Measuring and displaying bandwidth contention

1. A method to report transmit bandwidth contention at a network device coupled to a network, comprising:

building a histogram representing transmit bandwidth demand for a plurality of time bins representing contiguous time intervals of equal duration including a current time bin and plural future time bins;

displaying the histogram; and

periodically updating the histogram at a periodicity equal to the duration represented by each of the plurality of time bins in accordance with the formulas:

B 0 ( t +1)= B 0 ( t )+ B 1 ( t )− T ( t ), and

B n ( t +1)= B n+1 ( t ) for n >0

wherein B n (t)= Bandwidth demand for time bin n at time = t;

=a total length of packets scheduled for transmission during time bin n;

n = an integer time bin number ranging from 0 to K, where time bin number 0 is the current time bin and K is a positive integer;

t =a time of a most recent update of the histogram;

t+1 = a time of a histogram update immediately following the most recent update; and

T(t) = a total length of packets actually transmitted during the time period from t to t+1.

2. The method of claim 1 , wherein transmit bandwidth demand is represented by a total length of packets scheduled for transmission by the network device during each of the plurality of time bins.

3. The method of claim 2 , further comprising:

normalizing the transmit bandwidth demand for each of the plurality of time bin to a line rate.

4. The method of claim 2 , wherein building a histogram further comprises:

determining a length and an estimated transmit time for each of a plurality of packets to be transmitted by the network device;

assigning each of the plurality of packets to one of the plurality of time bins based on the respective estimated transmit time; and

summing the lengths of the packets assigned to each of the plurality of time bins.

5. The method of claim 4 , wherein determining an estimated transmit time for each packet of the plurality of packets further comprises:

receiving the packet;

attaching a receive time stamp indicating a reception time for the packet;

determining a delay time for the packet; and

adding the delay time to the reception time stamp to determine the estimated transmit time.

6. The method of claim 5 , wherein determining a delay time for the packet further comprises:

determining an impairment class of the packet; and

determining the delay time for the packet based on an impairment profile associated with the impairment class.

7. The method of claim 6 , wherein determining an impairment class of the packet further comprises:

locating a test information block within a payload of the packet; and

determining the impairment class based on at least a portion of the content of the test information block.

8. A network test system, comprising:

an impairment engine coupled to a network; and

a processor coupled to the impairment engine;

wherein the impairment engine and the processor include circuits and software configured to:

build a histogram representing transmit bandwidth demand for a network device for a plurality of time bins representing contiguous time intervals of equal duration including a current time bin and plural future time bins;

display the histogram; and

periodically update the histogram at a periodicity equal to the duration represented by each of the plurality of time bins in accordance with the formulas:

B 0 ( t +1)= B 0 ( t )+ B 1 ( t )− T ( t ),and

B n ( t +1)= B n+1 ( t ) for n >0

wherein B n (t)= Bandwidth demand for time bin n at time = t;

=a total length of packets scheduled for transmission during time bin n;

n = an integer time bin number ranging from 0 to K, where time bin number 0 is the current time bin and K is a positive integer;

t =a time of a most recent update of the histogram;

t+1 = a time of a histogram update immediately following the most recent update; and

T(t) = a total length of packets actually transmitted during the time period from t to t+1.

9. The system of claim 8 , wherein transmit bandwidth demand is represented by a total length of packets scheduled for transmission by the network device during each of the plurality of time bins.

10. The system of claim 9 , wherein the transmit bandwidth demand for each time bin is normalized to a line rate.

11. The system of claim 9 , wherein, to build the histogram, the impairment engine and the process are configured to:

determine a length and an estimated transmit time for each of a plurality of packets to be transmitted by the network device;

assign each of the plurality of packets to one of the plurality of time bins based on the respective estimated transmit time

sum the lengths of the packets assigned to each of the plurality of time bins.

12. The system of claim 11 , wherein, to determine an estimated transmit time for each packet, the impairment engine and the processor are further configured to:

receive the packet;

attach a receive time stamp indicating a reception time for the packet;

determine a delay time for the packet;

add the delay time to the reception time stamp to determine the estimated transmit time.

13. The system of claim 12 , wherein, to determine a delay time for the packet, the impairment engine and the processor are further configured to:

determine an impairment class of the packet; and

determine the delay time for the packet based on an impairment profile associated with the impairment class.

14. The system of claim 13 , wherein, to determine an impairment class of the packet, the impairment engine and the processor are further configured to:

locate a test information block within a payload of the packet; and

determine the impairment class based on at least a portion of the content of the test information block.