The limited distance over which wireless transceivers can
operate, combined with the use of carrier sensing to determine
when it is safe to transmit, may also result in what is referred
to as a ‘hidden terminal’ problem, where two devices located
out of the range of each other try and communicate with a
third device that is located between them without knowledge
of the other’s actions. Wired carrier sensing technologies such
as Ethernet are able to avoid such problems by ensuring
that each device has knowledge of all others to which it is
connected, for example by limiting the total length of cable
allowed between any two stations. Even with careful planning
and device location, such knowledge cannot be guaranteed in
a wireless medium. Wireless transceivers are also only able
to operate at half-duplex, as their own transmissions would
overpower any signal they might be intended to receive.
Physical overhead on a wireless system is also significant in
comparison to wired systems, as most wireless protocols require
the transmission of predetermined data sequences before
or during data transmission in order to evaluate and correct
the effects of noise on the received information. Security of
wireless transmission is also of concern, as physical access
to the transmission medium cannot be restricted. Many wired
fieldbusses are also able to make use of passively-powered
field devices by supplying the energy required for the device’s
operation over the transmission medium. The existing wireless
technologies have no such capability and provision for energy
to remote devices is a concern, as is the energy efficiency of
the remote devices.