![](bg3.png)
if ((packet == B-packet) ||
(Len > Wi*Size ))
drop();
else
accept();
}else accept();
/* In the drop () procedure, LOW
increases by one when an I- or P-packet
gets accepted subject to ∆ remaining
greater than zero. */
Figure 2:.Pseudo Code of Buffer Management at Router
Input Buffer
Packet Scheduling is based on the original
MPAPS scheme. It is designed for per-flow queue
router structure. The E-MPAPS scheme thus
constructs virtual buffers in order to use MPAPS
with the above buffer management method. Each
virtual buffer holds a video stream. The virtual
buffer size (Bv) is computed according to the
following equation.
Bv
= W
i
*Size (1)
Due to the use of virtual buffer E-MPAPS has
one distinguished feature: it does not provide strict
isolation between different videos and video sources
could use the buffer reserved to others when load
level at the router is low.
MPAPS first maps the videos into the groups
with different transmission priorities based on their
upcoming packet type and current loss performance
(SL), and then a specific transmission schedule for a
video stream in a selected group is adaptively set to
respond instantaneously to needs based on the
Adaptive Priority Index (API). Here, the SL is
defined as ratio of the actual packet loss and the
maximum allowable packet loss, and the API is
defined as the product of SL and the normalized
length of an virtual input buffer holding stream i.
The rationale behind MPAPS is that the drop
probability of I packets will be lower than that of P
packets, which in turn, will be less than that of B
packets. Moreover, the videos that have a worse loss
performance than expected receive expedited and
more servicing, whereas videos that have
satisfactory or even better loss performance receive
slow and less servicing. Therefore, all the videos
will be transmitted with more acceptable loss
targets. Further details can be referred in [Bai and
Ito,2003].
3 RESULTS AND DISCUSSION
To demonstrate the advantages of E-MPAPS, we
compare its performance, including packet loss rate
and I-frame error rate with that of the original
MPAPS, and first-come-first-serve scheduling
(FCFS) schemes. FCFS is widely used in the current
Internet routers due to its simple implementation. In
FCFS, the incoming packets are accepted in order of
arrivals. The simulation details are presented in the
following.
Sources: real MPEG-1 video traces where the
number of bits per frame used by the MPEG coder is
described [http://www3.informatik.uni-
wuerzburg.de/MPEG/].
Parameters used in the simulation:
1)
Fixed:
• Packet Size: 1500 bytes or less
• Simulation Time: 40 minutes
• Video Starting Interval: 60 seconds
• Output Link: 100 Mbps
• Size: 150KB
• LOW: 0.90 [a threshold value of 0.90
means that the buffer length threshold is
90% of the buffer size (in packets)]
• HIGH: 0.95
• PLRi: 3% for first half of videos and
6% for the others
2) Variable:
• The number of background sources
varies in order to change the load level
at the router.
• The starting sequence of a video stream
was randomly selected in each run. The
results presented in this section show
the final values of the average of
different runs.
• The test-scenarios featured varying
degrees of load level and various traffic
patterns. The presented test results here
are an illustrative comparison of the
differences of the three schemes.
Table 1 and Figures 3 and 4 show the results
obtained for different number of transmitted videos,
namely, 10, 20 and 30 videos.
We see in Table 1 that E-MPAPS produces the
lowest number of videos whose actual packet loss
rate is greater than their maximum allowable packet
loss rate for all the cases.
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