Data transfer method and apparatus

Abstract: In a method and apparatus for transferring data from a computer (12) to a remote entity (16), a predetermined block count is stored in a counter (22) included in the computer (12). One or more data blocks are moved from the computer (12) to one or more buffers, respectively, in a dynamic RAM (26) included in an interfacing device (14). The block count is decremented for each of the data blocks so moved. One or more of the data blocks are then transmitted from the interfacing device (14) to the remote entity (16). An acknowledge signal is received from the remote entity (16) for each data block received by the remote entity (16), and the block count is incremented for each acknowledge signal so received.

Description: This invention relates to a method and apparatus for transferring data from a computer to a remote entity. In the past, the transfer of data between remotely located terminal equipment such as PCs has been relatively slow and characterized by transmission errors. Telephone lines used for data transfers have one or more links which only carry analog signals. Typically, the local telephone line or link between a PC and the first switching station handles only analog signals. In order to serially transmit data from a PC over an analog phone line, a device such as a modem converts digital data to an analog signal. Although many long distance calls are converted and transmitted digitally, there still remains a number of analog links for most transmissions. One problem with analog transmission is that amplifiers used to boost the analog signals can distort the original signal causing error rates in the data roughly equivalent to the distance of the transmission. One result of such transmission errors and the resulting retransmission of erroneously received data is to slow down the data transfer rate with typical rates at about 1200 to 2400 bits per second. In recent years, technology for digital data transmission has advanced. Digital transmission has a much lower error rate since amplifiers can easily and exactly restore a digital signal which only has two possible values. Digital transmission also permits multiplexing of various digitally encoded signals such as data, audio and video. Digital transmission is increasingly playing an important role in enhancing wide area computer networks which utilize telephone lines. One such digital network is referred to as Integrated Services Digital Network (ISDN). ISDN offers a wide range of services, most notably an increased transmissions speed on the order of 64,000 bits per second. The document EP-A-0 072 614 discloses a system including a digital bus coupled to a sending device for sending digital data words over the bus, and a receiving device coupled to the bus for receiving the sent words. The sending device includes a single-word register for storing a word to be sent over the bus, and the receiving device includes a FIFO queue store which can store up to eight received words. When a word is sent by the sending device over the bus, a write strobe is generated and also transmitted over the bus. When a word is removed from the FIFO store in the receiving device, a read strobe is sent back over the bus to the sending device. The write strobes and read strobes are applied to respective ring counters in the sending device. The ring counters are connected to a comparator and when a predetermined different value is detected, the transmission of further write strobes is inhibited. It is an object of the present invention to provide a method and apparatus for transferring data from a computer to a remote entity, which enables high data throughput to be achieved. Therefore, according to one aspect of the present invention, there is provided a method of transferring data from a computer to a remote entity, characterized by the steps of: (a) storing a predetermined block count in said computer; (b) moving one or more data blocks from said computer to one or more of a plurality of buffers, respectively, in an interfacing device; (c) decrementing said block count for each of said data blocks so moved; (d) serially transmitting one or more of said data blocks from said interfacing device to said remote entity; (e) receiving an acknowledqe signal from said remote entity for each data block received by said remote entity; (f) incrementing said block count for each acknowledge signal so received; and, in association with step (a), the step of (g) allocating a predetermined number of buffers in said interfacing device to receive data blocks sequentially moved from said computer to said interfacing device, wherein said predetermined block count and said predetermined number of buffers are the same. According to another aspect of the present invention, there is provided data communications apparatus, including a computer, an interfacing device connected to said computer and including transmission means, characterized in that said computer is arranged to generate a plurality of data blocks and to store a predetermined block count, in that said interfacing device includes a plurality of buffers corresponding in number to said predetermined block count and allocated for the reception of said data blocks from said computer, in that said transmission means is arranged to serially transmit said data blocks from said buffers to a remote entity and to receive an acknowledge signal in response to each successfully transmitted data block, and in that said block count is decremented in response to a data block transferred to said interfacing device and incremented in response to said acknowledge signal. It will be appreciated that in a method and apparatus according to the invention a high data throughput is achievable as a result of the data transfer from the computer to the interfacing device being independent of the data transfer from the interfacing device to the remote entity. In particular, a faster data transfer is achieved than if the computer needed to receive an acknowledge signal before transferring a further data block to the interfacing device. One embodiment of the present invention will now be described by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a block diagram of a data communications system according to the present invention; and Figures 2 and 3 show flow diagrams describing the operation of the present invention. Figure 1 shows a data communications system 10 having a computer 12, interfacing device 14 and a remote entity 16. In a preferred embodiment, computer 12 is a personal computer (PC). The components in computer 12 relevant to the present invention are a processor 18, memory 20, counter 22, and a program store 24. Data generated by computer 12 to be transferred therefrom is stored in memory 20 in a plurality of logically defined data frames or data blocks. Data blocks are transferred from computer 12 under the control of processor 18 running a program in store 24 and subject to the value in counter 22, as will be described more fully hereinafter. Interfacing device 14 includes a dynamic RAM (DRAM) 26, an I/O device 28, a program store 30 and a processor 32. In a preferred embodiment, interfacing device 14 is a modular board or card which may be plugged into an expansion slot on computer 12. DRAM 26 includes a plurality of logically defined memory units or buffers for receiving data blocks transferred from computer 12, as will be described more fully hereinafter. Typically, data is transferred in parallel from computer 12 to interfacing device 14. I/O device 28 serially transmits data blocks from DRAM 26, over an ISDN transmission line 34, to remote entity 16. I/O device 28 also receives acknowledge signals from remote entity 16 over ISDN line 34 for each data block successfully transmitted. In one embodiment, remote entity 16 may itself consist of a computer and interfacing device such as computer 12 and interfacing device 14 described above. In such embodiment, I/O device 28 may also receive data blocks from remote entity 16 as well as transmit acknowledge signals thereto. Counter 22 stores a block count of the number of data blocks which at any given time may be transferred from memory 20 to DRAM 26. The count is decremented in response to each data block so transferred, and the count is incremented in response to each acknowledge signal received by counter 22. The operation of the present invention may be understood by referring to the flow diagram shown in Figures 2 and 3. Referring to Figure 2, the first step in the process of transferring data from computer 12 to remote entity 16 is to initialize the data block transmission counter 22 (block 36) by storing a predetermined block count or number of credits in counter 22. Whenever a data block is ready to be transferred to interfacing device 14 for transmission over ISDN line 34 (block 38), the block count in counter 22 is checked (block 40). If the count is zero, the transfer enters a wait state indicated by line 42. If the count is greater than zero, the transfer can proceed. The block count in counter 22 is decremented (block 44), and the data block is moved to a buffer in interfacing device 14's DRAM 26 (block 46). The sequence of decrementing the count and moving a data block is not critical and the block count may be decremented after the data block is moved or contemporaneously therewith. As long as the block count is greater than zero, additional data blocks may be moved to respective buffers in DRAM 26 (line 48) irrespective of whether any data block has been transferred from interfacing device 14 over ISDN line 34. Figure 3 shows a flow diagram of data transfer from interfacing device 14 to remote entity 16. This process may commence as soon as a data block has been transferred to interfacing device 14 from computer 12 (block 46) (Figure 2). Data blocks are received by interfacing device (block 50) and are stored in respective data buffers in DRAM 26 (block 52). Data blocks stored in DRAM 26 are ready for transmission over ISDN line 34 (block 54). If a data block is not ready for transmission, the transfer enters a wait state (line 56). If a data block is ready for transmission, I/O device 28 is checked to see if it is ready to transmit data. If not ready, the transfer enters a wait state (line 58). If I/O device 28 is ready, a data block is serially transmitted from interfacing device 14 to remote entity 16 (block 60) over ISDN line 34. As long as there is a data block in DRAM 26 ready for transmission, the transmission process may continue (line 62 and blocks 56, 58 and 60) for additional data blocks. After one or more data blocks has been transmitted from interfacing device 14 to remote entity 16, an acknowledge signal will be received for each of such data blocks received by the remote entity (block 64). The acknowledge signal is transmitted from the interfacing device to computer 12 (block 66) in the form of a credit update, and the block count in counter 22 is incremented (block 68). Subsequent acknowledge signals will be received for each data block received by remote entity 16, and the block count will be incremented accordingly. The transmission of data blocks from interfacing device 14 generally proceeds independently of the movement of data blocks from computer 12 to device 14. In fact, each such transfer may be performed contemporaneously with the other. As long as the block count in counter 22 is greater than zero, data blocks may be moved to DRAM 26. As long as there are data blocks in DRAM 26 and the I/O device is ready, data blocks may be serially transmitted over ISDN line 34 to remote entity 16. When the block count reaches zero, further data blocks may not be transferred to interfacing device 14. However, data blocks may continue to be transmitted from interfacing device 14 to remote entity 16. The movement of data blocks from computer 12 to DRAM 26 may resume when the block count is incremented upon receipt of an acknowledge signal. As noted previously, the buffers for storing data blocks in interfacing device 14 are logically defined address spaces in DRAM 26. At the time the system is initialized, a predetermined number of such buffers are allocated to receive the data blocks sequentially moved from computer 12. Preferably, the allocation of buffers is performed in association with the counter initialization (block 36), and the number of buffers so allocated is the same as the maximum number of credits stored in counter 22. Data stored in memory 20 may be contained in a plurality of data frames which have a different size than that of the data blocks buffered and transmitted from interfacing device 14. For example, each such data frame may be larger than a data block. In this circumstance, each data frame will be segmented into a plurality of data blocks prior to being moved to DRAM 26. It may further be desirable to move all of the data blocks in a data frame to DRAM 26 prior to transmitting the data blocks to remote entity 16. Further, the block count would not be incremented until the transmission of all such data blocks is complete.

Claims: A method of transferring data from a computer (12) to a remote entity (16), characterized by the steps of: (a) storing a predetermined block count in said computer (12); (b) moving one or more data blocks from said computer (12) to one or more of a plurality of buffers, respectively, in an interfacing device (14); (c) decrementing said block count for each of said data blocks so moved; (d) serially transmitting one or more of said data blocks from said interfacing device (14) to said remote entity (16); (e) receiving an acknowledge signal from said remote entity (16) for each data block received by said remote entity (16); (f) incrementing said block count for each acknowledge signal so received; and, in association with step (a), the step of (g) allocating a predetermined number of buffers in said interfacing device (14) to receive data blocks sequentially moved from said computer (12) to said interfacing device (14), wherein said predetermined block count and said predetermined number of buffers are the same. A method according to claim 1, characterized in that a step (b) moving of a data block is performed contemporaneously with a step (d) transmitting of a data block previously moved to one of said buffers. A method according to claim 1, characterized by the steps of: (h) repeating steps (b) to (f) until said block count is zero; (i) performing steps (d) to (f); and (j) resuming steps (b) through (f). A method according to any one of the preceding claims, characterized in that said block count is stored in a counter (22), said interfacing device (14) receives said acknowledge signal, and in that said method further includes, after step (e), transmitting said acknowledge signal from said interfacing device (14) to said counter (22). A method according to any one of the preceding claims, characterized in that the data to be transferred is logically contained in a data frame which is larger than a data block, said method further including, prior to step (b), the step of segmenting said frame into a plurality of data blocks, in that all of the data blocks in a data frame are moved into said buffers prior to step (d); and in that step (f) is performed after all the data blocks in the data frame have been transmitted in step (d). Data communications apparatus, including a computer (12), an interfacing device (14) connected to said computer (12) and including transmission means (28), characterized in that said computer (12) is arranged to generate a plurality of data blocks and to store a predetermined block count, in that said interfacing device (14) includes a plurality of buffers corresponding in number to said predetermined block count and allocated for the reception of said data blocks from said computer (12), in that said transmission means (28) is arranged to serially transmit said data blocks from said buffers to a remote entity (16) and to receive an acknowledge signal in response to each successfully transmitted data block, and in that said block count is decremented in response to a data block transferred to said interfacing device (14) and incremented in response to said acknowledge signal. Data communications apparatus according to claim 6, characterized in that said computer (12) includes a counter (22) adapted to store said block count. Data communications apparatus according to claim 6 or claim 7, characterized in that said buffers are-formed by logically defined address spaces in a memory device (26).

Inventors: NANAVATY SAMIR J, PETERSON PAUL A, NANAVATY, SAMIR J., PETERSON, PAUL A
Applicants: NCR INT INC, NCR INTERNATIONAL, INC.
Application year: 1990
Main classification: G06F13
IPC codes: G06F13, H04L29, H04M11, H04Q3