High speed modem

• US 4,924,456 A
• Filed: 08/29/1988
• Issued: 05/08/1990
• Est. Priority Date: 09/18/1986
• Status: Expired due to Fees

• Current Assignee: Racal-Datacom, Inc. (Thales SA)
• Original Assignee: Racal Data Communications Incorporated
• Inventors: Hsieh, Ming-Chih, Maxwell, Ronald K., Prasad, Venkat S., Yoshikawa, Harunori, Smith, Fred G., Zorzi, Karl J.
• Primary Examiner(s): Orsino, Joseph A.
• Assistant Examiner(s): Scutch, III, Frank M.
• Application Number:
  US07/197,128
• Time in Patent Office: 617 Days
• Field of Search: 370/31, 370/32, 370/35, 370/110.1, 370/68.1, 370/24, 370/29, 370/84, 375/8, 375/7
• US Class Current: 370/296
• CPC Class Codes: H04L 1/0002 by adapting the transmission rate
• CPC Class Codes: H04L 1/0025 Transmission of mode-switching indication
• CPC Class Codes: H04L 1/1867 Arrangements specially adapted for the transmitter end
• CPC Class Codes: H04L 27/00 Modulated-carrier systems
• CPC Class Codes: H04L 5/1446 of transmission speed
• CPC Class Codes: H04L 5/16 Half-duplex systems; Simplex/duplex switching; Transmission of break signals non-automatically inverting the direction of transmission
• CPC Class Codes: Y02D 30/50 in wire-line communication networks, e.g. low power modes or reduced link rate

Abstract

A modem is disclosed having a data transmission protocol involving lower-speed, full-duplex operation during the connect sequence with a remote modem and an automatic switch to higher-speed, half-duplex operation for data transfer. Further, the modem data transmission involves transparently changing between lower-speed, interactive operation and higher-speed operation based upon data transmission demands. The operation is controlled by a processor monitoring the contents of a transmit data buffer and providing a mode control command to the modem transmitter. The modem also adapts its speed to the quality of the telephone line by fallback or fallforward to a different speed based upon predetermined data frame retransmission criteria.

Claims

1. Apparatus for providing communication of data over a telephone line, comprising:

a transmitter receiving frames of data, for placing a transmit signal on a telephone line;

said transmitter being selectively operable to transmit data at discrete data rates in response to a speed control command;

a processor for monitoring the transmission of data frames to determine data transmission errors, for selectively effecting the retransmission of data frames having errors in their transmission, and for producing the speed control command to select the data rate of the transmitter; and

said processor causing the data transmission rate to fallback to a lower data rate or to fallforward to a higher data rate based upon the number of retransmitted data frames.

2. A modem for providing asynchronous communication of data over a telephone line between a local data terminal and a remote modem, comprising:

a transmitter selectively operable at discrete data rates for transmitting frames of data and control information to the remote modem;

a receiver, selectively capable of receiving data and control frames transmitted from the remote modem in discrete data rates;

a controller processor, for selecting the data rate of said transmitter and receiver, the processor controlling the transmitter and receiver during a connect sequence with the remote modem and to thereafter effect an exchange of a data and control frames with the remote modem,

the processor evaluating received control frames, for identification of frames sent to the remote modem which were found to contain errors and effecting the retransmission of an identified data frame;

the processor maintaining a count of the number of times a data frame is retransmitted, and when the count exceeds a predetermined maximum value, the processor effects a fall-back in the transmission speed of the transmitter to the next-lower data rate, the processor further calculating the ratio of retransmitted frames to once-transmitted frames among the plurality of frames transmitted to the remote modem, and when the ratio is within a predetermined range, the processor effects a fall-forward in the transmission speed of the transmitter to the next-higher discrete data rate.

3. The modem of claim 2, wherein the processor disconnects the communication link with the remote modem in the event the count exceeds the predetermined maximum value when the modem is transmitting at the lowest of the discrete data rates.

4. A method of communicating information via a telephonic line between a first modem and a second modem, each having a receiver and a transmitter selectively operable in a plurality of discrete data rates and each having a controller processor, the method comprising the steps of:

formatting information to be transmitted by the first modem into frames of control information and frames of data;

establishing, under control of the controller processors, a communication link on the telephone line between the first and second modems and thereafter transmitting the control frames and data frames to the second modem;

determining, in the second modem, whether frames transmitted from the first modem contain errors, and transmitting an acknowledgement to the first modem identifying those frames received by the second modem which contained errors;

retransmitting, from the first modem, those frames identified in the acknowledgement as having been received by the second modem with errors;

counting, in the first modem, the number of times each frame is retransmitted before being received by the second modem without errors;

computing, in the first modem, the ratio of retransmitted frames to once-transmitted frames, among a plurality of frames transmitted from the first modem;

effecting a fall-forward in transmitting speed of the first modem and receiving speed of the second modem to the next-highest discrete data rate in the event that said ratio is within a predetermined range;

effecting a fall-back in transmitting speed of the first modem and receiving speed of the second modem to the next-lowest discrete data rate in the event that any frame is retransmitted more than a predetermined number of time from said first modem; and

effecting a disconnection of communication between the first and second modems in the event that a frame is retransmitted from the first modem more than the predetermined number of times when the first modem is transmitting at its lowest data rate.

5. The method of claim 4, wherein the step of determining whether frames transmitted from the first modem contain error comprises analysis of error-control information included in said transmitted frames.

6. The method of claim 4 wherein the steps of effecting a fall-forward and of effecting a fall-back in transmitting speed of the first modem and receive speed of the second modem comprises:

transmitting, from the first modem to the second modem, a control frame containing information in the form of a request for the desired speed change, and thereafter transmitting from the second modem to the first modem a control frame containing information in the form of an acknowledgement of the request for the speed change.