Core Insights
From the experts at The Broadcast Bridge.

TDM Mesh Networks- A Simple Alternative
To Leaf-Spine ST2110

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Core Insights: TDM Mesh Networks - A Simple Alternative To Leaf-Spine ST2110©The Broadcast Bridge

The limited working environments, rigging
and test availability, and rehearsal times
are not the exclusive to the ESC, but are
instead typical of ad-hoc international
events the world over. IP would lend itself
well to such a use-case, but a purely IP
solution proves difficult to manage andimplement.

Introduction by Tony Orme - Editor at The Broadcast Bridge.
broadcasters and we’re still ironingout all the wrinkles, not just in our
understanding, but also in how we apply
it to the specialist media streams wework with.

IP is delivering unprecedented
flexibility and scalability forbroadcasters. But there is a price to
pay for these benefits, namely, thecomplexity of the system increases
significantly as we add more video
and audio over IP.

Video and audio streams are quite uniqueas the data they create is relentlessly
constant, especially when working in theuncompressed domain. This leads totight time tolerances making interfacing
the synchronous SDI and AES transport
streams to IP packet networks even moreof a challenge.

The principal reason for this increased
complexity lies in timing andsynchronization. We may have moved
on from line and field syncs, but theirpresence still influences broadcast
workflows and video and audio
processing.

Building, installing, monitoring, andunderstanding IP networks for video
and audio transport is not for the fainthearted. It most definitely delivers
impressive benefits, but there areoccasions when these benefits are justnot needed. One example is that of thisyears European Song Contest (ESC).

Although timing reference signals may
be hidden in SDI and their influence isn’t
particularly evident, everything changes
when we move to ST2110.
PTP has its history in industry, hasbeen used for many years and iswell understood in the IT community.
However, it’s still relatively new for

Bringing together SDI, AES, analog
and IP all into a fully distributed meshnetwork is another option available tobroadcasters. Complete IP infrastructures
certainly have their place, but TDM not
only offers a viable alternative when
the complexity is not required, but also
provides the ultimate in hybrid systems.

Another solution is available through theuse of TDM (Time Domain Multiplexing).
Based on a mesh network topology,
TDM provides many of the benefits of IP
without having to get bogged down inthe detail of operating it. Switching nodes
are connected through fiber optic cabling
to provide high-speed interconnectivity,
flexibility, scalability, and resilience.
Each of the nodes not only acts as arouter between network connections,but provides interface gateways to all the
broadcast formats such as SDI, AES andanalog audio. Furthermore, IP interfaces
are available to provide IP routes acrossthe network without having to run aseparate IP network.
A software management and monitoring
system makes configuration and routingof devices easy as well as providing
resilient connectivity that self-heals
should a fault occur.
TDM has another benefit that may not
be immediately obvious, that is, very low
and predictable latency. The synchronous
nature of time-data slots within the
TDMs transport stream frame means
there is always availability for the data
being transported. There’s no congestion
and if a link runs the risk of becoming
oversubscribed, extra fiber cabling andnodes can be easily added to scale theinfrastructure accordingly.

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Part 1 - Balancing
Technical Requirements
workflows facilitate this especially if cloudsystems are used.

IP is well known and appreciatedfor its flexibility, scalability, andresilience. But there are times when
the learning curve and installation
challenges a complete ST-2110
infrastructure provides are just too
great.

Both video and audio streams are
inherently synchronous. This isn’t
just about maintaining lip-sync but
guaranteeing the number of video frames
and audio samples created by the source
devices are played back in time by thedisplay and loudspeaker systems. If a
microphone is creating audio samples
faster than a loudspeaker can decodeand play them then buffer overflow will
occur leading to horrible audio distortion.
Underflow occurs when not enoughsamples are created resulting in a similar
distortion.

Many broadcasters are moving to IP
because of the immense flexibility it
offers, especially when we consider theavailability of 4K, HDR, WCG and higher
frame rates. Distribution of multiple
audio streams lends itself well to IP andmany other services such as intercom,
monitoring, and control can be easily
added.

Synchronization Matters

The principle challenge we have with
distributing low latency video and audio
over IP is attributed to configuringmulticast streams and creating an outof-band timing plane. From the earliest
days of broadcasting the synchronizing
information was embedded in the video
signal and this system continued throughto SDI. It’s only when ST-2110 was
introduced did we separate the video andaudio essence from in-band timing.

Although the original reasons formaintaining video synchronization
may have disappeared into the history
books, the principle is still maintained.
That is, every video frame and audio
sample created, must be played backsynchronously for the viewer. And this iswhere our challenges for timing emanate.
PTP as an out-of-band timing systemprovides a reference that allows video
frames and audio samples to beuniformly created and synchronously
viewed. The time-invariant natureof signal sampling cannot be
underestimated as any variance in thesample rate will also result in non-linear
distortion that is difficult to correct.

Scalable Resource
Flexibility and scalability lend themselves
well to IP infrastructures. The IT industry
demonstrates this on a daily basis
through network scaling and resource
expansion. When COTS methodologies
are employed it’s relatively easy toincrease server resource and network
capacity. Software defined networks andTiming systems are well understood in

example, live studio and OB cameras
need OCPs for shading, line-up, andmonitoring, and these often connect to
the CCU using IP. System control, routing
and tally all benefit from IP as it saves
on having to install custom cabling forstandards such as RS422. Not only is thisdifficult, but custom routing systems areneeded to switch control signals, further
adding to the complexity and weight.
SDI is a relatively static system andworks with a limited number of formats,and SMPTE have done a fantastic jobof keeping up with the latest broadcast
standards to distribute signals. For
example, 4K120P can be distributed over
12G SDI cabling along with HLG HDR
and WCG. However, any non-broadcast
formats are difficult to transfer.

industry and broadcasters have adopted
PTP with its wealth of experiencealready well documented. However,this is a highly complex protocol
with many intricacies. Monitoring
isn’t as straightforward as checking
sync pulses on an oscilloscope, andsynchronizingdisparate systems requiresa deep understanding of networks andtheir topologies.

Solutions For Applications
That said, ST-2110 with PTP is continuing
to gain a lot of interest and broadcasters
are finding applications where it excels.
Predominantly, these tend to be fixedinstallations where IT and broadcast
teams come together to design, install,and maintain IP infrastructures. OB’s
do use ST-2110 and IP as its adoption
greatly reduces the amount of cablingthat results in reduced axle weight. But
again, these tend to be components ofmuch larger infrastructures where support
teams have had time to understand andtest this new technology.

Fiber Distribution
Limited to one service per cable, andsometimes less when dual and quad SDI
are used, SDI uses a massive amount ofcable that is heavy, cumbersome, anddifficult to maintain. It’s always the center
cable tie-wrapped in a large bundlethat breaks. As IP is a packet switched
network the topology is completely
different from circuit switched systems

There is more to IP than distributing video
and audio; it also includes transport ofcontrol and monitoring information. For

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TDM systems tend to be vendor specific,
and this is one of their greatest strengths
as managed networks simplify designs.
Multiple signal types are integrated into
aggregators that multiplex them onto
single fiber channels. This provides all
the advantages of SDI simplicity with theflexibility of IP networks and distribution,
but with much of the complexity taken
away.

such as SDI, facilitating many streams to
be distributed over single cables, whether
fiber or twisted pair.
Cable management is a concern formany remote installations. This may not
be the case for fixed broadcast facilities,
but even these have limited capacity
for cabling especially when considering
underfloor and elevator capacity.

Furthermore, cable management
becomes much easier as the aggregators
join through fiber connections to makea simplified network. Instead of having
to run a new cable between two trucks
or a truck and the arena for each signal,a new stream within the network isestablished through the management
interface to provide virtual circuits.
And the aggregators can also process
video and audio so that they act as
signal processors. This includes frame
synchronization, HD to SD and standards
conversion.

As in all things engineering there isalways a compromise and balance to be
struck. SDI may be static but is easy touse, whereas IP and ST-2110 is incredibly
flexible but difficult to configure andmaintain.

TDM - A Third Way
Until recently broadcasters have had
to make a stark choice between SDI
and IP for their installations. Balancing
the requirements, advantages, anddisadvantages, is often a difficult task.
However, a further compromise is now
available, that is TDM (Time Division
Multiplexing).

In the next two articles we will take a
deeper look at TDM and the associatednetworks to understand how they benefit
broadcasters.

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Part 2 - Knowing
What To Choose©The Broadcast Bridge

data packet has an allocated slot in the
TDM frame so buffer utilization is kept toa minimum at the receiver.

Broadcasters are no longer facedwith the binary choice of going
down the SDI or IP routes. The
hybrid method of using TDM (TimeDomain Multiplexing) combines theadvantages of distributed networks
with IP and SDI to deliver a fully
integrated solution that helps
broadcasters working across multiple
technologies.

IT Is Delivering

In recent years, the fiber opticalbackbone speeds have increased at a
massive rate. It was only in 2010 that
40Gb and 100Gb ethernet speeds wereconsidered state-of-the-art, by 2020 we
now have 400Gb with 800Gb, and 1.6Tb
is predicted for 2030. These speeds
have been empowered by advances infiber optic cable design and wavelength
division multiplexing (WDM) technologies.

Since the inception of IP broadcasters
have generally been thinking in terms ofspine-leaf architectures. These have theadvantage of being predictable in termsof latency and have a relatively goodlevel of resilience. However, they are
hierarchical and congestion points soon
manifest themselves if careful systemdesign isn’t employed.

The point is IT networks now surpass thesort of network speeds needed by thebroadcast industry. At least in point-topoint configurations. Although possible,
it’s unlikely that a studio or stadium will
have 130 cameras, and this leaves open
the option of using alternative topologies
to the traditional broadcast centralized
configurations.

Mesh networks differ from the spineleaf architectures as they can be
fully connected, dynamic and nonhierarchical. Each node within the mesh
acts as a switch or router and is awareof the other nodes within the network.
They have the ability to exchange routing
information and if each node has morethan one network connection the network
can self-heal if a fault occurs.

At these bitrates, the once extraordinarily
high broadcast speeds have almost
disappeared into the history books. With
progressive HD signals using 3G-SDI,
the bitrate needed is just short of 3Gbps,
and 4K requiring just short of 12Gbps.
This allows a staggering 130 progressive
HD signals to be transferred over a 400G
backbone or 30 progressive HD signals
over a 100G backbone. But it gets evenbetter as IT networks are often full duplex
thus allowing for the same number ofvideo feeds to be sent in both directions
simultaneously.

Synchronous Distribution
TDM is a synchronous method of
distributing data across network links. It
not only has the advantage of employing
fixed latencies but is predictable in termsof its capacity and signal throughput.
Unlike packet switched networks, every

Spin-leaf networks also use SDN typearchitectures but maintaining low
latencies and reducing the risk of overoversubscription within the switches
and networks is much more complex.
This is mainly due to the monitoring
methods adopted by the SDN system.
Network switches use systems such as
OpenFlow to gather monitoring statistics
on their ports including datarates, packet
loss and jitter. Although these are often
used, their accuracy is a function of theswitches ability to measure and report.

Through high-speed switching fabrics,
two network connections can have theirdata slots in their frames exchanged,
this in turn provides a routing system.
For example, if a mesh node has two
network connections NET1 and NET2,
and a video service is using TDM slot 10
in the NET1 input feed, the node will takethe data from this slot and insert it intoa free slot within the NET2 connection,
thus providing routing. This methodology
can be carried out for any type of service
as the node is switching
application agnostic
data within a time frame
between synchronous
networks.
Although the nodes areaware of each other within
the overall network, it isnormal practice to provide
a method of management
to facilitate routing andfault detection. Software
defined networking
lends itself well to thisapplication and an entire
mesh network can be built
using it.

Gathering these statistics is often therole of the SDN and it uses this data to
optimize the network, provide routing
and predict oversubscription and latency
issues. However, by virtue of the factthat the monitoring information has been
integrated, passed to the SDN servers
and then processed, the control is always
reactionary. Very occasionally predictive
systems can be used to announce thepossibility of congestion etc., but there is

Reducing Congestion
The non-hierarchical nature ofsynchronous mesh networks means that
there is less possibility of congestion.
It’s possible a network may reach its
full capacity as all the data slots within
a TDM frame could be utilized, but thisis highly predictable and well within
the control of the SDN management
software.

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always a compromise with such systems
and false positives can easily occur.

©The Broadcast Bridge

The mesh network can easily transportvideo, audio, control data and even
IP and ethernet packets. The nonhierarchical nature of the network
topology lends itself well to self-healing
systems and the synchronous time
domain multiplexing keeps latencies
predictably low and network congestionat an absolute minimum.

Comparing Leaf-Spine To MeshIf we contrast the leaf-spine and meshtopologies, it’s clear that the meshnetworks are much more predictable intheir latency and subscription capacity.
The very nature of the time domain
multiplexing means that every data
packet from an SDI, AES or control
channel, always has an allocated slot
within the TDM connection between
nodes. The SDN managing the network
knows exactly how much capacity isavailable and where the data packets arewithin their journey.

Mesh networks employing TDM work wellfor localized networks, but they can berouted through gateways to expand the
LAN to WANs. In the next article in thisseries, we look at the Eurovision SongContest as an application of TDM Mesh
Networks.

Just as an IP packet has no knowledgeof the data it is carrying and nor doesit care, the same is true for TDM meshnetworks. As far as the transport stream
is concerned, the data slots are justtransporting application agnostic datafrom one node to another. It’s mostprobable that the higher level SDN
software is aware of the data service, forexample an HD video service, and thisservice autonomy is what makes TDM
incredibly flexible for broadcasters.

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Core Insights: TDM Mesh Networks - A Simple Alternative To Leaf-Spine ST2110©The Broadcast Bridge

Part 3 - Applications –Eurovision Song Contest
With over 4000 signals to distribute,transfer and route, the EurovisionSong Contest (ESC) proved to be thisyear’s showpiece for Riedel’s TDM
based distributed mesh networked
system MediorNet. Understanding
the intricacies of such an event is
key to realizing why TDM is such a
powerful solution.

ESC is not only renowned for its
showcase glitz and glamour, but also for
its technical excellence and for pushing
the boundaries. First broadcast in 1956
to push broadcast technology to its
limits, ESC helped align many European
countries so they could exchange live
video and audio signals, a solution
unique for its time.

Video signals are known for using highbandwidth and bulky cables. It doesn’ttake too many video feeds for a bundleof cables to become bulky and heavy.
Then we add the audio; although the
bandwidths are much lower than video,
the cables are equally as bulky. The
network connections come next forthe myriad of computers, OCPs, andcontrol and monitoring units. And mostimportantly, the audio and control cables
for the intercom, without which, nobody
would be able to work.

If instead of running hundreds of video,
audio and network cables, single fibers
could be used, many of the problemsof rigging would be instantly fixed.
Complexity, resilience, and costs would
be fixed in an instant. In effect, this iswhat Riedel’s MediorNet has achieved.
In the previous article in this series, we
discussed the benefits of mesh networks
and how they assist broadcasters inlocalized network settings. Ad hoc
auditorium events are generally selfcontained with either a production crewon site or the camera and audio sources
being streamed back to a centralized
control room.

Running these cables throughout acustom-built TV station is a difficult andtime-consuming at the best of times,
especially when we start running cables
through the elevator conduits to move
between floors. But running this numberof cables through an auditorium with very
little time is a daunting task. It wouldn’t
be so bad if the TV riggers and engineers
were working in isolation, but the ESC
required a multitude of crews to create
the sets, rig the lights and configure
the auditorium, and that’s before theproduction teams arrived.

Signal Requirements
This year’s ESC production in the Ahoy
Arena, Netherlands, needed 24 cameras,
140 intercom panels, 75 microphones,
5 mobile units, a technical operations
center, 3 support vehicles, and 60 EVS
channels. A total of 130 video feeds wererequired. If we just look at the cameras
alone, each camera will need multiple
return feeds for the camera operator,
prompter feeds, tally, intercom, andprogram audio. Although most of thesePushing Technical Boundaries

completely flexible system is needed.

signals will be presented to the camera’s
CCU they still need to be routed to it.

The major advantage of the distributed
network is that extra feeds, receivers
and monitoring streams can be added
anywhere within the network at any time.
Admittedly, enough capacity will need
to be provisioned into the infrastructure,but even if it’s not then a single fiberand node can be installed with minimal
interruption. Adding an extra video
service requires connecting the physicaldevice to the node and then configuring iton the software manager.

With each MediorNet device acting as
both a node and signal interface, network
fiber cables provided the connectivity
between them. This facilitated theconnections for SDI video, AES audio,
control, tally, intercom and computerconnectivity into the network. The
management software not only provided
an instantaneous health check of theentire network, but also allowed ad-hoc
configuration after the initial installation.

Changing Production Requirements

Despite the resurgence in streaming
video and audio over IP, much of
the equipment currently available inbroadcasting still uses traditional SDI
and AES interfacing. Although interface
units to facilitate connection to IP exist,
configuring full ST-2110 systems for
ad-hoc events is time consuming andcomplex.

Anybody who has worked on any outside
broadcast, no matter how well planned,
will know that things always change after
the installation has been completed.
Extra monitoring positions, intercom
panels, and computer points are always
needed. And if we factor into this theforty countries contributing and receiving
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Core Insights: TDM Mesh Networks - A Simple Alternative To Leaf-Spine ST2110©The Broadcast Bridge

SFPs for bi-directional gateways from
MADI to ST-2110.

Aggregating Signals
The Riedel MediorNet system fixes thisby providing SDI and AES connectivity
into the TDM distribution system. Large
capacity fibers with up to 400Gbps
connectivity, easily facilitate the SDI and
AES integration. Once the feed is in the
network, the software interface is used
to route the signals anywhere within the
network. The simplicity of this cannot be
overstated as the network autoconfigures
to make the contributing signals available
anywhere on the network.

Monitoring is always a challenge forad-hoc events, and this was no different
at ESC. With such a complex network
containing in excess of fifty nodes
and interface units, knowing where asystem is experiencing problems iscritical. Cabling in outside broadcast
type events is always more difficult due
to the temporary rigging that increases
the susceptibility to damage. Being able
to detect cable faults and automatically
re-route signals is critical for today’s
broadcasters, especially as we consider
the ongoing effects of Covid-19.

Commentators often require last minutefeeds and running cables to them at an
event such as the ESC is impractical andpotentially dangerous. With adequate
cabling infrastructure, and with a fullknowledge of all the network, theReliable Low Latency Delivery
As the TDM is synchronous and data
packets are
multiplexed into
well-definedtimeslots, latency
becomes less of
an issue as there
is no contest on
the network andtherefore buffer
sizes can be kept
to a minimum. The
distance between
nodes is wellknown and theuse of fiber means
the transportdelay is insignificant. Consequently, thewhole system delivers low latency withincredibly high levels of reliability.

software management system was used
to route new video, audio and evencomms circuits to the commentators.
The commentary subsystem of the TDM
infrastructure was complemented by an
IP-based network built on the MediorNet
IP product range. Inside the commentary
control room, the FusioN standalone IPprocessing devices provided 16-PiP ST2110 multiviewers, while the commentarybooths relied on Riedel’s intelligent MuoN

Delivering to over 183 million viewers
across 36 markets demands the ultimate
in resilience and low latency. Advanced
monitoring methods and fault detection
meant the MediorNet was the perfectsolution for an ad-hoc event such as theEurovision Song Contest.

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This Core Insights document was written by the Editorial
team at The Broadcast Bridge in collaboration with oursponsor Riedel Communications.
The Broadcast Bridge publishes hundreds of freearticles like this every year. Subscribe online andcreate your own personalised home page andemail feed.

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