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where F i ( t ) is the remaining amount of data of frame i at time t . With t k ,
k
[ f n i ,
n i ], we can
then estimate B n i .
To estimate D i + 1 , we simply take the rate at which segment i was submitted into the server
buffer as the estimated value, i.e.,
n i
s k
k = n i 1 + 1
D i + 1 =
(8.7)
t n i
t n i 1
This is because the rate at which data are submitted into the server buffer is equal to the rate
at which data leave the server buffer, and so it reflects the transmission rate.
8.5 Rate Adaptation
Armed with a mean to estimate the client buffer occupancy and network bandwidth, the next
challenge is to devise an adaptation algorithm to control the video bit-rate to prevent client
buffer underflow.
8.5.1 Segment-Based Rate Control
As video data are transcoded and transmitted in fixed-duration segments, the server must de-
termine the target bit-rate before transcoding a video segment for transmission. The server
determines the target bit-rate based on two factors, namely, the estimated client buffer occu-
pancy and the estimated network bandwidth available, both can be estimated using techniques
described in Section 8.4.
Suppose segment i has just been submitted to the server buffer, with the estimated D i + 1
and B n i ,
we can predict the client buffer occupancy after transmitting the segment i
+
1 to the
client, i.e., B n i + 1 , from:
Mr i + 1
D i + 1
B n i + 1 =
B n i +
M
(8.8)
where the last term is the predicted time taken to send the whole ( i
+
1)th segment to the client.
Rearranging equation (8.8), we can obtain
1
D i + 1
B n i + 1
B n i
r i + 1 =
(8.9)
M
From equation (8.9), we can relate the video bit-rate r i + 1 with the estimated client buffer
occupancy (represented by B n i + 1 ). Our goal is to adjust the video bit-rate to maintain the client
buffer occupancy to above a given threshold B T so that short-term bandwidth variations can be
absorbed. In practice, B T
B p when B p is known, otherwise it can be configured according
to properties of the system/network or according to experiments. Simulations show that the
system performance is not sensitive to this setting.
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