# On the sojourn of an arbitrary customer in an $M/M/1$ Processor Sharing Queue

## TL;DR

This paper analyzes the distribution of arrivals and departures seen by a tagged customer in an M/M/1 processor sharing queue, revealing their equality in distribution and asymptotic behavior, with implications for queue approximation methods.

## Contribution

It derives the distributions of arrivals and departures seen by a tagged customer and proves their equality, providing insights into queue behavior and approximation validity.

## Key findings

- Distributions of arrivals and departures are equal in the queue.
- Asymptotic behavior of the common distribution is characterized.
- The approximation assuming random service among residual busy period is validated.

## Abstract

In this paper, we consider the number of both arrivals and departures seen by a tagged customer while in service in a classical $M/M/1$ processor sharing queue. By exploiting the underlying orthogonal structure of this queuing system revealed in an earlier study, we compute the distributions of these two quantities and prove that they are equal in distribution. We moreover derive the asymptotic behavior of this common distribution. The knowledge of the number of departures seen by a tagged customer allows us to test the validity of an approximation, which consists of assuming that the tagged customer is randomly served among those customers in the residual busy period of the queue following the arrival of the tagged customer. A numerical evidence shows that this approximation is reasonable for moderate values of the number of departures, given that the asymptotic behaviors of the distributions are very different even if the exponential decay rates are equal.