WLANs also differ from wired LANs as follows:
- WLANs connect clients to the network through a wireless access point (AP) or wireless router, instead of an Ethernet switch.
- WLANs connect mobile devices that are often battery-powered, as opposed to plugged-in LAN devices. Wireless NICs tend to reduce the battery life of a mobile device.
- WLANs support hosts that contend for access to the RF media (frequency bands). 802.11 prescribes collision avoidance (CSMA/CA) instead of collision-detection (CSMA/CD) for media access to proactively avoid collisions within the media.
- WLANs use a different frame format than wired Ethernet LANs. WLANs require additional information in the Layer 2 header of the frame.
- WLANs raise more privacy issues because radio frequencies can reach outside the facility.
The table summarizes the differences between wireless and wired LANs.
|Characteristic||802.11 Wireless LAN||802.3 Wired Ethernet LANs|
|Physical Layer||radio frequency (RF)||physical cables|
|Media Access||collision avoidance||collision detection|
|Availability||anyone with a wireless NIC in range of an access point||physical cable connection required|
|Regulation||different regulations by country||IEEE standard dictates|
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.
- Address1 – This usually contains the MAC address of the receiving wireless device or AP.
- Address2 – This usually contains the MAC address of the transmitting wireless device or AP.
- Address3 – This sometimes contains the MAC address of the destination, such as the router interface (default gateway) to which the AP is attached.
- 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.
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:
- 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.
- Sends a ready to send (RTS) message to the AP to request dedicated access to the network.
- Receives a clear to send (CTS) message from the AP granting access to send.
- If the wireless client does not receive a CTS message, it waits a random amount of time before restarting the process.
- After it receives the CTS, it transmits the data.
- All transmissions are acknowledged. If a wireless client does not receive an acknowledgement, 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 processes 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
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