Image Processing Reference
In-Depth Information
layer protocols. An intelligent bridge-like device is responsible for both translating the
different framing rules as well as interoperation of the different MAC protocols.
•
Wireless gateway approach
: In this approach integration happens at the application layer
or even in the application itself. Entirely different protocols can be used on the different
media types.
•
Some mixture of these approaches.
Any of these approaches require special
coupling devices
at the media boundaries. For the wire-
less cable-replacement and the MAC-unaware bridging approach, these devices can be simple. he
other approaches may require complex and stateful operations. Hence, the issues of failures and
redundancy need to be addressed.
24.2.4 Mobility Support
The potential station mobility is one of the main attractions of wireless systems. We can assume that
wireless fieldbus systems will be mostly infrastructure-based (meaning that there are base stations or
access points [APs]). A
handover
must be performed when a mobile station moves from the range
of one AP into the range of another AP. Typically, handover processes involve exchange of signaling
packets between the mobile and the APs. Ideally, a station can fulfill timing and reliability require-
ments even during a handover. he applicability and performance of handover schemes depend on
the maximum speed of a mobile station. In industrial applications, it is typically forklifts, robots, or
moving plant subsystems which are mobile, and it is safe to assume that these devices move with
speed of no more than km/h [].
A simple consequence of mobility is that stations may enter and leave a network at unforeseeable
times. To support this, a protocol stack at minimum must offer functionalities to make a new station
known to the network/the other stations and sometimes also address assignment is needed. On the
other hand, fieldbus systems and their applications often are designed with the assumption that the
network is set up once and not changed afterward. Consequently, some fieldbus systems do not sup-
port any dynamics in their set of stations. Consider, for example, the FIP/WorldFIP fieldbus [].
This system belongs to the class of real-time database systems and the producers of data items (called
process variables) are polled by a dedicated central station, the
bus arbiter
. The bus arbiter keeps a
table of variable identifiers and traverses it cyclically. To include a new station into the system, the
arbiters table has to be modified by a human operator. It is worth noting that the most widely used
fieldbus systems do not offer any support for dynamic address assignment.
24.2.5 Security Aspects and Coexistence
Security played no important role in the initial design of the fieldbus standards. his was reasonable,
because physical access to a wire is needed to eavesdrop or inject packets. However, the introduc-
tion of wireless media allows an attacker to eavesdrop packets at some distance, for example on the
factories parking lot. Even worse, an attacker could generate interference on the operating frequency
of a wireless fieldbus system and distort all transmissions (including the time-critical and important
ones). An attacker might also try to inject malicious packets into the network, for example, false valve
commands. herefore, security measures (integrity, authentication, authorization) have to be added
to wireless fieldbus systems [].
Noise and interference is not only generated purposely by some attacker, but can also come from
colocated wireless systems working in the same frequency band. As an example, both IEEE .
and Bluetooth use the . GHz ISM band and create mutual interference. his coexistence problem
is explored in [].
