Digital Television And The Converged IP
BY DAVID PECORELLA AND MARTY DUGAN
Though originally designed to transport data, Internet Protocol's
packet-based technology can provide high-quality digital video integrated
with interactive digital voice and data services over broadband networks.
Transporting multimedia such as digital television (DTV) or video on demand
(VoD) over IP has many advantages. It also has its weaknesses. For these
advanced services, quality can become an issue in the IP environment.
Emerging technology overcomes these quality challenges and enables IP to
transport high-end digital video on one converged network. IP allows cable
television and telecommunications service providers to transport
high-quality interactive digital video consumers want over their existing
IP AND THE FUTURE HEADEND
IP has become a leading contender for video transport because the
popularity of high-speed Internet access has spurred the creation of a
broadband network that is reaching into homes everywhere. A connectionless,
packet switching protocol, IP provides packet routing, fragmentation, and
re-assembly, allowing many users of multiple services to share bandwidth.
This new model of fast, multiservice delivery contrasts the traditional
community access television (CATV) plant, a branch and tree topology that
distributes content from a centralized headend out to customer premises.
These traditional CATV headends act as satellite and microwave reception
areas, and drive a one-way TV broadcast infrastructure. The new model is
bi-directional and therefore enables interactive services such as VoD.
Cable TV operators, as owners of the access broadband network, have
tapped into the high-speed residential data delivery business. The old model
of cable headends as a system of standalone, unidirectional broadcast
facilities have become nodes on a bi-directional or full-duplex wide-area
network (WAN). These WANs incorporate packet-based data networking
technologies like asynchronous transfer mode (ATM) and synchronous optical
network (SONET). Cable modems have evolved into high-speed digital
packet-based IP gateways to the home. First generation cable modems were
initially designed for IP-based data services only. While IP is evolving to
support video services such as VoD and DTV, these new media are
fundamentally different from data and bring quality of service (QoS) front
WHY IS QoS SO IMPORTANT?
Quality is always a priority in transporting video for any purpose, but
especially for entertainment. The de facto compression standard for
distributed, entertainment quality video like VoD and DTV has become MPEG-2
technology. The innate time-sensitive nature of video makes video stream
clocking management every bit as important as content management. MPEG-2
injects into the stream a free running 27 MHz timing clock called a program
clock reference (PCR). MPEG-2 DTV systems require that the encoder PCR time
clock and the decoder's clock be kept in close synch. MPEG-2 compressed
video is sensitive to propagation delay variance. Lost or corrupted data
cause noticeable interruptions. That's why MPEG-2 has stringent QoS
In this emerging broadband network, IP is the transport protocol for
MPEG-2. However, IP was originally designed as a data transport protocol.
Data is inherently more tolerant to error than media such as real-time voice
or digital video. IP technology breaks a digital information stream into
packets and gives each packet a destination. IP-based edge devices find a
path through available network links. Packet delay variations, packet loss
and bit error rate are secondary considerations. For high priority data,
where guaranteed delivery is necessary, protocols like transmission control
protocol (TCP) determine whether packets have arrived successfully. If they
haven't, the protocol initiates a retransmission. While this is fine for
non-real-time applications, such as e-mail, Web browsing, and file transfer,
it inhibits real-time digital video. Digital video is sensitive to packet
delay variation, packet loss and bit error rate. The high bandwidth demands
of a video feed make the problem worse.
Many point-to-point networks, such as those for phone service, don't have
this quality control problem. IP does not establish any comparable type of
point-to-point connection. However, tagging different services to be handled
differently by the network is a function of QoS that IP can accommodate.
Services can be assigned classifications based on sensitivity and priority.
The network is designed to handle them differently. IP can differentiate
classes of service (CoS), and distinguish different levels of service and
network handling. This is the essence of QoS.
IP MULTISERVICE QoS INITIATIVES
A number of differential traffic-handling approaches can provision
IP-based CoS, which in turn enables QoS. Approaches such as IEEE 802.1p, ToS
(type of service) marking, and DiffServ mark packets with a priority that
tells the network how to treat the packet. The DiffServ standard allows
networks to prioritize packets forwarded from the network edge devices based
on defined service codes. Video, for example, would be coded differently
than Web traffic. While this does not guarantee QoS, it does establish
priority relative to other traffic on the network.
Because a typical IP network itself has no direct knowledge of how to
optimize the path for a particular application or user, the IP protocol
provides a limited facility (ToS) for upper layer IP protocols to convey
what service tradeoffs should be made for the particular packet. The ToS
field in the IP header is not considered true QoS, but it is still a way to
identify special handling for a packet. Next-generation cable modems could
use ToS to drive IP-based multimedia over their hybrid fiber coax (HFC)
A number of resource reservation protocols like resource reservation
setup protocol (RSVP) and multi protocol label switching (MPLS) exist for
next generation systems to implement differentiated services, including
video. MPLS handles traffic management for media with different CoS and
injects predictability into the networks, dealing with QoS issues, such as
black and white management, latency, jitter, and packet/cell loss. MPLS can,
in theory, provision QoS over multiple types of network technologies. The
trick is getting MPLS provisioned end-to-end over multiple network types
(ATM, IP, SONET, and DWDM). An initiative spun off of MPLS -- called
multi-protocol lambda switching (MPlS) -- would provision QoS on the fiber
layer of the network. Currently, no standard regulates how MPLS should be
provisioned end-to-end in the optical (SONET/SDH), ATM, and IP layers
together. One should be set in the next two to four years.
CABLE MULTISERVICE QoS INITIATIVES
The cable industry has developed the Data Over Cable Service Interface
Specification (DOCSIS) to standardize high-speed data services
implementation over the cable infrastructure. DOCSIS v1.0 prescribes
universal ground rules for reliable IP packet routing between the CMTS and
the cable modem. More recent DOCSIS versions have included QoS and security
features to add packet-based voice services to the mix. Latest generation
DOCSIS standards do this by sub-partitioning bandwidth in the cable plant.
Each sub-partition is then assigned a different service type and allows
particular high priority, jitter, and time sensitive traffic priority over
non-real-time critical information.
The technologies and standards are maturing enough for cable operators to
consider IP-based video services in conjunction with data and voice
services. The next generation cable system includes standards-based network
equipment to ensure interoperability with out-of-system networks.
Collapsing the network layers into a single packet protocol ensures
maximum revenue/cost by offering flexibility, scalability, and simplicity.
Point-to-multipoint services over last mile infrastructure will exploit IP
routing's inherent benefits -- forwarding and multicasting -- while
addressing QoS issues. Video-aware network edge devices in conjunction with
new generation CMTS systems will give cable operators what they've wanted
for decades: a single converged multi-service network that allows dynamic
video, voice, and data provisioning. The new cable plant will evolve into a
multi-service network over a broadband infrastructure. As IP-based WAN
technologies advance in conjunction with DWDM, cable operators will be able
to allocate bandwidth not only to specific types of services, but also to
individual subscribers. The long-term advantages of such a vision are
obvious: a single integrated multimedia infrastructure that is flexible,
scalable, easy to maintain, and technologically poised to provision the next
generation of interactive video, voice, and data services.
David Pecorella is director of product marketing and Marty Dugan is
manager of corporate marketing for Artel
Video Systems, Inc. Artel provides video networking systems to cable TV
and telecommunications service providers who need to satisfy the consumer's
ever growing appetite for broadband digital entertainment. The company's
products are transforming the world's networking infrastructures by enabling
the transmission of broadcast and interactive TV over standards based
To The April 2001 Table Of Contents