Abstract:
Methods, systems, and computer program products for reconfiguring a modem. In an embodiment, fast reconfiguration of a modem occurs when a first modem determines there is a need for fast reconfiguration. The first modem signals its transition from showtime to fast reconfiguration. The first modem waits for acknowledgement from a second modem before transitioning to showtime. Once transitioned from showtime, the first modem estimates a signal to noise ratio and then exchanges parameters with the second modem. The first and second modems then transition to showtime.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Application No. 60/907,218, entitled “A Fast Sequence for Modem Reconfiguration,” filed Mar. 26, 2007, which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention relate to data communication. More specifically, certain embodiments of the invention relate to a method and system for communicating data in xDSL. 
     2. Background Art 
     In the field, DSL modems using discrete multitone (DMT) modulations, e.g., Asymmetric Digital Subscriber Line 2 (ADSL2), and Very High Speed Digital Subscriber Line 2 (VDSL2) are typically subject to retrains and/or re-initializations due to sudden variations in noise level. ADSL2 is described in ITU-T Recommendation G.992.3, Asymmetric Digital Subscriber Line Transceivers 2, January 2005, and subsequent Amendments: Amendment 1, September 2005; Amendment 2, March 2006; Amendment 3, December 2006; all of which are incorporated by reference herein in their entireties. VDSL2 is described in ITU-T Recommendation G.993.2, Very High Speed Digital Subscriber Line Transceivers 2, February 2006, and subsequent Corrigenda: Corrigendum 1, December 2006; all of which are incorporated by reference herein in their entireties. 
     For example, the noise varies in shape and amplitude, lowering the SNR on some tones and causing errors on the link. The modem(s) attempt on line recovery (OLR) procedures (e.g., bit swapping, seamless rate adaptation), but may not recover an error free link for various reasons (e.g., the link is broken in both directions, or the variation would exceed the minimal service requirements such as delay, impulse noise protection (INP), or rate parameters). One modem may essentially give up after a certain length of time, tear down the link, and the modems retrain or reinitialize. 
     The ADSL2 standard defines a fast initialization sequence to allow the modem to retrain in significantly less time than a normal initialization. This initialization consists in a restart of the modem where the handshake phase is skipped and the duration of the initialization signals are shorter. Handshake procedures are described in ITU-T Recommendation G.994.1, Handshake Procedures For Digital Subscriber Line (DSL) Transceivers, February 2007, and the superseded version of May, 2003 and its Amendments 1-4 (February 2004, June 2004, January 2005, January 2006, respectively); all of which are incorporated by reference herein in their entireties. 
     During the fast retrain, however, both modems must re-acquire loop timing, retrain equalizers (e.g., time domain and frequency domain equalizers), measure SNRs per tone, and exchange new bit-loading and framing parameters. Fast retrain is not very different than a normal initialization, and thus is not of significantly shorter duration than the normal initialization—except for the deletion of the handshake phase. 
     What is needed is a system and method to escape steady state (i.e., showtime) and quickly reconfigure or adjust the needed parameters to maintain communication. As used herein, “showtime” is the state of a modem after initialization including training is completed and data (e.g., bearer channel data) is being transferred. 
     BRIEF SUMMARY OF THE INVENTION 
     Methods, systems, and computer program products are provided for reconfiguring a modem. In an embodiment, fast reconfiguration of a modem occurs when a first modem determines there is a need for fast reconfiguration. The first modem signals its transition from showtime to fast reconfiguration. In an embodiment, the first modem waits for acknowledgement from a second modem before transitioning to showtime. Once transitioned from showtime, the first modem estimates a signal to noise ratio and then exchanges parameters with the second modem. In an embodiment, the modems then transition to showtime. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. 
         FIG. 1  illustrates an example VDSL2 functional model. 
         FIG. 2  shows an example initialization timeline. 
         FIG. 3  shows a timing diagram for an example fast initialization procedure for an example system. 
         FIG. 4  shows timeline  400 , illustrating an example fast reconfiguration for an example initiating modem and another example modem according to embodiments of the invention. 
         FIG. 5  shows timing diagram  500 , illustrating an example fast reconfiguration sequence for an example initiating modem and another example modem according to embodiments of the invention. 
         FIG. 6  shows flowchart  600 , illustrating exemplary methods of fast reconfiguration according to embodiments of the invention. 
         FIG. 7  shows an example computer system in which embodiments of the present invention may be implemented. 
     
    
    
     The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers may indicate identical or similar elements. Additionally, the left-most digit(s) of a reference number may identify the drawing in which the reference number first appears. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Overview 
     It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way. 
     While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the pertinent art will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the present invention. It will be apparent to a person skilled in the pertinent art that this invention can also be used in a variety of other applications. The scope of the invention is not limited to the disclosed embodiments. The invention is defined by the claims appended hereto. 
     References to “one embodiment,” “an embodiment,” “this embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment might not necessarily include the particular feature, structure or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such a feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     Although embodiments are applicable to any communication system, for brevity and clarity the well-known VDSL2 and ADSL2 environments are used as examples to illustrate various features of the present invention. 
     Example Environment 
       FIG. 1  illustrates the VDSL2 functional model that will be used to describe some embodiments of the invention. The ADSL2 environment is similar, and pertinent differences will be discussed at the time they become important. The environment  100  includes a VDSL2 Transceiver Unit Remote (VTU-R)  152 , which is typically located at the subscriber or customer site, and a VDSL2 Transceiver Unit Operator (VTU-O)  102 , which is typically centrally located. In ADSL, the equivalent units are the ADSL2 Transceiver Unit Remote (ATU-R) and the ADSL2 Transceiver Unit Central (ATU-C). There may be more than one central and more than one subscriber-side modem. 
     Each VTU-O  102  and VTU-R  152  includes a Physical Media Dependent sublayer (PMD)  112  and  162 , respectively. In the transmit direction, the PMD sub-layer receives input data frames from the PMS-TC sub-layer. Each data frame contains an integer number of data bits to be modulated onto one discrete multitone (DMT) symbol. Prior to modulation, the incoming bits are encoded into symbols, and the incoming bit stream is divided into small groups of bits. Each group is assigned to modulate a specific sub-carrier of the DMT signal. Each group is further encoded by a trellis encoder and mapped to a point in a signal constellation. The set of constellation points modulates the sub-carriers of the DMT symbol using an inverse discrete Fourier transform (IDFT). The number of bits assigned to each sub-carrier is determined during the initialization procedure based on the signal to noise ratio (SNR) of the sub-carrier and specific system configuration settings. After the IDFT, the resulting symbol is cyclically extended and windowed, and sent towards the transmission medium over the U interface. 
     In the receive direction, the signal incoming from the transmission medium via the U interface is demodulated and decoded to extract the transmitted data frame. The data frame obtained from a decoder, and the data frame output is sent to the PMS-TC sub-layer. 
     Each VTU also includes a Physical Media Specific Transmission Convergence sublayer (PMS-TC)  114  and  164 . Each PMS-TC  114  and  164  provides transmission medium specific TC functions, such as scrambling, framing, forward error correction (FEC), and interleaving. PMS-TCs  114  and  164  accepts incoming data in a uniform format including up to two bearer channels of transmit user data originated by one or more user data Transmission Protocol Specific TC sublayers (TPS-TCs)  108 ,  110 ,  158 , and  160 ; management data originated by Management Protocol Specific TC (MPS-TC) sublayers  106  and  156 ; and Network Timing Reference (NTR) sublayers  104  and  154 . The incoming user data and the overhead data are multiplexed into one or two latency paths. Each bearer channel is carried over a single latency path. A syncbyte is added to each latency path for overhead (OH) frame alignment. The multiplexed data in each latency path is scrambled, encoded using Reed-Solomon forward error correction coding, and interleaved. Interleaved buffers of data of both latency paths are multiplexed into a bit stream to be submitted to PMD sublayer  112  and  162 . 
     The TPS-TC layers of VTU  102  and  152  reside between the γ reference point and the α/β reference point as illustrated in the functional model shown in  FIG. 1 . The α and β reference points define corresponding interfaces between TPS-TC layer and PMS-TC  114  and  164  at the VTU-O  102  and VTU-R  152  sides respectively. Both interfaces are logical and application independent. The interfaces comprise the data flow; synchronization flow; and control flow of hypothetical signals between the TPS-TC layer and PMS-TCs  114  and  164 . 
     The TPS-TC layer contains one or more user data TPS-TC sublayers  108 ,  110 ,  158 , and  160  that provide transport of user data using different transport protocols; management TPS-TC (MPS-TC) sublayers  106  and  156  providing embedded operation channel (eoc) transport over the VDSL2 link; and network timing reference TC (NTR-TCs) sublayers  104  and  154  providing transport of a network timing reference. User data TPS-TC  108 ,  110 ,  158 , and  160  operates over a separate bearer channel, where PMS-TCs  114  and  164  allocate these bearer channels to latency paths. 
     User data TPS-TCs  108 ,  110 ,  158 , and  160  support different types of user data including synchronous transfer mode, asynchronous transfer mode, and Ethernet/generic packet transport via interfaces  118 ,  120 ,  168 , and  170 . Each data type is defined as an application option. VTU-O  102  selects the user data type for each bearer channel, both upstream and downstream, based on the type of higher layer data it chooses to support on that bearer channel. The enabled user data type for each of the bearer channels is indicated during initialization. 
     VTUs  102  and  152  each have a Management Protocol Specific TC sublayers (MPS-TC)  106  and  156  to support management data transport. MPS-TCs  106  and  156  facilitate transport of eoc data between VDSL2 management entities (VMEs)  116  and  166 . In the transmit direction, MPS-TC  106  or  156  gets an eoc message from VME  116  or  166 , and encapsulates it using a High-Level Data Link Control frame format, and submits it to be transported using a PMS-TC  114  or  164  overhead messaging channel. In the receive direction, MPS-TC  106  or  156  delineates HDLC frames, runs frame check sequence verifications, and extracts encapsulated eoc messages from correctly received HDLC frames. Received eoc messages are submitted to VME  116  or  166 . 
     VMEs  116  and  166  support management data communication protocols. VMEs  116  and  166  provide necessary management functions to communicate with the Management Information Base (MIB) and with the Network Management System (NMS) via the External OAM Interface Adapter (EIA). VMEs  116  and  166  also manage the Operations, Administration and Maintenance (OAM) communication channels, and support all internal management functions of VTUs  102  and  152 , including: performance monitoring, performance management, configuration management, and fault management. VMEs  116  and  166  also provide functionality to communicate the management data between VTU-O  102  and VTU-R  152 . Specifically, VME  116  and  166  originate eoc messages and IB to communicate management data, assign priority levels for eoc messages to share the overhead messaging channel, and maintain the protocol of eoc message exchange (re-send messages, abandon certain tasks, etc.). 
     Example Initialization 
     Initialization of a VTU-O  102  with a VTU-R  152  includes the following main tasks: definition of a common mode of operation (profile, band plan and initial values of basic modulation parameters); synchronization (sample clock alignment and symbol alignment); transfer from the VTU-O to the VTU-R of transmission parameters, including information on the Power Spectral Density (PSD) masks to be used, RFI bands (e.g., amateur radio bands) to be protected, and target data rates in both transmission directions; channel identification; noise identification; calculation of framer, interleaver, and coding parameters and the bit loading and gain tables; and exchange of modem parameters including RS settings, interleaver parameters, framer settings, bit loading and gain tables. 
       FIG. 2  illustrates an example initialization time line  200 . Time line  200  contains four phases illustrated in column  280  and column  290  for a VTU-O  102  and a VTU-R  152  respectively. First, a handshake phase  202  and  212  in accordance with G.994.1 is performed. Next, upstream power back-off is applied and a full duplex link between VTU-O  102  and the VTU-R  152  is established during the channel discovery phase  204  and  214  to set the Power Spectral Densities (PSDs) of the transmit signals and the main modulation parameters. During the training phase  206  and  216 , any existing time-domain equalizers (TEQs) and echo cancellers may be trained, and the timing advance is refined. During the channel analysis &amp; exchange phase  208  and  218 , the two modems shall measure the characteristics of the channel and exchange parameters to be used in showtime. 
     Example on Line Reconfiguration and Fast Initialization 
     On Line Reconfiguration (OLR) of PMDs  112  and  162  provides a means for adapting to slowly varying channel conditions. OLR procedures provide transparency to the higher layers by providing means for configuration parameter changes without introducing transport errors, latency changes, and interruptions of service. 
     Three example forms of OLR are Bit Swapping (BS), Dynamic Rate Repartitioning (DRR) and Seamless Rate Adaptation (SRA). Bit Swapping (BS) reallocates data and power (i.e., margin) among the allowed subcarriers without modification of the higher layer features of the physical layer. BS reconfigures the bits and fine gain (bi, gi) parameters without changing any other PMD or PMS-TC control parameters. After a bit swapping reconfiguration the total data rate is unchanged and the data rate on each latency path is unchanged. Bit swapping may be used for autonomous changes to maintain the operating conditions for the modem during changing environment conditions. 
     Dynamic Rate Repartitioning (DRR) reconfigures the data rate allocation between multiple latency paths by modifying the frame multiplexor control parameters. DRR may also include modifications to the bits and fine gain (bi, gi) parameters, reallocating bits among the subcarriers. DRR does not modify the total data rate but does modify the individual latency path data rates. DRR can include a change in the number of octets per frame bearer per Mux Data Frame. DRR is used in response to higher layer commands and is considered DRR is an application option. The ability to support DRR may be identified during the initialization procedure. 
     Seamless Rate Adaptation (SRA) reconfigures the total data rate by modifying the frame multiplexor control parameters and modifying the bits and fine gains (bi, gi) parameters. Since the total data rate is modified, at least one latency path will have a new data rate after SRA. The number of frame bearer octets per Mux Data Frame can also be modified in SRA transactions. Because SRA is used in response to higher layer commands, SRA is an application option. The ability to support SRA may be identified during the initialization procedure. 
     Changing line conditions may warrant a fast initialization. A fast initialization is a full initialization as described elsewhere herein, with handshaking  202  and  212  omitted. Fast initialization may be initiated by either VTU  102  or  152 , and may be signaled by a specific symbol. A timing diagram for an example fast initialization procedure for an example system (a ADSL2 transceiver pair as defined by G.992.3) is illustrated in  FIG. 3 . A fast initialization may be initiated by either the central (e.g., ATU-C) or the remote (ATU-R) transceiver. 
     Fast Reconfiguration 
     In some circumstances, an OLR may be insufficient or inefficient to address SNR degradation, but fast initialization is not required or simply takes too much time. A Fast Reconfiguration procedure that enables a rapid modification of the bits and gains per tone and framer settings may be used. A fast reconfiguration may be desirable when a modem (e.g., a VTU, ATU, etc.) detects a sudden variation of SNR per tone which causes errors on the link. An example of an indication that the OLR mechanisms do not work is when the other modem does not respond to requests for or after OLR. These are simple examples, a person of skill in the art may identify other situations where a fast reconfiguration is necessary. 
       FIG. 4  illustrates a timing diagram  400  of an example fast reconfiguration. Columns  480  and  490  show the timing for the initiating and the other modem respectively. This example begins with both modems in showtime; however, fast reconfiguration may be desirable at other times, depending on the communications system incorporating an embodiment of the invention. This example illustrates the specific messages used for a G993.2 compliant system. Other systems, current and future, would use different messages to accomplish similar results. 
     Both modems are initially in showtime  402  and  422  respectively. Showtime is the state of a modem after initialization including training is completed and data (e.g., bearer channel data) is being transferred. Upon determining a fast reconfiguration is desirable, the initiating modem sends a synchronization sequence during Sync Sequence  404 . Note that either the central modem (e.g., VTU-O, ATU-C, etc.) or a remote modem (e.g., VTU-R, ATU-C, etc.) may be the initializing modem. 
     A synchronization sequence is a specific sequence of DMT symbols. For example, a synchronization sequence may be a modified Sync Symbol, a modified Sync Symbol followed by QUIET symbols, or a different DMT symbol(s). The initiating modem has escaped showtime: it is no longer transmitting bearer channel data. After some finite amount of time, the other modem enters Sync Sequence  424 , and also generates a specific sequence of DMT symbols to indicate that it has transitioned from showtime. 
     The synchronization DMT symbols have the same symbol timing as showtime symbols, i.e., the beginning of the DMT symbols are aligned with the showtime ones and have the same symbol duration. In other words, the synchronization symbols include the same cyclic prefix, suffix and transmit window length as the showtime symbols. In an embodiment, the DMT symbol alignment for symbols transmitted by the initiating modem is the same from the initial showtime  402  to new showtime  412 . In a further embodiment, the DMT symbol alignment is the same for symbols transmitted by the other modem from showtime  422  to new showtime  430 . The PSD of the synchronization symbols must be well defined and known to the receiving modem. For example, the PSD may be identical to the PSD of the DMT symbols sent during the analysis and exchange phase of the previous initialization (i.e., all gains set to 1). 
     In the next phase, SNR estimation sequence  406  and  426 , the initiating modem and other modem send symbols and estimate the SNR per tone. They may also perform some training of any equalizers. For example, ADSL2 modems may enter MEDLEY as defined in G.992.3 and VDSL2 modems may send O- and R-P-MEDLEY as defined in G.993.2. 
     Next, the modems enter their respective exchange sequences  408  and  428 . The modems exchange new bits and gains tables and framer setting carried by DMT symbols. For example, ADSL2 modems may enter C- and R-PARAMS states and send the appropriate messages (as described in G.992.3) and VDSL2 modems may send O- and R-PMS and O- and R-PMD messages. 
     After this exchange, the modems send a set of DMT symbols during showtime entry sequences  410  and  430  to indicate a transition back to showtime  412  and  432  with the new bits and gains tables and framer settings. For example, ADSL2 modems may send REVERB and SEGUE in G.992.3, and VDSL2 modems may send O- and R-P-Synchro in G.993.2. 
     Because this new sequence is functionally similar to the analysis and exchange phase of the normal initialization of G.992.3 or G993.2, similar signals and messages may be used to estimate SNR and exchange parameters, thus simplifying implementation. 
       FIG. 5  illustrates timing diagram  500  for the initiating modem and the other modem during an example fast reconfiguration sequence. This example uses VDSL2 (i.e., G.993.2) modems, but the principles are equally applicable to other current and future communications systems, including ADSL2 systems. 
     The two inner columns  582  and  592  show the sequences of signals that are transmitted from the initiating and the other modem respectively. The two outer columns  580  and  590  show the messages that are sent over the SOC by the initiating and other modem respectively. 
     In this example, the initiating modem is VTU-O  102  and the other modem is VTU-R  152 , however, either VTU-R  152  or VTU-O  102  may initiate fast reconfiguration. Both modems are initially in showtime as shown by blocks  502  and  552  respectively, and both special operations channels (SOCs) are inactive as shown by blocks  572  and  522 . Upon determining a fast reconfiguration is desirable, the initiating modem sends a synchronization sequence during Sync Symbol  504 . The synchronization sequence may be a sync symbol, another specific sequence of DMT symbols, or any other method of signaling. The initiating modem has escaped showtime  502 : it is no longer transmitting bearer channel data. After some finite amount of time, the other modem enters Sync Symbol  554 , and also generates a synchronization sequence to indicate that it has transitioned from showtime  552 . 
     The O-PMS message sent during block  524  conveys the initial PMS-TC parameter settings that will be used in the upstream direction during the next showtime  512 . It also specifies the portion of shared interleaver memory that VTU-R can use to de-interleave the downstream data stream. O-PMS is fully described in section 12.3.5.2.1.3 of G.993.2. 
     The O-PMD message sent during block  526  conveys the initial PMD parameter settings that will be used in the upstream direction during the next showtime  512 . O-PMD is fully described in section 12.3.5.2.1.4 of G.993.2. 
     The R-PMS message sent during block  574  conveys the initial PMS-TC parameter settings that will be used in the downstream direction during the next showtime  562 . R-PMS is fully described in section 12.3.5.2.2.3 of G.993.2. 
     The R-PMD message sent during block  576  conveys the initial PMD parameter settings that shall be used in the downstream direction during the next showtime  562 . R-PMD is fully described in section 12.3.5.2.2.4 of G.993.2. 
     O-P-MEDLEY sent during block  506  is used by VTU-R  152  to estimate the downstream SNR and to communicate the SOC messages O-PMS and O-PMD. During transmission of O-P-MEDLEY, the SOC is in an active state. The duration of O-P-MEDLEY is variable. The VTU-O terminates O-P-MEDLEY by transmitting O-P-SYNCHRO  6 . O-P-MEDLEY is fully described in section 12.3.5.3.1.1 of G.993.2. 
     O-P-SYNCHRO  6  is sent during block  510 , and provides an exact time marker for the transition from O-P-MEDLEY to the next showtime  512 . During transmission of O-P-SYNCHRO  6 , the SOC is in its inactive state. The duration of O-P-SYNCHRO  6  is 15 DMT symbols. O-P-SYNCHRO  6  is fully described in section 12.3.5.3.1.2 of G.993.2. 
     R-P-MEDLEY is sent during block  556 , and is used by the VTU-O to estimate the upstream SNR and to communicate the SOC messages R-PMS and R-PMD. During transmission of R-P-MEDLEY, the SOC is in an active state. The duration of R-P-MEDLEY is variable. The VTU-R terminates R-P-MEDLEY by transmitting R-P-SYNCHRO  6 . R-P-MEDLEY is fully described in section 12.3.5.3.2.1 of G.993.2. 
     R-P-SYNCHRO  6  is sent during block  560  and provides an exact time marker for the transition from R-P-MEDLEY to the next showtime  562 . During transmission of R-P-SYNCHRO  6 , the SOC is in an inactive state. The duration of R-P-SYNCHRO  6  is 15 DMT symbols. R-P-SYNCHRO is fully described in section 12.3.5.3.2.2 of G.993.2. 
     An embodiment of the invention implemented in the G.992.3 ADSL2 framework would be similar, and the ATUs would transmit and be in the REVERB, SEQUE, MEDLEY, EXCHMARKER and PARAMS symbols and states as described in G.992.3. 
     Example Method 
       FIG. 6  shows flowchart  600  illustrating an example embodiment of a fast reconfiguration. The steps may be performed in any order or concurrently unless specified otherwise. Some embodiments of the present invention do not require the performance of each and every step. 
     In step  602 , a need for fast reconfiguration is determined. The need may be for any reason. For example, a fast configuration may be desirable when a modem (e.g., a VTU, ATU, etc.) detects a sudden variation in SNR. The sudden variation of SNR may cause errors on the link. Also, if OLR is ineffective, a fast reconfiguration may be desirable. Other reasons for fast configuration are apparent to a person of ordinary skill in the art. 
     In step  604 , a transition to fast reconfiguration is signaled. The initiating modem may signal the transition in any way. For example, the initiating modem may send a specific sequence of DMT symbols to indicate the transition, such as a modified sync symbol or a modified sync symbol followed by a QUIET symbol. The modem transitions from showtime in this step. 
     In step  606 , a signal for fast reconfiguration is received by another modem. 
     In step  608 , a fast reconfiguration signal is acknowledged by the modem. For example, the modem receiving the fast reconfiguration signal may transmit a specific sequence of DMT symbols. 
     In decision step  610 , whether a response was received is determined by the initiating modem. If the initiating modem received an acknowledgement of receipt of its fast reconfiguration signal, control proceeds to step  614 . If not, then control proceeds to step  612 . 
     In step  612 , an initialization, either full or fast, is performed. This step may be reached, for example, if SNR degraded to the point that the modems are unable to communicate. In this case, the initiating modem may attempt a full initialization. For systems with a fast initialization (e.g., G.992.3), a fast initialization may be performed. 
     In step  614 , an SNR is estimated. The SNR may be estimated by any means. In an embodiment, the SNR is estimated for a tone by an exchange of symbols such as in MEDLEY in G.992.3 or O/R-P-MEDLEY in G.993.2. 
     In step  616 , parameters are exchanged. The modems may exchange parameters by any means. In an embodiment, a modem sends DMT symbols carrying new parameters. The new parameters describe the new modem parameters; for example, new bits and gains tables and framer settings. In an embodiment, the new parameters are exchanged in C/R-PARAMS states (as defined in G.992.3). In another embodiment, new parameters are exchanged in O/R-PMS and O/R-PMD (as defined in G.993.2). 
     In step  618 , transition to showtime is signaled. A modem may signal transition to showtime in any way. For example, a modem may send a specific sequence of DMT symbols (e.g., REVERB/SEGUE in G.992.3, O/R-P-SYNCHRO in G.993.2, etc.). 
     In step  620 , a showtime state is entered. 
     The sending modem in the above steps sends DMT symbols with the same symbol timing as the showtime symbols; that is, the beginning of these DMT symbols are aligned with the showtime ones, and have the same symbol duration as the showtime symbols. Other words, these DMT symbols have the same cyclic prefix, suffix, and transmit window length as the showtime symbols. The power spectral density (PSD) of these DMT symbols used is known by the receiving modem. For example, the PSD may be identical to the PSD of the DMT symbols sent during the analysis and exchange phase of the previous initialization. 
     Various aspects of the present invention can be implemented by software, firmware, hardware, or a combination thereof. Calculations may be approximated using table look-ups. Hardware implementations of individual components are not limited to digital implementations and may be analog electrical circuits. Additionally, embodiments may be realized in a centralized fashion in at least one communication system, or in a distributed fashion where different elements may be spread across several interconnected communication systems. Any kind of computer system or other apparatus adapted for carrying out the methods described herein may be suited. 
       FIG. 7  illustrates an example computer system  700  in which the present invention, or portions thereof, can be implemented as computer-readable code. For example, the methods illustrated by flowchart  600  of  FIG. 6  can be implemented in system  700 . Various embodiments of the invention are described in terms of this example computer system  700 . After reading this description, it will become apparent to a person skilled in the relevant art how to implement the invention using other computer systems and/or computer architectures. 
     Computer system  700  includes one or more processors, such as processor  704 . Processor  704  can be a special purpose or a general purpose processor. Processor  704  is connected to a communication infrastructure  706  (for example, a bus or network). 
     Computer system  700  also includes a main memory  708 , preferably random access memory (RAM), and may also include a secondary memory  710 . Secondary memory  710  may include, for example, a hard disk drive  712 , a removable storage drive  714 , any type of non-volatile memory, and/or a memory stick. Removable storage drive  714  may comprise a floppy disk drive, a magnetic tape drive, an optical disk drive, a flash memory, or the like. The removable storage drive  714  reads from and/or writes to a removable storage unit  718  in a well known manner. Removable storage unit  718  may comprise a floppy disk, magnetic tape, optical disk, etc. which is read by and written to by removable storage drive  714 . As will be appreciated by persons skilled in the relevant art(s), removable storage unit  718  includes a computer usable storage medium having stored therein computer software and/or data. 
     In alternative implementations, secondary memory  710  may include other similar means for allowing computer programs or other instructions to be loaded into computer system  700 . Such means may include, for example, a removable storage unit  722  and an interface  720 . Examples of such means may include a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an EPROM, or PROM) and associated socket, and other removable storage units  722  and interfaces  720  which allow software and data to be transferred from the removable storage unit  722  to computer system  700 . 
     Computer system  700  may also include a communications interface  724 . Communications interface  724  allows software and data to be transferred between computer system  700  and external devices. Communications interface  724  may include a modem, a network interface (such as an Ethernet card), a communications port, a PCMCIA slot and card, or the like. Software and data transferred via communications interface  724  are in the form of signals which may be electronic, electromagnetic, optical, or other signals capable of being received by communications interface  724 . These signals are provided to communications interface  724  via a communications path  726 . In an embodiment, communications path  724  includes the U interface, as illustrated by  FIG. 1 , and communications interface includes at least on component of a VTU-O  102  or a VTU-R  152 . Communications path  726  carries signals and may be implemented using wire or cable, fiber optics, a phone line, a cellular phone link, an RF link or other communications channels. 
     In this document, the terms “computer program medium” and “computer usable medium” are used to generally refer to media such as removable storage unit  718 , removable storage unit  722 , and a hard disk installed in hard disk drive  712 . Signals stored elsewhere and carried over communications path  726  can also embody the logic described herein. Computer program medium and computer usable medium can also refer to memories, such as main memory  708  and secondary memory  710 , which can be memory semiconductors (e.g. DRAMs, etc.). These computer program products are means for providing software to computer system  700 . 
     Computer programs (also called computer control logic) are stored in main memory  708  and/or secondary memory  710 . Computer programs may also be received via communications interface  724 . Such computer programs, when executed, enable computer system  700  to implement the present invention as discussed herein. In particular, the computer programs, when executed, enable processor  704  to implement the processes of the present invention, such as the steps in the methods illustrated by flowcharts  600  of  FIG. 6  discussed above. Accordingly, such computer programs represent controllers of the computer system  700 . Where the invention is implemented using software, the software may be stored in a computer program product and loaded into computer system  700  using removable storage drive  714 , interface  720 , hard drive  712  or communications interface  724 . 
     The invention is also directed to computer program products comprising software stored on any computer useable medium. Computer programs or software in the present context means any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: a) conversion to another language, code or notation; b) reproduction in a different material form. Such software, when executed in one or more data processing device, causes a data processing device(s) to operate as described herein. Embodiments of the invention employ any computer useable or readable medium, known now or in the future. Examples of computer useable mediums include, but are not limited to, primary storage devices (e.g., any type of random access memory), secondary storage devices (e.g., hard drives, floppy disks, CD ROMS, ZIP disks, tapes, magnetic storage devices, optical storage devices, MEMS, nanotechnological storage device, etc.), and communication mediums (e.g., wired and wireless communications networks, local area networks, wide area networks, intranets, etc.). 
     SUMMARY 
     The above described embodiments may be realized in hardware, software, or most commonly a combination thereof. Additionally, embodiments may be realized in a centralized fashion in at least one communication system, or in a distributed fashion where different elements may be spread across several interconnected communication systems. Any kind of computer system or other apparatus adapted for carrying out the methods described herein may be suited. A typical combination of hardware and software may be a general-purpose computer system with a computer program that, when being loaded and executed, may control the computer system such that it carries out the methods described herein. 
     Alternatively, the above described embodiments may also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods described herein, and which when loaded in a computer system is able to carry out these methods. Computer program in the present context means any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: a) conversion to another language, code or notation; b) reproduction in a different material form. 
     While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims.