Patent Publication Number: US-4922438-A

Title: Method and apparatus for reading packet-oriented data signals into and out of a buffer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to a method for controlling the write-in and read-out of data signals transmitted serially or in parallel on a network packet-oriented with a determined maximum plurality of bits per packet into and out of a buffer cyclically addressable by memory location and provided with data input and data output in a station connected to the network via a network access controller. The processing clock frequency of the station approximately corresponds to the serial or parallel data bit clock rate, whereby a packet start bit combination and a packet end bit combination and/or bit combinations indicating stuffing information between the data packets are insertable and the size of the buffer is adapted to the maximum deviation between the processing clock frequency and the data transmission rate as well as to the maximally-occurring jitter. 
     2. Description of the Prior Art 
     An exchange of digital data signals is occurring to an increasing degree in communications and in data processing technology at transfer locations or, respectively, interfaces between systems, system components, etc., for example between two subscriber stations connected to a public or private network. In terms of bit clock frequency and phase relation, the data signals transmitted from one system to another system and received at the latter usually do not agree with the bit clock frequency and phase relation of the processing clock of the receiving system. These deviations are primarily caused by the clock frequency deviations of the clock oscillators implemented in the systems. When the distance between the two systems requires a transmission of the data signals by way of standard transmission technology then, caused for example by noise voltage influences, transient responses in synchronizing devices, etc., the bit clock frequency deviations can slightly increase and the phase fluctuations, also referred to as jitter, can considerably increase. The jitter is generally defined as phase fluctuation about the characteristic points in time of a digital signal or, respectively, about the ideal, equidistant points in time. A possibility of matching the bit clock frequency and the phase relation of the data signals to that of the processing clock of the receiving system is represented by the insertion of a buffer between the transfer location and the further-processing system components. The received data signals are thereby written into the buffer memory with the data bit clock derived from the data signals and are, in turn, read out from the buffer with the processing system clock after a prescribed time adapted to the maximum clock frequency and phase deviation. The write-in and read-out usually occurs with devices which generate memory location addresses and produce write-in and read-out signals, the write-in and read-out and address inputs of the buffer being selected with these devices. The write-in or, respectively, read-out procedure is usually initialized by a specific initialization procedure which sequences before the write-in or, respectively, read-out event. Buffers controlled in this manner are known, for example, from time-division multiplex data transmission systems, particularly pulse code modulation (PCM) transmission systems, or from access equipment of data processing systems, whereby the data signals may also exist in parallel and the adaptation then occurs in a parallel buffer, for example in a 1-byte buffer. The introduction of packet-oriented data transmission or, respectively, processing technology and, therefore, of the burst or, respectively, packet transmission of data signals requires initialization and termination procedures of the write-in and read-out events of the buffer adapted to this technology given simultaneous consideration of continuously increasing transmission rates of the data signals. A predetermined packet start or, respectively, packet end bit combination defines the beginning and, respectively, end of an individual data packet. Bit combinations indicating stuffing information can be inserted between the individual data packets in order to recognize, for example, operational or down status. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method and an apparatus with which the write-in and, respectively, the read-out of packet-oriented data signals present serially or in parallel into and out of a buffer addressable by memory locations is initialized and terminated packet-related at the earliest possible point in time. 
     Proceeding on the basis of the method initially set forth, the above object is achieved and is characterized in that the write-in of the serial or parallel data signals into the buffer controlled by the memory location addresses and clocked by the data bit clock is initiated after the identification of the presence of a packet start bit combination at the data input of the buffer and is terminated after the identification of the presence of a packet end bit combination or of a bit combination indicating stuffing information. It is further characterized in that the read-out of the data signals from the buffer controlled by memory location addresses and clocked by the processing clock is initiated after a prescribed time delay following the identification of the presence of a packet starting bit combination, the time delay representing approximately half the clock steps of a buffer cycle covering the addressing of all memory locations. Furthermore, the read-out of the data signals from the buffer controlled by memory addresses and clocked by the processing clock is terminated either after the identification of the presence of a packet end bit combination and/or bit combination indicating stuffing information at the data output of the buffer, or dependent on the presence of a packet end bit combination or bit combination indicating stuffing information at the data input of the buffer after the identification of the address equality of the buffer address of the packet end bit combination most recently read into the buffer and/or of the most recent bit combination indicating stuffing information and the buffer address of the buffer location currently to be read. 
     The advantage which may be obtained in practicing the present invention is, in particular, that the write-in or, respectively, read-out can be initialized or, respectively, terminated at the earliest possible point in time after the reliable recognition of a packet start or, respectively, packet end, due to the evaluation of the packet start bit combination or, respectively, of the packet end bit combination. Furthermore, the write-in and, respectively, the read-out event is limited to the duration of a packet. As a result thereof, the size of the buffer can be considerably reduced since only the bit clock frequency deviations and phase deviations occurring in a relatively short packet duration, in comparison to data signals to be continuously transmitted, for example, in time-division multiplex systems have to be compensated. This is also true of data practice between which bit combinations indicating stuffing information are inserted. The write-in of the data signals into the buffer and the read-out of the data signals from the buffer are thereby respectively terminated after the recognition of the bit combination indicating stuffing information and the most recently addressed memory location which contains the bit combination indicating the stuffing information is subsequently continuously read until a further, incoming data packet reinitializes or, respectively, re-terminates the write-in or, respectively, read-out events. The read-out of the data signals from the buffer is terminated when either a packet end bit combination or a bit combination indicating a stuffing information is identified at the data output of the buffer or when the memory location address of the data signals most recently read-in coincides with the memory location address of the data signals currently being read, assuming a packet end bit combination or a bit combination indicating stuffing information was identified at the data input of the buffer. As such, the packet end bit combination and the stuffing information function as packet termination bit combinations. Two arrangements are set forth for implementing the method and are directed to the aforementioned alternatives in view of the termination of the read-out of the data signals from the buffer. 
     The first arrangement is characterized in that a data line or, respectively, data lines carrying serial or, respectively, parallel data signals is, or, respectively, are connected to the data input or, respectively, data inputs of a buffer and to a first comparator identifying the packet start bit combination and packet end bit combination or bit combination indicating stuffing information. That output of the first comparator indicating the packet start is connected to a start input of a write-in device cyclically generating the memory addresses and is connected to a start input of a read-out device cyclically generating the memory addresses, being connected to the latter by way of a first delay device provided with a defined delay time. That output of the first comparator indicating the packet end and/or the stuffing information is connected to a stop input of the write-in device by way of a second delay device provided with a second, prescribed delay time. Data outputs of the buffer comprising serial or parallel structure are connected to one another and to a second comparator identifying the packet end bit combination and/or the bit combination indicating stuffing information and can be continued on to further-processing devices. An output of the second comparator indicating the packet end and/or the stuffing information is connected to a stop input of the read-out device. The address outputs of the read-in or read-out device are fed to read-in or, respectively, read-out address inputs of the buffer. 
     The second arrangement for implementing the method is characterized in that data lines comprising serial or, respectively, parallel data signals are connected to a data input or, respectively, data inputs of a buffer and to a first comparator identifying the packet start and packet end bit combinations or a bit combination indicating filler information. An output of the first comparator indicating the packet start is connected to a start input of a write-in device cyclically generating the memory addresses and is connected to a start input of a read-out device cyclically generating the memory addresses, being connected to the latter via a first delay device provided with a defined delay time. An output of the first comparator indicating the packet end and/or the stuffing information is connected to a stop input of the write-in device via a second delay device provided with a second, prescribed delay time and, secondly, is connected to an enable input of a third comparator, half of whose comparison inputs are connected to address outputs of the write-in device and half of whose comparison inputs are connected to the read-out device and whose comparator output is connected to a stop input of the read-out device. 
     An improvement may be realized in the second arrangement in that the comparators can be realized with bit-comparing integrated circuits. 
     An improvement in both arrangements is characterized in that the delay device can be realized with clocked registers. 
     A further improvement in both arrangements is characterized in that the write-in and read-out devices can be realized with clocked shift registers. 
     The aforementioned possible realizations are aimed at a utilization of the method and networks, for example ring network, insofar as the individual components, such as buffer, write-in device, read-out device and comparator can be realized with integrated circuits operating at high clock rates, i.e. above 10 MHz. The two exemplary embodiments of the invention shall be set forth in greater detail below with reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the invention, its organization, construction and operation will be best understood from the following detailed description, taken in conjunction with the accompanying drawings, on which: 
     FIG. 1 is a block circuit diagram of an arrangement for implementing the method of the invention wherein the read-out of the buffer is terminated after the identification of the presence of a packet end bit combination or of a bit combination indicating stuffing information; and 
     FIG. 2 is a block circuit diagram of an arrangement for the implementation of the method wherein the read-out of the buffer is terminated when the memory location address of the data signals most recently write-in coincides with the memory location address of the data signals currently being read, assuming a packet end bit combination or a bit combination indicating stuffing information was identified at the data input of the buffer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a buffer memory PS constructed of four shift registers SR1 . . . SR4, whereby each shift register SR contains registers serially following one another. For example, such a shift register SR can be realized with an integrated circuit of the type SN 74 AS 821 containing 10 serially-connected registers and manufactured by the Texas Instruments Company. This integrated circuit further contains a write-in address input EAE and a read-out address input AAE. A positive signal edge change of an input signal at the write-in address EAE effects the simultaneous read-in of the data signals d present at the 10 shift register inputs. By analogy therewith, a positive signal edge change at the read-out address input AAE of the shift register SR effects a simultaneous read-out of the information respectively present in the registers at the respective output of the register. Each of the data inputs of the shift register SR is connected to the data inputs exhibiting the same ordinal number 1 . . . n is all shift registers. By analogy therewith, all data outputs of the shift registers SR are connected to one another. The data inputs or, respectively, data outputs connected to one another in this manner therefore represent a 10-bit parallel data input PSE and a 10 bit parallel output PSA of the buffer PS. In order to avoid a mutual influencing of the data outputs of the shift registers SR, those data outputs of the shift registers SR which are not situated in a read-out condition are switched inactive or, respectively are disconnected from voltage. 
     Each write-in address input EAE of the shift registers SR is fed by way of an appropriate connection to an address output ADA provided with the same ordinal number and a read-in device ELE. The read-in device ELE is realized with a shift register constructed of four registers connected in succession. The shift register representing the write-in device ELE further comprises a start input SE and a hold input HE as well as a clock input TE. Clock signals t 1  whose bit clock frequency is derived from the data signals d incoming to the buffer input PSE are supplied to the clock input TE. The data inputs (not shown) of the write-in device ELE are connected in such a manner that, after a start information has been applied to the start input SE of the write-in device ELE, the data outputs or, respectively, address outputs ADA comprise an information effecting the read-out of the respectively selected shift register SR, comprising this information in a prescribed, constantly repeating sequence. Each read-out address input AAE of the shift registers SR of the buffer PS is connected to a data output or, respectively, address output ADA of a read-out device ALE. The read-out device ALE is again realized with a shift register which is formed of four registers connected in succession. Here, also, the inputs of the shift register are connected in such a manner that an information effecting the reading of the shift register SR of the buffer PS is applied at the address outputs ADA in a prescribed, constantly repeating sequence. Likewise, the shift register representing the read-out device ALE comprises a start input SE, a hold input HE and a clock input TE. A clock signal t 2  which corresponds to a processing clock signal of a device for further processing of the data signal d is supplied to this clock input TE. 
     The n-pole data line, selected as n=10 in this exemplary embodiment, is connected to the buffer input PSE and is additionally connected to an n-pole input of a first comparator V1. In the first comparator V1, the incoming data signal d are examined for the presence of a packet start combination pab and for a packet end bit combination peb. When a packet start bit combination pab is identified, then an information representing this state is applied to an output AA1 indicating the packet start. By analogy therewith, an information representing the corresponding state is applied to an output AE1 indicating the packet end when a packet end bit combination peb is identified. The output AA1 indicating a packet start is connected to the start input SE of the read-in device ELE and is connected to the start input SE of the read-out device ALE by way of a first delay device VZ1. The output AE1 of the first comparator V1 indicating a packet end is connected to an input of a second delay device VZ2 whose output is connected to the hold input HE of the write-in device ELE. The n-pole (already defined as n= 10 in this exemplary embodiment) data output PSA of the buffer PS is connected to an n-pole input of a second comparator V2 as well as to a device (not shown) which further-processes the data signals d. In this second comparator V2, the data signals d present at the data output PSA of the buffer PS are investigated for the presence of a packet end bit combination peb. The identification of such a packet end bit combination peb is signaled at an output AE2 of the second comparator V2 by outputting an information representing the state. The output AE2 of the second comparator V2 indicating the packet end is connected to the hold input HE of the read-out device ALE. 
     The packet-oriented data signal d provided with packet start and packet end bit combinations pab, peb which are serially transmitted, for example, in a ring-shaped network proceed via a series-to-parallel converter (not shown) to the 10-bit parallel data input PSE of the buffer PS and to the input of the first comparator V1. Simultaneously with the recognition of a packet start bit combination pab, the write-in of the packet start bit combination pab and of the following data signals d into the buffer PS beginning with one of the four shift registers SR is initialized via that output AA1 of the first comparator V1 indicating the packet start and via the start input SE of the write-in device ELE. After a time delay prescribed in the first delay device VZ1, the read-out of the data signals d read into the buffer PS is initialized via the start input SE of the read-out device ALE. This first time delay is adapted to the buffer size and, therefore, to the maximum deviation between the data bit clock t 1  derived from the data signals d and internal processing clock t 2 , caused by the clock oscillator deviations and jitter. After the identification of the packet end bit combinations peb in the first comparator V1, the hold input HE of the write-in device ELE is selected via the output AE1 of the comparator V1 indicating a packet end and via a second delay device VZ2, whereby the read-in event is terminated after the write-in of the packet end bit combination peb. The second time delay is required in order to guarantee a reliable write-in of the packet end bit combinations peb. When a packet end bit combination peb then proceeds to the data output PSA of the buffer PS, then the presence of this packet end bit combination peb is identified in the second comparator V2 and the read-out of the buffer PS is terminated via its output AE2 and by way of the hold input HE of the read-out device ALE. When bit combinations pfb indicating stuffing information are inserted between the individual data packets, then the data signals d can be investigated for the presence of these bit combinations both in the first comparator V1 and in the second comparator V2. The advantage of this modification is that, after the write-in and read-out of the data signals d, a bit combination indicating the filling or stuffing information is available at the data output PSA of the buffer PS and, therefore, is constantly forwarded to the further-processing device. This bit combination indicating stuffing information is applied until a further packet start bit combination pab is recognized and a new write-in event or, respectively, read-out event is initialized or, respectively, terminated. 
     FIG. 2 illustrates an identical buffer PS constructed with the same integrated circuit technology, shows a write-in device ELE, a read out device ALE and a first comparator V1. Both the connections between these components as well as the functions of the individual components correspond to the connections and functions shown or, respectively, set forth with respect to FIG. 1. The second comparator V2 is replaced by a third comparator V3 whose inputs are connected to the address outputs ADA of the read-in device ELE and to the address outputs ADA of the read-out device ALE. Furthermore, that output AE1 of the first comparator V1 indicating the packet end is connected to a further, enable input FE of the third comparator V3. Assuming that a packet end bit combination peb or a bit combination pfb indicating stuffing information has been recognized, a hold signal is generated in the comparator V3 given equality of the information at the address outputs ADA of the write-in device ELE and of the read-out device ALE, and this hold signal is supplied by way of an output VA3 of the third comparator to the hold input HE of the read-out device ALE. This means that the buffer PS is read-out until the memory location address most recently generated for the write-in of the data signal d corresponds to the current read-out memory location address, assuming a packet end bit combination peb or bit combination pfb indicating stuffing information was identified in the first comparator V1 and this condition has been communicated to the third comparator V3. 
     Possible realizations of the system components are set forth below. 
     
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Buffer PS   SN 74 AS 821                                                  
                       Circuit of Texas Instruments                       
Write in and Read                                                         
            74 F 194                                                      
out means ELE,                                                            
ALE                                                                       
Comparator V1,                                                            
            74 F 521                                                      
V2, V3                                                                    
Delay means VZ1,                                                          
            74 F 109   Circuits of the advanced                           
VZ2                    Schottky TTL Technology of                         
                       Fairchild                                          
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     Although I have described my invention by reference to particular illustrative embodiments thereof, many changes and modifications of the invention may become apparent to those skilled in the art without departing from the spirit and scope of the invention. I therefore intend to include within the patent warranted hereon all such changes and modifications as may reasonably and properly be included within the scope of my contribution to the art.