Asymmetrical duplex error-controlled modem
First Claim
1. A modem for connecting digital data terminal equipment to an analog communication link comprising, in combination,frequency division filtering means serially connected with said link for establishing a high speed channel having a bandwidth in a first range of frequencies and a low speed channel having a bandwidth in a second and more limited range of frequencies,digital-to-analog conversion means connected between said equipment and said filtering means for translating digital signals from said equipment into analog signals for transmission over said link in a selected one of said high or low speed channels,analog-to-digital conversion means connected between said equipment and said filtering means for producing digital signals supplied to said equipment from the analog signals received over the other one of said channels, andmeans operative when said digital-to-analog conversion means is transmitting data over said low speed channel for sending a channel reversal command over said link whenever the amount of data awaiting transmission over said low speed channel exceeds a predetermined amount.
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Accused Products
Abstract
A modem for connecting data terminal equipment (DTE) to a remote DTE via a general switched telephone network or leased lines at data rates of 300, 1200 and 2400 bps using standard modulation techniques, and additonally providing virtual full duplex transmission capability at 9600 bps using trellis code modulation (TCM). The high-speed 9600 bps path is implemented by asymmetrical frequency division of the available bandwidth into a high speed, wideband forward channel (9600 bps) and a low speed, narrowband backchannel (300 bps). The high speed transmitting channel is assigned to that modem having the greatest data demand and the direction is dynamically reversed whenever the amount of data awaiting transmission over the low speed channel exceeds a predetermined maximum backlog, provided that a direction reversal has not taken place for at least a specified minimum interval during which use of the forward channel is guaranteed. To improve the efficiency of the backchannel, extended data formats are added to the MNP link protocol for use in backchannel transmission and to control direction reversals on the line. The modem is implemented with three available general purpose microprocessors with interprocessor communications being synchronized by a timer driven interrupt mechanism. A hardware timer/counter built into one of these processors is used to monitor the signals on the telephone line to detect and distinguish answer tones, voice signals, busy signals, ringback signals and dial tones. A hardware counter/timer is also used to measure the width of the start bit from the serial port to form the basis for a calculation of the baud rate being used by the DTE.
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Citations
5 Claims
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1. A modem for connecting digital data terminal equipment to an analog communication link comprising, in combination,
frequency division filtering means serially connected with said link for establishing a high speed channel having a bandwidth in a first range of frequencies and a low speed channel having a bandwidth in a second and more limited range of frequencies, digital-to-analog conversion means connected between said equipment and said filtering means for translating digital signals from said equipment into analog signals for transmission over said link in a selected one of said high or low speed channels, analog-to-digital conversion means connected between said equipment and said filtering means for producing digital signals supplied to said equipment from the analog signals received over the other one of said channels, and means operative when said digital-to-analog conversion means is transmitting data over said low speed channel for sending a channel reversal command over said link whenever the amount of data awaiting transmission over said low speed channel exceeds a predetermined amount.
Specification