Packet Queueing Delay in Resilient Packet Ring Network Nodes

SHI Guowei (史国炜) et al:Packet Queueing Delay in Resilient Packet Ring Network Nodes 409
is simple to deduce that
2
q j = ,
N − 1
for N is odd (27)
2
qj= ,
N
for N is even (28)
Combined with Eq. (5), the average number of transit
buffers traversed by each packet can be obtained:
N −1
N − 3
α=
∑ qj( j− 1 ) = , for N is odd (29)
4
j=
2
N −1
N − 2
α=
∑ qj( j− 1 ) = , for N is even (30)
4
j=
2
From Eq. (26), the maximum total system utilization
of the ring is obtained,
max N
ρtotal
= (31)
1+
α
We now analyze the queueing delay of different
priority traffic in the two different transit queue modes.
We assume that N = 63, the ring is loaded with Class B
traffic of 1 ρ and with Class C traffic of 1 ρ ,
3 3
max
total
and Class A traffic is varied from 0 to
max
total
1 max
ρ
3 total . The
queueing delay is normalized by the average service
time X ( X = 14.7 µs for a 2.5-Mbps link bit rate) to
mask the effect of different link rates. The function w
given in Figs. 4-7 is the scaled time. The actual
simulation time W can be obtained by multiplying the
factor X
W = wX
(32)
Figures 4 and 5 show the average transmit queueing
delays for the three classes of traffic in the SQ and DQ
modes, respectively. It is obvious that the queueing delays
for all three classes of traffic in transmit buffers
increase with increasing system utilization of Class A
traffic, and that Class A traffic has the shortest queueing
delay.
Fig. 4 Queueing delay in transmit buffers
(in the SQ mode)
Fig. 5 Queueing delay in transmit buffers
(in the DQ mode)
Figure 6 shows the queueing delays of Class A traffic
in transmit buffers for both the DQ and SQ rings. It
is clear that Class A traffic is delayed more in an SQ
ring than in a DQ ring.
Fig. 6 Comparison of queueing delays in transmit
buffers in the SQ and DQ modes
Figure 7 shows the traffic queueing delays in transit
buffers for the two kinds of transit queue modes.
Figure 7 shows that the STQ buffer in the DQ ring
contributes more traffic delay than the PTQ buffer in
the SQ ring which in turn causes more delay than the
PTQ buffer in the DQ ring.
Fig. 7 Queueing delay in transit buffers STQ and PTQ
4 Conclusions
The packet queueing delay is one of the most important
performance measures of a data network and is a
key factor to be considered in the scheduling buffer
(Continued on page 415)