodiment of the novel technique employed at the base station level of the network hierarchy, may be summarised with reference to FIG. 5. FIG. 5 depicts, in the from of a flowchart, a preferred method for resolving which of a set of duplicate valid packets (where the method is also used to determine packet validity) is to be sent uplink to GCC 101 by a specific one of a plurality of base stations receiving the duplicate packets. The flowchart functionality describes the operation of each base station, whether it is the target base station or not, where it is assumed that the base stations are able to communicate with one another using, for example only, the LAN type interconnection referred to hereinbefore. Block 510 of FIG. 5 indicates the routine function of listening for packets transmitted by any terminal within the reception range of a given base station. Once a packet is detected, the base station must determine if the base station ID transmitted by the terminal corresponds to the receiving base station's ID. This is indicated at block 515 of FIG. 5. In the event that the base station's actual ID and the base station ID transmitted via the terminal are the same, path 517 of FIG. 5 is taken and what now is known to be the target base station determines packet validity. This is indicated as taking place at block 520 (at the base station level of the network hierarchy; not at the GCC level). If the packet is determined to be valid, it is then passed to GCC 101 (as indicated at block 530 along path 527), and GCC 101 will receive only the one valid packet as desired. If it is determined at block 515 that the receiving base station is not the target base station, the embodiment of the invention depicted in FIG. 5 then temporarily stores the packet. This is indicated at block 540 down path 537, in FIG. 5. During a predetermined time period, t i , where time period t i is different for each of the plurality of base stations in the network, the non-target base station receiving the temporarily stored packet checks on the LAN to see if any other base station (the target base station or other non-target base station) has forwarded the packet from the transmiting terminal to GCC 101. This determination is indicated as being made at block 545 of FIG. 5. If this has occured (i.e., if another base station has already forwarded the packet to GCC 101), there is nothing further for the base station to do with respect to the received packet. If the packet was not forwarded to GCC 101 by another base station within time period t i , then the base station must determine the validity of its temporarily stored packet (of course this step may be performed before temporarily storing the packet without departing from the spirit or scope of the invention), as indicated at block 550 down path 549. If the packet is not valid, it is not forwarded to GCC 101, and the base station being desired (a non-target base station) has nothing further to with respect to the temporarily stored packet. However, if the packet is determined to be valid at block 550, it is forwarded to GCC 101 (as shown at block 560), thereby providing GCC 101 with the one and only valid packet it will receive, even where duplicate packets have been received by the plurality of base stations. Finally, with reference to FIG. 5, if the base station is the target base station as determined at block 515, and the packet is invalid, as determined at block 520, then path 577 is taken leading to block 580. In this situation the target base station also checks to see if any of the non-target base stations has sent the packet to GCC 101 within time period t max , where t max is greater than any time period t i within which the non-target base stations must either succeed or fail to send a valid packet to the GCC level of the hierarchy. If the determination at block 580 is "yes", than GCC 101 has its valid packet (and therefore also has the terminal/base station pair ID as selected by the terminal, even throught the actual routing of the packet to GCC 101 is via another base station), and the function of the target terminal with respect to the received packet is complete. However, if no base station has forwarded a packet to GCC 101 within time period t max , as determined by the target terminal, then it can be presumed that a retransmission request for the packet needs to be issued by the target base station which, as indicated in FIG. 5, proceeds to issue such request, as shown at block 590. The above described procedure clearly assures that GCC 101 will be passed only a single valid copy of a packet being sent uplink and clearly does not rely on (or require) any signal strength measurements being taken or analysed at either the base station or GCC levels of the network architecture. Accordingly, the certain aspects of the aforementioned uplink message management overhead burden presently suffered by GCCs and base stations, in networks similar to the one described herein, is either eliminated or significantly reduced. In summary, the network aspects of the preferred embodiment of the invention, as described hereinabove, can be realised by modifying existing networks, of the type depicted in FIG. 1, as follows: (1) Programming the terminals of the network to select a base station with which to establish an optimal RF link. Simply locking onto the strongest echo message from a base station within range of the terminal, for example, using Automatic Gain Control (AGC) circuitry within a terminal, using the capture characteristics of FM signals, etc., provides a well known and suitable technique for making the terminal/base station pair selection at the terminal level of the network; (2) Eliminating signal strength determination equipment at the base station level of the network and substituting the new technique described hereinabove for resolving which of a set of duplicate packets to send to the GCC (including determining packet validity), will assure that the GCC receives only one valid packet from the terminal side of the network and eliminate GCC overhead associated with interpreting signal strength measurements and validity information heretofore provided by the base stations; and (3) Storing, at the GCC level, only the terminal/base station pair ID signal from a packet being sent uplink, to identify the "return" path of choice, before passing what is now presumed to be valid data on to host computer 110. Of course, other packet components may be stored at the GCC level (without any analysis having to be performed with respect to uplink communications), such as downlink power level information, the terminal analysis record described hereinbefore (if passed uplink), etc. What has been described in detail hereinabove are methods and apparatus for providing a radio frequency data communications network in which (1) uplink network control functions are decentralisation away from the GCC level of the network hierarchy; (2) the overhead expended in making power lever (signal strength) determinations and assessments at both the GCC and base station levels of the network hierarchy is minimised or even eliminated; and (3) individual subscriber radios (terminals) dynamically determine the most desirable base station to communicate with when exchanging information with the host computer via a shared RF communication channel (the link to the base station) and the GCC.

Claims: A data communications network (100) of the type in which packets of digital information signals are exchanged between a general communications controller (101) and at least one terminal located within a predefined geographical area, the data communication network including: a plurality of base stations (102), including a target and non-target base station, each coupled to said general communications controller (101), for transmitting and receiving signals over said predefined geographical area; a single channel, shared by the at least one terminal and said plurality of base stations, over which communications between the at least one terminal and said plurality of base stations (102) may be performed; the at least one terminal including: means for dynamically selecting an optimal one of said plurality of base stations (102) to communicate with over said channel; the data communication network further including means for forwarding a packet received by said non-target base station to said general communications controller (101) only if the packet is valid and has not been forwarded to the general communications controller (101) by said target base station or by any other of said plurality of base stations (102). The network according to claim 1 wherein the at least one terminal further comprises: means for transmitting a sign-on packet; means for locking onto a signal that is the strongest in a set of signals concurrently presented to the terminal by a set of base stations (102), included in said plurality of base stations (102), in response to the transmitted sign-on packet. The network of claim 2 wherein said sign-on packet includes a terminal ID identifying the terminal signing on. The network of any preceding claim, wherein each response to a transmitted sign-on packet by any given base station includes a base station ID signal identifying the responding base station. The network of any preceding claim, wherein said selector includes means responsive to the capture characteristics of the signals concurrently presented to the particular terminal. The network of any preceding claim further comprising means for including a terminal/base station pair ID signal in packets transmitted to the general communications controller by the terminal once it has selected a base station with which to communicate. The network of claim 6 wherein such packets transmitted to the general communications controller further include at least a data portion, the terminal/base station pair ID signal indicating the target base station selected by the terminal, and information from which a given base station may ascertain the validity of a given packet after transmission over the single channel. The network of any preceding claim which is intended for use in a radio frequency data communications network. The network of any preceding claim further comprising a checking device, located at each of said plurality of base stations, for determining which one, if any, of copies of a given packet received by at least two of said plurality of base stations, is to be sent uplink to said general communications controller. The network of claim 9, wherein each packet transmitted by the terminal includes a terminal/base station pair ID signal which may be used by said checking device to identify a target base station. The network of claim 9 or 10 further comprising means for requesting the retransmission of a packet not forwarded to the general communications controller within a time period t max . A method for exchanging packets of digital information signals in a data communication network between a general communications controller (101) and at least one terminal located within a predefined geographical area, comprising the steps of: coupling a plurality of base stations (102), including a target and a non-target base station to said general communications controller (101), wherein each of said base stations (102) is capable of transmitting and receiving signals over said predefined geographical area; sharing a single channel over which links between said at least one terminal and said plurality of base stations (102) may be established; dynamically selecting a terminal/base station pair, for each active terminal, between which a link may be established to form an optimal communications path, wherein selection is performed locally by each active terminal; and wherein a packet received by said non-target base station is forwarded to said general communications controller (101) only if the packet is valid and has not been forwarded to the general communications controller (101) by said target base station or by any other of said plurality of base stations (102). The method of claim 12 further comprising the step of determining, via a checking device included in each of said plurality of base stations, which one, if any, of copies of a given packet received by at least two of said plurality of base stations, is to be sent uplink to said General Communications Controller. The method of claim 13 wherein said step of determining further comprises the step of sending said general communications controller any packet received by an identified target base station, so long as the packet is ascertained to be valid. The method of claim 13 or 14 where in said step of determining further comprises the step of following a predetermined priority ordering of non-target base stations for forwarding a received packet to the general communications controller whenever the base station receiving a packet is not the target base station. The method of claim 15 further comprising the step of forwarding a packet received by a non-target base station to the general communications controller only if the packet is valid and has not been forwarded to the general communications controller by the target base station or by a base station having a higher priority in said predetermined priority ordering. The method of claim 15 or 16 further comprising the step of establishing said priority ordering of non-target base stations by arranging said plurality of base stations into a local area network with each non-target base station being assigned a different time period for monitoring the local area network as a function of its priority, to determine if the target base station or a non-target base station with a higher priority has sent a valid packet to the general communications controller. The method of claim 17 wherein said step of determining, whenever said checking device is located in said target base station and the target base station has received an invalid packet from a given terminal, further comprises the step of monitoring said local area network to determine if any of the plurality of base stations forwards a valid copy of the packet received by the target base station, to the general communications controller, within a time period t max , where t max is at least as long as the longest time period for monitoring the local area network assigned to the set of non-target base stations.

Inventors: AMES RICHARD NORMAN, MOORE VICTOR STUART, AMES, RICHARD NORMAN, MOORE, VICTOR STUART
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES
Application year: 1992
Main classification: H04B7
IPC codes: H04Q7, H04L12, H04B7