General description -1

General description

4.1 General description of the architecture
This clause presents the concepts and terminology used within this standard. Specific terms are defined in
Clause 3. Illustrations convey key IEEE 802.11 concepts and the interrelationships of the architectural
components. IEEE Std 802.11 uses an architecture to describe functional components of an IEEE 802.11
LAN. The architectural descriptions are not intended to represent any specific physical implementation of
IEEE Std 802.11.
4.2 How WLAN systems are different
4.2.1 Introduction
Wireless networks have fundamental characteristics that make them significantly different from traditional
wired LANs.
4.2.2 Wireless station (STA)
In the design of wired LANs it is implicitly assumed that an address is equivalent to a physical location. In
wireless networks, this is not always the case. In IEEE Std 802.11, the addressable unit is a station (STA).
The term implies no more than the origin or/and destination of a message. Physical and operational
characteristics are defined by modifiers that are placed in front of the term STA. For example, in the case of
location and mobility, the addressable units are the fixed STA, the portable STA, and the mobile STA. The
STA is a message destination, but not (in general) a fixed location.
A STA might take on multiple distinct characteristics, each of which shape its function. For example, a
single addressable unit might simultaneously be a portable STA, a quality-of-service (QoS) STA, a
dependent STA, and a hidden STA.
4.2.3 Media impact on design and performance
The PHYs used in IEEE Std 802.11 are fundamentally different from wired media. Thus IEEE 802.11
PHYs:
a) Use a medium that has neither absolute nor readily observable boundaries outside of which STAs
with conformant PHY transceivers are known to be unable to receive network frames
b) Are unprotected from other signals that are sharing the medium
c) Communicate over a medium significantly less reliable than wired PHYs
d) Have dynamic topologies
e) Lack full connectivity, and therefore the assumption normally made that every STA can hear every
other STA is invalid (i.e., STAs might be “hidden” from each other)
f) Have time-varying and asymmetric propagation properties
g) Might experience interference from logically disjoint IEEE 802.11 networks operating in
overlapping areas
Because of limitations on wireless PHY ranges, WLANs intended to cover reasonable geographic distances
may be built from basic coverage building blocks. When providing QoS services, the MAC endeavors to
provide QoS “service guarantees” within the limitations of the medium properties identified above. In other
words, particularly in unlicensed spectrum, true guarantees are often not possible. However, gradations of
service are always possible; and in sufficiently controlled environments, QoS guarantees are possible.