Image Processing Reference
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to be taken about the increased propagation delays. In many cases, such as for example in half-duplex
Ethernet, CAN, or Profibus networks, this leads to limitations on the maximum number of repeaters
that can be inserted between any two nodes or the maximum cable length for every segment.
Pure repeaters are quite unusual in hybrid wired/wireless networks, as the resulting physical com-
munication support in most practical cases is not seen as a sufficiently uniform medium by the MAC
protocol. This is because the signaling schemes and physical transmission techniques adopted by
wireless networks are, typically, rather different from wired solutions, which means some form of
non-trivial buffering has to be carried out by ISs.
An interesting example of hybrid wired/wireless interconnection carried out at the lowest layer of
the protocol stack (i.e., below the MAC) is provided in Ref. []. In that case, the authors refer to a
hybrid architecture between Profibus and Radio Fieldbus [] that relies on “cut-through” forwarding
devices located between wired segments and the wireless medium. Both networks use the same data-
link layer protocol—specifically, the FDL as defined by Profibus, including its access scheme based on
token-passing—but rely on completely different physical layers. Indeed, Profibus is based on the well-
known RS standard, whereas Radio Fieldbus uses a direct sequence spread spectrum technique
similar to the one adopted in IEEE . WLANs.
26.3.2 Interconnections at the Data-Link Layer
An IS operatingat thedata-linklayer iscalled“bridge.” Itsmain purposeistransferringframesbetween
systems that are not (or cannot be) treated as a uniform communication support by protocols at the
MAC level (e.g., switched LANs that are made up of several separate collision domains). Such devices
usually operate according to the “store & forward” principle, that is, when a frame is completely
received on one port of the bridge, it is forwarded (possibly, according to a selective strategy based on
adaptive message filtering) to the other subnetwork(s) through the related port(s). As an alternative,
forwarding may start as soon as a sufficient amount of information is received about the incoming
frame (e.g., cut-through bridges). Even though featuring shorter transmission latencies, in this latter
case erroneous frames are not blocked by the bridge, hence reducing the net available bandwidth.
It is worth noting that, thanks to this decoupling mechanism, physical signaling, transmission
speeds, medium access techniques, and even the frame formats might differ in the interconnected
networks. A bridge, which is equipped with more than ports, is commonly referred to as a “switch”
(this kind of device is currently present in almost every real-life Ethernet network).
Even though the MAC mechanism is not required to be the same for the subnetworks intercon-
nected through bridges/switches, the sets of communication services provided at the data-link layer
should be at least similar. Limitations on the kinds of networks that can be interconnected concern,
for example, their addressing scheme (which has to be uniform on the whole network) and the max-
imum transfer unit (MTU) that affects the allowed payload size (this is because fragmentation is not
permitted at the data-link level). When subnetworks with different MTUs are interconnected, care
has to be taken so that the payload of the exchanged frames never exceeds a threshold equal to the
smallest among the supported MTUs.
Figure . shows an example of interconnection taking place through a bridge for the hybrid
networks considered in this chapter. From a practical point of view, the bridge used in hybrid
wired/wireless networks is a device equipped with two (or more) transmitting/receiving interfaces
(one for each subnetwork) and, optionally, a protocol converter. A frame originated from whatever
segment (either the wired network or the wireless extension) is propagated to the other one by the
bridge, which receives the frame, converts it into the suitable (target) format, and then retransmits it.
When real-life devices and solutions are considered, access points (APs) are a very popular example
of bridge. They are used to set up hybrid networks consisting of one (or more) wired IEEE .
segments and an IEEE . wireless extension. Data-link layer services, in this case, are exactly
thesameforthetwokindsofnetworks(i.e.,thoseforeseenfortheIEEEfamilyofprotocols),
