We have all seen the growth of Internet
TV and all the associated stuff - better satellite service, the increase of digital cable, and the beginning of HDTV...they
have all changed the way TV is delivered to people.
Now, a new delivery method is upon us in a very large way. It's called Internet
Protocol Television (IPTV), and it is here to stay...it has come to us, and backed by the big
bank accnts of the telecommunications industry, it's starting to offer more interactivity and
bring a large amt of competition to the business of selling TV.
Here
are some of the basic facts:
IPTV describes a system capable of receiving and displaying a video
stream encoded as a series of Internet Protocol packets. If you've ever watched a video clip on your computer, you've used
an IPTV system in its broadest sense. When most people discuss IPTV,
though, they're talking about watching traditional channels on your television, where people demand a smooth, high-resolution,
lag-free picture, and it's the telcos that are entering into this market fast.
Once
known as phone companies, the telcos now want to turn a new trick of voice, data, and
video that will put them in a powerful position.
In this Training_Cloud Session, we'll explain how IPTV works and what the
future holds for the technology. Though IP can (and will) be used to deliver video over all
sorts of networks, including cable systems, we'll focus in this article on the telcos, which
are the most important people.
And we will see that
many of the Corporates-Enterprises we work with are new generating a lot of these IP Mobile
enabled Devices into the IT asset disposal pipeline.
They're pumping billions into new fiber
rollouts and backend infrastructure. Why the big rush for the
TV business? Because the telcos see that the stakes are far higher than just some television:
companies want to become your place's sole communications link, and IPTV is a major part of
that strategy.
More on IPTV
Remeber the bedroom set-top box...it's been declining in the cable world....but
will make a big gain in IPTV systems. The box will connect to the home DSL
line and is responsible for reassembling the packets into a coherent video stream and then decoding the contents. Your computer
could do the same job, but most people still don't have an always-on PC sitting beside the TV, so the box will make a stay
here for a long while.
Where will the box get its picture from? To answer that question, let's start at the beginning.
Most
video enters the system at the telco's national headend, where
network feeds are pulled from satellites and encoded if necessary (often in MPEG-2, though H.264
and Windows Media are also possibilities). The video stream is broken up into IP packets and
dumped into the telco's core network, which is a massive IP network
that handles all sorts of other traffic (data, voice, etc.) in addition to the video.
Here the advantages of owning
the entire network from beginning to end (as the telcos do) really come into play, since quality
of service (QoS) tools can prioritize the video traffic to prevent delay or fragmentation of
the signal. Without control of the network, this would be a gamble, since QoS requests are not
often recognized between operators. With end-to-end control, the telcos can guarantee enough
bandwidth for their signal at all times, which is key to providing the "just works" reliability consumers have come
to expect from their television sets.
The video streams are received by a local office, which has the job of getting them out to the people
at home or bedroom. This office is the place that local content (such as TV stations, advertising, and video on demand) is
added to the mix, but it's also the spot where the IPTV middleware
is housed. This software stack handles user authentication, channel change requests, billing, VoD
requests, etc.—basically, all of the necessary infrastructure.
All the channels in the lineup are multicast
from the national headend to local offices at the same time, but at the local office, a bottleneck
becomes apparent. That bottleneck is the local DSL loop, which has nowhere near the capacity
to stream all of the channels at once. Cable systems can do this, since their bandwidth can be in the neighborhood of 4.5Gbps, but even the newest ADSL2+ technology tops out at around 25Mbps (and this speed drops quickly as distance from the DSLAM [DSL Access Multiplier] grows).
So how do you send hundreds of channels out to an IPTV
subscriber with a DSL line?
Simple: you only send a few at a time. When a user changes
the channel on their set-top box, the box does not "tune" a channel like a cable system. (There is in fact no such
thing as "tuning" anymore—the box is simply an IP receiver.) What happens instead
is that the box switches channels by using the IP Group Membership Protocol (IGMP)
v2 to join a new multicast group.
When the local office receives this request, it
checks to make sure that the user is authorized to view the new channel, then directs the routers in the local office to add
that particular user to the channel's distribution list. In this way, only signals that are currently being watched are actually
being sent from the local office to the DSLAM and on to the user.
No matter how well-designed a network may
be or how rigorous its QoS controls are, there is always the possibility of errors creeping
into the video stream. For unicast streams, this is less of an issue; the set-top box can simply
request that the server resend lost or corrupted packets. With multicast streams, it is much
more important to ensure that the network is well-engineered from beginning to end, as the user's set-top box only subscribes
to the stream—it can make no requests for additional information. To overcome this problem, multicast
streams incorporate a variety of error correction measures such as forward error correction (FEC),
in which redundant packets are transmitted as part of the stream.
The fact of owning the entire network is important
since it allows a company to do everything possible to guarantee the safe delivery of streams from one end of the network
to the other without relying on third parties or the public Internet.
Though multicast technology provides
the answer to the problem of pumping the same content out to millions of subscribers at the same time, it does not help with
features such as video on demand, which require a unique stream to the user's home. To support VoD
and other services, the local office can also generate a unicast stream that targets a particular
home and draws from the content on the local VoD server.
This stream is typically
controlled by the Real Time Streaming Protocol (RTSP), which enables DVD-style control over
a multimedia stream and allows users to play, pause, and stop the program they are watching.
The actual number of simultaneous video
streams sent from the local office to the consumer varies by network, but is rarely more than four. The reason is bandwidth.
A Windows Media-encoded stream, for instance, takes up 1.0 to 1.5Mbps for SDTV,
which is no problem; ten channels could be sent at once with bandwidth left over for voice and data. But when HDTV enters
the picture, it's a different story, and the 20-25Mbps capacity of the line gets eaten up fast.
At 1080i, HDTV bit rates using Windows Media are in the 7 to 8 Mbps range (rates
for H.264 are similar). A quick calculation tells you that a couple of channels are all that can be supported.
The bandwidth situation
is even worse when you consider MPEG-2, which has lower compression ratios. MPEG-2
streams will require almost twice the space (3.5 Mbps for SDTV,
18-20 Mbps for HDTV), and the increased compression found in the newer codecs
is one reason that AT&T will not use MPEG-2 in the rollout
of its IPTV service dubbed "U-verse."
Simultaneous delivery of channels is necessary to keep
IPTV competitive with cable. Obviously, multiple streams are needed to support picture-in-picture,
but they're also needed by DVRs, which can record one show while a user is watching another.
For IPTV to become a a real whole-house solution, it will also need to support enough
simultaneous channels to allow televisions in different rooms to display different content, and juggling resulting bandwidth
issues is one of the trickiest parts of implementing an IPTV network that will be attractive
to consumers.
