Image Processing Reference
In-Depth Information
Application
program(s)
Application
program(s)
Application
Application
Wireless
extension
Transport
Transport
Data-link protocol converter
Network
Network
Frame
Transceivers
Frame
Data-link
Data-link
Data-link
Data-link
Physical
Physical
Physical
Physical
Bridge
(Access point)
Wired
network
Wired
device
Wired
device
Wireless
device
FIGURE .
Interconnection of networks via a bridge.
as well as the -byte addressing scheme. On the contrary, frame formats and MTUs differ. For the
sake of truth, it is worth remembering that APs actually have a twofold role: they are used both to
enable communication in an infrastructure WiFi network (also known as basic service set [BSS]), by
relaying frames among wireless stations, and to interconnect the BSS to an existing Ethernet backbone
(“portal” function).
26.3.3 Interconnections at Higher Protocol Layers
For ISs working at the network layer or above, the generic term “gateway” is often adopted. A gateway
is responsible for transferring protocol data units (PDUs), as well as for converting generic streams
of information and application services between application processes executing in the nodes of the
interconnected systems, by performing all required protocol translations. From a practical point of
view, there is no particular restriction on the kinds of networks that can be interconnected through
gateways. On the other hand, gateways are usually complex, expensive devices, often developed
ad hoc for the considered systems by means of suitable software modules, and the performance they
achieve are to several extents lower than bridges. Generally speaking, noticeably higher latencies have
to be expected in this case, which could result incompatible with timings requirements of control
applications.
ISs that operate specifically at the network layer are known as “routers.” At this level, a uniform
addressing scheme and protocol are used (i.e., the Internet protocol [IP]), which ensures worldwide
connectivity irrespective of the physical media, MAC mechanisms, and data-link services of the inter-
connected subnetworks. In this chapter, routers are not specifically addressed, as most industrial
networks are deployed as local networks. However, when timing constraints are not very tight, inter-
connection techniques that rely on routers allow devices, which are natively enabled to communicate
over local intranets and/or the Internet, to be directly embedded in the control system (this is the case
of remote monitoring and maintenance tasks).
Whentheinterconnectiontakesplaceattheapplicationlayer,theISshouldtakecareofthecon-
version of high-level services. Usually, this implies the gateway has to carry out non-trivial operations
for gathering and converting information which, in turn, often requires a number of simpler com-
munication services to be invoked on the interconnected networks. In these cases, the term “proxy”
is sometimes adopted. Indeed, a proxy is much more than an application-layer gateway: it usually
makes some parts of the network (e.g., a whole subnetwork) look like as if they were a single node,
possibly hiding the underlying structure for security or performance reasons. Sometimes, some form
 
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