WLANs
Wireless networks have fundamental characteristics that
make them significantly different from traditional wired LANs. Some countries
impose specific requirements for radio equipment in addition to those specified
in the 802.11 standard.
On wired LANs, a MAC address equates to a
physical location. This is implicitly assumed in the design of wired LANs. In
IEEE 802.11, the addressable unit is a station (STA). The STA is a message
destination, but it is generally not a fixed physical location, as shown in
Figure
.
The
physical layers used in IEEE 802.11 are fundamentally different from those used
with wired media. The following is true about the IEEE 802.11 PHY protocols:
Figure

- They use a medium that has neither absolute nor readily observable
boundaries, outside of which stations will be unable to send or receive network
frames.
- They are unprotected from outside signals.
- They communicate over a medium that is significantly less reliable than
wired media.
- They have dynamic topologies.
- They lack full connectivity. Normally, the assumption is made that every
STA can hear every other STA. This assumption is invalid for WLANs. STAs may be
hidden from each other.
- They have time-varying and asymmetric propagation properties.
Because of limitations on wireless PHY ranges, WLANs needing to cover
reasonable geographic distances must be built up from basic coverage building
blocks.
One of the requirements of IEEE 802.11 is to handle mobile as
well as portable stations. A portable station is one that is moved from
location to location, but is only used while at a fixed location. Mobile
stations actually access the LAN while in motion. It is not sufficient to
handle only portable stations, since propagation effects blur the distinction
between portable and mobile stations. Fixed stations often appear to be mobile,
due to these propagation effects.
Another aspect of mobile stations is
that they are often battery powered. Therefore power management is an important
consideration. For example, it cannot be presumed that a station receiver will
always be powered on.
IEEE 802.11 is required to appear to higher
layers, such as LLC, as an IEEE 802 LAN. The IEEE 802.11 network must handle
station mobility within the MAC sublayer.