Abstract:
A system and method for detecting an error in a transmission of data through an ATM cell in a ATM switching system is disclosed. The present invention allows a system manager to easily detect errors as the errors occur in the ATM switching system.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a data communication and more particularly to monitoring an Asynchronous Transfer Mode cell bus. 
     DISCUSSION OF THE RELATED ART 
     The Asynchronous Transfer Mode (ATM) is the technology selected by the Consultative Committee on International telephone &amp; Telegraph (ITU) International standards organization (now called the ITU-T) to realize a Broadband Integrated Services Digital Network (B-ISDN). ATM is a very high speed transmission technology utilized in data communication. A general concept of ATM is disclosed by Harry Neviton in  Newton&#39;s Telecom Dictionary , Flatiron Publishing, pp. 67-8, 1998 and is fully incorporated herein. 
     ATM is a high bandwidth, low-decay, connection-oriented, packet-like switching and multiplexing technique. The usable capacity is segmented into 53 fixed size cells, consisting of header and information fields, allocated to services on demand. The small, fixed length cells require lower processing overhead and allow higher transmission speeds than traditional packet switching methods. Also, ATM automatically adjusts the network capacity to meet the system needs and can handle data, voice, video and television signals. 
     All broadband transmissions (whether audio, data, imaging or video) are divided into the fixed length cells and routed across an ATM network consisting of links connected by ATM switches. Each ATM link comprises a constant stream of ATM cell slots into which transmissions are placed or left idle, if unused. The cell-switching technology of ATM combines the best advantages of both circuit-switching (for constant bit rate services such as voice and image) and packet-switching (for variable bit rate services such as data and full motion video) technologies. 
     Particularly, each cell consists of 53 eight-bit bytes, called octets, wherein five octets are header field and the remaining 48 octets are user data. The header contains data that identifies the related cell, a logical address that identifies the routing, header error correction bits for priority handling and network management functions. The user data contains the “payload” or the information to be transmitted. 
     An ATM switching system in the related art includes a bus master and a plurality of bus slaves connected to the bus master. Also, a system manager may monitor the status of ATM cell buses utilizing a Cyclic Redundancy Check (CRC). Namely, the cell buses are monitored during the transmission and reception of Packets by periodically requesting and receiving responses to analyze the CRC value of the transmitted and received data. 
     As an error detection method for detecting group error in the transmitted data, the CRC utilizes polynomial codes for error detection as well as the minimum bits required for expressing the data to detect errors. For CRC, additional bits need not be attached to each character, like a parity checking method. However, a Frame Check Sequence (FCS) calculated based upon the actual contents of a frame is attached to the end of the frame and transmitted. Accordingly, generation of polynomial to calculate the FCS is required. 
     As described above, CRC allows a system manager to periodically detect errors in the ATM switching system. Therefore, errors cannot be detected as the errors occur in between the periods while the ATM switching system is in operation. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve at least the problems and disadvantages of the related art. 
     Another object of the present invention is to provide a hardware based method of monitoring ATM cell buses. 
     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims. 
     To achieve the objects and in accordance with the purposes of the invention, as embodied and broadly described herein, 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein: 
     FIG. 1 is a general structure of an ATM switching system in the related art; 
     FIGS. 2A to C Illustrate the flow of control signals in ATM cell buses according to the present invention; 
     FIG. 3 is a block diagram of an ATM cell bus monitoring apparatus according to the present invention; 
     FIG. 4A is a timing diagram illustrating a normal operation of the ATM switching system; and 
     FIG. 4B is a timing diagram illustrating an abnormal operation of the ATM switching system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a general structure of ATM cell bus including a bus master  1  which controls an allocation of the ATM cell bus; a plurality of bus slaves  2 ˜n which use the ATM cell bus allocated by the bus master  1 ; a control bus B 1  which transmits a control signal from the bus master  1  to each bus slaves  2 ˜n for controlling each bus slaves  2 ˜n; a transmitting cell bus B 2  which transmits data from the bus master  1  to each bus slaves  2 ˜n; and a receiving cell bus B 3  which transmits data from each bus slaves  2 ˜n to the bus master  1 . 
     In the ATM cell bus, the bus master  1  transmits the necessary data to a corresponding bus slave via a transmitting cell bus B 2 , and each bus slaves  2 ˜n transmit the necessary data to the bus master  1  via a receiving cell bus B 3 . Also, the bus master  1  controls the bus slaves  2 ˜n by transmitting a control signal via a control bus B 1 . 
     As shown in FIGS. 2A to C, the control bus B 1 , the transmitting cell bus B 2  and the receiving cell bus B 3  respectively include buffers  5 ˜ 7  generated, in effect, due to the backbone of the ATM cell bus system. In the present invention, the signal prior to the buffer is a control signal output from the bus master  1  and the signal after the buffer is a control signal from the bus master  1  input to the bus slaves  2 ˜n. Accordingly, the buffer  5  differentiates the bus master output control signal from the bus slave input control signal. 
     Because of the difference between the bus master output control signal and the bus slave input control signal, the transmission of data through an ATM cell bus must be monitored in order to determine whether the system is normally operating. Accordingly, a plurality of ATM cell bus monitoring apparatuses as shown in FIG. 3 is implemented in the present invention. Particularly, an ATM cell bus monitoring apparatus is implemented in each one of the bus slave  2 ˜n. 
     Referring to FIG. 3, the ATM cell bus monitoring apparatus includes an AND Gate  10  which receives and performs a logical AND operation on the bus slave input control signal and the bus master output control signal; a counter  20  which receives and counts the signal output from the AND Gate  10  for a period enabled by the input clock signal; a comparator  30  which compares the count value from the counter  20  with a prestored count value; and a pre-counter  40  which updates, if necessary, and stores the lower count value from the counter  20 . The period enabled by the input clock signal, i.e. the enabled time, represents the period in which the logic levels of the bus slave input control and the bus master output control signals are equivalent as shown in FIGS. 4A and 4B. The operation of the ATM cell bus monitoring apparatus will next be explained. 
     First, an AND logic operation is performed on the bus slave input control signal and the bus master output signal. The logically added signal is counted by the counter  20  for the enabled time. If noise or Jitter is generated due to circumstances of the internal or external environment while the counter  20  counts, the counter  20  resets and restarts counting the logically added signal. 
     The count value of the counter  20  is stored in a memory device (not shown) and input to the comparator  30  which compares the received count value to a prestored count value. Namely, the prestored count value is a count value of a period necessary for the ATM switching system to transmit or receive an ATM cell via the ATM cell bus and is stored in the pre-counter  40 . Accordingly, the counter  20  compares the count value received from the counter  20  and the prestored count value from the pre-counter  40 . 
     Upon comparison, the comparator  30  outputs the lower of the two count values to the pre-counter  40 . Here, the comparator  30  would output either the count value from the counter or the prestored count value if the two values are equivalent. The pre-counter  40  receives the count value from the comparator  30  and updates the prestored count value if necessary, i.e. if the newly received count value is lower than the prestored count value. Thereafter, an error message may be transmitted and displayed on a monitor if the received count value is lower than the prestored count value. 
     Particularly, if the ATM switching system is operating normally, the enabled time for the counting would be constant and the count value would be constant. In contrast, if the ATM switching system is operating abnormally, the enabled time for the counting of the logically added signal would be inconsistent. Because of jitter or noise, the counting would be restarted, thereby shortening the enabled time. FIG. 4A shows an enabled time in a normal state and FIG. 4B shows a shortened enabled time in an abnormal state. As seen from a comparison of FIGS. 4A and 4B, the count value of an abnormally operating system would result in a lower count value than a count value of a normally operating system. Thus, an update of the prestored count value would indicate that data is not being normally transmitted in the ATM cell bus. 
     Accordingly, a transmission of the error message may be triggered by an update of the prestored count value or upon a request by the system manager. With such an error message, the system manager can also determine the location in which an error has risen within the ATM cell. Moreover, after a transmission of an error message, the prestored count value is reset to the initial count value, i.e. the count value of a period necessary for the ATM switching system to transmit or receive an ATM cell via the ATM cell bus. Alternatively, the prestored count value may be reset to the initial count value after each period equivalent to an enabled time period in an normal operation. 
     For example, if the period necessary for the ATM switching system to transmit or receive an ATM cell via the ATM cell bus is 64 clocks, the count value obtained during the enabled time would be 64 in a normal state. Also, the prestored count value would initially be 64. However, the count value would be less than 64 in an abnormal state due to a shortened enabled time, as shown in FIG.  4 B. 
     Thus, when the comparator  30  compares the count value from the counter  20  with the prestored count value, 64 in the above example, the count value from the counter  20  would be smaller than 64. Accordingly, the pre-counter  40  would update and store the lower count value as the prestored count value. The update would trigger a transmission and display of an error message on a monitor and then prestored count value would be reset to 64. 
     As described above, according to the present invention, the system manager can easily recognize the status of an ATM cell bus in the ATM switching system. Therefore, the system manager may take appropriate remedial actions such that the system normally operates, thereby increasing the system reliability. 
     The foregoing embodiments are merely exemplary and are not to be construed as limiting the present invention. The present teachings can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art.