Multicarrier transmission system with low power sleep mode and rapid-on capability

[0001] The invention relates to multicarrier transmission systems, and comprises method and apparatus for establishing a power management sleep state in a multicarrier system.

[0002] This application is based on U.S. Provisional Application Ser. No. ______, filed Jan. 26, 1998 by John Greszczuck, Rick Gross and Halil Padir and claims the benefit thereof.

[0003] Multicarrier transmission systems provide high speed data links between communication points. Such systems have recently been introduced for communications over the local subscriber loop that connects a telephone service subscriber to a central telephone office; in this important application they are commonly referred to as xDSL systems, where the x specifies a particular variant of DSL (digital subscriber loop) communications, e.g., ADSL (asynchronous digital subscriber loop), HDSL (High-Speed Digital Subscriber Loop), etc. These will be referred to generically herein simply as DSL systems.

[0004] In such systems, a pair of transceivers communicate with other by dividing the overall bandwidth of the channel interconnecting the subscriber and the central office into a large number of separate subchannels, each of limited bandwidth, operating in parallel with each other. For example, one common system divides the subscriber line channel into two hundred and fifty six subchannels, each of 4.3 kilohertz bandwidth. A first group of these (e.g., one hundred ninety six) is allocated to communications from the central office to the subscriber (this is known as the downstream direction); a second group (e.g., thirty-two) is allocated to communications from the subscriber to the central office (this is known as the upstream direction). The remaining subchannels are allocated to administrative, overhead and control (AOC) functions.

[0005] Data to be communicated over the link is divided into groups of bits, one group for each subchannel. The group of bits allocated to a given subchannel is modulated onto a carrier whose frequency is specific to that channel. Typically, quadrature amplitude modulation (QAM) is used for this purpose, and the group of bits is mapped into a vector defined by one of the points of a constellation which specifies the allowable data points for transmission over that subchannel at a particular time. Each vector or data point thus comprises a unique symbol representing a specific bit configuration for transmission as a group over its associated subchannel. During the time period allocated for transmission of a symbol (commonly referred to as a symbol period or frame), each subchannel transmits its symbol in parallel with all other subchannels so that large amounts of data can be transmitted during each frame.

[0006] The number of bits carried by a symbol is dependent on the characteristics of the subchannel over which it is to be transmitted. This may vary from one subchannel to another. The principal determinant is the signal-to-noise ratio of the subchannel. Accordingly, this parameter is measured from time to time in order to ascertain its value for each subchannel, and thus determine the number of bits to be transmitted on the particular subchannel at a given time.

[0007] The telephone channel is subject to a number of impairments which must be compensated for in order to ensure reliable transmission. Phase (delay) distortion of the transmitted signal is typically the most limiting of these impairments. This distortion is frequency-dependent, and thus components of a signal at different frequencies are shifted by varying amounts, thereby distorting the signal and increasing the likelihood of erroneous detection unless provision is made to combat it. To this end, frequency domain equalizers (FDQ) and time domain equalizers (TDQ). are commonly incorporated into the transmission channel in order to equalize the phase (time) delay across the channel frequency band. Other impairments also exist. For example, frequency-dependent signal attenuation adversely affects signal transmission on the telephone line. This is compensated by the use of gain equalizers on the line, while echo on the line is handled by the use of echo cancellers.

[0008] The problem of signal impairment is especially serious in those xDSL configurations which carry the DSL communications on a common line with ordinary voice communications but which omit the use of a splitter at either the subscriber premises or the central office or both. A splitter is basically a filter which separates the low-frequency voice communications (e.g., from zero to four kilohertz) from the higher-frequency data communications (which may extend up into the megahertz band) and provides a strong degree of isolation between the two. In the absence of a splitter, unique provisions must be made to accommodate voice and data communications on the same line. For a more detailed description of the problem and its solution, see the co-pending application of Richard Gross et al. entitled Splitterless Multicarrier Modem, Serial No. PCT/US98 21442, filed Oct. 9, 1998, and assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference.

[0009] Because of their extensive use in Internet communications as well as in other applications, DSL transceivers are commonly maintained in the on state, ready to transmit or receive once they have been installed and initialized. Thus, such modems consume a significant amount of power, even when they are not actively transmitting or receiving data It is generally desirable to limit this power consumption, both for environmental reasons as well as to prolong the life of the equipment. Further, such modems may be implemented or incorporated in part or in whole in computer equipment such as in personal computers for home and business use, and such computers increasingly incorporate power conservation procedures. See, for example, U.S. Pat. No. 5,428,790, Computer Power Management System, issued Jun. 27, 1995 on the application of L. D. Harper. Thus, it is desirable to provide an ADSL modem which can accommodate power conservation procedures in equipment with which it is associated, as well as independently of such equipment as may be appropriate.

[0010] Because of the complexity of DSL transceivers, and the conditions under which they must operate, it is necessary to initialize them prior to the transmission and reception of data. This initialization includes, inter alia, channel corrections such as training the frequency-domain and time-domain equalizers and the echo cancellers; setting the channel gains; negotiating the transmission and reception data rates; adjusting the fine gains on the subcahnnels over which communication is to take place; setting the coding parameters; and the like. Additionally, it includes measuring the signal-to-noise ratio of each of the subchannels, calculating the bit-allocation tables characteristic of each under given conditions of transmission, and exchanging these tables with other modems with a given modem communicates. For more detailed discussion of these procedures, refer to the application of Richard Gross et al., cited above and incorporated herein by reference. These procedures can require from seconds to tens of seconds. In a new installation, the time required is inconsequential. However, in an already-operating installation, the time required to initialize or re-initialize the system after a suspension of operation in connection with power conservation is generally unacceptable, since it is typically desired to have the modem respond to request for service nearly instantaneously.

[0011] Accordingly, it is an object of the invention to provide a multicarrier transmission system having a low power sleep mode and a rapid-on capability.

[0012] Further, it is an object of the invention to provide a multicarrier transmission system for use in digital subscriber line communications that can rapidly switch from a sleep mode to a full-on condition.

[0013] Still another object of the invention is to provide a DSL system that can readily be integrated into a computer having a low power sleep mode and which is capable of rapid return to full operation.

[0014] The invention description below refers to the accompanying drawings, of which:

[0015] FIG. 1 is a block and line diagram of a multicarrier transmission system in accordance with a preferred embodiment of the present invention;

[0016] FIG. 1A is a portion of an exemplary chart showing a possible bit distribution among subchannels;

[0017] FIG. 1B illustrates a timing signal used in accordance with the invention;

[0018] FIG. 2 is a flow diagram of the operation of the present invention; and

[0019] FIG. 3 is a block and line diagram of still another aspect of the present invention.

[0020] For purposes of explanation, the present invention will be described in the context of an ADSL system having a first transceiver located at the site of a customer's premises (referred to hereinafter as the CPE transceiver) and a second transceiver located at a local central telephone office (hereinafter referred to as the CO transceiver). The two are interconnected for communication by means of a common telephone line over which voice and data are to be transmitted, and the CO transceiver is commonly connected into a broader network such as the Internet to and from which data is to be communicated. The system will be described as using Fourier transform technology for modulation and demodulation of the data to be transmitted. It will be understood, however, that the invention is not limited to this environment, and is applicable to point-to-point communications is other environments, and with other forms of modulation/demodulation. Further, since the CPE transceiver and CO transceiver are very similar, the invention will be explained in connection with a detailed illustration of the CPE transceiver only.

[0021] In FIG. 1, a DSL transceiver 10 in accordance with the present invention has a transmitter section 12 for transmitting data over a digital subscriber line 14 and a receiver section 16 for receiving data from the line. The transmitter section 12 is formed from an input buffer and converter (IBC) 18 that receives a serial string of data (e.g., binary digits) b_i to be transmitted and converts the data into a plurality of pairs of complex-valued symbols X_i and their conjugates X_NiX*_i, i0, 1, . . . N. Typically, the buffer 18 holds at least a frame of data (a frame comprising the amount of data to be transmitted during one symbol period). The pairs of symbols X_i and X*_i are applied to an Inverse Fast Fourier Transform (IFFT) 20 to provide real time output signals x_j, j0, 1 . . . N/21. The latter in turn are converted to serial form in a parallel-to-serial converter (PSC) 22 and then applied to a digital-to-analog converter (DAC) 24 for application to a line driver 26. The converter 24 may apply a cyclic prefix to the signals x_j to combat intersymbol interference caused by the transmission medium. The driver 26 may incorporate a gain control section (GC) 26a for controlling the signal amplitude (and thus power) as it is applied to a communication channel such as the digital subscriber line 14.

[0022] IFFT 20 may be viewed as a data modulator. The symbols X_i, and their conjugates X_Ni, correspond to data points defining signal vectors in a quadrature amplitude modulation (QAM) constellation set. The converter 18 forms the respective symbols from the input data with the aid of a bit allocation table (BAT) 28 which specifies, for each subchannel, the number of bits to be carried by the symbol transmitted over that subchannel, and thus defines the data point to be associated with the symbol. This table is typically calculated at the transceiver and is transmitted to other transceivers with which the instant transceiver communicates, to thereby enable them to decode the symbols received by them from the instant transceiver.

[0023] The number of bits which each symbol carries is determined by the characteristics of the subchannel over which the symbol is to be transmitted, and particularly by the signal-to-noise ratio of the subchannel. Procedures for this calculation are known. FIG. 1A shows an example of such a table as formed and stored at transceiver 10. Thus, the symbol to be transmitted over subchannel 50 may be determined to have an allocation of six bits; that of subchannel 51, six bits; that of subchannel 52, seven bits, etc.

[0024] A Clock 30 controls the timing of the operation of the transmitter 12. It supplies input to a Controller 32 which controls the individual units of the transmitter. In the case of the CO transceiver, the clock 30 typically is a master clock to which a remote transceiver, such as at a subscriber premises, will be synchronized. In the case of a transceiver at the subscriber premises, such as is shown here for purposes of illustration, the clock is derived from the master clock at the central office as described more fully below in connection with the receiver portion of the transceiver.

[0025] A Frame Counter (FC) 34 connected to the controller 32 maintains a count of the number of frames of data transmitted from or received by the transceiver 10. The clock 30 maintains the count in counter 34 synchronous with that of a corresponding counter (not shown) in the CO transceiver. In DSL systems, typically, data is communicated in the form of a sequence of data frames (e.g., sixty-eight frames for ADSL as specified in ITU Document G.992.2), followed by a synchronization frame, each frame having a duration of one symbol period of approximately two hundred and fifty microseconds. Together, the sixty-nine frames comprise a superframe. Thus, the counter 34 typically maintains a count modulo sixty-nine. Finally, a State Memory (SM) 36 connected to the controller 32 records the state of the transceiver for reasons discussed more fully below.

[0026] Turning now to the receiver section 16, it is formed from a line conditioner 50; an analog-to-digital converter (ADC) 52; a serial-to-parallel converter 54; a Fast Fourier Transform (FFT) section 56; a decoder 58; and a parallel-to-serial converter 60. The conditioner 50 compensates for transmission distortions introduced by the line 14, and commonly includes a frequency-domain equalizer (FDQ) 50a; a time-domain equalizer (TDQ) 50b; and an echo canceller (EC) 50c, among other elements. The ADC 52 converts the received signal to digital form and applies it to the serial-to-parallel converter 54. The converter 54 removes any cyclic prefix that may have been appended to the signal before it was transmitted, and applies the resultant signal to the FFT 56 which effectively demodulates the received signal. The output of the FFT is applied to decoder 58 which, in conjunction with a bit-allocation-table 62, recovers the symbols X_i and X_i* and the bits associated with them. The output of detector 58 is applied to the parallel-to-serial converter 60 which restores the data stream, b_i, that was originally applied to the transmitter. The controller 32 also controls the operation of the receiver portion 16 of the transceiver 10.

[0027] During normal (non-sleep mode) operation, a phase-lock loop (PLL) 62 receives from the FFT 56 a timing reference signal 62a (see FIG. 1A) via a line 62b. The timing reference signal 62a is transmitted from the transmitter with which the receiver 16 communicates (e.g., the CO transmitter). This signal is advantageously a pure tone of fixed frequency and phase which is synchronized with the Master Clock in the transmitter; its frequency defines the frame rate of the transceivers. Other forms of timing signal may, of course, be used, but use of a pure tone has the advantage of simplicity and reliability even when portions of the transceiver are powered down in accordance with the invention. The PLL 62 locks itself to this signal and drives clock 30 in synchronism with the Master Clock in the driving transmitter. This also synchronizes frame counter 34 of the CPE transceiver to the corresponding frame counter of the CO transceiver. Control of the receiver section is provided by the controller 32.

[0028] In the sleep mode, the FFT 56 is preferably dormant. Accordingly, the timing reference signal for PLL 62 is provided from the output of the analog to digital converter 52 via a detector 64 which extracts the timing signal from the signal appearing on line 14 during sleep mode. by calculating the DFT of the synchronizing pilot tone. Controller 32 controls the switching of the input to PLL 62 between these two sources so that the PLL remains locked to the CO transceiver timing reference.

[0029] As noted earlier, the transceiver of the present invention will commonly be incorporated in a computer such as a personal computer; indeed, it may be implemented as an integral part of such a computer, which may have a power conservation capability for activation when the computer is not in active operation. It is thus desirable that the transceiver be able to suspend operations and enter a sleep mode in which it consumes reduced power when it is not needed for data transmission or reception, but nonetheless be able to resume transmission or reception almost instantaneously, e.g., within a few frames.

[0030] Further, when the ADSL transceiver is implemented as an integral part of a computer, it may often be the case that the processing power of the computer is, at a given moment, devoted to another task such as graphics, word processing, and the like, and is thus unable to service the transceiver. In such circumstances, it is possible that a frame that is assembled for transmission to the CO transceiver; and thence to some network connected to it, is incomplete, and thus would generate errors if transmitted. Accordingly, the transceiver of the present invention is responsive to such conditions by entering an idle state in which it ceases active transmission of data while the computer is elsewhere occupied. This state is similar in many ways to the sleep mode state although, of course, its purpose is not power conservation, and thus in the idle state the power to selected portions of the transceiver may, but need not, be reduced. The idle state maintains synchronous signaling between the CPE and CO transceivers but no data is transmitted. On receipt of the idle indicator from the CPE, the CO transceiver transmits idle cells to the network to maintain connection with the network.

[0031] Referring now more particularly to FIG. 2, the power down operation of the CPE transceiver begins on receipt of a power down indication (step 80) by the CPE transceiver controller 32. The power down indication may be applied to the controller 32 from an external source such as a personal computer in which the transceiver is included; it may be generated within the transceiver itself as a result of monitoring the input buffer 18 and determining that no data has been applied to it for a given time interval or that the buffer has not been filled despite passage of a symbol time; it may be responsive to a power down command from the CO transceiver; or it may be generated in response to other conditions.

[0032] Considering for the moment the first two cases, the CPE transceiver responds to the indication by transmitting to the CO transceiver an Intend To Enter Sleep Mode notification (step 82). This notification indicates that the transceiver is about to undergo a change of state, and may take any of a variety of forms; preferably it is a message, but may also comprise a tone, an inverted sync signal, a flag, or even the cessation of data transmission itself. The notification may advantageously be transmitted over one of the embedded operations channels of the system. It provides the CO transceiver an opportunity to prepare itself to enter sleep mode, and also to signal the CPE transceiver if entrance into sleep mode is not appropriate or desirable at the particular moment.

[0033] If entrance into sleep mode is permissible at this time, the CO transceiver responds to the power down or idle signal by transmitting an Acknowledge Sleep Mode notification (step 84) to the CPE transceiver. This and subsequent notifications described in connection with the sleep or idle mode may similarly take any of a variety of forms such as described above for the Intend To Enter Sleep Mode notification, but again preferably is in the form of a message transmitted over an embedded operations channel.

[0034] After it has received acknowledgment from the CO transceiver, the CPE transceiver transmits an Entering Sleep Mode notification (step 86) to the CO transceiver and ceases transmission, either immediately or after a given number of frames. The CO transceiver detects this notification; transmits its own Entering Sleep Mode notification (step 88); and enters sleep mode (step 90). In pursuance of this, the CO transceiver stores its state in its own state memory corresponding to the state memory 38 of CPE transceiver 10. The state of the CO or CPE transceivers preferably includes at least the frequency and time-domain equalizer coefficients (FDQ; TDQ) and the echo-canceller coefficients (ECC) of its receiver portion and the gain of its transmitter portion; the transmission and reception data rates; the transmission and reception coding parameters; the transmission fine gains; and the Bit Allocation Tables. The CO transceiver continues to advance the frame count and superframe count during the period of power-down in order to ensure synchrony with the remote CPE transceiver when communications are resumed. In order to maintain synchronization during the power down or idle state, the CO transceiver continues to transmit to the CPE transceiver the synchronizing pilot tone 62a. It may, at this time, perform its own power reduction. In particular, it may reduce or cut off power to the digital modulator/demodulator portions of its transmitter and receiver sections (corresponding to the IFFT 20 and FFT 56 of the CPE transceiver, FIG. 1); this provides a significant power reduction. Further, it may reduce power to parts of the analog circuitry. Power will be maintained, of course, to at least that portion of the analog driver circuitry which transmits the pilot tone and other control signals to the CPE transceiver, and to line circuits required to monitor the line 14 for signals from the CPE transceiver.

[0035] In a central office, the rest of the equipement is still operating, and while the link between the CO transceiver and the CPE transceiver is in a sleep state, user data provided by the CO transceiver will be benign idle data such as ATM Idle Cells or HDLC Flag octets, and not generate false data errors to other equipment.

[0036] In response to the Entering Sleep Mode notification from the CO transceiver, the CPE transceiver enters the sleep mode (step 92). In particular, it stores its state (step 94) in state memory 38; as noted above in connection with the CO transceiver, this includes preferably at least the frequency and time-domain equalizer coefficients (FDQ; TDQ) and the echo-canceller coefficients (ECC) of its receiver and the gain of its transmitter; the transmission and reception data rates; the transmission and reception coding parameters; the transmission fine gains; and the Bit Allocation Tables. The phase and frequency offset of the phase-locked loop 62 is maintained by continued opertion of the loop. The CPE transceiver 10 then reduces power to the digital modulator/demodulator circuitry comprising IFFT 20 and FFT 56, as well as to and transmitter data line drivers 26. However, it continues to advance the frame counter 34 in accordance with the received synchronizing signal 62a. However, the CPE controller 32 now causes this signal to be applied to the PLL 62 from the output of the ADC 52 (FIG. 1) via the detector 64, which implements the DFT of a single tone, instead of directly from the output of the FFT 56 as was previously the case. This enables the FFT 56 to be powered down. The CPE and CO transceivers then operate in sleep mode (steps 95 and 97, respectivley) until they awaken.

[0037] During the sleep mode state, the CO transceiver continues to monitor (step 90) the data subscriber line 14 for an Exiting Sleep Mode signal from the CPE transceiver (step 96). The CPE transceiver transmits this signal when its controller receives an Awaken indication (step 98) from an external source such as a computer in which it is installed or from other sources, or when its controller detects the presence of new data in the input buffer 18. In response to the Awaken signal, the CPE transceiver retrieves its stored state from the state memory 38; restores full power to its circuitry; and restores the output of the FFT 56 to the input of the PLL 62 (step 96). The CO transceiver, on detecting the Exit Sleep Mode notification from the CPE transceiver (step 98), thereupon exits sleep mode by restoring its state and restoring power. On waking up from sleep mode, the CPE transceiver can begin transmitting immediately or after only a few frames delay, since it need not repeat the initialization that was earlier required to establish the requisite parameters (e.g., frequency and time-domain equalizer coefficients (FDQ; TDQ, echo-canceller coefficients (ECC), transmitter gains; transmission and reception data rates; transmission and reception coding parameters; transmission fine gains; and Bit Allocation Tables) required for reliable communications. The same is true for the CO transceiver.

[0038] The present invention enables rapid resumption of transmissions, whether recovering from a power down or from an enforced idle condition due to temporary unavailability of processor resources in the case of an embedded transceiver, i.e., a transceiver implemented largely software and sharing CPU processing power with other applications which may from time to time divert CPU resources from the transceiver. In particular, the transceiver of the present invention is capable of recovering full data transmission capabilities within a period of a few frames.

[0039] On resuming communication, it may be desirable for the CPE transceiver to transmit several frames of test (known) data (step 100) before resuming transmission of user data. This enables the system to verify that system conditions have not changed so significantly as to require renewed initialization. If the CO transceiver receives these without error, it notifies the CPE transceiver (step 102) and the latter resumes full user data transmission (step 104 and 106). Otherwise, reinitialization must be performed (steps 104 and 108) before user data transmission occurs. A similar procedure may be employed for the transmission by the CO transceiver, and need not be described in further detail.

[0040] It will be understood that the order of certain of the steps described above may be changed, and that some steps may be omitted or added. For example, instead of initiating sleep mode at the CPE transceiver as shown in FIG. 2, the CO transceiver may initiate sleep mode. In such a case, the flow of notifications will be as shown in FIG. 2, but with the positions of CO and CPE transceivers reversed. Further, in some cases it may be desirable to omit the preparatory notifications of steps 82 and 84 and commence entrance into sleep mode as at steps 86 and 88. This will speed entrance into sleep mode, but is more prone to error.

[0041] It should also be understood that it is possible, and in various circumstances may be desirable, to operate in a partial sleep mode, in which only part of each transceiver is powered down. For example, where data transfer is one-way (when, for example, receiving video at the CPE transceiver from the CO transceiver without any upstream data being sent in return to the CO), the CO receiver and the CPE transmitter may operate in the sleep mode, while the CO transmitter and the CPE receiver are operating in full power mode.

[0042] As noted earlier, it is possible to implement major portions of the CPE and CO transceivers in software. In some applications, a dedicated CPU will be used for this purpose; in others, the CPU will be shared with other applications. Even when the sleep mode power conservation procedures described above are not necessary, when the transceivers are implemented in a shared-CPU environment, it may often be necessary to enter an idle mode that is similar to the sleep mode described above in order to maintain synchronization between the transceivers whenever the CPU is unable to service the transceivers. This is accomplished in accordance with the present invention by providing an Interface Transmission Unit (ITU) between the CPU and the data subscriber line which generates an idle indicator whenever the CPU is unavailable for servicing the transceiver which and transmits it to the remote transceiver to maintain the latter in synchronism with the former.

[0043] In particular, referring now to FIG. 3, an Interface Transmission Unit (ITU) 200 in accordance with the present invention is interposed between a CPU 202 and the data subscriber line 14. The unit 200 includes a PCI bus interface 204, a data buffer 206, an analog-to-digital and digital-to-analog converter 208, a controller 210, an idle indicator generator 212, and an analog front end 214.

[0044] In normal operation, the CPU passes data and control signals to the ITU 200 via the bus interface unit 204. Data from the CPU that is to be transmitted over the line 14 is stored in the buffer 206 and passed through the digital-to-analog converter portion of unit 208 for application to the line 214 through the analog front end. Conversely, data that is received from the line 14 is passed through the digital-to-analog converter portion of unit 208 to the buffer 206 and thence to the CPU 202 via the interface 204. The CPU processes the data flowing in the different directions in the manner illustrated in connection with FIG. 1, i.e., it performs the requisite IFFT and FFT transforms on the data as appropriate. The idle indicator generator may be inactive in this state.

[0045] When the CPU must divert its resources elsewhere for servicing applications having higher priority at the moment, the controller 210 activates the idle indicator generator 212 which generates a synchronous idle indicator symbol for transmission to the other transceiver with which the present transceiver is communicating. For example, the CPU may affirmatively notify the controller 210 that this is about to occur. Alternatively, the controller may monitor the buffer 206 and note its filling with data by the CPU has been interrupted for some time period. The idle indicator symbol is preferably of the same duration as a data frame, so that the remote transceiver can process it in the normal manner. However, it must be distinct from any valid data frame so that it can be detected and distinguished without ambiguity. It may comprise a tone, a collection of tones, a collection of tones modulated by bits, with similar of different phases, an inverted sync signal, or even the absence of signal. The CO transceiver responds to receipt of the idle symbol by itself transmitting idling data to the network or other devices with which it is connected so that erroneous data will not be transmitted. When the CPU returns to servicing its associated transceiver, the idle generator suspends operation, preferably on command of the controller 210.

[0046] Even when the transceiver is not implemented as part of the shared resources of a CPU, it may nonetheless be beneficial to transmit an idle signal as described above when the transceiver is in sleep mode. This minimizes the likelihood that erroneous data may be emitted, for example, to a network by a CO transceiver when a CPE transceiver with which it is communicating is in sleep mode.

[0047] From the foregoing, it will be seen that we have provided a multitone transceiver system that capable of operating wholly or partially in a sleep mode, both for purposes of power conservation as well as to accommodate itself to integration with, or incorporation into, computer systems having a power conservation mode. The transceiver system also is adapted to implementation in software under circumstances where the CPU on which the software runs is shared with other applications which may sometimes preempt resources needed for the transceiver.