  Recently, the problems of evaluating performances of switches and routers
have been formulated as online problems, and a great amount of results have
been presented. In this paper, we focus on managing outgoing packets (called
{\em egress traffic}) on switches that support Quality of Service (QoS), and
analyze the performance of one of the most fundamental scheduling policies {\em
Priority Queuing} ($PQ$) using competitive analysis. We formulate the problem
of managing egress queues as follows: An output interface is equipped with $m$
queues, each of which has a buffer of size $B$. The size of a packet is unit,
and each buffer can store up to $B$ packets simultaneously. Each packet is
associated with one of $m$ priority values $\alpha_{j}$ ($1 \leq j \leq m$),
where $\alpha_{1} \leq \alpha_{2} \leq \cdots \leq \alpha_{m}$, $\alpha_{1} =
1$, and $\alpha_{m} = \alpha$ and the task of an online algorithm is to select
one of $m$ queues at each scheduling step. The purpose of this problem is to
maximize the sum of the values of the scheduled packets. For any $B$ and any
$m$, we show that the competitive ratio of $PQ$ is exactly $2 - \min_{x \in [1,
m-1] } \{ \frac{ \alpha_{x+1} }{ \sum_{j = 1}^{x+1} \alpha_{j} } \}$. That is,
we conduct a complete analysis of the performance of $PQ$ using worst case
analysis. Moreover, we show that no deterministic online algorithm can have a
competitive ratio smaller than $1 + \frac{ \alpha^3 + \alpha^2 + \alpha }{
\alpha^4 + 4 \alpha^3 + 3 \alpha^2 + 4 \alpha + 1 }$.
