WLAN Operation

802.11 Wireless Topology Modes

Wireless LANs can accommodate various network topologies. The 802.11 standard identifies two main wireless topology modes: Ad hoc mode and Infrastructure mode. Tethering is also a mode sometimes used to provide quick wireless access.

Ad hoc mode

Ad hoc mode – This is when two devices connect wirelessly in a peer-to-peer (P2P) manner without using APs or wireless routers. Examples include wireless clients connecting directly to each other using Bluetooth or Wi-Fi Direct. The IEEE 802.11 standard refers to an ad hoc network as an independent basic service set (IBSS).

 

 
Infrastructure mode
Infrastructure mode – This is when wireless clients interconnect via a wireless router or AP, such as in WLANs. APs connect to the network infrastructure using the wired distribution system, such as Ethernet.
Tethering
Tethering – A variation of the ad hoc topology is when a smart phone or tablet with cellular data access is enabled to create a personal hotspot. This feature is sometimes referred to as tethering. A hotspot is usually a temporary quick solution that enables a smart phone to provide the wireless services of a Wi-Fi router. Other devices can associate and authenticate with the smart phone to use the internet connection.

BSS and ESS

Infrastructure mode defines two topology building blocks: A Basic Service Set (BSS) and an Extended Service Set (ESS).

BSS and ESS

Infrastructure mode defines two topology building blocks: A Basic Service Set (BSS) and an Extended Service Set (ESS).

ESS

Extended Service Set

When a single BSS provides insufficient coverage, two or more BSSs can be joined through a common distribution system (DS) into an ESS. An ESS is the union of two or more BSSs interconnected by a wired DS. Each ESS is identified by a SSID and each BSS is identified by its BSSID.

Wireless clients in one BSA can now communicate with wireless clients in another BSA within the same ESS. Roaming mobile wireless clients may move from one BSA to another (within the same ESS) and seamlessly connect.

The rectangular area in the figure depicts the coverage area within which members of an ESS may communicate. This area is called the Extended Service Area (ESA).

802.11 Frame Structure

Recall that all Layer 2 frames consist of a header, payload, and Frame Check Sequence (FCS) section. The 802.11 frame format is similar to the Ethernet frame format, except that it contains more fields, as shown in the figure.

All 802.11 wireless frames contain the following fields:

  • Frame Control – This identifies the type of wireless frame and contains subfields for protocol version, frame type, address type, power management, and security settings.
  • Duration – This is typically used to indicate the remaining duration needed to receive the next frame transmission.

From a wireless device:

  • Address 1 Receiver Address – MAC address of the AP.
  • Address 2 Transmitter Address – MAC address of the sender.
  • Address 3 SA/DA/BSSID – MAC address of the destination which could be a wireless device or wired device.

From the AP:

  • Address 1 Receiver Address – MAC address of the sender.
  • Address 2 Transmitter Address – MAC address of the AP.
  • Address 3 SA/DA/BSSID – MAC address of the wireless destination.
  • Sequence Control – This contains information to control sequencing and fragmented frames.
  • Address4 – This usually missing because it is used only in ad hoc mode.
  • Payload – This contains the data for transmission.
  • FCS – This is used for Layer 2 error control.

CSMA/CA

WLANs are half-duplex, shared media configurations. Half-duplex means that only one client can transmit or receive at any given moment. Shared media means that wireless clients can all transmit and receive on the same radio channel. This creates a problem because a wireless client cannot hear while it is sending, which makes it impossible to detect a collision.

To resolve this problem, WLANs use carrier sense multiple access with collision avoidance (CSMA/CA) as the method to determine how and when to send data on the network. A wireless client does the following:

  1. Listens to the channel to see if it is idle, which means that is senses no other traffic is currently on the channel. The channel is also called the carrier.
  2. Sends a request to send (RTS) message to the AP to request dedicated access to the network.
  3. Receives a clear to send (CTS) message from the AP granting access to send.
  4. If the wireless client does not receive a CTS message, it waits a random amount of time before restarting the process.
  5. After it receives the CTS, it transmits the data.
  6. All transmissions are acknowledged. If a wireless client does not receive an acknowledgment, it assumes a collision occurred and restarts the process.

Wireless Client and AP Association

For wireless devices to communicate over a network, they must first associate with an AP or wireless router. An important part of the 802.11 process is discovering a WLAN and subsequently connecting to it. Wireless devices complete the following three stage process, as shown in the figure:

  • Discover a wireless AP
  • Authenticate with AP
  • Associate with AP

In order to have a successful association, a wireless client and an AP must agree on specific parameters. Parameters must then be configured on the AP and subsequently on the client to enable the negotiation of a successful association.

  • SSID -The SSID name appears in the list of available wireless networks on a client. In larger organizations that use multiple VLANs to segment traffic, each SSID is mapped to one VLAN. Depending on the network configuration, several APs on a network can share a common SSID.
  • Password – This is required from the wireless client to authenticate to the AP.
  • Network mode – This refers to the 802.11a/b/g/n/ac/ad WLAN standards. APs and wireless routers can operate in a Mixed mode meaning that they can simultaneously support clients connecting via multiple standards.
  • Security mode – This refers to the security parameter settings, such as WEP, WPA, or WPA2. Always enable the highest security level supported.
  • Channel settings – This refers to the frequency bands used to transmit wireless data. Wireless routers and APs can scan the radio frequency channels and automatically select an appropriate channel setting. The channel can also be set manually if there is interference with another AP or wireless device.

Passive and Active Discover Mode

Wireless devices must discover and connect to an AP or wireless router. Wireless clients connect to the AP using a scanning (probing) process. This process can be passive or active.

Passive mode

In passive mode, the AP openly advertises its service by periodically sending broadcast beacon frames containing the SSID, supported standards, and security settings. The primary purpose of the beacon is to allow wireless clients to learn which networks and APs are available in a given area. This allows the wireless clients to choose which network and AP to use.

Active mode

In active mode, wireless clients must know the name of the SSID. The wireless client initiates the process by broadcasting a probe request frame on multiple channels. The probe request includes the SSID name and standards supported. APs configured with the SSID will send a probe response that includes the SSID, supported standards, and security settings. Active mode may be required if an AP or wireless router is configured to not broadcast beacon frames.

A wireless client could also send a probe request without a SSID name to discover nearby WLAN networks. APs configured to broadcast beacon frames would respond to the wireless client with a probe response and provide the SSID name. APs with the broadcast SSID feature disabled do not respond.

12.3.8 Check Your Understanding – WLAN Operation

 

CAPWAP Operation