Abstract:
The penalties associated with relying on layer- 4  to handle packets lost as a result of a handoff can be reduced by forwarding, in response to a handoff request from a first base station to a second base station, at least one layer- 2  frame of a layer- 3  packet destined to/from the wireless terminal that has been passed down from layer- 3  to layer- 2 , and so is indicated to have been transmitted at layer- 3  even though not all of the layer- 2  frames of the layer- 3  packet have actually been transmitted. In one embodiment of the invention, each of the at least one layer- 2  frames may be encapsulated together in a special layer- 3  packet that is transferred from the first base to the second base station in the usual manner of inter-base-station communication. Advantageously, packets are not lost at layer- 4  due to handoffs. Thus, layer- 4  retransmissions are not required, and so delays in the network are reduced. Furthermore, if only the layer- 2  frames that were not transmitted over the air to the wireless terminal by the first base station are transferred to the second base station, a savings may be had in use of the air interface.

Description:
TECHNICAL FIELD 
     This invention relates to the art of wireless communication, and more particularly, to a method of improving the quality of data communication at the time of handoff of a call from one base station to another. 
     BACKGROUND OF THE INVENTION 
     Prior art wireless systems during wireless data transmission typically rely on the transport layer, or layer- 4  in the International Standards Organization (ISO) Open Systems Interconnection (OSI) seven layer reference model for data communications to insure reliable data transfer end-to-end for a data call. Therefore, if a data packet is lost due to a handoff of a wireless terminal from a first base station to a second base station during transmission of the packet, some layer- 4  protocols will cause the retransmissions of the packet from its source all the way through the network to the destination. This leads to network delays and additional loading of the network. Other layer- 4  protocols are not designed to retransmit lost packets, resulting in a gap in the data at the destination. Thus, with either protocol type there is a penalty associated with relying on layer- 4  to handle packets lost as a result of a handoff. 
     SUMMARY OF THE INVENTION 
     We have recognized that the penalties associated with relying on layer- 4  to handle packets lost as a result of a handoff can be reduced, in accordance with the principles of the invention, by forwarding, in response to a handoff request from a first base station to a second base station, at least one layer- 2  frame of a layer- 3  packet that is destined to/from the wireless terminal that has been passed down from layer- 3  to layer- 2 , and so is indicated to have been transmitted at layer- 3  even though not all of the layer- 2  frames of the layer- 3  packet have actually been transmitted. In one embodiment of the invention, each of the at least one layer- 2  frames may be encapsulated together in a special layer- 3  packet that is transferred from the first base to the second base station in the usual manner of inter-base-station communication. Advantageously, packets are not lost at layer- 4  due to handoffs. Thus, layer- 4  retransmissions are not required, and so delays in the network are reduced. Furthermore, if only the layer- 2  frames that were not transmitted over the air to the wireless terminal by the first base station are transferred to the second base station, a savings may be had in use of the air interface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing: 
     FIG. 1 shows an exemplary network arrangement in accordance with the principles of the invention; and 
     FIG. 2 shows an exemplary process, in flow chart form, for forwarding, in response to a handoff request from said first base station to said second base station, at least one layer- 2  frame of a layer- 3  packet destined to/from the wireless terminal that has been passed down from layer- 3  to layer- 2 , and so is indicated to have been transmitted at layer- 3  even though not all of the layer- 2  frames of the layer- 3  packet have actually been transmitted, in accordance with the principles of the invention. 
    
    
     DETAILED DESCRIPTION 
     The following merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
     Thus, for example, it will be appreciated by those skilled in the art that the block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the invention. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudocode, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown. 
     The functions of the various elements shown in the FIGs., including functional blocks labeled as “processors” may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, read-only memory (ROM) for storing software, random access memory (RAM), and non-volatile storage. Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the FIGS. are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementor as more specifically understood from the context. 
     In the claims hereof any element expressed as a means for performing a specified function is intended to encompass any way of performing that function including, for example, a) a combination of circuit elements which performs that function or b) software in any form, including, therefore, firmware, microcode or the like, combined with appropriate circuitry for executing that software to perform the function. The invention as defined by such claims resides in the fact that the functionalities provided by the various recited means are combined and brought together in the manner which the claims call for. Applicant thus regards any means which can provide those functionalities as equivalent as those shown herein. 
     Unless otherwise explicitly specified herein, the drawings are not drawn to scale. 
     FIG. 1 shows an exemplary network arrangement in accordance with the principles of the invention. Shown in FIG. 1 are a) wireless terminal  101 ; b) N base stations  103 , where N is an integer greater than or equal to 2, including base station  103 - 1  through  103 -N; c) N antennas  105 , including antennas  105 - 1  through  105 -N; d) N structures  107 , including structures  107 - 1  through  107 -N; e) N cells  109 , including cells  109 - 1  through  109 -N; f) network  111 ; g) base station authentication unit  113 ; h) N communication links  115 , including communication links  115 - 1  through  115 -N; i) communication links  117  and  121 ; j) security center  119 . 
     Wireless terminal  101  is able to communicate with multiple base stations which transmit with sufficient signal strength to be detected and useable for communication at the current location of wireless terminal  101 . Once a signal of sufficient strength is detected for a particular base station, wireless terminal  101  may engage in communication with that base station. The particular types of wireless link and protocol, i.e., the air interface, employed by wireless terminal  101  are not essential to the invention and may be any type desired by the implementor, although of course the radio link and protocol employed by wireless terminal  101  must be the same type employed by base stations  103 . 
     Wireless terminal  101  may achieve communication with multiple base stations in any manner desired by the implementer. For example, wireless terminal  101  may have only a single receiver, and it may receive signals, when not occupied with the exchange of information with the base station currently serving it, from other base stations that have signals of sufficient strength reaching wireless terminal  101 . Alternatively, wireless terminal  101  may receive signals from multiple base stations simultaneously, e.g., by employing multiple parallel receivers in wireless terminal  101 . Further alternatively, wireless terminal  101  may have more than one receiver, but the number of receivers is less than the number of base stations from which wireless terminal  101  can receive a signal of sufficient strength at its current location, so wireless terminal  101  needs to perform scanning on at least one of its receivers to obtain signals for some of the base stations. 
     Base stations  103  are substantially conventional base stations except for the following. First, base stations  103  need not be connected to a dedicated network for inter-base-station communication. Instead, base stations  103  can employ a shared public network, e.g., an internet protocol (IP)-based network such as the Internet. Second, each base station  103  need not contain any “map” information. Instead, each of base stations  103  is capable of discovering its necessary portions of the “map” information. Preferably, base stations  103  are small base stations that can easily be incorporated into a small space, e.g., one that is already available, rather than requiring dedicated construction and site preparation. Advantageously, such small size, coupled with the ability to discover the necessary portions of the “map” information, enable the rapid construction of a new wireless communication network. Furthermore, such a wireless communication network is flexible in its architecture, i.e., base stations can easily be added or removed, and it is also easy to maintain. 
     Each of antennas  105  are coupled to a respective one of base stations  103 . Each of antennas  105  radiates the signal developed by its respective one of base stations  103 . Each combination of a one of base stations  103  and its respective one of antennas  105  yields a one of cells  109 , which is a particular coverage area. The shape of cells  109  in FIG. 1 do not represent actual cell shapes but instead are merely conventional notation for cells. Note that the shape of the actual various cells  109  are all independent. 
     Each of structures  107  provides a facility in which to place one or more of base stations  103 . Furthermore, structures  107  may also provide a place on which to mount antennas  105 . For example, some of structures  107  may be already existing homes in which a one of base stations  103  is located in an unused space and to which a one of antennas  105  is exteriorly affixed. 
     Network  111  provides a way for base stations  103  to communicate with each other, as well as with base station authentication unit  113  and security center  119 . Network  111  may be made up of various subnetworks, which may be networks in their own right. Furthermore, the various subnetworks may be of different types and may employ different protocols. In one embodiment of the invention, network  111  is a packet based network, e.g., an asynchronous transfer mode (ATM) network or an IP network. 
     Each of base stations  103  is connected to network  111  via a respective one of communication links  115 , which may be construed as part of network  111 . For example, where network  111 , or at least a subnetwork thereof, is an IP network, and one of base stations  103  are located within structures  107  that are homes, communications link  115  may be an Internet connection, e.g., over cable television lines or a fiber-to-the curb connection, that is shared by the base station for communicating with other base stations and by the occupants of the home for Internet browsing. 
     Base station authentication unit  113  contains a list of all valid base stations  103 , and any associated information such as security keys and alternative identifiers or addresses of the base station. A base station may be listed in base station authentication unit  113  at any point. However, the base station only becomes valid once it is listed in base station authentication unit  113 . Although shown herein as a single unit, in practice base station authentication unit  113  may be made up of several parts, which need not be geographically collocated. Furthermore, to improve reliability and performance, some or all of the various parts or functions of base station authentication unit  113  may be replicated, as will be readily recognized by those of ordinary skill in the art. 
     Base station authentication unit  113  is connected to network  111  via communication link  117 . Of course, when base station authentication unit  113  is made up of more than one part, or is replicated, communication link  117  is construed as covering all the necessary communications paths between network  111  and the various parts or replicas. 
     Security center  119  contains a list of all valid wireless terminals that may be served. In addition, security center  119  contains security information, such as authentication challenge-response pairs and/or encryption keys associated with each wireless terminal. The security information may be distributed by security center  119  to base stations  103 , as necessary. A wireless terminal may be listed in security center  119  at any point. However, the wireless terminal only becomes valid once it is listed in security center  119 . Although shown herein as a single unit, in practice security center  119  may be made up of several parts, which need not be geographically collocated. Furthermore, to improve reliability and performance, some or all of the various parts or functions of security center  119  may be replicated, as will be readily recognized by those of ordinary skill in the art. 
     Security center  119  is connected to network  111  via communication link  121 . Of course, when security center  119  is made up of more than one part, or is replicated, communication link  121  is construed as covering all the necessary communications paths between network  111  and the various parts or replicas. 
     FIG. 2 shows an exemplary process, in flow chart form, for forwarding, in response to a handoff request from a first base station, e.g., base station  103 - 1  (FIG. 1) to a second base station, e.g., base station  103 - 2 , at least one layer- 2  frame of a layer- 3  packet destined to/from the wireless terminal that has been passed down from layer- 3  to layer- 2 —and so is indicated to have been transmitted at layer- 3 , even though not all of the layer- 2  frames of the layer- 3  packet have actually been transmitted—in accordance with the principles of the invention. Note that the same process may be advantageously used to reduce the latency of packet transmission from the wireless terminal to the base stations or from the base stations to the wireless terminal. 
     The process is entered in step  201  (FIG.  2 ), when layer- 3  passes a packet to layer- 2  for transmission. In step  203 , layer- 2  fragments the layer- 3  packet into N layer- 2  frames. N is a variable integer greater than or equal to 1 that is a function of the frame size and the size of the layer- 3  packet, as is well known in the art. Thereafter, a counter variable i is initialized to  1 , in step  205 . Next, conditional branch point  207  tests to determine if there is a request for a handoff from the first base station that is currently serving the wireless terminal, e.g., base station  103 - 1  (FIG. 1) to another base station, e.g., base station  103 - 2 . As will be recognized by those of ordinary skill in the art, the source of the request for handoff may be either one of the base stations or the wireless terminal. The particular requester depends on the system architecture and the quality of the signals that are being received by the wireless terminal and the base stations. 
     If the test result in step  207  is NO, indicating that there has not yet been received a request for handoff, control passes to step  209 , in which frame i is transmitted over the wireless link. Conditional branch point  211  tests to determine if the transmitted frame was successfully received. If the test result in step  211  is NO, indicating that some form of error occurred in the transmission of the frame, control passes back to step  207 , and the process continues as described above. Doing so will result in retransmissions of the frame. 
     If the test result in step  211  is YES, indicating that the frame was successfully transmitted, control passes to conditional branch point  213 , which tests to determine if i is equal to N. If the test result in step  213  is NO, indicating that there yet remains more frames to transmit for the layer- 3  packet, control passes to step  215 , in which the value of i is incremented. Control passes back to step  207  and the process continues as described above for the next frame. 
     If the test result in step  213  is YES, indicating all frames that made up the layer- 3  packet have been transmitted, control passes to step  217 , in which the receiver assembles all of the received frames back into the layer- 3  packet, which is passed up to layer- 3 . The process then exits in step  219 . 
     If the test result in step  207  is YES, indicating that a handoff from the first base station to a second base station has been requested, control passes to step  221 , in which the transmitter assembles the remaining frames, which number N-i+1, into a new, special for the purpose of transmitting the remaining frames, layer- 3  packet, in accordance with an aspect of the invention. In step  223 , the special layer- 3  packet is forwarded to the new base station, using conventional inter base station communication, in accordance with the principles of the invention. Also, in step  223 , the values of N and i are transmitted as part of the special layer- 3  packet, in accordance with an aspect of the invention. The transport of the special layer- 3  packet is achieved in the usual manner of interbase station communication. Thereafter, in step  225 , the process waits for the wireless terminal to successfully establish a connection with the second base station. Once a connection is established control passes to step  227 . 
     In step  227 , layer- 2  refragments the received special layer- 3  packet into the previously existing N-i+1 layer- 2  frames. Of course, to do this, the values of N and i are first extracted from the special layer- 3  packet. In step  229  frame i is transmitted over the wireless link between the new base station and the wireless terminal. Conditional branch point  231  tests to determine if the transmitted frame was successfully received. If the test result in step  231  is NO, indicating that some form of error occurred in the transmission of the frame, control passes back to step  229 , and the process continues as described above. Doing so will result in retransmissions of the frame. If the test result in step  231  is YES, indicating that the frame was successfully transmitted, control passes to conditional branch point  233 , which tests to determine if i is equal to N. If the test result in step  233  is NO, indicating there yet remains more frames to transmit, control passes to step  235 , in which the value of i is incremented. Control passes back to step  229  and the process continues as described above for the next frame. 
     If the test result in step  233  is YES, indicating that all the frames of the special layer- 3  packet have been transmitted over the wireless link, control passes to step  217 , and the process continues as described above. 
     In the embodiment described above, it is assumed, for pedagogical and clarity purposes, that there are no further requests for handoffs during the transmission of the frames received in the new layer- 3  packet. However, should an additional handoff request be received, those of ordinary skill in the art will be able to insure that the remaining frames of the special layer- 3  packet are forwarded along with the current values of N and i in the same manner as described herein for the frames of the original layer- 3  packet. 
     In other embodiments of the invention, the values of N and i may not be incorporated in the special layer- 3  packet transmitted in step  223 . For example, the values of N and i may be deduced from the frames already received and the frames received in the new layer- 3  packet. Alternatively, the values of N and i may be transmitted in a separate packet. 
     Note that the process as described in FIG. 2 is from the point of view of the base station to wireless terminal link, i.e., the downlink. Thus, the first base station forwards to the second base the frames that it has not yet transmitted to the wireless terminal station. To use the process for the wireless terminal to base station link, i.e., the uplink, step  223  must be modified to forward to the second base station in the special layer- 3  packet the frames that have already been received, as opposed to the frames that have already been sent, by the first base station from the wireless terminal.