""" cellphone.py
Simulate the operation of a BCC cellphone system.
Calls arrive at random to a cellphone hub with a fixed number of
channels. Service times are assumed exponential. The objective
is to determine the statistics of busy periods in the operation of a
BCC cellphone system.
The required measurements are
(1) the total busy time (all channels full) in each 1-hour period and
(2) the total number of busy times in a 1-hour period.
The simulation is continuous but the observing Process, a
Statistician, breaks the time into 1-hour observation periods
separated by 15-minute gaps to reduce autocorrelation. The total busy
time and number of busy times in each interval is printed.
"""
from SimPy.Simulation import *
import random as ran
## Model components ------------------------
class CallSource(Process):
""" generates a sequence of calls """
def execute(self, maxN, lam,cell):
for i in range(maxN):
j = Call("Call%03d"%(i,))
activate(j,j.execute(cell))
yield hold,self,ran.expovariate(lam)
class Call(Process):
""" Calls arrive at random at the cellphone hub"""
def execute(self,cell):
self.trace("arrived")
if cell.Nfree == 0: self.trace("blocked and left")
else:
self.trace("got a channel")
cell.Nfree -= 1
if cell.Nfree == 0:
self.trace("start busy period======")
cell.busyStartTime = now()
cell.totalBusyVisits += 1
yield hold,self,ran.expovariate(mu)
self.trace("finished")
if cell.Nfree == 0:
self.trace("end busy period++++++")
cell.busyEndTime = now()
busy = now() - cell.busyStartTime
self.trace(" busy = %9.4f"%(busy,))
cell.totalBusyTime +=busy
cell.Nfree += 1
def trace(self,message):
if TRACING:
print "%7.4f %13s %s "%(now(), message, self.name)
class Cell:
""" Holds global measurements"""
Nfree = 0
totalBusyTime = 0.0
totalBusyVisits = 0
result=()
class Statistician(Process):
""" observes the system at intervals """
def execute(self,Nperiods,obsPeriod,obsGap,cell):
cell.busyEndTime = now() # simulation start time
if STRACING: print "Busy time Number"
for i in range(Nperiods):
yield hold,self,obsGap
cell.totalBusyTime = 0.0
cell.totalBusyVisits = 0
if cell.Nfree == 0: cell.busyStartTime = now()
yield hold,self,obsPeriod
if cell.Nfree == 0: cell.totalBusyTime += now()-cell.busyStartTime
if STRACING:
print "%7.3f %5d"%(cell.totalBusyTime,cell.totalBusyVisits)
m.tally(cell.totalBusyTime)
bn.tally(cell.totalBusyVisits)
stopSimulation()
cell.result= (m.mean(),m.var(),bn.mean(),bn.var())
## Experiment data -------------------------
NChannels = 4 # number of channels in the cell
maxN = 10000
ranSeed = 3333333
lam = 1.0 # per minute
mu = 0.6667 # per minute
Nperiods = 10
obsPeriod = 60.0 # minutes
obsGap = 15.0 # gap between observation periods
TRACING = False
STRACING = True
## Experiment ------------------------------
m = Monitor()
bn =Monitor()
ran.seed(ranSeed)
cell=Cell() # the cellphone tower
cell.Nfree = NChannels
initialize()
s = Statistician('Statistician')
activate(s,s.execute(Nperiods,obsPeriod,obsGap,cell))
g = CallSource('CallSource')
activate(g,g.execute(maxN, lam,cell))
simulate(until=10000.0)
## Output -------------------------
print 'cellphone'
# input data:
print "lambda mu s Nperiods obsPeriod obsGap"
FMT= "%7.4f %6.4f %4d %4d %6.2f %6.2f"
print FMT%(lam,mu,NChannels,Nperiods,obsPeriod,obsGap)
sr = cell.result
print "Busy Time: mean = %6.3f var= %6.3f"%sr[0:2]
print "Busy Number: mean = %6.3f var= %6.3f"%sr[2:4]
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