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USRobotics 9600-BPS COURIER HST MODEM SETS NEW HIGH-SPEED STANDARD The new 9600-bps Courier HST modem from USRobotics, Inc., sets a new standard in high-speed modem technology. And its $995 list price sets a new standard in modem value. The Courier HST (High Speed Technology) modem, designed for use on voice-grade, dial-up telephone circuits, provides full- duplex 9600-bps data communication through an asymmetrical frequency division of the telephone channel. This design provides simultaneous 9600-bps and 300-bps data channels, with the high-speed channel direction automatically assigned according to data flow demand. At 9600-bps, Courier HST uses Trellis Coded Modulation, an advanced technology which enables the modem to achieve maximum speed over a wide range of dial-up line conditions. A proprietary error- and flow-control protocol allows Courier HST to transmit up to 1,100 characters a second, error free, over local and long-distance telephone circuits. Modem users will find the Courier HST easy to operate. It uses an extended version of the industry standard "AT" command set, and works with most popular data communications software. The modem incorporates the same user features as USRobotics Courier 2400 modem. It also automatically falls back to 2400, 1200 and 300 bps in both answering and originating calls to connect with nearly any modem. At 2400 and 1200 bps, Courier HST implements Classes 1 through 3 of MNP (an industry-standard error- and flow-control protocol), providing error-control compatibility with USRobotics Courier 2400e and other MNP-capable modems. New Standard in Value Courier HST advances a new standard in value. On a price- per-bit basis, the $995 Courier HST costs about ten cents a bit. A competitive 2400-bps modem priced at $899 costs about 37 cents a bit. Compared with conventional 1200-bps modems, Courier HST could pay for itself in just eight hours of high-speed data transmission. Sending 1,100 characters a second, the Courier HST could transmit over 31 million characters in eight hours. A 1200-bps modem would take 73 hours and 20 minutes -- more than 65 hours longer -- to send the same amount of data. At an average long-distance telephone rate of $15 an hour, the time saved with the Courier HST modem would equate to its $995 list price. Viewed in yet another way, a customer could purchase seven Courier HST modems for the same price of just two competitive 9600-bps modems that implement the V.32 recommendation from CCITT (the international communications standards organization), or five Courier HST modems for about the same price of two DCA Fastlink modems which use a proprietary multi-carrier half-duplex modulation technique. Optimum Use of Bandwidth "The asymmetrical modulation design responds to the way people really use PCs and data terminals to communicate," USRobotics Vice President of Engineering Dale Walsh explained. "It gives the user a high-speed channel for fast downloading or uploading of data files, and a low-speed channel that handles both manual data entry by the user and error-control coding. "Courier HST represents optimum use of the dial-up, voice grade telephone circuit," he continued. "It provides simultaneous two-way communication -- full duplex. Most currently available 9600-bps modems are half-duplex devices based on proprietary modulation techniques or the old CCITT V.29 recommendation." Half-duplex modems devote the entire telephone bandwidth to 9600-bps in one direction at a time, and "ping-pong" the data flow back and forth to approximate full duplex. This solution is ill suited to interactive online sessions. For example, in half-duplex a typed character will be sent at 9600-bps, but the echo of that character may take several seconds to appear on the screen of the system that sent it, due to long round-trip delay. As a result, the system fails to achieve the illusion of full-duplex that most interactive applications require. The Courier HST modem's aymmetrical full-duplex approach also is better suited to PC datacom applications -- and much less expensive -- than the symmetrical full-duplex technology representing CCITT's V.32 recommendation. Walsh explained that V.32 modems employ echo-cancelling as a solution to overlapping Answer and Originate frequencies. Simply stated, echo-cancelling allows each modem to ignore its own transmitter and pick up the remote transmitter. Measured by computations per second and bits of resolution, a V.32 modem is roughly 64 times more complex than a 2400-bps modem, Walsh said. This translates directly into higher costs. Trellis Coded Modulation While avoiding the cost and complexity of V.32 echo- cancelling, the Courier HST modem employs the V.32 modulation technique -- Trellis Coded Modulation (TCM). TCM is a multi-dimensional technique that makes transmission less vulnerable to data errors caused by telephone network conditions. The Courier HST implementation of TCM uses a 2400-baud signalling rate, transmitting four data bits per baud to achieve its 9600-bps speed. Unlike other modulation techniques, TCM allows migration to even high speeds, signalling five or six bits per baud to achieve bit rates of 12,000 and 14,400 bps. Future versions of the Courier HST may incorporate these higher speeds. TCM provides an inherent 4-dB signal-to-noise advantage over V.29 modulation. Four dB difference might not mean much at lower data rates that already have large built-in margins. But at 9600 bps, it is extremely meaningful in improving transmission quality. It is very important to note that the currently available 9600-bps modems based on the V.29 recommendation use Quadrature Amplitude Modulation (QAM). All other factors being equal, a QAM modem will experience at least twice as many block errors as a TCM modem. This translates to far fewer retransmissions and consistently higher data throughput for a TCM modem. Courier HST achieves its maximum speed on circuits with signal-to-noise conditions of 17dB or better. It also is more resistant to such impairments as low frequency phase jitter, satellite connections and impulse noise than either V.29 modems or the multi-carrier half-duplex technique used in the Telebit Trailblazer and DCA Fastlink modems. Enhanced Error Control While TCM itself provides virtually error-free data transmission, the Courier HST modem also incorporates a new proprietary ARQ (Automatic Repeat Request) block-check error- control protocol called USR-HST. This protocol is essentially an enhanced version of MNP that USRobotics devised specifically for the Courier HST modem's aysmmetric frequency division. Like MNP, USR-HST provides error-detection and retransmission to protect data integrity. It divides data into transmission frames that include an algorithmic calculation called Cyclic Redundancy Checking (CRC). The receiving modem performs the same CRC algorithm on each frame and responds positively or negatively to the sending modem. A negative acknowledgement is a request that the sending modem retransmit an errored data frame. The receiving modem accepts no more frames until the one in question is transmitted successfully. In this way, USR-HST protects against errors and ensures that the data arrives in sequence. Adapted for asymmetrical modulation, USR-HST provides faster response than MNP. USR-HST includes nine types of data frames compared to MNP's five types, including a faster method of signalling a negative acknowledgement and a faster training algorithm to command channel turnaround. USR-HST's design efficiencies result in less data overhead (control information) than MNP requires. Under optimal phone line conditions, two Courier HST modems can exchange data at rates up to 1,100 characters a second. This fifteen percent gain over the normal 960-cps speed for 9600-bps modems is achieved by removing start and stop bits from each data character so the data is in bit format, as in synchronous transmission. The receiving modem reinserts the start and stop bits before passing the data to the receiving terminal or computer. USR-HST, like other error-control protocols, requires that the modem control data flow from the terminal or computer to the modem, to prevent the possible loss of data that might otherwise occur. Line conditions, for example, might cause a number of retransmission requests that interrupt the normal flow over the data link. The modem is equipped with a buffer for storing incoming data from the computer or terminal, so that the data does not get lost while the modem is retransmitting. The modem monitors the buffer and, if it approaches full capacity, signals the computer or terminal to stop sending. The modem signals the computer to resume sending when the modem has sent enough data over the link to sufficiently empty the buffer. USR-HST's hardware-based flow-control protocol is known as "Request to Send/Clear to Send" (RTS/CTS), using Pins 4 and 5 of the RS232C interface. Software-based, or command-based, flow control uses ASCII "XON/XOFF" signals to perform the identical function of RTS/CTS. An important user benefit of flow control is the ability to set the data transfer rate of the computer or terminal as high as 19,200 bps, regardless of the transmission speed of the modem. With this arrangement, the computer or terminal data speed is the same from call to call. The computer or terminal sends data to the modem buffer as fast as possible, with the modem controlling the flow of data into the buffer. Courier HST automatically matches the speed of the remote modem when answering or originating a call. In addition to USR-HST error-control at 9600-bps, the Courier HST features Classes 1 through 3 MNP when connected at 2400 or 1200 bps to other MNP modems. Other Features The Courier HST also includes Non-volatile Random Access Memory (NRAM), which saves user-defind modem settings from session to session, even if the modem is turned off and on. NRAM also allows the Courier HST to store up to four frequently dialed phone numbers for automatic dialing, and can instruct the modem to dial the first number stored as soon as the modem is powered on. Courier HST responds to the industry standard "AT" modem command set at any of its operating speeds, providing compatibility with most data communications software programs. At fallback data rates, Courier HST is compatible with CCITT V.22 bis at 2400, Bell 212A at 1200, and Bell 103 at 300 bps. Courier HST includes automatic retraining (resynchronizing with the remote modem) if it detects line disturbances that might affect data reliability. At 9600 bps, the connection must be with another Courier HST modem. Retraining also occurs at 2400 bps with other V.22 bis compatible modems. Additionally, Courier HST incorporates the same design and operational features of other modems in USRobotics' Courier product line, including "help screen" summaries of the modem command set and S-registers, a printed operations summary on the modem's bottom panel, externally accessible programming switches and an extended twelve-function LED front panel display. Other features in common with the Courier modem family are call-duration reporting, call-progress detection, modem settings display, the ability to continuously repeat a command, an on- board speaker with volume control for audio phone line monitoring, automatic speed detection in answer and originate, analog loopback self-test in answer and originate, and internal test pattern generation. Courier HST includes a telephone cord, illustrated user's manual and quick reference card. USRobotics provides free parts and service coverage under a two-year warranty. The original purchaser also may buy a two-year warranty extension for $15. International Standard Proposed No international standard exists yet for an asymmetrical high-speed modem like the Courier HST. Support for such a standard is developing, however, within the U.S. Modem Working Party, a committee of industry representatives that reports to a study group of CCITT. USRobotics submitted proposals at two 1986 Modem Working Party meetings for an HST-based standard. Concord Data Systems, which manufactures a V.32 modem, became a co-sponsor of USRobotics' proposal subsequent to its submission. The only other technology being advanced as a potential high-speed modem standard is the multi-carrier half-duplex technique used by the Telebit Trailblazer modem. Dale Walsh is USRobotics representative to the Modem Working Party. He served from 1978 to 1984 as the group's chairman, and is well-acquainted with CCITT's standards-development methods. He said it is likely that CCITT will move cautiously in adopting a new standard for 9600-bps dial-up modems, especially since the V.29 and V.32 standards already exist. "The standards community is fairly conservative in adopting new recommendations," Walsh explained. "They favor incremental change and derivative technology, not radical departures from existing standards. "In that respect, the asymmetrical modem proposal would fit the expectations of the standards community much more closely than would the multi-carrier proposal. "The asymmetrical modem represents the synthesis of several existing standards -- the modulation technique of V.32, a derivative of an accepted error-control protocol, the incorporation of existing 2400 and 1200 standards, and even the concept of asymmetrical frequency division, which has been used successfully in other modems," Walsh continued. "The multi-carrier technique, however, is a complete break from existing dial-up modem technology. It remains to be seen whether the standards community will support this idea." USRobotics also is working with other modem companies which may be interested in developing Courier HST-compatible modems. ___________________ For more information on USRobotics Courier HST modem, call toll-free, 800-DIAL-USR (342-5877). In Illinois, call our sales department at 312/982-5001.