Image Processing Reference
In-Depth Information
approach. Hence, chances are that it will be more and more adopted in the next years as the wireless
solution of choice for many real-world industrial plants.
26.2.4 IEEE 802.15.4
The IEEE .. standard deals with
ad hoc
wireless interconnections of electronic devices within
limited ranges (some tens of meters) at low data rates (up to kb/s). he standard potentially targets
several application fields including, for example, home and building automation applications, smart
tags communications, cable replacement, and automotive sensing. Devices compliant with the IEEE
.. standard typically operate in the industrial, scientific, and medical (ISM) band, although a
second band is available as well, depending on the country considered, but not widely adopted due
to its limited data rate ( Kb/s).
The IEEE .. standard specifies two medium access protocols, namely, the beacon-enabled
MAC protocol, characterized by the presence of a “network coordinator,” and the non-beacon MAC
protocol. In the beacon-enabled mode, the network coordinator periodically issues “superframes”
that are divided into two parts. The first part, called contention access period (CAP), takes place
immediately after the beacon and is based on a distributed CSMA/CA mechanism that handles the
access to the channel. After the CAP, there might be an optional contention free period, in which
guaranteed time slots (GTSs) are exclusively allocated to the nodes. Conversely, in the non-beacon
mode a fully distributed channel access method is realized by means of a pure CSMA/CA medium
access technique.
26.3 Implementation of Hybrid Networks
From a general point of view, different (and often dissimilar) communication networks can be
interconnected through specific devices called intermediate systems (ISs) []. These devices have
different structure, functionality, and complexity, according to the layer of the OSI reference model
they refer to. In the following, some practical implementations of these devices are described, along
with their features and related issues.
26.3.1 Interconnections at the Physical Layer
The simplest forms of ISs are “repeaters.” They operate at the physical layer and are used to inter-
connect subnetworks that share the same MAC mechanism. Repeaters are usually adopted for
regenerating physical signals (electrical, optical, etc.) flowing across different network segments. In
Ethernet networks, for example, they operate according to the so-called “-R principle”: re-shaping,
re-timing, and re-transmitting. In this way, it is possible to face the problem of signal attenuation
over the cable when the network extension grows larger. Moreover, they enable the number of nodes
that can be networked—which is practically limited by the current drained by devices attached to the
bus—to be increased, and achieve segmented network topologies in addition to the plain linear bus.
Besides simpler two-ported devices, a special kind of repeater exists (called “hub”) that is provided
withseveralportsandcanbeadoptedtosetupnetworksbasedonstartopologies.Becauseofthe
increased reliability (a defective cable no longer affects the whole network operation, hence avoid-
ing partitioning faults), hubs are particularly suitable for deploying network infrastructures and for
the cabling of buildings. More advanced versions of repeaters may sometimes be employed to inter-
connect subnetworks that rely on different media (e.g., copper wires and fiber optics) or signaling
techniques (e.g., voltage/current levels).
When network segments are connected through repeaters, the MAC mechanism that regulates the
accesstothesharedtransmissionmediumisexactlythesameonthewholenetwork.hus,carehas
