wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

41-6 Industrial Communication Systems
• Acknowledged service
Each destination node returns an acknowledge message to the source to confirm the transmission.
If it is missing, the transmission will be repeated after a certain time period. Missing acknowledges
from individual group members will be inquired by resending the message accompanied
with special reminder messages. Duplicated messages are recognized by a transaction ID contained
in each packet. This service can be used for unicast and multicast packets.
• Unacknowledged repeated service
If acknowledge messages should be avoided, especially group messages can create many of them,
and additionally the probability of unrecognized message losses should be lowered, the unacknowledged
repeated transmission can be used. This service will send the message consecutively
several times. The number of repetitions is configured by a retry count property. As in
the acknowledged service, transaction IDs avoid the duplicate processing of messages. Also, this
service can be used for unicast and multicast packets.
The session layer offers a request/response service. This service is used to execute actions, which return
data to the sender, such as most of the network management messages. This data, contained in the
response packet, consists of a success or fail code and the requested data. Like in acknowledged service,
requests will be repeated if the response was not received for a certain time. This request/response service
is used alternatively to the services of the transport layer.
To avoid unauthorized execution of commands and requests, the authentication service can be
used in addition to the acknowledged and request/response service. The authentication service uses
a challenge–response authentication method to test the authorization of the sender. Therefore, each
LonWorks node in the network has the same shared secret key, set in the configuration phase during
the assignment of the node’s logical address (domain, subnet, and node ID). After receiving a message,
the receiver transmits a random number (the challenge) to the initial sender, who encrypts it
with his secret key and sends it back (the response). The receiver compares the returned value with the
value he has calculated locally. If they match, the authorization is confirmed and the initially received
command or request is executed.
In LonWorks, a 48 bit secret key is used to encrypt a 64 bit random number resulting in a 64 bit
encrypted value. Since the encryption algorithm is not published, its quality cannot be evaluated.
Moreover, a 48 bit key is not strong enough for brute force attacks on high-bandwidth channels. Another
weakness is the distribution of the shared secret key, which has to be sent in unencrypted network management
messages over the unsecure bus.
Authentication can be used for all network management transactions and can be activated for each
network variable connection, too.
41.3.5 application and Presentation Layer
The application and presentation layers form a unit in LonWorks. They offer services to support the
application on the, one hand and to execute network management and diagnostic functions, on the other
hand. These services and functions are
• Network variable propagation
Network variables are basic communication objects, which define the logical datapoints of the
application. Input network variables receive values from the network and output network variables
send values to the network. The connections between output and input variables are realized by
bindings, which are configured by setting special table entries in the source node via a system
integration tool. Whenever the value of an output network variable is changed, these table entries
are used for sending a network variable propagation message to those nodes, which contain the
bound input variables. According to these messages, the bound nodes update the values of their
input variables.
© 2011 by Taylor and Francis Group, LLC