The 802.11 standard identifies two main wireless topology modes: infrastructure mode and Independent Basic Service Set (IBSS). IBSS is also knows as ad hoc mode. With the ubiquity of wireless networks, mesh topologies are now common.
With infrastructure mode, wireless clients interconnect via an AP. Figure 22-4 illustrates infrastructure mode terminology. Notice that the configuration of the APs to share the same SSID allows wireless clients to roam between BSAs.
Figure 22-4 Example of ESS Infrastructure Mode
Infrastructure mode terminology includes the following:
Basic service set (BSS): This consists of a single AP interconnecting all associated wireless clients.
Basic service area (BSA): This is the area that is bound by the reach of the AP’s signal. The BSA is also called a cell (the gray area in Figure 22-4).
Basic service set identifier (BSSID): This is the unique, machine-readable identifier for the AP that is in the format of a MAC address and is usually derived from the AP’s wireless MAC address.
Service set identifier (SSID): This is a human-readable, non-unique identifier used by the AP to advertise its wireless service.
Distribution system (DS): APs connect to the network infrastructure using the wired DS, such as Ethernet. An AP with a wired connection to the DS is responsible for translating frames between 802.3 Ethernet and 802.11 wireless protocols.
Extended service set (ESS): When a single BSS provides insufficient coverage, two or more BSSs can be joined through a common DS into an ESS. An ESS is the union of two or more BSSs interconnected by a wired DS. Each ESS is identified by its SSID, and each BSS is identified by its BSSID.
IBSS, or Ad Hoc Mode
In the 802.11 standard, Independent Basic Service Set (IBSS) is defined as two devices connected wirelessly in a peer-to-peer (P2P) manner without the use of an AP. One device takes the role of advertising the wireless network to clients. The IBSS allows two devices to communicate directly without the need for any other wireless devices, as shown in Figure 22-5. IBSSs do not scale well beyond 8 to 10 devices.
Figure 22-5 802.11 Independent Basic Service Set
Having a wired DS connecting all APs is not always practical or necessary. Instead, APs can be configured to connect in mesh mode. In this mode, APs bridge client traffic between each other, as shown in Figure 22-6.
Figure 22-6 Example of a Wireless Mesh Network
Each AP in the mesh maintains a BSS on one channel used by wireless clients. Then the APs bridge between each other using other channels. The mesh network runs its own dynamic routing protocol to determine the best path to the wired network.