Campbell Scientific PakBus Networking Guide - Manual (Page 12)

PakBus Networking Guide
3.6 Concurrent Communication
The packets in PakBus networking are small and the communication links are
only active for a short period of time while the packet is being transmitted.
Longer messages can be broken into several smaller packets and sent
individually between devices. Since these packets do not require a dedicated
link, other related or unrelated packets can be sent over the same link at
approximately the same time.
In addition, individual packets contain the necessary information to facilitate
communication. Therefore, PakBus devices can simultaneously send and
receive packets to and from different nodes on the network. For example, two
researchers in different locations can collect data from a single datalogger
simultaneously over the same PakBus connection (if permitted by the physical
connection itself).
3.7 Keeping Track of PakBus Devices
Before devices acknowledge each other as neighbors, they must successfully
complete a hello exchange and periodically communicate with each other. For
communication between the devices to continue, they must keep an accurate
list of all known neighbor routes. A healthy PakBus network continually
updates the neighbor routes information between nodes. Once devices
establish each other as neighbors with a hello exchange, they expect to hear
from each other within a calculated verify interval.
A verify interval can be entered in the datalogger for a specific link. If the
verify interval is set to zero, the verify interval becomes 2.5 times the beacon
interval. However, if both the beacon interval and verify interval are set to
zero, the default verify interval for a device becomes 300 seconds.
During the hello exchange, neighbors pass each other their verify interval
information. A calculated verify interval is negotiated between the neighbors
from this information. The negotiated interval is the lesser of either the
neighbor node’s verify interval or 6/5 of the first node’s verify interval. The
calculated verify interval is used to determine how often neighbors must
communicate with each other to keep the link active.
A good practice is to set the verify interval to a value longer than the shortest
expected interval of communication inactivity. With that setting, regular
communication between devices is sufficient to preserve their status as
neighbor devices without causing the verify interval to expire.
If a node fails to hear from a neighbor and the calculated verify interval
expires, an attempt will be made to re-establish the connection. The device
will issue up to four hello messages, 2 to 12 seconds apart, directed at the
neighbor node in an attempt to make contact. If communication is still
unsuccessful after four hello message retries, the neighbor is removed from the
list of know routes in the device. The neighbor will only be added to the list of
known routes in the device after another successful hello exchange.
Devices set as routers must keep accurate lists of not only neighbors but also
other routers in the network. Routers use the same verification process as
neighbors when adding and removing other routers from their list. In addition,
when the list of known routes in a router on the network changes by having a
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