A Horizontal Taxonomy for the Industrial IoT
variation or “jitter”. Even a fast server with low average latency can experience large jitter under
load.
In a distributed system,
the most important
architectural impact is
the
potential
jitter
imposed by a server or
broker in the data path.
An architecture that can
satisfy a human user
Figure 4: Added Server Latency
annoyed by a wait longer
Although the hardware transmit time is often negligible, sending data
than 8 seconds for a
through a server “hop” requires traversing the sending machine’s transmit
website will never satisfy
stack, the server’s receive stack, the server’s processing queue, the server’s
an industrial control that
transmit stack, and finally the destination’s receive stack. Each of these has
threads, queues, and buffers that add uncontrolled latency. Worse, the server
must respond in 2ms.
cannot easily prioritize traffic as easily as the endpoints. Thus, systems that
We find the “knee in the
are sensitive to maximum latency often cannot use data servers.
curve” that greatly
impacts design occurs
when the speed of response is measured in a few tens of milliseconds (ms) or even microseconds
(µs). We choose 100ms, simply because that is about the unpredictable delay of today’s servers.
Systems that much respond faster than this usually must be peer-to-peer, and that is a huge
architectural impact.
Figure 5: IIoT Real-Time Applications
To provide quality feel to surgeons, medical robotics distributed control loops must run at rates up to 3 kHz and
control the “jitter” to only tens of microseconds. Similarly, autonomous cars must react to fast enough to safely
control the vehicle and prevent collisions. These fundamental performance needs imply a system architecture
that does not send data through intermediaries.
IIC Journal of Innovation
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