Abstract:
An ADSL termination unit, which at the transmission end includes a descrambler and a compressor between its scrambler and interleaver. The present invention provides for the use of compression on the data in a channel to remove some of the inherent redundancy, in order to yield much better throughput, particularly in conjunction with certain powerful FEC schemes. In this manner the compression may be performed on unscrambled data that has a higher redundancy than scrambled data, thereby improving compression. At the reception end, the ADSL termination unit includes a decompressor and a scrambler between its deinterleaver and descrambler.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to the field of data compression, and more particularly to data compression on a digital subscriber line (DSL) loop.  
           [0002]    With the increasing popularity of the Internet, there has been a corresponding increase in the demand for high rate digital transmission over the local subscriber loops of telephone companies. A loop is a twisted-pair copper telephone line coupling a user or subscriber telephone to a central office (CO).  
           [0003]    Traditionally, data communication equipment uses the voice band of the subscriber loop. Such equipment includes voice band modems, which operate at up to 56 kbps using compression techniques. On the other hand, Integrated Services Digital Network (ISDN) systems have boosted data rates over existing copper phone lines to 120 kbps. However, traditional voice band equipment is limited by the maximum data rate of the existing switching networks and Pulse Code Modulation (PCM) data highways.  
           [0004]    Utilizing the frequency bandwidth of the loop outside the voiceband has enabled other high-speed systems to evolve. However, because loops can differ in distance, diameter, age and transmission characteristics depending on the network, they pose some limitations and challenges for designers of these high-speed systems.  
           [0005]    Current high-speed digital transmission systems of the above type include asymmetric, symmetric, high-rate, and very high-rate digital subscriber loops, conventionally known as ADSL, SDSL, HDSL and VDSL, respectively. Normally these and other similar protocols are known as xDSL protocols.  
           [0006]    Of these flavors of xDSL, ADSL is intended to co-exist with traditional voice services by using different frequency spectra on the loop. An overview of ADSL is provided in the ADSL Forum&#39;s Technical Report TR-017, “ATM Over ADSL Recommendation” (March 1999), which is incorporated herein by reference for all purposes. More detail on ADSL can be found in the document ANSI T1.413, “Asymmetric Digital Subscriber Line (ADSL) Metallic Interface” (1998), which is incorporated herein by reference for all purposes.  
           [0007]    In the future, it is possible that multiple different transmission schemes may be employed in different frequency bands on the same loop, and that these transmission schemes may include traditional analog voice services as well as current and new forms of xDSL. In today&#39;s ADSL systems, the plain old telephone services (POTS) use the frequency spectrum between 0 and 4 kHz and the ADSL uses the frequency spectrum between 30 kHz and 1.1 MHz for data over the telephone line. ADSL partitions its frequency spectrum with upstream (subscriber to CO) transmission in a lower frequency band, typically 30 kHz to 138 kHz, and with downstream transmission in a higher frequency band, typically 138 kHz to 550 kHz or 1.1 MHz. ADSL uses a discrete multi-tone (DMT) multi-carrier technique that divides the available bandwidth into approximately 4 kHz sub-channels.  
           [0008]    In order to maximize the throughput on a given channel, it is important to minimize the redundancy in the transmitted data, followed by the careful addition of some redundancy (in order to enable the use of forward error correction (FEC)). Thus far, there has been a lot of activity to improve the performance of DSL (particularly on long loops) with the use of better FEC. Reed-Solomon encoding and Trellis Coded Modulation are already part of the G.DMT specification for ADSL, and further additions of concatenated convolutional and turbo codes are open issues for the G.DMT specification.  
           [0009]    It is appropriate to consider first whether data compression should be performed at the physical layer in an ADSL modem. Performing data compression at the physical layer for a DSL link is practical. If performed at higher layer protocols and applications, it is difficult to ensure that the union of all application programs, operating systems, network protocols, and content providers would present data to the ADSL link in a compressed format. Some users of some applications would use data compression, while many users of the Internet would continue to transfer files, download web pages, and exchange email without the benefit of compression.  
           [0010]    The inclusion of data compression in the DSL link does no harm, but it potentially provides a great benefit. It is possible to design a data compression scheme that will not provide a degradation of throughput.  
           [0011]    Finally, DSL provides data rates and services that are quite different from dialup modems and Ethernet-based local area networks (LANs). DSL specific issues need to be addressed by DSL standards and cannot be left to general networking solutions. One example of a DSL specific issue is the use of ATM cells over the link. It is possible to exploit the redundancy in the ATM cell headers to help compress the traffic on the loop by up to 10%. Another example is the multiplicity of services that can be run on the bandwidth of a DSL link. It is possible to download a file while browsing the web and listening to an audio broadcast. Compression algorithms in such an environment need to be agile to the interleaved traffic.  
         BRIEF SUMMARY OF THE INVENTION  
         [0012]    The present invention provides for the use of compression on the data in a channel to remove some of the inherent redundancy, in order to yield much better throughput, particularly in conjunction with certain powerful FEC schemes. At the transmission end, an ADSL termination unit includes a descrambler and a compressor between its scrambler and interleaver. In this manner the compression may be performed on unscrambled data that has a higher redundancy than scrambled data, thereby improving compression. At the reception end, the ADSL termination unit includes a decompressor and a scrambler between its deinterleaver and descrambler.  
           [0013]    The removal of redundancy on the transmitted data is independent of the data rate on the loop. However, for long loops with reduced data rates, the improvement can provide significant benefits to the end user of the DSL service.  
           [0014]    According to one embodiment of the present invention, a compression system for ADSL includes a central office ADSL termination unit (ATU-C) and a remote ADSL termination unit (ATU-R). The ATU-C includes a descrambler and a compressor between its scrambler and interleaver. The ATU-R includes a decompressor and a scrambler between its deinterleaver and descrambler. In this manner, compression may be performed on unscrambled data that has a higher redundancy than scrambled data, thereby improving compression.  
           [0015]    According to another embodiment of the present invention, a transmitting ATU includes an asynchronous transfer mode (ATM) transmission convergence circuit, a scrambler circuit, a descrambler circuit, a compression circuit, and an interleaver circuit. The descrambler circuit and compression circuit operate as described above regarding the compression system.  
           [0016]    According to yet another embodiment of the present invention, a receiving ATU includes a deinterleaver circuit, a decompression circuit, a scrambler circuit, a descrambler circuit, and an ATM transmission convergence circuit. The scrambler circuit and decompression circuit operate to decompress the compressed data received from the above-described transmitting ATU.  
           [0017]    One aspect of the invention provides for the limiting of the maximum compression bandwidth to assist ATM provisioning.  
           [0018]    Another aspect of the invention provides for limiting the average compression bandwidth to assist ATM provisioning.  
           [0019]    A still further aspect of the invention provides for ATM flow control over the ADSL loop to assist ATM provisioning in the presence of bandwidth variation. (Compression is one way to get bandwidth variation, among others.)  
           [0020]    A still further aspect of the invention provides for the use of multiple or hybrid compression algorithms to match the interleaved data traffic seen on ADSL loops.  
           [0021]    These aspects of the invention are further detailed with reference to the following figures and detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 shows the location of the compression function (COMP) in the ATU reference transmitter model;  
         [0023]    [0023]FIG. 2 is a schematic diagram showing a method for making compression compatible with ATM payload scrambling;  
         [0024]    [0024]FIG. 3 is a block diagram of a portion of the transmission side of a central office ATU;  
         [0025]    [0025]FIG. 4 is a block diagram of a portion of the reception side of a remote ATU;  
         [0026]    [0026]FIG. 5 is a block diagram of a portion of the transmission side of a remote ATU; and  
         [0027]    [0027]FIG. 6 is a block diagram of a portion of the reception side of a central office ATU. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    The compression function according to the present invention is performed by a compression block  16  between an ATM cell transmission convergence (TC) block  12  and an LS 1  input  14  to the interleaved data stream as shown in the ADSL termination unit (ATU)  10  of FIG. 1. Under ADSL standards, the LSO input corresponds to a “fast” path that generally has a higher bit error rate and lower latency than that of the LS 1  input, which corresponds to an “interleaved” path. The remaining functions of the ATU  10  are standard and are described briefly as follows with reference to functional sections  18 - 26 . In section  18 , aside from the novel compression performed by the compression block  16 , the ATU  10  performs transmission convergence, multiplexing, synchronization control, and cyclic redundancy checking. In section  20 , the ATU  10  performs forward error correction. In section  22 , the ATU  10  performs interleaving (if appropriate) and tone ordering. In section  24 , the ATU  10  performs constellation encoding and gain scaling. In section  26 , the ATU  10  performs inverse discrete Fourier transforming, parallel/serial buffering, digital-to-analog conversion, and analog processing. As FIG. 1 corresponds to transmission, similar blocks at the receiving end perform analogous inverse functions.  
         [0029]    Performing compression before interleaving provides the benefit that the compression algorithm can tackle the redundancy inherent to the ATM headers. In the downstream direction, the ATM cells have almost reached their destination, so some of their header information is no longer necessary. Header redundancy can account for 10% of the usable bandwidth. Furthermore, this placement allows the compression algorithm to work in the presence of scrambling within the ATM cell payload. This concept is described in the next section.  
       ATM Cell Scrambling  
       [0030]    Most compression algorithms perform poorly if the source data has been scrambled. Scrambling makes it impossible for the algorithm to identify redundancy in the source data. During header error check (HEC) delineation the payload might be interpreted as a valid ATM header. To eliminate this misinterpretation, the ATM layer is conventionally required to scramble its payload. This can be handled in the presence of compression as shown in FIG. 2.  
         [0031]    It can be seen from FIG. 2 that at the ATM transmission convergence (TC) level, the requirement for payload scrambling is met with a descrambler/scrambler pair as part of the compression function. The operation for the additional descrambler/scrambler pair is the same as that of the ITU-T Recommendation I.432 using the self-synchronizing polynomial X 43 +1.  
         [0032]    The decompressed data rate is source data dependent, which has the following implications:  
       ATM Permanent Virtual Circuit (PVC) Provisioning  
       [0033]    The unpredictable requirement for network bandwidth at the central office ADSL termination unit (ATU-C) required to service a DSL loop using compression is an issue that requires some careful consideration. An analogous problem exists in systems developed using present DSL technology. If a shelf is built to support 100 full rate modems, the theoretical maximum bandwidth required will be 6 Mpbs×100=600 Mbps of data. However, depending on the statistics of the number of active subscribers, and the length of the loops connected to the 100 modems, the actual bandwidth used can be significantly less. System designers have a choice to either accommodate the peak traffic, or allow some blocking for the average traffic they feel is reasonable. This determination of the bandwidth to be provided is termed “provisioning.” 
         [0034]    Similarly, with statistically variable bandwidth in the presence of compression, it is possible either to provision for the maximum compressed data rate, or to provision for a reasonable average. However, there are two differences in the analogy. First, in the shelf example the statistics are an ensemble average over many users, and in the compression case the statistics are a time average for a single user. Second, in the shelf example it is possible to calculate the absolute maximum traffic that can be generated, but in the case of compression it is more difficult to establish this value. The following two sections address these issues.  
       Limiting the Maximum Traffic Generated Through Compression  
       [0035]    With a simple implementation of data compression, it is possible to generate a very high peak data rate at the output of the decompression circuit. For example, using run length encoding, if a user is sending all 0&#39;s, it is possible to transmit a single 0 along with a count. If the count is large, a very large amount of data is instantaneously created at the output. Furthermore, the latency to produce the first output is determined by the length of the burst. Such pathological cases can be avoided by placing latency and coding gain requirements on the compression algorithm.  
         [0036]    Once this is accomplished, it is possible to determine the peak throughput, given an uncompressed line rate. This absolute maximum can be used to determine the network bandwidth that needs to be provisioned at the ATU-C.  
         [0037]    Provisioning Less Than the Absolute Maximum Possible Bandwidth:  
         [0038]    There are two possible solutions to provision less than the absolute maximum possible bandwidth. The first solution is to limit the provisioning of compression to longer loops with inherently less bandwidth. Assuming that the maximum compression gain is limited to a factor of 2×, in a system designed to service 6 Mbit modems, if a subscriber loop has an uncompressed bandwidth greater than 3 Mbits, then compression is disabled for that loop. A user on a long loop using compression is then indistinguishable from a user on a short loop with no compression.  
         [0039]    The second solution is to implement flow control over the DSL loop. If a subscriber&#39;s peak bandwidth exceeds the buffer space allocated in the central office, the flow of ATM cells is reduced at the remote ADSL termination unit (ATU-R) until the average bandwidth matches the provisioned PVC at the central office. A variation of this algorithm would be the use of feedback in the compression algorithm, which would ensure that the average data rate does not exceed a prescribed amount. This amounts to a cap on the average performance (rather than the peak performance) of the algorithm.  
         [0040]    In summary, compression maximizes the channel capacity on all loops, as well as complementing the benefits that would come from more powerful error correction techniques. The gains can be particularly important to subscribers with access to limited bandwidth on long loops.  
       Specific Implementations  
       [0041]    FIGS.  3 - 6  show specific implementations of these concepts. Although the description uses the term “circuit,” this term is intended to be broadly read to include both dedicated, specific integrated circuit hardware as well as general, programmable hardware under software or firmware control, as well as combinations thereof.  
         [0042]    [0042]FIG. 3 shows an ATU-C (transmission portion)  100  according to an embodiment of the present invention. The ATU-C  100  includes an ATM TC circuit  102 , a scrambler circuit  104 , a descrambler circuit  106 , a compression circuit  108 , an interleaver circuit  110 , and a control circuit  112 . The ATM TC circuit  102  performs transmission convergence on the ATM data from the network to be transmitted to the subscriber on the subscriber loop. The scrambler circuit  104  performs scrambling on the output of the ATM TC circuit  102 . The interleaver circuit  110  interleaves the data prior to transmission to the subscriber. The ATM TC circuit  102 , scrambler circuit  104  and interleaver circuit  110  may be standard components and are not further detailed, nor are the other standard components of ATU-C  100  that are not shown. Furthermore, the ATU-C  100  may perform other functions that are not necessary to the understanding of the present invention and are not described in detail. Such functions may include idle cell insertion, header error control generation, bit timing ordering, cell delineation, header error control verification, cell rate decoupling, signaling facilitation, connection admission control, resource management, and layer/plane management functions.  
         [0043]    Between the scrambler circuit  104  and the interleaver circuit  110 , compression is performed. As discussed previously regarding FIG. 2, descrambling is needed to allow redundancy to be exploited. The descrambler circuit  106  performs this function. After the data has been descrambled, the compression circuit  108  compresses it. The compression algorithm used may be any standard compression algorithm or combination thereof.  
         [0044]    The control circuit  112  controls the descrambler circuit  106  and the compression circuit  108 . As described above, such control may be exercised for various reasons. First, the maximum compression bandwidth may be limited. Such limiting may be implemented by changing the compression algorithm, by reducing or increasing the redundancy removed from the data, or by simply not performing compression under certain circumstances. Second, the average compression bandwidth may be limited. Third, the control may be exercised for ATM flow control over the ADSL loop to assist ATM provisioning in the presence of bandwidth variation, which may be caused by compression (or other factors). Fourth, the control circuit  112  may direct the compression circuit  108  to selectively implement one of a number of available compression algorithms.  
         [0045]    [0045]FIG. 4 shows an ATU-R (reception portion)  120  according to an embodiment of the present invention. The ATU-R  120  basically performs the inverse of the functions performed by the ATU-C  100 . The ATU-R  120  includes a deinterleaver circuit  122 , a decompression circuit  124 , a scrambler circuit  126 , a descrambler circuit  128 , and an ATM TC  130 . The deinterleaver circuit  122  deinterleaves the data after reception from the central office. The descrambler circuit  128  performs descrambling prior to input to the ATM TC circuit  130 . The ATM TC circuit  130  performs transmission convergence functions to the ATM data prior to its provisioning to the subscriber&#39;s various customer premises equipment (CPE) (e.g., telephone, computer, etc.). The ATM TC circuit  130 , descrambler circuit  128  and deinterleaver circuit  122  may be standard components and are not further detailed, nor are the other standard components of ATU-R  120  that are not shown.  
         [0046]    Between the deinterleaver circuit  122  and the descrambler circuit  128 , decompression is performed. The decompression circuit  124  performs this function using an inverse of the compression algorithm used by the ATU-C  100 . As discussed previously regarding FIG. 2, scrambling is needed so that the descrambler circuit  128  can operate as usual. The scrambler circuit  126  performs this function.  
         [0047]    As can be seen, the ATU-C  100  and the ATU-R  120  acting together at the central office and the subscriber location, respectively, effectively increase the downstream data rate. In ADSL implementations, the downstream data rate is generally the one designers are concerned about. However, the principles of the invention may also be applied in the upstream direction, as shown in FIGS.  5 - 6 .  
         [0048]    [0048]FIG. 5 shows an ATU-R (transmission portion)  140  according to an embodiment of the present invention. The ATU-R  140  includes an ATM TC circuit  142 , a scrambler circuit  144 , a descrambler circuit  146 , a compression circuit  148 , an interleaver circuit  150 , and a control circuit  152 . These components operate similarly to the components described above in the ATU-C  100  and are not further detailed.  
         [0049]    [0049]FIG. 6 shows an ATU-C (reception portion)  160  according to an embodiment of the present invention. The ATU-C  160  basically performs the inverse of the functions performed by the ATU-R  140 . The ATU-C  160  includes a deinterleaver circuit  162 , a decompression circuit  164 , a scrambler circuit  166 , a descrambler circuit  168 , and an ATM TC  170 . These components operate similarly to the components described above in the ATU-R  120  and are not further detailed.  
         [0050]    As can be appreciated, the ATU-C  100  and the ATU-C  160  may both be implemented as a single unit (e.g., in a line card), as may the ATU-R  120  and the ATU-R  140  (e.g., in a modem).  
         [0051]    Although the preceding description has focused on specific embodiments, numerous variations, alternatives and their equivalents are considered to be within the scope of the present invention, which is defined by the following claims.