The main component of an 802.11 wireless network is the radio card, which is referred to by the 802.11 standard as a station (STA) . The radio card can reside inside an access point or be used as a client station.
The 802.11 standard defines three separate 802.11 topologies, known as service sets, that describe how these radio cards may be used to communicate with each other.
These three 802.11 topologies are known as a basic service set (BSS), extended service set (ESS), and independent basic service set (IBSS). 802.11 radio cards can also be used in topologies not defined under the 802.11 standard.
Some examples of these nonstandard topologies are bridging, repeating, workgroup bridging, and mesh networking.
Before we discuss the different 802.11 topologies, we need to review a few basic networking terms that are often misunderstood: simplex, half-duplex, and full-duplex. These are three dialog methods that are used as communications methods between people and also between computer equipment.
In simplex communications, one device is capable of only transmitting and the other device is capable of only receiving. FM radio is an example of simplex communications.
Simplex communications are rarely used on computer networks. In half-duplex communications, both devices are capable of transmitting and receiving; however, only one device can transmit at a time.
Walkie-talkies, or two-way radios, are examples of half-duplex devices. IEEE 802.11 wireless networks use half-duplex communications. In full-duplex communications, both devices are capable of transmitting and receiving at the same time.
A telephone conversation is an example of a full-duplex communication. Most IEEE 802.3 equipment is capable of full-duplex communications.
The only way to accomplish full-duplex communications in a wireless environment is to have a two-channel setup where all transmissions in one direction are receiving while all transmissions in the other direction are transmitting.
Access Point
The CWNP definition of an access point (AP) is a half-duplex device with switchlike intelligence. A wired infrastructure device typically associated with half-duplex communications is an Ethernet hub.
A wired hub is effectively a shared medium in which only one host device can transmit data a time. Access points are half-duplex devices because the RF medium uses halfduplex communications that allows for only one radio card to be transmitting at any given time.
In reality, an access point is simply a hub with a radio card and an antenna. The radio card inside an access point must contend for the half-duplex medium in the same fashion that the client station radio cards must contend for the medium. Access points do have some switchlike cleverness that a wired hub simply does not possess.
For example, although not defined by the 802.11 standard, an access point can support virtual local area networks (VLANs) that can be created on managed wired or wireless switches. VLANs are used to reduce the size of broadcast domains and to segregate the network for security purposes.
Wired hubs do not support VLANs. Another example of switchlike intelligence used by access points is the ability to address and direct wireless traffic. Managed switches maintain dynamic MAC address tables that can direct packets to ports based upon the destination MAC address of the packet.
Similarly, an access point is a portal device that directs traffic either to the network backbone or back into the wireless medium. The 802.11 header of a wireless frame typically has three MAC addresses, but it can have as many as four in certain situations.
The access point uses the complicated layer 2 addressing scheme of the wireless frames to forward the upper-layer information either to the distribution system medium or to another wireless client station. The upper-layer information that is contained in the body of an 802.11 wireless data frame is called a MAC Service Data Unit (MSDU).
The 802.11 standard considers the radio card in an access point to be a unique station (STA) that provides connectivity between mobile 802.11 STAs (client stations) and a network infrastructure that may be either wired or wireless.
Access point directs traffic to ports much as a switch does. In the case of an AP, the traffic is directed to either the Ethernet portal or the radio card portal.
Because an access point operates in a half-duplex shared medium and possesses some switchlike intelligence, an AP is a hybrid device that might be humorously characterized as a wireless SWUB (half switch/half hub).
Client Stations
A radio card that is not used in an access point is typically referred to as a client station. Client station radio cards can be used in laptops, PDAs, scanners, phones, and many other mobile devices.
Client stations must contend for the half-duplex medium in the same manner that an access point radio card contends for the RF medium. When client stations have a layer 2 connection with an access point, they are known as associated.
Distribution System (DS)
Access points by their very nature are portal devices. Wireless traffic can be destined back onto the wireless medium or forwarded onto what is called the distribution system (DS).
The DS consists of two main components: Distribution System Medium (DSM) A logical physical medium used to connect access points Distribution System Services (DSS) System services built inside an access point usually in the form of software.
A single access point or multiple access points may be connected to the same distribution system medium. The majority of 802.11 deployments use an AP as a portal into an 802.3 Ethernet backbone, which serves as the distribution system medium.
Access points are usually connected to a switched Ethernet network, which often also offers the advantage of supplying power to the access points via Power over Ethernet (PoE). An access point may also act as a portal device into other wired and wireless mediums.
The 802.11 standard by design does not care, nor does it define onto which medium an access point translates and forwards data. Therefore, an access point can be characterized as a “translational bridge” between two mediums.
The AP translates and forwards data between the 802.11 medium and whatever medium is used by the istribution system. Once again, the distribution system medium will almost always be an 802.3 Ethernet network as pictured in Figure below.
Although rare, 802.5 token ring access points do exist, and the distribution system medium would be the 802.5 token ring infrastructure. In the case of a wireless mesh network, the handoff is through a series of wireless devices with the final destination being an 802.3 network.
Wireless Distribution System (WDS)
Although the DS normally uses a wired Ethernet backbone, it is possible to use a wireless connection instead. A wireless distribution system (WDS) can connect access points together using what is referred to as a wireless backhaul.
A WDS may operate using access points with a single 802.11 radio or dual 802.11 radios. Figure below depicts two 802.11b/g access points, each with a single radio.
The radios in the APs provide access to the client stations and communicate with each other directly as a WDS.
A disadvantage to this solution is that throughput can be adversely affected due to the half-duplex nature of the medium, particularly in a single radio scenario where an access point cannot be communicating with a client station and another access point at the same time.
The end result is a degradation of throughput. In Figure below, two dual radio access points are shown, each with radios operating at different frequencies.
The 2.4GHz 802.11b/g radios provide access for the client stations, and the 5GHz 802.11a radios serve as the WDS link between the two access points. Throughput is not adversely affected because the 2.4GHz radio cards can communicate at the same time as the 5GHz cards.
Service Set Identifier (SSID)
The service set identifier (SSID) is a network name used to identify an 802.11 wireless network. The SSID wireless network name is comparable to a Windows workgroup name.
The three 802.11 topologies utilize the SSID so that radio cards may identify each other in a process known as active scanning or passive scanning. The SSID is a configurable setting on all radio cards, including access points and client stations.
The SSID can be made up of as many as 32 characters and is case sensitive. Most access points have the ability to cloak an SSID and keep the network name hidden from non-legitimate end users.
Hiding the SSID is a very weak attempt at security that is not defined by the 802.11 standard; however, it is an option many administrators still choose to implement.
Basic Service Set (BSS)
The basic service set (BSS) is the cornerstone topology of an 802.11 network. The communicating devices that make up a BSS are solely one access point (AP) with one or more client stations.
Client stations join the AP’s wireless domain and begin communicating through the AP. Stations that are members of a BSS are termed as “associated.” Figure below depicts a standard basic service set.
Typically the AP is connected to a distribution system medium (DSM), but that is not a requirement of a basic service set. If an AP is serving as a portal to the distribution system, client stations may communicate, via the AP, with network resources that reside on the distribution system medium.
It should also be noted that if client stations wish to communicate with each other, they must relay their data through the access point. Stations cannot communicate directly with each other unless they go through the access point.
Basic Service Set Identifier (BSSID)
The 48-bit (6-octet) MAC address of an access point’s radio card is known as the basic service set identifier (BSSID). The BSSID address is simply the MAC address of a BSS access point. Do not confuse the BSSID address with the SSID address.
The service set identifier (SSID) is a network name that is user configurable, while the basic service set identifier (BSSID) is the layer 2 MAC address of an AP provided by the hardware manufacturer.
The BSSID address is found in the header of most 802.11 wireless frames and is used for identification purposes. The BSSID address also plays a role in directing 802.11 traffic within the basic service set.
The BSSID address is also used as a unique identifier of the basic service set. Furthermore, the BSSID address is also needed during the roaming process.
Basic Service Area (BSA)
The physical area of coverage provided by an access point in a BSS is known as the basic service area (BSA). Client stations may move throughout the coverage area and maintain communications with the AP as long the received signal between the radios remains above RSSI thresholds.
Client stations may also shift between concentric zones of variable data rates that exist within the BSA. The process of moving between data rates is known as dynamic rate switching.
The size and shape of a BSA depend upon many variables, including AP transmit power, antenna gain, and physical surroundings. Because environmental and physical surroundings often change, the BSA can often be fluid.
Extended Service Set (ESS)
While a BSS might be considered the cornerstone 802.11 topology, an extended service set (ESS) 802.11 topology would be analogous to an entire stone building. An extended service set is two or more basic service sets connected by a distribution system.
An extended service set is a collection of multiple access points and their associated client stations, all united by a single DS. The most common example of an ESS has access points with partially overlapping coverage cells, as shown in Figure below.
The purpose behind an ESS with partially overlapping coverage cells is to provide seamless roaming to the client stations. Most vendors recommend cell overlap of 15 to 20 percent to achieve successful seamless roaming.
Although seamless roaming is usually a key aspect of WLAN design, there is no requirement for ESS to guarantee uninterrupted communications.
For example, an ESS can utilize multiple access points with nonoverlapping coverage cells as pictured in Figure above. In this scenario, a client station that leaves the basic service area (BSA) of the first access point will lose connectivity.
The client station will later reestablish connectivity as it moves into the coverage cell of the second access point. This method of station mobility between disjointed cells is sometimes referred to as nomadic roaming.
One final example of an ESS deploys multiple access points with totally overlapping coverage areas, as pictured in Figure 6. This 802.11 ESS topology is called co-location, and the intended goal is increased client capacity.
It should be noted that all three of the previously mentioned extended service sets share a distribution system. As stated earlier, the distribution system medium is usually an 802.3 Ethernet network; however, the DS may use another type of medium.
In the majority of extended service sets, the access points all share the same service set identifier (SSID) network name. The network name of an ESS is often called an ESSID (extended service set identifier).
Although an ESSID is essentially synonymous with an SSID, there is no requirement for all the access points in an ESS to share the exact same network name. Access points that share a DSM may have different SSIDs and still be classified as an extended service set.
Independent Basic Service Set (IBSS)
The final service set topology defined by the 802.11 standard is an independent basic service set (IBSS). The radio cards that make up an IBBS network consist solely of client stations, and no access point is deployed. An IBSS network that consists of just two stations (STAs) is analogous to a wired crossover cable.
An IBSS can, however, have multiple client stations in one physical area communicating in an ad-hoc fashion. Figure below depicts four client stations communicating with each other in a peer-to-peer fashion.
All of the stations transmit frames to each other directly and do not route their frames from one client to another. All client station frame exchanges in an IBSS are peer-to-peer. All stations in an IBSS must contend for the half-duplex medium, and at any given time only one STA can be transmitting.
Nonstandard 802.11 Topologies
The three service sets defined by the 802.11 standard are basic service set (BSS), extended service set (ESS), and independent basic service set (IBSS). Wi-Fi vendors also utilize 802.11 radio cards in nonstandard topologies while still remaining compliant with the 802.11 standard.
The most common example is wireless bridging. 802.11 radios can be used to connect two wired networks together using a wireless bridged link. Another very common nonstandard 802.11 topology is the workgroup bridge (WGB).
A workgroup bridge acts as a gateway for a small wired workgroup, yet the workgroup bridge is a client station associated with an access point. A repeater is a special access point that forwards the data of client stations to a root access point.
The net effect of a repeater is that the root access point’s coverage cell is extended. Wireless mesh routers are essentially a combination of multiple repeaters using proprietary layer 2 routing protocols.
