Image Processing Reference
In-Depth Information
network and control requirements, the selection of the best sampling period is a compromise. More
details on the performance computation and analysis of points A, B, and C in Figure . can be
found in Ref. [], including simulation and experimental results that validate the overall shape of
the chart.
23.3 Differentiation of Industrial Networks
Networks can be differentiated either by their protocol (at any or all levels of the ISO-OSI seven-layer
reference model []) or by their primary function (control, diagnostics, and safety). hese dimen-
sions of differentiation are somewhat related. In this section, we first define how network protocols
are categorized technically with respect to timing, and then discuss the different types of protocols
that are commonly used in industrial networks. In Section ., we describe how these different types
of networks are used for different functions.
23.3.1 Categorization of Networks
When evaluating the network QoS parameters associated with timeliness, determinism, etc., the pro-
tocol functionality at the data link layer is the primary differentiator among network protocol types.
Specifically, the MAC sublayer protocol within the data link layer describes the protocol for obtaining
access to the network. he MAC sublayer thus is responsible for satisfying the time-critical/real-time
response requirement over the network and for the quality and reliability of the communication
between network nodes []. The discussion, categorization, and comparison in this section thus
focus on the MAC sublayer protocols.
There are three main types of medium access control used in control networks: time-division
multiplexing (such as MS or token-passing [TP]), RA with retransmission when collisions occur
(e.g., Ethernet and most wireless mechanisms), and RA with prioritization for collision arbitration
(e.g., CAN). Implementations can be hybrids of these types; for example, switched Ethernet com-
bines time-division multiplexed (TDM) and RA. Note that, regardless of the MAC mechanism, most
network protocols support some form of MS communication at the application level; however, this
appearance of TDM at the application level does not necessarily imply the same type of parallel oper-
ation at the MAC level. Within each of these three MAC categories, there are numerous network
protocols that have been defined and used.
A recent survey of the types of control networks used in industry shows a wide variety of networks
in use; see Table . and also Refs. [,,]. [,,].The networks are classified according to type: RA with
CD, CA, or arbitration on message priority (AMP); or TDM using TP or MS.
TABLE . Most Popular Fieldbuses
Network Type Users (%) Max. Speed Max. Devices
Ethernet TCP/IP RA/CD   Gb/s 
Modbus TDM/MS   Mb/s 
DeviceNet RA/AMP   kb/s 
ControlNet TDM/TP   Mb/s 
WiFi (IEEE .b) RA/CA   Mb/s not specified
Modbus TCP TDM/MS   Gb/s 
PROFIBUS-DP TDM/MS and TP   Mb/s 
AS-I TDM/MS   kb/s 
Source: Grid Connect. The Grid Connect Fieldbus comparison chart. http://www.
synergetic.com/compare.htm; Montague, J., Control Eng ., (), .
Note: The maximum speed depends on the physical layer, not the application-level
protocol. Note that the totals are more than % because most companies use more
than one type of bus.
 
Search WWH ::




Custom Search