Side_1_360

tains a stream of numbers as an input. The num-
bers are interpreted as the number of bits that are
to be put into a datagram (udp packet) the next
period. The module takes as parameters the
(fixed) maximum size of the datagram and the
(fixed) duration of each period. If the number of
bits in the trace file for one period does not fit
into one packet, several packets are created and
sent back-to-back.

As an example of use, let the trace file be the
streams of MPEG-1 coded video sequences
made available by Oliver Rose [Rose95]. Each
video trace contains nvideo frames X= {X 1 , X 2 ,
..., Xn} (n= 40000). A video frame will then be
the sending period. A frame consists of a vari-
able number of bits using MPEG coding and the
most common Group of Picturepattern
IPPBPPBPPBPPB, see Figure 6.

The Xibits in video frame iare converted to Yi
UDP packets of NPbytes, i.e.
A new video frame is sent every TPand the size
of the video frame is given by the current posi-
tion in the MPEG trace.

All YiUDP packets in position iare sent back-
to-back at maximum line speed. A video is ran-
domly, and uniformly, selected among the
K= 19 different video streams that are available.
The packet size NPand interference time TFare
parameters that can be set by the scenario de-
signer. Default values are set to NP= 1024 bytes
and TF= 40 ms.

Although this module originally uses a set of
MPEG-1 coded video sequences it is very sim-
ple to e.g. use MPEG-2 traces instead, as long as
the resulting input trace files have the same for-
mat as X. In fact, any trace with the format of X
applies. It is also easy to change the size of UDP

packets, and the time between each video frame.

This is very convenient when investigating the

sensitivity and importance of e.g. video coding

techniques and frame segmentation.

2.2.1.4 VoIP and CBR Modules – Sending

Constant Stream of Fixed Sized

Packets

Both the VoIP and the CBR (from the term con-

stant bit rate) interface modules send a determin-

istic stream of packets, i.e. a stream of equally

sized packets with a constant inter-packet arrival

time. Hence, the packet pattern is characterized

by the packet size Np(not included overhead like

8 byte UDP header and 20 byte IP header) and

the (constant) time between packets, Tp. This

gives a bitrate of (excluded overhead) 8Np/ Tp.

Figure 7 shows an example of a packet pattern

generating a 64 kbit/s stream, or 8 packets per

second (pps).

Figure 6 MPEG coded video:

An example of a Group of

Picture pattern

Figure 7 An example of a

64 kbit/s streaming of udp

datagrams for the constant

packet source

Figure 5 An example of a file

size distribution used by the

FTP interface module

cummulativ density

0,9

0,8

0,7

0,6

0,5

0,4

0,3

0,2

0,1

0

0,01 0,1 1 10 100 1000 10000

file size [kbytes]

Average file

size 154 kbytes

Increasing time

I BBP B BPBBPBB

Forward Predictions

Bidirectional Predictions

1024 bytes

Tp = 125 ms

1024 bytes

125 ms

1024 bytes

125 ms

1024 bytes

125 ms

time

Np =

Yi=
Xi/(Np·8).