ABB AWIN GW120, User Manual

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TECHNO LOGY OVERVIEW NETWOR K MANAGER
4JNO0 0030 8 A 13
Burst:
Message 1: Field device publishes pre-configured message data such as a primary variable or
loop current to the Host on a fixed interval.
From this example it is obvious that, the Burst method results in about half the network traf-
fic, freeing the network to take care of other tasks such as keeping track of device configura-
tion changes, communicating with joining devices, searching for lost devices, or gathering
statistics on the health and stability of the network. In most cases, an efficient network will
use a mixture of both communication methods.
The WirelessHART access point can send messages over the WirelessHART network using its
IEEE 802.15.4-compliant radio transceiver. WirelessHART uses the first 15 channels of the
IEEE 802.15.4 standard for communication and efficiently avoids interference and maximizes
the bandwidth in the 2.4 GHz spectrum by continuously frequency hopping between chan-
nels according to a frequency hop sequence. This should not be confused with message
hops, which refer to the paths that messages take between devices in a mesh network.
2.5 Network Manager
The WirelessHART network bandwidth is allocated by the network manager. The network
manager assigns Absolute Slot Numbers (ASN), or message slots, to field devices wishing to
publish burst messages over the network. Devices that have not been allocated bandwidth
on the network are not able to communicate with the gateway and are not able to publish
messages on the network. The network manager is responsible for the “joining” of field de-
vices, allocation of network bandwidth, managing network security, maintaining a list of net-
work devices, and providing communication paths through the mesh network.
The network manager is responsible for broadcasting the existence of the network to pro-
spective field devices and instructs devices already joined to the network to broadcast on its
behalf. Devices may discover the network via an advertisement from any other device already
on the network. This advertisement contains a network ID that can be used to provide a
unique distinction between otherwise overlapping networks. Only devices that have been
programmed to join the network will respond to advertisements of the network ID.
Once an advertisement is received, the device may request to join the network by responding
with the correct join key of the network. The join key is a 32-character, hexadecimal string
that prevents unauthorized devices from joining the network. The network manager rejects
device join attempts by any devices not providing the correct join key.
2.5.1 Network ID and Join Key
After the network manager validates the network ID and join key of the requesting device,
the device is allocated with a session key to use it for communicating with the network man-
ager. The session key may be a static key or it may be periodically changed or “rotated” by
the network manager as an increased security measure. Only devices which have been allo-
cated communication bandwidth and have valid session keys can communicate on the net-
work. To further increase security, each field device is given a unique session key such that
only the network manager can understand and decrypt the data from the field device.
2.5.2 Message Hops
The network forms around the access point, and all devices which can communicate directly
with the access point are described as being one “hop” from the access point. Other devices
may join the network as long as they can communicate with at least one other device that is