Patent Publication Number: US-2004047407-A1

Title: Pcm modem over all-digital connection

Description:
RELATED APPLICATIONS  
     [0001] The present application is a continuation-in-part (CIP) of PCT application PCT/IL00/00198, filed Mar. 29, 2000, the disclosure of which is incorporated herein by reference. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The present invention relates to communication systems and in particular to voice band modems.  
       BACKGROUND OF THE INVENTION  
       [0003] Voice band modems (VBMs) are used for transmitting data over telephone communication links. Generally, two modems on opposite ends of a communication link form a modem connection and send each other data on the connection by converting (i.e., modulating) the data into electrical signals suitable for transmission on the link. Generally, the modems of a link are referred to as a client modem (e.g., of a home user) and a server modem (e.g., of an Internet service provider). One of the modems, referred to as the call modem, generally dials the telephone number of the other modem in order to form the connection. In most cases, the call modem is the client modem, however in some cases the server modem acts as the call modem. Existing telephone links include analog lines, such as twisted copper pairs through which most residential homes are connected to the telephone network, and/or digital trunks, such as E1 and T1 links.  
       [0004] In order to allow modems of different vendors to transmit data to each other, standards (also known as protocols) have been defined stating exactly how the signals should be modulated by the modems. Usually modems implement a plurality of different standards. Generally, when two modems form a connection, they search, in a protocol negotiation phase (e.g., a V.8 stage), for a highest transmission rate protocol which they both implement, and use this protocol for transmission over the connection.  
       [0005] The V.34 standard describes a commonly used transmission protocol, which allows transmission of signals at rates of up to 33,600 bps. The V.34 standard is described in ITU-T Recommendation V.34, 2/98, the disclosure of which is incorporated herein by reference.  
       [0006] The end modems of the V.34 standard may be connected to the telephone link through a digital trunk, in which case the modem is referred to as a digital modem, or may be connected through an analog loop, in which case the modem is referred to as an analog modem. A digital modem provides samples at a rate of the digital trunk, generally 8000 samples per second. The analog and digital V.34 modems are substantially the same except that the linear data representation used in the modem is converted to PCM samples e.g., G.711 samples, for the digital modem and into analog symbols for the analog modem. The provisions of the V.34 do not differentiate between the direction of transmission and the same provisions are used for the upstream (from the client to the server) as for the downstream (from the server to the client).  
       [0007] In order to achieve higher transmission rates, the V.90 standard, described in ITU-T Recommendation V.90, 9/98, the disclosure of which is incorporated herein by reference, was defined. The V.90 standard imposes the limitation that the server modem is connected to the communication link through a digital trunk. Using this limitation, the downstream transmission rate is increased to about 56 Kbps. The V.90 standard allows for the transmitter of the digital modem to perform acts that compensate for analog line conditions, for example, spectral shaping which helps the receiver of the client modem to combat the effects of an intermediate digital to analog conversion. During a training phase, the client modem instructs the digital modem, as to which spectral shaping parameters are to be used. Generally, in the V.90 standard, the client modem is referred to as an analog modem, which is connected to the telephone link through an analog loop. In defining the first phase of the negotiation stage (the V.8 protocol negotiation), the V.90 standard states that when both modems identify themselves as being digitally connected to the telephone link, the call modem shall operate as the analog (client) modem and the answer modem shall operate as the digital (server) modem. Similar statements appear in the V.8 ITU recommendation (2/98), the disclosure of which is incorporated herein by reference. It is noted, though, that the V.90 and V.8 standards do not state any provisions for the operation of a client modem, digitally connected to the link, as an analog modem, thus possibly leaving the definition from the V.8 negotiation stage as a theoretical provision for farther development.  
       [0008] While the V.90 standard improves the downstream transmission rate, it leaves the upstream transmission rate as in the V.34 standard. In order to improve the upstream transmission rate of the V.90 standard, the V.92 standard was introduced. The V.92 standard is described in V.92 ITU draft PCM-00-062R1, the disclosure of which is incorporated herein by reference. The V.92 standard uses the downstream provisions of the V.90 standard but introduces a new method for upstream transmission so as to achieve upstream transmission rates of up to 48 Kbps. In order to achieve these upstream transmission rates, a third phase of the connection training in the V.92 standard determines the effect of the analog transmission link, on the uplink transmission, to allow compensation for the determined effect.  
       [0009]FIG. 1 is a schematic time chart of training signals transmitted according to a portion of the third phase of the V.92 standard, as is known in the art. Generally, only information required for the explanation of the present application are described herein, while the details of the signals shown in FIG. 1, appear in the V.92 standard. Responsive to receiving a TRN 1d  signal  500 , the analog modem transmits an S u  signal  502 , used for evaluation of the analog line, for a period of 144T (T being the sample transmission interval). That is, the analog modem transmits 144 samples of S u  signal  502  at a sample rate of 8000 samples per second. Thereafter, the analog modem transmits an S u -not signal  504  for a period of 24.5T, followed by transmission of S u  signal  506  until a J p  signal  508 , which includes a phase correction factor (ε), is received from the digital modem. When J p  signal  508  is received, the analog modem transmits a second instance of S u -not signal  510  for a period of (24+ε)T. The transmission of signals with fractional intervals (i.e., intervals which include a non-integer number of sample periods), for example, S u -not signal  504  and S u -not signal  508 , and the consequent transmission of out of phase signals, e.g., S u  signal  506 , generally requires the use of an analog modem, for example using a resampler and/or a digital to analog converter of the analog modem.  
       [0010] In recent years, increasing numbers of computers connect to ISPs through all-digital connections, for example, using cellular phone connections. The V.91 standard, described in ITU-T Recommendation V.91, 5/99, the disclosure of which is incorporated herein by reference, was defined for such connections, allowing rates of 64 Kbps in both directions. The V.91 standard, however, is not widely implemented. If one of the modems of an all-digital connection does not support the V.91 standard, contemporary modems will generally fall back to using the V.34 standard, which provides a maximal transmission rate of 33.6 Kbps in both the upstream and downstream directions, as the V.90 and V.92 standards are not implemented for all-digital connections.  
       [0011]FIG. 2 is a schematic illustration of a modem connection as is known in the art. A client modem  12 , connected to a public switching telephone network (PSTN)  19  through an analog loop  14 , forms a connection with a server modem  24  which is connected to PSTN  19  through a digital trunk  32 . Usually, modem  24  belongs to a modem pool, for example of an Internet service provider (ISP)  29 . Generally, a line card  16  translates the signals from analog lines  14  to digital link  36  of PSTN  19 , and vice versa. It is noted, that in most cases, except for analog loop  14  which connects line card  16  and client modem  12 , PSTN  19  is formed of substantially only digital links, represented in FIG. 2 by a digital network  23 .  
       [0012] Client modem  12  comprises a signal processing unit  13  and a sampler and reconstructer  15  which turns digital signals into analog signals for transmission and analog signals from lines  14  into digital signals. When a computer  10  requests to connect to ISP  29 , client modem  12  forms a negotiation V.8 connection with server modem  24 . During the negotiation connection, client modem  12  identifies as an analog modem and server modem  24  identifies as a digital modem such that the modems agree to use a V.90 connection for transmission of data. Thereafter, modems  12  and  24 , sequentially as defined by the V.90 protocol, transmit test signals used to check the characteristics of loop  14  (and trunk  32 ).  
       [0013] After the tests are concluded, modems  12  and  24  move into transceiving states according to the V.90 standard. Signals transmitted from computer  10 , are prepared for transmission by processing unit  13  of modem  12  at a rate of up to 33.6 Kbps and are then converted to analog signals by a D/A of sampler and reconstructer  15 . Signals transmitted by server modem  24  are transmitted from the modem at a rate of up to 56 Kbps as described above. The transmitted signals from server modem  24  are converted to analog signals by line card  16  and are passed on link  14  to modem  12 . An A/D of sampler and reconstructer  15  of modem 12 samples the analog signals at a high enough rate which allows proper operation of the modem, i.e., proper synchronization of a clock of sampler and reconstructer  15  in receiving modem  12  to the timing of transmitting modem  24 . Generally, to allow for rate correction, the A/D of sampler and reconstructer  15  samples the signals at a rate higher than 8000 samples per second. The sampled digital signals are then passed to processing unit  13  for processing.  
       [0014]FIG. 3A is a schematic block diagram of an exemplary data receiving path  40  of processing unit  13  of modem  12 , as is known in the art. Path  40  may be used, for example, for the V.90 and V.92 protocols. Path  40  receives samples from sampler and reconstructer  15  on a line  42 . The samples are added to correction values provided by an echo canceller  44  and are then filtered by a channel filter  46 . The filtered samples are passed to a timing recovery unit  48  and a rate converter  50 , which correct for timing drifts of the received samples. The samples are passed through an automatic gain control (AGC) unit  52  and are then provided to an equalizer  54  which corrects phase and amplitude distortions of the received samples. The samples from equalizer  54  are passed to a symbol decision module  56  which determines for each sample which symbol it represents. Symbol decision module  56  also detects attenuation pad impairments and performs robbed bit signaling (RBS) in order to better perform the determination of the samples which represent the symbols. The symbols are then passed through a symbol to bit converter  58  which translates the symbols into bits, and through a descrambler  60  which descrambles the bits, which were scrambled by server modem  24  before their transmission.  
       [0015]FIG. 3B is a schematic block diagram of an exemplary data transmission path  61  of processing unit  13  of modem  12 , as is known in the art. Path  61  may be used, for example, for the V.92 protocol. Transmission path  61  includes a modulus encoder  63  and a precoder  64 , which convert data bits to be transmitted into transmitted symbols. As is known in the art, an inverse mapper  65  and a convolution encoder  66  participate in generating the symbols with precoder  64 . The generated symbols are passed to a prefilter  67 , which filters the signals before they are transmitted onto a line  62 , to sampler and reconstructer  15  (FIG. 2).  
       SUMMARY OF THE INVENTION  
       [0016] An aspect of some embodiments of the present invention relates to a digitally connected modem which is adapted to operate as a V.92 analog client modem.  
       [0017] An aspect of some embodiments of the present invention relates to a digitally connected modem which is adapted to transmit signals in accordance with a transmission scheme defined only for transmission from analog to digital modems. Optionally, the transmission scheme comprises the upstream transmission method of the V.92 protocol.  
       [0018] An aspect of some embodiments of the present invention relates to a digitally connected client modem adapted to transmit one or more training signals for an interval having a length of a non-integer number of samples, e.g., the S u -not signal of the V.92 protocol.  
       [0019] In some embodiments of the invention, the digital client modem comprises a memory which stores a sample value which represents both a fractional portion of the signal transmitted for the non-integer interval and a complementary portion of a signal transmitted immediately thereafter. Optionally, the fractional portion comprises half of a sample. Alternatively or additionally, the modem resamples the training signal at a higher rate, for example twice the original rate, and immediately down samples the signal. At the interface between the transmission of the signal transmitted for the non-integer interval and the signal transmitted immediately thereafter, the down sampling skips one or more of the higher rate resampled samples, so as to generate a sample which represents fractional portions of both the signals.  
       [0020] An aspect of some embodiments of the present invention relates to a digitally connected client modem adapted to transmit two instances of a training signal, separated by an interval corresponding to a non-integer number of samples. Thus, the modem is adapted to transmit the training signal in two different instances shifted relative to each other by a plurality of symbols plus a fraction of a symbol, for example half a symbol. In an exemplary embodiment of the present invention, the training signal comprises the S u  signal of the V.92 protocol and the interval between the instances is 24.5 samples.  
       [0021] In some embodiments of the invention, the digital client modem comprises a memory which stores the two instances of the training signal, optionally in the form of two series of samples. Alternatively or additionally, the training signal is cyclic and the memory stores for each of the shifted versions of the training signal, a series of samples representing a single cycle of the signal. In transmitting the training signal, the modem uses the series corresponding to the required shift amount. Further alternatively or additionally, the memory stores a table which includes the samples from which the signal is formed, for each of the required shifts of the signal. In generating the training signal, the modem optionally accesses the table based on the required shift and the current position within the training signal. Alternatively or additionally, the modem resamples the training signal at a higher rate, for example twice the original rate, and immediately down samples the signal with the required shift.  
       [0022] An aspect of some embodiments of the present invention relates to a modem adapted to receive, during a modem training phase, a message which includes a value to be used in determining one or more parameters of a transmitted training signal. The receiving modem ignores the value in the message and uses predetermined values for the one or more parameters.  
       [0023] In some embodiments of the invention, the one or more parameters include a fractional transmission time of the training signal the modem is to transmit, e.g., as in the J p  message of the V.92 protocol. The training signal is transmitted with a predetermined fractional transmission time, optionally with a fractional transmission time of half a symbol. In some embodiments of the invention, the modem comprises a digital modem, which due to its digital connection does not cause phase shifts and therefore the fractional transmission time in the directive is generally half a symbol.  
       [0024] An aspect of some embodiments of the present invention relates to a digitally connected modem which performs, for one or more protocols, fewer tasks than required by an analog connected modem, for the protocol. Optionally, the one or more protocols differentiate between analog and digital modems. Alternatively or additionally, the one or more protocols state different provisions for the upstream and downstream transmissions.  
       [0025] Optionally, the fewer tasks are performed in the transmission path. For example, instead of performing all the tasks performed by an analog connected modem and in addition performing an analog to digital conversion or replacing the last conversion to analog signals by a conversion to PCM signals, the digital modem directly converts 16 bit linear level samples, used in the transmission path, into PCM signals. In some embodiments of the invention, the transmission path does not include one or more filters, for example, a precoder filter and/or a prefilter, which are used in transmission paths of analog modems.  
       [0026] Alternatively or additionally, the fewer tasks are performed in the reception path. For example, instead of performing all the tasks performed by an analog connected modem on the received samples, the digital modem ignores tasks not required for digitally connected modems. Optionally, the ignored tasks are tasks required for correction of analog line impairments, such as, echo cancellation, rate conversion, channel filtering, automatic gain control, time recovery and/or equalization.  
       [0027] An aspect of some embodiments of the present invention relates to a digital modem which purposely incorrectly identifies itself as an analog modem during a protocol negotiation procedure, e.g., a V.8 or V.8bis procedure. In some embodiments of the invention, the digital modem selectively determines whether to identify as a digital or analog modem according to a guess of the identity of the remote modem. Optionally, the digital modem identifies as an analog modem, for analog-to-digital modem protocols, e.g., V.90, V.92. Alternatively, the digital modem determines whether to identify as analog or digital according to a user instruction. Alternatively or additionally, the digital modem determines whether to identify as analog or digital according to the telephone number of the remote party of the connection.  
       [0028] In some embodiments of the invention, the digital modem identifies itself as an analog modem for one or more protocols and as supporting one or more all digital protocols (e.g., V.91) in a single connection. Alternatively, the digital modem consistently identifies itself as an analog modem for all protocols, i.e., it does not state support of all digital protocols.  
       [0029] By having the digital modem identifying as an analog modem, the digital modem is able to have a remote digital modem agree to form an analog-to-digital connection with the digital modem, for a protocol that is defined only between an analog and a digital modem.  
       [0030] There is therefore provided in accordance with some embodiments of the invention, a method of transmitting data, comprising establishing a modem connection between two digitally connected modems; and transmitting data on the modem connection in accordance with a transmission scheme defined only for transmission from analog to digital modems. Optionally, transmitting the data comprises transmitting in accordance with the upstream transmission scheme of the V.92 protocol.  
       [0031] Optionally, the transmission scheme defined only for transmission from analog to digital modems is defined for use concurrently on a same line with an opposite direction transmission scheme which has a higher maximal transmission rate.  
       [0032] There is further provided in accordance with some embodiments of the invention, a digitally connected modem, comprising a digital line interface adapted to transmit symbols at a predetermined rate defining respective symbol periods; and a signal generation unit adapted to generate, during a training phase of a modem connection, symbols of a pair of instances of a training signal, shifted relative to each other by a phase shift of a partial symbol period, for transmission by the line interface.  
       [0033] Optionally, the training signal comprises a signal defined by the V.92 protocol.  
       [0034] Optionally, the phase shift comprises a shift of half the symbol period. Optionally, the signal generation unit comprises an interpolator adapted to convert a generated training signal into a series of samples with a sampling rate higher than that of the signals transmitted by the digital line interface and a decimator adapted to down-sample the training signal to the sampling rate of the signals transmitted by the digital line interface, with a phase shift, when a phase shift is required.  
       [0035] Optionally, the signal generation unit comprises a memory which stores segments of the training signal with and without the phase shift and the generation of the signals is performed with reference to the segments stored in the memory. Alternatively or additionally, the signal generation unit comprises a memory which stores a sample series representing both the instances of the training signal together with at least one signal transmitted between the instances. Further alternatively or additionally, the signal generation unit comprises a memory which stores a table which includes all the sample values of the training signal with different phase shift values.  
       [0036] Optionally, the different instances of the training signal have different lengths. Optionally, the different instances of the training signal are shifted relative to each other by a shift period including a non-integer number of symbol periods, the phase shift being equal to a fractional portion of the shift period.  
       [0037] There is further provided in accordance with some embodiments of the invention, a method of transmitting modem training signals, comprising transmitting, from a first modem to a second modem, a message including a value of at least one parameter of a training signal expected to be received by the first modem, and transmitting, by the second modem, the training signal expected to be received by the first modem, with a predetermined value of the at least one parameter, which is independent of the value of the at least one parameter in the message. In an exemplary embodiment of the invention, transmitting the message comprises transmitting a J p  message of the V.92 protocol. Optionally, the at least one parameter is related to a length of the training signal. Optionally, the second modem comprises a digital modem.  
       [0038] There is further provided in accordance with some embodiments of the invention, a method of establishing a modem connection, comprising establishing a physical connection between a pair of digital modems, selecting a protocol which requires that at least one of the modems of the connection is a digital modem, to govern the transmission on the physical connection, and transmitting, on the established connection, at least one signal defined for evaluating analog effects of the physical connection.  
       [0039] Optionally, selecting a protocol comprises selecting a protocol which uses different provisions for upstream and downstream transmission. Optionally, transmitting at least one signal defined for evaluating analog effects comprises transmitting a signal for evaluating a phase shift caused by the physical connection. Alternatively or additionally, transmitting at least one signal defined for evaluating analog effects comprises transmitting a signal for determining a required echo cancellation and/or for channel equalizer training.  
       [0040] There is further provided in accordance with some embodiments of the invention, a modem, comprising a line interface adapted to receive signals transmitted in a downstream of a modem connection in accordance with a modem protocol which has different provisions for the upstream and the downstream, and a digital interpretation unit adapted to receive samples from the line interface, after passing through fewer than all of an echo canceller, an automatic gain controller, a rate converter, a time recovery unit, an equalization unit and a channel filter.  
       [0041] Optionally, the line interface is adapted to receive signals transmitted in a downstream of a modem connection in accordance with the ITU V.90 or V.92 protocol. Optionally, the digital interpretation unit is connected directly to the line interface. Optionally, the digital interpretation unit comprises a symbol decision module adapted to receive samples directly from the line interface.  
       [0042] Possibly, the line interface is adapted to receive symbols at a rate of 8000 symbols per second. Optionally, the line interface is connected to a digital trunk line. Optionally, the digital interpretation unit is adapted to receive samples from the line interface, without the samples passing through any unit for overcoming analog signal effects.  
       [0043] There is further provided in accordance with some embodiments of the invention, a modem, comprising a line interface adapted to transmit signals in accordance with an upstream of a modem protocol which has different provisions for the upstream and the downstream, on a communication line, and a digital transmission unit adapted to transmit samples through the line interface, without passing the samples through both a precoder filter and a prefilter.  
       [0044] Optionally, the digital transmission unit is adapted to transmit samples without passing through a precoder filter or a prefilter. In some embodiments of the invention, the line interface is adapted to transmit signals in accordance with the upstream of the V.92 modem protocol. Optionally, the digital transmission unit comprises a constellation point selector which is connected to the line interface without intervening filters.  
       [0045] Optionally, the digital transmission unit performs calculations using 16 bit linear level samples and the modem comprises a converter, which converts 16 bit linear level samples into PCM samples. Optionally, the line interface is adapted to transmit signals onto a digital trunk.  
       [0046] There is further provided in accordance with some embodiments of the invention, a method of forming a modem connection, comprising establishing a physical connection between a first and a second modem, and transmitting a message including data on the capabilities of the first modem, from the first modem to the second modem, the message purposely identifying the first modem as being of a different type, having a different capability or being connected differently than the first modem is actually connected.  
       [0047] Optionally, the first modem is digitally connected and the message identifies the first modem as being analog connected. Optionally, the message identifies the first modem as supporting an all digital protocol. Alternatively, the message does not identify the first modem as supporting an all digital protocol. Optionally, the second modem is digitally connected. Optionally, the message identifies the first modem as supporting at least one protocol which differentiates between analog connected and digital connected modems.  
       [0048] Optionally, the message identifies the first modem as supporting at least one of the V.90 and V.92 protocols. Optionally, the method includes receiving a user instruction on whether to identify as an analog or digital connected modem and wherein the message identifies the connection of the modem responsive to the user indication. Optionally, the method includes determining whether to identify as analog or digital connected responsive to a telephone number of the second modem and wherein the message identifies the connection of the modem responsive to the determination. Optionally, the message is transmitted during a protocol negotiation procedure. Optionally, the first modem does not perform, during the protocol negotiation procedure, at least one test required to determine analog connection parameters. Alternatively or additionally, the first modem disregards results of at least one test required to determine analog connection parameters. 
     
    
    
     BRIEF DESCRIPTION OF FIGURES  
     [0049] Particular non-limiting embodiments of the invention will be described with reference to the following description of embodiments in conjunction with the figures. Identical structures, elements or parts which appear in more than one figure are preferably labeled with a same or similar number in all the figures in which they appear, in which:  
     [0050]FIG. 1 is a schematic time chart of training signals transmitted according to a portion of the third phase of the V.92 standard, as is known in the art;  
     [0051]FIG. 2 is a schematic illustration of a V.90 connection as is known in the art;  
     [0052]FIG. 3A is a schematic block diagram of an exemplary receiving path of a V.90 or V.92 modem, as is known in the art;  
     [0053]FIG. 3B is a schematic block diagram of an exemplary transmission path of a V.92 modem, as is known in the art;  
     [0054]FIG. 4 is a schematic illustration of a modem connection, in accordance with an embodiment of the present invention;  
     [0055]FIG. 5 is a flowchart of the actions performed by a digital modem in forming a modem connection, in accordance with an embodiment of the present invention;  
     [0056]FIG. 6 is a schematic illustration of a transmission path for transmission of training signals of the V.92 protocol, in accordance with an embodiment of the present invention;  
     [0057]FIG. 7 is a schematic illustration of a transmission path for transmission of training signals of the V.92 protocol, in accordance with an embodiment of the present invention;  
     [0058]FIG. 8 is a schematic block diagram of a receiving path of a modem, in accordance with an embodiment of the present invention; and  
     [0059]FIG. 9 is a schematic block diagram of a transmission path of a modem, in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
     [0060]FIG. 4 is a schematic illustration of a digital modem connection  20 , in accordance with an embodiment of the invention. A mobile unit  22 , such as a wireless access protocol (WAP) unit, forms a communication connection With a server modem  24  (usually belonging to a modem pool) of an Internet service provider (ISP)  29 . The signals transmitted and received by mobile unit  22  are optionally passed over a cellular link  30  to and from a base station  26  of a cellular company. Optionally, the signals to and from mobile unit  22  are passed through a client modem  28  (also generally belonging to a modem pool) associated with base station  26  or with another base station of the cellular company. Modem  28  is connected to modem  24  through a digital network  23 , which is optionally formed only of digital links, such as E1 or T1 links. Digital network  23  may belong to a private or public communication network, such as PSTN  19  (FIG. 2), and/or may comprise a plurality of concatenated data links or may be formed of a single link. Modems  28  and  24  optionally connect to digital network  23  through digital lines  36  and  32 , respectively.  
     [0061] It is noted that FIG. 4 is brought by way of example, and the apparatus and/or methods of the present invention may be used advantageously for substantially any other digital modem connections. For example, a computer may connect to client modem  28  through an ISDN connection instead of through cellular link  30 .  
     [0062] In some embodiments of the invention, server modem  24  comprises a standard server modem, which is configured to connect to both digital and analog client modems. Alternatively, server modem  24  is configured to connect exclusively to analog client modems. In some embodiments of the invention, server modem  24  does not differentiate between analog connected client modems and digitally connected client modems, which operate according to the present invention.  
     [0063]FIG. 5 is a flowchart of the actions performed by modem  28  in forming a modem connection between mobile unit  22  and ISP  29 , in accordance with an embodiment of the invention. Upon receiving (70) a request to form a connection with ISP  29  from mobile unit  22 , modem  28  optionally establishes (72) a V.8 negotiation connection with server modem  24  of ISP  29 . During the negotiations of the V.8 connection, client modem  28  and server modem  24  exchange ( 74 ) ability messages (e.g., CM, JM) which state which protocols are supported by the modems. In some embodiments of the invention, client modem  28  identifies itself as supporting one or more analog-to-digital protocols, which optionally use different transmission schemes for the upstream and downstream, e.g., V.90 or V.92. It is assumed herein that server modem  24  supports at least one of the analog-to-digital protocols supported by client modem  28 .  
     [0064] Optionally, client modem  28  identifies itself as an analog modem, e.g., in the access category of the V.8, such that server modem  24 , which is a digital modem, will relate to the modem connection as an analog-to-digital modem connection and agree to use an analog-to-digital protocol for the connection. Alternatively, client modem  28  identifies itself as a digital modem, for example, when one or more analog-to-digital protocols state that in case both end modems of the connection are digital modems, the call modem (i.e., the client modem) acts as the analog modem.  
     [0065] Further alternatively or additionally, client modem  28  determines, before transmitting the ability message, whether the remote modem, e.g., server modem  24 , is an analog or digital modem. In some embodiments of the invention, the determination of whether the remote modem is an analog or digital modem is performed by receiving a user indication, from a user knowing or guessing whether the remote modem is an analog or digital modem. Alternatively or additionally, the determination is performed automatically based on a pre-configured table and/or function and one or more parameters of the connection, such as the telephone number of the remote modem. It is noted that the determination is not necessarily always correct. When the determination is incorrect, the modem connection will fall back to using a slower protocol, e.g., the V.34 protocol.  
     [0066] In some embodiments of the invention, the ability message of client modem  28  includes an indication of support of one or more all-digital protocols, e.g., the V.91 protocol. In these embodiments, the ability message includes a contradiction as it states both digital and analog connection for the same modem. Alternatively, the ability message of client modem  28  does not indicate support of all-digital protocols.  
     [0067] As is known in the art, the protocol selected for use on the connection is the fastest protocol supported by both server modem  24  and client modem  28 . After the V.8 negotiation stage, tests are performed ( 76 ) in order to evaluate the quality of link  32  and/or to determine parameters of the connection, in accordance with the selected protocol, as is known in the art. In some embodiments of the present invention, client modem  28  skips ( 78 ) tests required to determine analog connection parameters and/or to evaluate analog lines. For example, client modem  28  optionally does not transmit test signals used to estimate echoes which are to be canceled, e.g., the MD signal of the V.90 protocol, and/or signals used to perform channel equalizer training, e.g., the PP signal of the V.90 protocol. Alternatively or additionally, client modem  28  transmits signals related to non-required tests although the results of the tests are ignored, so that server modem  24  is not confused and/or does not realize that client modem  28  is not an analog connected modem.  
     [0068] In some embodiments of the invention, client modem  28  ignores results of tests, and/or test signals, conveyed from server modem  24 , when the tests relate to analog connections and therefore the results of the test are known or irrelevant. For example, the test results may include a parameter to be used in determining a training signal to be transmitted by client modem  28 . In an exemplary embodiment of the invention, when during implementing the V.92 protocol client modem  28  receives the J p  signal transmitted by server modem  24 , client modem  28  ignores the transmitted value for ε and uses a value of ½, which is the expected value for a digital connected modem.  
     [0069]FIG. 6 is a schematic illustration of a transmission path  100 , of a digital client modem, for transmission of training signals of the V.92 protocol, in accordance with an embodiment of the present invention. Transmission path  100  comprises a data generator  102  and a symbol generator  104 , which generate training signals as required by the V.92 protocol, e.g., the S u  and S u -not signals. A resampler  106  and a D/A converter  108  convert, under the control of a clock  110 , the generated symbols into analog signals, which are transmitted on an analog line  114 . An analog to digital converter (A/D)  112  reconverts the signals into a digital form, e.g., PCM samples, in which the signals are transmitted on digital network  23 .  
     [0070] As described above with reference to FIG. 1, during phase  3  of the V.92 protocol, transmission path  100  is required to transmit a first instance of the S u  signal for 144 samples, the S u -not signal for 24.5 samples and then transmit a second instance of the S u  signal which has a half-sample phase shift relative to the first instance of the S u  signal. Optionally, the half-sample shift is performed by resampler  106  and/or D/A converter  108  using methods known in the art.  
     [0071]FIG. 7 is a schematic illustration of a transmission path  150 , of a digital client modem, for transmission of training signals of the V.92 protocol, in accordance with another embodiment of the present invention. Transmission path  150  comprises data generator  102  and symbol generator  104 , which generate training signals, substantially as in transmission path  100 . Unlike path  100 , however, path  150  does not include D/A converter  108  and A/D converter  112 , which add to the cost of the modem  28  and are not really needed. Instead, transmission path  150  optionally comprises an interpolator  152  and a decimator  154 , optionally with 1:2 and 2:1 rates, respectively. When a phase shift is required, e.g., in the interface between S u -not signal  504  and S u  signal  506  (FIG. 1), decimator  154  will skip one of the output samples of interpolator  152  or will use one of the output samples twice. Alternatively, decimator  154  operates at the interface between signals  504  and  506  on one interpolated value from signal  504  and a second interpolated value from signal  506 , thus forming a decimated value formed from fractions of both S u -not signal  504  and S u  signal  506 .  
     [0072] Referring also to FIG. 1, in some embodiments of the invention, interpolator  152  and decimator  154  are used only during phase  3  or only during the transmission of the S u  signal and the S u -not signal. Optionally, the use of interpolator  152  and decimator  154  is terminated after the completion of the transmission of the second instance of S u -not signal  510  (FIG. 1) or responsive to receiving the J p  signal transmitted by server modem  24 . Alternatively, interpolator  152  and decimator  154  are brought into use immediately before the phase shift is required.  
     [0073] Alternatively or additionally to using interpolator  152  and decimator  154 , data generator  102  and/or symbol generator  104  generate signals S u  and S u -not using pre-stored samples of the signals. Optionally, modem  28  stores a series of samples of the entire signal to be transmitted including the phase shifted portions, which are prepared at the time of manufacture of the modem and/or at a later initialization process. In an exemplary embodiment of the present invention, data generator  102  stores a first sample series covering S u  signal  502 , S u -not signal  504  and a long interval of S u  signal  506  and a second sample series covering S u -not signal  510 . The stored length of S u  signal  506 , is optionally longer than ever expected to be transmitted. When S u  signal  502  is to be transmitted, data generator  102  transmits the stored signal until J p  signal  508  is received, and then transmits the stored sample series representing S u -not signal  510 .  
     [0074] Alternatively, S u  signal  502  and S u -not signal  504  are cyclic signals and modem  28  stores a single cycle of each of the signals, with the phase shift and without the phase shift. During transmission of S u  signal  502 , modem  28  repeatedly transmits the stored cycle of the S u  signal which does not have a phase shift. Thereafter, modem  28  transmits S u -not signal  504 , by repeatedly transmitting the stored cycle of the S u -not signal without the phase shift. After transmitting 24 samples of S u -not signal  504 , modem  28  optionally transmits an interface symbol formed of fractions of both of signals  504  and  506 , and then repeatedly transmits the stored cycle of the S u  signal, with the phase shift. After receiving J p  signal  508 , client modem  28  transmits the stored cycle of the S u -not signal with the phase shift.  
     [0075] Further alternatively or additionally, modem  28  manages a table which includes all the possible samples of the training signals, e.g., S u  and/or S u -not, with a phase shift and without a phase shift. The S u  signal and the S u -not signal, for example, each has six samples in each cycle, such that the table optionally includes 12 samples for each of the signals, six without the phase shift and six with the phase shift. Optionally, samples at different positions in the cycle which have the same value appear in the table only once. Alternatively or additionally, as the S u -not signal is a mirror image of the S u  signal, a single set of values is used for both signals. In generating the training signal, the modem keeps track of the current position in the cycle, whether the S u  or S u -not signal is transmitted and whether the phase shift is required. Each time a sample is to be generated, the table is optionally accessed based on the position in the cycle, which signal is transmitted and whether the phase shift is required.  
     [0076] Referring back to FIG. 5, after the modem training is completed, a modem connection ( 80 ) is established and data is transmitted ( 82 ) on the connection.  
     [0077] In some embodiments of the invention, during the data transmission, the entire transmission and reception paths defined for analog modems are used by client modem  28 , in addition to an A/D converter which converts the analog generated signals into digital samples, and a D/A converter for the opposite direction. Optionally, one or more of the units of client modem  28  which perform tasks not required for digital connections have a reduced complexity, e.g., use a shorter filter length than generally used.  
     [0078] Alternatively, for one or more protocols in at least one direction, substantially identical procedures from other protocols, which do not differentiate between analog and digital connections (e.g., V.34), are used. For example, the transmission path of the upstream of the V.90 protocol may be implemented as a V.34 transmission path, such as described in the above mentioned PCT application PCT/IL00/00198.  
     [0079] Further alternatively, as is now described with reference to FIGS. 8 and 9, client modem  28  includes, for one or more protocols, fewer units than required for an analog connected modem.  
     [0080] Referring back to FIG. 3A, it is noted that receiving path  40  of an analog modem known in the art may be viewed as formed of two major parts. A first part, an analog to digital conditioning unit, comprises units  44 ,  46 ,  48 ,  50 ,  52  and  54 , which are used to overcome analog signal effects. The analog to digital conditioning unit brings the analog signals received on line  42  substantially back to the state at which they were before the conversion performed by line card  16  (FIG. 2). A second part, a digital interpretation unit, comprises units  56 ,  58  and  60 . The digital interpretation unit translates the digital signals provided by the analog to digital conditioning unit into a form tangible by computer  10 .  
     [0081]FIG. 8 is a schematic block diagram of a receiving path  90  of modem  28 , in accordance with an embodiment of the present invention. Optionally, modem  28  receives digital signals on an input line  88  and passes the digital signals directly to a digital interpretation unit  86 . Optionally, the digital signals received on input line  88  are passed directly to symbol decision module  56  of digital interpretation unit  86 . In some embodiments of the invention, symbol decision module  56  performs detection of attenuation pad impairments and robbed bit signaling (RBS) in addition to determining the samples which represent the symbols. As the downstream digital signals from ISP modem  24  were not converted by a line card  16  (FIG. 2) to analog signals, modem  28  optionally does not perform the tasks of the analog to digital conditioning unit of path  40 , i.e., modem  28  does not perform echo cancellation ( 44 , FIG. 3A), and does not pass the signals through channel filter  46 , timing recovery module  48 , rate converter  50 , AGC unit  52  and equalizer  54  (FIG. 3A). Thus, modem  28  is simpler and has a lower CPU and power consumption than modems which would implement all the analog reception tasks. Reducing the CPU consumption of a single modem, allows increasing the number of modems implemented by a single processor of a modem pool.  
     [0082]FIG. 9 is a schematic block diagram of a transmission path  200  of client modem  28 , for transmission of data in accordance with the V.92 protocol, in accordance with an embodiment of the present invention. It is noted that the principles described with reference to transmission path  200  are not limited to the V.92 protocol and may be used for other protocols which differentiate between analog and digital modems. Transmission path  200  optionally includes modulus encoder  63 , convolution encoder  66  and inverse mapper  65 , as described above with reference to transmission path  61  (FIG. 3B). Transmission path  200 , however, does not include a prefilter  67  (FIG. 3B) as such a filter is required only for the analog connection of the modem. In addition, as is known in the art, precoder  64  (FIG. 3B) of transmission path  61  can be viewed as formed of a constellation point selector and a precoder filter. In some embodiments of the invention, as shown in FIG. 9, transmission path  200  includes only a constellation point selector  202  and does not include a precoder filter, which is required only for an analog modem.  
     [0083] Alternatively, to not implementing the filters, the filters are implemented using degraded values which require less processing resources and perform only partial filtering or no filtering at all.  
     [0084] Optionally, instead of the filters not used (or degraded), transmission path  200  includes a converter  210 , which converts 16 bit linear level samples, as provided by constellation point selector  202 , into PCM samples. Optionally, the conversion is performed by finding, for each 16 bit linear level sample the closest level in the 256 scale.  
     [0085] It is noted that the units of modem  28  shown in FIGS. 8 and 9 may be implemented in hardware, software on one or more processors and/or in any combination thereof. The illustration of the units of modem  28  as different blocks is for clarity of the explanation only.  
     [0086] It will be appreciated that the above described methods may be varied in many ways, including, changing the order of steps, and/or performing a plurality of steps concurrently. For example, the determination of whether the ability message transmitted by the client modem should indicate analog or digital support may be performed at any time until the actual transmission of the message, including before or after the establishment of the physical connection with the remote modem. It should also be appreciated that the above described description of methods and apparatus are to be interpreted as including apparatus for carrying out the methods, and methods of using the apparatus. The present invention has been described using non-limiting detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. It should be understood that features and/or steps described with respect to one embodiment may be used with other embodiments and that not all embodiments of the invention have all of the features and/or steps shown in a particular figure or described with respect to one of the embodiments. Variations of embodiments described will occur to persons of the art. Furthermore, the terms “comprise,” “include,” “have” and their conjugates, shall mean, when used in the claims, “including but not necessarily limited to.” 
     [0087] It is noted that some of the above described embodiments may describe the best mode contemplated by the inventors and therefore may include structure, acts or details of structures and acts that may not be essential to the invention and which are described as examples. Structure and acts described herein are replaceable by equivalents which perform the same function, even if the structure or acts are different, as known in the art. Therefore, the scope of the invention is limited only by the elements and limitations as used in the claims.