An 802.11 LAN depends on a cellular architecture in which the system is subdivided into cells. Each cell called Basic Service Set, or BSS, in the 802.11 nomenclature that is controlled by a Base Station called Access Point (AP).
Even though a wireless LAN may be formed by a single cell having single Access Point, but
most installations are formed by
several cells, where the Access Points are connected through some kind of backbone called
Distribution System (DS). This backbone is typically Ethernet mainly wireless itself. The whole interconnected Wireless LAN along with different cells having their respective Access Points and the Distribution System, is viewed as a single 802 network to the upper layers of the OSI model and is so called in Standard as Extended Service Set (ESS).
The following diagram shows a typical 802.11 LAN including the components described above:
The concept of a Portal is defined by the standard, as a device portal interconnects between an 802.11 and another 802 LAN. This concept is a complex description of a "translation bridge".
Although the standards not necessarily request it, also that typical installations will have the AP and the Portal on a single physical entity. This is the case with BreezeCOM's AP having both functions.
IEEE 802.11 Layers Description
In any 802.x protocol, the 802.11 protocol covers the MAC and Physical Layer. The Standard
currently give light to a single MAC which interacts with three PHYs (all running at 1 and 2
Mbit/s) as follows:
Frequency Hopping Spread Spectrum in the 2.4 GHz Band
Direct Sequence Spread Spectrum in the 2.4 GHz Band
Infrared
As far the standard functionality performed by MAC Layers, the 802.11 MAC performs
other functions that are usually related to upper layer protocols, such as Fragmentation, Packet
Retransmissions, and Acknowledges.
The IEEE 802.11 standard defines two basic network topologies.
ad hoc/IBSS
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 .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. It does not use APs-only STAs are involved in the communications.
depicts a sample IBSS that includes a mobile telephone, laptop computer, and a PDA communicating via IEEE 802.11 technology. The circle in Figure represents the signal range of the devices, which is important to consider because this determines the coverage area within which the stations can remain in communication. A fundamental property of IBSS is that it defines no routing or forwarding, so all the devices must be within radio range of one another.
One of the key advantages of ad hoc WLANs is that theoretically they can be formed anytime and anywhere, allowing multiple users to create wireless connections cheaply, quickly, and easily with minimal hardware and user maintenance. In practice, a number of different types of ad hoc networks are possible, and the IEEE 802.11 specification allows many of them. An ad hoc network can be created for various reasons, such as supporting file sharing activities between two client devices. However, client devices operating solely in ad hoc mode cannot communicate with external wireless networks. A further complication is that an ad hoc network can interfere with the operation of an AP-based infrastructure mode network that exists within the same wireless space.
infrastructure mode
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 .It has an AP that connects wireless STAs to each other or to a DS, typically a wired network. Infrastructure mode is the most commonly used mode for WLANs.
The ad hoc mode is depicted conceptually .This mode of operation, also known as peer-to-peer mode, is possible when two or more STAs are able to communicate directly to one another. Three devices communicating with each other in a peer-to-peer fashion without any wireless infrastructure or wired connections. A set of STAs configured in this ad hoc manner is known as an independent basic service set (IBSS). An ESS is a set of two or more BSSs forming a single sub network. 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.
In infrastructure mode, an IEEE 802.11 WLAN comprises one or more Basic Service Sets (BSS), the basic building blocks of a WLAN. A BSS includes an AP and one or more STAs. The AP in a BSS connects the STAs to the DS. The DS is the means by which STAs can communicate with an organization's wired LANs and external networks, such as the Internet. The IEEE 802.11 infrastructure mode is outlined in Figure by two BSSs connected to a DS.
The IEEE 802.11 Wireless LAN Architecture
The 802.11 architecture have several components and services that cooperate to give station mobility transparent to the upper layers of the network stack.
Wireless LAN Station
The station (STA) is the most fundamental module of the wireless network. A station is device that contains the functionality of the 802.11 protocol, that being MAC, PHY, and a connection to the wireless media. Characteristically the 802.11 functions are to bring about in the hardware and software of a network interface card (NIC).
A station is a laptop PC, handheld device or an Access Point. Stations maybe mobile, portable or stationary .All stations support the 802.11 station services of authentication, de-authentication, privacy, and data delivery.
Basic Service Set (BSS)
802.11 define the Basic Service Set (BSS) as the vital building block of an 802.11 wireless LAN. The BSS is a group of any number of stations. The BSS not amused one until it take the topology of the WLAN into attention.
IEEE 802.11 has two fundamental architectural components:
Station (STA). A STA is a wireless endpoint device. Typical examples of STAs are laptop computers, PDAs, mobile telephones, and other consumer electronic devices with IEEE 802.11 capabilities.
AP An AP logically connects STAs with a distribution system (DS), which is typically an organization's wired infrastructure. APs can also logically connect wireless STAs with each other without accessing a DS. In addition, APs can function in a bridge mode, which allows APs to create point-to-point connections to join two separate networks.
Today, a STA is most often thought of as a simple laptop computer using an inexpensive wireless network interface card (NIC) that provides wireless connectivity. As IEEE 802.11 and its variants continue to increase in popularity, many other types of devices could also be STAs, such as scanners, printers, and digital cameras.
The use of multiple APs connected to a single DS allows for the creation of wireless networks of arbitrary size and complexity. In the IEEE 802.11 specification, a multi-BSS network is referred to as an extended service set (ESS). Figure conceptually depicts a network with both wired and wireless capabilities, similar to what would generally be deployed in an enterprise environment. It shows three APs with corresponding BSSs, which comprise an ESS. The ESS is attached to the wired enterprise network or DS, which, in turn, is connected to the Internet and other outside networks. This architecture could permit various STAs, such as laptop computers and PDAs, to access network resources and the Internet. In addition, the use of an ESS provides the opportunity for IEEE 802.11 WLAN STAs to roam between APs while maintaining network connectivity.
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.
