Abstract:
A full services access multiplexer is described. A master DSL modem is coupled to a conductor pair. A POTS extender is coupled to the conductor pair and may sense the operation of a fallback or other signal on the conductor pair. A suppression signal may be transmitted to the master DSL modem upon occurrence of the fallback. The suppression signal may travel over a control circuit. Traffic over a backplane or other network segment may be uninterrupted to an Integrated Access Device by handling signals inbound and outbound to the backplane via packet assembler and disassembler (PAD). The PAD may transmit a data stream to vocoder and received a data stream from vocoder for injection onto the backplane. The vocoder connects duplexed traffic to the subscriber line interface circuit (SLIC), wherein traffic between the vocoder and the SLIC are in analog formats in or near the audible range of frequencies.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. application Ser. No. 09/820,029, filed Mar. 28, 2001, which is incorporated by reference in its entirety. 
     
    
     BACKGROUND  
       [0002]     The invention relates to broadband access methods and more particularly to providing a network-side surrogate when an access device is impacted by a fault.  
         [0003]     Current voice telephone system operators are generally engaging in a growth phase of deploying data services to their subscribers. One of the chief ways to do this is to offer digital subscriber lines (DSL) wherein both voice and data may be carried over a common twisted pair cable to a subscriber&#39;s residence or business. In one of the more popular versions of DSL, Asymmetric Digital Subscriber Line (ADSL), a twisted pair carries two types of duplexed signals over different frequency bands. The first signal is the voice signal, generally at 4 KHz and below. The second signal is the data signal, generally modulated at above 4 KHz.  
         [0004]     Other forms of DSL may inter-mingle voice signals with data, by, e.g. voice over packet (VOP). Such forms include: Symmetric DSL (SDSL), High bit-rate DSL (HDSL), and Very-high-bit-rate DSL (VDSL). In such cases, the voice signal may be modified by a Integrated Access Device (IAD) to be converted into data packets. A IAD is intended to provide access to the twisted pair by multiplexing at least one voice signal with other signals, such as data, which is used most commonly by computers for internet access. Such a device, coupled with DSL service can provide a great improvement to home-owners or small businesses because the cost to maintain a single twisted pair subscriber line that multiplexes various signals is much less than having a dedicated subscriber line for each device at the customer premises.  
         [0005]     Unfortunately, because the IAD is performing high level communications functions—essentially changing the signaling format between dissimilar networks—the IAD may be susceptible to power failures that leave none of the customer premises equipment operating. Although the data culture associated with computers has long accepted, in many situations, the possibility of intermittent failures, the opposite is true for the voice telephony world. Namely, in many parts of the world, a telephone is viewed as a necessity, and particularly, a valuable tool to avert disasters through timely call placement to emergency personnel, e.g. calling 911. For this reason, telephone switches are required to have an up time greater than 99.999% of the time.  
         [0006]     Thus the dilemma: how to offer fast data throughput, including that supporting voice, and maintain constant fault-free voice operation to a customer while keeping the number of subscriber lines between customer premises and data aggregator to a minimum.  
         [0007]     Part of the solution lies in the ability to adjust the operation of a remote device, by transmitting a minimalist signal from the local device that is failing—a last gasp, if you will. A signal may be the encoding of two or more values (sometimes voltages) that change over time on a medium. A packet may be a series of changing values or an arrangement of signals. A protocol may involve the exchange of one series of changes on a medium. A signal may occur when a voltage changes or where an established protocol requires a responsive reply, but none occurs. Under ordinary circumstances a master DSL master modem communicates with a slave DSL modem component of the IAD so that the slave DSL modem synchronizes or trains to signals provided by the master DSL. Any failure of the slave DSL modem to respond under this protocol may be taken as a signal that the slave DSL is in a fault mode.  
         [0008]     A data aggregator may be a Digital Subscriber Line Access Multiplexer (DSLAM) having at least one master DSL modem. Constant voice access has been accomplished for some forms of DSL by providing a secondary twisted pair line between a data aggregator and an IAD. Companies such as Coppercom provide a IAD that upon detecting a power failure, would route at least one of the telephones at a customer premises to the secondary twisted pair line—thus bypassing the data network altogether. For obvious reasons, it is twice as costly to maintain the two pairs of cabling from the data aggregator such as a switch or a DSLAM to the subscriber than in the situation where a single twisted pair is used. Nevertheless, the highly fault-resistant voice central office is much more reliably available than its counterpart, the IAD. This is true because central office (CO) equipment is more capable of providing reliability because of economies of scale, particularly due to CO backup batteries and other redundant power sources. Thus, there is a need to extend plain old telephone service (POTS) to an IAD that has failed in that the IAD is unable to transmit data packets.  
       SUMMARY  
       [0009]     Embodiments of the invention may provide a switch-over means at a data aggregator and other network elements when an integrated access device (IAD) is unable to transmit packets. Such a switchover may be accomplished in part by a detector that identifies the presence of analog signals on the subscriber line connecting the IAD to the data aggregator. Upon detecting such analog signal carrying e.g. voice frequencies, the data aggregator, according to the embodiments, may disable any DSL modem present at the data aggregator, and couple an analog to digital converter to prepare signals arriving on the subscriber line for transmission on a data network. Other devices such as a vocoder and packet assembler and disassembler may complete the conversion of the formerly analog signals for packet transmission through the data network. This accomplishes a couple of things. Continued telephone service may be maintained to a customer premises even though an IAD has a power failure—and this without the need for a dedicated backup analog subscriber line in addition to the subscriber line that carries digital packets. Consistent with the continued telephone service is the provision, at a central point, of power to the remote customer premise equipment, thus providing economies of scale that may be available where multiple diverse customer premises may need power back-up, and the power back-up is collectively provided ad the data aggregator.  
         [0010]     An embodiment may provide a fallback mode to a subscriber line. A POTS extender, which may convert a upstream voice signal on the subscriber line to at least one upstream packet. Similarly the embodiment may convert a downstream packet on a packet network to a downstream voice signal.  
         [0011]     The embodiment may have a subscriber line interface circuit (SLIC) connected to the subscriber line. A codec converts signals from the SLIC to an upstream digitized voice signal output. The codec also converts signals from a vocoder to downstream voice signals. A vocoder converts voice signals to datastreams in both the upstream and downstream directions. A packet assembler and disassembler (PAD) may convert the first data stream to at least one packet. The PAD may convert at least one packet into the second data stream, wherein the PAD is coupled to the packet network. The PAD may have at least one network address. The embodiment may include an output means for transmitting a Master Digital Subscriber Line modem control signal.  
         [0012]     Another embodiment may include a digital subscriber line (DSL) suppression circuit for suppressing DSL modem operation on a subscriber line. The embodiment may have a subscriber line interface circuit (SLIC) for sensing current drain on the subscriber line. A means for providing a suppression signal may connect to the SLIC and a master DSL modem may operate coupled to the SLIC, said master DSL modem operating in a quiescent state upon receiving the suppression signal.  
         [0013]     In the end, at least one embodiment may place a master DSL modem in a quiescent mode whereby voice traffic may operate on the subscriber line without any modulation by a master DSL modem interfering. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1   a  shows an architecture supporting voice over packet protocol according to an embodiment.  
         [0015]      FIG. 1   b  shows a comparison of two architectures supporting voice over packet protocol.  
         [0016]      FIG. 2  shows a block diagram of a Integrated Access Device (IAD) according to an embodiment of the invention.  
         [0017]      FIG. 3  shows a block diagram of a Full Services Access Multiplexer (FSAM) according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]      FIG. 1   a  shows an architecture supporting voice over packet protocol  100  including Integrated Access Devices (IADs) and data aggregators. The data aggregator may comprise a Digital Subscriber Line Access Multiplexer (DSLAM)  101  having a Full Services Access Multiplexer (FSAM) embodiment attached to at least one local loop  103 . A DSLAM is typically located at a central office and may interconnect voice packets of a subscriber line to a switch via circuit switched connection. In addition, the DSLAM may interconnect voice packets on a subscriber line to a packet network using Asynchronous Transfer Mode (ATM) and other network protocols. The FSAM terminates the local loop  103  that is served by an IAD  105 . The IAD  105  may be located at the customer premises.  
         [0019]     The IAD  105  may multiplex multiple terminals and may serve both an analog voice terminal  107  and a data terminal  109 . In the case that the terminal is a voice terminal, the IAD  105  may provide, among other services, conversion of voice and other analog signals to digital encoding; packetizing such encoding into packets; and addressing such packets according to conventional packet protocols. Other functions consistent with voice over Packet (VoP) may be provided by the IAD  105 , including providing signals of call progress over the analog line  111  at the customer premises, wherein the analog line is a customer premise line.  
         [0020]      FIG. 1   b  shows two example configurations of  FIG. 1   a  that include a data aggregator. In the first configuration, a voice terminal  145  connects to an IAD  105 , which provides access to a subscriber loop  150 . A data aggregator  101  may place signals arriving by subscriber loop  150  onto a data network  152 . A concentrator  155 , which may be compliant to GR-303 standards, concentrates signals with traffic arriving from other sources. A switch, e.g. a class 5 central office  157 , may route signals to one or more other voice terminals in a network, which includes the public switched telephone network (PSTN).  
         [0021]     In a second configuration, a data aggregator  101  may receive such packets or analog signals over the subscriber loop  150  and route the packets or analog signals according to commands given by a softswitch  173 . The softswitch  173  may direct such packets to one or more gateways  175 . Such commands, or signaling, may pass over a data network  115  capable of transmitting and routing packets, e.g. the Internet.  
         [0022]     In both configurations the data network  152  of the first configuration, and the data network  115  of the second configuration may provide a medium for receiving requests for service originating at the voice terminal  145  and  165  and controlled by the concentrator  155  or softswitch  173 .  
         [0023]     The data network  152  and  115  may operate cooperatively with the concentrator  155  or softswitch  173  and other network elements to perform call processing, billing and other functions related to transmission of voice traffic over a data network. A service provider may own or operate edge nodes of the data network  115  and secure agreements from owners or operators of neighboring nodes to assure continuing availability of packet transmission facilities. Since the operation of data networks such as the internet are known to have many levels of redundancy, traffic that reaches the network is virtually assured safe passage to a destination. Similarly, it is known that the data network may transmit a greater bandwidth with lower costs than may be accomplished in a more conventional circuit switched telephone network. The reliability and bandwidth of the internet is accomplished through the use of many redundant paths for transmission of data signals and control signals.  
         [0024]     Even though the local loop  103  of  FIG. 1   a  is without a redundant spare, the operation of the IAD  105  tends to be more failure prone—particularly due to transient power availability, including blackouts. The IAD may be provided with a kind of redundancy—and greatly improve the overall capability to reach the network for voice transmission purposes, even though data handling may be hampered by unavailable power.  
         [0025]     Thus, an IAD  105  according to an embodiment of the invention, may provide an IAD fallback signal in the local loop  103  to signal to the data aggregator  101  that the IAD  105  is operating in a fault mode, caused by, among other things, local power failure at the IAD  105 . The IAD  105  may operate passively, wherein either it fails to respond to packets that request a response, or the IAD  105  fails to provide a follow-up signal during a time-out expiration. In any event, the absence of signals from the IAD  105 , itself, is a signal that the IAD  105  is in a fault mode. The data aggregator  101  according to the embodiment, may adjust accordingly by refusing to pass along any data packets over the local loop  103  until the IAD  105  restores the packet transmission support to the local loop  103 . The restoration of packet transmission support may be signaled by a IAD-restore signal dispatched by the IAD  105  onto the local loop  103 . Such an IAD-restore signal may be part of the start-up handshake between a slave DSL modem  106  and a master DSL modem  126 , such as synchronization signaling.  
         [0026]      FIG. 2  shows the internal operation of an IAD  105  according to an embodiment of the invention having a switching means to bypass packet-transmitting circuits. At least one voice conductor pair  201  dedicated to a voice terminal, e.g. a conventional wired telephone, enter the IAD  105 . The architecture of the embodiment may function adequately without the embodiment of  FIG. 2 .  
         [0027]     In an unpowered situation, a relay  203   b  or other switching means provides a straight through connection between a voice terminal  205  and the data aggregator through the local loop  207 . Upon applying power to the IAD  105 , the central processing unit (CPU)  209  may do self testing, verify operation of IAD components, configure IP addresses and other activities of an IAD  105  necessary to route packets with correct headers and provide signaling functions. One or more of the foregoing steps occurs during a process called boot-up, or the step of booting up the processor. Upon completion of the boot-up, the CPU  209  may energize the relay  203   a  to switch in a vocoder  211  and a packet assembler and disassembler (PAD)  213 . The vocoder  211  may receive signals from the individual voice terminals  205  by way of a bank of SLICs  210 , which may provide basic power to each line  201 , among other signals. The bank of SLICs  210  may provide at least one upstream signal to codec  212 , wherein the codes  212  may provide an analog to digital conversion such that vocoder  211  receives digital signals. Similarly, codec  212  may convert digital signals received from the vocoder  211  to analog signals directed to the bank of SLICs  210 . Vocoder  211  may be a G.711 variety of vocoder. Alternatively, the vocoder  211  may provide compression to upstream signals and decompression to downstream signals using, e.g. G.726 algorithms. The state describing the IAD  105  upon the application of power and completion of boot-up is called the vocoder mode. Failure of power restores the relay  203  to its former condition, providing a straight-through connection between the voice terminal  205  and the local loop  207 . This state is called the bypass mode.  
         [0028]     Additional telephones may be attached to the IAD  105  through ports that connect voice either directly to the local loop  207  or to the bank of SLICs  211  depending on the operating mode. For each telephone that is connected via the vocoder  211  through a distinct port, a network address may be assigned. The address assignments may be made via a local man-machine interface or by remote control, perhaps using the packet network to carry the instructions. The addresses may be IP addresses and port pair combinations or Asynchronous Transfer Mode (ATM) Virtual Circuit (VC)/Virtual Path (VP) pair identifiers. Thus the IAD  105  inherently may have the capability to present to the network (at the subscriber line) multiple virtual circuit identifiers for so long as power is supplied to properly functioning IAD  105 . Storage of the address assignments may be in a local non-volatile memory to the IAD  105 . Unused ports may be jumpered by a short conductor. Additional packet ports  240  may provide an interface to the PAD  213  while power is available and the relays  203  are in the vocoder mode position. Similarly, one or more data streams may be sent upstream from vocoder  211  to PAD  213  for proper packet formatting and addressing. DSL modem  257  may provide physical layer support for packets bound from PAD  213  to the upstream data aggregator on the local loop  207 .  
         [0029]     The relay  203  may switch telephony ports  230  so that while the IAD  105  is in the bypass mode, all telephony equipment including facsimile equipment, are in the same circuit, commonly referred to as a party line. The operation of the relay  203 , may change the character of the local loop, by e.g. disabling the origination of data packets from the IAD  105 , while in bypass mode. Similarly, the IAD  105  may lose the ability to associate a voice circuit and other addressing of the various telephony equipment. Some other device in the data network must take on that job in order for a voice call to be routed over the data network, i.e. a device upstream from the IAD  105  must adopt this function, otherwise the local telephones will be unable to make emergency calls. In other words, a FSAM embodiment of the invention may provide a voice circuit association to the local loop  207 , among other functions.  
         [0030]     The detection of an off-hook condition of one or more of the telephones may be a preliminary test done by the CPU  209  following boot-up. The CPU  209  may periodically retest the line, if an off-hook condition has been detected, until all telephones connected to the IAD  105  are on-hook. Following power restoration, a boot-up and any off-hook period, the CPU  209  may energize the relay  203  to operate in a vocoder mode such that the vocoder  211  and the PAD  213  provide packet traffic to and from the local loop  207 . At that time, IAD  105  may multiplex and voice traffic and data traffic from local telephones and data sources on the local loop  207 .  
         [0031]      FIG. 3  shows a parallel configuration of a Full Service Access Multiplexer (FSAM)  315  comprised of a plain old telephone service (POTS) extender  301  and a master DSL modem  303  at the data aggregator, or FSAM. The POTS extender  301  may be a POTS extender, wherein POTS service is supported at a voice terminal through to the FSAM  315 . The FSAM  315  may occupy two slots in a frame of equipment wherein one slot is filled with the POTS extender  301  and another slot is a master DSL modem  303 . The frame of equipment may house other FSAMs and master DSL modems to service local loops to other subscribers. The frame of equipment may be homogeneous in the sense that all master DSL modems are slaved to at least one POTS extender. The frame of equipment may be heterogeneous in that not all local loops are terminated by a FSAM. Some local loops have no support for emergency fallback because a legacy master DSL modem terminates the local loop that is not controlled by an FSAM embodiment of the invention or other life-line supporting apparatus.  
         [0032]     Under ordinary circumstances where the master DSL modem  303  is in active communication with the IAD  105 , or while the master DSL modem  303  is training the IAD  105 , the master DSL modem  303  may be said to be in an active state. In this state, the master DSL modem may provide at least one voice path (VP) or other address at the backplane  307  which corresponds to at least one telephony device at the IAD  105 .  
         [0033]     The frame may provide an aggregating unit, such as a multiplexer or a router, to combine the traffic arriving at several FSAMs onto a common, high-bandwidth, trunk line. Such traffic may include upstream voice packets. Similarly, the multiplexer or router may provide downstream voice packets addressed to at least one FSAM. The frame may provide redundant trunk lines, to better assure a path to a data network, such as the Internet. In addition, load balancing, and other algorithms to distribute traffic, may be employed while sending traffic to the data network, as is known in the art.  
         [0034]     The POTS extender  301  may have a connection to the tip of the local loop  309  in common with a master DSL modem  303 . The POTS extender  301  may have a connection to the ring of the local loop  309  in common with the master DSL modem  303 . The POTS extender  301  may have a control circuit  305  to the master DSL modem  303 . Alternate ways to send a control signal from POTS extender  301  include transmitting such a signal through a backplane  307 . Such a control circuit may be under programmed control of a CPU  331  and a CPU  332 . POTS extender  301  may control the operation of the master DSL modem  303 , such as, by sending a suppression signal or master DSL modem control signal to the master DSL modem  303  to suppress attempts to synchronize with an IAD  105  and enter a quiescent state. The SLIC  311  may sense a current drain in the subscriber line  309  which indicates that the IAD  105  has closed relays that put remote telephones directly on the subscriber line  309  circuit. The master DSL modem  303  may enter quiescent state by means known in the art, including removing power to the master DSL modem circuits in response to a suppression signal arriving at the master DSL modem  303 .  
         [0035]     The POTS extender  301  may have a number of circuits that permit it to support call-processing functionality. A Subscriber Line Interface Circuit (SLIC)  311  may provide loop current, electrical terminating characteristics, e.g. impedance, switch hook detection, and a ringing source as well as power the subscriber line  309  under certain conditions, such as operation in fallback mode. Switch hook detection may be provided by a switch hook detector coupled to the subscriber line  309 . DTMF tones and call progress tones including dial tones may be provided by the vocoder  351  in response to signaling provided by a gateway or a concentrator. The SLIC  311  may provide a switch hook detector. In some systems, an off-hook state of the switch hook may be detected based on current drain in the twisted pair as is known in the art, thus indirectly sensing an off-hook condition of a voice terminal. The master DSL modem  303  may detect synchronization.  
         [0036]     There is a co-ordination function between the POTS extender  301  and the master DSL  303 , which may be transmitted over the control circuit  305 . Ordinarily, the POTS extender  301  may operate in a passive or quiescent state, wherein the POTS extender  301  may monitor the subscriber line  309 . In this state, the POTS extender  301  may not respond to packets that are addressed via the backplane  307  to an address that it may have in common with the master DSL modem  303 . In other words, the POTS extender  301 , while in the quiescent state, may provide no bridging function between the backplane  307  and the subscriber line  309 .  
         [0037]     An IAD fallback signal may be provided by the IAD  105  in several ways. One way is that the IAD  105  may fail to respond to synchronization signals sent by the master DSL modem  303 . Another way is that the IAD  105  may fail to provide a data packet on the subscriber line within a time schedule established by a communications protocol for which it is designed to adhere. In any event, the IAD  105  begins to perform out of conformance with a protocol it has established with an upstream device, which is detectable by the POTS extender  301 , which supervises or monitors the subscriber line  309 . When a subscriber line or local loop  309  IAD fallback signal is detected, master DSL modem  303  synchronization and data transmit functions may be suppressed by control signals sent from the POTS extender  301  to the master DSL modem  303  over connection  305 . Single circuit gateway  301  may then operate to interface the subscriber line  309  to the backplane  307 , which may be multiplexed onto one or more trunk lines. Single circuit gateway  301  may operate a single voice path (VP) and a single voice circuit (VC) during operation in fallback mode. The single voice path may be associated with a lowest telephone number assigned to the at least one telephony terminals at the customer premises. Thus call setup, as may be controlled by a softswitch or a concentrator, may occur normally for calls to that voice path. Only calls from that voice path may traverse the subscriber line  309  during the fallback mode.  
         [0038]     Referring to  FIGS. 2 and 3 , recovery from the fall-back may occur by sensing the subscriber line with loop current detector  255 . Before power is provided to the IAD  105 , the relay  203  is held in the bypass position, as shown in  FIG. 2 . When power is provided, CPU  209  may boot up. A program may be run on the CPU  209  wherein a test of any voice device  205  is made, wherein a determination is made if the voice device is off-hook. Off-hook voice devices will be detectable by loop current detector  255 . When the CPU  209  is running and such a program detects that all voice devices  205  are on-hook, two additional steps may be performed. The relay  203  may be energized, placing each voice conductor pair  201  in electrical contact with the bank of SLICs  210 , while placing the DSL modem  257  in electrical contact with the subscriber line  207 . An additional step may include enabling training by the slave DSL modem  257  so that the slave DSL modem and the master DSL modem  303  may be synchronized.  
         [0039]     When an IAD-restore signal is detected by the POTS extender  301  on the subscriber line  309 , the POTS extender  301  may stop responding to any the voice path (VP) established during the IAD fallback. The POTS extender  301  may end suppression of the master DSL modem  303 , by sending a master DSL modem control signal, thus permitting normal synchronization of the master DSL modem  303  with the IAD  105 . The master DSL modem control signal may be one of two kinds: a signal to activate, meaning that the master DSL modem should attempt synchronization; and a signal to suppress, meaning that the master DSL modem should stop synchronization. Where multiple telephony ports of the IAD  105  are connected to telephony equipment, the IAD  105  may supply the voice paths (VP) and other addresses to each telephony port.  
         [0040]     Thus, in fallback mode, the FSAM  315  may accomplish two things. First, provide an analog port to the customer premises equipment or IAD  105 , wherein the IAD  105  may interface to the data network unimpaired by operation of any DSL modem on the circuit. Attendant with providing the analog port, the FSAM may provide a telephony current source. The current source may be used to power a remote telephone apparatus when off-hook, as is known in the art. In addition, Subscriber Line Interface Circuit (SLIC)  311  may interconnect a ringing signal source to the subscriber line  309  when an incoming call appears as received packets from the backplane  307 .  
         [0041]     A second function of the fallback mode is that the FSAM  315  may provide a VP visible on the data network corresponding to at least one telephony port of the IAD  105 . A Subscriber Line Interface Circuit (SLIC)  311  may perform additional functions that enable call processing. The SLIC  311  may detect an on-hook condition by means known in the art. The SLIC  311  may detect an off-hook condition. These functions may include the provision of, ringing voltages, interconnect to battery or ring voltages, switch-hook detection and impedance matching. In addition, the FSAM  315  may convert analog traffic from the subscriber loop  309  to packets for transmission on the packet network, wherein the packets may be ATM (Asynchronous Transfer Mode) cells. The POTS extender  301  may provide a codec  345  interconnect to the subscriber line  309 , wherein the codec  345  performs analog to digital conversion of signals from the SLIC  311  to provide upstream digital voice signal. Similarly, codec  345  may convert digital signals received from the vocoder  351  to a downstream digital voice signal directed to the SLIC  311 . Data output  321  from the vocoder  351 , now a stream of bits, may be packetized by a PAD  353  for transmission on a broadband media. A connector  390  or other interface may link the PAD  353  to a backplane  307  having interconnect to broadband media, including those supporting T1, ISDN, and OC3 among others known in the art. Each of said functional blocks, vocoder  351  and PAD  353  may be performed by a DSP or a CPU operating alone or together.  
         [0042]     A reverse process may occur wherein the PAD  353  receives downstream voice packets addressed to it from the backplane  307  and assembles the downstream voice packets into a data stream for a vocoder  351 . Vocoder may convert the data stream to a format amenable to digital to analog conversion. Codec  345  may perform the digital to analog conversion and provide the analog voice frequency signals to an input of the SLIC  311 .  
         [0043]     Although the invention has been described in the context of particular embodiments, various alternative embodiments are possible. Thus, while the invention has been particularly shown and described with respect to specific embodiments thereof, it will be understood by those skilled in the art that changes in form and configuration may be made therein without departing from the scope and spirit of the invention.