How can I fix this counter I wrote used to find the number of blocked calls?

Question

What my code is meant to do:

Simulate 100 voice calls that occur over an N channel network and calculate the grade of service (that is, the percentage of blocked calls) over a period of 60 minutes.

My Code:

import numpy as np
import pandas as pd

#Uniformly distributed start times
startTimes = np.random.randint(0,60,100)
startTimes.sort()

#Average call time for 100 people 
callDuration = np.random.poisson(20, 100)

channelCounter = 0;
blockedCounter = 0;

endTimes = np.add(callDuration, startTimes)

numberChannels = 1

for x in range(0,60): 

    for y in range(0, startTimes.size):

        if startTimes[y] == x: 
            if channelCounter < numberChannels:
                channelCounter=channelCounter+1

            elif channelCounter == numberChannels: 

                blockedCounter = blockedCounter + 1

        if (endTimes[y] == x):
            if channelCounter >= 1:
                channelCounter=channelCounter-1

My Approach:

I generate a uniform distribution of 100 call start times beginning from 0 min to 60 minutes.

I generate a random poisson distribution of 100 call durations for an average call time of 20 minutes.

I set my channel counter variable and blocked counter variables to 0.

Finally, I create another array consisting of the sum of call start time + average call duration to get the call end time.

The pseudo code logic behind blocked counter incrementing is as follows:

if number of channels occupied < number channels available:
    put a call through 
else if number of channels occupied == number channels available ( ie full):
    call is dropped so counter incremements 

if a call that is ongoing finishes: 
    decrement number of channels occupied 

My blocked counter isn't incrementing as I expect it to. I have a vague idea of what is going wrong but I don't know how to fix it. With the current values im inputting, I should expect to see a value of around 95 for blocked counter. What I get however is a value that hovers around 70-75.

If anyone spots where im going wrong, I would greatly appreciate it!

Screen shot of the data im working with

Answer 1

Ok, so your problem here is, that you take ANY endTime value, which happens on given minute as a sign of freeing a channel. Whereas some of these were actually blocked. Which is why you are inflating the total number of non blocked calls. What you rather want is to compute number of non-overlapping < startTime , endTime > intervals (assuming possible max overlapping calls at given minute equal to numberChannels AND order priority of calls)

So assuming one channel you can do:

minute=0
nonBlockedCounter=0

while(minute<=60):
    minute = min(startTimes[minute<=startTimes])
    if(minute>=0):
        nonBlockedCounter+=1
        minute=endTimes[startTimes==minute][0]
blockedCounter=100-nonBlockedCounter
print(blockedCounter)

Output is around 96-97 most often.