TITRE Adaptive Packet Video Streaming Over IP Networks - LaBRI

packets. If the number of lost packets is not more than h i , then the decoder will be able to recover
Ui. Otherwise, Ui is completely lost. Figure 4-16 shows the format of packets sent on the IP
network.
d bytes
IP/UDP/RTP
header
4 bytes
FEC header
seq n k
RTP Payload
Figure 4-16: Header information and packet format
UEP augments the amount of the traffic sent in the network. To correctly control the volume
of transmitted data, we have to provide a certain ratio of traffic overhead called r, for each level of
priority score. We assume that moving from one priority score to other increases by a 10 percent
ratio. Then, the ratio r can be defined by:
r = 0 . 1×
p
(Eq.3)
Therefore, the traffic overhead is limited to 10 percent (i.e., r=0.1) for the data flow of priority
score 1, to 20 percent (i.e., r=0.2) for the data flow of priority 2, and so on. In order to find the
efficient value n i for the (n i , k ) i RS code, we proceed as below:
∗
Let ς
U i
be the reserved byte-budget for error protection. It depends on the number of bytes
used to send U i when no error protection is performed. It is given by:
( k ⋅ d m )
ς (Eq.4)
∗
U
= r ⋅
i i
+
i
Where d is the packet header size (i.e, when RTP/UDP/IP is used with the proposed UEP, d
= (20+8+12+4) = 44 bytes). The relation between the real byte-budget spent on error protection,
ς
U i
, and the RS code to be used can be stated as follows:
The error margin between
( n − k ) ⋅ ( t d )
ς (Eq.5)
U i
=
i i i
+
ς
U i
and ∗ Ui
∧
∗
ς is ς ς U
− ς
=
i Ui
, that can be positive or negative. It
cumulates along the data access unit arrivals. Using the formula (Eq.4) and (Eq.5), the fluctuation
of the error margin can be written as:
∧
ς
( n )
i
⎧
⎪
= ⎨
⎪
⎩
0 , i = 1
∧
⎪( r + 1) ⋅ m − n ⋅ t + ( r − n + k ) ⋅ d + ς ( n )
i
i
i
i
i
i−1 , i
> 1
(Eq.6)
To respect the constraint given on traffic overhead, the best value n i is the one that provides
the smallest error margin in formula (Eq.6). Then, n i is obtained by:
min
ni
∧
( ni
) such that ni
∈ℵ,
ni
≥ ki
, ∀mi
, ki
, ti
ς (Eq.7)
With the proposed UEP, the RS code evolves dynamically so the network bandwidth is
correctly controlled according to video application requirement.
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