Information Technology Reference
In-Depth Information
CDN distribution node
CDN edge node
Users
Figure 1.5
Content delivery network (CDN).
The CDN typically installs hundreds of servers in data centers throughout
the Internet. The content providers can upload their contents to a specific
server and the contents are then replicated and distributed to CDN edge
nodes. When a user requests a video, the CDN server uses DNS-based redi-
rection to redirect the request to the CDN edge node closest to the user. Thus,
the delivery delay is reduced (see Figure 1.5).
A CDN architecture is more scalable than a Media Server Farm. However,
when a big event happens and everyone wants to get to the same edge server
for the same event, that server will still be overloaded. Even though the scal-
ability problem would be solved to some extent, the cost and maintenance
of the servers are still very high. And the cost is passed on to the content
providers who need to pay more for better service.
Figure 1.6 shows the traditional IPTV architectures. The aforementioned
Media Server Farms and CDNs use multiple unicasts to deal with multiple
user requests. The arrows show the identical copies of the streams. From the
figure, we can easily tell the disadvantages of delivering video streaming
with this approach. As the number of connections increase, the server load
and the link stress also increase.
It is obvious that multiple unicasts is not sufficiently scalable. Therefore, mul-
ticast technologies have been proposed to address this scalability problem.
1.2.2 Multicasting Video Streams
Multicast technologies can be loosely classified into IP multicast, Application
Layer Multicast (ALM), and hybrid IP/Application-Layer Multicast. Figure 1.7
shows the taxonomy of multicast technologies.
