Architecture
4.1.1.1 Network Topologies
The standard addresses two topologies: ad hoc topology, referred to as Independent Basic Service Set (IBSS), and infrastructure topology, referred to as Basic Service Set (BSS). A topology that combines several BSS cells is referred to as Extended Service Set (ESS).
4.1.1.1.1 Ad Hoc Network
As shown in Figure 4.2, an ad hoc network or an IBSS consists of stations within mutual communication range of each other via the wireless medium. Such a network is created spontaneously, without preplanning, for ad hoc temporary situations with limited needs to access the Internet. The IBSS is the most basic type of an IEEE 802.11 WLAN and may contain only two stations. Figure 4.2 shows two IBSSs, each with two stations. If a station moves out of its IBSS, meaning out of range, it can no longer communicate with the other IBSS members.
- Figure 4.2. Ad Hoc Topology: Independent Basic Service Sets
4.1.1.1.2 Infrastructure Network
The infrastructure network or BSS includes an access point (AP) in addition to the stations. This AP acts as the BSS arbitrator, meaning that the AP will handle all the BSS traffic. The BSS traffic can be either internal traffic (i.e., among the BSS participants) or external traffic (between the BSS participants and outside the BSS). The AP integrates the BSS within the distribution network. For example, all traffic between the BSS participants and the Internet will be delivered through the AP.
Figure 4.3 shows an ESS composed of two BSSs, each with two stations and an AP. Each BSS is interconnected to the distribution system (DS) (which may connect to the Internet) through the AP. The key concept is that the ESS network appears as a single network entity to the upper layers including the applications.
- Figure 4.3. Infrastructure Topology: Extended Service Set
To allow effective communication for users who constantly move from one BSS geographical area to another BSS area, the BSSs should be physically overlapping. In the standard there is no limit to the distance between the BSSs and it is left up to the user or network installer to determine the BSS location. Moreover, other users may decide to install an IBSS in the same geographical area. In this case, multiple WLANs will be collocated and function effectively if the respective users coordinate the used frequencies.
4.1.1.1.3 Collocation
Wireless networks can be located in overlapping geographical areas (see Figure 4.4). Radio signals may propagate from one network to another. A change in the transmitter position and geographical layout may have a profound effect on the propagation characteristics. Hence such networks do not have a clear boundary. The possibility of having multiple collocated operational WLANs not only allows multiple independent user groups to work simultaneously in the same place but also aggregates and increases the WLAN capacity.
- Figure 4.4. Collocated WLANs
Collocation is possible if network installers make an effort to coordinate the frequencies used by their respective WLANs. If they use DSSS based WLANs they can coordinate the center frequency. Several centers are possible according to the region in which the networks are installed. For example, IEEE 802.11b in North America details three non-overlapping channels and six overlapping channels (see Figure 4.5). This means that up to three networks can be located without mutual interruptions.
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