- Independent Basic Service Set (IBSS)
- Basic Service Set (BSS)
- Extended Service Set (ESS)
Independent Basic Service Set (IBSS) allows two or more devices to communicate directly with each other without a need for a central device. This is known as Ad hoc mode where a peer to peer network between stations is formed without the need for an Access Point.
Basic Service Set (BSS) Wireless LAN is established using a central device called an Access Point that centralizes access and control over a group of wireless devices. All wireless devices do not communicate directly with each other but instead they communicate with the AP, and the AP forwards the frames to the destination stations. The Access Point manages the wireless network, advertises its own existence by broadcasting the Service Set Identifier (SSID) and any device that needs to use the wireless network must first send an association request to the Access Point. The Access Point can require any of the following criteria before allowing a client to join.
- A matching Service Set Identifier (SSID)
- A compatible wireless data rate
- Authentication credentials
After a client has associated itself with the Access Point, all communications to and from the client will traverse the AP.
The wireless coverage area of an AP is called the Basic Service Area (BSA), sometimes also referred as Wireless Cell. An AP can also be connected to a wired Ethernet Local Area Network through an uplink port connection unlike the Independent Basic Service Set in which the wireless network cannot be connected to the wired network.
The BSS is uniquely identified by the Basic Service Set Identifier (BSSID) which is the Layer 2 Mac address of the BSS access point. The wireless network although is advertised using an SSID which announces the availability of the wireless network to devices.
Extended Service Set (ESS) is created by connecting multiple Basic Service Set (BSS) via a distribution system. Two or more Access Points are connected to the same Local Area Network to provide a larger coverage area which allows the client to move from one AP to another AP and still be the part of the LAN. This process is known as roaming in which a client can physically change locations and still be connected to the LAN. When a client senses that radio signal from the current AP are getting weaker, it finds a new AP with stronger signals starts using that AP. An ESS generally includes a common SSID to allow roaming from access point to access point without requiring client re-conﬁguration.
The wireless coverage area created by joining two or more Access Points via distribution system is called an Extended Service Area (ESA). Stations within the same ESA may communicate with each other, even though these stations may be in different basic service areas and may even be moving between basic service areas, this requires that the wireless medium and the backbone of the ESS must be layer 2 link.
A Distribution System connects multiple Access Points to form an ESS, while doing so it provides the wireless stations with the option of mobility. It is the means by which an access point communicates with another access point to exchange frames for stations in their respective BSSs, forward frames to follow mobile stations as they move from one BSS to another, and exchange frames with a wired network. Network equipment outside of the extended service set views the entire ESS and all of its mobile stations as a single layer 2 network where all stations are physically stationary. Thus the ESS hides the mobility of the mobile stations from everything outside the ESS. This allows correct inter-operation with other network protocols that do not understand the concept of mobility.
Figure below show a Basic Service Set on left side and an Extended Service Set on the right side.
Although not shown in the figure an Access Point can act as a bridge between the wireless and wired LANs, allowing stations on either LAN to communicate with each other.
In this lesson we learned different types of Service Sets in a Wireless LAN. This topic is one of the foundation topics in the Wireless Section of Cisco CCNA Certification and anyone who is preparing for the CCNA Certification must be able to define and differentiate the services sets of a wireless LAN.