Digital Signal Processing Reference
In-Depth Information
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2
3
1
Cable Modem
Head End
5
4
User
Termination
Router
(Internet
Access)
Coax
6
7
1
4
Digital
Subscriber
Line Access
Multiplexer
2
5
User
Termination
Router
(Internet
Access)
3
6
Figure 18.2. Aggregated access to
internet.
(DSLAM)
Twisted pair
or twist pair wiring. These lower-speed connections are aggre-
gated into a single higher-speed connection by the service
provider, at what is called a central office (though this function
could just be an equipment cabinet).
The common technology used across WANs, last mile and
LANs is IP and its protocols such as TCP/IP, UDP, and RTP. In
particular, compressed video over IP can be streamed across
networks made up of all these technologies.
18.6 Video Over Internet Issues
The internet is a remarkable network for all sorts of commu-
nication, but it does have serious limitations for video trans-
mission. The available raw bandwidth is large, but as there is
a high variability in number of users and demand from those
users, so available bandwidth for a given user is never guaranteed.
Packet losses also occur, perhaps as much as 1%. More prob-
lematic is the amount of jitter, or variation on the latency of
transmission from packet to packet. A packet that arrives late is
the same as a packet that never arrives in a real-time system.
These issues are slightly mitigated by the latest video
compression algorithms, which reduce video bandwidth, partic-
ularly on low-resolution video delivery. Many program sources of
video are available on the internet.
Dedicated non-internet transmission is needed for high-defi-
nition video with good quality. This is why the great majority of
consumers subscribe to a cable or satellite service, with high
quality, dedicated transmission networks of video content.
