Image Processing Reference
In-Depth Information
partly be confined to individual areas of the network. his, however, requires caution in the planning
phase of the network when defining communication relations between end nodes. Nodes exchanging
lots of information should be kept in common subnets to avoid congestion on the backbone links.
Tree structures are very common in building automation networks like EIB, LONworks, or BACnet.
Some fieldbus systems permit “free topologies,” where nodes can be connected in a multi-point
fashion without restrictions. Essentially, this is a variant of the line topology because the nodes
areelectricallyinterconnected.Contrarytothestandardlinetopology,however,therearenostrin-
gent limitations concerning line length, the length of stubs, or branches that would cause severe
signal degradation. This makes cabling very convenient for the installer, but poses a substantial
challenge for the signal processing in the nodes and their transceivers. Fieldbus systems using such
free topologies are, e.g., LONworks (where the free topology transceiver is the most widely used
because of its robustness even in cases where conventional RS transceivers would be sufficient),
SERIPLEX, or ASI, which was designed for utmost simplicity and ease of handling in harsh industrial
environments.
“Mesh” networks, where multiple paths through the network exist, play only a subordinate role in
fieldbus systems because they require appropriate routing strategies to keep messages from circling in
the network and causing congestion. LONworks and P-NET offer the possibility for building meshes.
The general topology used by the fieldbus also influences the way access to the medium is being
handled. As a matter of fact, topologies were often selected according to the desired MAC method,
or vice versa.
20.5.4 Medium Access Control
Fieldbus designers have been very creative in what concerns allocation of the available communica-
tion bandwidth to the individual nodes. On the data link layer, virtually all medium access principles
also known from LANs are used, plus many different subtypes, refinements, and hybrid solutions
(Figure .). .).This has to be understood against the backdrop of desired efficiency. Resources both
MAC
protocols
FDMA
CDMA
TDMA
SDMA
Token
passing
Random
access
Polling
Time-slot-based
p
-persist.
CDMA
Node-based
Message-based
Explicit
Implicit
Centralized
Decentralized
CDMA/CA
CDMA/CD
CAN and
related
(SDS,
DeviceNet,
CANopen)
FlexRay
EHS
Batibus
EIB/KNX
PROFIBUS-DP
PROFIBUS-PA
P-NET
BITBUS
MODBUS
AS-i
HART
WorldFIP
PROFIBUS-
FMS
P-NET
ControlNet
INTERBUS
SERCOS
TTP/A
TTP/C
FlexRay
ARINC 629
LonWorks
FIGURE .
MAC strategies in fieldbus systems and examples.
