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

LonWorks 41-5
in LonWorks. In p-persistent CSMA, the bus is checked for activity before the frame is transmitted.
As it becomes idle, the sender can start to transmit its frames. To avoid collisions, there are n time slots,
from which each sender chooses one with a probability of p = 1/n. In LonTalk, the default value of n is
16. Each sender will start to transmit its frame in its chosen time slot as long as the bus is idle. Thus, the
sender who takes the earliest time slot will transmit its frame and all others have to wait for the next idle
phase. Collisions can only occur when two or more senders have chosen the same time slot. To lower the
probability of collisions, the number of time slots n is predictively increased in intervals of 16 according
to the estimated channel backlog, which is the estimated number of return messages.
Optionally, the first time slots can be reserved for priority messages. The number of priority time slots
is specified in each node and should be the same for all nodes on a channel to prevent collisions with nonpriority
slots. Each node, which wants to send priority messages, gets its dedicated priority slot, whose
number is configured in the node. So when n p priority slots are reserved, n p nodes can send priority
messages. The remaining time slots are used for non-priority frames as mentioned above. So, priority messages
always suppress the other messages, which can theoretically result in starvation of the non-priority
frames. But this would require permanently high network traffic, which is atypical for a well-configured
network. To prioritize the complete message transaction, priority messages are sent with a set priority bit
in the frame. This signals routers to forward messages and receivers to send the responses prioritized, too.
Optionally, the Neuron Chip supports the processing of collision detections, which results in canceling
and retransmitting of the frame. But the transceiver has to detect the collision and signal it to the
Neuron Chip, which is not supported by all transceivers.
The link layer has to ensure that frames are transmitted correctly. Therefore, a 16 bit cyclic redundancy
check (CRC) sum is appended. Additionally, the frame contains the preamble, the priority bit,
the change of the backlog, and the alternate path bit. A set alternate path bit is an indicator for the transceiver
to send the frame via an alternate channel. This increases the reliability on unreliable media, such
as power line and radio frequency. By default, the bit is cleared, but based on the node configuration,
the bit can be set automatically for the last two transmission attempts. For example, using a power line
transceiver, a set alternate path bit implicates that the modulation method will be changed and the data
transfer rate will be decreased to increase the signal quality.
41.3.3 Network Layer
The network layer offers connectionless, unacknowledged transmissions of packets within a domain.
Following services are supported:
• Unicast
The destination of the packet is one single node. It can be addressed by the unique Neuron ID or
by the logical address consisting of domain, subnet, and node ID.
• Multicast
The destinations of the packet are a group of nodes defined by its group ID.
• Broadcast
The destinations of the packet can be all nodes belonging to a specific domain or all nodes belonging
to a specific subnet.
41.3.4 transport and Session Layer
The network layer does not handle the repeated transmission of lost packets or the detection of packet duplicates.
This is done by the transport layer and the session layer. The transport layer offers the following services:
• Unacknowledged service
The packet is sent exactly one time. Since there are no acknowledgments, a loss of the message
cannot be recognized. This service is usually used for broadcasts but can be used for unicast and
multicast packets as well.
© 2011 by Taylor and Francis Group, LLC