United States Patent [19]
`Hansen
`
`[54] EMULATION OF ANALOG MODEM
`SIGNALING OVER IDSN FOR
`TRANSLATION-LESS INTEROPERABILITY
`WITH PSTN BASED H.324 SYSTEM
`
`[75]
`
`Inventor: Carl Christian Hansen, Aloha, Oreg.
`
`[73] Assignee: Intel Corporation, Santa Clara, Calif.
`
`[21] Appl. No.: 08/926,413
`
`[22]
`
`Filed:
`
`Sep_ 9, 1997
`
`[51]
`
`[52]
`
`[58]
`
`Int. Cl.6
`
`............................. H04N 7/15; G06F 13/42;
`G06F 13/38
`U_S_ Cl ........................... 709/205; 709/204; 370/264;
`345/330
`Field of Search ......................... 395/200.61, 200.34,
`395/200.36, 200.35; 370/260, 352, 309,
`496, 264; 455/416; 399/90.01, 202, 219,
`93.21; 348/17, 15; 345/330; 300/10; 709/205,
`204, 203, 206, 207
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`111111
`
`1111111111111111111111111111111111111111111111111111111111111
`US005961589A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,961,589
`Oct. 5,1999
`
`5,774,857
`5,815,505
`
`6/1998 Newlin .................................... 704/271
`911998 Mills ....................................... 370/522
`
`Primary Examiner-Daniel H. Pan
`Attorney, Agent, or Firm-Blakely, Sokoloff, Taylor &
`Zafman LLP
`
`[57]
`
`ABSTRACT
`
`What is disclosed is a system for translationless videocon(cid:173)
`ferencing interoperability between an analog modem based
`node and an Integrated Services Digital Network (ISDN)
`node having a videoconferencing engine coupled to the
`ISDN node, the videoconferencing engine generating vid(cid:173)
`eoconferencing data according to an analog modem proto(cid:173)
`col. Further the system has a modem emulator coupled to the
`videoconferencing engine, the modem emulator configured
`to model said data as analog modem data interpretable by the
`analog modem based node.
`
`5,666,362
`
`9/1997 Chen et al.
`
`............................. 370/354
`
`15 Claims, 6 Drawing Sheets
`
`Node 210
`
`PSTN-based
`Videoconferencing
`lli
`Software
`
`Node 220
`
`-
`
`-
`
`-
`
`r
`
`TELCO
`250
`
`PSTN-based
`Videoconferencing
`Software
`225
`-
`
`-
`
`-
`
`J
`
`l -
`
`-
`
`-
`
`

`
`u.s. Patent
`
`Oct. 5, 1999
`
`Sheet 1 of 6
`
`5,961,589
`
`r
`
`Node 110
`- - -
`-
`ISDN-based
`Videoconferencing
`ill
`Software
`
`ISDN
`Adapter M.Q
`
`.....J
`
`Node 120
`- - -
`
`r
`
`-
`
`PSTN-based
`Videoconferencing
`ill
`Software
`
`~--~------t----tAnalog
`~---+-----.-~ Modem 13Q
`
`L . . . . - ._ - - - - - : ;=
`
`____ .....J
`
`Fig. 1
`(Prior Art)
`
`

`
`u.s. Patent
`
`Oct. 5, 1999
`
`Sheet 2 of 6
`
`5,961,589
`
`Node 210
`
`PSTN-based
`Videoconferencing
`ill
`Software
`
`Modem 217
`Emulator
`
`ISDN
`Adapter ~
`
`l
`
`TELCO
`250
`
`Node 220
`-
`-
`
`-
`
`-
`
`-
`
`r -
`
`-
`
`Analog
`Modem
`
`230
`
`PSTN-based
`Videoconferencing
`Software ~
`
`__ ...J
`
`Fig. 2
`
`

`
`u.s. Patent
`
`Oct. 5, 1999
`
`Sheet 3 of 6
`
`5,961,589
`
`START
`
`Generate a set of analog modem
`videoconferencing protocol compatible signals on
`ISDN capable node (1 st set of signals)
`
`310
`
`Generate a second set of signals that encode
`the first set of signals into analog-modem type
`data
`
`Sample and transmit a third set of signals from
`the second set using an ISDN device
`
`END
`
`Fig. 3
`
`

`
`u.s. Patent
`
`Oct. 5, 1999
`
`Sheet 4 of 6
`
`5,961,589
`
`Transmitter
`1
`\MOdule 410
`I r - - - - - - - - - - - - - - -
`
`-0 T
`
`
`IS DN
`A
`dapter
`
`1 1
`DYNAMIC
`I ADAPTER -
`I 1 MULTI-RATE ....
`
`I
`I -
`
`460
`-
`
`-
`
`-
`
`MODULATOR
`MODULE
`
`415
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`SCRAMBLER
`MODULE
`
`I
`-
`.J
`1
`1
`- ~
`
`414
`
`UART
`
`1
`I
`I
`I
`I
`I
`I
`
`Jjl
`
`430
`
`I
`I
`""
`I
`Videocpnferencing
`Engine
`450
`l
`I
`I
`
`Receiver
`_ \ Module 420 _
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`DESCRAMBLER
`
`.. MODULE
`
`I
`1
`I
`1
`DYNAMIC
`1
`I .. MULTI-RATE
`ADAPTER
`424 1
`1
`1
`I
`470
`~---------------~
`
`rom
`F
`
`IS DN
`dapter
`A
`
`DEMODULATOR
`
`.. MODULE
`..
`
`425
`
`-c;---------
`
`Modem
`Emulator 400
`
`Fig. 4
`
`

`
`u.s. Patent
`
`Oct. 5, 1999
`
`Sheet 5 of 6
`
`5,961,589
`
`Processor 510
`
`--
`
`Memory 520
`
`.. - - - /
`
`t"
`I Modem
`' - I L Emulator I
`I_~
`
`525
`
`"
`
`Video Conferencing
`Engine 527
`
`Bridge 530
`
`550
`
`560
`
`""
`
`Display
`Adapter
`
`Display
`Device
`
`570
`
`1 Sound Card
`
`59~
`
`Speakers
`
`System Bus 515
`
`Peripheral Bus 53 5
`
`580
`
`"-
`
`Image
`Capture
`Device
`
`540
`
`1 ISDN I
`
`Adapter
`
`585
`
`Microphone I
`
`Fig. 5
`
`

`
`u.s. Patent
`
`Oct. 5, 1999
`
`Sheet 6 of 6
`
`5,961,589
`
`START
`
`Receive a set of analog-modem compatible
`videoconferencing signals on ISDN capable node
`(1 st set of signals)
`
`Generate by modeling the first set of signals
`into a second set of signals that are encoded
`analog modem data
`
`Generate a third set of signals from the second
`set of signals that correspond to
`videoconferencing information
`
`END
`
`Fig. 6
`
`

`
`5,961,589
`
`1
`EMULATION OF ANALOG MODEM
`SIGNALING OVER IDSN FOR
`TRANSLATION -LESS INTEROPERABILITY
`WITH PSTN BASED H.324 SYSTEM
`
`BACKGROUND OF THE INVENTION
`
`5
`
`2
`or device would be incomprehensible and, thus, not com(cid:173)
`municate any useful information for facilitating videocon(cid:173)
`ferencing or any other activity requiring data decoding
`precision.
`One proposed solution for these incompatibilities, illus(cid:173)
`trated in the prior art of FIG. 1, has been to build a gateway
`or gateways at telephone company switching centers (Telco
`150) which convert H.320 data (generated by ISDN-based
`videoconferencing software 115) to H.324 (PSTN-based
`10 videoconferencing software 125) and vice versa. However,
`such gateways are expensive to build and maintain and
`significantly interfere with the overall connection speed and
`connection quality for the session. These gateways add extra
`delay because of the need for layers of buffering to accom-
`15 plish the translations and subsequent data streaming. The
`gateways themselves are complicated components and
`currently, have not been widely implemented. As such, if
`one Telco region does not provide such gateways, nodes
`within that region will be incapable of videoconferencing to
`20 other modulation incompatible nodes. The modulation
`incompatibility hinders also the use of software protocol
`translation/conversion schemes that will essentially be
`translating/converting incoherent data streams.
`Thus, there is needed a simple, low-cost method and
`25 apparatus which will allow an ISDN based node and a PSTN
`(analog modem based) node to carry out a videoconferenc(cid:173)
`ing session without the need for protocol translation and
`gateways.
`
`1. Field of the Invention
`The invention relates generally to telecommunications
`and videoconferencing. More specifically, the invention
`relates to telecommunications interfacing and provisioning
`for using compatible videoconferencing standards.
`2. Description of Related Art
`The Public Switched Telephone Network (PSTN) which
`facilitates voice (using telephones) and/or data (using
`modems) telephony throughout the world has in recent times
`been utilized to also deliver videoconferencing information.
`Towards that end, a standard has emerged for videoconfer(cid:173)
`encing within the PSTN environment. This standard, known
`as the H.324 Recommendation, entitled "Terminal for Low
`Bitrate Multimedia Communication," specifies the usage of
`modems for videoconferencing over the PSTN, and thus, is
`referred variously in this specification as an "analog-modem
`based videoconferencing protocol." Prior to the H.324
`recommendation, ISDN, or Integrated Services Digital
`Network, had been developed to evolve beyond ordinary
`PSTN modem networks. ISDN provides two "B" channels
`each of which can carry analog-encoded (ordinary voice
`telephony but not analog modem) information or digital
`(carrying data) signals. The advantage of ISDN over PSTN 30
`is that since ISDN is a digital transfer medium, the channels
`can be "bonded" or combined to provide a single channel
`with nearly twice the bandwidth of a single channel. Since
`each ISDN "B" channel provides up to 64 kilobits per
`second bandwidth, 2 bonded "B" channels can deliver 128 35
`kilobits per second, which is much higher than the 33.6 or
`56 kilobits per second currently given as the upper limit for
`Y.34 modems operating over PSTN. To take advantage of
`the added bandwidth of ISDN, a new videoconferencing
`standard over ISDN known as the H.320 Recommendation, 40
`entitled "Narrowband Visual Telephone Systems and Ter(cid:173)
`minal Equipment" was developed.
`When a user at one node of a videoconferencing session
`communicates with another user at the other node over the
`same type of carrier, either ISDN or PSTN, the session can 45
`be carried out translationless since both nodes will utilize the
`same protocols. FIG. 1, however, shows that, when a node
`110 utilizes an ISDN adapter 140, and consequently, the
`H.320 protocol, while the other node 120 uses an analog
`modem 130, and consequently, the H.324 protocol, a serious 50
`compatibility problem arises: the terminal at node 110 will
`be utilizing H.320 protocols while the terminal at node 120
`will be expecting to receive H.324 protocols. Additionally,
`there is a modulation incompatibility since node 110 using
`ISDN (digital) modulation while node 120 uses analog 55
`modem type modulation. Though an ISDN channel can be
`used for analog voice telephony, the voice data is nonethe(cid:173)
`less digitized when being carried over the network. While
`the ND (analog-to-digital) and subsequent (D/A) of voice
`telephony signals has been successfully achieved, the same 60
`cannot be said of analog modem signaling, such as Y.34
`(lTU -T (CCITT) Recommendation Y.34, entitled "A
`Modem Operating at Data Signaling Rates of up to 28,800
`bits/s for use on the General Switched Telephone Network
`and on Leased Point-To-Point 2-Wire Telephone-Type
`Circuits, published 1994) compatible signaling. As a result,
`an analog modem signal transmitted to a digital ISDN node
`
`SUMMARY
`
`What is disclosed is a system for translationless video(cid:173)
`conferencing interoperability between an analog modem
`based node and an Integrated Services Digital Network
`(ISDN) node. The system has a videoconferencing engine
`coupled to the ISDN node such that the videoconferencing
`engine generates videoconferencing data according to an
`analog modem videoconferencing protocol. Further, the
`system has a modem emulator coupled to the videoconfer(cid:173)
`encing engine such that the modem emulator is configured
`to model the data as analog modem data interpretable by the
`analog modem based node.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 illustrates the prior art of gateway-based ISDN to
`PSTN videoconferencing.
`FIG. 2 illustrates a system diagram of one embodiment of
`the invention.
`FIG. 3 illustrates a flowchart of transmitting videocon(cid:173)
`ferencing information according to one embodiment of the
`invention.
`FIG. 4 illustrates a diagram of a modem emulator accord(cid:173)
`ing to one embodiment of the invention.
`FIG. 5 illustrates a computer system capable of imple(cid:173)
`menting videoconferencing according to one embodiment of
`the invention.
`FIG. 6 illustrates a flowchart of receiving videoconfer(cid:173)
`encing information according to one embodiment of the
`invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`FIG. 2 illustrates a system diagram of one embodiment of
`65 the invention.
`FIG. 2 illustrates a videoconferencing session with two
`nodes 210 and 220 which communicate with each other via
`
`

`
`5,961,589
`
`3
`telephone company networks labeled Telco 250. Nodes 210
`and 220 can range from full-fledged computer systems to
`smart phones or other information devices which have the
`ability to communicate over lines provided by local tele(cid:173)
`phone companies. In the system illustrated in FIG. 2, node 5
`210 has the capability of communicating using ISDN
`(digital) while node 220 has the capability of communicat(cid:173)
`ing through PSTN (analog). Telco 250 includes the switches,
`hubs and other devices that are operated by telephone
`companies to facilitate both ISDN and PSTN type commu- 10
`nications.
`Node 210 includes a videoconferencing software inter(cid:173)
`face 215 (also referred to as a videoconferencing engine)
`which recognizes and operates a videoconferencing standard
`for PSTN (using analog modems) such as the H.324 stan- 15
`dard. Node 210 uses an ISDN interface to communicate via
`Telco 250, but yet according to this embodiment of the
`invention, can use analog modem (PSTN based) videocon(cid:173)
`ferencing software to communicate data to/from the ISDN
`interface. According to one embodiment of the invention, a 20
`Y.34 modem emulator 217 is interfaced with videoconfer(cid:173)
`encing software 215 so that the H.324 data originating from
`software 215 appears as though it was analog encoded by a
`modem when it exits emulator 217. The emulator 217 can be
`implemented in either hardware, software, or firmware or a 25
`combination thereof and performs the functions of an analog
`modem, such as a Y.34 modem, which transforms digital
`data into analog data and vice-versa to provide useful
`communication with a digital computer, for instance. If node
`210 is ISDN capable, there is likely to be provided, either 30
`within the operating system or as a separate application, an
`ISDN driver software 219. ISDN driver software 219 inter(cid:173)
`faces the modem emulator 217 with a physical ISDN device
`such as an ISDN adapter 240. ISDN adapter 240 encodes
`data from computer 210 into samples for transmission over 35
`the digital ISDN lines provided by telephone company 250.
`At the other end of the videoconferencing session is node
`220 which has the capability of communicating on telephone
`company lines using an analog modem 230 to which it is
`coupled. Computer 220 includes a videoconferencing soft- 40
`ware interface 225 which is capable of recognizing and
`operating the H.324 standard or any of the other analog
`modem videoconferencing protocols. Preferably nodes 210
`and 220 use software that utilize compatible videoconfer(cid:173)
`encing protocols. Videoconferencing software 225 is inter- 45
`faced to a modem driver 227 which interfaces modem 230
`to other components of node 220 such that the modem 230
`converts to analog any binary data received from node 220
`for transmission and converts to digital any analog data
`received from the telephone company 250 through the 50
`telephone lines.
`Modem emulator 217 provides a modem-like analog
`modeling to the digital data received by software interface
`215. ISDN driver 219 and adapter 240 encode or sample the
`analog-like data ("analog-like" since the modeled data 55
`resides as binary) and send it out to Telco 250. From the
`viewpoint of Telco 250, when node 210 initiates a session
`with node 220, Telco 250 will recognize that computer 210
`operates on a ISDN line and that node 220 operates on an
`ordinary PSTN line and, when routing data between the two, 60
`will provide digital-to-analog (D/A) and analog-to-digital
`(AID) conversion such that the two nodes 210 and 220 can
`communicate. Both node 210 and node 220 may have
`ordinary telephone numbers assigned them, but Telco 250
`will recognize that the telephone number assigned to the 65
`ISDN line for computer 210 is a digital line and will,
`therefore, convert the analog signals originating from
`
`4
`modem 230 to digital. Similarly, digital signals coming from
`adapter 240 will be converted by Telco 250 into analog
`signals which can be received and demodulated bv modem
`230.
`-
`The insertion of a modem emulator in the data stream
`between videoconferencing software 215 and videoconfer(cid:173)
`encing software 225 has the follmving effect. The frames of
`video and audio encoded by videoconferencing software 215
`are a sequence of bitmaps compressed according to a
`specified "codec" (compression/decompression) algorithm.
`The resulting data, which is binary, is passed to the modem
`emulator 217 and modulated virtually within the modem
`emulator which produces amplitude and phase information
`which approximates the output of an analog modem. If the
`modem emulator were purely software, the signal generated
`by modem emulator 217 would be a digital representation of
`analog modem signaling. However, if implemented in con(cid:173)
`junction with hardware elements, modem emulator 217
`could consist of a D/A(Digital to Analog) hardware element
`that could more precisely generate the output of an analog
`modem. The precision or resolution of the waveform mod(cid:173)
`eling needed will vary depending on the application, and the
`combination of hardware and software can easily be modi(cid:173)
`fied by one skilled in the art to achieve the desired result.
`One alternative solution is to introduce a hardware
`modem into the node and couple the hardware modem
`instead of the signaling generated by a modem emulator, to
`the ISDN adapter. The ISDN adapter has an analog input for
`voice calls which the hardware modem could be tied into to
`send signals through. However, this approach suffers in that
`the Telco now assumes that the ISDN adapter is dialing a
`voice call, and thus, the Telco may do voice compression,
`something not intended for the videoconferencing session,
`such that the data received at the other end is prone to error.
`A hardware modem, which is analog device will generate
`handshaking, ring tones etc. and may also be expecting
`voltage swings on the line. However, the ISDN adapter will
`likely interfere with and perhaps cancel such PSTN signal(cid:173)
`ing and thus, the hardware modem may not even operate
`properly. Furthermore, a physical hardware modem cannot
`sample at the rate of 64 Kilobits/Second as is required by the
`ISDN adapter and thus, the insertion of a physical modem
`(hardware) as opposed to software-emulated modem would
`not function properly (without sample rate conversion). The
`modem emulator can perform the proper sampling that an
`off-the-shelf modem cannot.
`The ISDN adapter driver provides straightforward com(cid:173)
`patibility since the signal generated by modem adapter 217
`is digital as the signal is to be passed through the system bus
`of the computer to the serial port (or network interface) to
`which the ISDN adapter is connected. The ISDN adapter
`transmits a rate adapted virtual analog waveform generated
`by the modem emulator to the Telco 250.
`Telco 250, recognizing that the originating device is ISDN
`and that the destination is PSTN will pass the transmitted
`digital signal through a D/Aconverter. The resulting analog
`signal will behave and emulate a true modem signal such as
`one generated under the Y.34 protocol and can be demodu(cid:173)
`lated by modem 230 at the destination node. The signal
`decoded by the modem is compatible with the H.324 system
`at the destination and, thus a videoconferencing session can
`successfully take place. Likewise, when data is sent by
`modem 230, the modem emulator will analog approximate
`the incoming signal to make it understandable by videocon(cid:173)
`ferencing software 215 of node 210.
`Beyond the pre-existing D/A and AID converters which
`the Telco 250 provides, the additional hardware, such as a
`
`

`
`5,961,589
`
`5
`gateway device, is extraneous for running a compatible
`videoconferencing session. No "interworking" is required
`and the system is translationless, providing for optimum
`signal definition. "Interworking" refers to the need to trans(cid:173)
`late dissimilar protocols and codecs which can lead to a loss 5
`of signal definition and quality due to buffering overhead.
`Further with a translating gateway, a user at one node must
`call into the gateway and cannot switch from a voice call
`seamlessly into a videoconferencing session while still
`on-the-line. The user(s) must essentially call into the gate- 10
`way to connect to each other which introduces a burden as
`a third-party is now required for videoconferencing which is
`preferably and optimally a direct dial.
`FIG. 3
`illustrates a flowchart of videoconferencing
`according to one embodiment of the invention.
`One methodology for translationless videoconferencing
`when the ISDN node transmits information is illustrated in
`FIG. 3. First, according to step 310, rather than generating
`a set of signals compatible with ISDN videoconferencing
`(e.g., H.320), a set of analog modem videoconferencing
`protocol compatible signals is generated by the videocon(cid:173)
`ferencing software on the ISDN-capable node. This first set
`of signals is then passed to a modem emulator or other
`modeling system which generates a second set of signals
`which encode the first set of symbols as analog modem
`signals (step 320). The exact protocol used will vary accord(cid:173)
`ing to the protocol used by the analog modem node. The
`ISDN adapter or network interface of the ISDN-capable
`node then transmits a third set of signals which are sampled
`from and approximate the second set of signals (step 330).
`This basic methodology may be extended or by the specifics
`of the videoconferencing or network components is merely
`exemplary. The ISDN node acting to receive videoconfer(cid:173)
`encing information is illustrated in FIG. 6.
`FIG. 4 illustrates a modem emulator a=ording to one
`embodiment of the invention.
`In general, modem emulator 400 can be configured to
`emulate any form of modulation, such as PCM (Pulse Code
`Modulation) or QAM (Quadrature Amplitude Modulation)
`and can provide a data rate concomitant with that particular
`modulation type. If data to be transmitted is not piped to the
`modem fast enough, a steady data rate can be maintained by
`"filling" bandwidth unused by the transmission with nulls.
`For videoconferencing, the modem emulator makes use of a
`standard known as H.223 Recommendation, lTV-V, "Mul(cid:173)
`tiplexing Protocol or Low Bit Rate Multimedia
`Communications", March 1996, in order to accomplish
`filling if the delivery of data from the videoconferencing
`software is less than the desired bit rate as specified by the
`particular modulation type.
`Most physical analog modems in the marketplace use a
`technique known as QAM (Quadrature Amplitude
`Modulation) which is also referred to as QASK (Quadrature
`Amplitude Shift Keying) to encode digital information (bits)
`into analog waveforms. Modem emulator 400 may be
`designed to emulate QAM to properly give the digital signal
`the appearance of analog modem data when received by the
`destination modem. Many modulation systems vary signals
`by phase or amplitude only, while QAM varies both phase
`and amplitude so that a larger number of distinct binary
`words can be represented by the combination of phase and
`amplitude. The modulation is balanced in that both sinusoid
`and cosinusoid components compose the modulated signal.
`The basic form of a QAM signal is Vqan,(t)=Ae(t)sin(wot+
`O)+Ao( + )cos(w ot+O). Ae and Ao take on the same number of
`different amplitude values as the number of bits represented
`
`6
`by each symbol. "Wo" is the angular carrier frequency at
`which the signal is modulated and 0 is the phase. By
`modeling signals a=ording to this form, a physical analog
`modem can be emulated in software or in a combination of
`hardware and software.
`The modem emulator 400 according to one embodiment
`of the invention may be logically divided into the following
`functional blocks when acting as data transmission interface:
`a scrambler module 414, a modulator module 415, and a first
`Dynamic Multi-rate Adapter 460. When acting as a data
`reception interface, modem emulator 400 may be divided
`into the following functional blocks: a descrambler module
`424, a demodulator module 425 and a second Dynamic
`Multi-rate Adapter 470. In addition both the receive and
`15 transmit modes of the modem emulator 400 may make use
`of a VART (universal Asynchronous Receive/Transmit) unit
`430.
`When data is passed to the modem emulator from a
`videoconferencing engine 450 (using, for instance, the
`20 H.324 protocol) for output via the ISDN adapter to the other
`videoconferencing node, the data may require serialization.
`As a further step in emulating an analog modem signal, the
`VART 430 should add three bits-a stop bit, a start bit and
`a parity bit-for every block (such as a byte). The start bit is
`25 added at the beginning of the data block with the parity bit
`and then the stop bit appended to the end of a data block.
`Therefore, the function of VART 430 is the affixation of the
`start, stop and parity bits to facilitate asynchronous transfer
`that analog modems currently utilize. The start and stop bit
`30 mark the beginning and end of the transfer block, while the
`parity bit is used for error correction. Depending on whether
`the parity bit is used as part of the 8-bit data block or in
`addition to it, the total length of the transfer block will vary
`from session to session depending upon negotiated protocols
`between the two videoconferencing nodes. Modem emulator
`400 is equipped with a VART module 430 which may
`perform the functions of a standard VART, but will disable
`its serialization function depending on the mode of data
`transfer (serial or parallel). Once the VART 430 has affixed
`40 start, stop and parity bits to the data block to produce the
`transfer block, the transfer block can, in one embodiment, be
`scrambled as with a physical modem. A data scrambler
`module 414 is used to ensure the data is random enough that
`clocking operations are reliable. Data scrambler is well-
`45 known in the art communications technology and one of
`ordinary skill in that art of would readily be able to design
`a data scrambler consistent with the operation of the modem
`emulator.
`Once the data is scrambled, it is passed on to the modu-
`50 lator module 415. To emulate a Y.34 compatible modem, a
`variation of QAM known as TCQAM (Trellis Code Quadra(cid:173)
`ture Amplitude Modulation) is used for modulating binary
`data into analog. "Trellis" coding is employed to facilitate
`real-time error correction by producing a trellis code which
`55 is unique to each combination of bits in a transfer block. The
`modulator module 414 signal that models a QAM (v.rith
`trellis coding, if desired). Modulator module 415 will per(cid:173)
`forming mathematically and logically the functions of a
`physical modulator by providing, among other functions,
`60 level conversion. As employed in QAM, level conversion
`involves transforming a sequence of N-bits into M different
`amplitude levels. When multiplied by P different phase shifts
`the modulator 415 will be able to represent all possible
`combinations (words) of N-bits using the modulation
`65 scheme.
`The modulator module 415 of modem emulator 400 will
`mathematically represent or model the scrambled data
`
`35
`
`

`
`5,961,589
`
`5
`
`7
`according to the basic sinusoidal and cosinusoidal forms
`given above. The modem emulator will thus be able approxi(cid:173)
`mate a sinusoid with the desired phase and amplitude and
`generate amplitude values corresponding to the amplitude of
`the modeled form at certain discrete sample points.
`Before the sinusoidally modeled samples can be passed to
`the ISDN adapter the issue of sampling and clocking should
`be considered. The ISDN adapter is very strict in its require(cid:173)
`ment of delivering and receiving 8 KiloSamples per second
`no matter what the actual rate of data delivery by the
`subsystems within the information generating device (e.g.
`computer system). By contrast, a physical analog modem
`delivers data at varying bit rates according to line conditions.
`The rate for a standard Y.34 28.8 modem, as it is known in
`the art, is around 30,000 bits per second (only 28.8 K is
`achieved because of decoding and overhead). But this is an
`ideal or optimal bit rate and depending upon the PSTN line
`conditions could widely vary. Whether the modulator mod(cid:173)
`ule 415 responds to presumed line conditions or not, the
`sample rate specified for ISDN will likely vary from the 20
`sample rate provided by the modulator data.
`To overcome this problem, the modulator module could
`be redesigned. However, then it would lose its functionality
`of emulating a modem signal which can be utilized for other
`applications. Further, redesigning the modulator module to 25
`generate a fixed number of samples per second may be
`difficult to achieve due to clocking differences between the
`ISDN and modulator. An ISDN adapter is clocked according
`to a clocking signal received from the network while, an
`analog modem is clocked internally and not from the PSTN 30
`network, and thus, a synchronicity problem may arise.
`Another solution to the sampling rate dilemma, the one
`adopted in this embodiment of the invention, is the use of a
`multi-rate adapter. As illustrated in FIG. 4, a dynamic
`multi-rate adapter (DMRA) 460 receives samples from 35
`modulator module and provides them out to the ISDN
`adapter for transmission. DMRA 460 uses techniques such
`as sample rate adaptation, filling and/or buffering to cause a
`sampling rate of 8 KiloSamples per second. The DMRA460
`is dynamic in that it can modify its operating parameters 40
`based on any changes in the bit-rate or sample rate provided
`by modulator module 415. Among other functions, the
`DMRA 460 may also perform phase and frequency adapta(cid:173)
`tion as a supplement/complement to adapting sampling
`rates. The process of sample rate adaptation is also referred 45
`to as "signal conditioning" and may techniques and appa(cid:173)
`ratus for implementing it are well-known in the art.
`Once the sample rate has been adapted, the sinusoidal
`samples are passed to the ISDN adapter. The ISDN adapter
`then transmits in digital form, the sinusoidal samples to the
`network. While in network transit, the digital samples are
`converted into analog signals via a D/Aconverter by virtue
`of the network's recognition that the recipient is non-ISDN
`and therefore, analog. The modem at the receiving node will
`pick up and demodulate the analog signals generating a 55
`non-return-to-zero (NRZ) serial digital signal. The physical
`modem at the receiving node will then decode and send the
`videoconferencing data to a software interface on the receiv(cid:173)
`ing node. When the videoconferencing engine on the receiv(cid:173)
`ing node translates this data, it will be translating data in a 60
`format it is capable of understanding, namely, H.324 data. In
`this manner, the ISDN node can utilize modem emulator 400
`to communicate videoconferencing information to the ana(cid:173)
`log modem receiving node using essentially its native vid(cid:173)
`eoconferencing protocol H.324. No protocol translation is 65
`needed since the protocols are identical. The modem emu(cid:173)
`lator 400 assures that the digital data will appear like an
`
`8
`analog-modem signal when decoded and since, the ISDN(cid:173)
`capable node uses H.324 have the behavior of analog H.324
`signaling.
`The ISDN-capable node uses modem emulator 400 to also
`receive videoconferencing information from the non-ISDN
`node. While in network transit, analog data from the non(cid:173)
`ISDN node will be converted to digital form in accordance
`with the network recognizing that the recipient of data is an
`ISDN node. When that data is received by the ISDN adapter
`10 it is first passed to second DMRA470. DMRA470 performs
`dynamic sample rate adaptation to accommodate the oper(cid:173)
`ating parameters of the demodulator module 425. The
`demodulator module 425 operates, in one embodiment,
`according to the same analog modem specification as modu-
`15 lator module 415 or according to a commonly agreed
`protocol/specification between the ISDN node and the non(cid:173)
`ISDN node. DMRA 470 can buffer the ISDN 8
`KiloSamples/second input such that the demodulator mod-
`ule can function appropriately at a lower bit rate, if desired.
`Demodulator module composes an analog modem signal
`from digital sinusoidal samples received by DMRA 470.
`Phase, frequency and amplitude are all analyzed and pro(cid:173)
`cessed by the demodulator module 425 and then the data in
`bits or words (blocks of bits) is passed to descrambler
`module 424. Descrambler module 424 reassembles the data
`in its proper sequence and presents sequenced date to UART
`430. UART 430, expecting to receive date in blocks, strips
`off any parity, stop and start bits and presents the raw data
`to videoconferencing engine 450. Videoconferencing engine
`450, using an analog modem videoconferencing protocol
`such as H.324, then processes and outputs videoconferenc(cid:173)
`ing information such as text or graphics audio on the ISDN
`node. To have the capability of receiving, transmitting, and
`interpreting H.324 data, the ISDN node utilizes modem
`emulator 400 and thus, communicates without translation or
`interworking with a node using an