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1.3.1.3
Hidden node
A collision occurs when two stations transmit simultaneously on the same medium because
each station has detected that the medium is free. This type of collision occurs when two
stations, which cannot contact one another, simultaneously transmit to a third station, which
they can both contact.
Transmitter 1
Maximum
range
Figure 1-7
Hidden node
The MAC level RTS/CTS protocol (Request to Send/Clear to Send) specified in IEEE
802.11 provides a solution in the form of a special handshake method. RTS or CTS
telegrams (with confirmation message) are exchanged to reserve a radio channel for a
specific period of time. To prevent collisions between RTS/CTS telegrams, which would
render them useless, these telegrams are very short. The telegram length at which the
RTS/CTS mechanism is used can be specified in the device. This function is useful when
using very long telegrams, e.g., when downloading.
1.3.2
Infrastructure mode - Basic Service Set
Infrastructure mode is the simplest form of a wireless network. Communication between all
devices is via a shared access point. This constellation is called the Basic Service Set (BSS).
If the wireless range of an individual access point is insufficient (e.g., because too many
devices are present, the transmit range or the available bandwidth is insufficient), several
physically overlapping BSSs can be grouped to form a shared wireless network. This
expansion is called an Extended Service Set (ESS). In an ESS, the access points must be
connected together. This connection can be via wires, but is usually via Ethernet or via radio
paths. The use of an ESS enables devices to communicate with one another, even if they
are not in the range of a shared access point.
In addition, with ESS mobile devices can be automatically forwarded from one access point
to another without interrupting communication (roaming).
Functions and wireless technology
Transmitter 2
PWR
POE
HOST
1
2
3
4
PWR
HOST
1
2
3
4
719000013
1-11
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