Abstract:
Data traffic such as cell streams in an ATM communication network frequently contain data destined for multiple output ports having different transmission data rates. In order to accurately schedule such traffic a clocking signal unique to each output data rate is required. This invention provides systems and methods for generating the necessary clock signals utilizing a single timing reference.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the scheduling of data traffic to multiple output ports having different data rates and more particularly to the scheduling of such traffic utilizing a single timing reference. 
     BACKGROUND 
     The present invention will be described with reference to data traffic management in an asynchronous transfer mode (ATM) network for broadband communications but it is to be understood that the basic concept disclosed is applicable to any data traffic management device which services multiple output ports having different data transmission rates. 
     Asynchronous transfer mode (ATM) is a networking technology that provides broadband communications and differentiated qualities of service (QoS). The technology, which utilizes a fixed length cell, is well-suited to the transmission of voice, video and data utilizing both real-time and non-real-time service categories. Statistical multiplexing is used in ATM switches to allow for a high efficiency of resource utilization. In order to take advantage of the statistical nature of ATM traffic and still ensure some guaranteed QoS to network data it is important that the streams of ATM data conform to certain specific parameters. These parameters and the guaranteed qualities of service make up the traffic contract as established by the ATM Traffic Management Forum. 
     In ATM switches it is often useful and/or necessary to shape or schedule ATM cells from various traffic streams. This scheduling ensures that the traffic conforms to the specified parameters for each particular stream which leads to better overall network performance. There are applications where a shaping entity in an ATM system needs to schedule a mixture of traffic which is bound for several output ports. Each of these output ports may have a different data rate parameter. The requirement to service multiple data streams to output ports having different data rates requires or at least implies that the scheduling entity must adopt a different timing reference for each output port. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide systems and methods for scheduling data traffic onto several ports with each port having a different data rate using only a single timing reference as an input to the traffic scheduling device. 
     In a specific embodiment of the invention the systems and methods apply to an ATM communication network. 
     Therefore, in accordance with a first aspect of the present invention, there is provided in a data traffic management system having a plurality of output ports for servicing data traffic of different data rates and a scheduling device having a plurality of data calendars for shaping data traffic destined for each of said output ports, a single timing reference to schedule transmission of data to appropriate output ports. The single timing reference comprises timing means to generate a clock pulse based on the data rate of the output port having the fastest data rate; counter means to continually count the clock pulses following an initialization event; increment means to dynamically compare the accumulated number of clock pulses with predetermined increments for each calendar based on the data rate assigned thereto; and threshold means responsive to inputs from said increment means to control transmission of data traffic from each of said calendars. 
     In a first embodiment the clock pulse is equal to the data rate of the data traffic of the output port having the fastest data rate. 
     In this embodiment a first-in, first-out buffer is positioned in advance of the output port to absorb the bursty nature of the calendar output. 
     In accordance with a second embodiment the clock pulse is ‘n’ times faster than the data rate of the output port having the fastest data rate. It is anticipated that ‘n’ will be at least 10. 
     In accordance with a second aspect of the present invention there is provided a method of providing a timing reference to a data traffic management system, the system having a plurality of output ports each configured to transmit data at a different data rate and a scheduling device having a calendar for shaping data traffic for delivery to each of said output ports. The method comprises the steps of continually generating a clock pulse based on the data rate of the output port having the fastest data rate; counting the accumulated clock pulses following an initialization event; dynamically comparing the accumulated clock pulses with predetermined increment values for each calendar based on an assigned data rate for each calendar; and controlling transmission of data from the calendars from appropriate output ports in response to input signals generated by threshold means when the accumulated clock pulses and increment values reach a threshold respecting each calendar. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in greater detail with reference to the attached drawings wherein: 
         FIG. 1  represents a simple calendar operation for connection scheduling; 
         FIG. 2  is a block diagram showing a single timing reference according to a first aspect of the invention; and 
         FIG. 3  is a block diagram of a timing reference input according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The basic concept of calendar-based scheduling is illustrated in FIG.  1 . Calendar scheduling is used in ATM applications to decide when to transmit a cell from a given connection. This scheduling is directly applicable to a shaping function. Time is generally referenced in cell times where one cell time is the time from when the first bit of one cell passes a reference point until the time when the first bit of the next cell passes the reference point. As an example, one cell time on an OC3 link is about 2.83 us. Each time the calendar current time is advanced, this is called a “tick”. As the current time moves forward, based on some generated or provided timing reference, the current time will pass a connection&#39;s scheduled transmit time. These are virtual connections (VC) in FIG.  1 . When the current time passes a VC&#39;s scheduled time, then a cell from that VC will be transmitted, or placed in a transmit queue, and that VC will be rescheduled on the calendar for a later time. In  FIG. 1  virtual channels (VC 1  and VC 2 ) are transmitting at the same speed while VC 3  is at a slower data rate. Thus, as shown in  FIG. 1 , when the current time advances by a “tick” a cell is transmitted from each VC and that VC is rescheduled for a later time slot in the calendar for the VC which his scheduled at that time. 
     If a scheduling device is scheduling for several output ports, then a calendar will exist for each port. It is within the scope of the present invention to have a physical calendar for each port or a system which uses a single calendar to emulate the operation of multiple calendars. For each of the calendars, the cell time may be different because the data rate of each port may be different. 
     The difference in cell times means that the current time marker/indicator for each calendar will be moved forward at a different rate. For example the data rate of ATM cells on an OC3 link is 149.76 Mb/s which means that one cell time is 2.83 us. On a DS3 link (Direct Mapped), the data rate is 44.21 Mb/s which means that one cell time is 9.59 us. Consequently, a calendar that services an OC3 link would step the current time marker forward once every 2.83 us while a calendar that services a DS3 link (Direct Mapped) would step the current time marker forward once every 9.59 us. 
     The present invention provides two separate embodiments relating to systems and methods for using a single time source to schedule traffic onto multiple ports wherein the data rate of each port is different. 
     The first embodiment is shown in a block diagram of FIG.  2 . As shown a plurality of output ports,  12 ,  14 ,  16 , are shown and further identified as port  1 , port  2  and port n. Each port includes a FIFO  18  for queuing and transmitting data traffic according to the data rate of the particular port. Each port continuously monitors the data rate of the data being transmitted therefrom and this data rate information is provided to Clock Selection Device  20  via lines  22 . Clock Selection Device  20  monitors each of the data rates and selects the fastest rate to provide a master clock pulse to Data Scheduling Device  24 . As shown in  FIG. 2  Scheduling Device  24  includes calendars  26 ,  28  and  30  wherein each calendar shapes data traffic for port  1 , port  2  and port n respectively. 
     For every clock event received by the Scheduling Device  24  a decision must be made for each calendar as to whether that calendar should be “ticked” or advanced. To decide whether to advance each of the calendars a counter, an increment device, and a threshold device are used. Each calendar has the following values associated with it:
     1) Counter,   2) Tick_Increment,   3) No_Tick_Threshold.   

     The Counter is initialized to zero and the other two parameters are initialized to specific values that are calculated based on the port speed associated with the calendar and also on the port speed of the fastest port. This is necessary since the fastest port provides the Master Cell Tick  32 . 
     Whenever a Master Cell Tick occurs each calendar will add Tick_Increment to the Counter. As long as the Counter is less than the No_Tick_Threshold then the calendar is ticked and a cell is sent to the port associated with the Counter. When the Counter is greater than or equal to the No_Tick Threshold then the value of Counter is decremented by the value of No_Tick_Threshold and the calendar is not ticked. 
     The following is a pseudo-code that illustrates the algorithm described above: 
     
       
         
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Service_Tick_Begin: 
               
             
          
           
               
                   
                 for each calendar 
               
             
          
           
               
                   
                 Counter = Counter + Tick_Increment 
               
               
                   
                 if Counter &gt;= No_Tick_Threshold 
               
             
          
           
               
                   
                 Counter = Counter − No_Tick_Threshold 
               
             
          
           
               
                   
                 else 
               
             
          
           
               
                   
                 tick the calendar &amp; send a cell 
               
             
          
           
               
                   
                 endif 
               
             
          
           
               
                   
                 next 
               
             
          
           
               
                   
                 Service_Tick_End 
               
               
                   
                   
               
             
          
         
       
     
     For the calendar that is servicing the fastest port, Tick_Increment will be zero and No_Tick_Threshold will be set to one. These values will ensure that the calendar is ticked for every Master Cell Tick. 
     For all the other calendars which are servicing ports that are slower than the reference provided by Master Cell Tick, values for Tick_Increment and No_Tick_Threshold must be chosen in a manner that ensures that the calendar is occasionally not ticked when a Master Cell Tick occurs. Thus, for every Master Cell Tick, one calendar (the fastest port) will tick but all the other calendars will go through a cycle that looks like: tick-tick-tick-tick-no tick-tick-tick-tick-tick-no tick. This occasional “no tick” has the effect of lowering the effective rate at which the calendar is being ticked. 
     Given the port rate for the fastest port and the rate of the port that a particular calendar is servicing, appropriate values for Tick_Increment and No_Tick_Threshold can be chosen so that the effective cell rate out of the calendar scheduling is equal to the cell rate of that port. The values used for all the calendar parameters should be in a representation that supports fractional values for better accuracy. FIFOs at each port are required to absorb the bursty nature of the calendar servicing created by this method. 
     In this manner, all calendars (that may service ports with different rates) can be effectively ticked at their appropriate rate using only the single timing input called Master Cell Tick. A given calendar will be ticked several times at the fastest rate, and then pause for a tick, and then tick again at the fastest rate, and then pause for a tick, and so on. 
     The second embodiment of the present invention is shown in FIG.  3 . In this embodiment the same output ports  12 ,  14  and  16  and FIFO  18  are illustrated. In this embodiment, however, the Clock Selection Device is not used but rather a Master Clock  42  which is ‘n’ times faster than the cell rate of the fastest output port is provided to scheduling device  24 . In a preferred embodiment the value of n is 10 or more. As an example, if the fastest cell rate is 100,000 cells per second, then the Master Clock Rate would be 1 MHz or 1,000,000 clocks per second. 
     The Master Clock is used to decide when to tick (and thus send a cell) each of the calendars managed by the scheduling device. In essence, each calendar will be scheduled by using the Master Clock and two per-calendar parameters. A counter that may be called Global Time is incremented by one for each Master Clock event. Thus, Global Time will continually increment at a rate at least 10 times faster than the cell rate of the fastest port. 
     Each calendar has the following values associated with it:
         1) Increment,   2) Next_Tick_Time.       

     The Global Counter is initialized to zero and Next_Tick_Time for each calendar is set equal to its calendar Increment. 
     For every Master Clock event, the Global Time value will increase. Each calendar will then compare its Next_Tick_Time to the Global Time. If the Global Time has passed the Next_Tick_Time, then that particular calendar must be ticked. The Next_Tick_Time will then be incremented by the Increment, which essentially schedules the calendar again for some time in the future. The Increment for each calendar is chosen based on the rate of the Master Clock and the rate of the port that the calendar services. Each calendar Increment can be chosen so that the calendar is ticked at the cell rate of the port. 
     Here is some pseudo-code that illustrates the algorithm described above: 
                                                                                                         Service_Master_Clock_Event_Begin:                Global_Time = GlobalTime + 1           for each calendar                if Global_Time &gt;= Next_Tick_Time                tick the calendar &amp; send a cell           Next_Tick_Time = Next_Tick_Time + Increment                endif                next                Service_Master_Clock_Event_End                        
With this method, all the calendars servicing ports with different data rates can be ticked at their appropriate cell rate, using only the single input timing reference.
 
     While particular embodiments of the invention have been described and illustrated it will be apparent to one skilled in the art that still other variations to the basic concept can be implemented. It is to be understood that such variations will also fall within the scope of the invention as defined by the appended claims.