dvd today feature
Wide Load Coming Through:
Designing Networks for MPEG Video Streaming
EMedia Magazine, October 2000
Copyright © Online
oday's network design is a far cry from
that of just five or six years ago. Back then, it seemed a nifty
trick just to get data back and forth from local workstations to
servers at some reasonable speed. Then someone decided to show off
what you could do on the Internet with streaming video. Sure, the
images were crude, worse than MPEG-1 from a 2X CD-ROM on a 486,
but they were moving and mostly in sync with the sound. That's when
we network administrator types began to talk seriously about things
like Quality of Service (QoS) and higher delivery speeds. The definition
of data shifted from text info to full multimedia.
Now, as we near the dawn of our new millennium, streaming video
technology continues to promise a richer, more satisfying experience
on the Internet. Unfortunately, the Internet's lack of bandwidth
still limits its ability to deliver on this promise, despite major
strides in strengthening the backbone. Fortunately, although this
promise remains elusive for most Internet users, streaming video
is an enjoyable reality for many intranet users today. In this
article, the first part of a two-part series, we'll look at practical
design for offering streaming video over local area networks (LANs)
and wide area networks (WANs), such as those found on university
and industrial campuses, where distance learning and training
programs increasingly demand good-quality streamed video. In part
two, we'll describe the exact components required for the various
As for image delivery, we are not just talking about MPEG-1
or using Real Video or Media Player formats; today's networks
can handle MPEG-2 (i.e. DVD-Video) streams and HDTV broadcasts
as well. Curiously enough, this latter capability may come as
a surprise. While the consumer market will wait years for HDTV
to penetrate the installed base, today's computer monitors can
display HDTV feeds just fine. Several vendors, including Sony
and the threesome of 3Com, TeraLogic, and 2NetFX, are already
working out the encoder/decoder hardware, software, and distribution
video network architecture to support this. So you can well have
a higher-performing video experience at your desktop more than
you can at your home theaterand without nearly the investment
THE GREAT LEAP FORWARD
Two things have contributed to the ease of creating effective streaming
video solutions on LANs and WANs: first, the plummeting price of
Gigabit Ethernet hardware; second, the incredibly increasing capabilities
of individual workstations.
With Gigabit Ethernet networksi.e., 1000BaseTprices
have fallen much faster than expected. By mid-year 2000, four-port
Gigabit Ethernet switches were available for under $1,500 with
network adapters going for under $300; part of what is driving
down the price is the onrush of newer technology itself.
At Networld+Interop in Las Vegas in May 2000, chip vendors were
expected to announce the new spec for 10Gb Ethernet. More remarkably,
several vendors actually showed working prototypes of the technology.
The buzz was that actual product wouldn't be years off, but rather
just a few months or maybe a year away.
Meanwhile, the average workstation four years ago was a Pentium
120mHz with a 2GB hard drive and 32MB RAM which sold for $2,500.
Today, a system with a 750mHz Pentium III with a 15GB drive and
128MB RAM sells for under $1,000 (plus $200 if you need a monitor.)
Along with the upgraded CPU also comes the faster bus100mHz
versus just 66\mHzand a wonderfully upgraded video card
with 16MB versus just 4MB with the older system. Simply put, it's
much, much easier for today's systems to display video.
WHAT ABOUT YOUR NEEDS?
Designing a network for MPEG streaming means addressing four basic
- What is your input?
- What is your available bandwidth?
- What is your destination?
- How many users do you want to service at once?
Network-ready digital video can be streamed in real time (such
as video conferencing or a live event broadcast), which is broadcast
from networked storage libraries or delivered on-demand to a single
user from those libraries. Solutions are available for both types.
For our discussion here, we will focus on a Video-on-Demand (VoD)
system from an optical disc source. IP network addressing distinguishes
between three kinds of streaming throughput:
- Unicast, in which the video packets stream to a single workstation
- Broadcast, in which the stream is sent to an entire subnetwork
- Multicast, in which the stream goes to a user-defined set
Of the three, Unicast is the most challenging, since you have
to design the system to support sending separate streams to each
workstation. Broadcasts are much simpler, since a single stream
supports as many workstations as want to tune in (à la TV broadcast).
Most network routers, however, limit broadcasts to just a single
subnetwork. (Routers do this to prevent overloads called broadcast
storms from flooding the entire network.) Multicasts are fully
supported in IP networks, but often are not implemented since
they do require setup. We'll see later that sending HDTV signals
over networks requires setting up a Multicast.
While Unicast is difficult, it is the most desirable of the
streaming options since it produces the Video-on-Demand structure
users look for. With all three types of video throughput, users
can typically request their own video when they want, but with
Unicast, they can pause, rewind, and replay the stream, something
they cannot do with broadcasts or Multicasts.
A complete system for on-demand delivery of optical disc-derived
video involves a variety of components including a DVD-Video disc
storage subsystem, which could be a RAID box or a jukebox; a dedicated
or shared network server; for network topology, Fast or Gigabit
Ethernet; a Hub or switch because Hub-based networks cannot support
MPEG-2 consistently; a PCI network card; an MPEG-2 decoder card,
which is essential unless the workstation is about 1gHz; and a
400mHz or better workstation CPU.
To anticipate and prepare for potential bottlenecks, users must
consider where the video will be expected to go and what it will
need to get there, with network bandwidth being the most critical
consideration. The video stream must pass to reach the user's
monitor. The stream must do so in a timely fashion as well. This
"disc-to-desk" time ideally should be the same as or less than
the time it takes for the user's local workstation to present
the same video.
Next, we need to have enough bandwidth to support our desired resolution
and there are a variety of compression types with a number of requirement
variables. It isn't the bandwidth of a single streaming video session
that is importantalmost every Fast Ethernet network can support
a single MPEG-2 stream or even HDTV for that matter. What is important
is in creating a network that scales for additional users. Each
new user requires a sustainable bitrate to the desktop, and this
bitrate must be consistent, otherwise the video or sound will be
choppy, losing frames.
In order to understand how a consistent video stream is delivered
across a network, let's look at a typical enterprise network. Most
networks have one or more hubs in a local wiring closet where user
workstations attach. Since the entire network bandwidth is shared
amongst all users, this may reduce the average bandwidth to 1Mbps
or less per workstation. This is good enough for teleconferencing
or future MPEG-4 (i.e., lower-bandwidth) applications, but not for
There are certainly layouts designed to make life easier for
the installer than the video user, and low-cost hubs easily support
typical office automation tasks, but unfortunately, leave each
user with only 1Mbps of bandwidthgood enough for teleconferencing
or future MPEG-4 applications, but not for MPEG-2. By substituting
Fast Ethernet (100BaseT) switches for the existing hubs, each
workstation can now access up to 10Mbps on a dedicated connection,
which is enough to support MPEG-2 video delivery. Note that this
throughput will require at least a Gigabit Ethernet backbone to
the network router. That's because each switch will require its
own 100Mbps connection to the video server in order to maintain
A Gigabit Ethernet backbone, when properly configured, can handle
up to 70-75 simultaneous MPEG-2 video streams. A Fast Ethernet-only
network, on the other hand, is limited to eight simultaneous streams.
With this type of system in place, a site could then upgrade each
switch and workstation to Gigabit Ethernet to support HDTV feeds
to the desktop as well.
THE EIGHT TRICKS OF STREAMED VIDEO TRADING
We just described how a switched Fast Ethernet network could sustain
MPEG-2 playback for up to 75 workstations from a video serverwhich
isn't quite true. In order for that to happen, we have to overcome
eight potential bottlenecks to achieving effective multiple-user
Trick Number 1 RAID Video Cache Storage
Many network servers create their video caches on RAID subsystems.
(While the DVD-Videos may reside in a jukebox or may have been retrieved
via a DVD-ROM drive, their contents are accessed by end-users from
a RAID box.) Most data RAID systems use RAID 5 for maximum data
protection. Streaming video, however, performs better from a RAID
0 configuration; data security is less important in this configuration
since the content is still available in the jukebox.
TRICK NUMBER 2 Good NAS Performance
For many sites, a dedicated NAS unit serves up video content to
users. However, most NAS units are not specifically tailored for
streaming video. A NAS video solution needs to handle a lengthy
connection with each requesting workstation to deliver its video
stream. Most NAS systems are tailored for discrete, small data packet
requests rather than a continuous stream of data. Newer systems
are now emerging that do support MPEG video as their primary job.
Trick Number 3 Good Network Cable
Gigabit Ethernet does support Cat5 copper cabling. Copper GigEther
networks are significantly less expensive to construct than their
fibre-based counterparts, since both network cards and switches
are less costly. However, you'll need to verify the quality of each
installation to ensure support for the higher speeds. You should
also verify the length of each connection to the switch; copper
GigEther connections can run only about 300 feet (100 meters). Beyond
that, you have to go with fibre. (Network bandwidth, of course,
is also a concern. For MPEG-2 purposes, we need at least switched
Fast Ethernet to support multiple MPEG-2 users simultaneously.)
Trick Number 4 Right Frame Size
Gigabit Ethernet, either as a backbone for router-to-server or router-to-switch
links, should support Jumbo Frames. This generates a five-fold increase
in throughput over conventional frames. Gigabit Ethernet introduces
a new frame type to the protocol. Jumbo frames are super-sized versions
of their regular cousinsthe 1615-byte framein Ethernet.
Jumbo Frames can be a whopping 9014 bytes, more than five times
larger. This significantly reduces overhead and vastly improves
Trick Number 5 Legacy Operating Systems
Network streaming users will be encouraged to discover that Microsoft
has improved Windows I/O with its latest OS release. Windows 2000
supports offloading various I/O tasks, such as packet checksum processing
and IP security, to the network card. Since every data packet has
a checksum, every Windows 95, 98, or NT workstation took quite a
hit in CPU utilization to handle this processing. For Fast Ethernet
(100BaseT), CPU utilization is 21% as compared with 58% for Gigabit
Trick Number 6 Right Network Card
If you have Windows 2000, then you'll also need a PCI network card
with onboard intelligence for handling the checksum and security
processing. Legacy ISA cards in particular will not deliver a consistent
Trick Number 7 Right CPU and Motherboard
Installing Gigabit Ethernet will not improve consistent data delivery
unless the workstations and servers have at least a 400-500mHz Pentium
II or III processor and a 100mHz motherboard. Legacy systems, such
as a Pentium 233 or 300, even if fully equipped for office automation,
do not process data fast enough for streaming video.
Trick Number 8 Software vs. Hardware Decoding
Once data arrives at your workstation, you will need an MPEG-2 decoder
to process it. (Contrary to what some users might expect, the network
transmission doesn't convert the stream to Real Video format.) If
you choose a hardware decoder, you'll need a minimum of a 233mHz
Pentium MMX. You'll also need an AGP video card to handle MPEG-2
streams at 30 frames per second. If you want to do everything in
software, however, expect to use an 800mHz-1gHz AMD Athlon processor
AND THE WINNER IS...
A streaming video network setup can be more effective than a standalone
workstation's performance in displaying smooth, full-motion, full-screen
video. As we've discussed, by sharing the tasks between the video
server, the network card, the video card, and the workstation's
CPU, the workstation can maintain a more consistent playback image
onscreen than it could from its own DVD-ROM drive.
Tune in next time, when we'll look at two scenariosDVD-Video
Unicast and HDTV multicastwith products and models as a
recipe for you to construct your own effective video network.
The Sweet Science: Boxing
MPEG in the Network Appliance
The fundamental difference between networked and so-called "standalone"
computing, naturally, is that networked computing requires that
multiple products and technologies work effectively in concert with
one another. With a network application as demanding as serving
MPEG-1 or MPEG-2 video over LANs and WANs, highly performing, highly
compatible componentry is a must.
This article looks at overarching architectural issues of creating
networks well-fortified for MPEG delivery, and elements essential
to avoid the inevitable bottlenecks that result when you attempt
to distribute high-bandwidth content over network configurations
that aren't up to the challenge. But there are other pieces to
the puzzle. Of course, there are the specific components, the
storage and server devices available for keeping and delivering
video online in intranet environments. These will be examined
in Doering's follow-up piece.
Another essential enabling factor of network streaming as we
know it comes, appropriately enough, from companies whose core
technologies are video-centric, or those who have at minimum set
out to tackle digital video encoding and network delivery in equal
measure. These companies have taken their MPEG encoding/decoding
technology and implemented it in so-called "network appliances,"
compact MPEG hardware designed expressly with the intent of connecting
to networks and serving MPEG video efficiently over them.
These products were examined in detail in Jeff Sauer's September
article, "Band-Aid: Network Appliances for MPEG Streaming" [pp.
39-45], which serves as something of a companion piece to Doering's
exploration of the architectural demands of network-streamed MPEG
and tells a crucial part of the story. Providers of "Network Appliance"
products looked at in Sauer's article include VBrick Systems (http://www.vbrick.com),
InnovaCom (http://www.transpeg.com), Optivision (http://www.optivision.com),
Minerva Networks (http://www.minervanetworks.com),
and Optibase (http://www.optibase.com).
These products help complete the picture of an MPEG-ready network.
Stephen F. Nathans
What's This Overhead?
It may surprise you that Fast Ethernet and Gigabit Ethernet don't
deliver a full 100Mbps (megabit per second) and 1Gbps (gigabit per
second), respectively. Often, the rationale is that Ethernet simply
is an inefficient protocol and suffers from too much overhead. Actually,
there's a workstation component to the problem outside of just protocol
overhead, which can be reduced using Jumbo Frames.
The CPU utilization issue hampers Ethernet throughput on all
but Windows 2000 workstations. With up to 50% of the CPU cycles
involved in packet processing, is comes as no surprise that there's
not enough throughput. But there's also the hard drive hurdle.
In fact, hard drive performance is the primary cause of slow throughput
on Gigabit Ethernet; even with 10,000 RPM SCSI drives, the networks
aren't fast enough to tackle Gigabit speeds, causing about a six-fold
reduction in throughput alone. Fortunately, 100 Mbps is fast enough
for consistent MPEG-2 playback on a workstation.
NETWORKOBSERVER columnist David Doering (firstname.lastname@example.org),
contributing editor, is also senior analyst with TechVoice Inc.,
an Orem, Utah-based consultancy.
Comments? Email us at email@example.com.