802.11 Architecture

 
802.11 Architecture
IEEE 802.11 supports three basic topologies for WLANs: the IBSS, BSS, and ESS. All three
configurations are supported by the MAC layer implementation.
The 802.11 standard defines two modes: ad hoc/IBSS and infrastructure mode. Logically, an ad hoc
configuration is analogous to a peer-to-peer office network in which no single node is required to function
as a server (see Figure 2-6). IBSS WLANs include a number of nodes or wireless stations that
communicate directly with one another on an ad hoc, peer-to-peer basis, building a full-mesh or partialmesh
topology. Generally, ad hoc implementations cover a limited area and are not connected to any
larger network.
Figure 2-6: Wireless ad hoc network
Using infrastructure mode, the wireless network consists of at least one AP connected to the wired
network infrastructure and a set of wireless end stations. This configuration is a BSS (see Figure 2-7).
Since most corporate WLANs require access to the wired LAN for services (file servers, printers, and
Internet links), they will operate in infrastructure mode and rely on an AP that acts as the logical server for
a single WLAN cell or channel. Communications between two nodes, A and B, actually flow from node A
to the AP and then from the AP to node B. The AP is necessary to perform a bridging function and
connect multiple WLAN cells or channels as well as connect WLAN cells to a wired enterprise LAN.
Figure 2-7: Wireless BSS
An ESS is a set of two or more BSSs forming a single subnetwork. ESS configurations consist of multiple
BSS cells that can be linked by either wired or wireless backbones. IEEE 802.11 supports ESS
configurations in which multiple cells use the same channel and use different channels to boost
aggregate throughput (see Figure 2-8).
Figure 2-8: 802.11 ESS
802.11 Components
802.11 defines two pieces of equipment, a wireless station, which is usually a PC equipped with a
wireless NIC, and an AP, which acts as a bridge between the wireless and wired networks. An AP usually
consists of a radio, a wired network interface (802.3, for example), and bridging software conforming to
the 802.11d bridging standard. The AP acts as the base station for the wireless network, aggregating
access for multiple wireless stations onto the wired network. Wireless end stations can be 802.11 PC
card, Peripheral Component Interconnection (PCI), or Industry Standard Architecture (ISA) NICs, or
embedded solutions in non-PC clients (such as an 802.11-based telephone handset).
An 802.11 WLAN is based on a cellular architecture. Each cell (BSS) is connected to the base station or
AP. All APs are connected to a DS, which is similar to a backbone, usually Ethernet or wireless. All
mentioned components appear as an 802 system for the upper layers of OSI and are known as the ESS.
The 802.11 standard does not constrain the composition of the DS; therefore, it may be 802 compliant or
nonstandard. If data frames need to transmit to and from a non-IEEE 802.11 LAN, then these frames, as
defined by the 802.11 standard, enter and exit through a portal. The portal provides logical integration
between existing wired LANs and 802.11 LANs.
When the DS is constructed with 802-type components, such as 802.3 (Ethernet) or 802.5 (Token Ring),
then the portal and the AP are the same, acting as a translation bridge. The 802.11 standard defines the
DS as an element that interconnects BSSs within the ESS via APs. The DS supports the 802.11 mobility
types by providing the logical services necessary to handle address-to-destination mapping and the
seamless integration of multiple BSSs. An AP is an addressable station, providing an interface to the DS
for stations located within various BSSs. The IBSS and ESS networks are transparent to the LLC layer.
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