MQTT is becoming a popular protocol for
Industrial IoT (Internet of Things) data.
Developed for connecting remote
devices to a central server, it is lightweight, efficient,
and secure. However, IoT implementations
are growing larger and more complex, and demand
is increasing for OT/IT connectivity. MQTT is now
being called on to aggregate and send diverse collections
of data values over increasingly complex
network topologies.
To meet these challenges MQTT must get
smarter. As a transport protocol, MQTT specifies
that messages are simply carried—not read—like
a letter in the post. But that doesn’t have to be
the case. What would happen if the letter carrier
could read the mail? In other words, what if we
gave an MQTT broker the ability to parse the
messages it carries? It would be able to handle
messages more intelligently and include some
information on the status of the data source or
quality of the connection.
Data collection
A smart broker should be able to collect data
in an intelligent way. For example, on large
systems data can come from a wide variety
of MQTT devices, each with its own message
format. A broker that parses messages could
convert these to a common message representation
and make that available to all clients.
Other data sources might include non-MQTT
protocols such as OPC UA, Modbus, DDE, and
others. A smart broker with protocol conversion
capabilities could act as a gateway for this
data to any MQTT client or cloud ser vice.
Data consistency
In real-time industrial systems, data consistency
from source to consumer is vital. Data that’s
stale or out of correct time sequence can lead to
incorrect decisions. Any disconnects or network
irregularities must be known.
Data can become inconsistent in several ways.
If messages arrive at an MQTT broker faster than
they can be delivered, some may be dropped. Or
data from multiple message streams may get sent
to a client out of sequence. Also, if a data source
goes offline, the client may not know whether an
unchanged value is current or stale.
A smart broker can ensure data consistency
by queueing incoming data in an intelligent way,
passing on only the latest values. It can also
parse timestamps on messages from different
data streams to sequence them properly, as well
as pass along data and connection quality information
with each value update.
Data security
Security is critical when accessing data from a
production system. The MQTT push architecture
that connects outbound through firewalls is
quite secure, but many corporate security policies
require isolating OT systems using a DMZ. This is
problematic for MQTT since messages must be
passed via two or more servers, while MQTT quality
of service guarantees are only valid for a single
sender-receiver hop. As a result, data at the end
of a multi-hop daisy chain can become unreliable.
A smart broker that parses messages and
converts protocols can solve this problem by
using a tunnel. The device producing the MQTT
data would connect to one instance of the smart
broker. The message data, along with quality
and timestamp information, gets tunnelled via
a secure, TCP-enabled protocol to a second
instance of the smart broker. That instance would
convert the data back into MQTT, with values,
timestamps, and quality codes intact.
