Abstract:
A modem comprising entirely digital components and circuitry for receiving a digital signal of a first format from a first device, converting the digital signal to a digital signal of a second format and transmitting the digital signal of a second format to a second device. In one embodiment, a digital signal processor replaces at least a portion of the digital components and circuitry for receiving a digital signal of a first format and converting the digital signal to a digital signal of a second format and transmitting the digital signal of a second format to a second device.

Description:
This non-provisional patent application claims the benefit of U.S. Provisional Application Ser. No. 60/058,444, filed Sep. 10, 1997, entitled, “METHOD FOR DYNAMICALLY DOWNLOADING A SOFTWARE IMAGE TO A DIGITAL MODEM.” 
    
    
     COPYRIGHT NOTICE 
     Contained herein is material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction of the patent disclosure by any person as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all rights to the copyright whatsoever. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to the field of high speed modems for use in telecommunications. More specifically, the present invention is related to a modem comprised entirely of digital components. No analog to digital (A/D) or digital to analog (D/A) converters are utilized by the modem of the present invention when modulating or demodulating data transmitted to or received from a digital transmission medium, thereby increasing performance while decreasing cost of the modem. Additionally, a DSP-based embodiment of the modem of the present invention may be dynamically configured by downloading a software image. 
     2. Description of the Related Art 
     With respect to FIG. 1A, a traditional telecommunications path is illustrated over which a computer such as Personal Computer (PC)  100  communicates with another computer such as PC  160 . The communication utilizes the Public Switched Telephone Network (PSTN) as may be necessary with communication between computers that are separated by a distance greater than can be accommodated by present Local Area Network (LAN) technology. The computers may communicate with each other as part of a client-server network, or to exchange electronic mail, or the like. 
     PC  100  is shown connected to a MOdulator/DEModulator (modem)  110  over line  170 . Line  170  is typically an asynchronous link, e.g., an RS- 232  line that transmits and receives data as a digital signal  160  as indicated in FIG.  1 A. Modem  110  converts the digital signal  160  originating from PC  100  to an analog signal  161  for transmission over analog line  171 , commonly referred to as a “local loop”. In this particular example, modem  110  is external to PC  100 . However, it is understood that modem  110  may be internal to PC  100 , in which case, line  170  is unnecessary. In either case, modem  110  operates in the same manner. Likewise, PC  160 , line  175  and modem  150  function in the same manner as described for PC  100 , line  170  and modem  110 , to provide analog signal  163  for transmission over line  174 , the analog local loop with respect to PC  160 . 
     Analog Transmission over the Local Loop 
     A computer, just like a telephone, is commonly connected to a Central Office Exchange (CO) over a “local loop”. This local loop is commonly a single pair of wires that transmits and receives voice or data transmissions in the form of an analog signal. For example, PC  100  transmits a digital signal  160  containing data to modem  110 , which converts the digital signal to an analog signal  161  for transmission over analog line  171 . Line  171  represents a local loop between PC  100 /modem  110  and CO  120 . 
     While the local loop today carries voice and data transmissions, it originally carried only voice transmissions. Voice signals arc, of course, characterized by an analog, not digital, waveform of varying amplitude and frequency. Voice frequency bandwidth, i.e., voiceband, is approximately 4000 Hz, but the frequency band actually passed in the telephone network is in the 300 Hz to 3000 Hz range, which is adequate for the human ear to recognize basic qualities associated with speech such as voice recognition, intonation, emotion, etc. This frequency range was chosen as a compromise—increasing the frequency range results in better voice quality, but at a greater expense. Hence, the local loop was initially designed to transmit analog signals of limited bandwidth. 
     Virtually every home or small business initially transmitted voice information as an analog signal over a local loop comprised of a twisted wire pair to a local telephone company&#39;s CO. Presently, particularly in metropolitan areas, the pair of wires comprising the local loop is being replaced by a transmission medium, e.g., wire, fiber optic or radio, capable of transmitting voice and data as a digital signal. However, the local loop portion of the worldwide telecommunications infrastructure remains overwhelmingly analog. Thus, there is an ongoing need for modems to convert from digital to analog signals when transmitting data over the local loop or to convert from analog to digital signals when receiving data over a local loop. 
     While the local loop remains mostly analog, the switching systems in the central office and transmission lines in the PSTN are commonly digital. Thus, a brief review of CO systems and digital transmission systems utilized in the PSTN, such as T1, is useful for an understanding of the present invention. 
     Digital Transmission over the PSTN 
     A Central Office (CO) is a switching system operated by a local phone company for connecting, maintaining and releasing a local call between two devices, such as telephones, facsimile machines, or modems, within the geographical area managed by the local office. A toll, or long distance connection between devices in different cities or states, however, requires utilization of the Public Switched Telephone Network (PSTN). For example, given a scenario in which telephones are located in different cities or states managed by separate COs, the PSTN provides an interconnection between the COs so that the telephones can establish a toll call for voice transmission between them. While a local loop transmits a single local call at a time, the PSTN multiplexes multiple toll calls at the same time over the same transmission medium utilizing, for example, a time division multiplexing (TDM) technique, to improve the efficiency of the PSTN. 
     Additionally, digital transmission techniques are often utilized to further enhance the performance and efficiency of the PSTN. As a result, analog signals transmitted by a telephone over the local loop to a local central office are generally converted to digital signals and coded, for example, using Pulse Code Modulation (PCM) techniques, at the local central office before being transmitted over the PSTN. The digital signals may then be decoded and converted back to analog signals at a receiving central office to recover the voice transmission before being transmitted over another analog local loop to a receiving telephone. Thus, a central office utilizes a COder/DECoder (CODEC) to convert analog signals received from an attached local loop to an encoded digital data stream for transmission over the PSTN. 
     Likewise, a CODEC in the receiving CO decodes the encoded digital data stream received from the PSTN into analog signals for transmission over the local loop. As will be discussed below, if the local loop to which a receiving telephone is connected is capable of carrying digital transmissions, the digital signals need not be converted back to analog signals at the receiving central office, but may be transmitted as digital signals until converted to analog signals at the receiving telephone. 
     Analog to Digital /Digital to Analog Conversion 
     Given that the local loop portion of the telephone network is generally analog and the PSTN portion is generally digital, the need for analog to digital and digital to analog signal conversion at the CO is readily apparent. For example, with respect to FIG. 1A, digital signals  160  representing data transmitted from PC  100  must be converted to analog signals  161  at modem  110 . At CO  120 , the analog signals are converted to digital signals  162  for transmission over PSTN  130 . Again at CO  140 , the digital signals are converted to analog signals  163  and transmitted to modem  150 , where the analog signals are converted yet again to digital signals  164  for transmission to PC  160 . 
     Recall that human speech produces an analog waveform of varying amplitude within a frequency range of 0 to 4000 Hz. To convert an analog signal with sufficient accuracy to a digital signal for transmission over a digital line requires sampling the analog signal at the Nyquist rate, that is, at a rate of at least twice that of the greatest frequency bandwidth of the analog signal, according to the well known Nyquist theorem. As discussed above, the voice channel bandwidth is approximately 4000 Hz. Thus, a sampling rate of 8000 samples per second is necessary to convert an analog signal in the voice channel bandwidth to a digital signal. Many methods for sampling analog signals exist in the prior art, including Pulse Code Modulation (PCM), briefly reviewed below. 
     Pulse Code Modulation Techniques 
     Pulse Code Modulation (PCM) samples an analog signal over a period of time and converts the sample to a number selected from a range of numbers, which number most closely approximates the amplitude of the sample. More specifically, a sample of an analog signal is taken over a certain period of time, e.g., 0.125 milliseconds for a signal sampled at a rate of 8000 times per second. The sample, in turn, is converted to a number assigned to represent a sample whose amplitude falls within a defined range of amplitudes. The process of approximating the sample with a number is known as quantization. The range of amplitudes within which a sample falls is known as the quantization interval. The difference between the actual amplitude value of the sample and the number representing the quantization interval between which the sample falls is known as quantization error. 
     An encoder, or simply coder, at the central office samples an analog signal, quantizes the sample into a number, and then codes the number into a series of bits representing the number. Typically, a limited number of bits, e.g., eight, are utilized to represent the number. Eight bits per quantized number multiplied by 8000 samples per second produces a bit rate of 64000 bits per second (bps). Thus, a CO samples, quantizes and codes an analog signal received over the local loop into a 64000 bps PCM signal for transmission over the PSTN. Likewise, a decoder in the CO receiving the PCM signal decodes it and converts the decoded results back to analog signals for transmission over a local loop connected to the receiving device, e.g., a telephone. (The COder and DECoder in a CO are often part of the same Integrated Circuit (IC) in a CO, which IC is generally referred to as a CODEC.) 
     The quantization error introduced in the analog to digital conversion of a signal produces noise, which is often heard on a telephone receiver as a hissing sound in the voiceband. Reducing quantization noise is therefore desirable. By compressing the size of quantization intervals with respect to the analog signal such that quantization intervals are smaller for relatively small analog signals and larger for relatively large analog signals, quantization noise is reduced. Adjusting the quantization intervals in this manner results in compression of the quantized and coded signal. Accordingly, upon receiving a compressed PCM digital transmission, the decoder portion of the CODEC in a CO first expands the digital signal. Typically, a COMpressor/exPANDER (COMPANDER) unit in a CODEC compresses the PCM signal and expands the corresponding compressed PCM signal. To achieve interoperability in a public switched telephone network, CODECs adhere to a standard for compressing and expanding a PCM digital signal such as the well known -law or “mu-law” compander (U.S.) or A-law compander (Europe) standards. 
     T1 Digital Transmission 
     As discussed above, voice and/or data signals are typically transmitted over the local loop as analog signals and received by a local Central Office (CO), where they are sampled, quantized, and coded into digital signals according to a modulation technique such as the companded PCM digital coding technique. As a result, the digital signals can be transmitted over the PSTN at much higher frequencies such that multiple digital signals can be time division multiplexed over a single transmission medium, e.g., a T1 line (in the U.S., or E1 line in Europe). Each signal is commonly referred to as a channel. 
     A T1 line is capable of transmitting 24 multiplexed digital channels (“DSOs”) plus 8000 bits of framing information per second. The framing information concerns where each DSO starts and stops in the T1 bit stream. Recall from the above discussion that each PCM digital signal transmits 64000 bits per second. 24 channels multiplied by 64000 bits per second plus 8000 bit per second for framing information equals 1.544 Mbps. Thus, a T1 line must be capable of transmitting data at a maximum rate of 1,544,000 bits per second (1.544 Mbps). Additional digital transmission systems such as T3, D4, ESF (Extended Superframe) operate on similar principles as the T1 line and offer similar or greater advantages. 
     Central Office Systems 
     As discussed above, the central office typically receives an analog signal from the local loop and converts it to a digital signal such as a compressed PCM digital signal for transmission over the PSTN. For example, with reference to FIGS. 1A and 1B, central office  120  receives an analog signal  161  from modem  110  over the local loop represented by line  171 . CO  120  receives the analog signal at local loop interface  120   a , which provides battery, voltage protection, ringing, supervision and other functions related to interfacing with the local loop. As is well known in the art, hybrid  120   b  receives the analog signal from local loop interface  120   a  and performs two wire to four wire conversion for transmission of the signal over longer distances. Before the analog signal can be sampled by CODEC  120   d , it first passes through a band pass filter  120   c   1  to ensure the proper frequency range, e.g., 300 Hz to 3000, is sampled. 
     A coder subsystem  120   d   1  of CODEC  120   d  then converts and compresses the analog signal received from bandpass filter  120   c   1  to a compressed PCM digital signal  162  appropriate for transmission over a channel in a T1 transmission system  172  or the like through PSTN  130 . Similarly, decoder subsystem  120   d   2  of CODEC  120   d  converts and expands a compressed PCM digital signal  162  received from a channel of T1 transmission system  172  to an analog signal, which is then smoothed at filter  120   c   2  before being transmitted over hybrid  120   b  and local loop interface  120   a  to the local loop  171 . Importantly, quantization error occurs only in the analog to digital conversion process performed by CODEC  120   d  when the analog signal received from the local loop is approximated, i.e., quantized. No further quantization error occurs, however, when converting the digital signal received from the PSTN to an analog signal. 
     Digital Local Loop 
     Analog modems are necessarily utilized to convert digital signals received from an attached device such as a PC to analog signals for transmission over an analog local loop to the central office. Conversely, analog modems are utilized to convert analog signals received over the analog local loop to digital signals for transmission to an attached device such as a PC. Analog modems use various standard modulation and demodulation techniques to perform the conversion (e.g., V.22bis, V.32bis). However, analog modems inefficiently utilize the telecommunications network since they transmit analog data at, for example, 14,400 bits per second over the local loop, yet each local loop is assigned to a DSO capable of transmitting a digital version of the data at 64,000 bits per second from the central office over the PSTN. 
     V.32 modems are capable of bit rates up to 57,600 bits per second using 4:1 data compression, but data compression is not always possible depending on the data being compressed and/or the line quality of the local loop. Using advanced signal processing techniques, V.34 modems can theoretically achieve bits rates of up to 115,200 bits per second using 4:1 data compression. However, as a practical matter, such modems generally provide significantly slower bits rates under typical line conditions. Additionally, CODECs are employed at the CO to convert analog signals received over an attached local loop to digital signals for transmission over the PSTN. The conversion results in quantization error as described above. A local loop capable of carrying digital signals removes the need for CODECs at the Central Office, and avoids the quantization error otherwise induced by the CODEC in converting analog signals to digital signals. 
     Thus, the analog local loop is increasingly being replaced by a digital local loop, particularly where the device attached at the end of the loop opposite the central office is a computing device such as a server, mainframe computer, or switch, providing access to online services or the like for multiple clients. The modem connecting the computing device to the digital local loop is a “digital modem”. However, the so-called digital modem is essentially a standard modem that converts analog signals otherwise transmitted over an analog local loop into a digital data stream for transmission over a digital local loop. 
     In such cases, a digital signal received at the central office from the PSTN does not need to be expanded and converted to analog before it is transmitted over the digital local loop to a destination computing device. Rather, the digital signal passes unchanged through the CO to the destination device. Under these circumstances, the CODEC and filtering functions typically performed at the CO necessarily migrate from the CO to the destination device or digital modem connected to or incorporated as part of the destination device. As with the CO, however, the prior art CODEC comprises analog components that induce quantization error as the digital signal received from the CO over the digital local loop is converted to analog and back to digital to recover the data transmitted by the device at the other end of the end-to-end connection. 
     For example, FIG. 2A illustrates a PC  100  capable of communicating with computing device  220  (which is perhaps a server, mainframe computer, or another PC) over a PSTN  130  and a Local Area Network or Wide Area Network (LAN/WAN)  212 . PSTN  130  and LAN/WAN  212  are communicatively coupled via a Remote Access Server (RAS)  210 . A modem  110  (not shown) is internal to PC  100 , which is corrected to a local CO  120  over an analog local loop represented by line  171 . A digital data stream  160  generated by PC  100  is converted to analog signals  161  via the internal modem before being sent over analog line  171  to CO  120 . A CODEC within CO  120  converts the analog signals into compressed PCM digital signals  262  for transmission over, e.g., a T1 channel in PSTN  130 . The compressed PCM digital signals are received at a remote CO  140  and transmitted unchanged over line  211  to RAS  210 . At RAS  210 , the compressed PCM digital signals are expanded and then converted to analog signals  265  as an intermediate step performed by a modem associated with RAS  210  (not shown) to recover the original data transmitted by PC  100  over local loop  171 . The conversion to analog signals is an intermediate step because the data stream is then converted by the modem to digital signals  266  for transmission over LAN/WAN  212  to computing device  220 . Line  211  is a digital local loop utilizing, e.g., an Integrated Services Digital Network (ISDN) or T1 transmission system. Thus, while the CODEC functionality remains with CO  120 , because line  211  between RAS  210  and CO  140  provides for digital signal transmission, the CODEC functionality migrates to a modem utilized by RAS  210 . 
     Interestingly, a modem such as utilized by RAS  210  in FIG. 2A is known as a digital modem because it transmits and receives digital signals in both directions, notwithstanding that it performs intermediate digital to analog (D/A) and analog to digital (A/D) conversions for interpolation from a digital signal of one format, e.g., companded PCM digital signals, to a digital signal of another format, e.g., Manchester encoded digital signals as may be utilized in an IEEE 802.3 LAN or the like. As a result of the analog to digital conversion, quantization error still occurs at RAS  210  (in addition to the quantization error that occurred at CO  120  in converting the received analog signals to a digital signals), even though both lines  211  and  212  arc digital transmission lines. 
     The communication network illustrated in FIG. 2A is one example of the use of a digital local loop  211  and so-called digital modem. While the digital local loop initially replaced the analog local loop between the CO and a server of an Internet Service Provider (ISP) or mainframe computer in a corporate intranet, increasingly, homes, home offices and small businesses have installed a digital ISDN line for digital transmission of data over the local loop to a client or PC. For example, a home office may have a PC that exchanges data with, and particularly, downloads extensive amounts of data from computing device  220  using online services such as Prodigy or CompuServe, the Internet, the World Wide Web (WWW) graphical portion of the Internet, a corporate intranet, or the like. In such case, a digital line such as an ISDN line, an ISDN terminal adapter and an ISDN compatible modem replaces analog line  171  and internal modem  110  to achieve faster response times and data throughput between PC  100  and computing device  220 . The ISDN terminal adapter and modem at PC  100 , just like the so-called digital modem in RAS  210  of FIG. 2A, comprises filtering and CODEC systems for expanding and converting a compressed PCM digital bit stream received over the ISDN line to analog signals to recover the data transmitted from computing device  220 . Utilization of the ISDN line avoids the use of filtering and CODEC systems at CO  120 . However, due to the analog components in the prior art modem, quantization error is still introduced by the D/A and A/D conversion performed by the modem in recovering the data transmitted from computing device  220 . What is needed is a truly fully digital modem capable of receiving a digital data stream of one format and converting it to a digital data stream of another format without performing the intermediate steps of D/A and A/D conversion. 
     Interestingly, in the prior art, this conversion of the data from digital signals to analog signals occurs regardless of whether the data was ever transmitted as analog signals over any portion of the end-to-end connection from computing device  220  to PC  100 . The conversion occurs because the digital modem in or associated with PC  100  is unable to ascertain over what transmission media the data traveled, and whether that media supported analog or digital transmission of data. In other words, the digital modem is unable to ascertain if the data was transmitted as analog signals at any point in along the end-to-end connection. Moreover, the digital modem associated with PC  100  is further unable to ascertain whether the data was converted from digital-to-analog and analog-to-digital by the analog components of a so-called digital modem at some point between the computing device  220  and the modem associated with PC  100 , e.g., at the digital modem associated with RAS  210 . Therefore, it must be assumed that analog transmission occurred at some point in the telecommunications network, given the historical and ubiquitous nature of the analog local loop and analog components in the central office. In other words, it must be assumed that the digital signal received at the digital modem is merely an approximation of an analog signal. However, because PC  100  communicates and processes data according to a digital format, the recovered analog signals are converted by the so-called digital modem associated with the PC to a digital format appropriate for the PC. 
     With respect to FIG. 2B, RAS  210  comprises a modem bank  230 . Modem bank  230  may be implemented on a single printed circuit board, and thus, may also be referred to as a modem card. Modem bank  230 , in turn, comprises a number of prior art so-called digital modems, e.g., modems  230   a - 230   d , each respectively connected to a separate DSO channel  225 - 228  to receive a compressed PCM digital bit stream from T1 line  211 . T1 Data Service Unit/Communication Service Unit (DSU/CSU)  215  receives time division multiplexed digital transmissions from T1 line  211  and forwards the transmissions to multiplexer  220  which demultiplexes the transmissions into the 24 channels defined by the separate DSOs. Alternately, each modem receives a digital data stream from a computing device such as computing device  220  connected to LAN/WAN  212  and forwards the data stream to a DSO. The digital data stream is multiplexed in a TDM manner with the data streams received by the other DSOs from a corresponding modem and sent out the DSU/CSU interface and over T1 line  211 . It should be noted that while the modem card is contained within RAS  210  in the prior art embodiment illustrated in FIG. 2B, the modem card may be separate from RAS  210 . 
     According to one prior art implementation, modem bank  230  comprises up to  24  prior art digital modems each connected to a separate DSO. Each modem in the modem card is connected to an internal data bus  240  to which is connected to a controller  250  via line  248  and a network interface card  260  via line  249 . Controller  250  manages access to bus  240  and stores and downloads appropriate software to each of the so-called digital modems in modem card  230 . Additionally, controller  250  provides a user interface for configuring RAS  210  and performing other well known tasks associated with transmitting and receiving data over a telecommunications network such as data compression, error correction, etc. Controller  250  may also be responsible for upper-layer protocol functions and communication associated with LAN/WAN  212 , e.g., routing, flow control, network management, and transmitting and receiving data packets over LAN/WAN  212  via Network Interface Card (NIC)  260 . 
     When a modem in modem card  230  has received data from its corresponding T1 channel and is ready to transmit the data to LAN/WAN  212 , controller  250  provides the modem with access to internal data bus  240 , at which time the data is transmitted over bus  240  to NIC  260 . NIC  260  typically encapsulates the data in packets with upper-layer protocol header/trailer information and then transmits it over line  261  to LAN/WAN  212  in accordance with the protocols for communication over LAN/WAN  212 , e.g., the IEEE  802 . 3  Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Media Access Control (MAC) protocol and Transport Control Protocol/Internet Protocol (TCP/IP) suite of protocols commonly utilized for baseband communication over a Local Area Network (LAN). 
     As shown in FIG. 3, a prior art so-called digital modem  230   a  further comprises filtering and CODEC systems for expanding and converting the compressed PCM digital bit stream received from the corresponding T1 channel to analog signals regardless of whether the data transmitted from PC  100  over its local loop was in the form of analog or digital signals. In any case, because LAN/WAN  212  utilizes a digital transmission medium, the modem converts the analog signals to a digital format appropriate for communication with computing device  220  over LAN/WAN  212 . 
     With further reference to FIG. 3, digital-to-Analog Converter (DAC)/expander  345  receives compressed PCM digital signals from DSO  225 , expands and decodes the signals, and converts the signals to analog format. The analog signals are then sent to bandpass filter  350  for smoothing before being converted back to digital signals at Analog-to-Digital Converter (ADC)  355 . Receiver  360  buffers the digital signals before they are transmitted on internal data bus  240  where they will be received by controller  250  and/or transmitted out NIC  260  to LAN/WAN  212 . 
     A similar process occurs when receiving a digital data stream from LAN/WAN  212  for transmission over the PSTN via T1 line  211 . Data packets are received by NIC  260  from LAN/WAN  212  and placed on internal data bus  240 . Controller  250  receives the data packets, decapsulates upper-layer protocol headers/trailers from the data packets if necessary, allocates a modem in modem bank  230  and corresponding DSO, and transmits the digital data stream corresponding to the decapsulated data over bus  240  to the modem, e.g., prior art digital modem  230   a . Modem  230   a  receives the digital data stream in transmit buffer  340  before it is sent to DAC  335  for conversion to analog signals. Before the signals are converted to digital and compressed by ADC/compressor  325 , low pass filter  330  filters frequencies above, e.g., 4000 Hz from the analog signals so that ADC/compressor  325  samples the signals properly and accurately. 
     As discussed above, the prior art so-called digital modems incorporate analog components, which are relatively expensive, large, and slow, compared to digital circuitry. Additionally, the conversion of digital signals to analog and back to digital introduces quantization error into the transmitted data, even if all communication links over which the data is transmitted are digital, and in addition to any quantization error already present in the data as a result of conversion from an analog signal to a digital signal, for example, at an analog local loop/CO interface. Thus, what is needed is a fully digital modem that receives a digital signal of a first format and converts the digital signal to a digital signal of a second format using all digital circuitry and techniques. 
     BRIEF SUMMARY OF THE INVENTION 
     Disclosed is a digital modem that is capable of receiving a digital signal having a first format and converting the digital signal to a digital signal having a second format using entirely digital components. The fully digital modem avoids the expense and performance problems associated with so-called digital modems that comprise circuitry such as D/A and A/D converters for converting the received digital signal to analog and back to digital in converting between, for example, a digital bit stream and a compressed PCM digital bit stream. The digital modem operates according to standard speeds and functionality such as defined by the V.22bis, V.32, V.32bis, and V.34 standards and is capable of operating at transmission rates of 14.4 Kbps, 28.8 Kbps, 33.4 Kbps, and 56 Kbps. Additionally, the digital modem interfaces directly with T1, E1 or ISDN Basic Rate (ISDN BRI) or ISDN Primary Rate (ISDN PRI) digital transmissions. 
     An embodiment of the digital modem is proposed in which a general purpose DSP is dynamically downloaded with a software image to configure the digital modem, including its functions and features. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The present invention is illustrated by way of example and not limitation in the following figures. Like references indicate similar elements, in which: 
     FIG. 1A is an illustration of a prior art telecommunications network utilizing analog local loops from the telephone company&#39;s central office. 
     FIG. 1B is a block diagram of the prior art CODEC utilized at the central office to convert analog signals received from the local loop to digital signals for transmission over, e.g., a T1 digital channel. 
     FIG. 2A is an illustration of a prior art telecommunications network utilizing a digital local loop between the remote access server and the local central office. 
     FIG. 2B is a block diagram of a remote access server having a bank of modems each connected to separate channels of the T1 transmission line. 
     FIG. 3 is a block diagram of a modem within the remote access server illustrated in FIG.  2 B. 
     FIG. 4 is a block diagram of a fully digital modem as embodied by the present invention. 
     FIG. 5 is a block diagram illustrating an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a modem comprised, in one embodiment, entirely of digital components capable of transmission bit rates of 56 Kbps. In another embodiment, the digital modem comprises a general purpose Digital Signal Processor (DSP) and software downloaded to the DSP to perform the modulation/demodulation of data transmitted to/from the modem. No analog to digital (A/D) or digital to analog (D/A) converters are utilized by the modem of the present invention when modulating or demodulating data transmitted to or received from a digital transmission medium, thereby increasing performance while decreasing cost of the modem. 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known architectures, circuits, and techniques have not been shown to avoid unnecessarily obscuring the present invention. 
     In alternative embodiments, the present invention may be applicable to implementations of the invention in integrated circuits or chip sets, wireless implementations, switching systems products and transmission systems products. For purposes of this application, the terms switching systems products shall be taken to mean private branch exchanges (PBXs), central office switching systems that interconnect subscribers, toll/tandem switching systems for interconnecting trunks between switching centers, and broadband core switches found at the center of a service provider&#39;s network that may be fed by broadband edge switches or access multiplexors, and associated signaling, and support systems and services. The term transmission systems products shall be taken to mean products used by service providers to provide interconnection between their subscribers and their networks such as loop systems, and which provide multiplexing, aggregation and transport between a service provider&#39;s switching systems across the wide area, and associated signaling and support systems and services. 
     A brief overview of the environment in which the use of an embodiment of the present invention is contemplated is now provided for a better understanding of the invention. With respect to FIG. 2B, a block diagram of a Remote Access Server (RAS)  210  is shown. The RAS, also referred to as a central site server, integrates remote access to a LAN or WAN by way of a modem bank  230  for multiple devices such as PC  100  or the like. Each modem in the modem bank  230 , in turn, respectively connects to a separate DSO channel, e.g., DSO channel  225 , to receive a compressed PCM digital bit stream from T1 line  211 . T1 Data Service Unit/Communication Service Unit (DSU/CSU)  215  receives time division multiplexed digital transmissions from T1 line  211  and forwards the transmissions to multiplexer  220 . Multiplexer  220  demultiplexes the transmissions into the 24 channels defined by the separate DSOs. 
     Alternately, each modem receives a digital data stream from a computing device such as computing device  220  connected to LAN/WAN  212  and forwards the data stream to a DSO. The digital data streams are time division multiplexed and sent over the PSTN via DSU/CSU interface  215  and T1 line  211 . The individual DSOs combined into the T1 bit stream can be dropped out of or inserted into another T1 data stream as some switching point in the PSTN depending on the CO to which the data stream is being transmitted. 
     In RAS  210 , modem card  230  comprises up to 24 modems each connected to a separate DS0. (Alternatively, the modem card may comprise 32 modems each connected to a separate channel of an E1 line). Each modem in the modem card is connected to an internal data bus  240 . The internal data bus  240 , in turn, is connected to a controller  250  via line  248  and a network interface card  260  via line  249 . Controller  250  manages access to bus  240  and stores and downloads appropriate software to each of the modems in modem bank  230 . Additionally, controller  250  provides a user interface for configuring RAS  210  and performing other tasks such as data compression, error correction, etc. Controller  250  is also responsible for upper-layer protocol functions and communication associated with LAN/WAN  212 , e.g., routing of data, transport services such as flow control of and acknowledgement for data, network management, and transmitting and receiving data packets over LAN/WAN  212  via Network Interface Card (NIC)  260 . Controller  250  provides a modem with access to internal data bus to transmit and receive data over bus  240  with NIC  260 . NIC  260  typically encapsulates the data in packets with upper-layer protocol header information and transmits the data packet over line  261  to LAN/WAN  212 . 
     While RAS  210  illustrates a single modem bank, wherein each modem in the bank is connected to a DSO of a single T1 line, it is appreciated that RAS  210  may comprise multiple modem banks, each housing a number of modems. Each modem bank may connect to a separate T1 line and communicate with controller  250  to access a separate internal data bus  240  for transmitting data over and receiving data from LAN/WAN  212  via a separate NIC  260 . Furthermore, each of the modems can be configured or optimized, via hardware and/or software means, to process different types of digital traffic, e.g., voice, video, audio, or data, depending on the type of bitstream to be processed by each modem. 
     With reference to FIG. 4, a block diagram of an embodiment of the digital modem of the present invention is illustrated. While the digital modem illustrated in FIG. 4 is embodied as a modem in a bank of modems in a remote access server, the fully digital nature of the modem is applicable to any modem technology, whether an external, stand-alone modem or one of the internal modems in a remote access server. 
     Digital modem  400  comprises digital signal processing circuitry that executes algorithms for expanding and converting the compressed PCM digital bit stream received from the T1 channel to which it is connected. The bitstream is converted to digital signals of a format and sampling rate utilized by the digital transmission medium of LAN/WAN  212  appropriate for communication with computing device  220 . Essentially, the digital modem embodiment illustrated in FIG. 4 interpolates a digital signal from one sampling frequency to another sampling frequency, utilizing only digital signal processing techniques. Unlike the prior art modem  230   a  illustrated in the block diagram of FIG. 3, no intermediate D/A and A/D conversion of the received digital signal is required. The embodiment of the present invention shown in FIG. 4 removes the need for the analog front end circuitry (DAA, A/D and D/A converters, and filters) and CODEC utilized by the prior art modem in FIG.  3 . 
     Transmitting data from LAN/WAN  212  through RAS  210  to T1 line  211  occurs as follows. A transmitter  340  receives digital signal samples from LAN/WAN  212  via NIC  260  and internal data bus  240  at a baud rate multiple synchronized by clock/phase lock loop circuitry  370  with the modem or the external clock coming from, for example, NIC  260 . The samples are forwarded to encoder/interpolator  430 , where samples are generated at 8 kHz in synchronization with T1 timing. The samples are converted in to mu-law (or A-law) 8-bit samples by compander  415  and stored in FIFO buffer  405 . The samples are transmitted serially from buffer  405  to multiplexer  220 , which multiplexes the samples in time domain fashion with other bitstreams to be transmitted over T1 line  211  via DSU/CSU  215 . The samples are then transmitted over T1 line  211 . 
     Receiving data from T1 line  211  through RAS  210  to WAN/LAN  212  occurs as follows. Compressed PCM digital signal samples are received at T1 DSU/CSU  215  and demultiplexed by multiplexer  220  into separate channels. FIFO buffer  410  receives the samples and forwards the samples to expander  425  where the samples are expanded (inverse mu-law or A-law). The expanded PCM samples are then interpolated by deocoder/serializer  435  from 8 kHz samples to samples at a baud rate multiple in synchronization with transmitter timing. A timing generation module (not shown) controls the encoder/interpolator and the decoder/serializer, taking into account the differences between the T1 timing and the modem timing. After subtracting far end echo from the signal and resampling the signal, the samples are output by decoder/serializer  435  and forwarded to receiver  360  before being placed on internal data bus  240 . Controller  250  controls access to bus  240  for receiver  360  when sending digital signals to NIC  260 . 
     DSP-based Embodiment of the Present Invention 
     With reference to FIGS. 4 and 5, although the embodiment of modem  400  illustrated in FIG. 4 comprises entirely digital circuitry to modulate between different digital signals, according to an alternative embodiment, modem  500  comprises a general purpose Digital Signal Processor (DSP) and accompanying software. Modem  500  offers very efficient as well as flexible implementation of the digital modem. A DSP-based embodiment is particularly advantageous because today&#39;s high speed modems must perform nontrivial signal processing tasks, such as data compression, error correction, etc., that are well suited for processing by a DSP. Just as the embodiment illustrated in FIG. 4, a DSP-based fully digital modem  500  does away with the need for the analog front end components such as DAA local loop hardware circuitry and A/D and D/A converters. It is appreciated that while there is a great advantage in utilizing the DSP-based digital modem  500  in a RAS  210  for the reasons stated above, in an alternative embodiment, a stand-alone digital modem might also incorporate the DSP and accompanying software. 
     The alternative embodiment  500  utilizing a DSP implementation for the digital modem of the present invention provides the ability to download software modules, or images, to the modem. For example, controller  250  can accept input regarding downloading of an image to the modem. Software may be downloaded to upgrade the modem to a new version of software, change the configuration or functionality of the modem, or provide standard or customized features, such as voice compression, error correction, etc., without requiring any hardware changes or onsite personnel to effect the reconfiguration. A flash EEPROM or the like may be utilized to upgrade or change the images available for downloading to the modem. For example, flash EEPROM can be loaded with a new a software module that could then be downloaded to change the configuration, with or without input relating to configuration settings from controller  250 , from a modem to a DSU, IDSN Terminal Adapter (ISDN TA), or X.25 PAD. Regardless of what configuration is selected, common hardware, i.e., the DSP, can be utilized. While the DSP-based embodiments described herein are with reference to modem technology, it is understood that such embodiments are generally applicable to related communication technologies, e.g., wire or wireless based forms of data and telecommunications. 
     Dynamic Downloading of an Image to DSP-based Modem 
     Moreover, the present invention contemplates dynamic downloading of an image to a DSP, e.g., a DSP in a digital modem. In this manner, diverse applications such as Internet phone, Internet video phone, fax, audio, video, etc., may be processed by the same modem simply by changing the image loaded in to the modem whenever switching between applications. An appropriate algorithm, or image, may be dynamically downloaded to the DSP from an image repository, e.g., EEPROM, flash EEPROM, CD-ROM, static or dynamic random access memory, magnetic media, etc, internal or external to modem bank  230  or RAS  210 , having stored therein different images. (In the case of a modem connected or internal to PC  100 , the image repository may reside in the modem, PC or an external data storage device). Modem  500  may directly access the image repository directly via an internal data bus, e.g., internal data bus  240 , or external data bus. Alternatively, controller  250  may control, with or without input from a user via a user interface, access to the repository. 
     While the embodiment illustrated in FIG. 5 shows a DSP dynamically configured with a software image in a modem  500  installed in a modem bank  230  of RAS  210 , it is contemplated that the DSP may be installed in any modem, which modem may be internally or externally connected to RAS  210 , or internally or externally connected to, e.g., PC  100 , or other computing device. 
     The timing of the dynamic download of an image to the DSP in modem  500  may be event driven. For example, an image may be downloaded during call set-up, power up, reset, or other event. Additionally, dynamic download of an image may be interrupt-driven. Finally, it is contemplated that dynamic download of a particular image may be driven by the content of the data stream received from a remote device, e.g., PC  100 , or a local device, e.g., computing device  220 . For example, a modem may request or command a download of a particular image based on different DTMF tones for fax, voice, video or data, or based on different telephone numbers received, for example, as a result of PC  100  initiating communication with computing device  220 .