Table of Contents
Advertisement
Figure 2. Infrastructure network (ESS).
An independent network, or BSS (Basic Service Set), consists of two or more stations that
communicate directly with each other through the wireless medium. This kind of network can
also be referred to as an ad hoc network. Physical limitations determine the direct station-to-
station distance that can be supported. For some networks this distance is adequate, other
networks require increased coverage.
An infrastructure network, or ESS (Extended Service Set), is constructed from multiple BSSs,
access points and a distribution system (DS).
DS enables mobile device support by providing the logical services necessary to handle
address to destination mapping and seamless integration of multiple BSSs. DS extends the
network and enables communication between stations that are too far from each other to
communicate directly.
An access point has station functionality and provides access to the distribution service. Data
moves between the BSS and DS via an access point.
The key concept of ESS is that it appears to the LLC layer as an independent network.
Stations within an ESS can communicate with the other stations, and mobile stations may
move from one BSS to other BSSs within the same ESS transparently to LLC. 802.11 does
not assume anything about the physical locations of the architectural components.
Integration with the existing wired LANs is possible through a portal. Portal is the logical point
at which MSDUs (MAC Service Data Unit) from a non-802.11 wired LAN enter the 802.11
DS. A portal can be implemented in the same physical device as an access point, but not
necessarily. Portal is not equivalent to a traditional bridge in the strict sense, since a bridge is
used to connect similar MAC layers.
2.1.4 Services
The 802.11 architecture allows for the possibility that the DS may not be identical to an
existing wired LAN. DS can be implemented with many different technologies, including
current 802.X wired LANs. 802.11 does not constrain the DS to be either Data Link or
Network layer based. Nor does 802.11 constrain the DS to be either centralized or distributed
in nature. Instead of specific implementation, 802.11 defines services. There are two
categories of 802.11 services: Station Services (SS) and Distribution System Services
(DSS). Both categories are used by the 802.11 MAC layer.
STA 1
STA 2
BSS 1
AP
DS
Portal
802.X LAN
ESS
AP
STA 3
STA 4
BSS 2
7/113
Advertisement
Table of Contents
