the switching ecosystem
surrounding it, is
the king of video transport
technology in the
media facility. Whether
installed as 0.27, 1.5 or
3G links, no one doubts
its universal dominance and ability to carry
real-time AV signals.
It has several worthy traits that make
it the ideal fit for AV transport. Key points
to its success are: 1) lossless digital transport;
2) line and frame aligned data format;
3) frame-accurate switching system with
point-to-multipoint outputs, and 4) an integrated
control system for time aligned,
switched path control.
However, SDI is falling behind the
needs of new high-end production and
cinematic workflows. With the advent
of Ultra HD television formats, including
4K UHD (2160p) and 8K UHD (4320p),
3G SDI lacks the ability to carry these
new streams. However, 6G, 12G and 24G
SDI link rates could augment the existing
family of links and support the new
video formats. These do not exist today
but could, in theory, be standardized
by SMPTE over time. The new high-rate
I/O ports would likely leverage Ethernet
SFP+ and C-Form Factor Pluggable optical
This does not mean that new SDI links
will use Ethernet data packet framing, rather
they will rely on Ethernet technology
for the physical I/O ports. The on-the-wire
modulation format may also borrow from
Ethernet, but this has not been decided.
For the moment, let’s ignore the need
for more speed. SDI is locked into a custom
ecosystem that is not friendly to IT.
SDI is not “routable” in the IP sense. It is
a local link, not easily bridged to WANs
and other long-distance transport. The
SDI ecosystem includes link routers, path
extenders, PCIe I/O cards and other SDI
specific gear. Simply put, SDI runs counter
to many of IT’s core values: commodity
equipment, networkable, cloud-friendly
and well understood by masses of technicians
|Fig. 1: Comparing SDI and Ethernet environments
So, what can “replace” SDI? Ethernet
and IP in tandem show the most promise.
“But wait,” you say. “Packetized Ethernet is
not video-friendly; it’s asynchronous and
packets get dropped all the time. Ethernet
will never replace SDI.”
True, these are real obstacles, but current
study and advances show there are
practical solutions to using Ethernet/IP
to transport SDI payloads with near SDI
equivalence. Ethernet currently offers
a variety of rates including 100 Mbps, 1
Gbps, 10 Gbps, 40 Gbps and 100 Gbps.
Whereas SDI I/O is an add-on to a data
server, 10G Ethernet ports will be included
on most servers’ motherboards.
Ethernet, which celebrated its 40th anniversary
in May, started life as a humble
10 Mbps link. Researchers see no end in
sight for higher rates. Ethernet’s substantial
rate support is partially responsible for
the bandwidth tsunami we see today. At its
current pace, expect to see 400 Gbps Ethernet
in 2017, 1 Tbps in 2024 and looking
far ahead, 1 Pbps in 2053. Robert Metcalfe
and his team are credited with inventing
Ethernet. There is no stopping this train,
so get onboard or get out of the way.
Fig. 1 compares SDI and Ethernet environments.
Due to their intrinsic differences,
Ethernet can’t exactly duplicate SDI.
However, can Ethernet and its surrounding
switching ecosystem be coerced to
provide an acceptable solution, one that
works for the majority of use cases?
This question is being answered on several
fronts. From real-world tests we know
that the total end-to-end latency, through
several switches, in a facility environment
can be less than one line of HD video
(about 15 μsec).
We also know that switched paths can
have lossless QoS using a variety of standard
techniques and that replicated outputs
are possible as on an SDI switch.
WORKING OUT THE DETAILS
There is accumulating evidence that
commodity Ethernet switches can be used
to switch AV streams frame accurately
as done in an SDI router. Expect to see
demos of this at the 2014 NAB Show. Sure,
there are details to work out, but the signs
are encouraging for an all-Ethernet facility
being a real possibility.
The SDI data payload can be mapped
onto Layer 2 (MAC) or Layer 3 (IP); this
has been called SPoE for SDI Payload over
Ethernet. There is wisdom to reusing the
SMPTE SDI data payload format with embedded
multichannel audio and complex
Let SMPTE continue to define the payload
formats and let the IEEE evolve Ethernet;
a perfect marriage. Mapping a 1.5G
SDI payload onto a 10G link results in
packing five separate SDI payload streams
or 10 if the Ethernet is used bidirectionally
(not possible with SDI).
This packing efficiency will reduce
cabling and I/O ports. 10G Ethernet can
be transported over long-distance optical
fiber or CAT 6A cables with common RJ45
connectors and spanning 100 meters.
It’s worth noting, the IEEE has standardized
Audio Video Bridging, a method
to send time-stamped AV streams over L2/
MAC networks while maintaining lossless
paths and end-point time sync for all signals.
There are efforts in the works to do
a SPoE mapping. AVB is not a direct SDI
replacement, but it has several characteristics
In 2013 the Video Services Forum,
SMPTE and the EBU (Geneva) formed the
Joint Task Force for Networked Media.
This cross-vendor and end-user group is
motivated to sort out and resolve all the
networking issues and submit their recommendations
to standardization. They
are collecting use cases for solutions that
would, at a minimum, replicate the key
traits of streaming SDI links using commodity
This does not mean that SDI has no future.
The transition to IT networking for
AV streaming may take many years; expect
a slow dissolve between technologies. Expect
both Ethernet and SDI to coexist for
Ethernet and its ecosystem have a
bright future in the media facility. There is
much work to be done before it will augment
custom AV transports. Only the future
will tell to what extent Ethernet will
replace all traditional AV links or will stop
at being just a subset. Stay tuned.
Al Kovalick is the founder of Media
Systems consulting in Silicon Valley. He
is the author of “Video Systems in an IT
Environment (2nd ed).” He is a frequent
speaker at industry events and a SMPTE
Fellow. For a complete bio and contact information,