Regulation 3.2 AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT DIVISIONAL APPLICANT: AWARE, INC. Invention Title: MULTICARRIER COMMUNICATION WITH VARIABLE OVERHEAD RATE The following statement is a full description of this invention, including the best method of performing it known to me: MULTICARRIER COMMUNICATION WITH VARIABLE OVERHEAD RATE 5 CROSS-REFERENCE TO RELATED APPLICATIONS The subject application claims the priority of U.S. Provisional Patent Application Ser. No. 60/090,891, filed Jun. 26, 1998, entitled "Multicarrier Modem with Variable 10 Overhead Rate." The disclosure of said Provisional Application is incorporated herein by reference in its entirety. BACKGROUND OF THE INVENTION 15 1. FIELD OF THE INVENTION This invention relates generally to communications, and more particularly, to a multicarrier communications system and method that are able to controllably change an overhead channel data transmission rate. 20 2. BRIEF DESCRIPTION OF THE PRIOR ART The public switched telephone network (PSTN) provides the most widely available form of electronic communication for most individuals and businesses. 25 Because of its ready availability and the substantial cost of providing alternative facilities, it is increasingly being called upon to accommodate the expanding demands for transmission of substantial amounts of data at high rates. Structured originally to provide voice communication with its consequent narrow bandwidth requirements, the PSTN increasingly relies on digital systems to meet the service demand.
A major limiting factor in the ability to implement high rate digital transmission has been the subscriber loop between the telephone central office (CO) and the prem ises of the subscriber. This loop most commonly comprises a single pair of twisted wires which are well suited to carrying low-frequency voice communications for which s a bandwidth of 0-4 kHz is quite adequate, but which do not readily accommodate broadband communications (i.e., bandwidths on the order of hundreds of kilohertz or more) without adopting new techniques for communication, One approach to this problem has been the development of discrete multitone digital subscriber line (DMT DSL) technology and its variant, discrete wavelet multi 10 tone digital subscriber line (DWMT DSL) technology. These and other forms of dis crete multitone digital subscriber line technology (such as ADSL, HDSL, etc.) will commonly be referred to hereinafter generically as "DSL technology" or frequently simply as "DSL". The operation of discrete multitone systems, and their application to DSL technology, is discussed more fully in "Multicarrier Modulation For Data Trans is mission: An Idea Whose Time Has Come," IEEE Communications Magazine, May, 1990, pp. 5-14. In DSL technology, communications over the local subscriber loop between the central office and the subscriber premises is accomplished by modulating the data to be transmitted onto a multiplicity of discrete frequency carriers which are summed to 20 gether and then transmitted over the subscriber loop. Individually, the carriers form discrete, non-overlapping communication subchannels of limited bandwidth; collec tively, they form what is effectively a broadband communications channel. At the re ceiver end, the carriers are demodulated and the data recovered from them. The data symbols that are transmitted over each subehannel carry a number of 25 bits that may vary from subchannel to subchannel, dependent on the signal-to-noise ra tio (SNR ) of the subchannel. The number of bits that can accommodated under speci fied communication conditions is known as the "bit allocation" of the subchannel, and is calculated for each subchannel in a known manner as a function of the measured SNR of the subchannel and the bit error rate associated with it. 30 The SNR of the respective subchannels is determined by transmitting a refer ence signal over the various subchannels and measuring the SNR's of the received sig- -3 nals. The loading information is typically calculated at the receiving or "local" end of the subscriber line (e.g., at the subscriber premises, in the case of transmission from the central telephone office to the subscriber, and at the central office in the case of trans mission from the subscriber premises to the central office) and is communicated to the s other (transmitting or "remote") end so that each transmitter-receiver pair in communi cation with each other uses the same information for communication. The bit alloca tion information is stored at both ends of the communication pair link for use in defin ing the number of bits to be used on the respective subchannels in transmitting data to a particular receiver. Other subchannel parameters such as subchannel gains, time and 10 frequency domain equalizer coefficients, and other characteristics may also be stored to aid in defining the subchanneL Information may, of course, be transmitted in either direction over the sub scriber line. For many applications, such as the delivery of video, internet services, etc. to a subscriber, the required bandwidth from central office to subscriber is many times 15 that of the required bandwidth from subscriber to central office. One recently devel oped service providing such a capability is based on discrete multitone asymmetric digital subscriber line (DMT ADSL) technology. In one form of this service, up to two hundred and fifty six subehannels, each of 4312.5 Hz bandwidth, are devoted to down stream (from central office to subscriber premises) communications, while up to thirty 20 two subchannels, each also of 4312.5 Hz bandwidth, provide upstream (from subscriber premises to central office) communications. Communication is by way of "frames" of data and control information. In a presently-used form of ADSL communications, sixty eight data frames and one synchronization frame form a "superframe" that is repeated throughout the transmission. The data frames carry the data that is to be transmitted; 25 the synchronization or "sync" frame provides a known bit sequence that is used to syn chronize the transmitting and receiving modems and that also facilitates detennination -4 of transmission subchannel characteristics such as signal-to-noise ratio ("SNR"), among others. A DMT standard has been set for DSL transmission by the ANSI Standards body for full-rate ADSL in the publication "TI E1.4/97-007R6 Interface between net s work and customer installation asymmetric digital subscriber line (ADSL) metallic in terface," published Sept. 26, 1997 - referred to hereinafter as "T1.413 Issue 2". This standard has also been recommended as the standard modulation technique to be used for splitterless DSL operation by the Universal ADSL Working Group (UAWG) (See:, "Universal ADSL Framework Document TG/98-OR.0," published by the UAWG on 10 April 22, 1998, and referred to hereinafter as "UADSL specification"). A variation of this standardized DMT technique is also expected to be approved as a standard, termed G.Lite," by the International Telecommunications Union. According to these stan dardized DMT techniques, hundreds of 4.3125 kiloHertz (kHz) subchannels are used for DSL transmissions between a telephone company central office (CO) and remote is terminal (RT) or customer premises (at a home or business). Data are transmitted in both the downstream direction (from the CO to the RT) and the upstream direction (from the RT to the CO). According to these standards, the aggregate bandwidth (i.e. the sum of the bandwidths used in both upstream and downstream transmissions) of a full rate ADSL system is over I megaHertz (MHz), while that of G.Lite is over 500 20 kHz. A superframe is 17 milliseconds in duration. A frame is effectively 250 micro seconds in duration (or conversely, the frame rate is approximately 4 kHz) and is made up of a collection of bytes (with one byte corresponding to 8 bits).
After one DSL modem has initialized and established an active communication session with another DSL modem, the modems enter a steady state or information transmission mode. In this mode, data are transported in the upstream direction and the downstream direction at data rates that were determined during the initialization 5 process in which the session was established. During steady state mode, each frame of data transmitted/received by the modem is made up of an overhead section and a pay load section. The overhead section carries information that is used to manage the communications between the two communicating DSL modems, while the payload section contains the actual (e.g., user) data to be communicated between the modems. 10 In DSL communications that conform to the DMT communications standards whose specifications are referenced above, the first byte of each frame of data is designated as an overhead byte. The overhead section may comprise cyclic redundancy check (CRC) data, indicator bit (IB) data, embedded operations channel (EOG) data and ADSL over head channel (AOC) data. Cyclic redundancy data are used to check the integrity of the is communications link between the two DSL modems. Indicator bit data are used to in dicate certain communications error conditions that may occur during the comnunica tions session. EOC and AOC data provide information concerning the status of the communications session. The format and information provided by these portions of overhead data are described in detail in T1.413 Issue 2. (See, e.g., Sections 6.4.1.3, 8. 1, 20 10.1 and Table 3 of the T1.413 Issue 2) As described in T1.413 Issue 2, data can be transported between the conununi cating modems during a given DSL communications session either with data inter leaving or without data interleaving. If data interleaving is employed, the transported data are channeled through an "Interleave Buffer". Conversely, if transported data are -6 not interleaved, the data may be channeled through a "Fast Buffer" As noted previ ously, the first byte in each frame is an overhead data byte. When data interleaving is employed, this overhead byte is termed a "sync byte"; however, when interleaving is not employed, the overhead byte may be termed a "fast byte. Table 1 below is taken from Table 7 of the TI.413 Issue 2, and illustrates how overhead data may be distributed in frames transmitted during a conventional DSL communications session, wherein a "reduced overhead mode" of operation is em ployed. As is described in detail in Section 6.4.4.2 of the TI .413 Issue 2, in the "re duced overhead mode" of operation, the sync or fast bytes are "merged." Table 1- Overhead functions for reduced overhead mode - with merged fast and sync bytes Frame Number (Fast Buffer Only) (Inrerleaved Buffer Only) Fast Byte Format Sync Byte Format 0 Fast CRC Interleaved CRC I IBO-7IB-7 34 IB8-15 IB8-15 35 ~~IB 16-23 1362 4n+2, 4n+3 EOC - B1-2 with n=0... 16 n;*8EO 4n, 4n+1 AOC -O with n=0.. .16, n#0AO As depicted in Table I above, the first overhead byte in the first frame is used to transport CRC data. The first byte in the second frame is used to transport the first 8 indicator bits. The first byte in the 34h frame is used to transport the eighth through the fifteenth indicator bits. The first byte in the 3 5h frame is used to transport the sixteenth through the twenty-third indicator bits. The first byte in all the remaining frames alter- -7 nates between either EOC data or AOC data. However, in this conventional scheme, when actual EOC or AOC data are not available for transport, which can often occur when according to the scheme, EOC or AOC data are to be comprised in a frame, pre determined dummy bytes are used instead of unavailable actual EOC or AOC data. s Unfortunately, since one byte out of each frame in each superframe during conven tional DSL communications is dedicated to overhead data, the corresponding overhead data rate is invariably fixed at 32 kbps, and is not changed when either the payload data transmission rate changes or when no actual EOC or AOC data are available for inclu sion in the frame. Further, some telephone lines used in DSL communications are of to such poor quality that the maximum possible DSL data transmission rate using such lines may not exceed 128 kbps. Unfortunately, this means that when DSL cormunica tions are carried out over such poor quality lines, an undesirably large proportion (e.g., up to twenty-five percent) of the DSL communications system's throughput may be used to transmit overhead data. At any given time during a given communications ses 15 sion, the total communications bandwidth is constant. Thus, since the total data com munications transmission rate either upstream or downstream, as the case may be, at any given time during a DSL communications session, is constant, this means that communications bandwidth that otherwise would be available to transmit payload data is unnecessarily consumed in transmitting overhead data. 20 Objects of the Invention In general, it is an object of the present invention to provide a multicarrier communications system and method that overcome the aforesaid and/or other disad vantages and drawbacks of the prior an. and more specifically, to provide such a sys- -8 tem and method wherein the overhead data transmission rate during a communications session may be changed and/or selected. SUMMARY OF THE INVENTION Accordingly, a multicarrier communications system and method are provided s that are able to overcome the aforesaid and other disadvantages and drawbacks of the prior art. In the system and method of the present invention, the overhead data trans mission rate may be changed and/or selected. More specifically, this rate may be se lected during an initial negotiation process and/or during a steady state mode of opera tion. 10 In one embodiment, the system of the present invention may comprise two DMT DSL modems, one located at a customer premises and another located at a telephone central office, connected by a conventional POTS line through which the modems communicate by transmitting and receiving discrete frames and superframes of data, Within each superframe are 68 data frames and a synchronization symbol. Within each is frame is a number of bytes that are allocated to payload and overhead data. The allo cation of the bytes to either overhead or payload data is flexible (i.e., changeable and/or selectable). Whereas in the prior art, the first byte in each frame is dedicated to over head data regardless of whether there is a need to transport overhead data or not, in this embodiment of the present invention, the overhead data transmission rate is determined 20 during start-up and can be modified during steady state mode, Due to the construction of frames in DSL systems, decreasing the overhead data transmission rate during steady state mode results in a higher payload data transmission rate, while conversely, in creasing the overhead data transmission rate during steady state mode results in a lower payload data transmission rate.
-9 Flexible Overhead A (location As noted previously, in conventional DSL systems, one byte per frame is dedi cated to overhead data. In the improved system of this embodiment of the present in vention, both the number of bytes and the frame(s) comprising overhead data may be s selected. By selecting the number of frames that comprise overhead data, and the nurn ber of bytes allocated to overhead data in those frames, the amount of throughput that is dedicated to overhead data can modified. This is a marked departure from conventional DSL systems wherein the amount of throughput that is dedicated to overhead data is unchangeably fixed at 32 kbps. 10 Similarly, in this embodiment of the present invention, it is possible to select which of the superfrarnes are to carry overhead data-containing frames. This introduces an other degree of freedom in allocating the overhead and payload data transmission rates. Also advantageously, since the overhead data transmission rate is selectable in this embodiment of the present invention, it is possible to select that rate based upon the is relative priorities that are desired to be given to transmission of payload and overhead data, and/or whether there is a need to have a high overhead data transmission rate be cause a given application requires it (e.g., if compressed voice data is to be transported via an overhead data channel). Control commands may be exchanged between the modems during their initial 2o negotiation or handshake phase that may govem how many and which of the frames and/or superframes may contain overhead data, and the number of bytes of such data in the effected frames. These control commands may comprise respective messages whose receipt by a modem during initial negotiation may cause the modem to select from a plurality of sets of parameters, a respective set of parameters that will govern -10 how many and which frames and/or superframes will contain overhead data, the num ber of bytes of such data in the effected frames, etc. during the communications session between the modems. These sets of parameters may be stored in table form in each of the modems. and may designate which the particular bytes, frame(s), and superframe(s) s are to be dedicated to overhead data. Dynamic Overhead Data Throughput Allocation. In addition to permitting the amount of throughput devoted to overhead data to be selectable, this embodiment may also permit dynamic adjustment of that throughput during steady state operation. 10 For example, after establishing the overhead data transmission rate during startup negotiation, a new messaging process may allow renegotiating of this data transmission rate during steady state operation, as necessary. For example, a 4 kbps overhead data rate may be initially negotiated during startup, and thereafter, if a large EOC data transfer is required, a new overhead channel data transmission rate (for ex is ample 32 kbps) could be negotiated, to pennit the overhead data to be quickly trans mitted. Upon completion of that data transfer, the overhead data transmission rate may then be renegotiated, as appropriate. The dynamic renegotiations of the overhead data transmission rate during steady state operations may be effected by exchange of control commands between the 2G central office and customer premises' modems, in a manner similar to that used to ini tially negotiate that rate. These control commands may be exchanged via the overhead channels. Similarly, the commands exchanged may comprise respective messages whose receipt by a modem during renegotiations of the overhead data transfer rate may cause the modem to select from a plurality of sets of parameters, a respective set of parameters that will govern how many and which frames and/or superframes will contain overhead data, the number of bytes of such data in the effected frames. etc. during fur ther communications between the modems. These sets of parameters may be stored in table form in each of the modems and may designate which the particular bytes, 5 frame(s), and superframe(s) are to be dedicated to overhead data. The messages may comprise one or more tones, or may comprise use of a predetermined protocol over an overhead channel, that identify the particular parameter set. Once the change in overhead data transmission rate has been renegotiated, in order to effectuate further exchange of overhead data, the modems involved in the re 10 negotiation must synchronize their transmission/reception of overhead data in accor dance with newly negotiated rate. In accordance with this embodiment of the present invention, there are several alternative techniques by which this synchronization may be accomplished. In the first such technique, the central office modem may keep an internal count of the frames/superframes that have been transmitted from that modem to is the customer premises modem with which it has been communicating, and the cus tomer premises modem may likewise keep an internal count of the frames/superframes that it has received from the central office modem. A message may be passed from one of the modems to the other modem that contains a frame/superfiame count value at which the two modems are to adjust their overhead data transmit/receive rates in accor 20 dance with the newly negotiated rate. Each modem then adjusts its overhead data transmit/receive rate when its respective internal frame/superframe count reaches that value. Alternatively, one of the modems may transmit to the other modem a flag mes sage indicating that, when the other modem transmits to the modem sending the flag -12 message a specified subsequent superframe (e.g., the next superframe), the overhead data transmission/reception rates are to be adjusted in accordance with the newly nego tiated rate. Upon transmission of that specified superframe, the modem that transmitted the superframe adjusts to the newly negotiated rate; likewise, upon receipt of the speci 5 fied superframe, the modem receiving that superframe adjusts to the newly negotiated rate. Of course, it will be appreciated that a request to renegotiate the overhead data transmission rate can originate from either the modem at the central office or from the modem at the customer site. Further, that request may be initiated by either the trans io mit block or the receive block in the modem initiating the request. These and other features and advantages of the present invention will become apparent as the following Detailed Description proceeds and upon reference to the Fig ures of the Drawings, wherein: BRIEF DESCRIPTION OF THE DRAWINGS 15 Figure 1 is a schematic diagram of a DSL system wherein the present invention may be advantageously employed. Figure 2 illustrates a conventional data superframe format. Figure 3 is a flowchart of a conventional process for generating a data frame. Figure 4 is a flowchart of an embodiment of a process according to the present 20 invention for generating a data frame.
-13 Figure 5 is a flowchart illustrating an embodiment of a process according to the present invention for renegotiating overhead data transmission rate during a steady state mode of operation. Although the following Detailed Description will proceed with reference being 5 made to specific embodiments and methods of use, it is to be understood that the pres ent invention is not intended to be limited to these embodiments and methods of use. Rather, as will be appreciated by those skilled in the art, many alternatives, modifica tions, and variations thereof are possible without departing from present invention. Ac: cordingly, it is intended that the present invention be viewed broadly as encompassing to all such alternatives, modifications, and variations as are within the spirit and broad scope of the hereinafter appended claims. DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Figure I shows a DSL communications system wherein the present invention may be advantageously used. As shown in Figure 1, a telephone central office ("CO") 15 10 is connected to a remote subscriber 12 ("CP: Customer Premises") by a subscriber line or loop 14. Typically, the subscriber line 14 comprises a pair of twisted copper wires; this has been the traditional medium for carrying voice communications between a telephone subscriber or customer and the central office. Designed to carry voice communications in a bandwidth of approximately 4 kHz (kilohertz), its use has been 20 greatly extended by DSL technology. The central office is, in turn, connected to a digital data network ("DDN") 16 for sending and receiving digital data, as well as to a public switched telephone net work ("PSTN") 18 for sending and receiving voice and other low frequency communi cations. The digital data network is connected to the central office through a digital - 14 subscriber line access multiplexer ("DSLAM") 20, while the switched telephone net work is connected to the central office through a local switch bank 22, The DSLAM 20 (or its equivalent, such as a data enabled switch line card) connects to a POTS "splitter" 24 through an ADSL transceiver unit -central office ("ATU-C") 26. The local switch s 20 also connects to the splitter. The splitter 24 separates data and voice ("POTS") signals received from the line 14. At the subscriber end of line 14, a splitter 30 performs the same function. In par ticular, the splitter 30 passes the POTS signals from line 14 to the appropriate devices such as telephone handsets 31, 32, and passes the digital data signals to an ADSL trans 10 ceiver unit-subscriber ("ATU-R") 34 for application to data utilization devices such as a personal computer ("PC") 36 and the like. The transceiver 34 may advantageously be incorporated as a card in the PC itself; similarly, the transceiver 26 is commonly im plemented as a line card in the multiplexer 20. In this approach, a communication channel of a given bandwidth is divided into is a multiplicity of subchannels, each a fraction of the subchannel bandwidth. Data to be transmitted from one transceiver to another is modulated onto each subchannel in ac cordance with the information-carrying capacity of the particular subchanneL Because of differing signal-to-noise ("SNR") characteristics of the subchannels, the amount of data loaded onto a subchannel may differ from subchannel to subchannel. Accordingly, 20 a "bit allocation table" is maintained at each transceiver to define the number of bits that each will transmit on each subchannel to the receiver to which it is connected. These tables are created during an initialization process in which test signals are trans mitted by each transceiver to the other and the signals received at the respective trans ceivers are measured in order to determine the maximum number of bits that can be 25 transmitted from one transceiver to the other on the particular line. The bit allocation table determined by a particular transceiver is then transmitted over the digital sub scriber line 14 to the other transceiver for use by the other transceiver in transmitting data to that particular transceiver or to any similar transceiver connected to the line 14, The transmission must, of course, be done at a time when the line is not subject to dis 30 turbances which may interfere with communications.
-15 It should be appreciated that although the system 1 has been shown as com prising splitters 24, 30, if appropriately modified as described in detail in copending PCT Application Serial No. PCT/US98/21442, entitled "Splitterless Multicarrier Mo dem," filed October 9, 1998, which is commonly owned by the owner of the subject s application, Aware, Inc. of Bedford, Massachusetts, U.S.A., splitters 24, 30 may in stead be wholly eliminated from system 1. The entirety of the disclosure of said co pending PCT application is incorporated herein by reference. Also. although not shown in the Figures, it should be understood that each of the transceivers or modems 26, 34 comprises a respective processor, read only and ran ]O dom access memories, and transmitter and receiver circuitry blocks which are intercon-. nected via conventional bus circuitry, and are operable to permit the transceivers 26, 34 to carry out the DSL communications processes and the various other processes ac cording to the present invention described herein. The read only and random access memory of these modems 26, 34 may store program code instructions which are ex iS ecutable by the modems' processors, and when executed by the processors, cause the modems to carry out these processes. Figure 2 shows the format of a conventional DSL data superframe 100. Super frame 100 is made up of sixty eight frames, the first frame 102 in each superframe is designated as frame 0, while each subsequent frame (collectively referred to by number 20 104) therein up to the sixty-seventh frame is assigned a number that corresponds to its ordinal sequence in the superframe (i.e., Frame 1, Frame 2 . .. Frame 67). Each super frame is terminated with a synchronization symbol 110 Each frame 102, 104, in turn, has structure 105. In frame structure 105, the first byte 107 is either the sync or fast byte, depending upon whether interleaving is being 25 employed, or not, respectively. The remaining bytes 108 in the frame structure 105 are either interleaved data bytes or fast data bytes, depending upon whether interleaving is employed, or not employed, respectively.
-16 Figure 3 depicts a flowchart of a conventional process 71 which heretofore as been used to determine allocation of overhead and payload bytes in each frame struc ture 105 to be generated for transmission. That is, prior to the present invention, the process 71 has been used by conventional DSL transceivers when generating frames for 5 transmission. Process 71 then begins by initializing a byte counter k to the value of k= I (step 70). Thereafter, the counter is incremented by 1 (step 75), and the incremented counter value is compared to zero (step 80). If the incremented counter value equals zero, then an overhead data byte is generated and inserted into the frame (step 90). -The type of overhead data byte that is generated at step 90 is determined in accordance with to the information presented Table 1, described previously. The incremented counter value is then compared to the number of bytes (kmax) to be included in the frame to be generated minus unity (step 95), and if the incremented counter value is equal to kmax minus unity, then the process flow loops back to step 70. Alternatively, if the incre mented counter value does not equal kmax minus unity, then the process flow loops is back to step 75. Conversely, if at step 80, the incremented counter value k is not zero, then a payload data byte is generated and concatenated with the last byte previously inserted into the frame. Thereafter, the process 71 continues to step 95. Step 95 determines whether the frame is full, i.e. whether all the bytes kmax that are to be transported in 20 the frame concatenated to the frame. As noted previously, using this prior art frame construction process 71 ensures a static overhead data transmission rate in a conventional DSL communications system. In accordance with one embodiment the present invention, system I implements nego- -17 tiation and frame generation techniques that permit the overhead data transmission rate in system 1 to be dynamically adjustable. New Overhead Allocation Table In accordance with the present invention, the value of a new variable "nma" is s negotiated by the transceivers 26, 34 at initialization and/or at steady state. By appro priately negotiating this value, the EOC/AOC channel can be programmed to have a transmission rate that is between a minimum of approximately 2 kbps and a maximum of approximately 30 kbps. The manner in which the value selected for nm. affects the overhead data transmission rates, and the frames in which overhead data exists, is 10 summarized in Table 2 infra. For example, by selecting nin, to be less than 16, more of the throughput of system I can be allocated to payload data when the EOC/AOC requirements are lim ited. For example, if nm.i is selected to be 2, then frames having numbers 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 34, 35 in each superframe will have a first byte that is an over is head byte. The remaining 54 frames (of the 68 total frames) in a superframe will not have an overhead byte as the first byte in the frame. The total overhead data rate (based upon all EOC/AOC, CRC and indicator bit data) therefore is reduced from 32 kbps to approximately 6.5 kbps. Table 2- Modified Overhead functions for reduced overhead mode - with 20 merged fast and sync bytes Frame Number (Fast Buffer Oniy) (Interleave Buffer Only) Fast Byte Format Sync Byte Format 0 Fast CRC Interleaved CRC I IB0-7 130-7 34 1B8-15 IB8-15 35 IB16-23 Ib16-23 4n+2. 4n+3 EOC or sync EOC or sync - 18 with n=0-.. , nm n*8 4n, 4n+1 AOC AOC with n=O.. nm,n 0 Figure 4 is a flowchart of a process 190 for constructing the portion 108 of each frame transmitted in system 1. That is, each transceiver 26, 34 executes the process s 190 when constructing a frame to be transmitted to the other transceiver 34, 26, respec tively, in system I. The value of n. is first negotiated (according to a process 193 that is described more fully below) at step 200. Thereafter, a frame counter L is ini tialized to -I and incremented by one (steps 210 and 215), and byte counter k is ini tialized to -1 and incremented by one (steps 220 and 230). The counter L is then com to pared to the Li values defined in block 241. If L is equal to one of the Li values as shown in block 241, then it is also determined whether the byte counter k is equal to 0 (step 260). If so, then an overhead byte is generated and inserted in the frame (step 270). The content of the overhead byte is determined as set forth in Table 2. If k is not equal to zero at step 260 or if L hot equal to one of the Li values at block 240, then a is payload data byte is inserted in the frame (step 250). From steps 250 or 270, the proc ess 190 continues to block 280, wherein it is determined whether the end of the frame has been reached by checking whether the byte counter k is equal to kmax minus 1. If kmax is not equal to kmax minus 1, the process 190 flow returns to step 230, and byte counter k is incremented in step 230 and the steps from 240 on are repeated. If k is 20 equal to k minus 1, then the frame counter L is evaluated to determine whether it is equal to 67, which is the maximum value permitted for the frame counter as there are 68 data frames in a superframe (step 290). If the frame counter L has not reached this -19 maximum value, the process 190 loops back to step 215. Conversely, if L is equal to this maximum value, the process 190 branches to step 300 where superfrane counter (which was initiated via a separate process step not shown) is incremented by one, and thereafter, it is determined whether the value of nm, is to be changed (step 310). If the s value of nm. is to be changed, then the negotiation process of step 200 is executed. Conversely, if the value of n,, is not to changed, then the process 190 continues to step 210 and the superframe counter is incremented by one, and the frame and byte counters are reset (steps 210, 220). The process of Figure 4 is a preferred because it allows flexibility in the over 10 head data rate with the introduction of a single new parameter, nma. Techniques that involve more parameters, and hence would be more complicated to implement (and would require more modifications to the existing DSL specifications), are also possible, without departing from the present invention. In this illustrative embodiment, the parameter, masuffices to give a significant is amount of flexibility in the overhead data transmission rate. By incrementing the nma parameter by integer increments, overhead data rates can be incremented in approxi mately 2 kbps steps. As overhead data rates are decreased, payload data rates increase and vice versa. Also, in this illustrative embodiment 1, a new EOC command is defined to al 20 low the EOC/AOC channel data rate to be increased after startup, thereby permitting the frame structure to be changed "on-the-fly" during on-going communications be tween the modems 26, 34. This EOC command causes renegotiation of the nm pa rameter from the value to which it was originally set during initial negotiation in order -20 to increase or decrease the EOC/AOC channel data rates. The format of this command may vary depending upon the particular manner in which the system 1 is constructed, With reference now being made to Figure 5, a process 193 that is implemented by the transceivers 26, 34 to negotiate the value nm, that is to be used by the transceiv s ers 26, 34 in constructing frames to be transmitted to each other is described. For pur poses of this negotiation, it is assumed that the transceivers 26, 34 in system 1 have al ready entered steady state mode prior to beginning process 193. The transmit circuitry block TX comprised within either or both of the transceivers 26, 34 notifies the receive circuitry RX block comprised in the other transceiver 34, 26, respectively that the 10 transceiver 26, 34 comprising the block TX desires to change the overhead data rate (step 301). The RX block in transceiver 34, 26, respectively, detects the change (step 311) and grants the request by sending a message to the TX block of the transceiver 26, 34 (step 330). Thereafter, in step 320, the TX block of transceiver 26, 34 detects the grant of the change request message and sends another message (step 340) to the RX is block of transceiver 34, 26 defining the new nm. that is to be used in communications between the transceivers 26, 34. This new nm. may be one of a collection of nm .op tions that are already stored in the TX and RX of the transceivers, and the message may therefore be a signal for choosing one of the options in the collection. However, in this embodiment, when the RX block receives the new nm., the new n. value has effec 20 tively been negotiated between the transceivers (step 350). In this embodiment, the new nm. is used at the beginning of the next superframe by the two transceivers 26, 34. The steps shown in Figure 4 are then followed to construct that superframe. Of course, the process 193 may also be used during initial startup of the communications session to negotiate the nm. value that is to be used by modems 26, 34.
-21- As described previously, there are numerous other ways in which this negotia tion/renegotiation of the value of n.. may be implemented. As has also been de scribed previously, the adjusting of the overhead data transfer rates in system 1 to the new nma, value need not occur on the next superframe boundary. It could change in s accordance with another message transmitted between the transceivers 26, 34, or it could change only when a counter value of transmitted superframes reaches certain values. It is also possible to establish and exchange many more parameters than just n,,,, and to thus enable a more flexible, albeit more complicated, system. The proposed framing mode will allow low overhead and high payload effi 1o ciency in G.lite systems while maintaining the ability to "open" a high bandwidth clear channel EOC if necessary. On the other hand, if a G.lite system does not require high data rate EOC channel it can simply negotiate it down to less than 2 kbps. This pro posal is simple in the sense that the flexibility is achieved by simply negotiating one variable (n,,.). Is One application that requires dynamic overhead allocation is the transport of compressed digital voice over the eoc/aoc data in a DSL system. Although DSL sys tems operate without interrupting POTS on the telephone line on which the DSL serv ice is being offered, the use of the DSL data for transporting virtual second line (or third line, etc.) voice over a single phone line is attractive. This digitized voice traffic 20 is likely to be compressed using any of the conventional industry speech compression techniques to bring the data rate of the compressed speech to below 24 kbps. The EOC/AOC could be "opened up" when a second line voice channel is needed and used to transport the digital voice traffic (as EOC/AOC bytes in the overhead data) by using the on-demand technique described above. During this time, the DSL payload data rate -22 would be decreased. Upon completion of the voice data transmission, the EOC/AOC data rate may be renegotiated so as to be lower, using the adaptive technique described above and the DSL payload data could go back to its higher rate. The framing method described in accordance with this embodiment of the pres 5 ent invention allows the EOC/AOC channel to be programmed to be within a range between a minimum of approximately 2 kbps and a maximum of approximately 30 kbps, at approximately 2 kbps granularity. The EOC channel data rate can be further increased or decreased by making additional changes to the framing structure. For ex ample, the maximum EOC/AOC overhead data rate could be increased by allowing to more that 1 sync byte (or fast byte) per frame. In this case a new variable, "K", that dictates the number of sync bytes (or fast bytes) per frame may be defined, and negoti ated by the transceivers at startup and/or during steady state mode. In the case shown in Table 1, K=1, since there is always one EOC/AOC byte per frame. But if K is made to be K=2, then the data rate of the EOC/AOC channel could be doubled, thus allowing a is maximum of 60 kbps. With this technique (and also as K is increased to even higher values), the EOC/AOC channel may be increased to utilize all the bandwidth available on the channel if necessary (i.e., no bytes allocated to payload). This could be benefi cial if the EOC/AOC channel is intended to be used for lengthy diagnostics tests or modem firmware upgrades during periods of the day or night when the user was not 20 running applications over the modem connection. Likewise the EOC/AOC channel minimum channel data rate can be further de creased by changing the framing format to allow the EOC/AOC bytes to be allocated to only certain superframes. With this framing format, a superframe counter defined as an 8 bit counter (modulo 256), is employed. The counter therefore counts from 0 to 255 as -23 superframes are transmitted (or received) and thereafter, recommences counting at 0 again. A new variable Smax, may also be used to dictate how many of the 256 super fames contain EOC/AOC data. For example if Smax=8, then the first 8 superframes of the 256 superframes counted contain EOC/AOC data. The remaining 248 superframes S would contain payload bytes in the place of EOC/AOC bytes in each frame. In this case, the EOC/AOC channel data rate is decrease by a factor of 8/256 (i.e., by a factor of 0.03125). In general with method, the EOC/AOC channel minimum data rate can be decreased to (2 kbps)/256= 0.0078 kbps in the case of a module 256 counter, and may be further decreased by employing larger modulo counters. 10 In both of the methods described above for further increasing or decreasing the EOC/AOC channel rates, the additional variables, "K" and Smax, may be negotiated by the modems 26, 34 during initialization and/or steady state operation. While the invention has been disclosed in connection with the preferred em bodiments and methods of use, it is to be understood that many alternatives, modifica 15 tons, and variations thereof are possible without departing from the present invention. Thus, the present invention is intended to embrace all such alternatives, modifications, and variations as may be apparent to those skilled in the art and encompassed within the hereinafter appended claims. Throughout this specification and the claims which follow, unless the context 20 requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge in Australia.