Basic service set

5.3 Architecture

The standard defines two types of services: the basic services set (BSS) and the extended service set (ESS).

5.3.1 Basic service set

IEEE 802.11 defines the BSS Service State BSS set as the building block of wireless LANs. It consists of fixed or mobile devices, mobile and a central optional device, known as Access Point (AP). Figure 5.1 shows two groups in this standard. BSS without AP is a separate network and cannot send data to other BSSs. It is called an ad hoc architecture. In this architecture, Stations can form a network without the need for an AP, they can find each other and agree to be part of a BSS. An BSS with an AP is often referred to as an infrastructure network.

Figure 5.1 Basic service set

5.3.2 Extended ESS service set

(Expanded Service State) ESS, consists of two or more BSS with AP. In this case, the BSSs are connected to a distribution system, which is usually a wired LAN. The distribution system connects the USSR APs. IEEE 802.11 does not limit the distribution system, but can be any type of IEEE LAN such as an Ethernet.

Note that the extended set of services uses two types of stations: mobile and stationary. Mobile stations are normal stations within a BSS. Stationary stations are AP stations that are part of a cable LAN. Figure 5.2 shows an ESS. When BSSs are connected, stations that have the ability to connect to each other can communicate without the use of an AP. However, communication between two stations in two different BSSs usually occurs through two APs.

Figure 5.2 Extended set of ESS services

Types of Stations

IEEE 802.11 defines three types of stations based on their movement on a wireless LAN: stations without transition, with BSS transition, and with ESS transition.

• A station without movement transit is stationary or moves only within a BSS.

• A BSS transition station can move from one BSS to another, but the movement is locked inside an ESS.

• An ESS transition station can move from one ESS to another.

5.4 MAC sublayer

There are two MAC sub-layers in this protocol, but the one most commonly used is based on CSMA / CA (carrier sense multiple access with collision avoidance).

Figure 5.3 shows the flow diagram.

Figure 5.3 CSMA / CA flow diagram

Wireless LANs cannot implement CSMA / CD for three reasons:

1. For collision detection a station must be able to send data and receive collision signals at the same time. This requires costly stations and increased demand for bandwidth.

2. The collision may not be detected due to the hidden station problem. We will discuss this problem later in the chapter.

3. The distance between stations can be large. Weakening of the signal may prevent the station on one side from hearing the collision of the station on the other side.

1. Before sending the frame, the source station listens to the transmission to check the energy level at the carrier frequency.

a. The channel uses a persistent back-off strategy until the channel is free.

b. Once the station is free, the station waits for a period of time called

Distributed InterFrame Space (DIFS) space, then the station sends a control frame called Request To Send-RTS.

2. After receiving the RTS it waits for a period of time called short interframe space

(Short InterFrame Space-SIFS). The destination station then sends a control frame, called Clear To Send-CTS, to the source station. This control frame indicates that the destination station is ready to receive data.

3. The source station also sends the data after waiting for an amount of time equal to SIFS.

4. The destination station, again waits a time equal to SIFS and then sends an acknowledgment signal to indicate that the frame has been taken. Recognition is necessary in this protocol because the station has no means to check for the successful arrival of its data at the destination. On the other hand, the lack of collision on the CSMA / CD is a kind of indicator of the source that the data came.

5.4.2 Network allocation vector

How do other stations delay sending their data if one station requires access? In other words, how is collision avoidance achieved through this protocol? The key is a feature called NAV. When a station sends a frame RTS, it also includes the time it takes to keep the channel busy. Stations affected by this type of transmission create a ringtone called Network Alocation Vector (NAV), which indicates how much time must pass before these stations are allowed to watch for free channel. Every time a station accesses the system and sends the RTS frame, other stations start their NAV. In other words before the stations check the transmission channel they check their NAV to see if the time is up.


The wireless environment is very noisy, a corrupt frame needs to be retransmitted. For this reason, the protocol recommends fragmentation (dividing large frames into smaller ones), as it is easier to retransmit a small frame than a large one.

Frame format, MAC layer Frame consists of 9 fields as in Figure 5.5.

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