Image Processing Reference
In-Depth Information
Control system
S
Nonrouting
device
R
Routing
device
G
Backbone
router
S
S
Nonrouting
device
Routing
device
R
M
G
Routing
device
S
R
G
Routing
device
Gateway and
manager
Backbone
router
S
Nonrouting
device
Gateway
Sensor
Router
Sensor with router
G
S
R
S
FIGURE .
ISA.a network.
that UDP is an unreliable delivery service. Delivery reliability for a DLL subnet is handled in the DLL,
whereasreliabledeliveryonthebackbonenetworkishandledbythebackboneprotocol,whichisnot
within scope of the standard. The transport layer also contains options for securing the delivery of
packets, e.g., encryption, authentication, integrity, etc.
.... Basic Functionality
As WirelessHART, ISA.a is a TDMA, frequency hopping, mesh network. In addition, it supports
theIPvprotocol,andtheUDPtransportprotocol(throughtheuseofloWPAN).hus,alldevices
in an ISA.a network have an IPv address, which makes them globally visible within the plant.
In an ISA.a network, there are several different kinds of devices, as it can be seen in
Figure .. ISA.a supports both “routing” and “nonrouting” devices, as well as a special kind
of routing devices, “backbone” routers, which utilize the backbone in order to transport messages
within, or between, DLL subnets.
The DLL provides time synchronization, frequency hopping, mesh networking, routing, within a
DLL subnet. A “time slot” in ISA.a can vary between and ms. his flexibility allows the use
of “duo-cast,” which is a procedure in which each message is sent to two receivers, who both answer
with an ACK on successful reception. Even if one transmission fails, the other might succeed, which
increases the communication reliability of the network.
Frequencyhoppingcanbeeither“slotted,”i.e.,thefrequencyischangedforeveryslotor“slow”
i.e., the frequency is only changed when several sequential slots have passed. Slow hopping relaxes
the time synchronization requirements for devices, allowing low-cost clocks and crystals to be used.
ISA.a DLL subnets can have many topological shapes: mesh, star, or hybrid (a mix of both),
and the network as a whole also incorporates the use of a backbone network. he Gateway/Manager
