Internet Queuing Delay Introduction How many packets in the queue? How long a packet takes to go through?

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Internet Queuing Delay Introduction How many packets in the queue? How long a packet takes to go through?

The M/M/1 Queue An M/M/1 queue has –Poisson arrivals (with rate λ) –Exponential service times (with mean 1/μ, so μ is the “service rate”). – One (1) server –An infinite length buffer The M/M/1 queue is the most basic and important queuing model for network analysis Terminology: A/B/c/K –A: interarrival time distr. B: service time distr. –c: number of servers K: size of buffer  M: expo. distr. G: general distr. D: deterministic

The Poisson Arrival Model Examples –Customers arriving to a bank –Packets arriving to a buffer The rate λ of a Poisson process is the average number of events per unit time The time interval between arrivals follows exponential distribution:

Exponential Distribution

State Analysis of M/M/1 Queue n : number of customers in the system (including queue + server) p n : steady state probability of finding n customers in the system  /  : Traffic rate (traffic intensity)

# of transitions  = # of transitions  State Analysis of M/M/1 Queue p i are probabilities: : prob. the server is working (  is called “server utilization”)

State Analysis of M/M/1 Queue N: # of customers in the system

State Analysis of M/M/1 Queue W: waiting time for a new arrival T: sojourn (response) time This T is the access delay at edge router in the problem 9 of Chapt. 2 : remaining service time of the customer in service Exponential r.v. with mean 1/  due to memoryless property of expo. Distr. : service time of i-th customer

M/M/1 Queue Example A router’s outgoing bandwidth is 100 kbps Arrival packet’s number of bits has expo. distr. with mean number of 1 kbits Poisson arrival process, = 80 packets/sec How many packets in router expected by a new arrival? What is the expected waiting time for a new arrival? What is the expected access delay (response time)? What is the prob. that the server is idle? What is P( N > 5 )? Suppose you can increase router bandwidth, what is the minimum bandwidth to support avg. access delay of 20ms?