Specifications

83
The term, Quality of Service, refers to a number of technologies such as Dierentiated Service
Codepoint (DSCP), which can identify the type of data in a data packet and so divide the packets
into trac classes that can be prioritized for forwarding. The main benets of a QoS-aware
network include the ability to prioritize trac to allow critical ows to be served before ows
with lesser priority, and greater reliability in a network by controlling the amount of bandwidth
an application may use and thus controlling bandwidth competition between applications. PTZ
trac, which is often regarded as critical and requires low latency, is a typical case where QoS
can be used to guarantee fast responses to movement requests. The prerequisite for the use of
QoS within a video network is that all switches, routers and network video products must sup-
port QoS.
Figure 9.4a Ordinary (non-QoS aware) network. In this example, PC1 is watching two video streams from cameras 1
and 2, with each camera streaming at 2.5 Mbit/s. Suddenly, PC2 starts a le transfer from PC3. In this scenario, the le
transfer will try to use the full 10 Mbit/s capacity between the routers 1 and 2, while the video streams will try to
maintain their total of 5 Mbit/s. The amount of bandwidth given to the surveillance system can no longer be guaran-
teed and the video frame rate will probably be reduced. At worst, the FTP trac will consume all the available band-
width.
Switch 1
Camera 1
Camera 2
PC 3
100 Mbit
100 Mbit
Router 1
Router 2
Switch 2
PC 1
PC 2
100 Mbit
10 Mbit
FTP 2
Video 5
HTTP 3
FTP
Video
Figure 9.4b QoS aware network. Here, Router 1 has been congured to devote up to 5 Mbit/s of the available 10
Mbit/s for streaming video. FTP trac is allowed to use 2 Mbit/s, and HTTP and all other trac can use a maximum
of 3 Mbit/s. Using this division, video streams will always have the necessary bandwidth available. File transfers are
considered less important and get less bandwidth, but there will still be bandwidth available for web browsing and
other trac. Note that these maximums only apply when there is congestion on the network. If there is unused
bandwidth available, this can be used by any type of trac.
Switch 1
Camera 1
Camera 2
PC 3
100 Mbit
100 Mbit
Router 1
Router 2
Switch 2
PC 1
PC 2
100 Mbit
FTP
Video
10 Mbit
FTP
Video
NETWORK TECHNOLOGIES - CHAPTER 9