IBSS – Independent Basic Service Set
The first type of service set is known as an IBSS (Independent Basic Service Set), which is often simply referred to as an Ad Hoc network. In this type of service set, all stations operate in a peer to peer manner, without the need for a Wireless Access Point (AP).
In an Ad Hoc network, client devices are simply need to be wireless enabled through the use of a wireless NIC (Network Interface Card) to enable communications with similar devices using the same wireless frequency. There is rarely any advance planning or site surveys involved prior to setting up an Ad Hoc WLAN. Wireless devices can communicate with any other devices within range, but situation can arise whereby one device maybe able to hear two other devices, but those devices cannot hear each other. This situation has become known as the ‘hidden node scenario. Because this is an Ad Hoc network, the device in the middle cannot relay information between the two external devices, therefore no communication can occur between the stations out of range of each other. As there is no AP involved to manage the WLAN, timing has to be achieved in what is known as a distributed manner. This is first achieved by the first station to transmit setting what is known as the Beacon Interval, and this station creates a set of TBTTs (Target Beacon Transmission Times). Client devices use this interval in the following way:
Once the TBTT has been reached by a client device, it will:
• Suspend any pending backoff timers from a previous TBTT.
• Determine a new random delay.
• If another beacon signal arrives before the end of the random delay, suspend the random backoff timers. If no beacon arrives then send a beacon and resume the suspended backoff timers.
The beacons have an embedded Timer Synchronisation Function (TSF), and each client device compares the TSF in received beaconswith it’s own timer and if the received value is greater, then it updates it’s own timer. The result of this is that every active client device will eventually synchronise with the station with the fastest timer. This has the effect that eventually every client will synchronise with the station that has the fastest timer. The number of client devices in the Ad Hoc network will determine the time elapsed before all stations have synchronized.
BSS – Basic Service Set
The second type of service set is the BSS (Basic Service Set), which does require the use of an AP - Access Point which acts as the controller for the network. All stations are associated with the AP and pass traffic through the Access Point.
ESS – Extended Service Set
Finally, an ESS (Extended Service Set) is a group of BSSs which are usually connected via a wired network. This wired network when associated with a WLAN is often referred to as the DS - Distribution System. Distribution Systems can also be other wireless networks. Another term for an ESS is a Multiple Infrastructure BSS.
Wireless technologies and concepts are discussed on a number of our Training Courses.