Abstract:
A transceiver for wireless data transmission provides for variable bit rates within a packet (intra-packet rate changes) to provide a high-speed adaptation to variations in link quality useful for continuous mobility applications. Intra-packet rate variations may be obtained with standard hardware by remapping payload data to a subset of the hardware transmission constellation symbols.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional applications 61/097,406 filed Sep. 16, 2008 and 61/095,216 filed Sep. 8, 2008, both hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to wireless transceivers for transmitting and receiving digital data and, in particular, to a transceiver system that varies the effective bit transmission rate within individual data packets or frames. 
         [0003]    The connection of electrical devices to the Internet using wireless protocols, for example WiFi, has provided what may be termed “discrete mobility” to users of laptop computers and other devices. With discrete mobility, the user of the device is free to work at a variety of locations but typically suspends use of the device while moving between locations. 
         [0004]    In providing discrete mobility, current wireless protocols adapt to different qualities of the wireless transmission link (for example, the amount of electrical interference in the transmission link or the signal strength of the transmission) at different locations by changing the transmission rate of the data packets and providing for retransmission of data packets that are corrupted. Generally, lower transmission rates provide improved transmission over noisy or low signal strength links In the retransmission of corrupted data packets, the corruption may be detected, for example, by error detection codes associated with each packet, or missing packet sequence numbers. 
         [0005]    The discrete mobility offered by current wireless protocols is often inadequate for wireless devices such as phones and music players where the user expects “continuous mobility”. Such continuous mobility requires a real-time Internet connection with low latency as the user moves between locations. Yet, measurements made by the present inventors using a mobile phone implementing voice over WiFi (VoWiFi) using standard transmission rate adaptation and retransmission mechanisms found that 80% of the data required retransmission. This level of retransmission wastes bandwidth, battery 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a transmission protocol that varies the effective bit rate of the transmission within an individual packet. This intra-packet rate modulation permits a higher speed, pro-active adaptation to varying link qualities necessary for continuous mobility. Intra-packet rate modulation is rendered practical with standard wireless hardware by mapping transmitted data to a subset of possible wireless symbols. Wireless symbols are generally the different signaling events that may be transmitted by a transmitter to represent one or more bits. Different symbols may, for example, be distinguished by one or more of phase, frequency, amplitude or the like. 
         [0007]    Using this approach, the actual bit rate of used by the hardware for a specific data transmission may remain unchanged as the logical bit rate is adjusted as a function of the position of a data bit in the packet. This approach also permits both intra- and inter-packet bit-rate adjustments, allowing the present invention to work with current inter-packet rate adaptation mechanisms. 
         [0008]    Specifically then, the present invention provides a transceiver system for the transmission of packetized digital data. The transceiver includes a transmitter circuit that receives a first data payload and a first rate map describing a desired transmission bit rate of the data payload where the desired transmission bit rate varies as a function of bit positions in the first data payload. The trans-bitter transmits the first data payload according to the first rate map together with control information in a first packet. 
         [0009]    The transceiver also includes a receiver circuit receiving a second packet including a second data payload and control information and decoding the second data payload according to a second rate map describing a transmission bit rate of the second data payload that varies as a function of bit positions in the second data payload. 
         [0010]    It is thus a feature of the invention to permit intra-packet data rate modulation for high-speed adaptation to varying link quality incident to continuous mobility use of wireless devices or caused by variability in wireless link conditions from other moving objects such as people. 
         [0011]    The transmitter circuit may transmit the first rate map in the first packet and the receiver may receive the second rate map from the second packet. 
         [0012]    It is thus a feature of at least one embodiment of the invention to permit dynamic rate modulation on a packet-by-packet basis. By embedding the rate map in the packet, changes in the rate may be affected instantaneously with the transmission of each new packet. 
         [0013]    The transmitter circuit may include an encoder circuit variably mapping payload data to transmission symbols according to the first rate map to produce a variable transmission bit rate. Similarly the receiver may include a decoder circuit variably mapping received symbol data to payload data according to the second rate map to decode the transmitted second payload. 
         [0014]    It is thus a feature of at least one embodiment of the invention to provide a software level rate modulation permitting the present invention to work with standard hardware and be readily combined with current hardware-implemented rate adaptation systems. 
         [0015]    The transceiver may further include a statistical payload error table recording a statistical probability of errors as a function of bit position in a payload and the bit rate encoder may encode the payload according to the packet error table to reduce bit rates at bit positions having high statistical probability of error and to increase bit rate at bit positions having low statistical probability of errors. 
         [0016]    It is thus a feature of at least one embodiment of the invention to exploit the present inventors&#39; discovery of a regular pattern of errors in spread-spectrum transceivers to anticipate those errors and thus improve data throughput by reducing mis-transmissions. 
         [0017]    The transceiver may further include an error detector detecting variation in error rates as a function of bit rate within a packet having different intra-packet bit rates and monitoring the function to deduce an improved error-corrected transmission rate. The bit rate encoder may then encode a payload according to the improved error-corrected transmission rate. 
         [0018]    It is thus a feature of at least one embodiment of the invention to use an individual packet and its varying bit rates to accurately deduce an ideal bit rate for a later packet. The varying bit rate packet provides an indication of not only when the bit rate is too high for the channel but also when the bit rate is too low for the channel. 
         [0019]    The bit rate encoder may vary the transmission bit rate within the first packet to provide transmission bit rates on either side of the improved transmission rate. 
         [0020]    It is thus a feature of at least one embodiment of the invention to continually bracket the optimal transmission rate to provide rapid adaptation to link degradation. 
         [0021]    The transmitter and receiver may provide for a transmitted symbol constellation and the transmitter may vary the transmission bit rate by using only a subset of the symbols of the constellation, the subset size being a function of the desired bit rate. The receiver may employ an error corrector re-mapping data received at constellation points outside of the subset to constellation points within the subset. 
         [0022]    It is thus a feature of at least one embodiment of the invention to capture the improved error correction qualities incident to lower transmission rates using a reduced constellation subset. 
         [0023]    These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0024]      FIG. 1  is a simplified diagram showing a mobile user moving between two stationary access points showing zones in which different transmission bandwidth will be available to the user&#39;s mobile device; 
           [0025]      FIG. 2  is a block diagram of the principal components of an access point and mobile device implementing transceivers suitable for use with the present invention; 
           [0026]      FIG. 3  is a detailed block diagram of the transceivers of  FIG. 2  having a programmable processor working with standard wireless hardware, together implementing a bit spreader and a bit de-spreader; 
           [0027]      FIG. 4  is a graph showing bit errors as a function of bit position in a packet for a spread-spectrum transmission protocol; 
           [0028]      FIG. 5  is a detailed block diagram of the bit spreader for Quadrature Amplitude Modulation (QAM) hardware; 
           [0029]      FIG. 6  is a constellation diagram of the QAM wireless symbols; 
           [0030]      FIG. 7  is a figure similar to that of  FIG. 6  showing mapping of data to be transmitted to a subset of the constellation of  FIG. 6 ; 
           [0031]      FIG. 8  is a figure similar to that of  FIGS. 6 and 7  showing error correction possible with the remapping of the present invention; 
           [0032]      FIG. 9  is a plot of data collected by the transceiver showing errors as a function of bit rate in an individual packet; 
           [0033]      FIG. 10  is a plot of inter-packet rate adjustment guided by multi-rate packet measurements; 
           [0034]      FIG. 11  is a figure similar to that of  FIG. 4  showing transmission rate and a projected transmission rate based on the introduction of a new synchronization clock sequence in mid-packet; and 
           [0035]      FIG. 12  is a flowchart of a program executed to assess points at which additional synchronization clock sequences may effectively be inserted into a packet. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0036]    Referring now to  FIG. 1 , a mobile device  10 , such as an Internet phone or Internet connected music player, may employ a wireless protocol such as 802.11 based WiFi to communicate with one or more access points  12   a  and  12   b.  The access points  12   a  and  12   b  may be connected to the Internet  14 , for example, by physical conductors  16 . 
         [0037]    As a user  18  moves away from a given access point  12   a,  he or she will pass rapidly through multiple zones  20  having different link quality with the access point  12   a  as measured by link bandwidth. As used herein, “bandwidth” generally refers to the achievable rate of transmission of data by the link in the zone  20  as may be affected by signal strength, channel noise, and other interference such as multi-path reflections. 
         [0038]    Referring now to  FIG. 2 , each access point  12  may provide an antenna  22  leading generally to transceiver circuitry  24 . The transceiver circuitry  24  may use any of a variety of different modulation techniques such as Orthogonal Frequency Division Multiplexing (OFDM) including, for example, Quadrature Amplitude Modulation (QAM) such as 16-QAM, or phase-based methods such as QPSK and BPSK as well as others. Normally the transceiver circuitry  24  will implement a spread-spectrum technique in which the carrier signal is spread in the frequency domain to reduce susceptibility to interference. The transceiver circuitry  24  provides both a transmitter and receiver for modulation and demodulation, respectively, according to these modulation techniques. 
         [0039]    The transceiver circuitry  24  may be connected to a network protocol circuit  26  implementing the wireless protocol, for example 802.11, or other wireless standards as is generally understood in the art. 
         [0040]    The network protocol circuit  26  may, in turn, connect with a processor  28  communicating with a memory  30  holding an operating system  32  and other programs needed for the access point  12  as well as program  34  implementing the present invention. The processor  28  may also connect with a standard network interface circuit  33  communicating with conductors  16 . The transceiver circuitry  24 , network protocol circuit  26 , and processor  28  together form a transceiver system. 
         [0041]    As described above, during operation, the access point  12  communicates with the mobile device  10  by means of radio signals  38  coupled between the antenna  22  of the access point  12  and antenna  40  of mobile device  10 . The antenna  40  of mobile device  10  leads to a transceiver circuitry  42  similar to transceiver circuitry  24  which in turn connects with a network protocol circuit  44  also generally identical to corresponding and network protocol circuit  26 . Again, a processor  46  may connect to the network protocol circuit  44  and execute an operating system  48  providing the basic functionality of the mobile device  10  and a program  34  for the present invention held in a memory  50 . The transceiver circuitry  42 , network protocol circuit  44 , and processor  46  together form a transceiver system. 
         [0042]    Depending on the purpose of the mobile device  10 , the mobile device  10  may also have a display screen  52  and user input device  54  such as the touch screen or button array or the like, both communicating with the processor  46 . An I/O port  58 , for example, providing for audio output or input may also communicate with the processor  46  to implement phone or music player features. 
         [0043]    Referring now to  FIG. 3 , the present invention as described may be implemented in software  34  executed by processors  28  and  46  and thus provides a simple migration path for implementing this protocol. Nevertheless, it must be understood, that the software functions may alternatively be implemented in hardware elements  26 ,  24 ,  42 ,  44  as the technique of the present invention gains acceptance. 
         [0044]    Because the circuitry involved in the transmission of the radio signal  38  is similar for mobile device  10  and access point  12 , only access point  12  will be described now, with it being understood that a similar description applies to the corresponding elements of the mobile device  10 . 
         [0045]    A transmission by access point  12   a  begins with the receipt by the processor  28 , within a buffer  60 , of data forming an unmodified payload  62  to be transmitted. The unmodified payload  62 , for example, may be audio data (music or spoken words) or text data, or other data, provided from the Internet or from the device itself according to the particular context. The unmodified payload  62  will be modified, as will be described, to produce modified payload  62 ′ forwarded to a buffer  66  in the network protocol circuit  26 . Within the network protocol circuit  26 , the modified payload  62 ′ is concatenated with control data  68 ,  70  and  72  to produce a data packet  64  to be transmitted by the transceiver circuitry  24 . The term “data packet” as used herein is generally a payload as will be transmitted with a single set of common control data  68 ,  70 , and  72 . The control data,  68 ,  70  and  72  will typically include a header  68  holding a destination, data type, synchronization clock sequence, and sequence number for the data packet  64 , a rate table  70  related to interpretation of a variable bit rate of the data packet  64  added by the present invention, and error correction and/or detection codes  72 . Control data  68  and  72  is well-known in the art and control data  70  will be described further below. 
         [0046]    Between the buffer  60  receiving the unmodified payload  62  and the buffer  66  holding the data packet  64  ready for transmission, the present invention employs an encoder  74  providing for a bit-spreading operation that converts the unmodified payload  62  to a modified payload  62 ′. 
         [0047]    Referring momentarily to  FIG. 6 , each transmitter of transceiver circuitry  24  will provide for a series of symbols  80 , typically representing various modulation states of the radio signal  38 , for example phase or frequency modulation, that may be uniquely decoded by a corresponding transceiver. An example constellation for quadrature phase shift (QPSK) is depicted consisting of eight symbols  80  defined by instantaneous phase shifts of two quadrature radiofrequency signals. This modulation system may be termed 8-PSK and allows the instantaneous transmission of three bits (a triplet) of data. These symbols  80  are depicted as locations on a circle defined by a phase angle θ having angles of 0° to 360° with the symbols  80  separated by 45° increments. The relationship between the symbols  80  and the given triplets may follow a Gray-code sequence in which successive triplets differ by only a single bit. Thus, starting at angle θ and proceeding counterclockwise around the circle, the symbols  80  map to triplets as follows: 000, 010, 110, 101, 111, 100, 001, and 011. 
         [0048]    In one embodiment of the present invention, the effective data rate of the transmitter (with respect to the communication of data of the unmodified payload  62 ) may be decreased by mapping the unmodified payload  62  to a subset of the symbols  80 . With this three mapping, even though the transmitter transmits a constant number of symbols per second, the number of bits of the unmodified payload  62  transmitted per second may be decreased. 
         [0049]    Referring also to  FIG. 5 , consider for example the subset of symbols  80  including only the triplets: 000, 110, 111, and 001. A mapping table  75  may be generated as follows: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Payload data 
                 Symbol 
               
               
                   
                   
               
             
             
               
                   
                 00 
                 000 
               
               
                   
                 01 
                 110 
               
               
                   
                 10 
                 111 
               
               
                   
                 11 
                 001 
               
               
                   
                   
               
             
          
         
       
     
         [0050]    For example, unmodified payload  62  of 101110 may be mapped using mapping table  75  to the modified payload  62 ′ equal to 111 001 111. This can be compared to straight transmission of unmodified payload  62  implemented by the symbols 101 110. The bit-spreading operation of encoder  74  thus reduces the effective rate of transmission of the data by approximately two thirds. Further, this reduction can be implemented without modification of the transceiver circuitry  24  or network protocol circuit  26 , but simply by modifying the payload before it is transmitted. 
         [0051]    While one example of a bit-spreading operation is shown, it will be understood from this example that different degrees of bit-spreading may be implemented simply by selecting among different mapping tables  75 . For example, a further decrease in effective transmission rate may be obtained by remapping of 0 in the payload to the 000 symbol and 1 in the payload to the 111 symbol. In addition, mapping tables  75  may be developed for other modulation schemes by simply selecting subsets of the symbols  80  of those modulation schemes and assigning them to elements of the unmodified payload  62  using mapping table  75 . 
         [0052]    Referring again to  FIG. 3 , in the present invention, the encoder  74  uses different mapping tables  75  to change the effective bit transmission rate as a function of the bit number in the unmodified payload  62 . Thus, different bits in the unmodified payload  62  may have different effective transmission rates. For example, the first three bits  101  may be mapped to the symbol  101  without compression while the next two bits  11  may be mapped  2001  as shown in  FIG. 5  producing a compression by two thirds. The last bit  0  may be mapped to 000 producing compression by one third. In this way, three different effective transmission rates of these bits may be realized. As shown in  FIG. 3 , the particular compression function (compression as a function of bit number) is captured and stored as a rate table  70  concatenated to the modified payload  62 ′ as will serve in assisting the decoding of the packet  64 . 
         [0053]    The particular function used by the encoder  74  may be generated in a variety of ways which will be discussed further below. 
         [0054]    Referring still to  FIG. 3 , the decoding of the packet  64  arriving at the transceiver circuitry  24  follows the reverse procedure as described above. After a data packet is received by the transceiver circuitry  24  and provided to a buffer  82  in the network protocol circuit  26 , the header  68  and error correction and/or detection codes  72  are removed from the modified payload  62 ′ and processed according to techniques well known in the art. The error correction and/or detection codes  72  may indicate an erroneous modified payload  62 ′ which may be used independently to provoke a retransmission of the packet  64  or a correction of the erroneous data. In either case, the bit number of the error identified may be forwarded to an error-histogram  76  which will be described below. 
         [0055]    A modified payload  62 ′ that is free from errors is passed to a decoder  84  which also receives the rate table  70 . The decoder  84  implements a bit-spreader which reads the rate table  70  and performs a recompression operating analogously to the bit-spreading described above with respect to the encoder  74  but with the mapping conducted in reverse. The particular mapping is determined by the rate table  70  so that a subset of the possible symbols  80  is mapped (by a mapping table  75 ) to produce unmodified payload  62 . This unmodified payload  62  is provided to a buffer  86  to be further processed according to the context of the device, for example, to be transmitted on the Internet (for access point  12 ) or generate an audio signal on mobile device  10 . 
         [0056]    Referring momentarily to  FIG. 7 , the use of a limited subset of the symbols  80  allows for a second level of error correction (or detection) of the received data in a hardware implementation of the present invention. For example, assume that data is received at phase angle θ=43° shown by an X in  FIG. 7 . In this case, if the transceiver circuit  24  is aware of the particular subset of active symbols (shown in solid circles in  FIG. 7 ) the transmitted data at 43° may be correctly resolved to the symbol 000 rather than the symbol 010 as may normally occur. Thus, the benefits of lower data rates by simple remapping can produce increased noise immunity. 
         [0057]    Referring to  FIG. 8 , in certain cases this benefit of additional error correction (or detection) can be implemented purely in software after the transceiver circuitry  24 . For example, when the subset of employed symbols  80  of 8-PSK are reduced to two, for example, 000 and 111, a symbol interpreted by the transceiver circuitry  24  to be symbol 010 may be confidently and easily corrected in software to symbol 000. Regardless of the error correction and detection, the bit-spreading operation may improve the reliability of the transmission by the distribution of the data of the unmodified payload  62  to a greater number of bits decreasing the chance of corruption on a per bit basis because of the independent statistical probability of corruption each transmitted bit. 
         [0058]    Referring again to  FIG. 3 , the function used by the encoder  74  to variably change the bit rate of the payload  62  may be received from a variety of different sources. 
         [0059]    Referring now to  FIG. 4 , a first option for controlling the bit rate using the encoder  74  employs a predefined error table  77  recording a statistical likelihood of errors as a function of bit position in the modified payload  62 ′. In this regard, the present inventors have monitored error rates as a function of bit position in the data packets  64  and noted a regular pattern of error rates  90 . While the inventors do not wish to be bound by a particular theory, it is believed that low error rates near the beginning of the packet  64  occur because of the high degree of synchronization of the early data of the packet  64  being most proximate to the synchronization clock sequence (a timing pattern) of the header  68 . Generally the synchronization clock sequence provides a “training set” that is predetermined and thus known by both the transmitter and receiver so as to be used by the receiver to adjust its reception circuitry with respect to the expected phase, frequency, and amplitude of the incoming signal. In addition, the present inventors have detected periodic spikes in error rates believed to be the result of the pattern of frequency hopping incident to spread-spectrum transmission that periodically moves the carrier frequency to the edge of the allotted bandwidth where interference is greatest. This empirically derived pattern of error rates  90  may be used to provide an exactly offsetting transmission rate  92  (held in error table  77  or computed therefrom) in which times of lowest error rates are associated with greatest effective transmission rates and times of highest error rate are associated with lower transmission rates. The transmission rate  92  may be used to guide the encoder  74 . In this way, a higher effective throughput (measured as error-free bits of transmission) may be obtained more cost-effectively in terms of time and/or transmission energy. 
         [0060]    Referring to  FIG. 9 , a second option for controlling the bit rate using the encoder  74  (which may be used alternatively or in addition to the first option) monitors error rates in received packets  64  as a function of bit position of the modified payload  62 ′ and effective transmission rate as indicated by rate table  70 . The errors are stored in the error-histogram  76  described above which generates an error plot  100  as a function of data rate (and bit position) for as little as a single packet  64  and thus on a very rapid basis. 
         [0061]    The error plot  100  may extract from a single packet  64  a sampling of transmission rates and their resulting error rates. The error rates and data rates may be used to derive an error-free throughput curve  102  indicating the effective error-free transmission of data by the system. The peak  104  of this curve  102  defines an optimum data rate for the next transmitted packet  64 . 
         [0062]    Referring to  FIG. 10 , the optimum frequency represented by the peak  104  of curve  102  from  FIG. 9  may be used as a target transmission rate  106  for the encoder  74  that evolves with time. The target transmission rate  106  may define a center of the transmission rate of the modified payload  62 ′ for the encoder  74  which may further vary the transmission rate of other bits of the unmodified payload  62  to be above and below the target transmission rate  106  as indicated by shaded regions  108 . In this way, every packet  64  tests for the target transmission rate  106  providing extremely rapid compensation for changes in the bandwidth zones  20  (shown in  FIG. 1 ). 
         [0063]    Alternatively, the target transmission rate  106  may be used to adjust the transmission rate of the packet by modification of the transmitter of the transceiver circuitry  24  according to techniques known in the art. 
         [0064]    The present invention providing for intra-packet rate modulation may be used in conjunction with inter-packet rate modulation (which directly affects the symbol transmission rate implemented by the transceiver circuitry  24 ) incorporated into various standards and often implemented in hardware. 
         [0065]    Referring now to  FIGS. 11 and 12 , the data of the predefined error table  77 , as concurrently refined, may be used to dynamically insert additional or augmenting synchronization clock sequences into the packets  64  in cases where the insertion provides for a net gain in effective transmission rate. In one embodiment, the offsetting transmission rate  92 , for example, ay be used to guide the actual transmission rate of the encoder  74 . In this case, as indicated by process block  200 , at periodic times  202 , the program  34  may forecast the transmission rate by extending the offsetting transmission rate  92  (calculated from the error rate  90  of error table  77 ) into the future as indicated by the solid line in region  206 . Region  206  begins at a time  208  at which data transmission would resume, after a hypothetical synchronization clock sequence transmission period  210  occupied by a hypothetical augmenting synchronization clock sequence that might be inserted into the packet  64  at time  204 . 
         [0066]    Also at process block  200 , a hypothetical transmission rate  92 ′ is determined under the assumption that there has been an introduction of a synchronization clock sequence at time  204 . The hypothetical transmission rate  92 ′, indicated by the dotted line in region  206 , may be derived from the offsetting transmission rate  92  by simply shifting the latter to align with time  208 . An area  205  between the solid line of offsetting transmission rate  92  and the dotted line of hypothetical transmission rate  92 ′ in region  206  represents a benefit in data transmission from the introduction of a synchronization clock sequence in period  210 . 
         [0067]    At process block  212 , the cost in terms of lost data transmission caused by the introduction of a synchronization clock sequence at period  210  is also calculated as the area  214  beneath the offsetting transmission rate  92  during period  210 . 
         [0068]    At decision block  216 , the areas  205  and  214  are compared and if the data gain represented by the difference between areas  205  and  214  exceeds a predetermined amount, a new synchronization clock sequence will be inserted at time  204  as indicated by process block  218 . Otherwise the program  34  returns to process block  200  to compute these values for later bit position or time within the packet  64  (assuming the existence of any previously selected augmenting synchronization clock sequences). 
         [0069]    While this process is described as if done contemporaneously with the transmission of data, it may also be done before the transmission of each packet  64  based on the expected offsetting transmission rate  92  for that packet  64 . 
         [0070]    The receiver may remove these additional synchronization clock sequences (added at process block  218 ) after using them to resynchronize the receiver, so as to extract the payload  62  being transmitted. The location of the added synchronization clock sequences may be marked by other header information to distinguish it from payload data. 
         [0071]    It will be understood that this process may be used together with the above described variation in effective transmission rate according to the offsetting transmission rate  92  or maybe implemented without intra-packet adjustment of the transmission rate of the transmitter by using an estimate of the effective transmission rate without intra-packet adjustment taking into account retransmission. This estimate will generally have a steeper downward slope than offsetting transmission rate  92  caused by data lost in retransmission. In addition, it will be understood, that locations for augmenting synchronization clocks sequences may be predetermined and inserted into the packet on a regular basis or according to gross metrics such as total packet error rate and the like, by selecting from predetermined insertion schedules linked to particular packet error rates. 
         [0072]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the invention should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.